Structure and Phase Stability of Polyelectrolyte-Macroion Solutions
Abstract: Polyelectrolytes are polymers bearing ionisable groups, which, in polar solvents, can dissociate into charged polymer chains (polyelectrolytes) and small counterions. In aqueous solutions, polyelectrolytes interact strongly with other macroions and in particular they tend to associate with objects of opposite charge and form complexes. Nearly all industrial and biological process involves solutions of charged macromolecules, i.e., paint, detergents, drug delivery, and cosmetics, but also many diseases are associated with malfunctions at the colloidal level. The complexation between one linear flexible polyelectrolyte and one or several oppositely charged macroions was examined by employing a simple model system with focus on the electrostatic interactions. The composition and the structure of the complex as well as conformational data of the polyelectrolyte were obtained by using Monte Carlo simulations. The binding isotherms obtained are Langmuir-like, and in excess of macroions the polyelectrolyte-macroion complex displays a charge reversal. These properties were investigated at different linear charge densities, different lengths, and flexibilities of the polyelectrolyte, and different macroion charges, all at different numbers of macroions at constant volume. The effect of adding simple 1:1 salt has also been investigated. The complexation, phase separation, and redissolution of concentrated polyelectrolyte-macroion solutions have also been examined. As oppositely charged polyelectrolytes were added, the stable macroion solution with repelling macroions became successively less stable. The strong electrostatic attraction brought macroions and polyelectrolytes closely together and slightly before macromolecular charge equivalence, distinct and repelling complexes were established. At macromolecular charge equivalence, the system became unstable and a large and loose cluster of macroions and polyelectrolytes was formed. Finally, in excess of polyelectrolytes, the large cluster was broken up and the macroions were dispersed again – a redissolution had occurred. The effect of the macroion radius, the chain length, and the chain flexibility on the phase separation has also been investigated. A semiflexible chain displayed a smaller tendency to promote phase instability as compared to flexible and stiff chains, the origin most likely arising from the similar chain persistence length and macroion radius.
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