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 surface tension, pyrene solubilization, fluorescence, viscosity, freezing point, light scattering and calorimetry. It is found that CHP forms small micelles 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 as SDS. This compares with the butyl and hexyl pyrrolidone micelles reported earlier. At high CHP/H₂O ratios the large exothermic heats of mixing suggest hydrate formation. The partial molar heats of dilution in the micellar range indicate that the micellar structure changes significantly with concentration. The two-dimensional (2-D) second virial coefficients of the adsorbed monolayer of CHP at the air-water interface become more negative with increase of temperature and the standard heat of adsorption is endothermic. The adsorption of CHP at the air-solution interface near the cmc corresponds to an area of 54±2 Ų per molecule.