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Advances in Colloid and Interface Science (v.167, #1-2)
Foreword
by Paul L. Dubin Guest Editor; A. Basak Kayitmazer Guest Editor; Q. Huang Guest Editor (pp. 1-1).
A review of the early development of the thermodynamics of the complex coacervation phase separation by Arthur Veis (pp. 2-11).
Coacervation was defined as the phenomenon in which a colloidal dispersion separated into colloid-rich (the coacervate), and colloid-poor phases, both with the same solvent. Complex coacervation covered the situation in which a mixture of two polymeric polyions with opposite charge separated into liquid dilute and concentrated phases, in the same solvent, with both phases, at equilibrium, containing both polyions. Voorn and Overbeek provided the first theoretical analysis of complex coacervation by applying Flory–Huggins polymer statistics to model the random mixing of the polyions and their counter ions in solution, assuming completely random mixing of the polyions in each phase, with the electrostatic free energy, Δ Gelect, providing the driving force. However, experimentally complete randomness does not apply: polyion size, heterogeneity, chain stiffness and charge density ( σ) all affect the equilibrium phase separation and phase concentrations. Moreover, in pauci-disperse systems multiple phases are often observed. As an alternative, Veis and Aranyi proposed the formation of charge paired Symmetrical Aggregates (SA) as an initial step, followed by phase separation driven by the interaction parameter, χ23, combining both entropy and enthalpy factors other than the Δ Gelect electrostatic term. This two stage path to equilibrium phase separation allows for understanding and quantifying and modeling the diverse aggregates produced by interactions between polyampholyte molecules of different charge density, σ, and intrinsic polyion structure.► The hypotheses of the Voorn–Overbeek theory of complex coacervation are described. ► The V–O theory requires random chains in both dilute and coacervate phases. ► Phase compositions of pI 5–pI 9 gelatin coacervates do not support random mixing. ► The Veis–Gates model proposes the formation of symmetrical aggregates in both phases. ► The V–G SA model predicts the observed MW fractionation in pauci-disperse systems.
Keywords: Polyelectrolyte complexes; Gelatin–gelatin coacervates; Free energy of mixing; Electrostatic free energy; Phase separation; Aggregate structure models; Phase diagrams; Collagen
Anisotropic domain growth and complex coacervation in nanoclay-polyelectrolyte solutions by Nisha Pawar; H.B. Bohidar (pp. 12-23).
In this review, the generalized domain growth in a coacervating solution is discussed. Associative electrostatic interaction between nanoclay (Laponite) and gelatin-A (a polyelectrolyte) is shown to drive complex coacervation at room temperature (25°C). Phase separation kinetics, leading to spontaneous coacervation transition occurring below spinodal temperature (315K) was studied by depolarized dynamic light scattering. Depolarization and axial ratio data clearly revealed that the domains formed of soluble complexes undergo time-dependent anisotropic growth during the initial period of phase separation (t<500s). The equatorial axis of these domains was observed to grow following a power-law behavior: a( t)~ t β and β=0.25±0.04 independent of quench depth that was not deep. In contrast, the polar axis shrunk with time following: b( t)~ t− δ and δ=0.15±0.05 independent of quench depth. These domains preferentially grew as oblate ellipsoids during this time. Effect of gravity on domain growth was not observed in our experiments. These results answer the basic issue of binding between discotic colloidal particles and polyelectrolytes in dispersion phase and the resultant phase separation kinetics.
Keywords: Anisotropic domains; Depolarized scattering; Gravity; Depolarization ratio; Coacervation
Complexation and coacervation of polyelectrolytes with oppositely charged colloids by Ebru Kizilay; A. Basak Kayitmazer; Paul L. Dubin (pp. 24-37).
Polyelectrolyte-colloid coacervation could be viewed as a sub-category of complex coacervation, but is unique in (1) retaining the structure and properties of the colloid, and (2) reducing the heterogeneity and configurational complexity of polyelectrolyte–polyelectrolyte (PE–PE) systems. Interest in protein-polyelectrolyte coacervates arises from preservation of biofunctionality; in addition, the geometric and charge isotropy of micelles allows for better comparison with theory, taking into account the central role of colloid charge density. In the context of these two systems, we describe critical conditions for complex formation and for coacervation with regard to colloid and polyelectrolyte charge densities, ionic strength, PE molecular weight (MW), and stoichiometry; and effects of temperature and shear, which are unique to the PE-micelle systems. The coacervation process is discussed in terms of theoretical treatments and models, as supported by experimental findings. We point out how soluble aggregates, subject to various equilibria and disproportionation effects, can self-assemble leading to heterogeneity in macroscopically homogeneous coacervates, on multiple length scales.
