Monolayer Films Prepared by the Spontaneous Self-Assembly of ...

Exposure of evaporated gold films supported on silicon rvafers to solutions of dialkyt sullides__(R-
(CH,ri-S(CH2),,R'; R and R' represent CH3 or CO.:H) or alkanelhiols (R(CH1)^SH; R represents CO2H or
CHri in merhinoi or ethanol iesults in rapid formation of a monolayer of the organosulfur cornpound
adsorbed onto the gold. The resulting films have been characterized by using a number of lechniques,
including X-ra1'pholoelectron spectroscop)'(XPS), infrared spectroscopy (IRS), ellipsometr)', qnd we.lting.
These self-a.se-bled, supported organic monolay'er films are s!'stems that can be used to sludy ptoblerns
in the phy.sical-organic chemistry and materials science of organic surfaces, especially the relation between
the molecular-level strucLure of the film constituents and the mecroscopic properties of the assembled
monolayers. The films are relalively robust: examples formed from both dialky'l sulfides and alkanethiols
withstand washing and prolonged soiking in neutral water, but f-rlms conta.ining diallll sultldes are destroyed
by'conlact with aqueow base at pfi 6-13. Srudi.'s of the thickness of the di,tlkf'l sultide films b.v ellipsome-r"ry'*,
oi the orientation of the hydrocarbon cheins in these filnrs by'IFtS, and of the polarity'of the interface
between the films and w'atei by' contact angle suggesl a ph1'sical model for the film of a relativell' disotdered
organic liquid, liquid cr1'stai, or glass pinned to the gold surface by gold-sulfur coordinative bonds.
Colresponding studies of thiol frlms suggest that lhey are more ordered than thedialkyl sulfide fiims. If
polar organic-functional groups are incorporated into the terminal positions of thealky'l moieties of the
aia.tt

.t
T,
-v
:i
p1
fl
l{
366 Langmuir, Vol. 4, I,{o. 2, ^l988
,!u::,^:
) r$.r.,./
I ln' rf\
_ffi0il
AAAIAT
uwu
uwu
n,ae
Figure l. Schematic illustration of the physicar-organic approach
to the design of interfaces with 'buried; functional-itv. tire attvt
chains are indicated here as extended only for iliusiration; they
are almost certainly less ordered than these diagrams suggest.
muir-Blodgelt films and are certainly'no more difficult to
characterize.r? Thel' are intrinsically simpler structura-lly
than the interfacial region of surface-functionalized poljrrers,l3-';
since their thickness is fixed al one molecular
layer. They are usuall)' more stable than organic crystals
and more easily obtained with large areas oi surface. Infrared
spectroscopic and electrochemical studies have indicated
that both alkanethiols and dialkyl disulfides form
monolayers that are highly ordered.r0 Similar claims ha'e
been made for monolayer films based on long-chain alkylsilanes
covalently attached to glasstT and silicars and
fatty acids adsorbed on oridized alumjnumle and silver,sil
but the interpretation of the spectroscopic evidence in
terms of interfacial structure remains conlroversial. Detailed
studies of interfacial properties as a function of
systematic variations in the structure of a sinsle adsorbate
system have remained an obvious need tolncrease the
understanding of organic inlerfaces.
Systems based on dialkyl sulfides are, in principle,
particularly attractive as the basis for studies of structure-property
relationships since the structures of the two
alkyl groups connected to sulfur can be varied independently
by straightforward slnthetic methods: this variation
permirs a degr'ee of conbrol of the local slructure of the
adsorbed monolayer thal is not easily possible with other
organosulfur compourds, or, broadly, w-ith monolayer filrns
prggaled by assembly' of components based on simple faity
acid derivati'es, The particular hypothesi. urud as the
basis for this work is summarized in Figure 1. Adsorption
9l un _unsymmetrical dialkl'l sulfide, CHr(CHz)^S-
(CH2)^CO2H, through sulfur onto a gold surface results
in a monolay'er film whose structure depends both on the
D.
=[u=
w
Polar Arm
Longer
Polar anC f.l
cn.polar
Arms Equal
irJcn-polar
Arm
LOng:r
Nl. J. Am. Chern. Soc.
T. J.; Deutch,
Langnuir 19E6, 2,
Nl. Langmuir 1997,3, 62-?5.
Nuzzo, R G ; Nfdarthy, T.
;Allara, D. L. Chem.
Troughton e
molecular composition of the dialkyl sulfide and on
details of its packing and ordenng. The bwo chairs of t.
types of dialkyl sulfides terminate in groups having
ferent hydrophilicity and polarity-methyl and carbo:,
acid. If the films are structurally ordered in a war. :
places the pollmethylene chains roughly perpendicula
the surface of the met^1, and if the length of the carbo:.
acid terminated chain is equal to that of the methy.l_
minated chain, we expect the poiar carboxy,lic acid t
exposed at the film-water inlerface anC to int'lui
welting. If the me:hy'l-terminated chain is longer rha,-.
carboxylic acid terminated chain, we expect the inter
belween the orga-ric film and a welting liquid tr,
CHr/CH2-like (thar is, nonpolar in characreri.
Ii the adsorbeC fiim is structuially disordered an
quidlike, tr'e cannor predict its wettabiiity' a priori,
expect to be able to infer at least certain of its sr,ruci
characteristics fron comparisons of wetrabilir-r.a:d s:
troscopic characier with those of better-characte r
model s!'slems. Thus, b1's5'stematic l'ariatiorr rn
structures of dialkr I suliides adsorbed on guld, r,r.e €\,
to be able to determine man)' f:aiures oi the strucI
of these monola,r'er films. In the event that th: fil:
highll' ordered, Be would have availabie an e.trreLflexible
system rrr:h which to studl' the properties o:
terfaces betrveen Iiquids and organic solids; in th= e..
that the film is less well ordered, this sl.stern nri;h:
pro!'e l'aluable in study'ing organic surface chenri.;rr
Tfris work relies on a number of conr.entional optir-al
spectroscopic tech:liques to characterize the aclsr_,rlt.,d :
but places particular emphasis on me:rsurt,!nini:
wettabilitl'. The riason for this emphasis is th,_. int'cr:
fronr our !!'orktJ ll
and from that oI olhers:r thet ,,r'e','
is one oI the mos: 'surface sensitive' techniqu..s p:r'se:
available. \\'etrir.. has the further useful ch:rr.r:rer
that it is applica:le to the stud_r' of interfu.gs [..gr.r.
condensed phases-a regime thai cannor be explore
using man!' speciroscopic techniques requi:i:r; r
vacuurn-and tha: ir is a technique thai is p.rriicrr.
simple experimen:ally'.
Early evidence that the free energl.of B sr.rlici-li
interface is influe:ced only by'atoms a'" or clirsr. tr) :
interface came frcn Langmuir's "Principle of I;'rrleler,
Surface Aclion':i:t and from invesr,igalions of' tl-.r rr.,
biliti' of poly'me:s and monolay'er films carrieci ou'
Zisman and others]!?r Zisman's studies incl'rrl.-.C a r,
variet.l' of subslra:+s and suggested lhal solids coirioi:
molecules with la;;e uncompensaied dipoles corrlC a:'
rvetting properties even if these dipoles w'erL'-i)Lr:
under the ouleri:1)sl layers of a solid suria... l-
I:
important study',r Shafrin and Zisman demons:rr::.ci '
the rvettability' of nonomolecular films of a series o:
sorbed acids hevin3 the structure CF3(CF:).rCF{,r .,C'
(n = 0, I,2, 4, axc 6) decreased rr'iih incre.r.;in; Ien::
the perfluoroalkl'l lroup up to n = i; aL thi-r p,)il:. ri....
had w'etting prope:,ies that" resernbled th,tse of nr.':rol
(21) ConSa:i A.rr'lr, !i';ll:brijl), onC A j.:es:on: Fo..r k:s, F. \l
Advances in Chemis:i {3; Americen Che.xica! Soci..i,,.: \l'.r;hir
D.C., r96{.
(23) Langrnuir, I. J. A,n. Chen. Soc. 1916,3.J, 2't-rrj The'Pnr
of hdependent Surface.{ction'is ba-oed on the hlpothesis thrr e-rclof
a molecule possesses r local surface enerfy and thai thr. rt.liros
interfac'ial free ener3l is made up of the sum of lhe moleculrr i
contribulions from th,xe atoms erposed ai the surface.
(2{) Langmuir, I. T^,:.-d Colloid Syrnpcsirrn Llonogrcpl-.; Chen
talog: New York, l9:15.
(25) Shafrin, E.C.;Ze mar, \V. A..L Phys. Chen 1960,6J,5li
(36) Shafrin, E.C;Z.scoan, \\'. rlu J. Phys. Chem. 19S7,6/, lC{&
(2?) Shafrin, E.G;Zrsman, \\'. A. J. Ph;s.Chem. 1962,6b, ?lri
(28) Adamson, A- \\'. Physical Chemi:try of Surlases,4th ed,; \'
New l'ork, l9$J; Chap'cr 3, pp 63-65.
, 1,'f:/.'.y!2'--,r,. .:.: :
'itlr,.
',. -.'.
-
',r
.

