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 aqu eous, monovalent ions on charged surfaces of solid oxides up to about 70 de greesC. For sorption of the M-th cation and L-th anion represented as: >SO- + M+ = >SO- - -MCand >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 epsilo n (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 io n, epsilon (k) and epsilon (w) are the dielectric constants of the k-th sol id and of bulk water, respectively, T is the absolute temperature, and Delt aG(i,k)(0) is the free energy of ion adsorption. The small values predicted for enthalpies suggest weak temperature dependence for electrolyte affinit ies. The reaction enthalpy is negative for all oxides considered, and is th e major contribution to the free energy of adsorption. Reactions are less e xothermic 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 q uartz and amorphous SiO2 where the reverse trend is predicted. Similarly, e xothermicity decreases as F- > Cl- > Br- > I- for all oxides excluding quar tz and amorphous SiO2. The entropic contribution to free energy is small, a nd is negative for all the oxides considered including quartz, but is posit ive for amorphous SiO2, suggesting an intriguing difference between the sur faces of quartz and amorphous SiO2. In order to determine the temperature dependence of surface-complexation, D eltaH(M+,k)(0) and DeltaH(L-,k)(0) are combined with the enthalpies for dep rotonation and protonation of the neutral surface site (respectively, yield ing DeltaH(M+,k)(0*),and DeltaH(L-,k)(0+) which correspond to the reactions : >SOH + M+ = >SO- - -M+ + Hand >SOH + H+ + L- = >SOH2+ - -L- Positive values of DeltaH(M+,k)(0*) (endother mic reaction) are obtained for all oxides considered (except pyrolusite and quartz) implying that M+ complexation should increase with temperature. Am orphous silica differs from quartz in that reactions are slightly endotherm ic to thermoneutral. Negative values of DeltaH(L-,k)(0*) (exothermic reacti on) are obtained for all oxides considered, suggesting that L- complexation decreases with temperature. DeltaH(M+,k)(0*) and DeltaH(L-,k)(0*) vary onl y 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*) d o not vary systematically with epsilon (k) because -DeltaH(H+,2)(0) and Del taH(H+,1)(0) depend not only on epsilon (k) but also on the Pauling bond st rength per metal-oxygen bond length (s/r) of the metal constituting the sol id oxide. Copyright (C) 2000 Elsevier Science Ltd.