The interactions of the chloride and bromide ions in nematic lyotropic liquid crystalline solution

Abstract

The binding interactions of chloride and bromide ions have been studied in nematic lyotropic liquid crystalline materials prepared using the mixed detergent system of potassium dodecanoate/dodecylpyridinium chloride. The results from this study show that a three-site model of binding is sufficient to describe the behaviour of the observed quadrupole splittings. The results clearly reveal that the selectivity of ion binding is a function not only of the counterion but also of the detergent ion to which it is bound. In the cationic mesophase prepared from decylpyridinium, Cl− binds simultaneously to two and Br− to three headgroups. This contrasts with the alkyltrimethylammonium bromide system, where Cl− binds to three cationic amphiphiles while Br− binds to two. Sodium ion appears indifferent to the type of cationic detergent present, a result in accordance with the expectation that its binding is determined only by the headgroup of the anionic detergent. Extension of the three-site theory to take into account the effect of surface charge on ion binding provides information concerning relative strengths of binding. In the dodecylpyridinium system Cl− binds only weakly compared to Br− whereas in the alkyltrimethylammonium system Cl− binds more strongly than Br−. K+ binds most strongly to dodecanoate anion, Cs+ least strongly, and the other alkali metal ions occupy intermediate positions.