| Ion. | n. | a. |
| Hydrion or hydroxylion | 20 | 10 |
| Kation of a metal | 15 | 7 |
| Organic anion | 10 | 8 |
| Inorganic anion | 6 | 6 |
For the sake of simplicity we can assume that these ions are all monovalent. The ions adsorbed by unit mass will then be 10c(1/20), etc. If these hypothetical adsorption isotherms be plotted as usual we get the fairly typical curves shown in Fig. 1.
Now in practice there are always two of these ions, each giving its own specific effect in opposite senses, and the difference (a1c(1/n1)-a2c(1/n2)) represents the nett charge adsorbed. Hence we have the following combinations:—
| Inorganic acid | 10c(1/20)-6c(1/6) |
| Organic acid | 10c(1/20)-8c(1/10) |
| Alkali | 10c(1/20)-7c(1/15) |
| Inorganic salt | 7c(1/15)-6c(1/6) |
If we plot these values of nett adsorption against the concentration we obtain the curves shown in Fig. 2.
Fig. 2.
On the assumption that the nett charge adsorbed is the dominant factor in determining the maximum swelling at equilibrium, one must therefore regard the curves of Fig. 2 as representing the changes in volume of the swollen gel as the concentration is increased. Now in type these curves correspond to those obtained by experiment from hydrochloric acid, acetic acid, caustic soda, and common salt. The maximum swelling with hydrochloric acid increases rapidly with the concentration at first and then rapidly decreases, though not at such a great rate. The swelling with acetic acid increases less rapidly and to a less maximum, but decreases more slowly. With common salt there is a slight swelling followed by contraction. Caustic soda gives a rapid increase in volume at first, afterwards much less so, and finally yields an exceedingly slow decrease. The correspondence of these facts with the type-curves inevitably suggests that the phenomenon of swelling might be accounted for, in part at least, along these lines.
Of course it is not likely that the simple figures selected for the illustration of the argument are either relatively or absolutely correct. Thus we know that the adsorption curve for hydrions and hydroxylions are not likely to be quite identical, as assumed above. As gelatin is primarily slightly positive, it is probable that the values of a and of n for hydrion adsorption will be relatively slightly greater. The relative values supposed, however, are near enough to illustrate the contention that the type of the maximum volume curve can be explained on this assumption of different adsorption isotherms for each of the ions.
If the remarks on the compression of the continuous phase be recalled, it will be obvious that in the present paragraphs we have been giving the question of equilibrium-volume a rather one-sided consideration. The volume of the gel when equilibrium is established may be determined in type by the nett charge adsorbed by the disperse phase, but it will be modified also by the lyotrope influence of the particular substance on the continuous phase. When gelatine swells in solutions the influences on both phases are always in operation, and either upon occasion may become predominant. In the case of neutral organic substances, such as cane sugar, the lyotrope influence is the determining factor. In the case of neutral salts the predominant influence is decided by the place occupied by the salts in the lyotrope series. If at either end of the series the lyotrope influence is uppermost and the effect of ionic adsorptions is practically swamped. Thus sodium sulphate and sodium iodide hinder and promote imbibition respectively as could be expected from their strong lyotrope power. On the other hand, in the case of sodium chloride, which has comparatively feeble lyotrope influence, the relatively different adsorptions of its ions comes to the fore. With acids and alkalies the relatively large adsorption of the hydrion and hydroxylion causes this to be the predominant influence, but we must concede the possibility that purely lyotrope influences may be at work in some cases, and especially at the greater concentrations. Indeed, it is sometimes a difficult problem to decide whether an increase or decrease in swelling is due to lyotrope or adsorptive influence, but, broadly speaking, we can expect strong lyotrope effects at either end of the series and also at large concentrations, and we can expect strong adsorptive effects in dilute solutions, in the middle of the lyotrope series and in the case of alkalies and acids.