Wetting Films of Two Ionic Liquids: [C8mim][BF4] and [C2OHmim][BF4]

The stability of wetting films of two ionic liquids, 1-octyl-3-methylimidazolium tetrafluoroborate, [C(8)mim][BF4], and 1-ethanol-3-methylimidazolium tetrafluoroborate, [C(2)OHmim][BF4], on alumina was assessed through the measurement of disjoining pressure isotherms, which represent the dependence of the disjoining pressure on the film thickness. Two experimental techniques were used at low and high disjoining pressure, respectively: captive bubble and modified thin film pressure balance. Theoretical predictions of the disjoining pressure isotherms were made using the microscopic approach of London and Hamaker based on the van der Waals contribution to the disjoining pressure. Good agreement was found between the experimental and the theoretical isotherms for [C(8)mim][BF4], whereas in the case of [C(2)OHmim][BF4], the experimental data are slightly shifted toward larger thickness, especially at higher disjoining pressures. An interpretation of these results is given in terms of a corresponding-states principle argument applied to the surface tension and the use of auxiliary molecular dynamics data. The conclusion is that although both Coulomb and dispersion interactions contribute to determine the bulk properties of these ionic liquids the surface properties are mainly related to the dispersive forces.