I I
I I
I I
i
I
I
\
Iv{onolayer Fitms Prepared from Dtathyt Sult'ides
fihns prepared from perfluorooctanoic acid. lvfore recent
studies have compared surface compositions in reconstructing
p-olymers using XPS and wettinglr-te and have
measured directly the interaction bebween solids, including
organic films, using film balance techniques.F
In none of these studies is interpretation unambiguous,
since in none is the structure of the interface unr-Jbig.rously
determined. Nonetheless, trvo general conclusiJns
emerge: where van der lvaals interaclions determine the
energ-y of a solid-liquid interface, the interactions that
determine wetting fall off roughly as r{ and are thus very
short rallge in character-probably'extending over no more
than one to two atomic lay.ers (3-6 A). Dipole interactions
fall off more slorvly (r-3) and may in parficular systems2?
extend into deeper layers of the solid il0 A). By contrast,
photoelectron spectroscopy-a'surface sensitive;
spectroscopy-typica]ly' samples 35-100 A of the surface
of nonmeta-llic materials.3o
In this work, we have used carboxylic acid groups as our
polar gr.o^upof
-choice in probing tire surfaJe for several
reasons.13-16'31'3:
- Firsl, carboxylic acid groups are hydrophilic
and strongly influence rvettabilitl,. secbnd, they can
be con'erted into
Tore
hy'drophilic carboxl.late anions by'
deprotonation, and this deprotonation reaction can be
conveniently followed by' measuring the contacl angle of
buffered aqueous solutions (conLact ansle titration).13" The
acidity inferred for carboxy'lic acids at interfaces can be
compared with the body of information concerning the
acidity of carboxylic acid groups in solution. Thiri, we
have used carboxy'lic acid groups and contacr angle tit,ration
extensi'ely' in examining the properties of surfacemodified
polyethylene and ha'e a'iilaLte a large amount
of information for comparison.13-td Fourrh, the beha'ior
of carborylic acid groups at interfaces is rere'anl to a w.ide
range of other phenomene in materials science33,3{ and
biologr-.ls-lr
\1'e note explicitll' thar the concepts of *surface' 'interface" and
are not uniquell' defined for s1'stems of the
types examined here and are probably. best described in
terms of specific erperimental measurements.ls \l'e wili
use the word 'surface' to indicate that part of a condensed
phase in direct'an der \\'aals contact wirh another phase.
This w'ord is reasonably'sharpl-l'defined in discussing the
outermosl atomic la1'er of gold atoms of the substrateand
its interaclions r'ith coordinated sulfur atoms but is more
difficult to apply tothe organic monolay.ers, particula;I1'
if these are disordered, mobile, or srr.ollen by,a contactin!
liquid.
Results
-
General considerations. organic monola.v'er films oi
dialk.vl sulfides *'ere prepared b!' dipping a t-hin, evapo-
..(29) Israelachvili, J.lntern,'tl,ecular cnd surlcce Forces: ll'ith Ap.
piicatton to CoiloiCc! and BiologiccJ S-r,sie^*; Acadecric: London, 1985.
_ (_39) Kelier, K. H.; Andrade, J. D.; Bai.r,
!. E.; DiXingham, E. O tEly,
J.; Klein, E.; lrlorrisey., B. \1,.; Alrieri, F. D. iuiCei,ie, 1o, enysrcochemical
charccter;,:ct!on,of Bioma:ei.aLs, ir-IH pubL,ication'No. 6G2l.s6;
N.aiional Heart Lung, and Blood Institute, NIH: Betherau, trro, tsso;
Chapter 4, p {t.
(31) Hotrues.Farley, S. R.; Bain, C. D.;\!hitesides, G. Nt., ur:publ.ished
res ul ts.
(32) Troughton, E. 8., unpublished resuls.
K. Composifey
^.^(3ll^lV*ltersson, Sc.ience and Technology l9gl, 22,
-(311
\\'altersson, K. Cornposites Science and Technology 1995,23,
303-32 L
(q5) Jencks, W. ?.
.Catalysis in Chemistry and Enzymology;
It{cCraw-Hill: New }'ork, 1960; pp 9c-9tj.
^ - (qql Susj, H.; Z.ell,T.;
fimasheif,
S. l.{. Arch. Biochem. Broptryr. 1959,
&i,,137-{{3
-^13]l
Eisenberg, D.; NfcLachlun, A. D. A'cjure (London) 19E6,3/9,
199-20J.
Langmuir, Vol. 4, No. .2,.IW
.- 367
rated gold filxo.supporred on a singlu;9*ririii4i.;^f..
into an alcoholic solution of the diarktl sulfide'at room
temperature for infervals of time ranging from minutes to
hours. The adsorption process, and ttrelharacteristics ol
the resultrlg films, were fouowed by number or t*n"iqu..,
qf ylich ellipsometry, X-ray phoioelectron spectro.lopr.
cxPS), infrared spectroscopy, and measuremunt of contaci
angle using aqueous solutions were the most useful. obtaining
reproducible results in these experiments required
careful attention to detail at a number of points, most
particularly in the preparation and purification of the
dialkyl sulfides. 1\'e discuss each of these techniques in
turn.
Preparation and Handling of Gold Substrates. The
gold subst-rates ye-re prepared by thermal evaporation of
a -2000'A'thick layer of high-iuriry gold onto chromium-primed,
electronic-g:ade single-cryital s ilicon wafe rs.
During the course of the experimental work, gold films
\r.e19 prepared by using fir'e different evapoltors, including
both thermal and, electron-bu^- rouices. Four of
these e'aporator. yielded fi[rnJthat ga'e rise to rronolay.er
films having similar properties; the one exception is discussed
further i1 th9 Experimental section. The gold Frlms
w'ere not annealed before use: upon annealing L o*y,gun
at 300 oC, the gold surface became clouC;.and adopi.i
sil'ery hue, probably'due ro rnigration of chromiun and/or "
silicon to the gold-air interiace. After prepara.,ion, samples
n'ere ordinaril-v' srored in wafer tray's w'rapped with parafilm
and *'ere nor inteniionally'exposed to Conhminanrs;
othe^r'ise, they'w'ere not protected. The-r'could te stored
before use for ai least 1 w'eek r'ithoui influencing thc.
properties of the film \\'irh minimal handlins of thu
samples, XPS (see belorr ) sho*.ed little carbon Jr sulfur
conlamination on the gold. It is clear, however, that the
surface of the gold was coniaminated b1, adsorbed impurities;
the conlact angle of water on these samples was
t-v'picalll' 0\ - 60o compared to 0n - 0-Bo reported for
clean gold.3*-r'l Nel'ertheless, the monolay,ers prepared
b1'.adsorption on this golc had reproducible prop..tils; \4'e
believe that the strongli' adsorbing org.ro.ulfur compounds
di.spl.aced residual elements of the adventitious
c()nraminating surface fil:r. Ellipsometrv was not useful
for the deterrnination of ii".e cbsol,uf e 'alues of the thick.
ness of the containinani_. preadsorbed on gold after remo'al
from the e'apora:,lr both because of poor reproducibilitl'
(at this level oi anall.sis: b-30 Ai of the opiical
properties of samples precared in different evaporitions
and due to the absence o: a suitable reference value for
q,lld. Gold samples storec in *'cer tra;'s or exposed to the
laborator)' B:mosphere ir: e!'en a fe*, hturs rapidly adsorbed
contarninants, as i:ldicated by the increase in the
\&'arer contact angle fronr 6 J" to g(lo and the accumulation
of a thick fiIm (+-tO At U'. ellipsometrl.. Before use in the
preparalion of monolay'e:s, the substrates were cleaned b1'
brief w'ashing rvith etha:ri. This rr.ashing appeared to
remo!'e man)'of the adso:bec contarninants; euipsometrr
shorved a decrease in th: rhickness of the adsoibed film
and the contact angle d=.-reased from 90o to b5-70".
The gold surface obLain=c by'e'aporation under r.acuunr,
exposure to the laboratoq,'ar_r:rosphere, and washing with
ethanol rvas, thus, a suita'cle substrate in the s.nse that
anf impurities adsorbed cn it had minimal effects on the
properties of the organic r:-ronolay'ers a:ld on the characterization
of these monola', ers but cerlainll' was hot 'clear'
in the sense used in high-vacuum phy'sics. In these and
(3S) Smith, T. J. Colloid In::.1.:ce Sci. 1930, 25, bl-65. "
(39) Schrader, Irl. E. J._Ccil:i'! In:erltte Sci. igSJ,rr0O,'B?2-3g0.
({0) Caines, C. t-. J. Coiloi: [".:erfc;i S;i. l9Bt, 79,-29S.;
.

368 Langmuir, Vol. 4, No. 2, Ig88
Figu rc. 2. Scenning elc'clrc,n nrie f ,J{rJph oi a gold sample reprr.sr-ntutire
ol'lhr,se rrsed in thr'se studi.-s A l,lr,Lnm bar is sho'*n.
t)thc.r slrrclir.s oI orglrnic suri.icc. cherrristrvli rr.e hal'e been
surpris..ci b_r' the relerirt' inserr.;irir it,,' cri tht' results to
cietails ttt' the hist()rv oi the sarr:irlu. il the cttse of organ()sLlllur
ct)rllf)t)ttocl. ads,,ritecl on g,rl,l, rr: b.:lieve that this
i n.se n-s i t i vi tl' re tlec t.s t rr, rt c i reu n ii l.rn(.r: l-r i) i, o rganosulfur
cont[)r)Lln(is c,t,rrclinltIr. sir rir1l,, 1,r g rlri ir;t,] displace mOSt
organic contlrrnirr.rnt.i prcsrn: ():r th._. g,rl lsrrr[ace. Independr.Frt
StLrdits t,r lrt, rri)r)iic'(l l.rlr'rr. ilt,licrte that organosul
irrI crtnl[)r)tr ncl.s r,xih.i llg.. b.,.Ir,, :cn the grtld surface
and a cottlitcIiltg Si)ltlli()it. !illts, 6cJ.1r, rS'.lll'ur compounds
ma)' e! en displ.ict oihc.r of g.lnosulr'ur conlaminants.
Second, the o rg.ur ic rrr,rn,rl.r-r'e ri thc rnst l'. es c()nstitute in-
Lerfaces hr,,'ing lorr intc'ri'aci.rl t'ree en,,'rgies and do not
adsorb c()ntarrrin.rnt.; strongl,,'. It ir p:,_ blbIe that some
oI the scatrer in our ellipsornc.Lric d,ri.r is clue lo variable
q,-rslttities ot' inrJruritic.s arlsori;irl orr tirr g ,lri srrrfa ce bet'ore
[r)trnali,rn ()t'Iire nr,'rVll,t,r.'-,r'arr,,i f',_.r1,,.['. f I residual Cont,trttirrtlltis
tir)[ cirsJrl rct,cl i". clr.tlx,, I - ri:iilc.s. Certain
pi',rlllcnr- irr rt-,Irirrcl,-r, il,ill:', r,.il.ir til.,n. ,::,,1]-rer systems
Cilai)LlltLert',.i F)fri,riir-'.tli,, Cl,l:i:r-i trli-',!,:^ Sc)€tTl tO be
t'orr.:!.'rteil rv11f1 lh',. tt.,'ci-l'1li,il L):f :r.1,',-, r.i i,rlJlile organic
thi,'lr ilt til'-'l.rll,tr.rfo5., .r.:nt,,il,it,-,r,,,ri .-, l,,aCtivitieS Of
t-'r t hr-' f !! r); lir. p:.
It i; clri't'i--uli t,'clr.rr.r,,'it-'n.ir, Iir,_, -,::i r.r] oi these gold
s'.rllstralr.s Exliiiirr.rii,rii t,i'tr?i,.,ri s'iir*ti.,tic'S b1'scanning
e l:.ct rrxr trr i.r,.r.c, r1ri' ( S [: \ l, sh, )',! : n r ci t : : i nct i!.e features
ai a Scitl'-. ot' - ll){){l r\ (F'r:L:r.,_, I r [)tilr.r_-ti,tn and CrOSS
secti,)ntl f i.,\l strrciir.s i;rclrcate tir.rl th.:::'(,;[d films are
[),)l_tcr-\'.itallin,,wit]r ali ri!t,r,t:a9 il-.uir sli: Oi the Order Oi
Stlr.,l-l()t-)() A anrcl cxhilrii t \c'i\ s:r,rn; r ii l) texlure, perh.rps
excer'(lirrg
'i'he
lrr);I ttI th. t,,:rl s,rii'.r,-'..i. films are
h,)r-lro9t'ilcL)Lls ancl r.rrrit',;rrrrlv anci sir:rlpl', r.-tlectil'e to the
eVr.. I)resen', ccucle thettri,,'. r)l weLiiil!'--.: ci,r not, however,
take i n tt) ilccoLr n t atonr ir:- le vel s'.l r i.rce rrr i .-:ohete rogeneit).,
esgleciali','irr circunlstelrices oi th: s,)rt encoLlntered with
adsorbeci thin iilms (F-igr.rrr 3 i. 'i'hrrs,
iui ex.tmple, a sLep
on the g,tlci sur[nr--e. oI onl_\'a fr.rr iing;rfonls may e.xpose
({ l.) 'I'r,r'rghton,
Fl B ; Flv:ll, J ; B,r.n e. D. unpublished resulb.
( { l) J,rhn.;orr, R. E , ,Jr.; Derrre, [i [{ I r -s*ri;,j t c^.J, Colloid Science,
l\le lijeric, 8., E.l ; \\' iler'. In te rscic ncr.. N.:,.r \'..ri. liil; \'ol. 2, pp &1-153,
({:J) J,:hrr;,rrr, li Fl , Jr; L)c,ilre, n l{ J Pr.,,; Chem. t96{, .A?,
t7l{-175{).
({t) ::?: t.
de Cennes, P. C RcL..t/.,d. fh,,i t9Si,5l , .c l;
(15) Schrt,rrt:, [, \\';Crrroif, S Lc..{.,,r,r'r l,)rj
-SdJ. .(:if :ti I
.
, /, 219-230..,-i . ' '
"r'I
_
Trough,ton
Figure 3. Sche:- alic illustration of the influence oi a
plateau on the i:.:erface presented by S[(CH:).COrH],
in a perfectly o::ented monolayer of carbox',.1i.- acr
nrinated dialkl'l sulfides methylene groups rhar ,,
olhenrise be hicien in a completely pla.rar surface.
surface free ene:at'is considered as an area-!',eighic
perposition of cc:tributions from directly exposed o:
groups,tJ'13'!t rouirness at- this sca.le might har.e a p:.
influence on th: characterisbics of the interf:t_-e.
In practice, \!. cannol comment on the inliue:
roughness on th: properties of these monola]'er fil'm.
than to say we have seen no evidence that roush:.
imporLant. Sa::ples of supported evaporateC g ,.
pared in difiere:: e!'aporators-sy'stems thlt al:.:-r-,:
tainll' produce cifierent degrees of microscopic s
roughness-yiel: organic nronolal'ers shorving r.r). s
m3croscopic su:i-ice properties.
PreparaIion of Dialky'l Sulfides. These (,rili]
r!'ere prepared l;' displacement of bromide iror:r
bromides b1' a :.riolate anion (eq 1). IL rva,. cri:
RBr * -S(CH.)rrCOr-
"'- RS(CHr) ,CO
method A
- -r* RS(cFi,)
r ,co !
method B
importanl to pu:'.:1'the dialkyl sultldes to honro{.:ne.
parlicularil' to ersure the absence of thiols as inrpL
For rnanl' oI ou: erperimenLs, the dialky'l sulr'id.'.s ,
adsorbates w'ere prepared by tw'o indepenCen: n.
(methods A an: B, eq 1), as a check for con:am.
introduced duri:.i slnthesis. Thiol impurisiss 6r-rrtr -:
to gold more strc:gly'tha,r dialkylsulfides aid co:rc::
ai the interface preferentialll'. Impure CH,tglt
(CFtt;pCO,H, p:epared b1'method A, iniiial,'. r:
monola)'er lLlm-. i-..rring B. < 10o, whjle the san-:e c,)r::
prepared using r,ethoC B gave d, = 111', Ca::iul
rified samples p:epared b1' using either meihod ga'.
same resuil: 0u - lOJo. Intentional dopin3 of Inrs c
sult-rde w'ith l Tc ,'.\ lw') oI a thiol demonstraied rn:r:
at these levels, triol impurities strongl;' afiect rhe ',,
bility' of the fil;:., For eKample, doping of impur:
(CH")r?S(CHr)r'(lO:H prepared by' method A r.i:
CHr(CH:)r;SH i:.creased the contact angle iror:. 0.
to 0, - 112o. Dtping o[ the same sulfide prspsr:
method B with i% HS(CH')r'CO'H decreaseC d,
111o to ( 20o: tl:se contact a.ngles are indicaiive o[ a
excluive incorpc:ation of the thiol into the monolal'e:
considered diallill sulfides to be pure when thev y'i'
monolayer fiLrns ehose wettabilit),was independent c
method used fo: their sl'nthesis.
Preparation of Self-Assembled l\Ionolay'er Filn
Adsorption of Dialkf'l Sulfides from Solution
Cold. Gencral Features. Thb gold sdbstrate \\as r
with'rireIhanol c: ethanol before-usd.' The dialki'l sr

