Effect of ionic charge
Complex coacervation may be brought about in water by the combination of two polymers, one with a positive charge and the other with a negative charge. The most common polymers used are gelatin, which is dissolved in water and the pH is adjusted so that it is below the isoelectric point, and acacia which is negatively charged because of the ionization of its carboxyl groups. The combination leads to a coacervate, which is polymer rich, and the other phase which is polymer poor (Luzzi, 1976). The interaction between the two polymers is also influenced by temperature and the presence of salts (Madan, 1978).
In a series of papers Nakajima and Sato (1972) reported upon the phase relationships and theory of complex coacervation of the sulfated polyvinyl alcohol-aminoacetalysed polyvinyl alcohol system. Phase relationships were examined for the polymer salt, water and sodium bromide. The experimental results were interpreted by the use of a theoretical equation for the free energy of mixing by taking into account the entropy and enthalpy contributions ascribed to a non-ionic polymer solution and the electrostatic free energy expression as derived by Voorn (1956). In two subsequent papers, Sato and Nakajima (1974a,b) investigated the effects of chain length of the polyelectrolytes, the thermodynamic interaction between the polymer and water and the number of charges of polyelectrolyte chain on the complex coacervate on the basis of a free energy equation. Conditions for the formation of coacervate droplets as a function of charge density and polymer concentration were also discussed.
Burgess and Carless (1984) investigated the electrophoretic mobility profile of polyions and showed that these profiles can be used to determine if complex coacervation will occur between two polyions. Furthermore, they showed that the pH range of coacervate, the pH of optimum coacervation and the salt tolerance of the system can be predicted. They also showed that the maximum coacervate volume occurred at the electrical equivalence pH, that is when the charges on the two polyions are equal and opposite.
A practical analysis of complex coacervate systems has been published by Burgess (1990) who reviewed several theories which are now briefly described. Overbeek and Voorn (1957) postulated that the coacervation which takes place between gelatin and acacia is a competition between ionic attractive forces, which tend to bring the polyions together, and entropy effects, which promote the dispersion. The coacervate phase binds water between the loops of the polymer chains. The water in the coacervate contributes to the entropy and permits a number of arrangements of polymers. As a result the coacervate is fluid. Another theory which attempts to explain complex coacervation is the 'dilute phase aggregate model', developed by Veis and Aranyi (1960) to take into account the formation of complex coacervation when the product of the charge density and the molecular weight is low. The model postulates that complex coacervation occurs in two steps, as oppositely charged gelatins fuse, aggregate, and then rearrange to form the coacervation phase. The rearrangement occurs slowly and is formed by the gain in configuration entropy. Several differences between the two theories are described by Burgess (1990). Burgess and Carless (1985, 1986b) confirmed the two-step process and detected the presence of small aggregates by light scattering. Tainaka (1979, 1980) modified the Veis and Aranyi theory to indicate that the aggregate pairs in the dilute phase did not have specific charge pairing. Again, the dilute phase aggregates condense to form the coacervate, but the aggregates are present in both the dilute and coacervate phases. The aggregates overlap with each other in the coacervate phase and, as a result, there is a gain in electrostatic energy due to the increase in ion density in the overlapped domain. High molecular weights and highly charged densities of the polyions enhance the attractive forces effecting phase separation. The Tainaka theory explains the suppression of coacervates at high polymer concentration as stabilization of aggregate structures at high concentration. Burgess (1990) concluded that, while the Tainaka model is not all-inclusive, as it does not explain the reduction of coacervation at low ionic strength, it is not as restricted as other theories and thus, at present, is the best general theory.
A recent paper by Van Oss (1988-1989) presents a somewhat different classification of coacervation, complex coacervation and flocculation based on polar (hydrogen) bonding components of interfacial interactions. Van Oss (1988-1989) has reviewed the classification for coacervation (simple) and complex coacervation for the system of gelatin and acacia (Table 1). A theoretical analysis of cohesion and adhesion in terms of the Lifshitz-van der Waals, or apolar, components and Lewis acid-base, or polar, components of free energy between two different bodies 1,2, through a liquid 3, indicates interfacial (hydrophobic) attraction when AG132 < 0 and interfacial ('hydration pressure' mediating) repulsions when AG132> 0. As a result of this theory, Van Oss provides a table which indicates the mechanisms and conditions for coacervation (simple) and complex coacervation. He indicates that coacervation (simple) takes place when polar and/or apolar repulsion between the two solutes, where one or both must be a polymer dissolved in the same solvent, results in phase separation. Complex coacervation takes place when there is electrostatic or polar attraction between two polymers of opposite charge (or of opposite signs of Lewis acid-base behaviour). Examples of coacervation (simple) due to polar interactions are negatively charged gelatin and gum arabic, agar and ethanol, polyvinyl alcohol and polyethylene glycol, the solvent in all cases being water.
Table 1 Comparison of coacervation (simple) and complex coacervation using a mixture of gelatin and acacia given by Bungenberg de Jong, adapted from Van Oss (1988-89)
Complex coacervation pH
Concentration of original solutions Indifferent salts
Composition of liquid layer
> Isoelectric point for gelatin Occurs with concentrated solutions
Tend to promote coacervation Place in lyotropic series is important
Drops show no disintegration Each layer contains essentially one species
Water deficit in the system
< Isoelectric point for gelatin Occurs with dilute solution
Tend to suppress coacervation Place in lyotropic series is minor Valency is important Drops show disintegration The coacervate layer is rich in the colloid which is a ratio of about one to one
Coacervation (simple) due to apolar interactions includes cellulose acetate and ethanol dissolved in chloroform, polyisobutylene and polystyrene in benzene. Complex coacervation always takes place in water and some examples resulting from electrostatic interaction are positively charged gelatin and negatively charged gum arabic, and positively charged gelatin and nucleic acid. Examples of polar (Lewis acid-base) interaction include polyacrylic acid and polyvinyl methylether.
