Points of Gelatin Gels

Meyer and van der Wyck (1937) calculated a value of 21,000 cal. for the molar heat of gelatin from solubility measurements of Mardles (1931) and setting point measurements of Pleass (1930) (see below). However, the solubility measurements quoted probably involved the extrac- 

6ft mplft Soureft Mol. weight pH Ionic strength 0 h Calories per mole Mell ing point for c = 40 g-/ liter C. Reference 

It may be concluded tentatively that the melting point depends but little on pH over a rather wide zone near the isoelectric point provided salt is present or the concentration is quite low. 

Dependence cf the Meliing Point of Odatin OeU upon Concentration of Sodium Chloride and Polaesium Thiocyanate (Cordon, 1916) 

Dependence of Melting Point on Other Added Reagents. In the course of studies of the effects of various substances on the optical rotation cS. gelatin (Section 3, /, page 39), Katz and Wienhoven (1933) noted that a 

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MELTING POINT AND VISCOSITY JUST ABOVE THE MELTING POINT OF GELATIN GELS ACCORDING TO OUWELTJES... 

OuwELTjES on account of his researches on the elastic extension and the liquefaction temperature of gelatin gels as affected by added salts, discusses at length the hypothesis that the partition between the amorphous and the crystalline portions of the gel framework is shifted according to the lyotropic value of the salt. The phenomena to be explained are also consistent with the assumption that there is a corresponding shift either of the number or of the cohesive energies of the non crystalline junction points in the amorphous portion of the gel, or of both. We shall revert to these problems later. 

Kraemer and Dexter found that the Tyndall light of gelatin gels prepared from 1% solutions depends on the pH of the solution, showing a maximum at the iso-electric point. Moreover, different values were obtained when the temperature at which the gelatination was effected was varied. Especially in the isoelectric point, the intensity of the scattered light increased when the temperature of setting was lower. 

In the literature, viscoelastic measuinnents before the gel point of gelatin solutions are largely unreported. As far as the author knows, Djabourov et aL [469] were the first to report measuronents of viscoelastic properties of aqueous gelatin solutions before the gel point (down to G = 7 x 10 N/m ) of a 4.7 /o aqueous solution. In about 3 h the system gradually changes from a Newtonian... 

Ferry [22] found linearity when log c and /T were plotted, where c was the concentration of gelatin and T was the melting point of the gel expressed in absolute temperature. Similarly, they also found linearity between logM and /T. The equation derived from the van t Hoff equation is used to express this linear relationship ... 

Wherever each molecular chain approaches, crystallites are formed, leading to the formation of a 3D network. This structure explains weU the main properties of the gel, such as the melting point, degree of optical rotation, modulus, and rubber-like elasticity. However, a few kinds of networks coexist in the photosensitive material depending on the manufacturing and development methods, the behavior of gelatin gels is very complex. 

The swelling of gels is markedly affected by the presence of electrolytes, this effect being a minimum at the isoelectric point of the material. In general, sulphates, tartrates, etc. inhibit swelling, while iodides and thiocyanates promote the swelling. Thus gelatine disperses completely in iodide solution even at low temperatures. 

It will be noticed that all the properties of the gel with the exception of the turbidity and foam have minimum values at or near the isoelectric point P = 4 7, whilst these two attain their maximum values at this point. Evidently as the data for the swelling and viscosity indicate the hydration of the gelatine particles is at a minimum at the isoelectric point (see Chiari, Bioohem. Zeit xxxili. 167, 1911) where as indicated by the alcohol number it is most readily precipitated from solutions to form large aggregates to which the turbidity and the foam formation are due. 

The influence of neutral salts as well as of acids and bases on the swelling of gelatine which we have seen can be attributed to an apparent change in the solvation of the gel fibrils and may be interpreted in the light of Donnan s theory of the effect of a non-diffusible ion on the osmotic pressure differences between the two phases, is likewise to be noted in the alteration of the viscosity and alcohol precipitation values of protein solutions. From the considerations already advanced there should exist two well-defined maxima in the viscosity and alcohol precipitation curves when these properties are plotted as functions of the Ph, the maxima coinciding with the points of maximum dissociation of the salts... 

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