lv[onolayer Films prepared,
from Dialhyt Sulftdes
ea
120
100
T I
l.
!
. . *Eoto t=o*'--::
//////'z'g
T TI
j=___.li
_ I
n
- cl-Q:! : --O--
:!
-c,---""9
2,' a
Ii
tl
-l- _ _l
Langmuir, VoL 4, Iy,o.2,
IW 369
o r, fxslcHr)ncH,
o ^,1. 12) were not "ihi;h'"rorecurar
jll
solu6re ar concentrations
in
of
methanol,
0.05
ethanol, h.*r.., or hexadecane. Illonol"gr:
( r o, w e ttln3, cll i peonre try., a-,rd X.raV photoelcctron
rtrongly rupport a
rpectroscopy
modet i" ,iU.l ti.,l"fr7
bound to gord through the rur.:-.rr *i,r, ,i.tt,l,lt*^n ft'-pbnolav-er,
prepared from.ut*"t.a solutions in these
had,
sorvents
hower,'er,,wetting prgp.rties that were similar
hlnu prepared
to those
from.Lirah;,drofuran solutions (_0.005
in which the
IvI)
sorubility is itGh... The gord
mersed
fi'o was
in
im-
a solurion of theiialkvf r"riia.-(tyiicaily
12-36 h), remol'ed,
ro,
washea witn a srnail
.1qu
arnount
solvenr,
of the
and dried briefly in a steam-;;;;".
of these s;,stems
Some
were nor ,uropt o[il;;
fron ffi..g.a
the *.t
solution used u"a t.quired washing
forn:arion
to avoid the
of mutrilay,e, fil-s.- U;l;r;;;;;;?;,
hac
washing
no.imporfanr efflcr on the J;;d i,rw'ettability..
ir".uess or
Fgi.e..xalple, films prepared
of
by adsorption
didocos}'l sulfide from ietrahydrofuran
h
solution
were not
for
affffted (eliipsom.i.y,,.ontrct
B
angle
or hexadecane)
with water
br l0 .onr..utiv.e cl,cles of "washing
ethanol
with
and dry-ing or.bi'roni.ution
(see.
for 10 min in
the
ethanol
ExperimJntat Secrionl.
- The stabilities of the -onoruyer films ,,.ere,
good'
in geheral,
-The1' could be stored ioi"extended periods
in rvafer
of
rravs
time
or grass via[. rit-r;h;;i.g
so me
."nii.n..
con lam ina
of
tion.( for. e.xample, *,rn
'alue of
lrigf,
contact angre with waier.suggesting "*iu.i.afy
of
adsorption
h1'drophobic conLminants) could
b;' brief
;.;;;^il"yli .1.*.d
washing with 0.t X HCt, water,
Exposure
and ethanol.
to st16ng b*u l.a to irreverriUf.
wettatility'.
.l,"nges in
The monorayer, *=r., rro*uo=r,i;ti.
least 2{
for
h in w'ater
at
o'er the pH range 1-?.
prepared
The monorayers
from dlalty] sutnaes were not stable
temperatures
at
(-60 oC) high
or to cerfajn .."guni.
30%
including
h-r'drogen peroride and ulnur.rt solutions
ald phosphorous
of diborane
pentachloride.
^ 9ootact-Angle- Techniqu.. ur.d to.measure
tact the
angle,
con-
d, have been descriu"a i" irt"il;b."rr.r.iinii
measurebents were mbde ,t:iOOZ r.lotiu. hu,midity.
ll,thoqqh,'in.principle, rhe ;H ;f ';h;';i;r;on=uraa
representative films as a fu::ction of chain Iength.
fered' deionized wate-ra: pFI b.6. The insensitiv'ity,of
of these samples
mosr
to pH is discussed ir
For pH
;;;i., a.t"if
titrations,
belo,,r.
ihu bufferidescribed previousr;,we
emplol'ed tt re
Th: monorryer films were uns,,able
oi pH greater
at varues
than T. ior the sake of .on.irtuncy,
measurements
all
of contact angle wich drop,
made
oi;H )7
wirhin
were
1 min afrei aplpiicarion oi tt,.
surface'
;;;p to
The.values
the
tupoit.J may not represent
stationary'
true
angles. rn this w'ork, as in most
of
ol
organic
our studies
surface chemistry, rr-pt.. .-liuit"a jgnificanb
hy'steresis in the measured .onta.t angres, and
report
we norma,r.
an adr.anc.ing angle, d., obtai;.e;f,;;;;;;;il:
neous advance of the drop edge t rJ rl"pp.al"it,.
teresis (.0r- t rr-
!,,
meqsured in d.ir..s) seemed.,.
more with
*rr#i.
the polaritl' of the
measuring d is important
i'
in thebe types
practice, of ,tuair.rlii
mos.t ii.,
measurernents were made by,using'uibuf-
-tt.t1"
"sample
than
roughness:
with surface
more polar s,;fa*. tr.*gi;J;
larger
;;J
h1'steresis.
sho*.ed
The ,*.rpl.r used here were
standards
flat by the
of mosr. sarnples ur.a in ;;;r;;i;i'.oit".t
gles; it seems unlikerf iir.r tt'r. ooserved hl.steresis (14 "n. =
0"- 0, = 10o at 0, = 110., C cos, = ao, d. _ cos d, = _O.17;
A4 = 25o aL g^ = 7So, J cos d = -O,30;
ig =
= 4-d50"
50o, A
d"
cos d = -0.36) is due either to,*fr..iJugl_,n.., "t
or to.heterogeneity at the micron scare. There
theories
are
that
no
currentll' ciscuss hysteresis in terms
omic-scale
of at-
heterogunui:;, and *.e note that
measure
A cos d,
of
a
the differen'-'e in rneasured interfaciar
energies
free
for ad'ancing ard receding drop., i=ri*l.r"tir.el,,.
Iittle' Ir is thus un.r-.u,
"herhei irr.
hlsteresis with oorarity b a ..J pi.nomenon "ppr*'f.fr"i.'i'"
of
or
cons.idering
an a.tifacr
d" - d, rather thai cos d" _ c9s gr. Receding
angles, d,, s-how.
:::l?_.t
the same general trends
!'anclng
as
contact
adangres.
\'alues of d. < 15o are unieriabr:,
and such values are sho*n oJpfot" as -0".
. Fol r-eference, Figure I ptot*?. as a function
length
of
for
chain
monolay-er fih,,?r;;;l from four
pounds:
sets of com-
dialkl'l sulfides t"r-i"rti"q in methyr groups
ca;boxy'lic
a.nd
acii groups a:d arkanethiors arso terminarinu
tn these groups. Evidence discussed uu.*i.rr1bl,it-r;;:
Dound to sold through the rut:.ur, *irL tl.-titi ;G";f;#.: ar rhe
1urfs.ce, Bain, C. D.;Troughto"- d B;;_;
b il ' xii;: R.6: mdnurcr:r: ,,., o,.o*ti,'o'1Tao,l''T.jtttitesides,

i:
!i
t!
li
l{
ri
n
370 Langmuir, Vol. 4, No. 2, IgSB
gests that a-lkanethiols form highly ordered, close-packed
monolayers. The contact angle of water on the het,hyl
surface of n-alkanethiols is among the highest knos; for
smooth surfaces (0, = ll4o, deionized HrO, - pH 6.6; for
comp-aris9n, d"
: 103.
for polyethylene). Similarly, the

