Influence of the Size of the Polymer Colloid

1. Equilibrated Conformations, Adsorption-Desorption Limit

Equilibrated conformations of particle-chain complexes (with N = 100, <3m = 3.57 A, f = 1, lB = 7.14 A) as a function of ionic concentration and size ratio are presented in Table 4. It can be seen that the adsorption-desorption limit is dependent on both particle size and ionic concentration. Adsorption is promoted by increasing particle size because of the concomitant decrease in chain entropy loss during adsorption on flat surface and increase of the particle charge to keep the surface charge density constant (+100 mC.m-2). As the the particle size is

FIG. 9 Number of collapsed monomers N"is as a function of the total number of monomers in the chain N, for different values of the ionic concentration. Due to an increase of the particle capacitance N"is increases with C..

increased, the charge of Q is also changed as an effect of the constant surface charge density imposed. No adsorption is observed when C > 1 M (the attractive surface-polymer interactions in this domain are not strong enough to overcome polyelectrolyte confinement near the particle), whereas adsorption is always observed when C < 0.01 M. To calculate the limit between the adsorption and desorption domain and critical particle size op for each particle size, the ionic concentration C was progressively and carefully decreased and monitored to satisfy the adsorption criteria (in contact at least with one monomer for more than 50% of the simulation time). A plot of the critical particle radius op (the critical value below which no adsorption occurs) as a function of k is presented in Fig. 10. No adsorption or desorption is observed in the region situated at the right of the curve.

TABLE 4 MC Equilibrated Conformations of Polyelectrolyte-Particle Complexes as a function of the Ionic Concentration C and Particle/Monomer Size Ratio C„/oma

aThe polymer size is constant and equal to N = 100. The central point charge Q of the particle is adjusted so as to keep a constant surface charge density. Depending on the size of the particle, three regions may be defined. Small particles do not permit adsorption of all polyelectrolyte monomers owing to the confinement energy of the chain. As a result, extended tails are formed in solution. The adsorbed polyelectrolyte conformation is quite similar to the isolated polyelectrolyte conformation. When the particle radius is large enough the polyelectrolyte collapses on it to form "tennis ball" conformations. By further increasing particle size, polyelectrolytes can spread to the same extent as on a flat surface. Finally, by increasing the ionic concentration, the charged polymer becomes more or less bonded to the particle surface. When screening is important, no adsorption is observed.

FIG. 10 Plot of the critical particle radius op versus K, the inverse of the Debye length (N = 100). It is shown that by increasing K, adsorption is achieved by increasing the size of the particle.

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