Tunable Coacervation of Well-Defined Homologous Polyanions and Polycations by Local Polarity

Abstract

The ionic complexation of polyelectrolytes is an important mechanism underlying many important biological processes and technical applications. The main driving force for complexation is electrostatic, which is known to be affected by the local polarity near charge centers, but the impact of which on the complexation of polyelectrolytes remains poorly explored. We developed a homologous series of well-defined polyelectrolytes with identical backbone structures, controlled molecular weights, and tunable local polarity to modulate the solvation environment near charged groups. A multitude of systematic, accurate phase diagrams were obtained by spectroscopic measurements of polymer concentrations via fluorescent labeling of polycations. These phase diagrams unambiguously revealed that the liquidlike coacervation is more stable against salt addition at reduced local polarity over a wide range of molecular weights. These trends were quantitatively captured by a theory of complexation that incorporates the effects of dispersion interactions, charge connectivity, and reversible ion-binding, providing the microscopic design rules for tuning molecular parameters and local polarity.

Recent progress in the science of complex coacervation

“…25 This is ca. 2-3 orders of magnitude lower than typical concentrations for coacervates, which are often at 0.5-3 M. 27,34,35,[40][41][42][43]45 2. Polymer charges are treated as unconnected electrolyte particles in Voorn-Overbeek.…”

“…27 Indeed, the phenomenology of coacervate phase behavior is largely captured by Voorn-Overbeek, in that phase separation is observed at low salt concentrations and leads to miscibility at high salt concentrations. 27,34,35,40 A number of observables are commonly used as an indicator of phase behavior, though the primary method is to use turbidity measurements that capture the light scattering of small coacervate droplets. 32,[41][42][43][44][45][46] Turbid mixtures correspond to phase separation.…”

“…32,[41][42][43][44][45][46] Turbid mixtures correspond to phase separation. However, other methods such as UV-Vis, 34,47,48 or fluorescence spectroscopy, 27,40,49 ionic conductivity, 34,50,51 thermogravimetric analysis (TGA), 35,51 NMR spectroscopy, 50 radiolabelling, 52 and quartz crystal microbalance (QCM) 53 have been used to measure concentrations of polymers and/or salt. Surface properties such as the interfacial tension between the surfactant and coacervate phase have been measured using atomic force microscopy 38,54 and surface forces apparatus.…”

“…A number of the assumptions built into the theory do not apply in the regime where coacervation occurs; namely, at the high charge densities and concentrations typical of these materials (i.e., synthetic systems typically have a concentration of charged species of the order of 1 M). 27,34,35,[40][41][42][43]45 1. Debye-Hückel theory is a limiting law for low salt concentrations.…”

“…[90][91][92][93][94][95][96] All of these limitations have been appreciated since the original development of Voorn-Overbeek theory, and most theoretical efforts since have focused on moving beyond this simplified picture. 33 Still, the fact that Voorn-Overbeek gets many aspects of the phenomenology qualitatively correct has led to its continued use; 27,28,39,40,97 there are only a few key experimental observations in simple homopolyelectrolyte coacervates that highlight the need to move beyond this simple picture. 35,40,47,98…”