RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Values of the point of zero charge, pK_a1^int, pK_a2^int, p∗K_cation^int, and the capacity of the innermost layer C₁ for magnetite suspended in KNO₃ solutions have been obtained in the temperature range 30–80°C from acid-base titration data. The dependency of the pzc on temperature indicates that ΔH*, the enthalpy of transfer of protons from bulk to surface minus the enthalpy of transfer of hydroxide ions from bulk to surface, is negative, and pzc and (½)pK_W become increasingly separated as temperature increases. These results are discussed in terms of the corresponding ΔH* and ΔS* values. The values of ΔS° and ΔH° of the ionization equilibria of the interface are also reported, and shown to be similar to equivalent magnitudes for aqueous Fe(II). C₁ increases with temperature, and a tentative rationale for this is given. The site binding model is less adequate to fit data at higher temperatures, especially at low ionic strengths. The calculated profiles for the various potentials as a function of pH are given; ψ₀/pH profiles deviate from the prediction of Nernst equation more markedly at higher temperatures. Ion pairs contribute less to the charging of the surface at higher temperatures, because of decreasing values of log K_anion^int and log K_cation^int.