Sept., 1922
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
795
Recent Chemical and Technological Advances in O u r Knowledge of Gelatin and Glue By Robert Herman Bogue INDUSTRIAL FELLOW, MELLON INSTITUTE OF INDUSTRIAL RESEARCH, PITTSBURGH. PA.
T
H E PAST few years have witnessed advances of a truly startling nature in almost every branch and subdivision of theoretical and applied chemistry, and the chemistry of gelatin has not lagged behind in this forward sweep. The gelatin ionogenic equilibria and their consequences have been studied from many a n g l e ~ ~ -by ~~~* Loeb, Smith, Sheppard, et al., Kuhn, Miss Lloyd, Procter, et al., Pauli, Davis, et al., Fischer, Bancroft, Bogue, and others. Theories of the structure of gelatin, or of elastic gels in general, have been discussed by Bradford, Hatschek, Barratt, Miss Lloyd, Zsigmondy, Robertson, Fischer, Bancroft, McBain, et al., Holmes, Sherrer, Loeb, and Bogue. Analytical data upon the composition have been accumulated by Dakin, and by Bogue. Still other aspects of the question have been presented by Alexander, Elliott and Sheppard, Thomas, Seymour-Jones, Mees, and others. Lastly, as an index of the awakened interest in gelatin and glue, books are in press or being prepared upon this subject by Sheppard, Alexander, Thiele, and Bogue.
IONOGENIC EQUILIBRIA
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The work which stands above all others R H. in this field is the very important series of investigations by Jacques Loeb.1 His accomplishments appear to be two in number: First, the protein colloids, as gelatin, casein, egg albumen, etc., are shown to be amphoteric bodies whose general behaviorisexplainable by an application of the classical laws of electrolytic dissociation and valency; and, second, the special colloid behavior of these bodies and the degree of such action are shown to be determined by an application of the Donnan membrane potential law. Gelatin is found to exist in three states: as non-ionogenic or isoelectric gelatin; as a metal gelatinate; and as a gelatin salt. At one definite hydrogen-ion concentration, namely, N, or pH 4.7, gelatin can combine with neither anion nor cation and is un-ionized. At a pH greater than 4.7 it can combine only with cations forming a metal gelatinate, as sodium gelatinate, and a t a p H less than 4.7 it can combine only with anions forming a gelatin salt, as gelatin chloride. Loeb has more recently shown that the swelling, the viscosity, and the osmotic pressure effects produced by varying the ionogenic equilibrium may be adequately explained and predicted by an application of Donnan's membrane potential law.32 By calculating from Nernst's formula for concentration cells a t 24" C., p. d. =0.059 log
a c1
where p. d. is the potential difference between the two sohtiOnS in which c1is the H-ion concentration inside the gelatin
* Numbers in text refer to reference list at end of article.
solution and CZthe same in the outside solution (separated by a semipermeable membrane). By comparing the observed and the calculated p. d. lrom the measurements of p H in the two solutions, Loeb obtained very satisfactory concordance. Procter and Wilsone had previously made use of the Donnan equilibrium to explain the swelling of gelatin. The viscosity of two-phase systems has been studied by H a t ~ c h e k ,who ~ ~ calculated formulas to cover the two main conditions of loosely packed and closely packed systems. The viscosity of gelatin sols was studied by Boguell and conformity with Hatschek's formulas could not be obtained. The value K which denoted the volume occupied by unit weight of the dispersed gelatin phase was found to rise to a maximum and thereafter decrease upon increasing the gelatin concentration. The increase was explained as due to increasing hydration, and the decrease to competition of the force of surface tension when the film separating the particles becomes very thin. Swelling and gelation were investigated by Rogue1' by the addition of increasing amounts of sodium hydroxide and several BOGUE silicates of sodium, and by Davis, Oakes and Brownes by the addition of acids and alkalies. The pH of maximum viscosity was found to be a t 3 to 3.5 on the acid side, and a t 8 to 9 on the alkaline side. If the dibasic calcium salts are not washed out, however, the rise on the alkaline side is less marked. Davis, et al., even failed to observe any rise at p H greater than 6.0. Sheppard, et C C ~have . , ~ investigated the effects which temperature, concentration, and H-ion concentration have upon the viscosity, jelly consktency, and melting point of gelatins. In this connection they have devised new apparatus for these measurements, which is more scientific than any before described. The jelly strength is measured by exerting a torque upon