Nonaqueous liquid/liquid interfaces: Surface activity at the interface between n-paraffins and N-alkyl derivatives of 2-pyrrolidone

While nonaqueous liquids are widely used commercially, the interfacial chemistry of nonaqueous liquid/liquid systems has received scant attention. In this paper interfacial properties of partially miscible two-component systems of n-hexadecane with 2-pyrrolidone, N-methyl-2-pyrrolidone or N- ethyl-2-pyrrolidone, and of n-heptane with N-methyl-2-pyrrolidone at 25°C are presented. The surface activity of the long-chain N-alkyl-2-pyrrolidones at these interfaces was also studied. The adsorption of the long-chain pyrrolidones increases with chain length following a modified form of Traube's rule. Partition coefficients for the long-chain pyrrolidones at low concentrations were measured, and standard free energies of transfer between the coexisting bulk phases and between each bulk phase and the interface were calculated. At higher concentrations, the interfacial tensions of the longer chain pyrrolidones go toward zero as the systems become more miscible. The temperature variation of the hexadecane/N-methyl-2-pyrrolidone and the hexadecane/N-ethyl-2-pyrrolidone interfacial tensions were also measured.