Optical microscopy, multinuclear NMR (2H,14N and35Cl) and X-ray studies of dodecyl-and hexadecyl-trimethylammonium chloride/water mesophases

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Abstract
The liquid crystal phases occurring with the cationic surfactants dodecyltrimethyl ammonium chloride (C12TAC) and hexadecyltrimethyl ammonium chloride (C16TAC) in water have been examined using differential scanning calorimetry, low-angle X-ray diffraction and, in particular, multinuclear magnetic resonance spectroscopy. The dependence of water (2H2O), chloride (35Cl), and surfactant (14N) quadrupole splittings (δ) on composition in the hexagonal and lamellar phase has been determined. For C12TAC, from the 14N δ values, the location and ordering of the NMe3 + was found to be almost independent of composition for the H1 phase, whereas at the higher surfactant concentration within the Lα phase the order increased gradually. Within the H1 phase the 35Cl δ values are highly sensitive to composition and temperature. This implies that the chloride ions reside between the head groups at the highest water concentrations and are squeezed out as temperature or surfactant concentration increases. Water 2H δ values show a maximum as a function of composition allowing an estimate of 4·3 bound water molecules per surfactant for the H1 phase and > 2 bound for the Lα phase to be made. For C16 TAC X-ray diffraction data shows that the gel phase occurring below 40°C has an interdigited monolayer structure. At temperatures above the gel phase, H1 and Lα phases occur, these being separated by V1 or intermediate phases. Broadly, similar NMR quadrupole splittings to those found for C12 TAC were measured for the H1 and Lα phases of C16 TAC. Only 2H δ measurements could be made on the gel phase. The values are generally larger than those for the Lα phase, indicating an increase in water ordering because of the increased order of the surfactant layer.