Ethanol exchange between two graphene surfaces in nanoconfined aqueous solution: Rate and mechanism

Abstract

We observe by computer simulations a remarkable long-distance rare but repetitive exchange of ethanol molecules between the two parallel graphene surfaces in nanoconfined aqueous ethanol solutions. We compute the rate of exchange as a function of the separation (d) between the two surfaces. We discover that the initiating (or, the launching) step in this exchange is the attainment of an instantaneous orientation of the carbon-oxygen bond vector relative to the graphene surface. This observation led us to construct a two-dimensional free energy surface for this exchange, with respect to two order parameters, namely, (i) the perpendicular distance of ethanol molecule from the graphene surfaces, z and (ii) the orientation of the O-C bond vector, θ of the tagged ethanol molecule. For d = 3 nm, the rate of exchange is found to be 0.44 ns-1 for the force field used. We also vary the force field and determine the sensitivity of the rate. From the free energy landscape, one could determine the minimum energy pathway. We use both the transition state theory and Kramers' theory to calculate the rate. The calculated rate agrees well with the simulated value as mentioned above. We find that the rate of exchange phenomenon is sensitive to the interaction strength of graphene and the hydrophobic group of ethanol. The Free energy landscape exchange shows dependence on the distance separation of the two hydrophobic surfaces and reveals interesting features.