Potential of Mean Force Calculation from Molecular Dynamics Simulation of Asphaltene Molecules on a Calcite Surface

In this paper, we present preliminary calculations of the potential of mean force (PMF) from molecular dynamics simulations of asphaltene molecules on a calcite surface. We calculate, for the first time, the PMF between an asphaltene molecule and the calcite surface directly by a constraint force method. The asphaltene molecule is obtained in a systematic way consistent with experimental information, using the quantitative molecular representation approach. The calcite surface has been defined as the {10.4} face, which is representative of the experimentally dominant crystal face. First, we calculate the time-averaged values of the constraint force as a function of the z distance between the asphaltene molecule and the calcite surface. We observe that, at a separation of 3 angstrom, the value of the force is positive, which corresponds to a repulsive interaction. At 3.5 angstrom, a minimum in the force is observed, corresponding to an attractive interaction. At longer distances, the average constraint force goes to zero, according to expectations. Then, we calculate the integral of the time-averaged constraint force as a function of the distance between the asphaltene molecule and the calcite surface. The difference between the minimum energy and the energy at large separations gives an estimate of the free energy of adsorption. In this case, the value of the free energy is of the order of 110 kJ/mol. This is a reasonable value for simulations in vacuo corresponding to an effective solvent.