Borue and Erukhimovich (1990) developed a microscopic statistical theory of symmetrical polyelectrolyte complexes. The complex was shown to form a polymer globule and the equilibrium density, the width of the surface layer and the surface tension were calculated as a function of salt concentration. Complex coacervation is considered as a precipitation of polymer globules due to a minimization of surface energy. The theory is based on the Lifshitz-Grosberg theory of polymer globules and the authors' previous work concerning the equation of state of polyelectrolyte solutions.
In a study of the encapsulation of hydrophobic compounds such as stearyl alcohol by complex coacervation with gelatin-acacia, Madan etal. (1972) found that only particles below 250 fim diameter could be encapsulated. It was proposed that the mechanism for encapsulation in this system was either a single coacervate droplet which encompasses a group of immiscible nuclei or individual coacervate droplets adsorbed to, or coalesced around the particles. Photomicrographs of 163 /*m particles indicate that encapsulation takes place by aggregation or coalescence of several droplets (with diameters usually under 40 /¿m) to surround the core stearyl alcohol particles. Larger particles were incompletely covered. The authors suggest that the affinity of the coacervate droplets for the core material is not great. They suggest that the velocity difference between the core particles and the coacervate droplets, as the mixture is stirred, tends to prevent the aggregation of droplets around the core particles. In addition, the probability that a sufficient number of coacervate droplets will aggregate and coalesce to surround a core particle decreases as the particle size increases. Experimental studies showed that larger particles could be encapsulated if the concentration of the coacervate was increased. In order to improve the encapsulation process, the stearyl alcohol was melted in an acacia solution and then congealed. The acacia is adsorbed more strongly to liquid stearyl alcohol than to the solid form. The adsorbed acacia then reacts directly with the gelatin in the encapsulation process to form the microcapsules.
Veis (1975) described the thermodynamics of phase separation in a mixture of oppositely charged polyelectrolytes. He indicated that homogeneous solutions will be formed as long as a plot of AGM, the free energy of mixing of a solute and solvent, versus <t>2, the volume fraction of the polymer, has a single minimum. However, if Xl2, the interaction parameter, which is proportional to the interaction energy per mole of solvent molecules, is sufficiently large and positive two minima will be present in the plot and any mixture prepared between these two will separate into two phases. Mathematical analysis shows that for polymers of moderate size, phase separation will occur at low solute volume fraction if X]2 is slightly greater than 0.5.
The thermodynamics of mixing of two dissimilar polymers in a single solvent are also discussed. The author discusses two cases. In the first case the polymeric ions have a very high charge density and phase separation occurs to give essentially solvated coprecipitates in equilibrium with an extremely dilute phase. These precipitates are the basis of certain membranes. The other case is that in which the polyions are of a moderate charge density and phase separation is driven by the more favourable electrostatic interaction in the concentrated phase. In this example, both phases contain both ionic polymers, as is the case for the gelatin-acacia interaction.
Based on two experimental findings, namely charge equivalence in the coacervate phase and molecular weight pairing in the coacervate phase, the author suggests the possibility of two mechanisms for the formation of the coacervate based on an unfavourable entropy change AGentropic > 0 and a favourable electrostatic free energy change AG electrostatic < 0 for the reaction:
where P+ is the cationic polymer, Q" is the anionic polymer and [PQ]Agg is the aggregate.
The aggregation may take two forms: the two molecules with the centres of gravity overlapped, or two molecules with explicit ladder-like charge pairing. The author argues in favour of the ladder type formation, based on the molecular weight pairing and the suppression of coacervation observed in most polydispersed mixtures. This new aggregate, PQ, behaves as a new component which should obey the basic Florey-Huggins polymer binding mixture phase separation rule.
The influence of cationic, anionic and non-ionic surfactants on complex coacervate volume and droplet size has been researched by Duquemin and Nixon (1985). The coacervate was prepared by dissolving the surfactant in the acacia solution and then adding an equal quantity of gelatin solution at the optimum pH of coacervation, 4.35. It was found that the per cent coacervate weight decreased with increasing concentration of sodium lauryl sulfate. At high surfactant concentrations, 0.20 and 0.35%, and at a low colloid concentration, 1%, formation of the coacervate was prevented. It was postulated that the additional ions from the surfactant prevented or restricted electrostatic attraction between the gelatin and acacia polyions. The effect of increasing concentration of cetrimide on the per cent coacervate by weight is not so clear and depends on the concentration of the colloid. At low concentration of the surfactant, there is a slight increase in weight and this has been attributed to an increase in water content of the coacervate. At a high colloid concentration, 4 and 5%, and high surfactant concentration, 0.075%, there is an appreciable decrease in the concentration of the coacervate. The authors suggest that this is due to the suppression of coacervation because the process is less energetically favourable. The effect of polysorbate 20 on coacervate weight is similar to that produced by cetrimide. It is suggested that steric hindrance of the large surfactant molecules suppresses coacervation.
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