Structure and Dynamics of Hybrid Colloid-Polyelectrolyte Coacervates

Surface-Immobilized Interpolyelectrolyte Complexes Formed by Polyelectrolyte Brushes

A scaling theory of interaction and complex formation between planar polyelectrolyte (PE) brush and oppositely charged mobile linear PEs is developed. Counterion release is found to be the main driving force for the complexation. An interpolyelectrolyte coacervate complex (IPEC) between the brush and oppositely charged mobile PEs is formed at moderate grafting density and low salt concentration. At higher grafting density mobile chains penetrate the brush, but the brush structure is controlled by the balance between entropic elasticity and nonelectrostatic short-range interactions, as happens in a neutral brush. An increase in salt concentration beyond the theoretically predicted threshold leads to the release of the guest polyions from the brush. For brushes with moderate grafting density, complexation with oppositely charged guest polyions is predicted to trigger lateral microphase separation and formation of the finite-size surface IPEC clusters. Power law dependencies for the cluster dimensions on the brush grafting density, PE length, and salt concentration are provided.

Structure and Dynamics of Hybrid Colloid-Polyelectrolyte Coacervates: Insights from Molecular Simulations

Electrostatic interactions in polymeric systems are responsible for a wide range of liquid-liquid phase transitions that are of importance for biology and materials science. Such transitions are referred to as complex coacervation, and recent studies have sought to understand the underlying physics and chemistry. Most theoretical and simulation efforts to date have focused on oppositely charged linear polyelectrolytes, which adopt nearly ideal-coil conformations in the condensed phase. However, when one of the coacervate components is a globular protein, a better model of complexation should replace one of the species with a spherical charged particle or colloid. In this work, we perform coarse-grained simulations of colloid-polyelectrolyte coacervation using a spherical model for the colloid. Simulation results indicate that the electroneutral cell of the resulting (hybrid) coacervates consists of a polyelectrolyte layer adsorbed on the colloid. Power laws for the structure and the density of the condensed phase, which are extracted from simulations, are found to be consistent with the adsorption-based scaling theory of hybrid coacervation. The coacervates remain amorphous (disordered) at a moderate colloid charge, Q, while an intra-coacervate colloidal crystal is formed above a certain threshold, at Q > Q*. In the disordered coacervate, if Q is sufficiently low, colloids diffuse as neutral nonsticky nanoparticles in the semidilute polymer solution. For higher Q, adsorption is strong and colloids become effectively sticky. Our findings are relevant for the coacervation of polyelectrolytes with proteins, spherical micelles of ionic surfactants, and solid organic or inorganic nanoparticles.

A molecularly informed field-theoretic study of the complexation of polycation PDADMA with mixed micelles of sodium dodecyl sulfate and ethoxylated surfactants

The self-assembly and phase separation of mixtures of polyelectrolytes and surfactants are important to a range of applications, from formulating personal care products to drug encapsulation. In contrast to systems of oppositely charged polyelectrolytes, in polyelectrolyte-surfactant systems the surfactants micellize into structures that are highly responsive to solution conditions. In this work, we examine how the morphology of micelles and degree of polyelectrolyte adsorption dynamically change upon varying the mixing ratio of charged and neutral surfactants. Specifically, we consider a solution of the cationic polyelectrolyte polydiallyldimethylammonium, anionic surfactant sodium dodecyl sulfate, neutral ethoxylated surfactants (C[Formula: see text]EO[Formula: see text]), sodium chloride salt, and water. To capture the chemical specificity of these species, we leverage recent developments in constructing molecularly informed field theories via coarse-graining from all-atom simulations. Our results show how changing the surfactant mixing ratios and the identity of the nonionic surfactant modulates micelle size and surface charge, and as a result dictates the degree of polyelectrolyte adsorption. These results are in semi-quantitative agreement with experimental observations on the same system.

Complex Coacervates as a Promising Vehicle for mRNA Delivery: A Comprehensive Review of Recent Advances and Challenges

Messenger RNA (mRNA)-based therapies have gained significant attention, following the successful deployment of mRNA-based COVID-19 vaccines. Compared with traditional methods of genetic modification, mRNA-based therapies offer several advantages, including a lower risk of genetic mutations, temporary and controlled therapeutic gene expression, and a shorter production time, which facilitates rapid responses to emerging health challenges. Moreover, mRNA-based therapies have shown immense potential in treating a wide range of diseases including cancers, immune diseases, and neurological disorders. However, the current limitations of non-viral vectors for efficient and safe delivery of mRNA therapies, such as low encapsulation efficiency, potential toxicity, and limited stability, necessitate the exploration of novel strategies to overcome these challenges and fully realize the potential of mRNA-based therapeutics. Coacervate-based delivery systems have recently emerged as promising strategies for enhancing mRNA delivery. Coacervates, which are formed by the aggregation of two or more macromolecules, have shown great potential in delivering a wide range of therapeutics due to their ability to form a separated macromolecular-rich fluid phase in an aqueous environment. This phase separation enables the entrapment and protection of therapeutic agents from degradation as well as efficient cellular uptake and controlled release. Additionally, the natural affinity of coacervates for mRNA molecules presents an excellent opportunity for enhancing mRNA delivery to targeted cells and tissues, making coacervate-based delivery systems an attractive option for mRNA-based therapies. This review highlights the limitations of current strategies for mRNA delivery and the advantages of coacervate-based delivery systems to enable mRNA therapeutics. Coacervates protect mRNA from enzymatic degradation and enhance cellular uptake, leading to sustained and controlled gene expression. Despite their promising properties, the specific use of coacervates as mRNA delivery vehicles remains underexplored. This review aims to provide a comprehensive overview of coacervate-mediated delivery of mRNA, exploring the properties and applications of different coacervating agents as well as the challenges and optimization strategies involved in mRNA encapsulation, release, stability, and translation via coacervate-mediated delivery. Through a comprehensive analysis of recent advancements and recommended future directions, our review sheds light on the promising role of coacervate-mediated delivery for RNA therapeutics, highlighting its potential to enable groundbreaking applications in drug delivery and gene therapy.

The Counterion-Mediated Controllable Coacervation of Nano-Ions with Polyelectrolytes

Nano-ions can complex with polyelectrolytes for coacervates with hierarchical structures; however, the rational design of functional coacervations is still rare due to the poor understanding of their structure-property relationship from their complex interaction. Herein, 1 nm anionic metal oxide clusters, PW₁₂O₄₀^3-, with well-defined, mono-disperse structures are applied to complex with cationic polyelectrolyte and the system shows tunable coacervation via the alternation of counterions (H⁺ and Na⁺) of PW₁₂O₄₀^3-. Suggested from Fourier transform infrared spectroscopy (FT-IR) and isothermal titration studies, the interaction between PW₁₂O₄₀^3- and cationic polyelectrolytes can be modulated by the bridging effect of counterions via hydrogen bonding or ion-dipole interaction to carbonyl groups of polyelectrolytes. The condensed structures of the complexed coacervates are explored by small angle X-ray and neutron scattering techniques, respectively. The coacervate with H⁺ as counterions shows both crystallized and discrete PW₁₂O₄₀^3- clusters, with a loose polymer-cluster network in comparison to the system of Na⁺ which shows a dense packing structure with aggregated nano-ions filling the meshes of polyelectrolyte networks. The bridging effect of counterions helps understand the super-chaotropic effect observed in nano-ion system and provides avenues for the design of metal oxide cluster-based functional coacervates.