Carboxyl Colloid

Rubilon Nichias 68 27 5 aluminum carboxylates colloidal silica... 

The dissociation of the various groups can be more or less strong according to the pH (and the electrolyte content) of the medium. Thus for instance the carboxyl colloids of group A in strongly acid medium are practically uncharged. 

On the middle curve are situated Na arabinate, Na pectinate, Na semen lini-mucilage and Na pectate all having carboxyl groups. They will be called carboxyl colloids. 

Thus Carboxyl colloids possessing only carboxyl groups, — the latter having a rather low dissociation constant — will lose their negative charge relatively easily by lowering the pH. 

At pH values not far from neutrality (pH 5—8) these different values of the dissociation constants no longer play a rdle, ionisation being practically complete even in the case of carboxyl colloids. ... 

Fig. 14 gives the ion spectra for three carboxyl colloids. The data for the third one, Na-pectate, are very incomplete because of technical difficulties. 

Fig. 14. Reversal of charge spectra of some carboxyl colloids (see text). ...

Comparing the results of carboxyl colloids with those of phosphate colloids, we may state that in the points 1 and 3 the resemblance is close, and that only in the points 2 and 4 is a different behaviour met with. 

As in carboxyl colloids UO shows no exceptional position with respect to the divalent ions of the B subgroups of the Periodic System. 

Comparing the results with those of the phosphate colloids, we may state, that in all four points the reverse is obtained. The differences with the carboxyl colloids are also considerable, as only point 4 and the sequence of the alkali — and alkaline earth cations in point 2 are alike. Still we may consider the ion spectra of the carboxyl colloids as intermediate between the two extremes of phosphate and sulphate colloids, though the carboxyl colloids stand much nearer to the phosphate colloids than to the sulphate colloids. 

Fig. 17. Reversal of charge spectra of Na pectate (carboxyl colloid) and of egg lecithin and alcohol soluble soya bean phosphatide (phosphate colloids) with alkali chlorides (and NH4CI).

After having discussed at some length the cation spectra of phosphate and sulphate colloids, we can be brief in discussing those of carboxyl colloids. 

Compared with the carboxyl colloids hitherto mentioned (arabinate, pectinate, pectate), the points 1, 2 and 4 are the same. 

It is even by this choice of 6, 5, 4, and 3 valent complex cations, that in 1 d the sulphate colloids differ only quantitatively from the phosphate and carboxyl colloids. For in this case the fundamental difference in polarisability between sulphate groups on the one hand and phosphate and carboxyl groups on the other has no, or practically no, influence on the results. 

If it were possible to use in Table 2 (p. 270) only ideal 6, 5, 4, and 3 valent mono-atomic cations (La and Ce being the only ideal cases of 3 valent cations, A1 and Th already behaving as large complex ions, see p. 291, and the still higher valent cations not existing at all) then the sulphate colloids would behave totally otherwise than the phosphate and carboxyl colloids (horizontal rows consisting presumably only in minus signs). 

The reversal of charge spectrum of SiO resembles fairly much that of sulphate colloids (p. 285, Fig. 15), that of TiOa the reversal of charge spectrum of carboxyl colloids (p. 284, Fig. 14). 

The ion spectrum of SiOs resembles nearly that of a sulphate colloid, the ion spectrum of TiOo that of a carboxyl colloid. It follows from the ion sequences that SiOg is less, TiOg is more polarisable than water. 

It is very interesting, that the sequence of the alkali cations is quite the reverse of that characteristic of the carboxyl colloids of 2d (seep.284, Fig. 14), further that the sequence obtained with the alkaline earth cations is also different, a characteristic transition sequence occurring. All this shows that the ionised carboxyl group in proteins is more polarisable than the same group in the carboxyl colloids of 2d. We have here once more a characteristic example of constitutional influences on the polarisability of the ionised groups, discussed already in 2k (p. 292). 

For this investigation a sulphate colloid (Na agar), a carboxyl colloid (Na pectinate) and three phosphate colloids (Na yeast nucleate, purified egg lecithin, and a soya bean phosphatide fraction soluble in alcohol) were used. 

The sequence of the curves is however quite the same as in colloids in which all or a larger number of reversal of charge points could actually be reached. The ion spectra of the latter colloids, viz, egg lecithin and soya bean phosphatide (phosphate colloids) and Na pectinate (carboxyl colloid) are given in Fig. 26, 27 and 28. 

The experimental results show that a simultaneous resemblance of ephedrine and Ca on the one side and of acetylcholine and K on the other side is present neither in sulphate colloids (agar) nor its substitute (Si02), nor in carboxyl colloids (pectinate) nor in the phosphate colloid nucleate but only in phosphatides. 

Compare the order phosphate > sulphate > carboxyl in the flocculability of acidoids with large organic cations (see note 2 on p. 405). Since proteins also appear here as organic cations the above series might be the sequence of the strength of the individual salt bond in the combinations of positive proteins with phosphate, sulphate and carboxyl colloids. 

There is strong evidence in favour of the point of view 1. in the variants colloid anion + micro cation or colloid cation -f micro anion in the specific ion sequences for reversal of charge or for coacervation or flocculation. We remind the reader for example of the sequences Cs < Rb < K < Na < Li which occur with sulphate colloids and carboxyl colloids (p. 289). Here polarisation phenomena are still in the background and here the largest ion, that is to say, the least hydrated ion, is most suitable for reversal of charge or coacervation. This points strongly therefore to a direct contact between cation and ionised group of the colloid. 

Now carrageen, pectate and nucleate have not, it is true, the same equivalent weights, but these are at least of the same order of magnitude and the observed differences in intensity of the tricomplex flocculation (strong, weak, absent) can indeed hardly be attributed to this. An explanation is however found in the different polaris-ability of the ionised groups of these colloids (carrageen = sulphate colloid pectate == carboxyl colloid nucleate = phosphate colloid). 

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