1
t-
,
I
t
L I
I I
I f
I
!r
f t'
t t'
,i
$
t
{
I'r
rt:
lv{onolayer Films prepared from Dialkyl Sulfides
35
6
o
c
..:<
o
o = Auf"r1anr1.an,
rr-,.f^,,(CH1)nCH,
v - Auf ".{cxr1ncx,
-r
20
Langmuir, Vol. 4, l,Io. 2, lggg 3; I
^'
l.,l3l:i:'.'"":"
e = m.thod A
O = mcthod B
3 te c
I I t--T-]-
020
Figure 6. Film thickness (by. ellipsometry,) of represe ntative films as a funclirtn of chain lenrth
n
Y c
ff#':l"fj#""{,,::i"#,ill=iiii,li":*:ntT ijif"tT,";iii:,::J,r;i""1,*:;t;
rglr*l*:":1,*i1"#:iilrj|Tl:*!..:""+:ll e:iirv qe."c'ed' siv"n 'lt" un"Jnil i'' o";";;;;''
thar the.c.rbo.xyric
";,0 ,";;",!T*j",i".,,ril:*il#t $:::=:iii]r .; "i.;,:*n::ru.Xii.l"i
11"4*:"*;i'n*:1*iii",','"'ill+#itffiT#:* iiT",ll,"'r..ii;i.ti:j;i;;jiHlii
:f,il'.*fr'e""'Jlil['rTllterminateddiarkvi;r;J;;; it{i:i]i{i:qlJ?"'i'i1:;;;ii};'i:::jf,nl#;
morogousriethvr-t".;in;'d;T,x,i:'"1i:$j."{r:i..,}i;
31,::,t:""1;ll""xj:;n:lli,s,i,lri:T?,
of the difference between carboxl.li.
tems and methyl-terminated
;
""iJ.".t"r"i"g'.iij
l"Jfflrij:is
I
0
r,
fxslcxrtncooH
^'l'{:x;i::::x
;::ii1tt-fiiJ:..,",t:,,:#
nared sysre' resernbres ,tii:.::-H
,n;;#uiX"r,T"ll,,J
.,'",.Tf:il: t'it-fiidiri1.':iiil?illiifi
mli:l,"mij*"llliiiii:i. , ,rrr;i;J ;i .;;
*nlttilfl*rthts}t#ilTnl;;:f mlff ilrrfi:,,slI!?d;',:.'d;jii,urrt l,'i:L"ffi [f ";'1""'".'""Tf :fi ".Ti'P'ft :".i:ff ;;rrr"lt'lili_']".ffi ii;fl irf;"]'i#ui:.*$
,.,i.i'lifi,T::.',F,0".T"+i::i,ll"data
ror the unsr,'rnme' 5",.$'J:iti!$J|.|i:*jf:Lif:,i.:,,,",
*ffii:#: ffir,t*lt',.*q.ii:'ffiT;: r.:"i::1:.:r,:q,*ucibiritv.a,d as an initiarestima or
row.sensirivitv
f*i'":'li"t":,y"iih',#qj1i{:l!1'".f iT!.,l:
u Q;;a:l:,"*i'.!:;#*["*iJ:dt
"ri,p."-"i.i',1Hi",'Tr:Xi"Ti,";ig r;1*";;:xr;""jtl:r*xil.l"*r*1iiyf
l}:*:T*::'Jff::*i'"""'i"Jll: :'l'p'i' i""gl'f i'"'iil #'
:frTiil:il,3i*ii*,1ffi"t,'i;,,1h:ri*ru:r;
;,'{:,.*1,:#*l:f,,:",d::::::
r:*3;*h tn" }'i:i:ili ;,""lif;
""n"t$lliii.il:ff
;"tt*..: *nrli: :f1,,,*.,. .","rui .1
lj*I.*yooii,:f,::ffi, "* " *1,*j i:ig *::ln' :iuffim"i,ilf; i:i::f .'H*:d+.+J:: lil
_r:,*.I";]*;,iiil#,f;:i:|t?il$(L?i,ii,".'# n*i:i.-a:,;:,1*tl;lti:,:;*n.r,
f-:l+fl ftq*:Hi:jrt'sil*i,{t"{i'$ ::*tlxffiI"'ffir'*t*n:iimt"
((Cu,;'g6rH),uy20carbon,-in_..uu,.sthetr,i.ud1ii.|
rne rum Dy -I0 A;
:}:.i9!l:l0A;lhu.',at.the'*upackirrg;;;i;;,il.,1,1'].:
thus, at the sarne packin! density, the
o.;.shiao, s. v.
Increasl
hlr"*r.yi c.al_a,
thain
s. D. Eil
by this
.in..one
number of .*lon. *oulJ i:i:i"r'r1:"rit:Tliff. "r!1l,l.B;p ttr; r-c,i.* dl.#"J s*r"ii irr-i.,1"'g"".
\ v.v.t LJ to,
PP ro.J-IH.
I,
l'l
I
,l
It
t.
r-l
I
-.f
LJrl
o.l
!lr
tr
U
or
o
B
o
I
tr
o
I
I
I
r
Y
v
c
c

37? Langmuir, Vol. 4,N9.?, 1988 Troughto,^.
o{
an
o c,)
-v
.9
l-
j0.
a; )
o
C)
r
-\J o
30
20
10
0
0.50
00
50
-1 .00
tr
o
o
So
B
I
*
12-36 h. This time is probably much longer than that
required to reach equilibrium; in other experiments, extensive
equilibration between preformed monolayers and
organosulfur compounds in solution has occurred over this
time.{l
X-Rey Photoelectron Spectroscopy. Obtaining reliable
XPS data for the monolayer films proved consistently
troublesome unlil it became clear that it was necessar!
io use lorv X-ray intensities and low temperatures
to minimize beam-stimulated desorption and damage to
the monolayer films. Previous investigators have also
observed serious decomposition of an adsorbed layer of
xafrthates on copper at high X-ray flures.sz Figrue 8 shows
spectra for a series of samples: three points are particularll'
relevant. First, the gold substrate transfened directly from
the evaporator to the spectrometer shoq'ed tittle e',"idence
of carbon, oxyggn, or sulfur on the surface. This result was
not typical, howeverj most 'clean'gold surfaces showed
a peak at -285 eV (referenced to Au 4t1/2 at 8-1.Q eV)
due to adventitious carbon adsorbed on the surface.
Second, the snmples of monolayers confaining carbosylic
ircid groups showed clear indications of the COzH catbon
of the saniple in the form of a broad (fwhm :" 1.6 eV)
Gaussian pe+ at a binding energy of about 269 eV supeiimposed
on a linear background. The identification of
thil"peak is important because certain of these samples
showed neither a titratable c.arborylic acid nor t}e presence
ql g stretch in
"_.EFonyl
their S r,p..tru. Ind.ependent
o
I
o
o
r
0 60 120 180 240 s400
Suonlnen, E.;
9, 691j605.
o o
I
| 1V
Yit
Frl
A wa
U
l,tcx.t..cx
o rs I'ii;;:;:l;;:
I lnr.r r AA
11 ^,, l"/en 2)lqcooH
v ^'1".(cH2)2,cH3
I rca,).^cooH
/r A,,l

?l
il
Iv{onolayer Fitms prepared,
from Dialhyl Sutftdes
I
I
^ ']l 4t 1s 2,,,,,-
|
Au4 |
z."t.7
I
/\ l
f^,il;lffJ",,':;-l
"Y'/A\ I I
p"*-^Jilll!{ r*p"dfr'/i\\,",
1 I
l**"*:jtf-',[| o,ndh'/,',l ,,,,,,/ ^,
I
h''0^r*u*-n,';**-'-,i,t1,*Jt , ,*,,,,.,,',,,,{,,f '/\n,,,,',,^,/ I
-
L f.:il1J.**''{-' ,,,1 il]
f'-'*t--.*",*',',J"'\__-*-._f-[ ;;i: I I
900 ^
Bt o||lc ENERGY (eV)
- , ",*',i'i*,,r*rr*l
2g0 280
n, ls-(Cnr)roCOOH
l-.tcnr)rocooH
-r
1c Hllr"c n,.
^,,1."(CHr),ncooH
a,,lq.-(CHr)rocooH
I "\(cHz),,c
H,
AU c-(CH2)2,cH.,
"\(cH2)r, cH.
Au subslrate
ffi:iilil?::::';lru:r:rl";;ifJ:"Tir:*i:ilri""x11ri:1;::xil,*T,"
rhisresultsupports
il n::'"T3;':l li';t:::vlgroup
clearlvshorvspeaksat21era
1 *s.upi';i;;;;;;;;;,,;:TF;s"p*$",'t;,,".,t
$rini|'iiitiitiil[:h1fi*j""
if blTsHhr#iilt;ifll.--ixl:lllt;i i',t$rl*l:,r'*,,n";::.;:"'" [:r1:
il discussions
and
will be
absenl in thi. ,drorb"Jl"-..=,.
rr"i"t:;:oo:i::fflJnterpretations
il :"*ii6tr,::i,[ithffriii]**,:;::*:t *"ry;9fi;ii*;i:1"'"',r;!?
reclpro.cal centimeters
t
r ;ff?i:iffnT#*r=ii[*li*"+t+r"*:,ru i:"":;":'#fif$;f"T""l :ttirt"l'ffil
i tr." '""'"r",=,,rr'";ii3',jir"33i5T#:rr'+'fxi:::
;q44::'":r,**tn:t'::iil;T*:,J
J :*::TJiilrtr;:o ag'ue."enl
"itt' tt'"'*etting *j tablish unesu-ivocatrv
"ilr;-
ur"ifne monorayer comorised tha
'
tilr*: lji:'F.ti$f,*."*1.ll.:t[:i!fi""ii *i,$:-t-.T1".ijfi*1,];']u
ffii;;:l''"'"ll'
, l*H;ffi:.:.1;,,".,ff.i."ff15,T*::*i:1,*i *: x*"",'ii]:x *iltiji:it"::#l$,.,:
Fii{idifi*dd[trtrr+,s"j!'Tixlti**il:{, i:f$iti'{f*
r i.'i{t"':"fl?j}j{1flili",t":',qtrl*i,,:"il-j,ff{
[i'".:-rT,'.llF,i:T"x
? ffii*gii"T:ffi5ffii:#."il5#:Hi Fiili "1,:ii:::rJ.:::;:l"un*e
(t" '"tt{-.fi
ii:l
: rue."oo r'* wss u*a'io.-**, used to e*ure u.", tlnt tr.,u the auuterium deuteriur rabelrjvoLld
tabel r;voini ::il:i"tr}fl*:ijl$::Tllyg,haq1..,!iiT;'''il;
il;;i.;;ih":;,J.ffitt;6"*t
gi 'ui# iHrt;f.,1illi';:li
mn*ii.:i:t+g"t-yi;+
m;bffili;;IJ *Tpr.: Arternativiiy, these ixurts rirightBe.iitionar
It4i:i.t.".i:i"...''';,*':'rneInlraredspectruro(I'igiirb9)
' on tire basis or" p"rJiiio,L"t"tion of the i:iiborvlic acid
,:.-,.,';.,',:.'ii;,i1":,r,, -' ". :.' - . '.;e.rd#f"---'-

