Diffusion of Cationic Polyelectrolytes into Cellulosic Fibers

Abstract

The penetration of cationic polyelectrolytes into anionic cellulosic fibers was evaluated with fluorescent imaging techniques in order to clarify the mechanism and time scales for the diffusion process. The bulk charge of the cellulosic fibers indirectly creates a driving force for diffusion into the porous fiber wall, which is entropic in nature due to a release of counterions as the polyelectrolyte adsorbs. The individual bulk charges in the fiber cell wall also interact with the diffusing polyelectrolyte, such that the polyelectrolyte diffuses to the first available charge and consequently adsorbs and remains fixed. Thus, subsequent polyelectrolyte chains must first diffuse through the adsorbed polyelectrolyte layer before adsorbing to the next available bulk charges. This behavior differs from earlier suggested diffusion mechanisms, by which polyelectrolytes were assumed to first adsorb to the outermost surface and then reptate into the pore structure. The time scales for polyelectrolyte diffusion were highly dependent on the flexibility of the chain, which was estimated from calculations of the persistence length. The persistence length ultimately depended on the charge density and electrolyte concentration. The charge density of the polyelectrolyte had a greater influence on the time scales for diffusion. High charge density polyelectrolytes were observed to diffuse on a time scale of months, whereas the diffusion of low charge density polyelectrolytes was measured on the order of hours. An influence of the chain length, that is, steric interactions due the persistence length of the polyelectrolyte and to the tortuosity of the porous structure of the fiber wall, could only be noted for low charge density polyelectrolytes. Increasing the electrolyte concentration increased the chain flexibility by screening the electrostatic contribution to the persistence length, in turn inducing a faster diffusion process. However, a significant change in the diffusion behavior was observed at high electrolyte concentrations, at which the interaction between the polyelectrolyte charges and the fiber charges was almost completely screened.

Prospects for Maintaining Strength of Paper and Paperboard Products While Using Less Forest Resources: A Review

“…Such results often can be attributed to more efficient retention on fibers during the papermaking process (Roberts et al 1986(Roberts et al , 1987. Horvath et al (2008) showed that cationic polyelectrolytes having a sufficiently high density of change tend to remain immobilized near to their first points of contact with a negatively charged material. Such behavior can, in principle, have a favorable effect of keeping a cationic additive at the fiber surface and thereby able to contribute to bonding.…”

Synthesis of Nonspherical Microcapsules through Controlled Polyelectrolyte Coating of Hydrogel Templates

“…35,36 The extent of diffusion of PLL into the matrix depends on a combination of steric and electrostatic effects. 37 We observed the contribution of steric effects by altering PLL molecular weight and PEGDA concentration. At pH 7/0.5 M NaCl, PLL 15−30 kDa diffuses into the entire hydrogel, while PLL 150−300 kDa (used in the majority of this work) forms a shell ( Figure S1), similar to what is observed by Bysell and coworkers in a PLL/poly(acrylic acid) microgel system.…”

“…First, there is a reduction in the hydrogel mesh size due to decreased swelling of the PEGDA-AAc particle ( Figure S2) by the Donnan effect. 37,41 Second, the charge screening at higher ionic strength enables denser packing of PLL molecules in the outer region of the particle, which sterically prevents further inward diffusion of other PLL molecules. A similar trend was observed by Kaufman et al during the fabrication of polyelectrolyte microcapsules across water/oil droplet interfaces.…”

Advanced-Microscopy Techniques for the Characterization of Cellulose Structure and Cellulose-Cellulase Interactions

“…Despite these difficulties, Horvath et al have recently reported a comprehensive study of the diffusion of cationic polymers into anionic cellulosic fibers using confocal laser scanning microscopy. (154) Through the use of polymer chains with different lengths and different charge densities they were able to show that the charge density of the polyelectrolyte had a larger effect on the diffusion times than did the molecule length. They also observed that the use of high ionic strength solutions could completely screen the interaction between the polyelectrolytes and the cellulosic fibers, leading to faster penetration into the porous structure of the substrates.…”