Proceedings of SerbiaTrib '13

day later, at a room temperature of 22°C, reveals asubstantially different SAM as shown in figure 7.c.Upon cooling of micellar CTAB solution from theinitial value ~30°C to 22°C, the solution thenconsists of crystals, monomers and micelles. As aconsequence of such structural changes in thesolution, the complex morphology (figure 7.c) isnot surprising. A distinction between monolayer orbilayer formation is not readily possible from AFMimages above (figure 7). The applied test, notdescribed here, on the samples in severalexperiments, suggested bilayer formation even atconcentration 10 -4 M.8. CONCLUSIONFigure 8. The structural changes in the CTAB solutionin the performed experiments, such as heating/coolingcycle of solution (marked as hysteresis) and the dilutionof the solution, both above the cmcNamely, solution used in the both experiments(10 -4 M) has been prepared by dilution of a 10 -2 Mstock solution. The room temperature in March(21±2°C) was slightly below the Krafft temperatureof CTAB and we could expect that this stocksolution mainly consisted of monomers. The sameargument applies, of course, also to the dilutedsolution (10 -4 M).In June, however, a significant daily temperaturevariation, with temperatures clearly above theKrafft temperature, has been recorded, particularlyin the days before the described adsorptionexperiment. It is to be expected that the solutionstructure of the stock solution is metastable andcomplex. The diluted solution may thus not be inthermodynamic equilibrium and consists ofmicelles and monomers. The clusters seen on thesample prepared under such conditions (figure 7.b)can be interpreted as micelle, adsorbed in differentshapes and sizes.The results shown in figures 6 and 7 clearlydemonstrate the temperature influence on thesurfactant films, morphology formed in differentseasons of 2003. The variety of adsorbed filmmorphologies in uncontrolled conditions above thecmc can also be explained by structural changes inthe stock solution. Namely, warming up the highlyconcentrated stock solution (10 -2 M) to some 30°C(above the Krafft temperature) will result in theformation of micelles. Since the film shown infigure 7.b. has been adsorbed at a temperature of24.5°C, which is near the Krafft temperature, wemight expect the presence of some aggregates inthe solution and also on the mica surface. Some ofthe micelles are expected to transform into surfacelayers.The repetition of the same experiment oneThe performed experiments describe theadsorption of quaternary ammonium surfactantsonto anionic, atomically smooth, muscovite mica.The surfactant films on mica, formed according todescribed experimental protocols, werecharacterized by contact angle measurements andby AFM. We have observed that SAMs can havecompletely different properties, depending on themeteorological conditions, influenced bytemperature. These results suggested thattemperature can influence all steps in adsorptionprocedure, from solution preparation to the rinsingstep. The fact that the Krafft temperature range ofCTAB (~25°) is around room temperature, makesthis system appear particularly complex.The dynamic CA experiments and AFMmeasurements have shown that the exact protocolof solution and self-assembled monolayer (SAM)preparation can substantially influence the stabilityof the hydrophobic layer, as well as thehydrophobicity. The results indicate that themorphology and the homogeneity of SAMs dependon many parameters, and the main reason for that isprobably the molecular structure of the solution,controlled by the temperature and concentration ofthe solution.In this model case of quaternary ammoniumsurfactants, the formation of homogeneous, wellorderedand reproducible monolayers is a verychallenging task. In order to assess such complexsystems, systematic variation of a great number ofparameters was a necessary procedure.ACKNOWLEDGEMENTSThe author would like to gratefully acknowledgethe support in obtaining and understanding thepresented results provided by her mentors, NicholasD. Spencer and Manfred Heuberger, members ofthe Laboratory for Surface Science and Technology(LSST), a part of the Department of Materials at the158 13 th International Conference on Tribology – Serbiatrib’13