Coacervate layer

Homogeneous, transparent solutions of proteins, carbohydrates, and other compounds can separate into two layers, one depleted and one enriched with these compounds. The process of separation of macromolecules into discrete entities is termed coacervation. The layer rich in molecules of the dissolved substance, referred to as the coacervate layer, actually consists of liquid "drops" or spherical microcapsules. The equilibrium liquid, which is the medium adjoining the coacervate layer, always contains less substance than the original solutions. The discrete liquid droplets resulting from macromolecular interactions might be made to serve as pseudocells from which pseudo tissues might be derived to constitute a restructured food. 

Viscosity. Since coacervates are heterogeneous liquid systems with a nonhomogeneous distribution of substances, the viscosity of a coacervate layer differs markedly from that of an equilibrium liquid. The viscosity of the equilibrium liquid is lower than the viscosity of the solutions from which the coacervate was obtained, whereas the viscosity of the coacervate layer or drops is higher than that of the initial solutions. The decrease of the viscosity of the equilibrium liquid results from the decrease in the total volume of the particles, since they concentrate into larger coacervate drops. Coacervates are formed most completely when the viscosity of the equilibrium liquid is lowest. 

The volume occupied by the coacervate drops is generally 3 to 5% of the volume of the entire coacervate system. An increase in the concentration of the solutions from which the coacervate is formed leads to an increase of the total volume occupied by the coacervate layer. For example, when 1% solutions of protein and carbohydrate were used to form coacervates, the volume of the coacervate layer was 5.31% of the total volume of the coacervate system, whereas when 4% solutions of these substances were employed, 26.2% of the volume was occupied by the coacervate drops. A negative temperature coefficient is characteristic of coacervates whose composition includes fats and fatty acids the higher the temperature, the smaller the volume occupied by the coacervate layer. 

Salts can not only break down coacervate layers and drops, but also promote their enlargement. The effect of salts on the formation of coacervates from lecithin and carrageen is seen in Figure 4. 

Desolvation of water-insoluble macromolecules in nonaqueous solvents leads to the deposition of a coacervate layer around aqueous or solid disperse droplets. Table 8.13 lists both water-soluble and water-soluble macromolecules which have been used in coacervation processes. Desolvation, and thus coacervation, can be induced thermally and... 

The latter system, corresponding to the coacervate layer, may be called a colloid liquid. As to the nature of the equilibrium between both systems of macro-units, we can characterise it as a liquid-vapour equilibrium of the macromolecular substance. 

Fig. 4), which latter coalesce more or less readily (see Fig. 2 and on p. 444 Fig. 11) and may under favourable conditions even coalesce in a relatively short time (order of hours) into one clear homogeneous colloid-rich liquid layer (coacervate layer, see Fig. 3). 

A thermometer inserted in the thermostat behind the glass vessel can be seen through both layers. The displacement which the thermometer shows in going from the colloid poor layer (upper) to the coacervate layer (lower) results from a difference in refractive index of the two layers. 

The transitional shapes are again readily deformable under the cover glass by pushing it backwards and forwards under pressure and they can also unite to an apparent homogeneous coacervate layer by centrifuging (up to a certain limit). 

If in the last two cases one adds water carefully (lowering of the alcohol concentration) the coacervate again becomes less viscous and the coalescence to a coacervate layer again becomes easier. 

The flocculate is transformed into a coherent coacervate layer after a sufficiently long time (See p. 234 1 c). 

Furthermore the composition of coacervate and equilibrium liquid is completely determined at a given total composition of the system. Coacervate layer and equilibrium liquid are in thermodynamic equilibrium the way by which one arrives at any particular final composition of the total system is of no influence on the composition of coacervate and equilibrium liquid. 

The combination gelatin — gum arabic can be considered as the most favourable object as yet for the study of coacervation. In this case the complex coacervate has relatively little viscosity and consequently readily fuses to a single transparent liquid layer whereby it becomes possible to take samples of coacervate layer and equilibrium liquid and investigate them as regards their composition. The two colloids can be kept in the dry state for unlimited times and show no denaturation phenomena in solution. The only factor to which one must pay attention is the temperature, since one otherwise obtains the complications mentioned above as a result of gelation. 

