Identification of Surface Basic Sites and Acid-Base Pairs of Hydroxyapatite

The Lewis and Bronsted basic properties of a stoichiometric hydroxyapatite (HAp) were investigated by infrared spectroscopy following the adsorption and desorption processes of a Lewis acidic molecule, CO2, and a Bronsted acidic molecule, C2H2. CO2 interacts with basic OH- and O2- of PO43- groups to form hydrogenocarbonates and surface carbonates, respectively. It also generates surface type A carbonates and related water upon substitution of two neighboring structural OH- groups. Water modifies the basic properties of the HAp by decreasing the surface carbonatation and enhancing the formation of hydrogenocarbonates, and promotes the substitution ability of OH- by carbonates. Due to the affinity of HAp for carbonatation, the thermodesorption experiment of CO2 accounts for the thermal decomposition of bulk type A and B carbonates rather than for the lone surface basicity. As for the acetylene probe, three nondissociative adsorption modes of acetylene on the HAp surface are observed: a pi-complex interaction with acidic POH, an interaction with an acid-base (POH-OH) pair, and finally, a a complex interaction with basic OH- that is the most stable upon desorption. There is no evidence of the involvement of basic O2- of PO43- in the interaction with acetylene. It is thus proposed that both acidic POH and basic OH- groups may play a determinant role in acid-base properties of hydroxyapatites.