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°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: ∆H i,k0=∆Ω iT[ 1/∊ k2( ∂∊ k/∂T)- 1/∊ w2( ∂∊ w/∂T)]+∆G i,k0, where i = M+ or L-, > SO- and > SOH2+ are charged surface sites, ∆Ω i is the interfacial Born solvation coefficient of the i-th monovalent ion, ∊ k and ∊ w are the dielectric constants of the k-th solid and of bulk water, respectively, T is the absolute temperature, and ∆ Gi, 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 + = NH 4+ > Cs + > TMA + (tetramethylammonium) for all oxides except quartz and amorphous SiO 2 where the reverse trend is predicted. Similarly, exothermicity decreases as F - > Cl - > Br - > I - for all oxides excluding quartz and amorphous SiO 2. The entropic contribution to free energy is small, and is negative for all the oxides considered including quartz, but is positive for amorphous SiO 2, suggesting an intriguing difference between the surfaces of quartz and amorphous SiO 2. In order to determine the temperature dependence of surface-complexation, ∆ HM+, k0 and ∆ HL-, k0 are combined with the enthalpies for deprotonation and protonation of the neutral surface site (-∆ HH+,20, ∆H H+,10), respectively, yielding ∆ HM+, k0∗ and ∆ HL-, k0∗ which correspond to the reactions: > SOH + M+ = > SO- - - M+ + H+and > SOH + H+ + L- = > SOH2+ - - L- Positive values of ∆ HM+, 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 ∆ HL-, k0∗ (exothermic reaction) are obtained for all oxides considered, suggesting that L- complexation decreases with temperature. ∆ HM+, k0∗ and ∆ HL-, k0∗ vary only slightly with ion-identity because their values are dominated by -∆ HH+, 20 and ∆ HH,10+. Also, ∆ HM+, k0∗ and ∆ HL-, k0∗ do not vary systematically with ∊ k because -∆ HH+, 20 and ∆ HH+, 10 depend not only on ∊ k but also on the Pauling bond strength per metal-oxygen bond length ( s/ r) of the metal constituting the solid oxide.