Brush osmotic

Whereas the above studies have attempted to identify the Na /H exchanger in renal brush border membranes (a resistant-type), at least one study has reported possible identification of a sensitive-type transport protein [49]. The Na /H exchanger in lymphocytes (a sensitive-type) can be activated by either 12-0-tetradeca-noylphorbol 13-acetate (TPA) or osmotic shrinkage. TPA or osmotic shrinkage... 

The non-aqueous system of spherical micelles of poly(styrene)(PS)-poly-(isoprene)(PI) in decane has been investigated by Farago et al. and Kanaya et al. [298,299]. The data were interpreted in terms of corona brush fluctuations that are described by a differential equation formulated by de Gennes for the breathing mode of tethered polymer chains on a surface [300]. A fair description of S(Q,t) with a minimum number of parameters could be achieved. Kanaya et al. [299] extended the investigation to a concentrated (30%, PI volume fraction) PS-PI micelle system and found a significant slowing down of the relaxation. The latter is explained by a reduction of osmotic compressibihty in the corona due to chain overlap. 

Fig. 19 Dependence of the brush thickness reduced by the number of polymer repeat units for monovalent co-ions, H/AT, on the concentration of the external salt, ( s> for strong (solid line) and weak (dashed line) polyelectrolyte brushes in neutral brush (NB), salted brush (SB), and osmotic brush (OB) regimes, a and ao denote the bulk and internal (for weak polyelectrolyte brushes only) degree of dissociation, respectively (Reproduced with permission from [89])...

In the nonlinear osmotic brush regime we combine the high-stretching (nonlinear) version of the chain elasticity in Eq. 22 with the nonhnear en-tropic effects of the counterions due to the finite volume of the polymer chains, i.e. we choose a finite effective diameter creff in Eq. 21. The final result for the equihbrium brush height is... 

If salt is present in the solution, counterions as well as co-ions do penetrate into the brush, which leads to additional screening of the Coulomb repulsion inside the brush. The amount of this screening, and the stretching of the polyelectrolyte chains, are now also controlled by the bulk salt concentration. Since the additional salt screening weakens the swelling of the brush caused by the counterion osmotic pressure, salt leads to a brush con-... 

The threshold salt concentration above which the brush contraction sets in is given by the salt concentration which equals the counterion concentration inside the brush. This means that the higher the grafting density (and consequently the higher the internal counterion concentration in the osmotic brush regime), the larger the salt concentration necessary to see any salt effects at all. 

From the measured core radii the grafting distance b can be calculated. Its variation as a function of added salt concentration is shown in Fig. 6 a. In the osmotic brush regime at low added salt concentrations the grafting distances are practically constant. At concentrations above c s 0.05 mol/1 ( salted brush ) the grafting distances decrease with increasing salt due to screening of the repulsive interactions between the corona chains [49]. 

Fig. 6 Experimentally determined grafting distance b (a) and shell thickness D (b) as a function of NaCl-concentration for PEE-PSSH. Different behavior at low salt ( osmotic brush ) and high salt concentration ( salted brush ) can well be distinguished [49]...

Alexander s treatment, the grafted brush is assumed to have a uniform segment density (step function distribution), and each chain to consist of connected semidilute blobs. The osmotic repulsion between blobs tends to stretch the chains, while the elastic free energy of the chains has the opposite effect. For a single brush, the minimization of the overall free energy with respect to the brush thickness yields the equilibrium brush thickness L0, given by... 

Figure 3. Particle-polymer brushed surface interaction profile, (a) particle-surface contact (b) dispersion interaction (c) electrical double layer interaction osmotic polymer-brush particle interaction.

The presence of the brush suppresses the adsorption of these model-proteins. At cT1 = 12 nm2 adsorption of HSA is more strongly reduced than for the smaller LSZ and PLG. Furthermore, at higher grafting densities, LSZ adsorption is relatively poorly suppressed. Apparently, the positively charged LSZ molecules are pulled into the brush by the negative electric field. Thus, there seems to be a trade-off between repulsive steric and osmotic interactions on the one hand and an attractive electrostatic interaction on the other. 

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