Adsorption monitoring of phospho-l-serine on hydroxyapatite

The adsorption of the aminoacid phospho-l-serine (PLS) on the surface of hydroxyapatite {Ca₅(PO₄)₃OH, HAP}, was investigated using streaming potential measurements. Solutions saturated with respect to HAP, containing different concentrations of PLS, were brought in contact under carefully controlled flow conditions through plugs made of well dispersed HAP powder. The measurement of PLS adsorption during the equilibration of the solute with the HAP substrate, showed a plateau regime corresponding, according to geometrical considerations, to monolayer surface coverage. The PLS uptake measurements on HAP suggested that it is possible to monitor in situ adsorption during the monolayer surface coverage measuring the streaming potential of HAP. Analysis of the surface potential measurements suggested that during the monolayer surface coverage step, the negatively charged (HL²⁻) PLS species were adsorbed. The adsorbed HL²⁻ was located at the inner Helmholtz plane of the electrical double layer, forming surface complexes with the positively charged ≡CaOH ₂ ⁺ sites on the surface of HAP. The rate constants of adsorption and desorption were calculated from the kinetics of adsorption of PLS on HAP.