Structure and Dynamics of Polyelectrolyte Complex Coacervates Studied by Scattering of Neutrons, X-rays, and Light

Abstract

We investigate the microscopic structure and density fluctuations of complex coacervates of flexible polyelectrolytes using scattering of neutrons, X-rays, and light. Poly­(acrylic acid) and poly­(N,N-dimethylaminoethyl methacrylate) offer a well-defined model system that allows for selective labeling and systematic variation of the strength of the attractive electrostatic interactions. Two neutron scattering experiments have been carried out: (i) we use deuterated polymeric tracers in a complex coacervate with an overall neutron scattering length density that is matched to that of the solvent, to probe the conformation of single polymer chains in the complex coacervates, and (ii) we measure complex coacervates in which all polymer chains of one type are deuterated, to probe their overall structure. The single chain static structure factors reveal that both polycations and polyanions have an ideal Gaussian chain conformation in the complex coacervates. At the same time, the overall structure is similar to that of a semidilute polymer solution, with polycations and polyanions strongly overlapping to form a network with a mesh size that is much smaller than the radius of gyration of the polymers. The mesh size decreases with decreasing salt concentration, following a scaling that is in good agreement with predictions from the corresponding salt–polymer phase diagram. These findings are confirmed by complementary X-ray scattering experiments. Finally, in all scattering experiments with light, X-rays, and neutrons, and for all polymer chain lengths and salt concentrations, we find a remarkable low-q excess scattering, following a power law with a slope close to −2. This points to the presence of equilibrium, large-scale density fluctuations in the complex coacervates. Dynamic light scattering experiments reveal two complementary diffusive modes in the complex coacervates, corresponding to fluctuations of the polymer mesh and diffusion of domains of varying density, respectively.

Recent progress in the science of complex coacervation

“…55,56 The structure and dynamics of bulk coacervates have been characterized using rheology 37,47,53,[57][58][59][60][61][62][63][64][65] along with X-ray and neutron scattering. 62,66,67 Finally, the thermodynamics of bulk coacervation has been characterized using isothermal titration calorimetry (ITC). 43,50,51,[68][69][70][71][72] Despite the wealth of reports describing and/or using coacervation over the decades, only recently has the coacervate phase diagram been mapped in a systematic way.…”

“…This model predicts the existence of a peak in the polyanion/polyanion scattering function, 119 which was verified by simulation and can in principle be tested by scattering experiments (Figure 5d, bottom). 62,66 Finally, this theory also offers a rough prediction for where the assumption of low linear-charge density is no longer applicable, 119 which is where the electrostatic blob is roughly the same size as an individual monomer, corresponding to the high charge-density limit.…”

50th Anniversary Perspective: A Perspective on Polyelectrolyte Solutions

“…Starting from the initial experimental observation 1 eight decades ago, the phenomenon of coacervate complexation has a long history. 1,4,6,8,10,12,16,20,29,41,45−49,52−61 There are many reviews on this subject providing compendia of various experimental results, theoretical approaches, and simulation studies. 61,132 The earliest theory of coacervate complexation is the Voorn–Overbeek (VO) theory, 66,67 which is found to be in good agreement with some experiments, despite its well-known approximations.…”

The Effect of Salt on the Complex Coacervation of Vinyl Polyelectrolytes

“…While the effects of salt on complex coacervation have been well studied in model systems, the majority of these studies have focused only on the impact of NaCl as a simple monovalent salt [1,2,[4][5][6][8][9][10]33,34,36,[46][47][48][49][50][51]. Those studies which have examined the impact of multivalent salts on polyelectrolyte complexation have either investigated systems which form solid complexes rather than liquid coacervates [39], or have utilized biologically-derived polymers and/or proteins [22,52,53].…”

“…This broadening of the compositional range for coacervation is thought to be a result of charge screening effects, where salt is able to interact with the polyelectrolyte and provide local compensation for the charge imbalance [56,57]. Similarly, the increase in overall signal could be the result of enhanced complexation (i.e., more droplets in solution), or swelling due to increased water content (i.e., larger droplets) [4,47].…”