37.1 Langmuir, Vol. 4, iY'o. 2, 1966
31 00
31 00
2700 1 800 1 200
Wavenumbers (.t't)
1 500
I z (cHr),.rcH.
AulSl
| \'(CHr),,CH.
| -/ (cHr)roco2H
--
Au lS ' \(cHr)loco2H
|,/(cHr)roco2H
AulS--
' -cH,
2923 | sxt o'' aos
I zeoo 21 'i?onu
s6
ilJ '!*,',,6.n,,J[{*..],ii_^^ I
| -/ (CHr)r oCO2H
AulSl
| \'
(cDr), rcD,
Figure 9. polarized infrared external reflection spectra oIrepresentative filrns prepareC bi'sp.,lnrineous assembll'oiCiunC"cy'l s
ii.iii-."iu"*yuna*l;tj .uit-,a*, rr-(meihyirnio).ina.canoic acid, and n-(hexacec-rl-cs-thio)undecanoic acid ad:orbrtes i:'-r::r
ethanol solutions o"if gofJ. Signals at 29i5 and 2g23 cm-t are altribuied to the CH2 asimmetric slretch' those al 2353 anc 2S';
io ifr. CHrsymmetric stretch, those at 17.{0 and 1?l? cm-r to the C:O strelch, a.nd those at' 2l9tl and 2095 cm-r to the CD; a-i}'m:
;|.;a;h ;'d'CD, ,y--etric stretctr, r..p..t-ueiy. Other pe+s are no|, interpr.eted in deta.il bul correspond roughl,v' to those ob'
in pui. .o-pou1i.. Note the absence of an observubie C:O stretch in the spectrum of the film prepared frorn 1l-(ner:
J*:tfri"ll"Ji.^nol. acid, even rhough rig"^f. to the methylene and deulerium'labeled methylene chains a:e ob'
Scale bars in absorbance units are "1iiiUglUt.
froviied for the high- a;rd low-frequenc)'spectra oi the deuteriated sulfide'
carbonyl group r.ith respect to the plane of the gold sub- minated chain is longer. For comparison, this r1:u:
strate.
"Flr rehection IR speciroscopy, the stroigest ad- includes data for two homologous series o[ referenc!
sorption occurs for vibrational nrodes with transition di- pounds (Fig:ure 4): methrl-terminSjed ard caibr)Ktl
p"i!. lerpendicular to the gold surface, and ierminated slmmetrical dialkyl sulfides. Recedin:c'
iirtually "ri""t"a no absorpiion o..urc for dipoies parallel to the angles, d,,.showed the same trends as adr'.rncir': c
surface, It is conceivable in Bome of these sa-Dples thai. angles, d.. bui a;e uniformly smaller (see the Exp':rir
ior gorie reason, the ca-rboxyl.ic acid groups tend to orient Section)
paraLlel to the substrate. If, for e*ariple, this orientation Ntethfl'terminated s1-ste.ms (both symmetrifrl s'j
i"*i-i*a hydrogen bonding, then ih. b*d" would be and methyl-terminated. thiols; Figure'l) nainuin a
diminished in ariplitude, drie to both broadening and hydrophobicit.'-- until the number of methl lene g
ori"nt"rion, *d mignt thus be lost in the noise. ihere between the su!'JJ atom and the methy'l groups fall:
"pp"*" t" l. a p"riori
"" reason to expect this particular approximately five. ghorter
-The
films becon: p:
oiilentatiorr, and independent evidence is needed for con- sivell' more trl drophilic, reaching a value of C. - i
fumation. i11rateuer ihe originofthe low intensity ofthis (CHs),;S. \1'e hrve_not established the ori3ir, oitrr
absorption, there must be sJme significant difference be- crease in hldrophobicitv *ith de.creasing chair len'
twee;the structures ofthe \rnslm;letrical dialkyl sulfides any detail. Itisplausible to ascribe.it to th.e irllu::
havirg one carboxylic acid terminated chair and the sym- Iocal gold-sulfur o.r sul fu r-carbon d dipol"' c:
Eehi;al bis(c;boiylic acid)-terminated sulfrdes, sincithe tacting water, to the interaction of "bon the hiShl' poi
latter do show distinct IR absorbance in the carbonyl re- metal *'ith the !vater,- or, -perhaps,
to direct co:1:ac!
gion.
water with suLt-ur arrd gold atoo by penetraiion be
,'idootact Angle of Water on Dialkyl Sulfide Mono- adsorbed organosulfuimolecules. .Independent el
'layerg.
. Fisuri fO gummarizes the measured wetting cheoicel studies of thiols on-gold electrodes injil'a'
'' pi'.;p",ii.. ;f ;;d;;;i;;;;;;;i;i;;';i;y;,:
the densitv of.pinhole defects incrbdiee as the chrin l
,,.'^
.,Lving the structure " CH3(CH,I,*.S(CHJroCO2H prepared - of alkairethiols decreases,to,l.,,',,'.t,' .. ,
,., ^ t'.
r r t'tat. ^ ^^-L^---.1 :^ ^^:J r^-*i-^t^I o.-l
, ':ti
i.o- ett inot.-S'iriritar iita were ibserved The carboxylic acid terminated slntems are mote
j^--:-^r-,1 .,;l;;. ^.,or thp llllL
"a-.orption t.h,'.f:l
..- ;J.:iil;;;;il""d;ffi;;p;;il;;i;F.'d;il;; tnan ihe rnetlvt'te:*i"ti:A't:!*r.q:"
.tr, -:i'.'u-ii.iiiiii"r, aiir,,liiil,"riiii.'ii.ill 'i,l,u.yi."" r*gtl'i.';t"i""a. rnb'ti'. p-ivti-. acid term'
*,+ft.ii;l!al*.";li;;h;i-L;;;i"a;Jti. n th e methyl- te rminated a"d ttte "r *tJttri. criibo xyli "i"r,"i" ihiol" u" tinirorrolv-wettable:,!4i* o' ror all s5
*liifii?itiiiii:T'.ffi;;'';;" ;".rtil il#;;;;,)ii;i; : 6tsd;t.j c-b.ivfii *ig.t"ffis ej--'t'i'd c
*l.t:t*ff:i,
ft! :taOps.be twee
I sxto'' aus

s
I
II
I
I
t,
I
'\[onoiayer Fitms prepared from Dialkyt Sulfides
ffinn
ffiu M
rn
Langmutr, Vol. 4, No. 2, Iggg A7.i
I
O, f .-{CHl)ro..CHr
l-\(c Hr),c.-cx!
I
- ^,, -,-11CHr)ro.-CHr
1",{CH,)'ocooH
mcthod A
method g
I
lu lslctr),0.-cooH
l\(cHr)1:.ncooH
-:o-^." Figure 10.
1'' Adv.anci
"uuan:rng contact angles{dei?nir.{ w3.ter, pH s.6) for monolal.
6i,:T:ir.iri3igr;.*r'9":::1";riii"lt:.'ds."T:j::[T';t,:,tr*$lt*i*
enr '/arues ror rfre iniu,iu.l,";;;J;;,l.iil::r-" .1ff,,1?i,1,?li*?$;:,;i:::|:fJ:*Htr*;T:*Xlri
J:
r#:;iiliS l$l;.*J'l"l:::l* i;,.t'::::ladsorbates prepared uy -u,r,ilt'i:T:"ii:r^T:if.:3 tr-(n arkrrthio)undecanoi
sulfides shorr"large variation in
length.
their w,ettability
For sho.j.il;i;;;i,
with chain
groups) d"
tr"", *I[ilui
is also _0o. ..tnylene
fo,
methvlene
.nrin. h;r:i";io
units berw'een
o, more
'z
*.*"rr*;j
group' the varues
rh.;Lio*1,Ii.
of contaci
((cHJ,o) angle i".r.".. rronc -400 ".id
to
:!0'
ricHr)rlj.
angle
For-,,.oarison,
of water on polyeriii;";,
the conLacr
(CHr;^,
simplest
is d. _
interpretLtion
103". The
oiih.iuu
termina
ro.
ted
the
smme
carboxyric
trical acid
diurkyi.urnd.r,
with orher data,
and on
b
e
;ir"t
consis te
dffJv'rt"-,
n t
as viscous
shourd
liquid be considered
o. liquiJcffstalline
r n c r eas i n
filnrs
r h'd rop
and
rr
that
o u i c i ti
the
dilurion
.o'., u r u t..-;
oi *'u
t
porli';;;-ir;;icid
il h ;'in.r.". i n g
methy'lene
moiei;;;;i
groups.
nonporar
r he dah for unslmmetrical
bo t h m e
gidfryi
thrt
sulfides conLaining
q.*i b"-/;.' ;:i j. expec
t"
red,
r_.
q
i,.,a
uaritiitir'.rv
ted ch ains are,
in rr
as
correspondinr
r,,,.i i"t. u. ;;;';ilse
symmetricar
of the
boxvric
rnethyl-terminated
acid terminrruj-ailir.iii.urna... aad car-
central concern
The quesrion
is whetfreiil;;. of
for i:
the
*y
unsvmmetricar
eridence in the data
Ji"itvr ,,irriJ.. 'tiJ'
group a' the methl',l-terminated.hs
to the carboxy.lic
becornes
acid
comparabre
t.rrninut.a .i.r.i"-i.
srructure t" ,\: .ugg.rt
plane of the ^O.i1l1"ri* fu'.p.ndicular
gord
to
subsfrate
the mean
u,'*g
and imprying
and re'ealing
systematic
of the joiar cov-
cnanses
c*6"i;L,?
in the length
,'.ij'iloup *ttr.,
of
roost
thf o'.tfryf-t";l;"rli.irin.
interesting,ieature
fr.,u
this quesrion "r},.'i"u
lrr,o
is
ii. u".r',"gu or
= dr.ol
6 and n::f;,
; 0"
rn = observed
4' This
berween
s-harp
rn
o e
inciease
creas in g
in hy,drophilicity
l e net h. i n t h
with
unlike
e rI e r'rr-ii-i r r..
the
i n ; re
smJoth
d ;i;"in i,
;;;;;'ir,r*.-a
u..y
djal.kyt sulfidesfor
rhe symmetrical
not all *:";*.",i
il.|.a,ta
compound:.r*:eie
is appreciabte,
exarnined
and
to co n tarn
for possibre
in a r in q artifacts
thjols. N#il.t..;
due
dialkyl sulfides prepared :il;;ffift tu
uy
in g
compatible.
u"ir.. methods
\\'e .on.tua.
A and
,r-rit
B are
re.q and
,i.
estabrish
trends in these
u r.ig"nififilt'.i*ge
daia are
1'ih a small
in
change
hydrophobicitv
i,i *.,.-Jrliiuru or.the adsorbid un-
:ffi;f:* **.
.ume;: ;ril;;;.
compariblawith ft ,rt
j."grn.
A secondrl.:l:r" of in.
diarky'r
data for
surfides
the unsymmetrical
suggesting structure
long merhyl.termiffi:;.;;;,
is the uuiu. of d. for
are comparable
Th;;;lr; (r" _
to those ror polyeth;b*,ii?Lulish
100")
t h e c ar bo x v- U c a c i d grg p.'
that
i i-t*:q
hidden from waterl i^j; ;J ;: : m p
Thus,
r e t e i,,.
a
enrl1'
a*.r116. "
sufficie.nty]fg. ,a"rf
group is appar-
,?El.
carboxyric
comptete"ty
acid moietr"
* i,na.rly.ing
A sim'ar
for long'chain
uffu_.t has
cster.-or
been obse*.ed
tnu-"t.ohor
or pol;'ethvtelScaruoryiorli;ipE;'g,"i derived b1. reducti,n
;to
sorbed monolay.ers hirinn-'r
for act.
( c H2), 6 co ; H T'il,! ffi ; il? ;1,; J:l;:'ff: jt
unsymmerri.ll^::lfldes
!i i; hauTng i.,;
chains (g^ -
fr", o.,..i,oijr.rn",inared
190"t -ir l.rr}run.ti
terface presenting
obsen'ed
",
for an in-
"l_o.ii*.tlr.iuut;. methyl g:roups (HS-
(CHr)r6CHr, d.
:-115") arj
predomin-t,t:,9|^,
..rpqrrut. to onu'.Jmprising
11w. 1r;iy.thy.l.ne ,0^ _ difference bei*'een
103").
the conLaci
Thrs
and
a-ngre
methl'rene-cont"jninf of water on rneth'r.
i"turrr.u. may arise from the
ii,';i;J,'||J:1T :: :!' rit.' .'.u, ,.du.ing-tie rarerar
rr r.,p,.i" ; sulfides :r ifif''fl1J i: : lX
having ".t #j: ii* o.t" T,..1
rJiti.i]t.r*inrted *,,
straightfo'o'*J. chains
e-*p..i**Hir., is ress
was greater
,nu scatter
than
in these
that for
d:rr
other
note here
systems
some
examined.
of the r..tu..r^that
\i.e
the interfacial
might contiibute
free ..";r;;;;;rr t..
the adsorbed
o'onoiry.;;G..
films i'. ii;;*ig*
Firsr,
that thev'are
are sufficiently
rnore hi'i;;;;iri. rhin
dependent
rr.,.n thicker
of any
systems,
s.tructure
in.
in
ably
the
due
organic
to inberactioro
layer (presum-
beroJn -i;;;;j,,the
gold-sutfur
*.r",
interphase). T4. *.u
goli and/or
drocarbon-air
stabiliri
interfl:q'"
or rhe hy..
".",", **.
acid-air that
interface;
oii J"rto*1.ti.
' thi;;;;;i
probably
of
reversed r"J .relative .er-abilities
furl;fr*r
is
*itrr wahr.'T.here is thu->
ilr 11 f e'";o it;.il;:il;X'i,ff fr,Ti:XHt* ft *
,..:,t,;' ll.+-i. r
..
.5;:.;il+-{i"i -\, a.' '
;:f i:T"",1,J,1?:flr;J'io',**,.ndqiliuo',in
.
)
" ln these urulmmetricat-eu'ido-tl-onu
;,.,-..^.,-.o
in
t

