Colloidal suspensions are represented as a mixture of macrospheres immersed in a multicomponent solvent of small spheres. The behavior of the macrospheres is analyzed on the basis of the Percus–Yevick theory when the ratios of the small to large sphere diameters go to zero at fixed packing fractions. Within the Baxter formalism the recent results of Biben and Hansen for the one component solvent are generalized. It is shown that the macrosphere suspension reduces to a one component system in which Baxter’s function goes exactly over to that of a one component adhesive sphere fluid. The expression for the adhesiveness is exactly expressed in terms of the packing fractions of the solute and the solvent components. Such a model permits the interpretation of the flocculation phenomenon induced by the addition of free polymers in a colloidal suspension.
Structural trends in multicomponent mixtures of adhesive spheres are analyzed by using the Baxter formalism and the Percus–Yevick approximation (PYA). The Orsnstein–Zernike (OZ) equations in q space are cast in a form which allows a fully analytical expression of the effective adhesiveness coefficient of the large (solute) spheres in the asymptotic limit of vanishing size ratio and no solute self-stickiness. This allows a simple discussion of the factors which determine the effective solute adhesiveness and suspension stability: while the steric effect and like particles stickiness are found to favor suspension instability, hetero stickiness is found to act in one side or in the opposite depending on the concentration of the smaller species. These qualitative predictions are paralleled with studies on solvent effects in ordinary colloidal solute–solvent systems and with the behavior of pseudobinary systems such as colloid–polymer or bidisperse colloidal mixtures. Results from the literature for hard sphere mixtures and calculations in the PYA including solute self-stickiness at nonzero size ratio are finally used to discuss the reliability of the trends deduced from the asymptotic limit.
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