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

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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.

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