Temperature and Pressure Dependence of the Properties of the Liquid-Liquid Interface. A Computer Simulation and Identification of the Truly Interfacial Molecules Investigation of the Water-Benzene System

The properties of the interface between water and benzene are investigated in detail on the basis of 23 Monte Carlo computer simulations performed at various temperatures and pressures. The interfacial properties are analyzed in terms of the novel identification of the truly interfacial molecules (ITIM) method, by which the intrinsic (i.e., capillary wave corrugated) surface of the two phases can be detected. The obtained results show that the use of a simple, nonintrinsic definition of the interface (made on the basis of the average density profiles of the components) not only leads to a systematic error in determining the list of the truly interfacial molecules, but this error is also reflected in the erroneous calculation of the thermodynamic properties of the system. The obtained results show that the reciprocal width and reciprocal amplitude of both surface layers decrease linearly with the temperature and reach the value of zero (i.e., the corresponding parameters become infinite) at the point of mixing of the two phases. Similar linear relation is observed between these reciprocal quantities and the logarithm of the pressure, but only above a certain temperature. This temperature is thought to be the upper end of the lower critical line of the phase diagram of the system; however, any reliable support of this conjecture would require a considerably larger number of simulations in the temperature range close to this line. The orientational preferences of the surface water molecules, governed by the principle of maximizing the number of their hydrogen-bonded neighbors, are found to be insensitive to the thermodynamic conditions but become weaker with increasing temperature and decreasing pressure. The lateral hydrogen-bonding network of the surface water molecules, spanning the entire water surface at ambient conditions, is found to undergo a percolation transition well, i.e., 200-400 K below the mixing of the two phases, indicating that the existence of such a percolating lateral network is not a universal feature of the water surface but depends also on the thermodynamic conditions.