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Significance
Studies involving polyelectrolytes have attracted significant attention amongst researchers. This can be attributed to their wide range of applications in various fields and processes. Unfortunately, the organization of charged macromolecules has not been fully explored due to the varying polyelectrolyte chain sizes in different solutions. The sizes depend on the strength of the particular solution. Generally, the strength of the electrostatic interactions results in their widespread over long distances. To this note, various research works have investigated the contribution of electrostatics to the segment length of polyelectrolytes. For example, in low ionic strength solutions, the large polyions sizes have been assumed to be due to the impact resulting from the intermolecular long-range interactions.
Recently, the total persistence length of polyelectrolytes chains has been described as the sum total of the electrostatic and bare components. However, the main features of a polyelectrolyte chain comprise of the hydrophobic or hydrophilic property of the bare chain as well as the charge density. Therefore, charged copolymers based on bare hydrophilic chains are very crucial in the evaluation of pure electrostatic interactions and their effects on the chain sizes. This is because they remain soluble in water for the entire range of the charge densities. For bare hydrophobic chains, the competition between the hydrophobic interactions and electrostatics determines the behavior of the low charged macromolecules.
Dr. Georges Pavlon, Olga Dommes, Dr. Olga Okatova, Irina Gavrilova and Professor Evgenii Panarin at Institute of Macromolecular Compounds, Russian Academy of Sciences investigated the hydrodynamic sizes and volumes of macromolecules of copolymers based on hydrophilic poly(N-methyl-N-vinylacetamide) in water solution. They used polymer chains of similar contour lengths and different low or moderate charge densities. They purposed to determine the dependence and trends of the average charge density on the viscosity at low ionic strength. Consequently, they compared the polyions sizes in salt-free solutions with the degree of charge in the entire range of the charge density. Eventually, they established a new parameter to describe the correlation between the charge average density of the bare linear chain and the statistical hydrodynamic length and volume of the segment. Their research work is published in the journal, Physical Chemistry Chemical Physics.
From the proposed parameter that is represented as lqq/Abare, a ratio of the average distance between the neighboring charges along the chain to the statistical segment length of a non-charged homologue, the authors observed that it indicated regions dominated by electrostatics short-range or long-range interactions. For instance, the three ranges observed were lqq/Abare>2 indicating a moderate increase in viscosity, lqq/Abare<2 indicating a sharp increase in viscosity and lqq/Abare <0.7 indicating saturation level with invariable intrinsic viscosity.
According to the authors, the first range represented an increase in the hydrodynamic volume due to the increase in the electrostatic long-range contribution to the chain dimensions. Consequently, the third range represented the adverse effects of the contributions of the short-range interactions. However, the intermediate range takes into consideration both the contributions of the short and long-range interactions with first increase in the charge density.
Reference
Pavlov, G., Dommes, O., Okatova, O., Gavrilova, I., & Panarin, E. (2018). Spectrum of hydrodynamic volumes and sizes of macromolecules of linear polyelectrolytes versus their charge density in salt-free aqueous solutions. Physical Chemistry Chemical Physics, 20(15), 9975-9983.
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