On the temperature dependence of intrinsic surface protonation equilibrium constants: An extension of the revised MUSIC model

The revised multisite complexation (MUSIC) model of T. Hiemstra et al. (J. Colloid Interface Sci. 184, 680 (1996)) is the most thoroughly developed approach to date that explicitly considers the protonation behavior of the various types of hydroxyl groups known to exist on mineral surfaces. We have extended their revised MUSIC model to temperatures other than 25°C to help rationalize the adsorption data we have been collecting for various metal oxides, including rutile and magnetite to 300°C. Temperature-corrected MUSIC model A constants were calculated using a consistent set of solution protonation reactions with equilibrium constants that are reasonably well known as a function of temperature. A critical component of this approach was to incorporate an empirical correction factor that accounts for the observed decrease in cation hydration number with increasing temperature. This extension of the revised MUSIC model matches our experimentally determined pH of zero net proton charge pH values (pH_znpc) for rutile to within 0.05 pH units between 25 and 250°C and for magnetite within 0.2 pH units between 50 and 290°C. Moreover, combining the MUSIC-model-derived surface protonation constants with the basic Stern description of electrical double-layer structure results in a good fit to our experimental rutile surface protonation data for all conditions investigated (25 to 250°C, and 0.03 to 1.0 m NaCl or tetramethylammonium chloride media). Consequently, this approach should be useful in other instances where it is necessary to describe and/or predict the adsorption behavior of metal oxide surfaces over a wide temperature range.