DRIFT characterization of polyacrylates and amino acids on bioceramic surfaces

Interactions between fluorapatite, hydroxyapatite, bioactive glasses and several organic species were studied. Each substrate was exposed to dilute aqueous solutions of amino acids or polyacrylates for 24 hours, at several initial pH values. Solution elemental analysis, pH measurement, and DRIFT spectral characterization were employed to understand the resulting interactions. Solute species suppressed pH increases resulting in increased apatite dissolution. This effect enhanced alkali leaching, gel-layer formation, and suppressed network dissolution in the glasses. Bonding in the apatite-acrylate system at pH > 8.3 was explained by electrostatic interactions between the ionized polyelectrolytes and the positively charged surface. Below pH = 8.3, chemisorbed water was displaced by carbonyl groups, suggesting interaction between carboxyl groups and basic sites. In the higher pH hydroxyapatite-amino acid samples, surface recrystallization of the amino acids was observed, with carbonyl groups fully ionized, while all other apatite samples showed residual carboxyl groups and no amino acid recrystallization. At high pH, evidence suggested water-mediated bonding between calcium-involved surface sites and the amino acids via the carboxylate group. In the glass-acrylate system, at low starting pH, carbonyl groups were ionized, with little evidence of -CO2-M + species. Above pH ∼ 9, -CO2-M + complexes formed, suggesting that the negatively charged gel layer at low pH suppressed glass-acrylate bonding. Adsorption of glycine and L-alanine occurred in a zwitterionic form, with conversion to anionic forms increasing slightly at high final pH. L-alanine crystallized at the surface, while glycine did not. Regardless of substrate or amino acid, no direct evidence of the formation of -CO2-M+ bonds was observed.