Studies on the behaviour of some mono-carboxylic acids at the solution-vapour interface

Abstract

The behaviour of acetic, propionic and butyric acids at the solution-vapour interface at 30 has been studied systematically. The surface tensions and vapour-pressures of fourteen systems were measured and the Gibbs adsorption equation used to calculate surface excesses, ⌐ (_2) (^N), whilst the amounts of the individual components at the interface were calculated by well known methods. It is assumed that the thickness of the adsorbed phase is essentially monolayer in character. The orientation of the acids at the interface is assumed to be the ‘parallel’ one for adsorption from benzeno, carbon tetrachloride and cyclohexane and the 'perpendicular’ one for adsorption from ethyl alcohol and water. Two factors, - affinity between acid and solvent in the bulk and rate of lowering of surface tension of acid + solvent mixtures by the acids, have been need to account for the extent of acid adsorption at the interface. In general adsorption of acid increases with increasing chain length, although for the cyclohexane systems it decreases with increasing acid chain length. The occurrence of a mixed monolayer at the interface is observed for all the systems, and a uni-monolayer of acid is formed only from the pure component. For the water systems it is possible for the adsorbed layer to contain two water molecules along the length of the acid molecule due to the much smaller size of the water molecule. The present investigations have thrown further light on adsorption behaviour at the solution-vapour interface. A complete understanding of adsorption phenomenon at the interface, involving a reasonable assessment of thickness of the adsorbed layer, composition and orientations at the interface, etc., depends on the particular systems involved, on the properties of the solutions, and on the intermolecular interactions possible both in bulk solution and at the interface.