Equilibrium thickness of foam films and adsorption of ions at surfaces: Aqueous solutions of sodium hydrogencarbonate and sodium carbonate.

HYPOTHESIS

The origin of the negative charge at water/air interface was proved to be not only specific adsorption of OH ions but that of HCO and/or CO ions in the previous study [1]. To determine which anionic species is primarily responsible for the surface charge, the surface density of ions in the Stern layer is numerically evaluated from foam-film thickness of aqueous solutions of NaHCO and NaCO.

EXPERIMENTS

Equilibrium thickness (equivalent thickness at equilibrium) of the foam films formed from aqueous solutions of NaHCO and NaCO was measured as a function of electrolyte concentration at 298.15 K.

FINDINGS

Applying a modified Poisson-Boltzmann (PB) equation developed for various kinds of electrolytes to the equilibrium thickness gave the surface density of ions in the Stern layer for NaHCO and NaCO systems. From the concentration dependence of the surface density together with that for NaCl and NaOH in the previous study [1], the negative surface charges for water and very dilute solutions were found to be due to specific adsorption of HCO ions. The surface charge at high electrolyte concentration is determined by the specific adsorption of electrolyte anions. The specific-adsorption ability of anion increases in the order CO≫ HCO> OH≫ Cl.