Composite coacervate drops

As a result of this partial miscibility the two coexisting coacervates contain all three complex components. The one besides gelatin contains mainly A together with a little iV the other besides gelatin contains mainly N together with a little A, To distinguish them these coacervates can be denoted by the symbols G + -4 + n and G + iV + a respectively, whereby the components mainly present are therefore denoted by capital letters. In Chapter XI, If-h (p. 438) we discuss further the mutual wettir properties of these coexisting coacervates, as a result of which drops of G + + 72 and of G + AT + a unite to form composite coacervate drops. 

Fig. 5. Composite coacervate drops consisting of two coexisting complex coacervates (124 x lin.). Enclosed drops belong to the G -f N a coacervate (weakly vacuolised), the surrounding coacervate shells to the G -f- A + n coacervate.

Fig. 6. Position of inclusions (a) or of gelation vacuoles (b) in composite coacervate drops. The inclusions or vacuoles are situated at the boundary of the two coexisting coacervates.

If we assume that the position of the vacuoles in Fig. 6 b really represents an equilibrium position at an instant shortly before the gelation, at which therefore the two coacervates were still liquid, it then follows necessarily from this that the hypothesis tacitly assumed up to now, as regards the complete wetting of the enclosed G + N + a coacervate by the surrounding G -f- A + n coacervate, is in need of revision. This hypothesis was indeed based on the microscopic picture which these composite colloid bodies exhibit when observed vertically (p. 438, Fig. 5). We are here confronted with the same difficulty which we have mentioned already in the beginning of 1 e (p. 437) for inclusions with horizontal observation one observes the picture B of Fig. 3 (p. 437) for our composite coacervate drops. 

G+N+a/E G+A+n/E which we have already deduced above (relation (3)) from the morphological picture of the composite coacervate drops. 

The typical morphological pictures of composite coacervate drops discussed up to now are encountered in total mixtures within the ellipse which lie on or near the reversal of charge line (for example a in Fig. 8). The contrast in colloid composition of the coexisting coacervates is the greatest here. 

Finally we may mention that composite coacervate drops are also known in which the complex coacervates belong to the type colloid anion -f micro cation (p. 384, Ch. X, 3). They are produced for example with hexol nitrate in a mixture of sols of ... 

The morphological pictures of these composite coacervate drops show that in this case also the equiv- ... 

Fig. 9. Composite coacervate drops consisting of two coexisting complex coacervates of the type colloid anion + micro cation (308 X linear).

The G + N + a coacervate drops which are surrounded by a shell of G + A + n coacervate in composite coacervate drops (see p. 438), also behave like inclusions. If the G + A - - n coacervate is electrophoretically negative, then the enclosed coacervate drop is displaced inside the G 4- A + n coacervate shell in the direction of the anode if the surrounding coacervate is positively charged, then it is displaced towards the other side. If the surrounding coacervate is at its reversal of charge point then the enclosed coacervate is not displaced. 

The enclosed G + N -1- a drop in the composite coacervate drops can also transform into a hollow sphere in still another way than addition of salt 2. 

F%. 25. Changes of state in the composite coacervate drops on addition of much distilled water, a) original state in which the G + N -f- A coacervate (indicated throughout in the figure by N) was weakly vacuolised. a, c, d and f reproduce changes discussed in the text-vacuolation occurring when the pH is lowered in d). 

Coacervate drops may be converted into a layer or a flocculent precipitate on standing, but they may be reconverted into drops and solutions. This reversibility may have definite limits, because it is related to the conditions under which the coacervate was formed and its chemical composition. For instance, if the composition of a coacervate includes serum albumin, the number of conversions is usually limited to three or four, after which it is irreversibly denatured. 

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. 

For explaining that the former is smaller than the latter, we must assume that this coacervate of equivalent composition still carries a negative charge at the surface of the coacervate drops. 

If hexol arabinate situated at the surface of the coacervate drop has the tendency to dissociate hexol ions such a negative charge of the surface can indeed arise. We thus arrive at the following formulation D for the coacervate of equivalent composition ... 

The dotted portions of the curves (clear mixtures measured with SiC particles) join on to the continuously drawn portion (measured on small separated coacervate drops), which points to the formation of a complete film containing both colloids on the surface of the SiC particles. The intersections of the two curves (measurements at and cell height respectively) with the level U == 0 coincide, which shows that the glass wall also is covered with a complete film, the composition of which is the same as that present on the surface of the coacervate drops (interaction product of G and A). 

For the explanation of the properties (shape, changes of state, etc.) of a colloid body of microscopic dimensions which consists of a limited amount of coacervate surrounded by equilibrium liquid, one has in general to take into account 1. the properties of the three-dimensional contents and 2. those of the two-dimensional surface. The first mentioned properties (e.g. viscosity, composition) can be studied on coacervates in bulk and these have already been described in previous chapters (VIII and X). The properties of the two-dimensional surface are equally important. The electrophoretic charge of complex coacervates drops has already been discussed in chapter X 2e (p. 345) but for this chapter it is the interfacial tension which is especially of interest. 

In Fig. 24a the G + A + n shell is already abolished before the remainii G +N + a coacervate drop is transformed into a hollow sphere. In Fig. 24b on the other hand the enclosed G -h N + a coacervate drop can transform into a hollow sphere while retaining the peripheral coacervate shell. The differences between the salts used must be associated with displacements in the material composition of the coexistit] coacervates, in fact we know that salts do this (p. 365) and that the salts arrange themselves in this case in the sequence of the so-called continuous valency rule (p. 452-455). 

The cell walls are permeable to some relatively low molecular colloids so that they are more or less rapidly washed out from the cell compartments. This is for example the case with yeast nucleic acid, as a result of which the composition in a cell containing G -I- N -F A changes continually. As the coalescence of very fine coacervate drops to the states represented in Fig. 39 requires a comparatively long time and as the composition of the content of the cell must lie within narrow limits (c.f. Ch. X, 2t, Fig. 33, p. 380) the realisation of satisfactory morphological pictures meets with serious difficulties and indeed has only succeeded one or two times. Moreover the states of Fig 39 do not represent the final equilibrium but in the end the coacervate G -h N f A disappears completely through continued loss of nucleic acid. 

The pH can also determine the type or composition of the coacervates formed from a three-component mixture, such as gum arabic, gelatin, and yeast nucleic acid, for example. In such a coacervate, at pH 3.36, the drops consist mainly of gum arabic and gelatin at pH 3.48, of gelatin, gum arabic, and nucleic acid at pH 3.8, of gelatin and nucleic acid. 

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