376 Ldngmuir, l'ol. 4, No, 2, 1988
ea
1t2-l
98 -tr
95-l
I
I r-ttt
OEEtru
Lrcl
rl Ia
rrl
o o-82
r-78
^"1'

lvfonolayer Fitms Prepared from Diolkyl Sulfides
60
50
40
30
20
10
0
60
50
40
ua 30
20
10
bU
trn
$U
30
20
10
0
arlralc
ln
-rB
0 2 4 6 8101214
pH
Figuru' 12. Eiiecl oI exposu-re to air on the advancing contact
a;:eles of selected uqu.orJU"ifers for- monolly'er surfaces prepared
i,;"; ;; ai i:;;;;1il;d.;/i
.I
rtl
I
e,
!
crr
I- C
rt
I
I
E
t
I
eu
. l-lcxr),ncoox
lr:.1CHr)loCOOH
I
lnltlrl arpoau?. to ad!orbrli
tolutlon 24 h.
S.mgl. 1.
Erpoa. lo Argon,
t oh
El 2h
O zltr
Sampla 2.
t) ErPost to Afgon lor 2{ h.
2) Erpo3E to dolonl!ccl
w3trr PH:5.6,
. utr'a u p d s o rt e d o nto gold'
might then be faced with different structures, depending
on"the environment. This phenomenon of surface recon'
,irultion is well estabtished for poll'ethylene carboxylic
r.ia and its a.riurtitles, although tht is
*ttconstruction
kinetically slow at room temperature"'
We have tried to establisfr the importance of surface
reconstruction in J.i.tn'tining the characteristics of the
interface between -onolol.erJ of dialkyl sulfides on go.ld
unJ tor*- or high-dielectric constant media. The results
oI these eftorts do not indicate any large influences of
reconstruction but are open to at least two interprelations:
f-rrsl, that there is no reconstruction, that is' the charac'
teristics of the interface are e*sentially independent of the
environment; second, that reconstruction occu*l too rapidly'
to be detected.
- \\'ith the exceplion of CHrS(CHz)roCOzH and
SttCHrl19CO2H)2, ..*po.u'e of monolayer films to argon
oi'ai, (io:"-aiutu.tri.-constant media) or water (a highdielectric
constant medium) for prolonged periods at room
temperaturu ,.tuit.a in no thun.gt- in the interfacial
characteristics of tn. monolayers (as measured by their
contact angle *'itt *'ut.t). These two carborylic acid
i.r.in"t.daiutt yt sulfides show small changes in inter'
facial characteri.ii.s reflecting the previous history of the
sample. Figure 12 shows tlie dala' In brief' samples
.onditioned"by exposure to argon for periods of 2-24 h at
room temperaturu uru consistently siightly more hydrophobic
than those condifioned by exposure to water for
I0h
o
96h
Sr-?i.3.
t; Erpore to delonlred
wsldr PH-3.6,
I2rh
E 96h.
2; ErPosr.]re tc ArgOn,
f_l t! h
(61) stricrly apeaking, it is a.decreP. ir", the tolal free coer3y of lhe
ryrtem that drives reconstructron.. we use the.qP,,T-tf:l*rfi::
:i.':1"#' ff :'liiiJr' ; lil t -iii. tt' u ir, t.'.. t io n. af the o rgan lc-fluld in'
te rface that dominatu .tt"niu. in the total freo energy upon lccog"truc'
tion.
Lcngmuir, Vol. 4,l'lo. 2, 1988 \ii
Figure 13. Schematic iliustration of.rhe influence of the orderinS
;i?-,1;, on rhe i.;;l;;;fresenred _b.1'
pe.rfecrly ordered fi6ns
ii;';il ir.,.-iol..iv oia.r.a sulfide firms in this srudy (B).
similar periods of tine. These data are consistent wtth a
..rJiaJgree of interfacial reconstruction reflecting prei'
eiential Jegtegation of the polar carboxylic acid groups a:
;;;;;r; Jtgi.'i.-\e'aier inierface and e:

378 Langmuir, I'ol.
i: -
4, t\-o. 2, tggg
t.,
Lr , r:
lVe have no di
litx*,ttf,*ul**ffi i:**itln,u
*[*N':r**nffi,,.g*
ggil-.;ffiht*:'''"'"{. '
;i11*ffi ""
m*Hff*ffig
*tnfitfi#:*u"x;l:'lr
nm*xf;: t
**ffiffiffi
l***** l*,*:*rn,:.';t', [:'*t ffi::"**ii:**ru1[:ll ll,t
$nt*sfi$ffi qn':,;f
t? u. r. t'hJ.r. Chem.l9g6, g0.
t r.s_rr
Engr. ll,_
tgzs,lsli,il6s,
!,or-,.trr.t.,,a,,,
:

;(
I
(.
I
i
I
'.,
I
t;
!:
t
HR,
E trl
il
;t
[,-l
$l
L{onolayo-r Films prepared
from Dialhyl Sutiides
baskets and comprised a home.made diffr.rsion.p'roped
(Ha'ard u"tlt^"J:{lFL)
evaporator
equipp"d wirh a riquiiiitrogen
operaring with
trap
a base pressur. of-t x lb; i;il;'NRC
3 I l 6 d iffusio n- pum
tvtodel
ped- evaporato r [p ro fesso * riu
equipped
u'*d
with
rinr*m)
a riquid nitiogen trap.and
presiri'rre
a
of I x
base
l0{ Torr, a h-ome-ourlr evaporator "p*rti"iwith
fprof. Kreml"_:..r]
j_1:ltryd^:,ith a.liquid nitrogen rrap and
a
operarins
bas€ pressure wirh
of I x l0-? Torr, and a modified
(AT&T
V;; evaporator
Bell Latora'-"-'i:t) .q",tr:_g with a clro-pump
erating with
and op-
a.base pres.ur.-of i0-r Tor.. The
evapo
erectron-bearn
ra to r (\f IT il{i croe I ectro n ics La boia tory,l'
vacuum
*."'i' frorne_bu
system
il
utilizing
t
a sroan Technorogyii..t.*-u."the
evaporator
;;;
1::^._q;ipo.o *.tl
operated wirh l_li;;le;;;;;. rrap and
a.base pressure of r0-? To;r \f
of
ith-ihe
rhe home-made
exception
evaiorato. fft.i.*d Unir..riifin'rl,
similar resulrs.
all gave
The unsatisra.tory resurrs obtain;
substrates
using gord
orenl;e! r_n thi.^u"apo.ator may have
coe!'aporation
been caused
of
by
copper from the erectroau.
contamination of the gold substrates. At Harvard "na-aonsequent
Laboratories,
and Beu
electroniJfradu, .ingt.-..y,.tal (
(2t
t l) silicon
or 3-in.
wafers
diame!e.) o,er-..t.*.Jin rr"i r,l-n.,blteozl/Hrso,
and rinsed wish distilred, deionizid o.ater and absolute
at NI'I'T',
ethanor;
the standard RCA crea'ing proceduren
wafers
was
were
used.
coated
The
w"ith 5o-150 A
-
of cr to promote
2^tlo0
adheslon
A of Au,
and
as measurea uv crlstar thickness
Gold-coated
"quurt
monitor.
wafers were remor.d a;;;-'."rp"r"tor
Teflon-coated
with
tweezers and w.ere stored in w.afer
in parafirm. trly;, *^ppua
Gord-coated wafers;i;;rd'b.';;;;iih.
since the1,
I week
conranrinare irr.r.urribly
Before use, gord-coated
;;;i";;; storage.
wafers *'uru .ut "p""
(usudly
into appropriate
l-l.S
aizes
cm in w,idth and 3_i c*. in length)
wafer
byscoring
on the
the
gold.coated side wirh
(Brookstone)
a d;r;";iLip cutter
a-rd.breaking th" *af., with Teflon-coated
silicon
rwe€ze,.,.
dust w'e; hrown fr"om thelubstrates by'a
gas.
streanr
The gold
of argon
substrates were then weshed 3 ti.es
by
with
usin3 a
ethanor
l'mL disposabre gr"*';ip.,te equipped *irh
bulb and
a rubber
dried ir
i '.^ J i u r. i;:
- i': ft
',':ilil;::
? : :.t: : :;l : .: euipsometry'and
I::il. n: Tt
conracr angre in order to determine
ability
their
for use
suit.
in the pr.porltion Jr -onoral,er
subslrabes
firms. suitable
for the pr.potaiion of monoray.. fid;
characteristics:
;ll;e
(t)
fouowing
They *.ru uiru"lly free of defects. (2)
being washed
After
wirh erhanol rh;y h;;;.;rjd;;tb*tu
properlies
.[ipro_.tri.
indeoendent of the [ocation or the spol
the substrate.
erarnined
In
on
oractice, .lrip.;;;;ric measurements
at three
were
locarioni
taken
on the g"le r;b;;rare thar were
al leut
sepruated
I cr.r. If
by
the porarize, ,oudittg. feu *ittrin
(this ra.nge
,, *0.15.
corresponds to abouirS-n of a film "'r-*ri *ith
refracrion
aa index
o,
of
t 0".)l,lhe gold,r*as jyis.a t"'U.
ellipsometrl'
."ii.t.t"ry for
experiments. (3) c;rd subsirates
washed
that
with
had been
etha.rol.shorvej ," = Si-;g" (deionized-o.utur,
5'6)' The
pH
contacr angre on freshiy eiaporated gord-coated
after remor.al
wafers
from Ihu .ropor;i*
i., qrni.uut. d:;b'. increased on
urrd
slor.lge to aboul 9()..
t re a r e d, o u d . h u. t
",
e r i zc. d i n r h i s *, ;;::
sulcs.
;;:J :'.1 r;t"liiL' i3:
There ale sev'erer issues concerning 'First, the preparation
g'rld substr.res
of suitabre
for these .rrJi"r.
wafers
precreaning of
beiore
siricon
the e,,.aporation was crirical
substrates
for preparing gold
thrt rvere free of rrritrt. a.ru.,., siricon substrates that
ltij-t i-properl,vcleaned
resulred in gord
oetects
substrates with
and
visibre
cloud' regions. such def'ects w.ere
films
arso observed
evaporaled'on
for
glo.., so silicon is a preferred
b eco
substrate.
n d, e v'a po ra t io ns of gold o n to s i r icon'
oul
* urr.rt*u-r!'."r.i.d
in such a fa-hion as to minimize heating of the dgriog
l?:r.:t,"o"ration
""f;; process: heating of the
orttusion
wafers couid result
o[ silicon
in
or chromium ihrough
not
ilr. g"ril"ylr.'
necessary.,
rt **
ho*.ev,er, to cool the ,i-li.on itring the
;1'T'r:ilion
""i.r,
process
Ldngmuir, Vol. 4, No..2, lggg BTg
Third, attem_p_ts fo clean gold
wirh chemical :u-b.jr-q'e-for rerxe by treatmenr
reagents (e.g;., hor l:.{
plasma cleaning (in {ro;ii0;;7Hrso.) or
oxygen"""a7o,
bv
hydrogen
been
plasmas)
corurs ten{ty.u..u"!iut;;;;.
have noi
r,i pr.liifi
substrares
[*ing
for r-epeaLed
the gold
use in monorav.; ;-;-;ri;;nLs
additiond handling
invorved
of the sub.t rte. th;r inrrfo;.ri.r.",.;
it was usuaLry moru
thus,
desirabie to pr.pur. fresh
evaporation
substrates
than to
by
clean and reuse thur.
svnthesis of Diarkvr surfide_s. The
prepared
dialkyr -Ji;rrh; surfides
by two
were
merhois, A and e
p
t.q'il
reparin g (n -alkyl A invol,,.ed
rh io) u ndeca noic.' ^.ia. ar;;'lj -_ercap
undecanoic acid
ro.
and n-alky'l biomides, *.t-r.,J'g^from
nethiols and r
n-arka-
l'bromounaui*oi. acid. The purity
*'as established
of compounds
bl,meltins poini" (

