Surface and colloidal properties of cyclic amides. 5. N-cyclohexyl-2-pyrrolidone-water mixtures aggregation in solution and adsorption at the air-solution interface

The aggregation of N-cyclohexyl-2-pyrrolidone (CHP) in aqueous solution and its adsorption at the air-solution interface were investigated using surfa ce tension, pyrene solubilization, fluorescence, viscosity, freezing point, light scattering and calorimetry. It is found that CHP forms small micelle s in water at a relatively high critical micelle concentration (cmc) (0.45 M). The aggregation number increases with temperature. The interior of the micelle is more polar than that of the common long-chain surfactants such a s SDS. This compares with the butyl and hexyl pyrrolidone micelles reported earlier. At high CHP;H2O ratios the large exothermic heats: of mixing sugg est hydrate formation. The partial molar heats of dilution in the micellar range indicate that the micellar structure changes significantly with conce ntration. The two-dimensional (2-D) second virial coefficients of the adsor bed monolayer of CHP at the air-water interface become more negative with i ncrease of temperature and the standard heat of adsorption is endothermic. The adsorption of CHP at the air-solution interface near the cmc correspond s to an area of 54 +/- 2 Angstrom (2) per molecule. (C) 2001 Elsevier Scien ce B.V. All rights reserved.