The effect of
glycerol on the micellization of the cationic
surfactant cetyltrimethylammonium bromide (
CTAB) and of the ethoxylated nonionic
surfactant Brij 58 has been investigated by various experimental techniques. For both
surfactants the critical micellar concentration (cmc), determined by surface tension measurements, is almost unaffected by the presence of
glycerol in the mixture; only at high
glycerol concentrations (>/=20% w/w) does the cmc significantly increase. The area per
surfactant molecule at the air-
solution interface, A, increases with increasing
glycerol weight percentage, w(g). Fluorescence quenching measurements indicate that the presence of
glycerol induces a lowering of the aggregation number of both
surfactants. The
glycerol intradiffusion coefficient has been measured by the pulsed-gradient spin-echo NMR technique as a function of
glycerol content at constant
surfactant concentration. It is almost unaffected by the presence of the
surfactants, indicating that no direct
glycerol-
surfactant interaction occurs in the mixture. The
surfactant intradiffusion coefficient has been also measured. In the case of
CTAB, it increases with increasing
glycerol concentration, a reflection of the decreased aggregation number. For
Brij 58, in spite of the lowering of the aggregation number, the
surfactant intradiffusion coefficient decreases with increasing
glycerol concentration, suggesting an increase of the intermicellar interaction. The experimental evidence shows that for both
surfactants the micellization is affected by the presence of
glycerol through an indirect,
solvent-mediated mechanism. In the case of
CTAB, the main effect of
glycerol is a lowering of the medium dielectric constant, which enhances the electrostatic interactions in
solution. In the case of
Brij 58, the results can be interpreted in terms of a salting-out effect according to which
glycerol competes with the
surfactant for water molecules, causing a
dehydration of the
surfactant ethoxylic headgroup.