Recent progress in the science of complex coacervation

Complex coacervation is an associative, liquid–liquid phase separation that can occur in solutions of oppositely-charged macromolecular species, such as proteins, polymers, and colloids. This process results in a coacervate phase, which is a dense mix of the oppositely-charged components, and a supernatant phase, which is primarily devoid of these same species. First observed almost a century ago, coacervates have since found relevance in a wide range of applications; they are used in personal care and food products, cutting edge biotechnology, and as a motif for materials design and self-assembly. There has recently been a renaissance in our understanding of this important class of material phenomena, bringing the science of coacervation to the forefront of polymer and colloid science, biophysics, and industrial materials design. In this review, we describe the emergence of a number of these new research directions, specifically in the context of polymer–polymer complex coacervates, which are inspired by a number of key physical and chemical insights and driven by a diverse range of experimental, theoretical, and computational approaches

Vegetable proteins in microencapsulation: a review of recent interventions and their effectiveness

Proteins from vegetable seeds are interesting for research at present because they are an abundant alternative to animal-based sources of proteins and petroleum-derived polymers. They are a renewable and biodegradable raw material with interesting functional and/or physico-chemical properties. In microencapsulation, these biopolymers are used as a wall forming material for a variety of active compounds. In most cases, two techniques of microencapsulation, spray-drying and coacervation, are used for the preparation of microparticles from vegetable proteins. Proteins extracted from soy bean, pea and wheat have already been studied as carrier materials for microparticles. These proteins could be suitable shell or matrix materials and show good process efficiency. Some other plant proteins, such as rice, oat or sunflower, with interesting functional properties could be investigated as potential matrices for microencapsulation

Relaxation Behavior by Time-Salt and Time-Temperature Superpositions of Polyelectrolyte Complexes from Coacervate to Precipitate

Complexation between anionic and cationic polyelectrolytes results in solid-like precipitates or liquid-like coacervate depending on the added salt in the aqueous medium. However, the boundary between these polymer-rich phases is quite broad and the associated changes in the polymer relaxation in the complexes across the transition regime are poorly understood. In this work, the relaxation dynamics of complexes across this transition is probed over a wide timescale by measuring viscoelastic spectra and zero-shear viscosities at varying temperatures and salt concentrations for two different salt types. We find that the complexes exhibit time-temperature superposition (TTS) at all salt concentrations, while the range of overlapped-frequencies for time-temperature-salt superposition (TTSS) strongly depends on the salt concentration (Cs) and gradually shifts to higher frequencies as Cs is decreased. The sticky-Rouse model describes the relaxation behavior at all Cs. However, collective relaxation of polyelectrolyte complexes gradually approaches a rubbery regime and eventually exhibits a gel-like response as Cs is decreased and limits the validity of TTSS.Comment: 12 pages, 5 figures, Follow Gels journal link for latest versio

Electrospinning Fibers via Complex Coacervation

Electrospun fibers are high-surface-area materials widely used in applications ranging from batteries to wound dressings. Typically, an electrospinning precursor solution is prepared by dissolving a high-molecular-weight polymer in an organic solvent to form a sufficiently entangled solution. Our approach bypasses the requirement for entanglements and completely avoids toxic chemicals by focusing on using an aqueous complex coacervates solution. Coacervates are a dense, polymer-rich liquid phase resulting from the associative electrostatic complexation of oppositely charged macroions. We were the first to demonstrate that liquid complex coacervates could be successfully electrospun into polyelectrolyte complex (PEC) fibers. A canonical coacervate system was formed with poly(4-styrene sulfonic acid, sodium salt) and poly(diallyldimethylammonium chloride). Characterization of the binodal phase behavior demonstrated the thermodynamic linkage of the polymer and salt concentrations (CP and CS): greater CP indirectly controlled by decreasing CS. Our results showed that electrospun fibers had smaller and more uniform diameters with increasing applied voltage, separation distance, and CP. The resulting fibers were ultra-stable to heat and organic solvents because of the strong electrostatic attraction between polymers. Coacervates have the potentials to be developed into an environmentally benign electrospinning precursor platform. Having demonstrated coacervates electrospinnability, we hypothesized that the associative interactions that drive coacervation can also enable electrospinning. Therefore, we synthesized a set of backbone-matched methacroloyl polymers of different chain lengths and formed coacervates. Amazingly, all the coacervates were successfully electrospun into continuous fibers, including an oligomeric Nanion/Ncation 6/9 coacervate system where no physical chain entanglements were possible. After correlating the spinnability of coacervates with their rheological behavior, we found out that spinnable coacervates had prolonged relaxation behaviors due to interpolymeric electrostatic interactions. The ability to electrospin oligomeric coacervates has significant impacts on decoupling polymer chain length from electrospinnability thus, enabling fibers formation from chemical species that were previously considered non-spinnable. Knowing the long history of applications where complex coacervates were used for encapsulation, we also investigated the ability to electrospin cargo-loaded PEC fibers via coacervation. We used a family of six fluorescent dyes with systematic structural differences as model drugs. All dyes preferentially partitioned into the coacervates phase, allowing the subsequent electrospinning of highly-loaded fibers. Dyes that were electrostatically attracted to PSS to undergo π-π interactions partitioned more favorably in the coacervate phase, slowed the release from within PEC fibers when exposed to aqueous solutions, as well as exhibited enhanced uptake by fibers. These findings have the potentials to use the PEC fibers in applications related to biomedicine, energy, and separations, where controlling the uptake and release of cargo into sponge-like mats is needed. In summary, this dissertation demonstrated the first electrospinning of aqueous complex coacervates solution into PEC fiber mats, identified that the spinning mechanism was electrostatic interactions as an alternative of entanglements, and studied the associated dye encapsulation and release properties of the mats to enable their use across a range of applications

