Solute transport and perturbation at liquid/liquid interfaces

Abstract

Solute transfer kinetics across aqueous-solution/organic-liquid interfaces have been measured by two techniques. First, a new method based upon the capillary-tube procedure for self-diffusion has been developed. A short capillary containing radio-labelled solute dissolved in one phase is immersed in a large stirred volume of the second liquid phase, into which the movement of marker is followed. Demonstration and validation of the approach has been performed with a number of systems including the transport of salicylic acid (SA) at a water/isopropyl myristate (IPM) interface. In the second procedure, a rotating diffusion cell has been used to determine whether the rate and energetics of the transfer process are altered by the presence of dissolved anaesthetic alcohols. SA and methyl nicotinate (MN) transport across an aqueous-solution/IPM interface has been studied with various concentrations of different alcohols dissolved in the aqueous phase. The effects of alcohol are solute dependent: SA is not affected by any of the anaesthetics studied whereas 100 mmol dm^–3 ethanol retards MN transfer by a factor of 2. For MN, furthermore, the favourable entropic contribution to the free energy of activation for interfacial transfer (which is observed in the absence of ethanol) is negated by the presence of 100 mmol dm^–3 ethanol. This change may reflect a destabilization of interfacially ordered water molecules by the alcohol and appears similar to the effect of poly(ethylene glycol) on the same system.