Adsorption of oil into surfactant monolayers and structure of mixed surfactant plus oil films

We describe a tensiometric method for determining the adsorption isotherm of an oil on a surfactant monolayer adsorbed at the air-water surface. The method involves measuring the surface pressure of oil, pi(oil), as a function of its activity, a(oil), varied by changing the relative vapour pressure. We compare the isotherm of dodecane adsorption onto a C12E5 monolayer determined in this way with that measured directly using neutron reflectivity. The agreement between the two allows us to conclude that, at least for this system, addition of oil does not result in a change in the chemical potential of the surfactant. Structural analysis of the dodecane + C12E5 mixed film has been performed with neutron reflectivity using two contrasts. In one, only the surfactant chain region is highlighted, whilst in the other only the oil film is visible. We document, for the first time, changes in thickness and packing density of both the oil and surfactant chains in the mixed surfactant + oil layer upon increasing oil content. For a diverse range of other oil + surfactant systems, we have determined the initial (S-j) and equilibrium (S-e) spreading coefficients of oil by measuring pi(oil) following addition of liquid oil (at unit activity) to surfactant solution surfaces. Those systems in which S-e is close to zero display a repulsive van der Waals component of the disjoining pressure-oil film thickness isotherm, whilst with toluene as oil the calculated isotherm is attractive, consistent with non-spreading observed for this oil. (C) 1999 Elsevier Science B.V. All rights reserved.