Effect of alkyl chain length on the wetting behavior of imidazolium based ionic liquids: A molecular dynamics study

Abstract

Molecular dynamics (MD) simulations are performed to investigate the wetting behavior of imidazolium-based ionic liquids (ILs), which mainly consist of an imidazolium ring with a flexible alkyl chain attachment. The length of the alkyl chain can be easily manipulated to modulate the wetting properties of the ILs. To understand the role of the alkyl chain, we investigate the wetting behavior of two room-temperature ionic liquids (RTILs), 1,3-dimethylimidazolium tetra-fluoroborate [DMIM][BF4] and 1-propyl-3-methylimidazolium tetra-fluoroborate [PrMIM][BF4], on a graphite surface. Furthermore, the effects of the droplet size and temperature on the contact angle of the IL droplets are investigated. The wetting behaviors of both IL droplets are characterized using a density profile and orientation order parameter profile along the axis normal to the surface. The length of the alkyl chain is found to be crucial for modulating the wetting properties of the ILs. With an increase in the length of the alkyl chain, the contact angle of the droplet decreases, which is also in line with experimental results [1,2]. The wetting of the IL droplets strongly depends on the temperature and the relative fluid-fluid and fluid-substrate interactions. With the increase in temperature, the contact angle of the IL droplet decreases, resulting in the formation of a single layer film at a high temperature. In addition, the behavior of the contact angle of the IL droplet is analyzed through the surface tension as well as the graphite-IL interfacial tension. Furthermore, considering the anisotropic properties of the imidazolium-based ILs, we calculate the contact angles for cylindrical filaments (2D) on the graphite surface and compare the obtained values with the contact angle obtained from the spherical droplet (3D). The contact angle of the cylindrical droplet is found to have a negligible system size effect as compared to the case with the spherical droplet.