Theoretical prediction of single-site enthalpies of surface protonation for oxides and silicates in water

Surface protonation is the most fundamental adsorption process of geochemic al interest. Yet remarkably little is known about protonation of mineral su rfaces at temperatures greater than 25 degrees C. Experimentally derived st andard enthalpies of surface protonation, Delta H(r,1)degrees, Delta H(r,2) degrees, and Delta H(r,ZPC)degrees, correspond to the reactions > SOH + H+ = > SOH2> SO- + H+ = > SOH > SO- + 2H(+) = > SOH2respectively, and provide a starting point for evaluating the role of surfa ce protonation in geochemical processes at elevated temperatures. However, the experimental data for oxides do not have a theoretical explanation, and data are completely lacking for silicates other than SiO2. In the present study, the combination of crystal chemical and Born solvation theory provid es a theoretical basis for explaining the variation of the enthalpies of pr otonation of oxides. Experimental values of Delta H(r,1)degrees,, Delta H(r ,2)degrees, and Delta H(r,ZPC)degrees consistent with the triple layer mode l can be expressed in terms of the inverse of the dielectric constant (1/ep silon) and the Pauling bond strength per angstrom (s/r(M-OH)) of each miner al by equations such as Delta H(r,ZPC)degrees = Delta Omega(r,Z)[(1/epsilon) - (T/epsilon)(2)(parti al derivative epsilon/partial derivative T)]-B-Z'(s/r(M-OH)) + H-Z'. The Born solvation coefficient Delta Omega(r,Z) was taken from a prior anal ysis of surface equilibrium constants. The coefficients B-Z' and H-Z' were derived by regression of experimental enthalpies for rutile, gamma-alumina, magnetite, hematite, and silica. This approach permits widespread predicti on of the enthalpies of surface protonation. Predicted standard enthalpies of surface protonation for oxides and silicates extend over the ranges (in kcal.mole(-1)): Delta H(r,1)degrees, approximate to -3 to -15; Delta H(r,2) degrees approximate to -0.5 to -18; Delta H(r,ZPC)degrees approximate to -4 to -33. Minerals with the largest values of s/r(M-OH) (e.g., quartz and ka olinite) are predicted to have weakly negative enthalpies and a weak temper ature dependence for their protonation equilibrium constants. Conversely, m inerals with the smallest values of s/r(M-OH) (e.g., garnets and olivines) should have strong negative enthalpies and a strong temperature dependence for their protonation equilibrium constants. Copyright (C) 1998 Elsevier Sc ience Ltd.