74 SOAPS AND PROTEINS 



VI 



DEFINITION OF HYSTERESIS, SWELLING, LIQUEFACTION, 

 GELATION CAPACITY, SOLVATION CAPACITY, SYNERE- 

 SIS, SOL 



We should now like to emphasize how this concept of the 

 changes which the soaps suffer in passing from liquid sols to dry 

 gels may help to explain a number of the " strange " character- 

 istics of colloid systems. 



1. A first question under this heading is that of the nature 

 of hysteresis, more particularly that observed when a colloid is 

 subjected to changes in temperature. The importance of the 

 thermal history of a colloid system is constantly stressed. It 

 is generally true of the lyophilic colloids that when subjected 

 to heat manipulation they tend to hold fast to the characteristics 

 of their previous states. A colloid on cooling, for example, first 

 sets at a certain temperature; yet the same colloid after setting, 

 fails on reheating to liquefy at this temperature it usually 

 first " melts " at a higher one. In fact it may be said quite 

 generally that the curve showing the increase in viscosity of a 

 lyophilic colloid with lowering of temperature is rarely identical 

 with that portraying the decrease in viscosity when the temper- 

 ature is raised through the same range. If the fact is remembered 

 that the absolute values of two mutually soluble substances are 

 rarely the same, and that the rates at which they go into solution 

 in each other are usually different, many of these difficulties 

 disappear. Figs. 48 and 49 show diagrammatically not only 

 what happens when the temperature of a solution of soap in some 

 solvent is lowered but also, in the lower halves of the pictures, 

 the effects of warming a gel. Increasing the temperature of the 

 original solvated-soap shown in region Z increases the solubility 

 of the soap in the water, and so the colloid dispersion Y results, 

 consisting of soaped-solvent in solvated-soap. Further increase 

 in temperature yields the regions X and W, but, because of the 

 persistence of the solvated-soap as the external phase, all these 

 regions continue to show a rigidity or viscosity higher than that 

 of systems of the same quantitative composition produced by 



