Bicontinuous phases in coulombic systems. The role of specific interactions

The effects of specific interactions in a two dimensional coulombic system are investigated. In the framework of a field theoretical approach, we have shown that the electric properties of the system are strongly modified whe n two ionic species have a tendency to demix. By means of a Langevin dynami cs, we investigate here a model system where, in the absence of electrostat ic interaction, the specific interactions are such that the system demixes. This phase separation cannot take place in the presence of the coulombic p otential, as a result of the electroneutrality condition in the system. The snapshots of the system obtained in the simulation, show large local conce ntration inhomogeneities, which are also charge inhomogeneities reminiscent of the tendency of the two species to separate. This is an unusual feature in a charged system, as it corresponds to the gathering of ions of the sam e sign. These inhomogeneities are constituted by a continuous network of li nes of ions of the same species, the adjacent line being of the opposite si gn. This structure is similar to a bicontinuous phase. One property of thes e lines: the average distance between the lines, has been studied by means of a simple field theoretical model. The Hamiltonian includes entropy, elec trostatic and specific interactions. In the mean field approximation, we ob tain a distance between the Lines in agreement with what is found in the si mulation. (C) 2000 Elsevier Science B.V. All rights reserved.