Long-range attractive interaction between macroparticles in a binary fluid mixture

Abstract

In a previous paper (Kinoshita, M., Iba, S., Kuwamoto, K., and Harada, M., 1996, J. chem. Phys., 105, 7184) we analysed the interaction between macroparticles immersed in a binary fluid mixture using reference hypernetted chain theory. The bulk mixture treated in the analysis separates into two immiscible phases in a certain composition regime, but the thermodynamic state considered was far from the spinodal line and the mixture was stable as a single phase. The singular behaviour of the reduced density profiles of mixture components near the macroparticle surface and the potential of mean force between macroparticles revealed in that study have been examined further. It has been found that the thickness of the surface-induced layer and the range of macroparticle interactions suddenly grow to a submacroscopic scale which is far longer than molecular dimensions (but not infinitely long) near the singular point. The macroparticle interactions always become strongly attractive. This is a transition phenomenon and can be interpreted as ‘partial wetting’ of one of the components accompanied by ‘partial drying’ of the other. It has been pointed out that trace amounts of impurity can affect the macroparticle interactions to a dramatic extent. For instance, the presence of trace amounts of small, nonpolar solutes in water can cause aggregation of large, hydrophobic molecules or particles.