Silica thin layer chromatographic separation of cetylpyridinium ...

tures in different volume ratios (M 2 –M 10 ) differential migration of the surfactants
was observed. The results are presented in Table II.
From the data presented in Table II it is evident that for low concentrations
of THF (up to 30%) almost all the cationic surfactants except
DTAB remained at the point of application and all the non-ionic surfactants
except Triton-X 100 migrated as tailing spots (R L − R T ≥ 0.3). When
mobile phases containing 40–70% THF were used (i.e. M 7 –M 10 ) most of
the surfactants (cationic and non-ionic) migrated as compact spots; this range
of concentrations of THF is therefore useful for separation of cationic
from non-ionic surfactants.
The low mobility of cationic surfactants may be attributed to their
strong electrostatic attraction to negative silanol groups on the silica gel
surface whereas the non-ionic surfactants are adsorbed on the silica surface
by hydrogen bonding [42,43]. A plot of R F against volume fraction of
THF in the mobile phase (Fig. 1) shows that the best separation of CPC
from Tween-20 is achieved by use of the mobile phase containing 0.06
mole fraction of THF (0.94 mole fraction of water). Mobile phase M 7
(THF–DDW, 6:4) was therefore selected for detailed study.
1
0.8
Rf CPC
Rf Tw 20
0.6
0.4
0.2
0
-0.2
0 0.2 0.4 0.6 0.8 1
Volume fraction of THF
Fig. 1
Plot of R F against volume fraction of THF for mutual separation of CPC from Tween-20
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