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1. The effect of comb architecture on complex coacervation

   https://doi.org/10.1039/c7ob01314k  

   Johnston, Brandon M.; Johnston, Cameron W.; Letteri, Rachel A.; Lytle, Tyler K.; Sing, Charles E.; Emrick, Todd; Perry, Sarah L. (January 2017, Organic & Biomolecular Chemistry)

   Complex coacervation is a widely utilized technique for effecting phase separation, though predictive understanding of molecular-level details remains underdeveloped. Here, we couple coarse-grained Monte Carlo simulations with experimental efforts using a polypeptide-based model system to investigate how a comb-like architecture affects complex coacervation and coacervate stability. Specifically, the phase separation behavior of linear polycation-linear polyanion pairs was compared to that of comb polycation-linear polyanion and comb polycation-comb polyanion pairs. The comb architecture was found to mitigate cooperative interactions between oppositely charged polymers, as no discernible phase separation was observed for comb-comb pairs and complex coacervation of linear-linear pairs yielded stable coacervates at higher salt concentration than linear-comb pairs. This behavior was attributed to differences in counterion release by linear vs. comb polymers during polyeletrolyte complexation. Additionally, the comb polycation formed coacervates with both stereoregular poly( l -glutamate) and racemic poly( d , l -glutamate), whereas the linear polycation formed coacervates only with the racemic polyanion. In contrast, solid precipitates were obtained from mixtures of stereoregular poly( l -lysine) and poly( l -glutamate). Moreover, the formation of coacervates from cationic comb polymers incorporating up to ∼90% pendant zwitterionic groups demonstrated the potential for inclusion of comonomers to modulate the hydrophilicity and/or other properties of a coacervate-forming polymer. These results provide the first detailed investigation into the role of polymer architecture on complex coacervation using a chemically and architecturally well-defined model system, and highlight the need for additional research on this topic. 

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2. Fluctuations, structure, and size inside coacervates

   https://doi.org/10.1140/epje/s10189-023-00335-1  

   Muthukumar, Murugappan (September 2023, The European Physical Journal E)

   Aqueous solutions of oppositely charged macromolecules exhibit the ubiquitous phenomenon of coacervation. This subject is of considerable current interest due to numerous biotechnological applications of coacervates and the general premise of biomolecular condensates. Towards a theoretical foundation of structural features of coacervates, we present a field-theoretic treatment of coacervates formed by uniformly charged flexible polycations and polyanions in an electrolyte solution. We delineate different regimes of polymer concentration fluctuations and structural features of coacervates based on the concentrations of polycation and polyanion, salt concentration, and experimentally observable length scales. We present closed-form formulas for correlation length of polymer concentration fluctuations, scattering structure factor, and radius of gyration of a labelled polyelectrolyte chain inside a concentrated coacervate. Using random phase approximation suitable for concentrated polymer systems, we show that the inter-monomer electrostatic interaction is screened by interpenetration of all charged polymer chains and that the screening length depends on the individual concentrations of the polycation and the polyanion, as well as the salt concentration. Our calculations show that the scattering intensity decreases monotonically with scattering wave vector at higher salt concentrations, while it exhibits a peak at intermediate scattering wave vector at lower salt concentrations. Furthermore, we predict that the dependence of the radius of gyration of a labelled chain on its degree of polymerization generally obeys the Gaussian chain statistics. However, the chain is modestly swollen, the extent of which depending on polyelectrolyte composition, salt concentration, and the electrostatic features of the polycation and polyanion such as the degree of ionization. 

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3. Salt-Dependent Phase Re-entry of Weak Polyelectrolyte Complexes: from Associative to Segregative Liquid–Liquid Phase Separation

   https://doi.org/10.1021/acs.macromol.3c01468  

   Li, Huiling; Liu, Ying; Lan, Fujie; Ghasemi, Mohsen; Larson, Ronald G (October 2023, Macromolecules)

   Hillmyer, Marc A (Ed.)

