Protein Transport upon Advection at the Air/Water Interface: When Charge Matters
Résumé
The formation of dense protein interfacial layers at afree air−water interface is known to result from both diffusion andadvection. Furthermore, protein interactions in concentrated phasesare strongly dependent on their overall positive or negative net charge,which is controlled by the solution pH. As a consequence, aninteresting question is whether the presence of an advectionflow ofwater toward the interface during protein adsorption producesdifferent kinetics and interfacial structure of the adsorbed layer,depending on the net charge of the involved proteins and, possibly, onthe sign of this charge. Here we test a combination of the followingparameters using ovalbumin and lysozyme as model proteins: positiveor negative net charge and the presence or absence of advectionflow. The formation and the organization of the interfacial layers arestudied by neutron reflectivity and null-ellipsometry measurements. We show that the combined effect of a positive charge oflysozyme and ovalbumin and the presence of advectionflow does induce the formation of interfacial multilayers. Conversely,negatively charged ovalbumin forms monolayers, whether advectionflow is present or not. We show that an advection/diffusionmodel cannot correctly describe the adsorption kinetics of multilayers, even in the hypothesis of a concentration-dependent diffusioncoefficient as in colloidalfiltration, for instance. Still, it is clear that advection is a necessary condition for making multilayers througha mechanism that remains to be determined, which paves the way for future research.
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