45522-AC7
Self-Assembly of Charged Macromolecules: New Simulation Techniques
In the first period of this grant, we have extended the geometric cluster Monte Carlo algorithm—a simulation method that is highly efficient for macromolecules in an environment of small molecules or atoms—to systems in which the constituents interact via electrostatic potentials. We have demonstrated that this greatly accelerates the numerical simulation of multicomponent systems of charged particles. A particular application involves solutions of macromolecules (colloids or polyelectrolytes) and multivalent counterions. It is know that such counterions can induce effective attractions between the macromolecules, but the resulting phase behavior is not well examined. For this reason, we have now, in the second period of this grant, expanded our work to incorporate a recently proposed technique into our cluster algorithm. This technique, referred to as the “restricted Gibbs ensemble” involves setting up two simulation cells and permitting exchange of particles between both cells. In the situation of charged particles this is particularly complicated, since it is desirable to maintain electroneutrality in each of the boxes individually. We have resolved this by performing a so-called “cluster move” in which a large number of particles are exchanged between both cells, followed by the move of individual particles to restore each cell to a neutral configuration. Tests have demonstrated that this is indeed a feasible and valid simulation technique. Until present, our actual application has been limited to colloids and monovalent salt, and we have found that in this case phase separation does not occur. This finding is at variance with other recent claims in the literature, but in strong agreement with theoretical and experimental observations. We explain the discrepancy from the fact that our method permits density and composition fluctuations that drive the phase separation, but that have been omitted in earlier work.
It is our expectation that this approach can be extended to a large variety of other systems, and will provide a new tool to the exploration of phase separation in size-asymmetric mixtures of charged components.
