Molecules 2023 , 28 , 7984
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4.2. Machinism of Interface Interaction 4.2.1. The Conformation of Polyelectrolytes
The conformation of linear polyelectrolytes in solution changes continuously. Numer- ous studies have shown that polyelectrolytes mainly exist in the configuration of circular adsorption or flat [103–106]. For high-level potential energy surface‚ polyelectrolytes have a small molecular weight, high charge density, and strong electrostatic interactions. So poly- electrolytes are prone to exist on solid surfaces in a flat configuration. Figure 3 displays the adsorbed polyelectrolytes, which was divided into three segments by Böhmer et al. [107]: train, loop, and tails. The length of each segment depends on its charge density, molecular weight, negative charge on the solid surface, etc. For example, the stronger the electrostatic effect, the smaller the molecule‚ thus resulting in more trains‚ and fewer loops and tails. Theoretically, it is predicted that when polyelectrolyte chains driven by strong electrostatic forces are absorbed on the solid surface, they are composed of 80% trains‚ 20% loops‚ and rare tails.
ȱ Figure3. Conformation of adsorbed polyelectrolytes on solid surfaces. 4.2.2. Adsorption between Polyelectrolytes and Fibers
Generally speaking‚ the adsorption process between cationic polyelectrolytes and fibers can be divided into three stages [108]: adsorption, reconstruction, and diffusion, as shown in Figure 4. For cationic polyelectrolytes with different structures, the stages will vary.
Figure4. Schematic diagrams of adsorption, reconstruction, and diffusion of polyelectrolytes.
For polyelectrolytes with low molecular weight and high charge density‚ they rapidly adsorb on the fiber surface in a flat configuration by ion-exchange mechanism. And it almost immediately diffuses into the fiber without any molecular reconstruction process. Cationic polyelectrolytes with medium molecular weight display a flat configuration during initial adsorption‚ resulting in low positive charge density on the surface of pulps. Subsequently, only minor reconstruction on the surface of pulps happens. Therefore, the charge decay at the second stage is also smaller than that of polyelectrolytes with high molecular weight. After adsorption of cationic polyelectrolytes with highly branched structures and high molecular weight on pulp fibers‚ the surface charge of fibers rarely decays. This is because it is difficult for the reconstruction and diffusion process of polyelectrolytes on the surface of pulp fibers to occur.
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