Effect of a Polymer Brush on Capillary Condensation

Abstract

We consider colloidal particles in a binary solvent. The solvent mixture has a solubility gap. The system is studied, from a solvent point of view, in the one-phase region near the binodal. The colloidal stability is significantly influenced by a mechanism called capillary condensation. In the confined space between particles, the minority solvent can form a solvent bridge resulting in an attractive force. This occurs when the particles prefer the minority solvent over the majority one. One way to protect such particles from capillary condensation flocculation is to end-graft polymers onto them. The main solvent should then be the better solvent for the polymer. However, if the chains are amphiphilic, in the sense that they have similar affinity for both solvents, it is possible to create conditions such that the colloidal particles are only kinetically stabilized by the brush chains. The true thermodynamic equilibrium may be the flocculated state, where two particles are held together by a thin minority-liquid film. This state can be reached only when a repulsive barrier, due to the compression of a polymer brush, is overcome. The opposite case also exists. It is possible that the global minimum is the dispersed one, and the particles are held together by a metastable film of the minority solvent. Now, the separation of the particles is retarded by the work of formation of two polar−apolar interfaces. These scenarios were proven by self-consistent-field calculations of the Scheutjens−Fleer type.