Interaction forces between mesoscopic objects are fundamental to soft-condensed matter and are among the prime targets of investigation in colloidal systems.Surfactant molecules are often used to tailor these interactions.The forces are experimentally accessible and for a first theoretical analysis one can make use of a parallel-plate geometry.We present molecularly realistic self-consistent field calculations for an aqueous nonionic surfactant solution near the critical micellization concentration,in contact with two hydrophobic surfaces.The surfactants adsorb cooperatively,and form a monolayer onto each surface.At weak overlap the force increases with increasing compression of the monolayers until suddenly a symmetry braking takes place.One of the monolayers is removed jump-like and as the remaining monolayer can relax,some attraction is observed,which gives way to repulsion at further confinement.The restoring of symmetry at strong confinement occurs as a second-order transition and the force jumps once again from repulsion to attraction.It is anticipated that the metastable branch of the interaction curve will be probed in a typical force experiment.Under normal conditions pronounced hysteresis in the surface force is predicted,without the need to change the adsorbed amount jump-like.
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