Thermotropic Behavior of a Cationic Surfactant in the Adsorbed and Micellar State: An NMR Study

Knowledge about how temperature affects the internal structure and dynamics of surfactant aggregates can lead to a better understanding of their behavior in complex environments and processes. ¹³C chemical shifts of the cationic surfactant tetradecytrimethylammonium bromide (TTAB) in micellar solution and when adsorbed on silica particles are recorded in the temperature range 8–78 °C, and give information on the conformation of the alkyl chain carbons. Adsorbed TTAB has conformational disorder similar to free TTAB at about 70 °C, with an increase in the rate of conformational changes occurring above 50 °C. Furthermore, no significant change in TTAB adsorption density was observed in the temperature range studied, and the results indicate a bilayer arrangement of the adsorbed surfactants. The number of <i>gauche</i> conformers increases linearly with temperature for the alkyl chain carbons in TTAB micelles. However, the total increase in <i>gauche</i> conformers is significantly smaller for micellar than for adsorbed TTAB within the temperature range studied. The fraction of micellized TTAB molecules in solution is found to increase with temperature