Computational Study of Polyelectrolyte Multilayer Films

Experimentalists have found that the growth of polyelectrolyte multilayer films are not always linear and can be exponential. The size of these films is very important in applications such as photovoltaics where exact depths are needed for ideal power eifficiency. This work is a computational study of such growth, attempting to find which conditions (such as length of the polyelectrolyte chains, solution densities and charge distributions) constitute the transition between exponential and linear growth. The study was performed using the GROMACS molecular dynamics simulation package and the Martini force field coarse graining method. The polyelectrolytes used were Polylysine cations and Polyglutamic Acid anions with different parameters, such as chain length, in each simulation. The simulation consisted of equilibrating a solution of polyelectrolytes and counter-ions in coarse grain water with periodic XY-boundaries onto a charged substrate at the Z=0 axis and then removing unbound matter. The next steps added solutions of alternating charges until the multilayer film is sufficiently grown. The rate at which the film grows was studied based on the depths of each layer, where a layer is defined as the polyelectrolytes bound during a single step