Surface and colloidal properties of cyclic amides. 1. Two-dimensional virial coefficients for adsorbed monolayers of N-alkyl-2-pyrrolidones at the air/water interface

The N-(n-alkyl)pyrrolidones are low vapor pressure polar solvents which are surface active in aqueous solutions. From measurements at low surface pressures and low concentrations the two-dimensional second virial coefficients B₂(T) at the air/water interface are obtained for the series of even-numbered alkyl substituents from ethyl to octyl. At 25 °C, B₂(T) is positive for the ethyl compound and becomes increasingly negative as the chain length increases. The effect of temperature on B₂(T) is unusual, increasing temperature leading to more negative B₂(T) values. The virial coefficients, heats of adsorption, and Traube coefficients are discussed in terms of molecular packing, chain-chain van der Waals attraction, and the interaction of the chains and large molecular dipoles with water.