Osmotic repulsion force due to adsorbed surfactants

When considering interaction forces in surfactant-stabilized colloidal dispersions a factor that has been rarely discussed is the possible effect of osmotic force due to overlapping adsorbed surfactant monolayers. In the present work, the osmotic repulsion force is built-in on the basis of DLVO mechanics and based on Fischer's consideration of the analogous situation for adsorbed polymer layers on solid surfaces [E.W. Fischer, Kolloid Zeitschrift 160 (1958) 120-141] and on Langmuir's earlier concept of osmotic pressure excess due to overlapping adsorption layers [I. Langmuir, J. Chem. Phys. 6 (1938) 873-896]. The advanced method for calculation of the net repulsion force in overlapping surfactant monolayers is developed and applied to real adsorbed surfactant systems. We show that the value of disjoining pressure can reach values as high as 8 MPa for the condition of fully overlapping surfactant adsorption layers, based on the calculation of the first virial term of the general expression for osmotic pressure. Thus, we have shown that osmotic forces can be substantial at distances of close interfacial approach, and that they can easily be of the same or greater order of magnitude than the forces that have been more conventionally considered.