Keywords: Complex coacervation; Polyelectrolyte-colloid interactions; Coacervate; Structure
Controlling electrostatic co-assembly using ion-containing copolymers: From surfactants to nanoparticles by J.-F. Berret (pp. 38-48).
In this review, we address the issue of the electrostatic complexation between charged-neutral diblock copolymers and oppositely charged nanocolloids. We show that nanocolloids such as surfactant micelles and iron oxide magnetic nanoparticles share similar properties when mixed with charged-neutral diblocks. Above a critical charge ratio, core–shell hierarchical structures form spontaneously under direct mixing conditions. The core–shell structures are identified by a combination of small-angle scattering techniques and transmission electron microscopy. The formation of multi-level objects is driven by the electrostatic attraction between opposite charges and by the release of the condensed counterions. Alternative mixing processes inspired from molecular biology are also described. The protocols applied here consist in screening the electrostatic interactions of the mixed dispersions, and then removing the salt progressively as an example by dialysis. With these techniques, the oppositely charged species are intimately mixed before they can interact, and their association is monitored by the desalting kinetics. As a result, sphere- and wire-like aggregates with remarkable superparamagnetic and stability properties are obtained. These findings are discussed in the light of a new paradigm which deals with the possibility to use inorganic nanoparticles as building blocks for the design and fabrication of supracolloidal assemblies with enhanced functionalities.Display Omitted► Polymers and colloids of opposite charges in water aggregate. ►Examples studied this review are surfactant micelles and magnetic nanoparticles. ► Ways to control aggregation are also proposed. ► The desalting transition pathway allows to fine-tune the kinetics of association. ► Highly resilient and rigid magnetic nanowires are fabricated.
Recent progress in biopolymer nanoparticle and microparticle formation by heat-treating electrostatic protein–polysaccharide complexes by Owen G. Jones; David Julian McClements (pp. 49-62).
Functional biopolymer nanoparticles or microparticles can be formed by heat treatment of globular protein–ionic polysaccharide electrostatic complexes under appropriate solution conditions. These biopolymer particles can be used as encapsulation and delivery systems, fat mimetics, lightening agents, or texture modifiers. This review highlights recent progress in the design and fabrication of biopolymer particles based on heating globular protein–ionic polysaccharide complexes above the thermal denaturation temperature of the proteins. The influence of biopolymer type, protein–polysaccharide ratio, pH, ionic strength, and thermal history on the characteristics of the biopolymer particles formed is reviewed. Our current understanding of the underlying physicochemical mechanisms of particle formation and properties is given. The information provided in this review should facilitate the rational design of biopolymer particles with specific physicochemical and functional attributes, as well as stimulate further research in identifying the physicochemical origin of particle formation.
Keywords: Biopolymers; Nanoparticles; Microparticles; Protein; Polysaccharide; Delivery systems; Complexation; Coacervation; Electrostatics
Protein/polysaccharide complexes and coacervates in food systems by Christophe Schmitt; Sylvie L. Turgeon (pp. 63-70).
Since the pioneering work of Bungenberg de Jong and co-workers on gelatin–acacia gum complex coacervation in the 1920–40s, protein/polysaccharide complexes and coacervates have received increasing research interest in order to broaden the possible food applications. This review focuses on the main research streams followed in this field during the last 12years regarding: i) the parameters influencing the formation of complexes and coacervates in protein–polysaccharide systems; ii) the characterization of the kinetics of phase separation and multi-scale structure of the complexes and coacervates; and iii) the investigation of the functional properties of complexes and coacervates in food applications. This latter section encompasses various technological aspects, namely: the viscosifying and gelling ability, the foaming and emulsifying ability and finally, the stabilization and release of bioactives or sensitive compounds.
Keywords: Protein; Polysaccharide; Electrostatic complexes; Coacervation; Microstructure; Functional properties
The model Lysozyme–PSSNa system for electrostatic complexation: Similarities and differences with complex coacervation by F. Cousin; J. Gummel; S. Combet; Boue F. Boué (pp. 71-84).
