Kinetics of solute transfer across aqueous phase-liquid hydrocarbon interfaces

Rate constants for the transfer of salicyclic acid across eight aqueous phase-hydrocarbon liquid (nonane through hexadecane) interfaces have been determined at 25°C using a rotating diffusion cell. For each system the interfacial transfer rate constant (k₁) organic → ;aqueous was approximately four times the value in the reverse direction (k_-1 ≈ 22 × 10^-6 m s^-1). The rate constants for salicylic acid transfer in both directions were significantly less for interfaces formed between water and even-numbered hydrocarbons than for those established between the aqueous phase and odd alkane homologues; the possibility of a more structured interfacial region in the former systems is suggested to account for this observation. Experiments performed using methyl nicotinate (3-pyridinecarboxylic acid, methyl ester) as the substrate traversing three of the water-hydrocarbon interfaces showed substantially the same pattern of kinetic behavior.