a cylinder of the jelly and noting the angle of twist a t break, and the breaking load. The results obtained have shown a great disparity of uniformity. It seems that the only conclusion which may be drawn thus far is that the several gelatins used differed from each other in such a way that while each produced a typical curve, these bore no exact or parallel relation to each other, and the variitions in elasticity were not R simple function of the concentration or the pH. THEORIES OF STRUCTURE Differences of some importance continue- with respect to the structure assumed to exist in gelatin gels and sols. Procter6 pictures a compound of gelatin with a n acid as a coherent mass from which the gelatin cannot diffuse 01 separate. The net structure is advocated by Robertson,16 and a honeycomb or sponge structure by Rancroft.lo
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THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
Bradford1%affirms the existence of a definitely crystalline structure in gelatin gels, claiming to have obtained gelatin crystals, but Schemer19 has found no trace of interference figures of the characteristic space lattice arrangement upon examining gelatin gels by means of the Rontgen photograph. Fischerg continues to urge his hydration theory, in which he regards gelatin gel as a solution of solid hydrated gelatin in gelatin solntion, or, if the solvent is limited, a reversal in type of system is indicated and there is produced solid hydrated gelatin holding within itself gelatin solution. Miss Lloyd5 has proposed that gelatin gel consists of a solid and a liquid phase, both of which are continuous. Gelation occurs only on the cooling of a sol which contains in solution isoelectric gelatin and gelatin salts in equilibrium with free electrolytes. A catenary or filamentous structure for both gels and sols was urged by Bogue20 in 1920 and again in 1922. According to this theory, gelatin sols appear to consist of slightly hydrated molecules united together into short threads resembling streptococci floating about in a dilute gelatin solution. A lengthening of these threads seems to take place as the temperature fdls, and at the same time the water-absorbing powder of the gelatin increases. This accounts for the rapid increase in viscosity with drop in temperature. A solid jelly will result only when the relative volume occupied b y the swollen molecular threads has become so great that freedom of motion is lost, and the adjacent heavily swollen aggregates cohere. The rigidity seems to depend on the relative amount of free solvent in the interstices of these aggregates, and on the amount of solvent that has been taken up by the gelatin in R hydrated or imbibed condition. The resiliency or elasticity is probably dependent upon the length and number of the catenary threads. LoeW has recently attempted to explain the differences in the behavior of different proteins upon treatment with water and electrolytes by an occlusion theory, The idea is advanced that proteins form true solutions in water which in some instances (e. g., in gelatin sols) contain, side by side with isolated ions and molecules, submicroscopic solid particles capable of occluding water, the quantity of which is determined b y an application of the Donnan equilibrium. Those substances which change this equilibriunl in the direction of an increased number of solid particles a t the expense of the molecularly dispersed particles will produce an increase in viscosity and a decrease in osmotic pressure, while the reverse holds equally true. CHEMICAL CONSTITUTION An accurate determination of the amino-acid constitution of proteins has been unknown until very recently. Fischer, Levine and A n d e r ~were ~ ~ able to recover only 42.1 per cent of the total nitrogen of gelatin in the amino acids by Fischer’s esterification method, and Skraup and von Biehler35 only 70.7 per cent. By an important modification of Fischer’s method, DakinZ1in 1920 recovered 91.31 per cent of the total nitrogen of gelatin in the amino acids. By employing the Van Slyke method36 for thc separation of amino-acid groups, BogueZ2has shown that the gelatins or glues obtained by long action of the water on the stock differ from the material that is first extracted by containing in the solution the hydrolgrsis products of proteins other than gelatin, as mucin, keratin, elastin, etc. These Rolutions, as well as the products obtained by the partial hydrolysis of pure gelatin, were shown to contain also increasing amounts of proteose, peptone, and amino acids. The first extractions are nearly pure solutions of the protein gelatin, but the last extractions contain larger amounts of proteose than protein, and these are derived to an increasing extent from noncollagen (non-gelatin) material.
