An approach to surface tension based on the thermal fluctuation of the microscopic area

PEREZ SIRKIN, YAMILA A.; FACTOROVICH, MATÍAS H.; SCHERLIS, DAMIAN A.

Congreso; Gordon Research Seminar of Chemistry and Physics of Liquids; 2015

Gordon Research Conference

Different routes exist to the computation of surface tension from molecular simulations. Their application to non-planar interfaces of arbitrary shape, is, however, far from trivial. In this work we propose and explore a scheme to compute this property based on its thermodynamic definition and on the thermal fluctuations of the microscopic area S. The free energy is calculated as function of S, from the distribution function P(S). The area is obtained from a numerical method, and then the surface tension is calculated from the slope of the free energy with respect to S in a linear region. Our objective is to implement this technique to study the surface tension of water droplets and other species in the nanoscopic limit, when it is difficult to obtain either from experiments or simulations.Many definitions are possible for the microscopic area at the molecular level: in our approach, we consider a numerical scheme where S depends on two parameters. Our calculations on the SPC-E water model show that the surface tension depends on the area definition through these parameters. Regardless of these parameters, however, the dependence of surface tension with respect to temperature is consistent with the experimental trend and with previous simulations. The dependence with respect to the radius of curvature, on the other hand, indicates that this property decreases with the size of a droplet, implying a negative Tolman length.