Salt concentration, polyelectrolyte adsorption

According to the polyelectrolyte character of ODNs, the effect of salt concentration on adsorption should be considered. Indeed, the ionic strength affects (i) the electrostatic interactions between the ODN and the adsorbent and (ii) the lateral repulsive electrostatic interactions between adsorbed ODNs. 

Rojas OJ, Claesson PM, Muller D, Neuman RD. The effect of salt concentration on adsorption of low charge density polyelectrolytes and interactions between polyelectrolyte coated surfaces. J Colloid Interface Sci 1998 205 77— 88. 

Features of polyelectrolyte adsorption are that both the adsorbance and the thickness can be easily varied by changing the concentration of added salt as well as pH in bulk solution since such changes cause variation of the electrostatic repulsions of polyelectrolyte chains adsorbed, i.e., the excluded volume effect. 

For homopolyelectrolyte, we first studied the ellipsometric measurement of the adsorption of sodium poly(acrylate) onto a platinum plate as a function of added sodium bromide concentration (5). We measured the effect of electrolyte on the thickness of the adsorbed layer and the adsorbances of the polyelectrolyte. It was assumed that the Donnan equilibrium existed between the adsorbed layer and the bulk phase. The thickness was larger and the adsorbance of the polyelectrolyte was lower for the lower salt concentration. However, the data on the molecular weight dependence of both the adsorbance and the thickness of the adsorbed polyelectrolyte have been lacking compared with the studies of adsorption of nonionic polymers onto metal surfaces (6-9). 

The adsorption isothems obtained first rise with the polyelectrolyte concentration and level off to a plateau, as shown in Fig. 24. The adsorbance at the polymer concentration of 0.1 g/dl, which is well in the plateau region, decreases as the salt concentration is lowered, and it varies linearly with the square root of the salt concentration, as shown in Fig. 25. This linear relationship agrees with the theoretical prediction by Hesselink23. ... 

For polyelectrolytes, the salt concentration has a strong influence on the molecular conformation and the adsorption. The charges on the polymer repel each other electrostatically and in this way tend to elongate the chain. This elongation is reversible if a salt is added, the charges on the polymer chain are screened and the conformation becomes more compact (Fig. 10.9). Addition of salt has a similar effect as improving the quality of the solvent (see Section 6.7). As a result they tend to adsorb in a more bulky conformation. 

Figure 5.32. Schematic overview of polyelectrolyte adsorption behaviour. The adsorbed amount is plotted as a function of the salt concentration c. for various combinations of the segment charge the surface charge density a°. and the chemical peu-ameters x rid The curves are numbered to facilitate the discussion (see text).

The decrease in the amount of polyelectrolyte adsorbed on the silica surface whilst increasing the salt concentration is in contrast to results obtained for adsorption of poly(diallyldimethylammonium chloride) on different silica samples [75, 76]. Probably the chloride ions shield the segment-segment interactions between charged groups inside the coil of an individual polyelectrolyte chain and the silica surface [77-80]. This explanation is also consistent with the assumption that electrostatic forces determine the adsorption mechanism of PVFA-co-PVAm chains on silica. 

Apart from polymer adsorption for uncharged macromolecules, charged macromolecules (polyelectrolytes), such as proteins can also adsorb at surfaces [20, 21]. Adsorption of a charged macromolecule is different from adsorption of an uncharged polymer in that there is a high dependency on the salt concentration. At a low salt concentration, repulsive electrostatic forces between charged polymer chains will inhibit formation of loops and tails (Fig. 4). This has been predicted and confirmed, for instance for adsorption of humic acids on iron-oxide particles [22]. 

For similar reasons, ampholytic polyelectrolytes show maximum adsorption around their isoelectric point. This maximum is less pronounced at higher salt concentrations. 

The mean polyelectrolyte layer thickness was found to be 1.5 nm for the case of PSS/PAH assembled from 0.5M NaCl. It should be noted that the average layer thickness of polyelectrolyte multilayers strongly depends of kind of polyelectrolytes and salt concentration used at polyelectrolyte assembling. More rigid polymers and increasing of salt concentration lead to thicker adsorption layer. 

Fig. 1 Adsorption of a polyelectrolyte chain onto spherical colloidal particles for various salt concentrations C (from [35]). The ratio ajb between the colloid radius a and monomer bond length b increases from top to bottom. The colloid surface charge density is constant, hence, the colloidal charge increases with a. The adsorption threshold depends on the salt concentration and the size ratio. More details of the underlying Monte Carlo simulations are provided in Ref. [35]...

Experimentally, a large number of similar studies on the influence of chain stiffness, colloid charge density, and salt concentration have been performed [43, 44, 121-138]. Some theoretical trends regarding the effect of surface curvature and salt concentration on critical adsorption and polyelectrolyte-colloid complexation are supported by experimental observations. These studies, however, also revealed a number of discrepancies and additional physical parameters to be taken into account, as compared with the outcomes of theoretical studies and computer simulations. 

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