To characterise the maximum coacervation in an isohydric series of mixtures one can choose provisionally all kinds of criteria, for example, maximum turbidity, maximum volume of the coacervate layer, etc. It is however desirable to have at one s disposal a criterion which is calculable from the analytical results and in which the water content of the coacervate layer itself plays no part. 

If the relative stability of the coacervate sol be removed, for example, by addition of electrolyte the system will flocculate. In the process clusters of very small coacervate drops are formed in the first instance which with sufficient fluidity of those drops may coalesce into larger drops or into a single coacervate layer. 

Complex gels can also be obtained in another — really much more natural —way namely by cooling complex coacervates containing gelatin. If for example the separation into layers has taken place at 40° to a clear equilibrium liquid and clear coacervate layer and one subsequently cools the tube to room temperature both layers become turbid. The coacervate layer becomes solid thereby and on microscopic examination the turbidity is seen to be caused by a large number of small vacuoles in the gel. The equilibrium liquid becomes turbid because some fresh coacervate separates out of it in small drops. These drops also gelate and stick to each other forming loosely built flakes. 

The conditions already discussed are expressed in Fig. 29 from which one can read ojEf at what mixing ratios and pH s of the original sols foam bodies and hollow spheres are formed after shaking the isolated coacervate layer with a certain amount... 

Fig. 37. Complex coacervation of a mixture of gelatin and gum arabic sols enclosed in cell compartments, a The complex coacervate has separated out as drops they coalesce with each other and flow on contact with the wall over it with the formation of a coacervate layer adjacent to the wall.

If one adds KCl to a solution of Na-oleate, the viscosity rises rapidly at a particular concentration so that the system can even gelatinise. If more KCl is added a separation into two layers begins, the upper one of which contains practically all the soap. With rising KCl-concentration this coacervate layer becomes smaller and smaller. 

In this paper, novel method for microencapsulation by coacervation is presented. The method employs polymer-polymer incompatibility taking place in a ternary system composed of two cellulose derivatives, anionic- sodium carboxymethyl cellulose (NaCMC) and nonionic- hydroxypropylmethyl cellulose (HPMC), and anionic surfactant- sodium dodecylsulfate (SDS). In the ternary system, various interactions between HPMC-NaCMC, HPMC-SDS and NaCMC-(HPMC-SDS) take place, which were investigated by detailed conductometric, turbidmetric, tensiometric, viscosimetric, and rheoiogicai study. Interactions were employed to obtain coacervate of controlled rheological properties. Deposition of thus obtained coacervate at the surface of dispersed oil droplets in emulsion and emuision stabiiity were investigated. Emulsions stabilized with coacervate layer of different properties were spray dried and microcapsules in a powder form were obtained. Dispersion properties of microcapsules and microencapsulation efficiency were investigated. 

Influence of SDS concentration on relative volume of coacervate phase in the ternary system, and turbidity of the system is shown in Figure 11. Coacervate layer formation appears at the onset of HPMC-SDS interaction. Relative volume of coacervate phase increases with SDS addition, and reaches maximum at the end of HPMC-SDS interaction (i.e. at PSP). The relative volume of coacervate phase increases on addition of SDS due to increased hydrophilic properties of HPMC/SDS complex, as more SDS molecules are bound to HPMC, which results in better solvation of HPMC/SDS complex. At SDS concentrations larger than PSP the coacervate disappears. On increase in SDS concentration turbidity of the ternary mixtures decreases until it reaches constant value, which is the same as is turbidity of binary HPMC/NaCMC mixture (Measured turbidity of the ternary mixtures having lowest SDS concentrations is low due to fast sedimentation of coacervate during the measurement.). The decrease in turbidity is attributed to increase in HPMC/SDS complex solvation as SDS concentration in the ternary system is increased. 

Figure 11. Changes of turbidity and relative volume of coacervate layer in 1.0% 0.7/0.3 HPMC/NaCMC mixture at various SDS concentrations...

Three distinct regions can be observed with respect to stability of coacervate layer formed around the droplets. Figure 22. These are region of stable coacervate layer (coacervate is adsorbed at the droplets surface during the course of the experiment), region of unstable... 

Figure 22. Influence of SDS concentration on time during which coacervate remains adsorbed at surface of emulsified oil droplets (i.e. the stability of coacervate layer).

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