Colloid equivalent weight

Indeed in the field of the complex colloid systems it is just the characteristic charge elements of the colloid (equivalent weight, polarisability of the ionised groups) and of the ions (valency, radius, polarisability) which play a decisive role. It is for this reason that these charge elements were already discussed in detail in chapter IX (p. 259). 

Alongside the colloids are given the reciprocal hexol numbers (p. 270, as a measure of the colloid equivalent weight) determined on the colloid preparations used. 

It then appears that beside the colloid equivalent weight another specific factor is present which depends on the nature of the ionised group of the colloid anion. One must distinguish between phosphate, carboxyl and sulphate colloids and for the same colloid equivalent weight the flocculability also decreases in this order. 

Since now however we saw in 3d that other colloids can indeed be brought by 3 — 1 or by 3 — 1 and 2 — 1 to dicomplex coacervation of flocculation, it is obvious to suppose that with gum arabic these salts do indeed bring about true complex relations but that these — in view of the relatively high colloid equivalent weight — are not considerable enot h to be able to force the separation as coacervate or flocculi against the forces which otherwise hold the arabinate in solution. 

The abnormal behaviour of the pho hatides is closely connected with the association character of these colloids. Their negative charge is due to impurities of an acid nature (probably phosphatidic acids) which are tenaciously attached to the actual phosphatide molecules by London — v. d. Waals forces. The relative amount of them compared with the actual phosphatide molecules (amphoion ) determines their colloid equivalent weight, the magnitude of their reversal of charge concentrations with cations and the position of the isoelectric point (see p. 295). 

The soya bean phosphatide insoluble in alcohol has a very low I. E. P. and also a very low colloid equivalent weight, with which is connected the fact that even in aqueous medium (without addition of auxiliary substances, see note 3, p. 405) complex flocculation occurs with 3 and 2-valent cations. According to the N P ratio = 2 1 one can here expect a considerable admixture of phosphatidic acid. 

In the two other combinations where the colloid equivalent weights differ appreciably, morphological pictures can occur (see fig. 9) which show more analogy to the typical pictures in the combination G 4- A -f- N. 

Chapter ix, entitled Reversal of Charge phenomena, Equivalent Weight, and Specific properties of the ionised groups, not only concerns the phase boundary of coacervates, but also the boundary of floccules and even the boundary of adsorbed colloid films on particles (e.g., on SiOe). 

Fig. 1. Macromolecular colloids of acidic nature arranged in a series of increasing linear charge density (decreasing equivalent weight) considered at a pH, at which the ionogenic groups are completely ionised, rectangles monomeric residues, black dots ionised groups (negatively charged).

The linear charge density increases by way of C and D to E (limiting case) to a maximum and the equivalent weight decreases to a minimum. If the ionisation is decreased by lowering the pH (for example COO —y COOH), then one and the same colloid can pass through a displacement of its charge density or equivalent weight in the opposite sense (for example E —> D —> C —> B — A or C—> B A). 

As will be shown in the followii sections the value of the equivalent weight is already of the greatest importance in determining the differences in the behaviour of different colloids. 

Table 1 shows that in the colloids investigated the equivalent weight increases twelvefold and that the reciprocal hexol number also increases to the same order. But as with Na-arabinate the latter is always smaller than the former. Thus apart from a systematic difference — which is... 

In column 3 this association colloid has been charsLCterized by its apparent equivalent weight (p. 273, 1 f) and it then shows quite the same behaviour as the other substances which are macromolecular colloids (p. 188). 

The colloids with very low reciprocal hexol numbers show flocculation or coacervation with 6—1, 5—1, 4—1, 3—1 and in some instance even with 2—1. Thus it seems that the equivalent weight (which multiplied by approximately 0.85 is the reciprocal hexol number) is an important factor in determining flocculability with salts of the types considered here. 

With decreasing equivalent weight of the colloid flocculability increases, in so far as flocculation or coacervation is already realisable with lower valent cations the lower the equivalent weight (see however also p. 295, 2 m),... 

Fig. 5. Relation between tendency to flocculation or coacervation and reciprocal hexol number (upper) or equivalent weight of the colloid anion (lower). (See text).

We may speak of an apparent equivalent weighty playing for colloid behaviour towards added salts a role similar to the true equivalent weight, if not all ionogenic groups are ionised but only a fraction of them. 

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). 

Further the magnitude of the apparent equivalent weights of both colloids is here of great importance, the interaction being the more intense, the lower the apparent equivalent weight (see p. 374). 

Proteins taking part in the formation of tricomplex colloid systems (see p. 415), act however preferably as amphoions. Therefore the above no longer applies here, for at the most favourable pH, namely the the apparent equivalent weight is... 

The results obtained show that from a physico-chemical point of view the division of the biocolloids considered accordii to the composition of rhe ionised groups into phosphate, carboxyl and sulphate colloids is a fertile one. Their behaviour depends not only on their equivalent weight, but also on the said composition. 

Both apparent equivalent weights are functions of pH and to get an impression of the relative change of these equivalent weights with pH, we can use the electrophoresis pH curves of both colloids. See Fig. 44. 

The particular form of curves similar to that of Fig. 43 must in principle be calculable from data on the apparent equivalent weight (see p. 273) of both colloids involved. 

If for instance two colloids are used, which show a practically constant equivalent weight in a certain pH range, then in curves analogous to Fig. 43, a horizontal tract must occur, that is the combining ratio of the two colloids must in a certain pH range be independent of pH. The electrophoresis — pH curve of clupein, a basic protein, shows in a certain pH range an electrophoretic velocity independent of pH (see Fig. 45) Thus it may be expected that in certain colloid mixtures, in which clupein is taken as the positive component, the above mentioned form of curve may occur. 

A second possibility for such curve forms is of course if in a certain pH range the equivalent weights of both colloids are changed (both decreased or both increased) in such a way that their ratio is not altered. 

From this we may conclude, that also quantitatively a charged colloid (here Na arabinate) behaves towards an oppositely charged one (here clupein) in the same way as a large organic ion (here germanin), but with the simplification that the systematic difference between reciprocal anion number and equivalent weight is much smaller (or perhaps absent) if the anion in question is a colloid anion. 

The complex relations are more intense the smaller the equivalent weight. One thing and another agree with the ideas already developed in 2 o (p. 370). The complex is stronger the greater the number of bonds of salt-like nature per gram of colloid-colloid salt. 

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