Colloids Complex coacervates

Ducel, V., Richard, J., Saulnier, P., Popineau, Y., Boury, F. (2004). Evidence and characterization of complex coacervates containing plant proteins applications to the microencapsulation of oil droplets. Colloids and Surfaces A Physicochemical and Engineering Aspects, 232, 239-247. 

Weinbreck, F., de Kruif, C.G. (2003). Complex coacervation of globular proteins and gum arabic. In Dickinson, E., van Vliet, T. (Eds). Food Colloids, Biopolymers and Materials, Cambridge, UK Royal Society of Chemistry, pp. 337-344. 

Complex coacervation (3) can be induced in systems having two dispersed hydrophilic colloids of opposite electric charges. Neutralization of the overall positive charges on one of the colloids by the negative charge on the other is used to bring about separation of the polymer-rich complex coacervate phase. 

Kruif, C.G. de., Weinbreck, F., and Vries, R. de (2004). Complex coacervation of proteins and anionic polysaccharides. Curr. Opin. Colloid Interface Sci. 9, 340-349. 

Sato, H. Nakajima, A. Complex coacervation in sulfated polyvinyl alcohol-amino acetalyzed polyvinyl alcohol system. I. Conditions for complex coacervation. Colloid Polym. Sci. 1974, 252 (4), 294-297. 

In the forementioned laboratories, we started with a new strategy based on phase separation in order to prepare natural particles. Simple or complex coacervation methods involving proteins or protein and polysaccharide mixtures3 were used to create new matrices dedicated to controlled release applications. The colloidal carriers produced were in the micrometre or nanometre size range depending on the substrates or the methods used. Wheat proteins, gliadins, were implicated in simple coacervation to produce nanospheres. Controlled release experiments with model compounds were conducted in order to evaluate... 

Burgess, D. J., Practical analysis of complex coacervate systems. J. Colloid Interf. Sci., 140 (1990) 227-238. 

Coacervation consists in separating from the solution the colloidal particles that agglomerate into a separate liquid phase called coacervate. Coacervation can be simple or complex. Simple coacervation involves only one type of polymer with an addition of strongly hydrophilic agents to the colloidal solution. For complex coacervation, two or more types of polymers are used. Active molecules are entrapped in the matrix during coacervate formation by adjusting precisely the ratio between the matrix polymer and the entrapped molecule (Figure 38.2). 

Genuinely liquid coacervates can indeed be produced in which a macromolecular colloid of the linear type takes part besides a corpuscular protein, for example, the complex-coacervation of serum albumin (positive) — gum arabic (negative). See p. 233, Fig. 2. 

Again in the mixture of the same two colloids simple or complex coacervation can appear according to the circumstances, a striking example of which we have still to discuss. See p. 255 8. 

Complex coacervation is dealt with more extensively in the Chapter, Complex Colloid Systems , Ch. X p. 335. For a brief characterisation see however the right-hand column of the summary on p. 256. 

The said apparent equivalent weights play a great part in complex coacervation of oppositely charged colloids, and explains for instance the shift in optimal mixing ratios by altering the pH (see p. 322, 6b and p. 359, chapter X, 2i). 

Bungenberg de Jong and coworkers found this also to hold in those cases of "hydrophilic colloids, in which coacervation occurs ("complex-coacervation , see p. 338, Chapter X, 2). An example is given in Fig. 41, giving the electrophoretic... 

If one wants a provisional characterisation — which however leads one to expect too concrete ideas — one could say that complex systems are salt-like combinations either of colloids among themselves or of colloids and micro ions whereby one has not committed oneself to the character which the system may have in addition according to the classification principle chosen in Volume II (sols — colloid crystals —coacervates — flocculi — gels). 

Now similar to the nomenclature in la (p. 336) one can again go over to a binary nomenclature for the characterisation of a complex system, in which first the variant and then the nature of the system is mentioned. Of each of these variants one can thus foresee sols, colloid crystals, coacervates, flocculi and gels. 

To distinguish this from other types of coacervation (p. 250 and 255, Ch. VIII, 5 and 8), this was called complex coacervation. The prefix complex is hereby meant to express that the two colloids which form the coacervate together with water, have united as a result of a contrast of charge. 

One can in fact also have the case that a coacervate contains two colloids because each of these colloids, if present alone, forms a coacervate under the conditions of the experiment and these two coacervates are mutually miscible. Such a coacervate can be called a mixed coacervate. In 21 we shall even encounter examples in which two complex coacervates form a mixed coacervate containing three colloids (see p. 378). 

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. 

In the case also when one studies complex coacervation by means of turbidity measurements (a method which can be employed at smaller colloid concentrations) one arrives at similar results. Compare Fig. 3 and 4, which refer to isohydric series of mixtures of 0.05% sols. 

When we continue to cling to the nature of complex coacervation as a process which is allied to salt formation between gelatin cations and arabinate anions, it will be clear that not one single equivalently constituted colloid-colloid salt of this kind exists but a whole series whose composition depends on the pH. (This is further confirmed in the discussion of the analytical composition of the complex coacervates. See p. 359, 2i). 

We conclude from Fig. 6 that optimum coacervation and reversal of charge coincide very closely. The various points fit with the view of complex coacervation as the separation of a colloid-colloid salt, which at the reversal of charge point is composed of equivalent (or nearly equivalent) quantities of gelatin cations and arabinate anions (see p. 326, Ch. IX, 6c). 

A simple explanation of these disintegration phenomena is obtained by assuming that the two colloid components in the complex coacervate are not really bound into a rigid salt but that the gelatin cations and the arabinate anions are displaceable in an electric field. The gelatin cations will move in the direction of the cathode, the arabinate anions in the direction of the anode. If further we assume that these colloid... 

If one regards the complex coacervate as a sightly soluble compound of polyvalent colloid ions, one can understand the occurrence of the double valency rule as increase of solubility as a result of the shielding of the colloid cations by the anions of the added salt and of the colloid anions by the cations of the added salt. 

Fig. 22. Changes in the colloid composition of complex coacervate and equil- ibrium liquid in an isohydric (pH 3.5) series of mixtures of equally concentrated (2%) sols of gelatin (G) and gum arabic (A).

Fig. 24. Changes in the colloid composition of complex coacervate and equilibrium liquid on varying the pH while the mixing proportion (1 1) of the 2% gelatin and gum arabic sols remains constant.

Fig. 26. Scheme in the discussion of the influence of the colloid concentration of the isohydric sols on complex coacervation ). See text. 

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