Studies in membrane processes. IX. The bilayer structure in mixed dodecanoate/decyltrimethylammonium micelles of a novel type II mesophase

Abstract

Nematic lyotropic liquid crystalline materials, which spontaneously align in a magnetic field (Type II) have been prepared from a mixed detergent system consisting of potassium dodecanoate and decyltrimethylammonium bromide. This system has an advantage that the charge on the interface of the finite disc micelles can be readily varied. Sodium and bromide ion interactions with the interface have been investigated by nuclear magnetic resonance of the ions. Deuterium magnetic resonance spectra of deuteriated water and specifically deuteriated detergent chains have been obtained. The results show that there is a water structure built around the carboxylate headgroups which ceases to be stable when about 30% of potassium dodecanoate is replaced by decyltrimethylammonium bromide. Bromine magnetic resonance studies show that the bromide ion interacts only weakly with carboxylate but indicates, as would be expected, much stronger interaction with the trimethylammonium groups. The sodium ion in contrast was found to bind strongly with carboxylate and weakly with trimethylammonium. The sodium spectra indicate that the association of sodium with carboxylate is complex, involving two specific binding sites. Deuterium spectra from deuteriated hydrocarbon chains are interpreted in terms of the motional freedom of the individual chains. The results re-emphasize that the motional properties of a particular hydrocarbon chain of an amphiphile in the micellar bilayer is determined to a large extent by its headgroup and that the behaviour of one chemical species cannot be inferred from that of another.