ESTIMATING ADSORPTION ENTHALPIES AND AFFINITY SEQUENCES OF MONOVALENT ELECTROLYTE IONS ON OXIDE SURFACES IN AQUEOUS SOLUTION

Abstract

A new expression is developed for estimating the adsorption enthalpy of aqueous, monovalent ions on charged surfaces of solid oxides up to about 70°C. For sorption of the M-th cation and L-th anion represented as: >SO- + M+ = >SO- - -M+and >SOH2+ + L- = >SOH2+ --L-the enthalpy at 25°C is given by: ΔHi,k0=ΔΩiT[1#k2(##k#T)-1#w2(##w#T)]+ΔGi,k0,where i = M+ or L-, >SO- and >SOH2+ are charged surface sites, ΔΩi is the interfacial Born solvation coefficient of the i-th monovalent ion, εk and εw are the dielectric constants of the k-th solid and of bulk water, respectively, T is the absolute temperature, and ΔGi, k0 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, ΔHM+, k0 and ΔHL-, k0 are combined with the enthalpies for deprotonation and protonation of the neutral surface site (-ΔHH+,20, ΔHH+,10), respectively, yielding ΔHM+, k0* and ΔHL-, k0* which correspond to the reactions: >SOH + M+ = >SO- - -M+ + H+and >SOH + H+ + L- = >SOH2+ - -L- Positive values of ΔHM+, k0* (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 ΔHL-, k0* (exothermic reaction) are obtained for all oxides considered, suggesting that L- complexation decreases with temperature. ΔHM+, k0* and ΔHL-, k0* vary only slightly with ion-identity because their values are dominated by -ΔHH+, 20 and ΔHH,10+.