Molecular Insights into the Abnormal Wetting Behavior of Ionic Liquids Induced by the Solidified Ionic Layer

Figuring out the wetting behavior and interfacial structure of ionic liquid (IL) droplets on solid surfaces is imperative for the new IL-based application in the field of chemical engineering. In this work, the quantitative relations between the wetting properties and solid surface property are explored through molecular dynamics simulations. Interestingly, two different equilibrium configurations, nonspreading structure and partial spreading structure, are identified for ILs when the solid interface property changes from hydrophobic to hydrophilic. Considering the quantitative relation between the contact angle (CA) and the solid surface energy (epsilon(s)), both EmimCl and EmimPF(6) show an abnormal wetting behavior compared with water, where the CA-epsilon(s) relation for ILs deviates the theoretical linear model when e s is beyond the critical surface energy epsilon(c). The reduced density distribution and orientation distribution show that ILs could form a denser solidified ionic layer near the solid surface, which will not happen to water. Moreover, the dynamical properties including the retention rate and vibrational displacement for ions and water molecules confirmed that the solidified ionic layer almost lost the fluid nature, while the adjacent water layer still possesses relatively high fluidity, especially when epsilon(s) > epsilon(c) That is to say, the dense solidified ionic layer which lost its mobility can bring about such an abnormal wetting behavior of ILs. These factors can serve as key indicators in characterizing the mechanism of the structural transition of ILs wetting the solid surface and facilitate the rational design of the surface modification or strain engineering of the solid substrates.