Wetting behavior of aqueous 1-alkyl-3-methylimidazolium tetrafluoroborate {[Cn MIM][BF4] (n = 2, 4, 6)} on graphite surface

Abstract

Herein, molecular dynamics simulations have been conducted to investigate the effect of cation chain length on the wetting and interfacial properties of aqueous imidazolium-based ionic liquids (ILs) consisting of 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], and 1-hexyl-3-methylimidazolium [HMIM] cations with common tetrafluoroborate [BF4] anions on a graphite surface in the IL concentration range of 10–50 wt%. The difference between the contact angles of aqueous [EMIM][BF4] and [BMIM][BF4] droplets is very small regardless of the IL content. However, a noticeable reduction in the contact angle is observed for the IL with a longer alkyl chain [HMIM][BF4] at high IL concentrations. Furthermore, interfacial tensions and works of adhesion are calculated to quantify the wettability of the studied ILs; as a result, lower works of adhesion are obtained for the longer alkyl chain compounds (Sali et al., 2017). Droplet characteristics near the surface are obtained from two-dimensional density contours and orientation order parameters. The majority of IL molecules are located near the solid–liquid interface in all the studied cases. However, the [BMIM][BF4] and [HMIM][BF4] ILs with longer alkyl chains are also detected at the liquid–vapor interface. Finally, IL interactions with water molecules through hydrogen bonding are investigated at various concentrations. The obtained interfacial tensions are in good agreement with the experimental values reported in the literature. The results of this study can help elucidate the wetting and interfacial properties of aqueous imidazolium-based ILs.