STRUCTURE OF COLLOIDAL DISPERSIONS WITH AN ATTRACTIVE PRIMARY MINIMUMAND FAR-FIELD REPULSION IN THE PAIR-POTENTIAL - ADHESIVE-HARD-SPHERE AND RANDOM-PHASE APPROXIMATIONS

An analytical method to estimate the static structure factors of charg ed dispersions dominated by the presence of primary minima in the inte rparticle potentials is presented. The near-field attraction is approx imated 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-sp here (AHS) reference and random-phase approximation (RPA). The accurac y of the AHS + RPA combination in predicting structure factors of SWY fluids is examined using Monte Carlo simulations for the SWY fluids ov er a range of conditions. The results show that AHS + RPA describes th e structure factors very well at low to moderately high volume fractio ns 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 o f 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 r elatively long-ranged Yukawa repulsive interactions. These results pro vide an unambiguous test of the AHS + RPA model. The AHS + RPA model h as been found to describe small-angle neutron scattering intensities o f a micellar system consisting of cationic dodecyl tributylammonium br omide surfactant in water and D2O reasonably well.