Behavior of the fullerene/water system under liquid and liquid–vapor conditions: A molecular dynamics approach

Abstract

Classical molecular dynamics simulations were carried out to quantify the behavior of fullerenes in water, under liquid phase and liquid–vapor conditions. Different molalities of fullerene were considered in the study. Radial distribution functions, static dielectric constant and equation of state were evaluated in liquid phase. Direct liquid–vapor coexistence simulations of water with fullerene added were conducted to explore density profiles, liquid–vapor coexistence curve, pressure tensor components and surface tension variations with respect to water. An all-atom fullerene model studied by Monticelli and the SPC/ɛ description of water were used. At ambient conditions fullerene–fullerene and fullerene-water correlations decreased as molality increased. In turn, the static dielectric constant decreased due that fullerenes hinder the dipolar orientation of water molecules. As temperature increased, the static dielectric constant lowered. Part of the study was conducted at 373 K from 10 to 24 kbar, revealing that density of the system increased as molality was higher while fullerene-fullerene correlations decreased and fullerene-water correlations increased slightly. Concerning the static dielectric constant, a decrease was observed due that fullerenes and fullerene aggregates disturbed water structure as molality increased. As the system is compressed at a fixed molality, fullerenes become more uniformly distributed and the dielectric constant increases. Therefore, increasing the pressure is a mechanism to force the dipolar orientation of water molecules. On the other hand, liquid–vapor coexistence was studied from 280 to 650 K using molalities from 0.037 to 0.695 mol/kg. Liquid–vapor simulations revealed that the location of fullerenes and the formation of aggregates affect the behavior of surface tension. Fullerenes can locate at the liquid–vapor interfaces forming aggregates that obstruct the pass of water molecules to the vapor, so surface tension increases slightly with respect to water. At 0.695 mol/kg, larger clusters were observed but they located predominantly within the liquid phase and therefore acted in the opposite direction, that is, they favored the migration of water molecules to the vapor and surface tension dropped. In addition, a peculiar behavior was observed for temperatures above 500 K: surface tension increased its value as temperature increased. This behavior could be attributed to the fact that at such high molality the calculated surface tension might not longer correspond to liquid–vapor interfaces, instead it could be related to a sort of solid–liquid interface that have formed into the system. These calculations set the basis for improving the description of fullerene/water interactions as more experimental information becomes available and also serve as reference data in the design of interaction parameters to model water-soluble fullerenes.