Understanding the influence of Coulomb and dispersion interactions on the wetting behavior of ionic liquids

Abstract

We study the role of dispersion and electrostatic interactions in the wetting behavior of ionic liquids on non-ionic solid substrates. We consider a simple model of an ionic liquid consisting of spherical ions that interact via Lennard-Jones and Coulomb potentials. Bulk and interfacial properties are computed for five fluids distinguished by the strength of the electrostatic interaction relative to the dispersion interaction. We employ Monte Carlo simulations and an interface-potential-based approach to calculate the liquid-vapor and substrate-fluid interfacial properties. Surface tensions for each fluid are evaluated over a range of temperatures that spans from a reduced temperature of approximately 0.6 to the critical point. Contact angles are calculated at select temperatures over a range of substrate-fluid interaction strengths that spans from the near-drying regime to the wetting regime. We observe that an increase in the relative strength of Coulombic interactions between ions leads to increasing deviation from Guggenheim's corresponding states theory. We show how this deviation is related to lower values of liquid-vapor excess entropies observed for strongly ionic fluids. Our results show that the qualitative nature of wetting behavior is significantly influenced by the competition between dispersion and electrostatic interactions. We discuss the influence of electrostatic interactions on the nature of wetting and drying transitions and corresponding states like behavior observed for contact angles. For all of the fluids studied, we observe a relatively narrow range of substrate-fluid interaction strengths wherein the contact angle is nearly independent of temperature. The influence of the ionic nature of the fluid on the temperature dependence of contact angle is also discussed.

Monte Carlo Simulation Strategies to Compute the Interfacial Properties of a Model Octane–Water–Silica System

“…Our approach involves calculation and subsequent analysis of two interface potentials. − This free energy-based technique provides the macroscopic contact angle. Our group has utilized these methods for several years, and examples of their use can be found in a number of our publications. ,− …”

Effect of Carboxylic Acid on the Wetting Properties of a Model Water–Octane–Silica System

“…We employ an interface potential-based approach − to study the wetting properties of the water–octane–acetic acid mixture at a silica surface. Detailed information related to this approach is available in earlier reports from our group. ,,− We work with two variants of the interface potential. The water spreading interface potential, W w,os ( l w ) provides the surface excess free energy associated with the growth of a water-rich film of thickness, l w from the silica surface in a mother octane-rich liquid.…”

Spontaneous transition of a water droplet from the Wenzel state to the Cassie state: a molecular dynamics simulation study

“…The other model is based on a virtual solid surface with a certain lattice structure. 28,29 To construct surfaces with different hydrophobic properties, the interaction potential energy parameters between the surface and water vary artificially. The latter model is more convenient and efficient for building surfaces that range from hydrophilic to hydrophobic.…”