On the Theory of Gels. IV
Samuel Clement Bradford · Biochemical Journal · 1923
THE investigation of the physical properties of the natural emulsoids has produced an enormous literature.For the correlation of this, and for the study of many problems of biology and industry a theory of the sol-gel trans- formation is needed which fits the facts.The present condition of uncertainty is illustrated by the multitude of mutually incompatible ad hoc theories of gel structure which are current.The many hypotheses may be classified in three groups: (1) one-phase or molecular systems, (2) two-phase liquid-liquid systems and (3) two-phase liquid-solid systems.To theories in the first class it must be objected that they are unable to explain the loss of mobility that occurs on setting.Those in the second group have the additional difficulty that no liquid-liquid systems can be imagined which would have the elastic properties of gels [Hatschek, 1917].Nor is there any direct evidence for hypotheses in either of these two categories.Thus we are obliged to assume that gels have a solid phase.This idea is the most natural and was the earliest to be held [Frankenhein, 1835].The suggestion of K. von Niigeli [1858] is well known, that gels are composed of molecular complexes or micellae with crystalline properties, in the interstices within and between which the water is held by molecular attraction.It has been inferred that von Niigeli intended a geometrical framework, and attempts have been made to devise a network that would account for the elastic and thermal properties of gels.If, however, it is realised both that the elastic properties of gels differ greatly and that the directive forces inherent in the ultimate particles of gels are probably different, it seems unlikely that a single framework could be found to account for the different properties of different gels.It is more probable that the structure of gels varies according to the nature of the gel substance.Biitschli's hypothesis [1892, 1896, 1898, 1900] that gels have a honeycomb structure is disproved by the work of Zsigmondy [1911], Anderson [1914] and Bachmann [1917] on the vapour pressure isotherms of gels, which shows that they must contain fine pores with a radius of from 2X5 to 5pt, some 300 times smaller than Biitschli's honeycombs.These experiments are particularly interesting as they give an explanation of van Bemmelen's [1878, 1880] curious hysteresis cycle.From microscopic work on soap curds and gels, Zsigmondy and Bach- mann [1912] have favoured a fibrillar structure.On the other hand McBain [1920] considers that identical colloidal particles are present in the sol and gel