Equilibrium thickness of foam films and adsorption of ions at surfaces: Water and aqueous solutions of sodium chloride, hydrochloric acid, and sodium hydroxide.

HYPOTHESIS

The origin of negative surface charge at water/air interface is still not clear. The most probable origin is specific adsorption of OH ions. From diffuse layer potential, we can evaluate the surface density of ions in the Stern layer which can be a measure for the specific adsorption of ions and determines whether the surface charge is solely due to the specific adsorption of OH ions.

EXPERIMENTS

Equilibrium thickness of foam films of pure water and aqueous solutions of NaCl, HCl, and NaOH was measured as a function of disjoining pressure for water and as a function of concentration for the aqueous solutions at 298.15 K. Quartz-glass cells thoroughly cleaned and immersed in pure water before use were used for the measurement.

FINDINGS

Application of a modified Poisson-Boltzmann equation to the equilibrium film thickness gave the diffuse layer potential and the surface density of ions in the Stern layer. From the concentration dependence of the surface density, it was concluded that not only OH ions but also Cl ions and HCO and/or CO ions adsorb specifically at the water/air interface.