Coacervation/precipitation

For alginates, the copolymer composition (ratio of mannuronic to guluronic acid units) can influence the ultimate complex properties. These include elasticity as well as permeability and mechanical resistance of coacer-vates cast into 2D or spherical membrane structures. The type of polymer-polymer coacervate (precipitate, sol, network) will also often be highly molar mass dependent, with useful membranes formd within a narrow window. This often does not correspond to the molar mass range required for bioapplications, which is dictated by factors such as cell toxicity and biocompatability. 

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Commercial lecithin is insoluble but infinitely dispersible in water. Treatment with water dissolves small amounts of its decomposition products and adsorbed or coacervated substances, eg, carbohydrates and salts, especially in the presence of ethanol. However, a small percentage of water dissolves or disperses in melted lecithin to form an imbibition. Lecithin forms imbibitions or absorbates with other solvents, eg, alcohols, glycols, esters, ketones, ethers, solutions of almost any organic and inorganic substance, and acetone. It is remarkable that the classic precipitant for phosphoHpids, eg, acetone, dissolves in melted lecithin readily to form a thin, uniform imbibition. Imbibition often is used to bring a reactant in intimate contact with lecithin in the preparation of lecithin derivatives. 

One of the first methods for making capsules involved polymer coacervation. In this method, macromolecules are dissolved in either the dispersed or continuous phase of an emulsion and are induced to precipitate as a shell around the dispersed phase. Coacervation can be brought about in several ways, such as changes in temperature or pH, addition of salts or a second macromolecular substance, or solvent evaporation (Bungenberg de Jong 1949). 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

The electrostatic interaction between oppositely charged protein and polysaccharide can be utilized for encapsulation and delivery of hydro-phobic nutraceuticals. As a result of this interaction, we may have either complex coacervation (and precipitation) or soluble complex formation, depending on various factors, such as the type of polysaccharide used (anionic/cationic), the solution pH, the ionic strength, and the ratio of polysaccharide to protein (see sections 2.1, 2.2 and 2.5 in chapter seven for more details) (Schmitt et al, 1998 de Kruif et al., 2004 Livney, 2008 McClements et al, 2008, 2009). The phenomenon of complex... 

There is mobility of both the protein and the polysaccharide in the liquid coacervate phase, i.e., the coacervate structure is dynamic, with enormous time-scale variations from milliseconds to several days (or even longer in precipitates). 

If the polysaccharide is a strong polyelectrolyte, then precipitation occurs instead of coacervation. 

For the production of good capsules, it is desirable to promote complex coacervation, rather than precipitation. 

Citrus oils readily form oxygenated products that are likely to congregate at oil/water interfaces and thereby cause a detectable change in IFT. The aldehydic components of citrus oil could react with the amine groups of the gelatin molecules present in the aqueous phases formed by complex coacervation and thereby affect IFT. In addition to chemical reactions, physical changes can occur at an interface and alter IFT. A visible interfacial film can form simply due to interfacial interactions that alter the interfacial solubility of one or more components. No chemical reactions need occur. An example is the formation of a visible interfacial film when 5 wt. per cent aqueous gum arabic solutions are placed in contact with benzene (3). Interfacial films or precipitates can also form when chemical reactions occur and yield products that congregate at interfaces. 

A discontinuous precipitate often was observed at citrus oil/ aqueous phase interfaces kept at 50 C for prolonged periods. The precipitate particles congregate on the water side of the citrus oil/aqueous phase interface and disperse into the aqueous phase upon agitation. If the aqueous phase is distilled water, or a supernatant phase, the precipitate particles cause the aqueous phase to become noticeably cloudy. Precipitate particles or interfacial films were not detected at citrus oil/complex coacervate phase interfaces. However, such interfaces normally were not kept for prolonged periods, because their IFT values rapidly decayed to a value too low to measure. 

When two different polymer solutions are mixed, they frequently undergo one or several distinct types of interaction, which in each case can lead to phase separation at polymer concentrations above a certain critical level [12]. In one case, two solution phases of approximately equal volume are formed, consisting of polymer A- and polymer B-rich solutions, respectively. This phase separation is called incompatibility, or simple coacervation. In the second case, two phases are formed but both polymers are concentrated in one of the phases (the precipitate ) while the other phase (the supernatant ) may be essentially polymer free. This separation is called complex coacervation. The two phase separation phenomena are shown in Fig. 2. 

While for the complexation with poly(sodium styrene sulfonate) or sodium cellulosesulfate 1 1 stoichiometry has been reported [150] a non-stoichiometric complex results with sodium carboxymethylcellulose [150]. Optimized conditions make it possible to create membranes with various properties using the PDADMAC/sodium cellulosesulfate system [166-168]. However, the symplex formation with PDADMAC or copolymers mostly results in flocculated precipitates [27,150,169]. Highly ordered mulilayer assemblies were prepared by alternate reaction of PDADMAC and various polyanions [170,171]. Recently, the efficiency and selectictivity of protein separation via PEL coacervation were examined using PDADMAC [172]. 

This method used the physicochemical properties of polymers like chitosan, which is insoluble in alkaline pH medium and therefore precipitates/coa-cervates when it comes in contact with alkaline solution. Particles are produced by blowing chitosan solution into an alkali solution like NaOH using a compressed air nozzle to form coacervate droplets (Agnihotri et al. 2004). 

Polymers may be induced to encapsulate other molecules by a variety of means (Risch and Reineccius, 1995) as diverse as dipping, spray-drying, extrusion, evaporation, and coacervation each technique has its special applications, strengths, and weaknesses. Advantages in common are the protection and slow release of the encapsulate. In any of the mechanisms, a coagulable polymer precipitates around a core of labile material. Polysaccharides are regular encapsulating polymers (Risch and Reineccius, 1995) acacia gum is particularly efficacious because of its protein content. 

Coacervation. If an oil phase is emulsified in a polymer water solution, and the polymer is precipitated (for example) by changing the pH, the polymer precipitate (coacervate) has a tendency to accumulate at the interface. This is the coacervation process called simple if one polymer is involved and complex if two polymers are involved. If the coacervation is obtained by dropping one polymer solution into a polymer solution of opposite charge, this is termed interfacial coacervation, or polyelectrolyte complex formation . 

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