Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

Abstract

<div>The role of hydrophobicity, and particularly nonionic hydrophobic comonomers, on the phase behavior of polyelectrolyte complex coacervates is not well-understood. Here, we address this problem by synthesizing a library of polymers with a wide range of charge densities and nonionic hydrophobic side chain lengths, and characterizing their phase behavior by optical turbidity. The polymers were prepared by post-polymerization modification of poly(N-acryloxy succinimide), targeting charge densities between 40 and 100% and nonionic aliphatic sidechains with lengths from 0 to 12 carbons long. Turbidity measurements on pairs of polycations and polyanions with matched charge densities and nonionic sidechain lengths revealed a complex salt response with distinct charge density-dominated and hydrophobicity-dominated regimes. The polymer solubilities were not directly correlated with the phase behavior of the coacervates, indicating the difficulty of understanding the coacervate phase behavior in terms of the polymer-water interaction parameter. This result suggests that there is significant room for further work to understand the mechanisms by which specific molecular-scale interactions moderate the phase behavior of complex coacervates.</div>

Influence of “Hydrophobicity” on the Composition and Dynamics of Polyelectrolyte Complex Coacervates

“…Longer alkane chains C6−C12 yielded more salt-stable complexes. 18 Sadman et al, 27 using PVP alkylated with one to three carbons, found the LVR of QPVP/PSS was relatively insensitive to hydrophobicity, in line with results seen here. However, in contrast to Figure 3A, they also found that more hydrophobic PECs were harder to dope.…”

“…24−26 The question of hydrophobicity in PECs was recently explored by Huang and Laaser using a library of acrylamide copolymers modified with C2−C12 alkanes. 18 In these polymers the hydrophobic side chains were located on neutral comonomers. Measurement of the CSC showed a complex dependence of stability on chain length and charge fraction.…”

“…From a synthetic perspective, it is relatively straightforward to program increasing hydrophobicity into a polyelectrolyte to investigate its effect on PEC formation and properties. 18 Recently, −CH 2 − pendant groups on the polyelectrolyte repeat unit, 27 or interspersed with charged units, 18,28 of the polymer backbone 29 or the pendant groups. 30 These studies have found that an increase in hydrophobic content results in an increase in PEC stability, consistent with the notion that hydrophobic interactions reinforce electrostatic ones.…”

Structure and Dynamics of Hybrid Colloid–Polyelectrolyte Coacervates

“…In the latter, increasing the total charge of PEs for a fixed length (i.e., without changing the translational entropy) is achieved by incorporating a higher fraction of ionic monomers, which, in a similar way, leads to enhanced coacervate stability against salt. 14,15 The more subtle role of charge blockiness in conventional interpolyelectrolyte coacervation is analogous, albeit other determinants are required to define the 1D blockiness in the linear (primary) sequence of PE monomers 16,17 as compared to the 2D interfacial charge patchiness encountered in globular proteins.…”