Estimating adsorption enthalpies and affinity sequences of monovalent electrolyte ions on oxide surfaces in aqueous solution

A new expression is developed for estimating the adsorption enthalpy of aqueous, monovalent ions on charged surfaces of solid oxides up to about 70 degreesC. For sorption of the M-th cation and L-th anion represented as: >SO- + M+ = >SO- - -MC+ and >SOH2+ + L- = >SOH2+ - -L- the enthalpy at 25 degreesC is given by: DeltaH(i,k)(0) = Delta Omega T-i[1/epsilon (2)(k)(partial derivative epsilon (k)/partial derivativeT) - 1/epsilon (2)(w)(partial derivative epsilon (w)/partial derivativeT)] + DeltaG(i,k)(0), where i = M+ or L-, >SO- and >SOH2+ are charged surface sites, Delta Omega (i) is the interfacial Born solvation coefficient of the i-th monovalent ion, epsilon (k) and epsilon (w) are the dielectric constants of the k-th solid and of bulk water, respectively, T is the absolute temperature, and DeltaG(i,k)(0) is the free energy of ion adsorption. The small values predicted for enthalpies suggest weak temperature dependence for electrolyte affinities. The reaction enthalpy is negative for all oxides considered, and is the major contribution to the free energy of adsorption. Reactions are less exothermic for solids with smaller dielectric constants. Ion-specific trends are also noted, with exothermicity of enthalpy decreasing as Li+ > Na+ > K+ > Rb+ = NH4+ > Cs+ > TMA(+) (tetramethylammonium) for all oxides except quartz and amorphous SiO2 where the reverse trend is predicted. Similarly, exothermicity decreases as F- > Cl- > Br- > I- for all oxides excluding quartz and amorphous SiO2. The entropic contribution to free energy is small, and is negative for all the oxides considered including quartz, but is positive for amorphous SiO2, suggesting an intriguing difference between the surfaces of quartz and amorphous SiO2. In order to determine the temperature dependence of surface-complexation, DeltaH(M+,k)(0) and DeltaH(L-,k)(0) are combined with the enthalpies for deprotonation and protonation of the neutral surface site (respectively, yielding DeltaH(M+,k)(0*),and DeltaH(L-,k)(0+) which correspond to the reactions: >SOH + M+ = >SO- - -M+ + H+ and >SOH + H+ + L- = >SOH2+ - -L- Positive values of DeltaH(M+,k)(0*) (endothermic reaction) are obtained for all oxides considered (except pyrolusite and quartz) implying that M+ complexation should increase with temperature. Amorphous silica differs from quartz in that reactions are slightly endothermic to thermoneutral. Negative values of DeltaH(L-,k)(0*) (exothermic reaction) are obtained for all oxides considered, suggesting that L- complexation decreases with temperature. DeltaH(M+,k)(0*) and DeltaH(L-,k)(0*) vary only slightly with ion-identity because their values are dominated by -DeltaH(H+,2)(0) and DeltaH(H,1)(0+). Also, DeltaH(M+,k)(0*) and DeltaH(L-,k)(0*) do not vary systematically with epsilon (k) because -DeltaH(H+,2)(0) and DeltaH(H+,1)(0) depend not only on epsilon (k) but also on the Pauling bond strength per metal-oxygen bond length (s/r) of the metal constituting the solid oxide. Copyright (C) 2000 Elsevier Science Ltd.