'l.l
IL
all tJd
a:..
lr
il'
!I
380 .I-cn gmuir, Vol. {, t'io. 2, 19ffi
Table I. Phyricel Propertler of Adstiibitir
Troug,:.ton
method
'of
compd
rynthesis EoP, oC
tH N.t'fR
cH3s(cHJrocOsH
data
analyses
cH3(CHJ55(CH2) rocO2H
CHs(CHrgS(CHz) roCOzH
CH3(CHr roS (C H:) roCO:H
CHJ(CH2) r rS(CH2) rocO:H
CH j(C H:) r25 (C Hi) toCOrH
C Hr(C Hr) rrs(C H2) tocOrH
C H3(C H2) r{S(C Hr) 43.5-45.0 300 NtHz (CDCU; 2.47 (t, 2 H), 2.33 (t, 2 H), 2.07 (s,
'
3 H), t.60 (m), 1.29 (s)
calcd for CI2H2.SOl
C,62.02; H, 10..{l; S,
found
C, 62.31; H, 10.{3; S,
50.5-5t.5 250 lvlHz (CDClj; 2.49 (t, 4 H), 2.3{ (1, 2 H), 1'58
(m), 1.2? (s),0.88 (t, 3 H)
calcd for Cr;Hr.S0r
C, 67.50; H, 11.3.1: S,
found
C, 67.58; H, ll.5,l; S,
63.H1.5 300 lvlHz (CDCU; 2.'t8 (t, 4 H), 2'33 (t, 2 H), 1'55
(m), 1.31 (s),0.88 (t,, 3 H)
calcd for C2rHr:SOr
C, ?0.33; H, 1L.Si; S,
found
C, ?0.58; H, 11.01; S,
65.G-65.5 3C$ l\lHz (CDCtr); 2.49 (t, 4 H), 2'34 (t, 2 H), 1'58
(m), 1.30 (s),0.87 (t, 3 H)
calcd for C:2H{.,S0r
C, 70.91; H, 11.9l: S,
found
C,71.05; H, lt.S.i; S,
?0.O-?1.0 300 lllHz (CDClr); 2.'tg (t, 1H),2'33 (r,2 H)' 1'55
(m), 1.2{ (m),0.87 (t, 3 H)
calcd for C:3H.,dSOr
C, ?r.qr; H, I1.9:, S,
found
C, ?1.68; H, I I 6i; S,
?0.5-?1.0 300 lvlHz (CDCI:); 2.19 (t,4 H),2'3{ (r,2 H), l'5?
(m), 1.2? (s),0.87 (t, 3 H)
calcd for C:rH.. jSO,
C,71.9-{; H, 12.trl; S,
found
C, 72.03; H, 12.?v; S,
1J:-it.1 3u1r) \IHz (CDCI:); 2.{9 (r, 1H),2.3{ (t,2 H), 1.58
(g1), l.2S (s),0.8? (t, 3 H)
^^l^l t.-. /- LI C.r^)
LdlLq tvt v^5r r-i.\-.J_
C, 72..{0; H, 11.11; S,
fou nd
C, 72.65; H, l2.CL-: -s,
rocO2H
?5 0-?5.? 3Co \lHz (CDCh): 2.18 (t, 4 H), 2'3{ (:, 2 H), 1'60
(m), 1.2? (s), 0.88 (t, 3 H)
calcd for C:il-{,rS,) -
C, ?2.83; H, l: : j S,
fou nd
C, ?3.29; F{, II : . :
C, 72.81; H, 12.2:
cHr(cHJ rss (C Hr) rocO:H ?6.5-79 { 3tlrl lltHz (CDCh); 2.'{3 (t, 4 H), 2.33 (r, 2 H), 1.56
(n), 1.2{ (m),0'8? (t, 3 H)
calcd for C:.H5.SOr
C, ?3.2{; H, I2.?:r, S,
found
C, ?3.55; H, 12.'{i; S,
cH3(c H2) r6s(cH2) roco2H
//.D-/Y.J 250 lvtHz (CDCI3); 2.49 (t, 4 H), 2.3{ (t,2 H), 1'57
(m), 1.25 (m),0.86 (t, 3 H)
calcd for C?.9H$SOi
C, 73.62; H, ll 3'1, S'
found
C, 73.96; H, ll.'{ 1;
cH:(cHz) us(c H:) rocozH ?9.5-8:.0250
IvtHz (cDcl3); 2.1i (r' 4 H)' 2.33 (t,2 H), 1.57
(na), 1.23 (s), 0.86 (t, 3 H)
calcd for C39H;^SO
C,73.95; H, ll-ll: :r
found
C, ?.t.O?; H, ll l;,
cH3(cH2) r?s(c H2) rocozH
-=--:'' -
C H:(CH:) res (C H2) rocOlH
I V.J-d i U 3u.0 lltHz (CDCI3); 2.{9 (1, 4 H), 2.3{ (1, 2 H),
(m), 1.29 (m),0.E? (t, 3 H)
8{.0-Es.32a0
lltHz (CDCI:); 2.13 (t, 4 H), 2.3{ (t, 2 H)' 1 57
(m), 1.28 (s),0.88 (t, 3 H)
calcd for C:..,Fi:,S0
\f
C, 73.9S; i-i, I I {l:
found
C, 7{.19; H, ll '{1;

Iv{onoloYer Films PrePared from
compd
lHOsCCH2lrs
IHO2C(CHz)z]zS
IHO2C(CH:):l:S
IHO2C(CH,:),ol:S
IHO:C(CH.,),s]:S
IHO:C(CHr)rrl]S
IHO(CH:)rrl:S
IC HrO.,CtCHr) 15]rS
method
of
synthesis
Dialhyt Sulfides
mP, oC
128.Fl30.5
130.0-132.5
t t t-112
1 r0.o-t 1r.5
lrri-llq
I I r . u I L v
r1?-1lq
l l I
8{.5-86.0
78.5-?9.5
Table I (Continued)
IH N}IR
data
300 lllHz (CDCtr); 2'19 (t' 4 H)' 2'35 (t' 4 H)' l6l
(m), 1.4 3 (m)
300 lvtHz (CDCU; 2.68 (r,4 H)' 2'3{ (t' 4 H)' 161
(n:),1.23 (rn)
3C0 l\lHz (THF-ds); 2.'t6 (t, 1H),2'20 (t' { H)' l'5';
(m), 1.29 (m)
insoluble
TableII.
Values of Contact Angle (Deionized Water' p[I
5.6) onR;;';;;;iu,iu"
Fil;; Preiared by Ardsolntion for 2't
ft 10 Days from Tetrahydrofuran
""a
compd
s[(cH:) roco2H]:
c H3(c H:)^s (c H2) roco2H
n=0
n=9
n=16
n=21
s[(cH:):rcHJl:
O.(Zf it),
deg
4l
41
83
89
9t
99
99
110
250 l\lHz (CDCI:); 3 61 (t' 1 H)' 2'50 (t' t Ht' t.5l (m
E H). 1.23 (m)
300 lvtHz (CDCt3); 3'65 (s,3 H)' 2"{3 (t' 4 H)'9
oi lr
d.(l0.daYs),
deg
36
80
83
100
113
beaker containingjce waler (2CI1 mL)' concentrated HCI (35 mL)'
a;rd diethyt.ri.;T'ili"'C0 Ji.- in. eih.real layer was separated
and extracted with deionized water (3 x 100 mL) and with sat'
urated uqu.o*'N"Cf if SO rnl). ethereal layer was dried o'er
.The
anhy&ous ,"di;rdiafe and frlteied, and the ether was removed
by rotoeVaporuion to viura a white solid. After recrystallization
-fiom
hexan..,^il g ii;2,-y.ield) of product *,as recovered.
A number dialkvf sulfides were p-repart{
It
reaction of tfr.'c#erp*ai.g
"i.;;;;t'itur alkyl bromides *'ith lithium sulfrde'
This procedure -srrni". is represented by the following:
Di.n -decyr To a lfij-ml flask conl"ining 50 mL of
degassed ,rr.tf.,"I'roi*as added l.bromodecane (2.00 e,9'0'{ mmol)
follorved Uy *nilarous lithium sulhde (0'416 g,9'0't mmol)' The
reaction mlrture was stirred at reflu-r under atgon overnighl The
reaction mixtlie *u. Jto*rd to cool to room temperalu-re and
poured into iiuityr-.th;t (200 mL)' The-eih.ereal laver was
erhacted q.ith deionized waber tg i fOO mL), &ied over anhl'd-rous
s4firrm r,'rlf"b:ij;;, ;e;"*uupottttd' leaving a clear liquid
The liquid piJr.t wL purified by cold recrysfallization frorn
'-
absoluie ethanol'
:
Di-n'docotvf'btffiae' 22'Br6modocoeane (5 g' 2'8 mmol) was
dissolved in 20 mL of chloroform. A eolution of anhydrous lithium
rulfrde' (0.9 I ;, G.6' -.oD and tr.imetlylh.e rad.qcylammonium
ilfi;ii0 &ih 3;i;i;;;tas adied to the bromide, and
i the resultinc two-phase mirture stkred at reflur for 8 b' A white
: i. .'ff i;r*i t r f,; ;[ "* tai;'r.'t r ;' ;al .bt l.d t6 roo m te m irc iiii u r e'
t' rTbe'*lid " *a.l;il;d.i;y nftiutio'o'and dried in.vacuo to gi"le
Langmuir, Y'o'.4,lio.2, 1988 331
analyses
c{H6so{
calcd for
c,32.00; H, 4.03; S,
found
C, 32.21; H, 3.?{; S,
calcd for C6HroSO{
C, {r).-t-{; H, 5.66; S,
f,;und
f , {r).fii); H, 5.39; S,
calcd for Cr:HxSO,
c, 5+.9+; H, 8.{5; S,
fcund
c, {-t.ls; H, 8.?3; s,
calcd for C22H{:SO{
L, bJ.bJ; H, 10.51; S,
tound
c, 65.90; H, 10.57; S,
calcd for c3:Hd25O{
c, ?0.?9; F{, 11.51; S,
found
c, 70.82; H, 1r.39; S,
crlcd for c{{H36so{
/- "r 11.
\--, / l.,J l, H, 12.19; S,
fr)und
c, ?-l 39; H, 12.l?; S, {.51
c.rlcd f,;r C::H.60?
C, ?r) 5:l; F{, 12.3S; S,8';';
i,-ru nd
C, ?r).S0; H, 12.58; S, E.S;
3.a g (80 7o) oldidocosy'l suliide The produtl recr1'stalli'r
"'9
i*i.'.'front hot ethanol to give shiny' *'hite leaflefs'
Formatior of 5.ii'Ar"r.-bl.''l I\lonolayers' Ceneral'
monolayer nt-, *f'e piepu"d by' the immersion of gold sr'
strales r-using f.non-toit"dtott"" into precleaned 2G.mL gl'
bottles thal contai"J tppftximately' l5 mL of an unstirred
lution (e.g., 0.01'o.i"t4'aialkY'l tuii-tdu in elhanol) at ambi'
temperaltue (usually 2C-i25oC) ior t2-3b h' Afterwards' the t-rl
were walrhed by ,i"Ji"g both sides oi rhe coated substrate 3 ti:
;i;i ;";pto'priuiu il"enr (e'8 : I mL of ethanol) bv usi:'
disposable gl"r. ;;t;tt uquipptd wirh a rubber bulb and ti
dried in a stream oitgon ff ' ;r'1 The properties of the fi
1
*-r. rn.u.ured immeji"ttly b1'eliipsomelry and contacL ar'
;"';i";;;;v. bora substrales '*ere Drepared and treatec
standardp,o..d*-..-tt,"t,*.describedfurtherinthe-pafa3:
concerned with th;;refaration oi gold subslrates and were u
immediatelyaftertheywererreshedrr-ithethanol.GlassweiS
bortles that were *"d'fo, adsorption r*'ere cleaned either by f-ri'
and empLy'ing the bottles 3 times each with melhylene chlor
deionized water, and pure aCsorption solvent ot:i
"th"nol,
mersion of the weighing bottles i1a glass conlainer conlai;
a solution oi 1:'t H"prigOzti I-{'S.o, (concn) at 80 oC for
followed by filling i"i.'.11pti.in3'ri-.e'borrles 3limes each *
*u'*t, .tt,*ot, *i p*u adsorption sol'r'ent' The latter proce(
;;;";*= .-ployt if lhe gla-r;'care hac p::riouslv beer exp
t lf*f"l GIa; Uottt.. *.rJht.t4:ed u.ing T.eflon'co.ated tq'e'
during the clea,rin! p'l"ta"tt and aliow'ed to drain and dr
a wooden rack. Adsorption solutions were prepaled by weig
a few milligrnms oirutiidu and approximately' 15 g of pure sol
into a precleunJ ZO--f' gla-'s *'ei3hing bottle' lr.lost o:
-ntnttated
monolayer films in this study *t"
by gelf-assei
from solutions n'ithin 1-'t h oi p"pitution' Nlonolayer fil:
similar qualitv.;il;; pttp"'!a ti'uting adsorption solu
up to several weeks old" Afier several hours of adsorptior
monolayer nf-r.;;-iutuin.d their limiting properties [F
Z ). I n add it io n, tl. p'op9 tt 11 s i r e p r e s e n ta t i v1 n r e i
.o
-l-t-[1'
by adsorption t oao-fiSfor 2{ h.and 10 days were sinilar ('
I I) . . The r*.up,iiit'Uryo n c s ol u t i o ns o f mercaltoalL'
acids to oridatiJn *ul .1.,..ted "*9li afrer 2 rt.eeks by rH NNIR
troecopy and indicit"a U;t less than 3% of the mercaptoal!
acid had oridiz,jJ-to ihe disulnde. occasionally,'adsorpti
lutions ,.ru orJoI;J-Ut:
*ing degessed ethanol':Eet:

