A Coarse-Grained Molecular Dynamics Study of Strongly Charged Polyelectrolyte Coacervates: Interfacial, Structural, and Dynamical Properties

HY Liang and JJ de Pablo, MACROMOLECULES, 55, 4146-4158 (2022).

DOI: 10.1021/acs.macromol.2c00246

Polyelectrolyte coacervates are molecular complexes consisting of oppositely charged polyelectrolytes. Their properties, such as interfacial tension, microscopic structure, and rheology, depend on the level of salt doping. In this work, we develop a coarse -grained model to study the interfacial, structural, and dynamical properties of coacervates under different salt concentrations. Using molecular dynamics simulations, we prepare coacervates from strong polyelectrolytes in equilibrium with a supernatant phase. The interfacial tension of the coacervate???supernatant interface follows a scaling law near the critical salt concentration (csalt,cr) of the form ?? ??? (1 ??? csalt/csalt,cr)3/2 but exhibits deviations at lower salt concen-trations. The structure and dynamics of these coacervates are similar to those of neutral polymer solutions with the same polymer concentration, since electrostatic interactions are strongly screened and become essentially short-ranged due to the high concentration of charged species. In coacervates, polyelectrolyte chains adopt ideal conformations, and the correlation length decreases with increasing polymer concentration in a manner that is reminiscent of concentrated polymer solutions in a poor solvent. The relaxation of polyelectrolyte chains, as well as the stress relaxation modulus of coacervates, can be described by the Rouse model. The relaxation time decreases with increasing salt concentration and decreasing polymer concentration. To distinguish the effect of salt and polymer on relaxation, we compare the dynamics of coacervates to those of neutral concentrated polymer solutions at the same polymer density. At low salt concentrations, when the Debye screening length is larger than the correlation length, the coacervate relaxation time depends only on polymer concentration, in a similar way to that observed in a neutral system. At high salt concentrations, the screening of electrostatic interactions accelerates coacervate dynamics. We also demonstrate that by applying appropriate time and modulus shifts, the stress relaxation modulus of coacervates with different salt and polymer concentrations can be superimposed to form a master curve, in agreement with the so-called ???salt???time superposition??? principle observed in experiments.

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