The parallel lives of polysaccharides in food and pharmaceutical formulations

The present opinion article discusses how polysaccharide structures can be used in both food and pharmaceutical formulations. We distinguish two regions depending on moisture content where polysaccharides form structures with distinct functional properties. Some trends in key areas of active research are assessed and in particular edible films, encapsulation, polycrystalline polysaccharides, protein-polysaccharide coacervation and fluid gels. We unveil that the physicochemical principles that are shared across the food and pharmaceutical disciplines provide a great opportunity for cross-disciplinary collaboration. We finally argue that such co-operation will help tackling polysaccharide functionality issues that are encountered in both areas

Charge symmetry broken complex coacervation

Liquid-liquid phase separation has emerged as one of the important paradigms in the chemical physics as well as biophysics of charged macromolecular systems. We elucidate an equilibrium phase separation mechanism based on charge regulation, i.e., protonation-deprotonation equilibria controlled by pH, in an idealized macroion system which can serve as a proxy for simple coacervation. First, a low-density density-functional calculation reveals the dominance of two-particle configurations coupled by ion adsorption on neighboring macroions. Then a binary cell model, solved on the Debye-H\"uckel as well as the full nonlinear Poisson-Boltzmann level, unveils the charge-symmetry breaking as inducing the phase separation between low- and high-density phases as a function of pH. These results can be identified as a charge symmetry broken complex coacervation between chemically identical macroions.Comment: 11 pages, 7 figure

Whey protein and gum arabic encapsulated Omega-3 lipids. The effect of material properties on coacervation

The effect of material properties on complex coacervation of whey protein and gum Arabic from various sources was investigated. In this study, it was demonstrated that material properties of whey protein isolates and gum Arabic affect the complex coacervation process significantly. For whey protein, the coacervation capability could be correlated with their level of denaturation and calcium content. For gum Arabic, both material sources and salt content were found to be attributing factors to their coacervation capability. This study facilitated the development of Omega-3 lipids microcapsules with promising performances in certain food applications.<br /

Perfume Microencapsulation by Complex Coacervation

Many techniques are known for the microencapsulation of substances. Complex coacervation is one of them. Generally, gelatin and gum arabic are used to form the capsule wall. Gum arabic may be replaced by other colloidal polymers such as pectin. Most often, pure substances are microencapsulated, but microencapsulation of complex mixtures such as perfumes is also possible. This paper reports on microencapsulation of perfumes using complex coacervation. The influence of the wall material on the microcapsule characteristics is given

SEQUENCE CONTROL OF COMPLEX COACERVATION

Complex coacervation is a liquid-liquid phase separation driven by the complexation of oppositely charged polyelectrolytes. The resulting coacervate phase has been used for many applications, such as underwater adhesives, drug delivery, food and personal care products. There also has been increasing interest in coacervate-like droplets occurring in biological systems. The majority of these “membraneless organelles” involve a combination of intrinsically-disordered proteins and RNA, and phase separate due to long-range charge effects and short-range hydrophobic effects. While evolution has optimized the self-assembly of these types of biological polymers, our ability to design such materials remains limited, in part because the relevant interactions that occur over a wide range of different length scales. The goal of this research is to establish molecular-level design rules as to how chemical sequence can modulate the formation and properties of complex coacervates. While studies to date have focused on the effect of parameters such as the charge stoichiometry, temperature, pH, salt concentration, stereochemistry, polymer architecture, and the density of charges present, the ability to pattern the sequence of charges and other chemistries has been rarely studied. However, polypeptides represent a model platform for the synthesis and study of polyelectrolytes with precisely controlled polymer architecture and sequence patterning at the molecular level, while retaining relevance to a variety of biological, medical, and industrial applications. Experimental measures such as turbidimetry and optical microscopy, coupled with isothermal titration calorimetry were used to study how variations in the patterning and overall fraction of charged groups along the polymer affect coacervate phase behavior. Increasing the number of charged residues increased the salt resistance and the size of the two-phase region. More interestingly, a comparison between polypeptides with the same overall charge fraction, but different periodic repeating patterns of charged monomers showed an increase in coacervate stability with increasing charge block size. Thermodynamic data, coupled with insights from simulation showed that the increase in stability was entropic in nature, resulting from differences in the one-dimensional confinement of counterions along the patterned polymer. We have also explored arbitrary sequences, hydrophobicity, and the identity of salt, as well as the self-coacervation of polyampholytes