INFLUENCE OF SOLVENT HEADGROUP INTERACTIONS ON THE FORMATION OF LYOTROPIC LIQUID-CRYSTAL PHASES OF SURFACTANTS IN WATER AND NONAQUEOUS PROTIC AND APROTIC-SOLVENTS

A fast method for the analysis of the phase diagrams of lyotropic compounds was employed to study the formation of liquid crystals from two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPBr) in water. Studies were also carried out in the protic solvents, glycerol (G), formamide (FA), ethylene glycol (EG), and N-methylformamide (NMF), and the aprotic solvents, dimethylformamide (DMF) and N-methylsydnone (NMS). While the normal succession of ordered phases appeared to be governed by geometric constraints of interface curvature, the differences in behavior were accounted for by the differences in cohesion energy of the solvents and the different natures of the polar heads of the two surfactants. In DMF, a solvent with low cohesion energy, both surfactants showed only lamellar phases, whereas CPBr with a highly delocalized charge on the polar head displayed a succession of conventional phases in all the other solvents. CTAB with a localized charge formed only lamellar phases in NMF and NMS. This behavior was interpreted as resulting from headgroup solvation due to dipole-dipole interactions or hydrogen bonding. The particular case of NMS was accounted for by better stacking between the planar molecules of this solvent and the pyridinium rings of CPBr.