Junction-points spectrum

Fig. 8. Various types of junction points arranged in the order of decreasing cohesive energy (junction point spectrum).

Some changes to which the gel is subjected, owing to varying reactions or conditions like swelling, contraction, temperature change and so on, may be expressed in terms of a shift of its junction point spectrum. 

This implies that a narrow junction point spectrum is required Gels in which the spectrum is broad, (ranging from lower to higher energies of cohesion) will be devoid of thixotropy. 

In odier words, we may say that the junction point spectrum of the gel (cf. p. 497) is chaiiged in the corresponding sense. The idea suggested by Hofmeister, the... 

Freshly prepared gels often show a tendency to contract their volume decreases and solvent is spontaneously pressed out. This phenomenon has been termed syneresis by Thomas Graham. We have seen that, after the setting of a gel, equilibrium condition is usually not attained and further processes, generally embodied in the term ageing , occur. Recrystalli ation and the formation of more junction points, or, in general terms, an extension of the junction point spectrum, may have a bearing on these phenomena. They are, however, of a rather complex nature and can by no means be considered as fully elucidated. 

A similar effect could be reached in drying gelatin and casein gels. These effects, which are of some interest from a technical point of view, can be interpreted in that the junction point spectrum of the gel is broadened and more permanent junction points, resistant to the action of water, are being formed as a result of the special treatment. The phenomenon can be classified as representing a hysteresis of contraction. 

Since macromolecular systems will generally tend to form broad spectra of junction points, thixotropy will only occur under special conditions giving rise to a narrow spectrum, as e. g., at great dilution or in the initial stages of gel formation, when the first chain to chain contacts are being formed and no recrystallisation has, as yet, occurred to an appreciable degree. 

The concept solubility is used here in a somewhat different sense from the usual We may perhaps rather express it thus the spectrum of stable junction points between the molecular chains is shortened the border line of solubility shifts towards the chain to chain contacts of greater energy. 

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