Adsorption Kinetics of Lysozyme on Silica at pH 7.4:  Correlation between Streaming Potential and Adsorbed Amount

The adsorption of lysozyme in phosphate buffer (10 - 2 M + NaCl 10 - 2 M) at pH 7.4 on silica was studied at different flow rates. We measured simultaneously as a function of time (i) the streaming potential between the extremities of the capillary (internal diameter, 0.530 mm; length, 22 cm) and (ii) the amount adsorbed at x 0 = 14 cm from the entrance by means of 125 I-radiolabeled proteins. We observed two regimes in the kinetics of adsorption, the first one being a transport-controlled process. They are separated at a critical interfacial concentration Γ cr depending on the initial ζ 0 potential of the interface, from 0.035 μg/cm 2 at low density of charges (|ζ 0 | < 25 mV) to a mean value of 0.055 μg/cm 2 for higher density of charges (26 mV < |ζ 0 | < 45 mV). A direct interpretation of those results concerns the electrostatic interactions: more molecules, which are slightly positively charged at pH 7.4, are adsorbed on the more negative surfaces. The ζ potential at the transition between both regimes (or final value) was dependent on the initial value ζ 0: no charge reversal occurred when ζ 0 = −25 mV, while the interface became slightly positive when ζ 0 = −45 mV. While both adsorbed amount and streaming potential increased in the first regime, in the second regime streaming potential showed very small variations, if any, and interfacial concentration continued to increase. Hence, we focused mainly on the correlation between both quantities in the first regime. The study of the initial adsorption kinetics as a function of flow rate was in accordance with the presence of multimers (diffusion coefficient D = 4.0 × 10 - 7 cm 2 s - 1; adsorption constant k a ≈ 4.0 × 10 - 3 cm s - 1 ) in the shear zone near the wall. With these parameters, we performed numerical simulations to estimate the ratio between average and local interfacial concentrations, 〈Γ〉/Γ( x 0 ). The variations of Δζ/〈Γ〉 versus time were then deduced. The data are also presented as Δζ/ζ 0 as a function of 〈Γ〉 and analyzed according to the recent model of M. Zembala and Z. Adamczyk ( Langmuir 2000, 16, 1593) for uniformly charged spheres. At low density of charges (ζ 0 ≈ −25 mV) we observed Δζ ∝ 〈Γ〉 almost over the whole first regime, while a slight departure from linearity occurred above 0.3Γ cr for higher density of charges. A semiquantitative analysis suggests that the molecules are oriented at the interface with their more positive parts facing the solid surface, the conformation of the molecules being different at small and high charge densities of silica.