Linear Charge Density Effects

The LCD, which is related to the charge number and charge fraction on the polyelectrolyte backbone, is an important factor controlled by the solution chemistry and polyelectrolyte concentration.

The influence of the LCD on polyelectrolyte adsorption has been investigated. In Fig. 19 the mean square radius of gyration and mean square end-to-end distance are plotted against the LCD. The analysis shows two different regimes corresponding to nonadsorbed (A) and adsorbed (B) polyelectrolytes, respectively. In regime A, one observes high <Rg> values (>3 x 103 [A2]), which are identical to those obtained for isolated chains under similar conditions. In regime B, these values are decreasing rapidly (~2 x 103 [A2]) immediately after polyelectrolyte adsorption. It is worth noting that for small ionic strengths (00.01 M) the polymer is always adsorbed on the surface of the colloid. When Ct is in the range 0.1-0.3 M, the curves show an increase in the <R2g> value which is the same as for the free chains (regime A); then a sharp transition is observed to the lower values of <^2> (regime B) corresponding to adsorbed conforma-

Linear Charge Density

FIG. 19 Mean square radius of gyration <R> (a) and expansion factor r = <Lle>/<R2g> (b) vs. the linear charge density (LCD) at different ionic concentrations for the polyelec-trolyte in the presence of the oppositely charged particle. Nonadsorbed (A) and adsorbed (B) regimes are delimited by dashed lines.

Linear Charge Density

FIG. 19 Mean square radius of gyration <R> (a) and expansion factor r = <Lle>/<R2g> (b) vs. the linear charge density (LCD) at different ionic concentrations for the polyelec-trolyte in the presence of the oppositely charged particle. Nonadsorbed (A) and adsorbed (B) regimes are delimited by dashed lines.

tions. Finally, when Ct = 1.0 M the curve exhibits the same increase in the <R2> value as for the free chains. Since there is no adsorption at this ionic strength, no transition in the <R2g> value is observed. Similar trends are also observed for the <Lee> values.

A qualitative picture of the polyelectrolyte-particle complexes has also been achieved by representing equilibrated structures as a function of Ct and LCD (Table 6). The absorption-desorption limit is moved from the higher to the lower ionic strengths with decreasing LCD. Hence, when the linear charge density is small, adsorption is promoted by increasing the attractive interactions between monomers and particle via the ionic strength. For example, when C, = 1.0 M the polymer is never adsorbed on the colloid whereas when C, = 0.1 M adsorption occurs only when LCD > 0.3.

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