Brian A. Pethica

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 2A 2 per molecule.

Adsorption equilibria in surface force balance studies

Abstract On the assumption of adsorption equilibrium in surface force balance measurements, the effects of solutes such as salts and surfactants on the force-separation profiles are necessarily related to changes in solute adsorption with the separation distance of the opposing surfaces. The thermodynamic relations given by Hall (J. Chem. Soc., Faraday Trans. 2, 68 (1972) 2169) are applied to published data on the effects of inorganic salts and long-chain surface-active solutes in aqueous solution on the force-distance profiles between mica sheets, allowing for the possibility of ion exchange. The results show that the adsorption changes with separation can be complex, and that rather small changes in adsorption are sufficient to account for the measured effects. If adsorption equilibrium is not achieved the Derjaguin approximation is not generally applicable. Problems relating to defining the zero of separation, the reversibility of experimental conditions and the effects of adventitious components are considered.

Surface and colloid properties of cyclic amides III. Surface activity and micellization of N-butyl-2-pyrrolidone in water

The aggregation and surface activity ofN-butyl-2-pyrrolidone (BP) in water is investigated using surface tension, solubilization, fluorescence and NMR methods. The micelle point is high (-1.0 M) as expected for the butyl chain length, and the aggregation number is approximately 5, with pre-association below the c.m.c. Despite the low aggregation number, the fluorescence data indicate that the interior of the aggregate is similar to pure liquid BP. The adsorption ofBP at the solution/air interface goes to a maxmum near the c.m.c., corresponding to an area of 4S ~ 1 N/molecule.

Two-dimensional colloids. The linear thermodynamics of monolayers

Abstract A variety of two-dimensional microscopic structures resembling emulsions and other colloidal forms have been reported in spread and adsorbed monolayers at fluid interfaces. These two-dimensional colloids may be described as the mutual dispersion of first-order co-existing surface phases associated with the linear tension between the phases, in a manner analogous to the role of surface tension in the dispersion of three-dimensional phases. A formal description of linear tension in monolayers is given, and several relationships, notably the one-dimensional analogues of the Gibbs adsorption isotherm and Kelvin equation, are discussed in the context of potential new experiments. The discussion is primarily for spread insoluble monolayers. The limitations of the utility of the linear tension concept are also considered.

Surface and colloid properties of cyclic amides II. phase separation, surface activity and micellization in mixtures of N-hexyl-2-pyrrolidone and water

N-hexyl-2-pyrrolidone (HP) and water are partially miscible above a lower consolute temperature (LCT) of 19.1°C. The interfacial tension of the two phase system was measured as a function of temperature. HP is surface active at the air/solution interface and forms micelles in solution at all temperatures from the freezing point up to the LCT and up to 25°C in the water-rich phase above the LCT. At low water contents, water is positively adsorbed at the air/solution interface.