Confinement-induced phase transition and hysteresis in colloidal forces for surfactant layers on hydrophobic surfaces

In this paper we consider surfactant solutions near a pair of interfaces. It is well-known that strong lateral interactions between surfactant molecules give rise to a step in the adsorption isotherm. In a self-consistent field theory, such a step in the adsorbed amount shows up as a van der Waals loop. The consequence of such a loop for surface force experiments is analyzed. From adsorption isotherms at fixed confinement we extract the relevant adsorbed amounts for a fixed chemical potential as a function of the confinement. A cusped structure is found for the relation between the interaction energy and the slit width: there is a confinement-induced first-order phase transition. The corresponding interaction curve has a kink at the binodal slit distance. Metastable branches as well as an unstable branch (bracketed by the two spinodal points) are presented. The metastability is expected to give rise to force hysteresis in, e.g., atomic force microscope or surface force apparatus experiments, distinctly different from those due to mechanical instabilities of the cantilever system.