Depleted layer effect surface exclusion

Fig. 3. Preferential protein hydration in the presence of precipitating agents used in crystallization experiments. When high concentrations of salts are used as precipitants, a precipitant-poor layer forms near the protein (P) surface due to a higher affinity of the protein for water than for the precipitant. Other precipitants (e.g., polyethylene glycol polymers) induce formation of a similar precipitant-depleted region near the protein by solvent exclusion effects. In either case formation of the precipitant-depleted layer is energetically unfavorable. Consequently, the overall effect of precipitants is to promote molecular associations that decrease the total protein surface area exposed to solvent. After Timasheff and Arakawa (1988).

The mechanism of this apparent slip effect is thought to be molecular surface exclusion, resulting in a layer depleted in polymer at the pore wall (Auvray, 1981 Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Chauveteau et al, 1984). This phenomenon is caused by the entropic exclusion of polymer molecules from the wall of the porous medium and is of particular significance when the dimensions of the macromolecule approach those of typical pore sizes. This effect has been observed in both non-adsorbing (Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Lecourtier and Chauveteau, 1984) and adsorbing (Chauveteau et al, 1984) porous media. 

The surface exclusion effect is illustrated schematically in Figure 6.6, in which the concentration profile (segment density) of the polymer across a narrow capillary, C(r), is shown. The quantity 6, is the effective thickness of the depleted layer and is of the order of the molecular size, /. The reason for... 

With some concentrated suspensions of solid particles, particularly those in which the liquid has a relatively low viscosity, the suspension appears to slip at the pipe wall or at the solid surfaces of a viscometer. Slip occurs because the suspension is depleted of particles in the vicinity of the solid surface. In the case of concentrated suspensions, the main reason is probably that of physical exclusion if the suspension at the solid surface were to have the same spatial distribution of particles as that in the bulk, some particles would have to overlap the wall. As a result of the lower concentration of particles in the immediate vicinity of the wall, the effective viscosity of the suspension near the wall may be significantly lower than that of the bulk and consequently this wall layer may have an extremely high shear rate. If this happens, the bulk material appears to slip on this lubricating layer of low viscosity material. 

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