Diffusiophoresis of hydrophobic spherical particles in a solution of general electrolyte

Abstract

The present article deals with the diffusiophoresis of hydrophobic rigid colloids bearing arbitrary ζ-potential. We derived the generic expression for the diffusiophoretic velocity of such a colloid exposed in an externally applied concentration gradient of the general electrolyte solution. The derived expression takes into account the relaxation effect and is applicable for all values of surface ζ-potential and hydrodynamic slip length at large κa (κa≥ca.50), where κ−1 is the thickness of the electric double layer and a is the particle radius. We further derived several closed-form expressions for particle velocity derived under various electrostatic and hydrodynamic conditions when the particle is exposed in an applied concentration gradient of binary symmetric (e.g., z:z), asymmetric (1:2, 2:1, 3:1, 1:3), and a mixed electrolyte (mixture of 1:1 and 2:1 electrolytes). The results for diffusiophoretic velocity are further illustrated graphically to indicate the mutual interaction of chemiphoresis, induced electrophoresis due to unequal mobilities of cations and anions of the electrolyte, and the mechanism by which the sufficiently charged particle migrates opposite to the direction of the applied concentration gradient. The impact of hydrophobicity is further discussed.