(257j) Influence of Surface Ions on Adsorption of Water on Hydrophilic Surface: Molecular Simulation Study

Hydrophilic surface with liquid interfaces are encountered in various academic and indus-
trially relevant systems. Understanding the interaction of surface moieties with the solvent
molecules is of prime importance. Using molecule simulations, we investigate the influence
of surface ions on adsorption of water molecules on hydrophilic surfaces. The adsorption
isotherms were obtained using muscovite mica surface populated with two different surface
ions, H⁺ and K⁺ ions, in grand canonical ensemble. The structure of water molecules on two
surfaces are remarkably different yet the obtained adsorption isotherm on two different surface
show a similar characteristic. The adsorption isotherm of water on both mica surface show
three prominent regimes, initial rapid adsorption at p/p₀ < 0.1, linear development regime
between p/p₀ = 0.1 to 0.7 and rapid film growth beyond p/p₀ = 0.7. The film thickness
of adsorbed water molecules obtained from the simulations are in good agreement with inter-
ferometery experimental data of Balmer et. al.(Langmuir, 2008, 24, 1566-1569) for both the
surfaces. The film thickness of adsorbed water layer in the linear regime of p/p₀ = 0.1 − 0.7
is lesser in case of H⁺ -mica surface compared to K⁺-mica surface, which is consistent with
experimental observations. The differences between the structural arrangement is contrasted
between two cases by analysing water density distribution, dipole orientation distribution, hy-
drogen bonding and isostearic heats of adsorption, to illustrate the reason behind the observed
differences in adsorption isotherms.