Structure of colloidal dispersions with an attractive primary minimum and far-field repulsion in the pair-potential: Adhesive-hard-sphere and random-phase approximations

Abstract

An analytical method to estimate the static structure factors of charged dispersions dominated by the presence of primary minima in the interparticle potentials is presented. the near-field attraction is approximated by square-well potentials and the far-field repulsion by Yukawa tails. Analytical representation of the structure factor is obtained for the square-well/Yukawa (SWY) dispersions using an adhesive-hard-sphere (AHS) reference and random-phase approximation (RPA). The accuracy of the AHS + RPA combination in predicting structure factors of SWY fluids is examined using Monte Carlo simulations for the SWY fluids over a range of conditions. The results show that AHS + RPA describes the structure factors very well at low to moderately high volume fractions when the range of the square-well potential is less than 10% of the particle diameter and that of the Yukawa tail is sufficiently large. At high volume fractions the AHS + RPA results agree well with those of simulations in the low- q region, but deviate appreciably in the high- q region. At low volume fractions, the model can be applied to even relatively long-ranged Yukawa repulsive interactions. These results provide an unambiguous test of the AHS + RPA model. The AHS + RPA model has been found to describe small-angle neutron scattering intensities of a micellar system consisting of cationic dodecyl tributylammonium bromide surfactant in water and D 2O reasonably well.