A Fourier Transform IR Study of the Phase Transitions and Molecular Order in the Hexadecyltrimethylammonium Sulfate/Water System

Variable temperature Fourier transform infrared (FTIR) spectroscopy was used to study the transition between the micellar and the coagel phases, and between the cubic and the coagel phases, for the hexadecyltrimethylammonium sulfate (CTAS)/water system. The phase transition takes place at 15°C for the coagel to micelle transition and 17°C for the coagel to cubic phase transition. CTAS in the solid state at two temperatures was also studied by FTIR to provide comparison with the aqueous samples on the molecular motion and packing in these phases. The present FTIR data also suggest the formation of rod-like micelles upon the addition of electrolyte, NaCl, to the CTAS/water system.