Long-Range Interaction between Macroparticles in Binary Fluid Mixture

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In analyses using the RHNC theory for two-component fluids near surfaces (e. g., macro-particles), we have found a sudden growth in the thickness of the surface-induced layer whose structure and composition are quite different from those of the bulk. Moreover, the sudden growth is accompanied by appearance of strong, long-range attractive force between surfaces. This is a sort of transition phenomenon : formation of the second phase near the surface that can be called the surface-induced phase transition. We have measured forces between mica surface-silicon nitride probe (case A) and mica surface-silica sphere (case B) in methanol-hexane system using an atomic force microscope. We have plotted the range of the attractive force L against the methanol concentration X in case A. We have also plotted the strength of the attractive force F at the surface separation 7.5nm against X in case B. As the saturation concentration of methanol in hexane is approached, both L and F increase very sharply, which implies a sudden growth in the methanol-rich layer near the hydrophilic surfaces. The forces under high methanol concentration are far longer-range than the conventional van der Waals forces. We have then measured forces between mica surface-silicon nitride probe in ethanol-hexane system. This system is thermodynamically stable as a single phase in the entire composition range. Near ethanol concentration of 。ォ1.9wt%, the range of the attractive force exhibits a sudden growth that can be interpreted as a signal of the surface-induced phase transition. We have also examined the experimental data reported by other researchers in our own way and concluded that the surface-induced phase transition does exist. It will be very useful in understanding the systems where surfaces or interfaces play critical roles (e. g., colloids, biological systems, etc.).