Thermodynamics of adsorption of inorganic electrolytes at air/water and oil/water interfaces

Abstract

Extensive surface and interfacial tension data for air/water and oil/water systems in the presence of high concentrations of inorganic salts were presented. The boundary tensions increased considerably in all cases in the presence of electrolytes, except in the nitro benzene/water case where a decrease was observed in the presence of KSCN. From thermodynamic analysis, the absolute amount of water and electrolyte present at the interface was determined. In most cases the amount of water and electrolyte present at the interface was of the order of 10 −9 and 10 −11 mole/cm 2, respectively. The interface was found to contain almost a monolayer of water in most cases, although a multilayer was also found in the presence of some salts. The interfacial region of the oil/water system in the presence of several salts was found to contain an appreciable amount of oil. Based on the integration of the Gibbs adsorption equation followed by a hypothetical dilution process, the standard free energy of transfer of 1 mole of water from bulk phase to the interface in the presence of various inorganic salts has been calculated for the change of a bulk mole fraction of water from 1.0 to 0.5. These free energies, which are all positive, are strictly comparable for different salts and for different types of interfaces under identical states of reference.