Acid/Base Properties and Phenylphosphonic Acid Complexation at the Aged γ-Al2O3/Water Interface

Abstract

Acid/base properties and phenylphosphonic acid (H2L) complexation at the water-suspended-and-aged γ-Al2O3/water interface have been studied in 0.1 M Na(Cl) medium at 25.0°C in the range 5.0 < -log[H+] < 9.5. Equilibrium measurements were performed as a series of potentiometric titrations supplemented with spectrophotometric phosphorous analyses. In the evaluation of the experimental data, the contribution from electrostatic forces was accounted for by using the constant-capacitance model. The adsorption of phenylphosphonic acid onto the studied phase was found to be strong at -log[H+] < 7. Desorption was observed when -log[H+] was increased toward 9.5. The specific capacitance was evaluated from data collected in the absence of phenylphosphonic acid. The model describing the acid/base reactions and phenylphosphonic acid binding onto the studied alumina surface, (≡AlOH), is given by the following equilibria, intrinsic constants, specific capacitance, and site density: ≡AlOH + H+ = ≡AlOH log β110(int) = 7.51 ± 0.09≡AlOH = ≡AlO- + H+ log β-110(int) = -8.87 ± 0.11≡AlOH + H2L = ≡AlLH + H2O log β011(int) = 9.58 ± 0.18≡AlOH + H2L = ≡AlL- + H+ + H2O log β-111(int) = 2.94 ± 0.08≡AlOH + H2L = ≡AlLOH2- + 2H+ log β-211(int) = -5.30 ± 0.13C= 1,40 F m-2Site density = 1.03 sites/nm2In a series of modeling calculations, the complexation features of phenylphosphonic acid are compared to those of orthophosphoric acid.