saz'
tiiiiaiii v"ii.
n=9
n=15
Table
n=2L
S[(CHr;.,9gr,"
.,oAdsorpcion . fron
va r u es o i .o ;;:;':"'j :l.rTi"r;J;'i:,T;
45,
526
/o'
92'
90.
9{.
I I1., 46d
936
6v'
91,
ggt
l-
: .. : -.!. i,;i:sni;i;"* et at.
ethanol :
lt"pl:ga uy ' "
eiro.btro; :: i-,f'€'i
fronr selectedSolventr
"' '--t.'
compd
s{(cHJ,o-b,
CHr(CHz)^S(CHrroC02H
n=0
thicknese, A 0r, deg
L2
4gd, 56.
8
484, 51'
t2
36t, 54'
45t
8
_ {.ohyarofud
thickness,
heradecane
I 0r, deg thickness, A 0r, deg
4lt
I ln
59.
I t.
l0
n
416.
80
60
o-
25"
I 106, 45a
:j,
::tu.,.
a
o
U
t
fito
pH 6
I'alues of contact angle I l?.,
pH
(deionized
45c
5.6) measureducer elijpsometry
5.6) measured
experiments.
befcrr
d eJlip;cr::e
Values try
of
erperinn ^.
con:.:-.t angle (,1exadec- . -'
Table IV.
by :i:"."ff:y,"^,I, lq wit Adsorptioo "trtt h. Et r-rrlilnol han ot f"o-'i'
Fi Fil6t rm s
Tetrn r..r_,:li" Didocosyl Prepared
Tetra h)'droiurio'fo.
Sutnau in
120
g h
times
thickn..r,--dlfr]|
was hed
d.(HD),.
100
00
dug deg
9<
I
ul .{6
Ol
at
il0
.to
^-
JJ
ll0
I
oi
46
gto
I
li0
r
ti
!
Ol
'U
t'
o
109
ro I
l0
21
15
109
t:
,1
AJ
(sonica::on
f,.rr l0 ll0
t:
rnin
AJ
/<
in l5 u0
.t
mL
!!
of ErOiJi'
t:
AJ
o
}feasurernenh
u
o, t{!l'l',t1?.n
;il;:f fSff",.',*ilntHT::r:i::"gi:T;,,Ji,.Ji
j;,i#_?:bl
I
ff nT:
"qiffi;*lf
;',il r ::"""::. i ir i r a r_r o r h o s e o. b t a i n e d b y
.t:;iii,:il:JT:'i{3ii,i;'t*:jffiffi f f:#F[Hffi
ii HTTT; ::;T. : [: Tl
I u t
H
i
:]i
o n s . o n,r n io * s u s p e n d e d
"':
ti{"',:Ti:n';ffi :?trt#gi,f, "ii1l'il"-',:'"'"ff },?i*i
If, ii "x f; i?:ll.' m"
Figure l{. Rece<
i:ii .&$*i ::", :*i:,j {in ff iiil lpff i ffii: i,*i;:.
B..gT.d. br- merhod A;;
represenr
J,;':-:l:'.',-.:
adsorbares
" i i i ai$r ::fr i ::-" : :?; :9J'l- tr nfiljl*Tiltx,l,T.1;?i,,;';;:: jffi ,_i1, jJ#::t::
a,i,l.,i,i; .i': :'ff : fri$ii; H : il ffi :.'- #: ;* #,Tilf
s u
T'"""0T
I
r e rn
i*[tj:]1":
e n t, c
;, g I ?. ! an g r es w e r e d e r e r.
I i,'i"::,'{'lJ rrrd: r i[ffi i{,H } lT i : I *ir ru; : r
intersection
*ith
::;fi: the
,it'J:r:'re
ff i."fi :i;ft ,"r,','"3iljfi :T*,*s,}il*x,.ffi il.{f i:
determi"*aHlit its
l .,j
:d ;;, *. ; ;;";" Jl; ",i;+i.; lll3.,_*u,.n s t f " Ii," r r. a . " j
:Jiil'-":ffi3t :::i:': ",'srs "i;; i:'u'' or the drop used w as
,:il.":;:tr.j;:jf iTdil:i*.";triii:,ii:'+ff ::ii't:
i:::1.,:.,;;;;i:l'f :,;,nH*;'.'":T::i-:f;, jfr'fii,ii jj
,,Hli:T "E !,.Tfl iti-fr : hi; :.TI{*H":: fllilTi:i
, ::{: '!!, :tFil ":Ti; fffl:t+:xT: * il];fru: Ti il j
F - lf
,e -
Ll
9\
s[(cH,;,,611r;"
Q'1
llt
t t r /^ i .
rrr (J CJ).S)
i,5!.li :fl T:::iffi ;::#;,,;J : 1 n i. ::t : e,.8' f. r r e c e d i n :
r* 41t'ff .it;,' ;i;:ii il : ti j T t ti i+!:,jl; ffi ';: ;
,ru:If,
.l : ilHl iffi ' :: :;::." ?tr i;:.i ndT fi d. g{;si:
,"H
Ol
-10
m
r-',t/tt.nL nl
J
. I m.liod A
'O
nr16o6 t
Table l'. Contact Aagles
Films
of tVater
before
(pH
and after
oa
Soakiog uoi", ,i.'6) If onola_r.er
-"!". for 2 Da1,s
d. (initial),
conr pd
d.(linaJ),
cug
rl SIrCH,),oCo]E
-'5 "-
r".-.-
5l

I"{onolayer Films Prepared from Dialhyl Sutfides
120
100
80
ea 60
40
aoooo
,ooooo
IIltt
trootrtr
Table vI. Atomic concentrations of Representative Films
Estimated bY XPS
compd %Au %C %S 7oO
32.3 5{.9 3.0 9.8
I t.l '1.1 l{.5
80.0 6.7 13.3
31.7 62.2 1.5 8.6
83.8 2.4 13.8
85.0 5.0 10.0
29.0 55.9 2.5 r2'7
?8.6 3.5 17.9
88.9 3.7 7.1
33. I
35.9
Time (min)
58.? r.2 ?.0
81j 1.8 10.5
91.2 2.9 5.9
61.? 2.3 0. t
96.3 3.5 0.2
92.3 7.7 0.0
. . ^. '...:
Langmuir, Vol. 4, No;-?, !988 333
. -) : :'
I
. ..l. -( C Hr)r rC Hr
^"1--,cHr)rrcHt
o ^,1,1:[:]::3?l'
^..1" -(CH2)16CooH
a ^"1'\( cH2),oCH,
L^.,. ? t
E A,i'.l!lii;5;"-
^ ^.1"/(CHr)roCOOH
t ^"i'-, cH2)rocooH
rro:i
ll*ffr',";,.,S'jl'J,:,'":*:*nU::!* illH"l1;fli'.i'.:i:;: i"""';:1,:TfflJ:i"*l,]:iIm: flfsti"i'adso'iption
cHrs(cH2) roco?H
obsd for Au, C, S, O
obsd for C, S, O
calcd for Cr2SOr
C H3(C H:)rS(C H2) toCO2H
obsd for Au, C, S, O
obsd for C, S,O
calcd for Q,7SO,
cHr(cH:) r2s(c H2) roco2H
obsd for Au, C, S, O
obsd for C, S, O
calcd for C2{SO:
cHr(cHJ,F(CHJ rocOzH
obsd for Au, C, S, O
obsd for C, S, O
calcd for CrrS02
HS (CH:)r,c H3
obsd for Au, C, S, O
obsd for.S,-S, O
calcd for C12S
HS(CHt rocO2H
obsd for Au, C, S, O
obsd for C, S, O
calcd for CITSOZ
2 r.5 58.9 2.2 17.5
?.{.9 2.8 22.3
?8.6 7.r 1{.3
ellipsomefer equipped with a HrNe laser light source (6328 A).
The angle of inciience was ?0.0o, and tbe compensalor was sel
ar -.{5.01. Further details concerning ellipsomelry experiments
have been described previously by AUara and Nuzzo.rs NIea'
euremenLs necessary ior the calculation of the fiho thickness
consisted of the deteimination of two sels of polarizer and analyzer
readings (averaged values of P1, 41, P3, and 43) for the 'bate"
gold s;bshate arid the conesponding set of values for the substrate
coated with a monolayer fiLno.
Values for each set of polarizer and anallzer aettings, messured
in zone 1 (Pl and At) and zonrj 3 (P3 and 43), are the aversge
of al least three rneasutemenfs taken at different locations
(separated by at least 1 cm) on thg sqmple and
lrad
a rosd'mum
sc"ite, of +tj.15'. These me{ulurements wdre determined with
thg snmple erposed to air and witbin r0.4-in S:
"ttl ramples
Table VII. Effect of Variation of pH on Contact Angles
(deg)
compd
S[(CHr):rCH:]:
c Hr(c Hr)^s(c Hr) roc0rH
n=2L
n=19
r.=Li
n=16
n=1{
n=10
n=9
n=5
n=0
st(cH:)rocOrHl:
0, 0, Ad.
(pl{ 5.6) (pH t3)' (PH 5.6-t3 ''
rrl
VD
9.1'
Yl,
/o
/o
80
lo
13
{l
ti
r r ob
I t-
60
85'
bJ
?n
bJ
h /
z.)
8
I I
.t 't
LI
t3
9'
6
1l
8
l5
1{
9\
nlll0nolayer fil.cos of cialky'l sulfides are not atable to 0.1 )
NaOH soluiion. Initial confact angles are reportcd with.in I min ':
applic"tion of test drops. lContag-t angle values for both acicr
and basic drops were reproducible if the monolayer was in conta':
"iti, O.t N |i;OH solution for less than I min. 'After erposure t
d;;;, of 0,1 N NaOH, d. for 0'l N HCI decreased from 9'{" to 8l
and d. for 0.1 N NaOH decreased from 85o to 66o'
(P'1, A'1, P'3,and A 3) rl'as determined for the monolayer film'
i .b"tt of ualues of thickness a:rd p (where p = (P'1+ P3- 90" )
*J tnu phase shift A upon reflection. is given by 2p 90') rr :
.+
gu..rtt.a with a computer program based on the algorithnr '
"ivt.Crr.t
rnm using thaaveraged sels of values for P[, Al, P3, a:
el io, the bare stibstrates and optical constants fgt !!9 films t
n,='.0;andhl=0.Valuesofthicknessforthefilmsw,e:
determined by comparisons o[ lhe obsen'ed aver-age values oI
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