The wetting behavior of aqueous imidazolium based ionic liquids: a molecular dynamics study

Abstract

Aqueous ionic liquids are of particular interest due to their tunability of physical and chemical properties and a deeper understanding of their structure–property relationship is desired. Molecular dynamics (MD) simulations were conducted to study the wetting behavior of aqueous imidazolium-based ionic liquids (ILs), consisting of a 1-ethyl-3-methylimidazolium [EMIM]⁺ cation and either a hydrophilic boron tetrafluoride [BF₄]⁻ or a hydrophobic bis(trifluoromethylsulfonyl)imide [NTF₂]⁻ anion mixed in water. To understand the effect of anion and concentration of ILs at the graphite solid–liquid interface, wettability studies were performed with IL concentrations from 0–50 wt%. The contact angle of aqueous IL droplets decreases with increasing IL concentration. Droplet characteristics near the surface were investigated by profiling the density perpendicular (z-direction) and horizontal (r-direction) to the graphite sheet; this was further quantified by an orientation order parameter. Due to the preferred adsorption of ILs, water depletes near the surface as IL concentration increases. The hydrophobic [NTF₂]⁻ anion forces the IL toward the interface from the bulk, whereas the hydrophilic [BF₄]⁻ anion causes the IL to remain in the bulk of the droplet. Differences in water–anion hydrogen bonding, the nature of the anions, and their interfacial tensions are crucial factors in the wetting behavior of aqueous ionic liquids.