Vol. 14, No. 9
TECHNOLOGY The advances in the technology of gelatin and glue have not been so spectacular as in the more theoretical aspects. This is due partly to the disposition of manufacturers to keep secret such improvements as are discovered, and partly to the relatively small number of able colloid chemists who are directly interested in the technological phase of the problem. A number of patents have been taken out upon improved machinery for glue- or gelatin-making. The old method of drying by allowing the liquid to gel in pans, and then cutting the jelly with wires, has been largely replaced by more efficient processes. The Kind-Landesmann machine37 permits the liquid to run onto a wide endless belt which conveys the liquid through a cool chamber where it gels, and on emerging a t the opposite end is transferred to the frames of mire or cloth net for drying. Spray drying has been tried out with some success, and seems to be a highly desirable innovation, as the liquid is rapidly converted into a fine, dry, white powder. Steam-roller drying is used for some products. Improved machinery has also been devised for many other of the manufacturing operations. Continuous extracts for obtaining the gelatin from the stock are of special importance, as much higher grade material may be obtained. A new system for the evaluation of glues and gelatins has been proposed by B o g ~ e . A ~~ method for evaluation that is based upon some fundamental property is needed very sorely, for the methods in common use are both incompetent for accurate differentiation, and the results are purely arbitrary and expressed in terms that mean little or nothing except to the salesman. The proposed system regards the undergraded gelatin content and (for glues) the maximum joint strength obtainable as the most fundamental properties. These are found to be parallel to the viscosity a t a temperature near the melting point. The method consists in a measurement of the viscosity a t 35” C. of an 18 per cent solution (moisture-free basis) by means of the MacMichael viscosimeter. The results are expressed in centipoises, and the designation of grade referred to this value. But the forward movement has only begun. We are in an era of chemical progress in industry, and what advance has already been made is but the early glow of that which is later to become the full day. REFERENCES 1-Jacques Loeb, J . Gen. Physiol, 1 (1918-9), 39, 237, 363, 483, 559; 2 (1919-20), 87; 3 (1920-21), 85, 247, 391, 827; 4 (1921-22), 73, 97, 107. “Proteins and the Theory of Colloidal Behavior,” New York, 1922. 2-C. R. Smith, J . A m Chem. Soc., 4 1 (1919), 135, 43 (1921), 1350; J . Ind. Eng. Chem., 12 (1920), 878. 3-S. E. Sheppard, S. S. Sweet and F. W. Scott, J . Ind. Eng. Chem., 1 2 (1920), 1007; S. E. Sheppard and S. S. Sweet, Ibid., 13 (1921), 423; J . A m . Chem. SOG., 43 (1921), 539; S. E Sheppard and P. A. Elliott, Ibzd., 44 (19221, 373; S. E. Sheppard, Nature, 107 (1921), 73. 4-A. Kuhn, Kollozdchem Beihefte, 14 (1921), 147. 5-D. J. Lloyd, Biochem. J., 14 (1920), 73. 6-H. R. Procter, J . Chem. Soc, 106 (1919), 316; Collegium, 1915; H . R. Procter and J. A. Wilson, J . Chem. S o c , 109 (1916), 307; J A. and W. H. Wllson, J . A m . Chem. S O L ,40 (1918),886 7-Wo. Pauli, Trans. Faraday Soc., 9 (1913), 54; Report of the Faraday and Physical Soc., 1921, 14; Wo. Pauli and Hirschfeld, Bz’ochem. Z.,62 (19141, 245. 8-C. E. Davis, E. T. Oakes and H. H. Browne, J . A m . Chem. S O G . , 43 (19211, 1526; E. E. Davis and E. T. Oakes, Ibid., 44 (1922), 464. 9-Martin Fischer, “Soaps and Protelns,” New York, 1921. l&W. D. Bancroft, “Applied Colloid Chemistry,” New York, 1921. Ii-R. H. Bogue, J . A m . Chem. Soc., 43 (1921), 1764; J . Ind. Eng. Chem , 14 (1922), 32. 12-S. C. Bradford, Biochem. J., 12 (1918), 382; 14 (1920), 91. 13-E. Hatschek, Report of Faraday and Physical Soc., 1921, 35. 14-J. 0. W. Barratt, Ibzd., 1921, 49. 15--Zsigmondy, Ibid., 1921, 51. 16-T. B. Robertson, “The Physical Chemistry of the Proteins.” 1918, 325.
