Effective charges of polyelectrolytes in solution
| Autor: | Li-Ying Wang, 王儷穎 |
|---|---|
| Rok vydání: | 2006 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 94 A polyelectrolyte dissolves in water, leaving ionized charged groups on the polymer backbone and counterions in the solution. Counterions are not distributed uniformly in the solution but prefer to stay in the neighborhood of the oppositely charged polymer. The extent of counterions residing near the polyelectrolyte mediates how polymers interact among themselves and thereby influence the physical properties of a polyelectrolyte solution. Counterion condensation around linear polyelectrolyte chains (sodium polyacrylate & sodium polystyrene sulfonate) is investigated in terms of the degree of ionization α, which is proportional to the effective charge. On the basis of counterion chemical potential, the effective charge of polyelectrolytes is directly determined by the ion-selective electrode and the degree of ionization is simply defined as the ratio of measured to intrinsic (complete ionization) counterion concentration, α ≣ co/ci. In the dilute regime, the degree of ionization decreases with increasing the molecular weight (Mw) and the polymer concentration (cp). This result is consistent with the prediction by Monte Carlo simulation and can be qualitatively explained by a simple model of two-phase approximation. In the semi-dilute regime, the degree of ionization becomes insensitive to cp. The counterion entropy loss seems to tie with the decrease of Coulombic attraction due to polyelectrolyte - polyelectrolyte interactions. As the molecular weight is increased at constant monomer concentration, α declines first but becomes insensitive to Mw. This consequence reveals that as the polyelectrolyte is long enough, however, the entanglement effects are indistinguishable. The degree of ionization based on ion-selective electrode agrees with that obtained from osmometry but deviates from that by conductometry. This result indicates that the primary contribution to the osmotic pressure of a polyelectrolyte solution comes from the free counterion concentration. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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