Conformation of the Adsorbed Polyelectrolytes

The conformation of adsorbed polymers are usually described in terms of trains, loops, and tails. In Fig. 20a-c we present the average number of monomers in trains, loops, and tails as a function of Ct and LCD for the colloid-polyelectro-lyte complex.

TABLE 6 Monte Carlo Equilibrated Conformations of Isolated Charged Polymers as a Function of Ionic Concentration C and Polyelectrolyte Linear Charge Density (LCD)a

TABLE 6 Monte Carlo Equilibrated Conformations of Isolated Charged Polymers as a Function of Ionic Concentration C and Polyelectrolyte Linear Charge Density (LCD)a

"Bright monomers represent charged monomers. The LCD is clearly controlling the adsorption/ desorption limit and polymer conformation at the particle surface. When screening is important, no adsorption is observed.

(a) Trains. In the absence of trains, according to our definition of polymer adsorption, the polyelectrolyte is never adsorbed. This is observed when Ci = 1.0 M. In the other situations, after a critical value of the LCD has been reached, one can observe a monotonic growth of monomers in trains with the increasing LCD. At a given LCD, with decreasing ionic strength the number of trains is growing, which means that polyelectrolyte chains adopt flat conformations at the interface.

(b) Loops. According to Fig. 20b, globally the number of monomers in loops decreases with increasing LCD and with decreasing Ci. Here one can also observe a sharp transition between the nonadsorbed and adsorbed regimes. A maximal value is reached just after adsorption; then a smooth decrease with increasing LCD is observed. The monomers in loops are then transferred to trains.

Loops

Loops

Trains

(c) Tails. When Ct = 0 and 0.01 M, the number of monomer in tails mono-tonically decreases with the increase in LCD. In the other cases, a maximal value is obtained at the adsorption-desorption limit.

As the polymer is only weakly adsorbed at the small values of LCD and low ionic strength, an important balance is observed in favor of loops and tails instead of configurations where the monomers are in trains. As the three parameters are correlated, the proportional changes can be easily observed; with increasing LCD the amount of monomers in trains increases whereas the amount of monomers in tails and loops decreases. At higher values of LCD the number of monomers in trains reaches a maximum and finally decreases slowly, further increasing the values of LCD.

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