THERMODYNAMIC ANALYSIS OF SOLUTE EFFECTS ON OSCILLATORY SOLVATION FORCES

The effects of cyclohexane and water on the oscillatory solvation force found between mica surfaces in the nonpolar liquid octamethylcyclotetrasiloxane (OMCTS) are analyzed by applying the Gibbs adsorption equation. At low solute concentrations and large separations adsorption of the solute oscillates with the periodicity of the molecular size of OMCTS. The amplitude of the adsorption oscillations is a linear function of concentration and an exponential function of separation. The solute molecules are depleted from the gap at the positions of the force minima. At the force maxima the gap is enriched with the solute. The adsorption behavior of water is reversed at short separations due to capillary condensation. The adsorption of cyclohexane in the gap at high concentrations diminishes and changes sign during the transition to pure cyclohexane. This example illustrates the utility of thermodynamic analysis of surface force data. The Gibbs equation enables the calculation of the composition of the medium between the surfaces from the force measurements.