Optical microscopy, multinuclear NMR (²H, ¹⁴N and ³⁵Cl) and X-ray studies of dodecyl-and hexadecyl-trimethylammonium chloride/water mesophases

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 L_α 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 L_α 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 L_α 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 L_α phases of C₁₆ TAC. Only ²H δ measurements could be made on the gel phase. The values are generally larger than those for the L_α phase, indicating an increase in water ordering because of the increased order of the surfactant layer.