Adsorption equilibria in surface force balance studies

On the assumption of adsorption equilibrium in surface force balance measurements, the effects of solutes such as sails and surfactants on the force-separation profiles are necessarily related to changes in solute adsorption with the separation distance of the opposing surfaces. The thermodynamic relations given by Hall (J. Chem. Sec., Faraday Trans. 2, 68 (1972) 2169) are applied to published data on the effects of inorganic salts and long-chain surface-active solutes in aqueous solution on the force-distance profiles between mica sheets, allowing for the possibility of ion exchange. The results show that the adsorption changes with separation can be complex, and that rather small changes in adsorption are sufficient to account for the measured effects. If adsorption equilibrium is not achieved the Derjaguin approximation is not generally applicable. Problems relating to defining the zero of separation, the reversibility of experimental conditions and the effects of adventitious components are considered.