   A high-salt phase-separation re-entry is observed in mixtures of poly (diallyldimethyl ammonium chloride) (PDADMAC), a strong polycation, and poly (acrylic acid) (PAA), a partially charged polyanion, within the pH range 4.7 to 5.3. This intriguing phenomenon exclusively occurs at salt concentrations exceeding the critical salt concentration required for dissolving the coacervate formed at low salt concentrations, here named the “Upper Critical Salt Concentration” (UCSaC), and at monomer concentrations exceeding 0.1M for each polymer. The transition from associative phase separation at low salt concentration, to a single solution, and ultimately to segregative separation at high salt concentration called the Lower Critical Salt Concentration (LCSaC), arises from the interplay between electrostatic interactions and the hydrophobicity of neutral PAA monomers in a high-salt solvent. To explain this transition, we use a theory combining short-range ion pairing and counterion condensation with long-range electrostatics using the random phase approximation (RPA), and with hydrophobic interactions between PAA neutral monomers and water. The latter is modeled through a Flory-Huggins χ parameter of around 0.6. Literature observations of a continuous transition from associative to segregative phase transition with increasing salt concentration, without a homogeneous single-phase solution at intermediate salt concentration, are also predicted and discussed. 

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4. Self-Exchange of Polyelectrolyte in Multilayers: Diffusion as a Function of Salt Concentration and Temperature

   https://doi.org/10.1021/acs.macromol.1c01464  

   Abbett, Rachel L.; Chen, Yuhui; Schlenoff, Joseph B. (October 2021, Macromolecules)

   null (Ed.)

   Polymer chain diffusion within a hydrated polyelectrolyte complex, PEC, has been measured using an ultrathin film format prepared by the layer-by-layer method. Isotopically labeled self-exchange of deuterated poly(styrene sulfonate), dPSS, with undeuterated PSS of the same narrow molecular weight distribution permitted reliable estimates of whole-molecule diffusion coefficients, D. Narrow molecular weight distribution poly(diallyldimethylammonium), PDADMA, was used as the polycation for the PEC. Extensive pretreatment of starting films was undertaken to remove residual stress, anisotropy, and layering. PSS/PDADMA “multilayers,” PEMUs, thin enough to provide substantial exchange of polyelectrolyte, even with diffusion coefficients as low as 10–16 cm2 s–1, as a function of salt concentration and temperature were measured for this PEC, which has a glass-transition temperature, Tg, close to room temperature. Two molecular weights of dPSS, about 15 and 100 kDa, presumed to be below and above the entanglement molecular weight, respectively, both diffused faster at higher temperatures with respective activation energies, Ea, of about 21 and 53 kJ mol–1, the latter about the same as Ea for the place exchange between two pairs of PSS:PDADMA. Studies of the linear viscoelastic response of macroscopic PECs showed a difference of about 8 °C in the Tg of the two lengths of PSS complexed with the same PDADMA. Increasing concentrations of NaCl influenced D of 100 kDa PSS but not 15 kDa PSS at room temperature. D was faster in the region of the film near the solution interface, again attributed to a lower Tg caused by greater water content at this interface. 

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5. Segregative phase separation of strong polyelectrolyte complexes at high salt and high polymer concentrations

   https://doi.org/10.1039/D4SM00994K  

   Chee, Conner H; Benharush, Rotem; Knight, Lexi R; Laaser, Jennifer E (October 2024, Soft Matter)

   The phase behavior of polyelectrolyte complexes and coacervates (PECs) at low salt concentrations has been well characterized, but their behavior at concentrations well above the binodal is not well understood. Here, we investigate the phase behavior of stoichiometric poly(styrene sulfonate)/poly(diallyldimethylammonium) mixtures at high salt and high polymer concentrations. Samples were prepared by direct mixing of PSS/PDADMA PECs, water, and salt (KBr). Phase separation was observed at salt concentrations approximately 1 M above the binodal. Characterization by thermogravimetric analysis, FTIR, and NMR revealed that both phases contained significant amounts of polymer, and that the polymer-rich phase was enriched in PSS, while the polymer-poor phase was enriched in PDADMA. These results suggest that high salt concentrations drive salting out of the more hydrophobic polyelectrolyte (PSS), consistent with behavior observed in weak polyelectrolyte systems. Interestingly, at the highest salt and polymer concentrations studied, the polymer-rich phase contained both PSS and PDADMA, suggesting that high salt concentrations can drive salting out of partially-neutralized complexes as well. Characterization of the behavior of PECs in the high concentration limit appears to be a fruitful avenue for deepening fundamental understanding of the molecular-scale factors driving phase separation in these systems. 

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