Different Models

Figure 9 illustrates the two models ofrearrangements ofthe dipoles roughly normal or parallel to the surface to obtain an attractive component between adsorbed proteins. We proposed the model of the ferroelectric-antiferroelectric arrangement normal to the surface to explain the change in variation of the Z potential (Vasina and Dejardin 2004; Fig. 9, left). Saving almost the same orientation of the proteins with respect to the surface (Robeson and Tilton 1996), cooperative clustering could appear also by dipolar interactions in the direction parallel to the surface (Fig. 9, right). The complete analysis would, however, require a careful description of the balance between attractive and repulsive electrostatic components, probably in connection with the local concentration of small ions. Such ferroelectric to antiferroelectric order transition was already considered for

Fig. 9. Schematic representation of possible ordered domains at the interface, with a top view of the charge repartitions. a Alternate orientation of the dipoles normal to the surface for intermolecular attraction, but with a repulsive component in front of the surface. b Alternate orientation of the dipoles parallel to the surface for intermolecular attraction, but with a repulsive intermolecular component between the likely oriented dipoles normal to the surface. Ellipsoid or cylinder representation is arbitrary

Fig. 9. Schematic representation of possible ordered domains at the interface, with a top view of the charge repartitions. a Alternate orientation of the dipoles normal to the surface for intermolecular attraction, but with a repulsive component in front of the surface. b Alternate orientation of the dipoles parallel to the surface for intermolecular attraction, but with a repulsive intermolecular component between the likely oriented dipoles normal to the surface. Ellipsoid or cylinder representation is arbitrary small dipolar molecules like carbon monoxide and halogenated methane (Burns and Dennison 1998; Burns et al. 2004).

Recently, it was shown by TIRF and streaming-current measurements on the lysozyme/silica system at high wall shear rates that the TIRF overshoot did not correspond to any particular event in the interfacial potential (Daly et al. 2003). Therefore, it appears that the concomitance of the changes in kinetic regime and interfacial potential is not always the rule. One important phenomenon behind the process is surface diffusion, which has to be taken into account. Different kinds of behavior are probably linked to the relative rate of arrival of molecules compared to their surface diffusion rate (see Chap. 2).

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