DEPLETION FORCE BETWEEN A COLLOID PARTICLE AND A WALL - SIMPLE DETERMINATION BY MEANS OF SCALED PARTICLE THEORY

Scaled particle theory is used to calculate depletion interactions bet ween a hard wall and a single hard sphere solute in a hard sphere solv ent. By determining the work required to insert and grow a cavity of a given size, it is possible to capture the qualitative and some quanti tative behaviour of the entropic potential between the solute and the wall for separations less than the solvent diameter. The method is bas ed upon simple geometric arguments (statistical geometry) and does not require knowledge of the internal structure of the solvent. As the di ameter of the solute particle is increased, the method predicts the en tropic potential with greater accuracy. However, for packing fractions in excess of 0.2, it begins to underestimate the entropic potential, although for large solute diameters the predicted barrier heights and force profiles are reasonably accurate. Possible improvements in the t heory are discussed. Besides capturing the qualitative (and some quant itative) behaviour more accurately than the simple Asakura-Oosawa idea l gas approximation, the method can be extended readily to the determi nation of depletion forces between the solute and a hard wall of arbit rary curvature.