616 



SCIENCE 



[N. S. Vol. XLIX. No. 1278 



absorption capacities for the difierent alcohols 

 lie lower, but their order remains the same 



If we attempt to say why we obtain these 

 typical colloid systems from such a variety of 

 materials we may begin with the fundamental 

 and now generally accepted conclusion that 

 colloid systems result whenever one material 

 is divided into a second with the degree of 

 subdivision coarser than molecular. A sus- 

 pension colloid results whenever the colloid- 

 ally dispersed phase is not a solvent for the 

 " dispersing medium " ; a hydrophilic or lyo- 

 philic colloid whenever the dispersing medium 

 is such a solvent (and independently of the 

 fact that the subdivided phase is solid, liquid 

 or gaseous at the temperature employed). 

 When soap is dissolved in acetone and the 

 temperature is lowered the soap falls out as a 

 coUoidally dispersed suspension colloid because 

 the acetone is not soluble in the soap; but the 

 same soap dissolved in an alcohol, toluene or 

 carbon tetrachloride, comes out as a lyophilic 

 colloid because these solvents are soluble in 

 the precipitating soap. 



But the physical characteristics of the ulti- 

 mately resulting system are not yet explained 

 when we have thus taken into account the 

 mutual solubility characteristics of their 

 phases. In any given case, as with a given 

 soap and its " solvent," four possible results 

 and consequently four main types of ultimate 

 system may be foreseen. At the top exists a 

 non-colloid, " molecular " or " ionized " " solu- 

 tion" of soap (soaped-solvent). For example 

 may be cited a fairly concentrated solution of 

 soap at a higher temperature. At the bottom 

 is found another " solution " but of the sol- 

 vent in the soap (solvated-soap). Between 

 these extremes exist two main types of mixed 

 systems, namely, one below the top which is 

 a dispersion of solvated-soap in soaped-solvent, 

 and another, above the bottom, which is a 

 disi)ersion of soaped-solvent in solvated-soap. 

 These are respectively the sols and gels about 

 which we talk. A concentrated solution of 

 soap in any solvent, it will at once be apparent, 

 passes successively, on lowering of the tem- 

 perature and when not too much solvent is 



used, from the top of this series through the 

 two middle zones to the bottom. 



All the systems below the true solution at 

 the top and above the true solution at the 

 bottom are " colloid." Gel formation is char- 

 acteristic of the middle zones. Such gels are 

 " dry " anywhere below the point where enough 

 solvated-soap falls out on lowering the tem- 

 I)erature to yield a continuous external phase 

 enclosing the soaped-solvent. Just above this 

 point they sweat, the amount of such " synere- 

 sis " obviously increasing progressively as the 

 amount of solvated-soap becomes inadequate 

 to form a continuous external phase. If the 

 " syneresis " is very great we no longer apply 

 the term, for the syncretic liquid (soaped- 

 solvent) now forms the continuous external 

 phase. The colloid system is said to have re- 

 mained or to have passed into the " sol " state. 



Since change or rate of change in tempera- 

 ture (as well as other factors) affects the 

 solubilities of the two phases in each other 

 unequally it is obvious that the sum total 

 of changes in any system need not be identical 

 at any given moment and at any given tem- 

 perature when the temperature is' being ap- 

 proached from a higher level with the smn 

 total of these same changes when the same 

 temperature is being reached from a lower 

 level. The attainment of equilibrium takes 

 time and so the systems hold over the char- 

 acteristics of the systems from which they 

 came. This is the " hysteresis " of lyophilic 

 colloid systems. 



m 



The effects of adding different hydroxides 

 and different neutral salts in increasing con- 

 centration to standard soap " solutions " has 

 received further study. In order to under- 

 stand the effects observed and their explana- 

 tion it is well to divide the experimental find- 

 ings into three groui>s while keeping in mind 

 the solubility characteristics of the pui-e soaps 

 themselves in water and for water.^ 



1. Soaps are formed more soliible in the dis- 

 persion medium. The viscosity of the soap 

 mixture regularly falls. This happens when 



2 See our previous paper, Martin H. Fischer and 

 Marian O. Hooker, Science, 48, 143, 1918. 



