Dominance of Dispersion Interactions and Entropy over Electrostatics in Determining the Wettability and Friction of Two-Dimensional MoS2 Surfaces

The existence of partially ionic bonds in molybdenum disulfide (MoS2), as opposed to covalent bonds in graphene, suggests that polar (electrostatic) interactions should influence the interfacial behavior of two-dimensional MoS2 surfaces. In this work, using molecular dynamics simulations, we show that electrostatic interactions play a negligible role in determining not only the equilibrium contact angle on the MoS2 basal plane, which depends solely on the total interaction energy between the surface and the liquid, but also the friction coefficient and the slip length, which depend on the spatial variations in the interaction energy. While the former is found to result from the exponential decay of the electric potential above the MoS2 surface, the latter results from the trilayered sandwich structure of the MoS2 monolayer, which causes the spatial variations in dispersion interactions in the lateral direction to dominate over those in electrostatic interactions in the lateral direction. Further, we show that the nonpolarity of MoS2 is specific to the two-dimensional basal plane of MoS2 and that other planes (e.g., the zigzag plane) in MoS2 are polar with respect to interactions with water, thereby illustrating the role of edge effects, which could be important in systems involving vacancies or nanopores in MoS2. Finally, we simulate the temperature dependence of the water contact angle on MoS2 to show that the inclusion of entropy, which has been neglected in recent mean-field theories, is essential in determining the wettability of MoS2. Our findings reveal that the basal planes in graphene and MoS2 are unexpectedly similar in terms of their interfacial behavior.