Confinement-Induced Symmetry Breaking of Interfacial Surfactant Layers

• Published in: The journal of physical chemistry. B 2006-05, Vol.110 (17), p.8756-8763
• Main Authors: Leermakers, F. A. M, Koopal, L. K, Goloub, T. P, Vermeer, A. W. P, Kijlstra, J
• Format: Article
• Language: English
• Subjects: adsorbed layer; adsorption regulation; aqueous film; association; field lattice model; hydrophobicity; ionic surfactants; nonionic surfactants; phase-behavior; statistical thermodynamics
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp061299w

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.