The thermodynamics of monolayer penetration at constant area: Part 1

The Gibbs adsorption isotherm is applied to the penetration of long-chain ionic molecules into an insoluble monolayer held at constant area at the air/water interface. Uncertainties arise in the calculation of the surface densities of the penetrating molecules. These uncertainties are due firstly to the possibility of ion exchange (surface hydrolysis), and secondly to the perturbation of the insoluble component and the introduction of assumptions as to the chemical potential of the insoluble component of the mixed film. These uncertainties are largely avoided with a condensed insoluble film spread on a solution containing excess neutral electrolyte. The theory is applied to the interaction of cholesterol monolayers with sodium dodecyl sulphate in solution.