Thermodynamic properties of surface species at the mineral–water interface under hydrothermal conditions: a Gibbs energy minimization single-site 2p KA triple-layer model of rutile in NaCl electrolyte to 250°C

Abstract

Standard partial molal thermodynamic properties of surface species in a single-site 2p K A triple-layer model (TLM) for rutile in NaCl electrolyte, consistent with the SUPCRT92-98 data, were estimated at temperatures T = 0 to 250°C and saturated vapor pressure P SAT using a Gibbs energy minimization (GEM) approach (Kulik, 1999) and hydrothermal potentiometric titration data (Machesky et al., 1998a). Evaluation strategy was based on conventional setting to zero of standard state entropy and heat capacity effects of the “surface hydronium desorption” reaction >O 0.5H 2 + = >O 0.5H 0 + H aq +, with one-term temperature extrapolation appropriate. The standard state properties of the “surface water dissociation” reaction >O 0.5H 0 = >O 0.5 − + H aq + in a three-term extrapolation, with Cp ° 298(>O 0.5 −) assumed equal to Cp ° 298(OH aq −) and Δ S ° 298 estimated from enthalpy of rutile surface protonation in water Δ H ° prot,298 , yielded a very good match of predicted pristine point of zero charge (pH PPZC) and measured point of zero charge (pH PZC) on rutile to T = 250°C. Correlation of Δ H ° prot,298 for various oxides versus pH PPZC suggests that the entropy Δ S ° prot,298 = 25 ± 4 J K −1 mol −1 may be constant for all oxide surfaces, pH PPZC( T) be hence a function of pH PPZC at T 0 = 25°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 surface complexes on rutile were estimated from GEM TLM fits of potentiometric titration data at different T and m NaCl. Adjustable parameters were: standard partial molal Gibbs energy g 0(>O 0.5 −Na +), interdependent with inner capacitance density C 1 (increasing from 1.6 F · m −2 at 25°C to 2.3 F · m −2 at 250°C), and maximum site density Γ max (>O 0.5H 2 +Cl −). No surface activity term (SAT) ln Ξ = f(Γ max ) corrections were needed for >O 0.5 −Na + species, i.e., Na + adsorption on rutile occurs in the “TLM ideal region” up to 300°C and 1 m NaCl. From the GEM fits, parameters of three-term temperature extrapolations were found for a reaction >O 0.5H 0 + Na aq + = >O 0.5 −Na + + H aq +. Directly fitted g 0 (>O 0.5H 2 +Cl −) values were less certain because the SAT with low Γ max(>O 0.5H 2 +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 >O 0.5H 2 +Cl − species were found using a zero-term extrapolation of an isocoulombic reaction >O 0.5H 2 +Cl − + >O 0.5 − + Na aq + = >O 0.5 −Na + + >O 0.5H 2 + + Cl aq −. This balanced like-charge reaction can serve as a very useful constraint reducing the set of adjustable TLM parameters at given T and pH PZC to C 1, p K Na and Γ max (>O 0.5H 2 +Cl −), optimal values of which could have been found from GEM fits of the rutile potentiometric titration data at different T and m NaCl.