Sols and Gels — Relation to Protoplasmic Structure 91 



to produce thixotropy. Alcohol added to a suitable iron oxide sol 

 makes it thixotropic.--^ We are probably dealing with a coagulation 

 due to "dehydration." These iron oxide sols may be considered to 

 be sufficiently hydrophilic to allow one to expect such effects of 

 dehydration as are discussed by Kruyt and Bungenberg de Jong 

 in their theory of the stability of hydrophilic sols and of coacerva- 

 tion.-^ If the alcohol is removed from the thixotropic gel by exposing 

 it to sulfuric acid in a desiccator, the gel is liquefied to a sol. 



All these results show that substances causing a certain degree I 

 of coagulation lead to the formation of thixotropic systems. Inversely, 

 a strong peptizing agent, making the particles independent of each 

 other, can transform a thixotropic system (having the proper par- 

 ticle size) into a dilatant one. A technical dispersing agent, "Horn- 

 kem," a sulfonated product of vegetable origin, applied in a suitably 

 high concentration in aqueous solution, acts upon ZnO particles in 

 such a way that it produces a strongly dilatant suspension parallel 

 with a maximum degree of dispersion.-' In pure water, the same 

 ZnO produces a very stiff paste without any indication of dilatancy. 

 In this case it can even be observed that thixotropy requires a 

 medium degree of coagulation: In pure water the degree of cluster- 

 ing is too great to allow a thixotropic behavior of the suspension; 

 if, however, the right amount of a dilute solution of Hornkem is 

 added, the paste becomes thixotropic, whereas at higher concen- 

 trations of Hornkem a state of high dispersion and dilatancy is 

 reached.-' 



It fits in well with these results that a dilatant paste can be 

 transformed into a thixotropic one by causing a certain degree of 

 coagulation of the particles. Hydrophilic colloids at low concentra- 

 tions favor the coagulation of hydrophobic particles, the phenome- 

 non of sensitization; whereas at high concentration they protect the 

 hydrophobic system. In this way, lecithin at low concentration may 

 coagulate aqueous quartz suspensions to a certain degree-'^; this is 

 proved by the marked increase in the rate of sedimentation of the 

 clustered particles after lecithin has been added. Only in this range 

 of sensitization by the lecithin are these quartz suspensions found 

 to be thixotropic, whereas they are dilatant in pure water. 



The following facts cannot, presumably, be applied to biological 

 phenomena, but they deserve to be mentioned briefly, nevertheless. 

 The thixotropic or dilatant state depends strongly on the dispersion 

 medium. Whereas suspensions of quartz or of intact starch grains 



