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.