The effect of temperature and urea on the apparent molal volumes and heat capacities of n-nonyltrimethylammonium bromide in water

Abstract

The densities and volumetric specific heats of the cationic surfactant n-nonyltrimethylammonium bromide (NTABr) have been measured in water and in urea-water mixtures in the temperature range 5 to 50°C, using, respectively, a flow digital densimeter and a flow microcalorimeter. From the experimental data, the apparent molal volumes ϕ v and heat capacities ϕ c have been determined. The apparent molal volume at infinite dilution ϕ v 0 in water increases with temperature and the transfer function Δ V NTABr 0 (H 2O → 3 M urea) is positive and decreases with temperature. The apparent molal heat capacity at infinite dilution ϕ C 0 in water passes through a maximum when plotted against temperature, while the transfer function Δ V NTABr 0 (H 2O → 3 M urea) is negative and tends to zero at low temperature. The concentration dependence of ϕ V and ϕ C in the premicellar region varies significantly with temperature and with the presence of urea and is consistent with the behavior of typical hydrophobic solutes. The absolute values of the change in volume and heat capacity during micellization, Δ V m and Δ C m , decrease with temperature and in the presence of urea and can be used to predict the temperature and pressure dependence of the CMC.