Titration calorimetry of aqueous alumina suspensions part II. Discussion of enthalpy changes with pH and ionic strength

Between pH 9.5 and 5.75, proton adsorption enthalpies for alumina decrease; most of the decrease occurs in the pH 7.5-5.75 range. Enthalpies are also lower at 0.1 than at 0.001 M ionic strength (NaCl) in this pH range. Below pH 5.75, however, enthalpy values do not vary with pH and ionic strength given the precision of the data. Possible explanations for these effects are discussed including inherent heterogeneous site distributions, steric interactions, counterion association with the charged surface and electrostatic contributions. The enthalpy decrease with decreasing pH is most likely due to the presence of 2 or 3 distinct proton binding sites on the alumina surface. At high pH the predominant H⁺ binding sites may be singly coordinated surface hydroxyl (SOH) groups bound to Al atoms in octahedral coordination with a smaller contribution from doubly coordinated SOH groups bound to one Al atom in tetrahedral and one in octahedral coordination. Below pH 5.75, the predominant H⁺ binding site may be a singly coordinated SOH group bound to an Al atom in tetrahedral coordination. Extensive disruption of lateral H-bond formation between adjacent, oppositely charged SOH groups by bound counterions (Na⁺, Cl^-) at 0.1 M ionic strength in the pH range 5.75 to 9.5 may be responsible for the lower enthalpies found in this pH region. The more constant enthalpy values below pH 5.75 may reflect the prevention of extensive lateral H-bond formation due to significant counterion binding at both ionic strengths.