The binding of monovalent electrolyte ions on alpha-alumina. I. Electroacoustic studies at high electrolyte concentrations

An electroacoustic technique has been used to monitor the binding of variou s monovalent inorganic ions on alpha-alumina at different pHs, nondilute so lids, and moderate to high (0.01-1.0 mol dm(-3)) electrolyte concentrations . The zeta, potential versus pH data show that NaNO3, KNO3, CsNO3, KBr, KCl , and KI are indifferent electrolytes for the alpha-alumina surface. Howeve r, LiNO3 causes a significant change in the isoelectric point of alpha-alum ina, indicating that Li+ adsorbs in a specific manner to the surface over t he range of concentrations investigated. At high (1.0 mol dm(-3)) electroly te concentrations, the monovalent cations bind to the negative alpha-alumin a surface in the order Li+ > Na+ > K+ approximate to Cs+. By contrast, the Br-, Cl-, I-, and NO3- anions adsorb to an almost identical extent over the entire range of concentrations and pH conditions investigated. The cation binding sequence is consistent with the water "structure making-structure b reaking" model first proposed by Gierst et al.(1) and Berube and de Bruyn,( 2) which is based on the hydration enthalpies of the ions and the heat of i mmersion of the colloidal substrate. The comparable anion adsorption behavi or is believed to arise because of the similar and/or low hydration enthalp ies of the anions, which lead to a similar anion-surface interaction in eac h case.