Ice and water droplets on graphite: A comparison of quantum and classical simulations

Abstract

Ice and water droplets on graphite have been studied by quantum path integral and classical molecular dynamics simulations. The point-charge q-TIP4P/F potential was used to model the interaction between flexible water molecules, while the water-graphite interaction was described by a Lennard-Jones potential previously used to reproduce the macroscopic contact angle of water droplets on graphite. Several energetic and structural properties of water droplets with sizes between 10(2) and 10(3) molecules were analyzed in a temperature interval of 50-350 K. The vibrational density of states of crystalline and amorphous ice drops was correlated to the one of ice Ih to assess the influence of the droplet interface and molecular disorder on the vibrational properties. The average distance of covalent OH bonds is found 0.01 Å larger in the quantum limit than in the classical one. The OO distances are elongated by 0.03 Å in the quantum simulations at 50 K. Bond distance fluctuations are large as a consequence of the zero-point vibrations. The analysis of the H-bond network shows that the liquid droplet is more structured in the classical limit than in the quantum case. The average kinetic and potential energy of the ice and water droplets on graphite has been compared with the values of ice Ih and liquid water as a function of temperature. The droplet kinetic energy shows a temperature dependence similar to the one of liquid water, without apparent discontinuity at temperatures where the droplet is solid. However, the droplet potential energy becomes significantly larger than the one of ice or water at the same temperature. In the quantum limit, the ice droplet is more expanded than in a classical description. Liquid droplets display identical density profiles and liquid-vapor interfaces in the quantum and classical limits. The value of the contact angle is not influenced by quantum effects. Contact angles of droplets decrease as the size of the water droplet increases which implies a positive sign of the line tension of the droplet.

Line tension and its influence on droplets and particles at surfaces

“…The line tension magnitude is also consistent with the estimates of Density Functional Theory [61] using the sharp kink approximation [54]. Very large values of the line tension in the range 10-100 pN (positive) have been reported for water droplets adsorbed on graphite [65][66][67]. In one of these studies it has been shown that quantum nuclear degrees of freedom do not influence the value of the contact angle [67].…”

“…Very large values of the line tension in the range 10-100 pN (positive) have been reported for water droplets adsorbed on graphite [65][66][67]. In one of these studies it has been shown that quantum nuclear degrees of freedom do not influence the value of the contact angle [67].…”

“…As discussed in section 2.1 computer simulation studies have followed the experimental approach, discussed above, to compute line tensions of droplets on solid surfaces [61][62][63][64][65][66][67]. The simulated line tensions in general involve either simple fluid droplets modeled with the Lennard-Jones potential or water droplets.…”

“…It would be very interesting to explore other interfaces (e.g. carbon-water), which involve different type of interactions, and for which larger estimates of the line tension (10-100 pN) have been reported [64][65][66][67].…”

“…the reduced line tension Eqs. (65) and (67) would correspond to a point lying approximately on the unstable-residual stable boundary on Fig. 24.…”