Reaction enthalpies for Bronsted acid-base reactions on pyrogenic silica were determined by titrating an aqueous suspension of solid particles with standard acids and bases in an isoperibol calorimeter. Formation of negative surface charge by proton desorption from a hydroxyl functional group is endothermic. In the limit of an uncharged surface, the reaction enthalpies range from 0-10 kJ/mol of charge. This range for silica is smaller than that measured on other oxide minerals (15-30 kJ/mol) by other researchers. The reaction enthalpies on silica become increasingly endothermic by approximately 1-2 kJ/mol of charge for each muC/cm2 change of charge densities as the pH (and total charge density) increases from the point of zero charge (p.z.c.). These enthalpy changes are interpreted with a modified form of the Bragg-Williams model of interactions among adsorbed species, yielding mixing parameters and activity coefficients for the surface complexes. (C) 1994 Academic Press. Inc.