(757d) The Role of Dispersion Interactions, Electrostatics, and Entropy in the Interfacial Behavior of MoS2

Authors: 
Govind Rajan, A., Massachusetts Institute of Technology
Sresht, V., Massachusetts Institute of Technology
Pádua, A. A. H., Institut de Chimie de Clermont-Ferrand, Université Blaise Pascal and CNRS
Strano, M. S., Massachusetts Institute of Technology
Blankschtein, D., Massachusetts Institute of Technology
The synthesis and applications of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) in membranes, sensors, and microfluidic devices, involves an inevitable contact between these materials and various liquids. The existence of partially ionic bonds in MoS2, as opposed to covalent bonds in graphene, suggests a plausible role for polar (electrostatic) interactions in determining the interfacial behavior on two-dimensional MoS2 surfaces. Surprisingly, not only the equilibrium contact angle, which depends solely on the total interaction energy between the surface and the liquid, but also the friction coefficient and slip length, which depend on the spatial variations in the interaction energy, are independent of the inherent polarization in MoS2. While the former is found to result from the negligible electrostatic interactions between MoS2 and liquids, the latter results from the tri-layered sandwich structure of the MoS2 monolayer. Specifically, the tri-layered sandwich structure results in spatial variations in the dispersion interactions that dominate over those in the electrostatic interactions. Furthermore, the wettability of MoS2 is found to be controlled by a delicate balance between dispersion interactions and entropy. Our findings reveal that in spite of the existence of heteropolar bonds in MoS2 compared to the existence of homopolar bonds in graphene, there is an unexpected similarity between the interfacial behaviors of these two surfaces. This finding, in turn, elucidates the similar physiosorption behavior of MoS2 and graphene, including the use of similar liquids to exfoliate these 2D materials.