Surface Tension of Acid Aqueous Solutions: An Analytical Theory

Many acidic aqueous solutions have lower surface tension than that of pure water, mainly due to surface adsorption of hydronium ions. However, an analytical theory for the surface tension of these acid solutions is yet to be developed. In this work, we propose an analytical theory to explain this phenomenon. We map an acid solution to a restricted primitive model (RPM) and a spherical bubble, serving as the detector of surface tension, to a neutral hard sphere solute in the acid solution. The surface adhesive interaction is described by a length parameter of nonadditivity. The cavity formation energy of the spherical solute is determined analytically using integral equation theory, which, combined with the morphological thermodynamics theory, leads to a formula for the surface tension of acid solutions. The theory is applied to four 1:1 acid solutions (HCl, HBr, HNO3, and HClO4), and good agreement with experimental data is found for concentrations up to 1 mol/L. This work completes the final piece of our series of studies on the influence of ions on the surface tension of electrolyte solutions. It demonstrates that the analytical theory based on cavity formation energy and morphological thermodynamics theory can quantitatively explain most ion-specific effects on surface tension.