Thermodynamic properties of surface species at the mineral-water interfaceunder hydrothermal conditions: A Gibbs energy minimization single-site 2pK(A) triple-layer model of rutile in NaCl electrolyte to 250 degrees C

Standard partial molal thermodynamic properties of surface species in a sin gle-site 2pK(A) triple-layer model (TLM) for rutile in NaCl electrolyte, co nsistent with the SUPCRT92-98 data, were estimated at temperatures T = 0 to 250 degrees C and saturated vapor pressure P-SAT using a Gibbs energy mini mization (GEM) approach (Kulik, 1999) and hydrothermal potentiometric titra tion data (Machesky et al., 1998a). Evaluation strategy was based on conven tional setting to zero of standard state entropy and heat capacity effects of the "surface hydronium desorption" reaction >O0.5H2+ = >O0.5H0 + H-aq(+) , with one-term temperature extrapolation appropriate. The standard state p roperties of the "surface water dissociation" reaction >O0.5H0 = >O-0.5(-) + H-aq(+) in a three-term extrapolation, with Cp-298(o)(>O-0.5(-)) assumed equal to Cp-298(o)(OHaq-) and Delta S-298(o) estimated from enthalpy of rut ile surface protonation in water Delta H-prot,298(o) yielded a very good ma tch of predicted pristine point of zero charge (pH(PPZC)) and measured poin t of zero charge (pH(PZC)) on rutile to T = 250 degrees C. Correlation of D elta H-prot,298(o) for various oxides versus pH(PPZC)(T) suggests that the entropy Delta S-prot,298(o) = 25 +/- 4 J K-1 mol(-1) may be constant for al l oxide surfaces, pH(PPZC)(T) be hence a function of pH(PPZC) at T-0 = 25 d egrees C only: pH(PPZC,T) = -29.134 + T-0/T(pH(PPZC,T0) + 3.2385) + 4.545ln T. The standard partial molal properties of Na+ and Cl- outer-sphere surfa ce complexes on rutile were estimated from GEM TLM fits of potentiometric t itration data at different T and m(NaCl). Adjustable parameters were: stand ard partial molal Gibbs energy g(0)(>O0.5-Na+), interdependent with inner c apacitance density C-1 (increasing from 1.6 F . m(-2) at 25 degrees C to 2. 3 F . m(-2) at 250 degrees C), and maximum site density Gamma(max)(>O0.5H2Cl-). No surface activity term (SAT) In Xi = f(Gamma(max)) corrections were needed for >O0.5-Na+ species, i.e., Na+ adsorption on rutile occurs in the "TLM ideal region" up to 300 degrees C and 1 m NaCl. From the GEM fits, pa rameters of three-term temperature extrapolations were found for a reaction >O0.5H0 + Na-aq(+) = >O0.5Na+ + H-aq(+). Directly fitted g(0)(>O0.5H2+Cl-) values were less certain because the SAT with low Gamma(max)(>O0.5H2+Cl-) = 0.9 +/- 0.1 sites nm(-2) had to be involved. Hence, symmetric electrolyte adsorption (pH(PZSE) = pH(PPZC)) has been assumed, from which the standard partial molal properties of >O0.5H2+Cl- species were found using a zero-te rm extrapolation of an isocoulombic reaction >O0.5H2+Cl- + >O-0.5(-) + Na-a q(+) = >O0.5-Na+ + >O0.5H2+ + Cl-aq(-). This balanced like-charge reaction can serve as a very useful constraint re ducing the set of adjustable TLM parameters at given T and pH(PZC) to C-1, pK(Na) and Gamma(max)(>O0.5H2+Cl-), optimal values of which could have been found from GEM fits of the rutile potentiometric titration data at differe nt T and m(NaCl). Copyright (C) 2000 Elsevier Science Ltd.