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Forces and fluid structure between patterned solutes : the influence of solvent phase behaviour Overduin, Sarah Danielle

Abstract

The behaviour of planar surfaces and spherical solutes immersed in a liquid near liquidvapour or liquid-liquid coexistence is examined using computer simulation and integral equation theories. Of interest is the structure of the solvent, and the resulting forces between the solutes. Both uniform and chemically patterned solutes are examined. Grand canonical Monte Carlo calculations are used to investigate the phase behaviour of a binary mixture of Lennard-Jones particles (species A and B) confined between planar, parallel, chemically patterned plates. Attention is focussed on the influence of surfaceinduced transitions on the net force acting between the plates. In addition to the stable and metastable bulk states that play a crucial role for homogeneous surfaces, for certain patterns and surface separations a bridge phase analoguous to that recently reported for one-component systems is observed. We find that the separation at which bridge formation occurs is limited by the unfavourable interfacial tension between the A-rich and 5-rich regions of the fluid. It is found that bridge phase formation leads to strongly attractive plate-plate forces that are equal in magnitude to those observed for homogeneous surfaces. The effect of surfactant particles on the confined mixture is also examined. We show that these liquid bridges can be extended by reducing the interfacial liquid-liquid tension when surfactant particles are added to the system. In addition, other fluid structures that are not observed in the binary fluid can be stabilized. We give a qualitative discussion of the surface-surfactant induced liquid structures and examine in detail the associated forces acting between the plates. Isotropic and anisotropic hypernetted-chain (HNC) integral equation theories are used to obtain the interaction of solutes both near and far from the solvent liquid-vapour or liquid-liquid coexistence. Uniform and chemically patterned (patched) solutes are considered, and the influences of particle and patch sizes are investigated. Solvophilic (or A-philic, in the case of mixtures) and solvophobic (A-phobic) solutes (or patches) are examined. Near liquid-vapour coexistence in the one-component fluid, drying-like behaviour occurs between solvophobic solutes (patches) of sufficient size. This gives rise to relatively long-ranged attractive forces that are strongly orientation dependent for the patched solute particles. Similar results are obtained in the yl-rich mixture; "drying" of species A, and "wetting" by species B occurs near A-phobic solutes (patches) in an A-rich fluid. In a 73-rich fluid, "wetting" by species A and drying of species B occurs between A-philic solutes (patches). We also report grand canonical Monte Carlo results for a pair of uniform solutes and demonstrate that the anisotropic HNC theory gives qualitatively correct solvent structure in the vicinity of the solutes. Comparison with previous simulations also shows that the solute-solute potentials of mean force given by the anisotropic theory are more accurate (particularly at small separations) than those obtained using the isotropic method.

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