Influence of organic counterions on the structure of lyotropic mesophases
Diluted and concentrated solutions of cationic surfactants (cetyltrimethylammonium- or cetylpyridinium-cations) with substituted benzoates as counterions were studied by means of CMC measurements, polarizing microscopy, and x-ray diffraction. A clear correlation was found between the viscosity and the existence of a nematic phase. The effect of the hydrophobicity of the anion on the CMC values is obviously superimposed by steric effects of the substituents. The conclusion can be derived that the counterion is fixed in a definite position at the interface. This was confirmed by small angle x-ray diffraction experiments on the lamellar phase of ternary mixtures with decanol. The headgroup area of the surfactant is the more significantly increased in the presence of organic counterions, the more the CMC is decreased.