NEMATIC STATES IN MICELLAR SYSTEMS OF MIXED COMPOSITION

The correlation between the molecular composition of micelles and the temperature stability of micellar nematic states is investigated on mixtures of cetyltrimethylammonium bromide (CTAB) or decyltrimethylammonium bromide (DTAB) with decylammoniumchloride (DACl) or potassium laurate (KL), 1-decanol, and NaBr. The phases were identified using a polarizing microscope. Replacing, in a CTAB/1-decanol system with a nematic phase, CTAB mole by mole with KL increases the temperature stability of the lamellar smectic state but the width of the nematic range is reduced. Similarly, replacing CTAB with decanol increases the stability of the smectic state and reduces the width of the nematic range. In contrast, replacement of CTAB and DACl has little effect on the width of the nematic range up to 35% replacement and no enhancement of the temperature stability of a latent smectic state is observed. The results are qualitatively interpreted in terms of the electrostatic interactions of the ionic surfactants and of the surface areas per headgroup.