Monte Carlo study of structural ordering in charged colloids using a long-range attractive interaction

摘要

Monte Carlo simulations have been carried out for aqueous charged colloidal suspensions interacting via an effective pair potential U-s(r), which has the long-range attractive term in addition to the usual screened Coulomb repulsion. Simulations are performed over four orders of magnitude of particle volume fractions (phi) under salt free as well as added salt conditions. The computed pair correlation functions g(r) at high values of phi show a face centered cubic (fcc) crystalline order, which is found to transform to a body centered cubic (bcc) crystalline order upon lowering of phi,. The crystalline order is found to melt into a liquidlike order upon the addition of salt. A purely repulsive potential based on Derjaguin-Landau-Vervey-Overbeek theory was earlier claimed to be responsible for the formation of bcc and fee phases and the associated order-disorder transitions. It is clearly shown here that U-s(r) also explains equally well such phenomena and the results are shown to be in close agreement with experimental observations. Simulations in the dilute regime show a vapor-liquid transition upon variation of phi and the coexistence of ordered and disordered regions with voids upon variation of the charge on the particles. These results explain the reported experimental observations, which suggested the existence of a long-range attraction in the interparticle interaction. For very low volume fractions calculated pair correlation functions show only a single peak and are found to be independent of phi. The reported results on the direct measurement of the pair potential are discussed in the light of the present results. [References: 67]