We review, based on structural information, the mechanisms involved when putting in contact two nano-objects of opposite electrical charge, in the case of one negatively charged polyion, and a compact charged one. The central case is mixtures of PSS, a strong flexible polyanion (the salt of a strong acid, and with high linear charge density), and Lysozyme, a globular protein with a global positive charge. A wide accurate and consistent set of information in different situations is available on the structure at local scales (5–1000Å), due to the possibility of matching, the reproducibility of the system, its well-defined electrostatics features, and the well-defined structures obtained. We have related these structures to the observations at macroscopic scale of the phase behavior, and to the expected mechanisms of coacervation. On the one hand, PSS/Lysozyme mixtures show accurately many of what is expected in PEL/protein complexation, and phase separation, as reviewed by de Kruif: under certain conditions some well-defined complexes are formed before any phase separation, they are close to neutral; even in excess of one species, complexes are only modestly charged (surface charges in PEL excess). Neutral cores are attracting each other, to form larger objects responsible for large turbidity. They should lead the system to phase separation; this is observed in the more dilute samples, while in more concentrated ones the lack of separation in turbid samples is explained by locking effects between fractal aggregates.On the other hand, although some of the features just listed are the same required for coacervation, this phase transition is not really obtained. The phase separation has all the macroscopic aspects of a fluid (undifferentiated liquid/gas phase) — solid transition, not of a fluid–fluid (liquid–liquid) one, which would correspond to real coacervation). The origin of this can be found in the interaction potential between primary complexes formed (globules), which agrees qualitatively with a potential shape of the type repulsive long range attractive very short range.Finally we have considered two other systems with accurate structural information, to see whether other situations can be found. For Pectin, the same situation as PSS can be found, as well as other states, without solid precipitation, but possibly with incomplete coacervation, corresponding to differences in the globular structure. It is understandable that these systems show smoother interaction potential between the complexes (globules) likely to produce liquid–liquid transition. Finally, we briefly recall new results on Hyaluronan/Lysozyme, which present clear signs of coacervation in two liquid phases, and at the same time the existence of non-globular complexes, of specific geometry (thin rods) before any phase separation. These mixtures fulfill many of the requirements for complex coacervation, while other theories should also be checked like the one of Shklovskii et al.Complex structure of polyanions (flexible or not) with oppositely charged Lysozymes (SANS, 5–1000Å) are correlated to phase separation, and interaction potential profiles are proposed. PolyStyrene Sulfonate (most studied since deuteriable) form gels or neutral core globules, with Reaction limited aggregation implying fluid-solid precipitation, blocked at high concentration. Globules for semi-flexible Pectin precipitate but coacervate incompletely for low charge. Semi-flexible Hyaluronan gives small rodlike complexes, possibly neutral, and fluid-fluid coacervation.Display Omitted
Keywords: Small angle neutron scattering; Polyelectrolyte–protein complexes; Coacervation; Colloids phase transitions; Interaction potential
Complex coacervates as a foundation for synthetic underwater adhesives by Russell J. Stewart; Ching Shuen Wang; Hui Shao (pp. 85-93).
Complex coacervation was proposed to play a role in the formation of the underwater bioadhesive of the Sandcastle worm ( Phragmatopoma californica) based on the polyacidic and polybasic nature of the glue proteins and the balance of opposite charges at physiological pH. Morphological studies of the secretory system suggested that the natural process does not involve complex coacervation as commonly defined. The distinction may not be important because electrostatic interactions likely play an important role in the formation of the sandcastle glue. Complex coacervation has also been invoked in the formation of adhesive underwater silk fibers of caddisfly larvae and the adhesive plaques of mussels. A process similar to complex coacervation, that is, condensation and dehydration of biopolyelectrolytes through electrostatic associations, seems plausible for the caddisfly silk. This much is clear, the sandcastle glue complex coacervation model provided a valuable blueprint for the synthesis of a biomimetic, water-borne, underwater adhesive with demonstrated potential for repair of wet tissue.
Keywords: Adhesives; Complex coacervates; Polyelectrolytes; Biomaterials; Biomimicry; Bioinspired
Coacervation of tropoelastin by Giselle C. Yeo; Fred W. Keeley; Anthony S. Weiss (pp. 94-103).
The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures.Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate.Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains.Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.
Keywords: Abbreviations; A; alanine; G; glycine; GAG; glycosaminoglycan; MAGP-1; matrix-associated glycoprotein 1; SVAS; supravalvular aortic stenosis; P; proline; V; valineCoacervation; Self-assembly; Tropoelastin; Elastin