December 22, 1923] 



NA TURE 



907 



layer. Other things being equal, the probability P of 

 an encounter leading to coherence will be a diminishing 

 function of the electric intensity tt of the similarly 

 constituted double layers, i.e. dVld-w will be negative. 

 Hence, of the total number of encounters in a given 

 small period of time, the number which lead to coherence 

 should be a maximum at the point of zero potential 

 difference (iso-electric point of Hardy). 



Now the experiments of Powis brought out the very 

 important fact that when the interfacial P.D. (whether 

 positi\e or negative) is above a certain value, which 

 was about o'o3 volt for his conditions, the rate of 

 ( oagulation or coherence of the oil drops is relatively 

 small, but rapidly increases when the P.D. falls inside 

 the zone -0*03 +o'o3volt. Under definite conditions 

 there exist, therefore, what we may, speaking broadly, 

 • all a critical potential and a critical potential zone. 

 When the P.D. is outside this zone the emulsion is 

 comparatively very " stable." Very small concentra- 

 tions of electrolytes, which, as we have seen, increase 

 the P.D., increase this stability. As soon as the 

 <;oncentration of any electrolyte is sufficient to bring 

 the P.D. into the critical zone, the stability of the 

 emulsion undergoes a sudden and very marked decrease, 

 and relatively rapid coagulation occurs. Take, for 

 example, the case of thorium chloride. On increasing 

 the concentration we find that the interfacial P.D. 

 traverses successively the following regions : 



(i) Above the critical value (and negative). 



(2) Inside the critical zone (negative and positive). 



(3) Above the critical value (and positive). 



(4) Below the critical value (and positive). 



In exact correspondence with this series we find that 

 the emulsion goes through the following states : 

 (i) Stable (oil particles " negative "). 



(2) Unstable and flocculating (oil particles negative 



or positive). 



(3) Stable (oil particles positive). 



(4) Unstable and flocculating (oil particles positive). 

 Here we see a very striking analogue and explanation 



of the phenomena observed by Joly in studying the 

 effect of aluminium salts on the sedimentation of clays, 

 and of the numerous examples of the so-called "irregular 

 series " observed in the flocculation of suspensoid 

 hydrosols by salts with polyvalent cations. 



As Linder and Picton showed, when two suspensoid 

 hydrosols, one negative and the other positive, are 

 mixed, then, depending on the ratio, a stable hydrosol 

 (either positive or negative) can be obtained. In 

 continuation of this work, W. Biltz demonstrated the 

 existence in such cases of a " zone of coagulation," 

 i.e. a zone of concentration ratios leading to coagulation. 

 A study of the mutual behaviour of a negative oil 

 emulsion and the positively charged ferric oxide 

 hydrosol provides a complete explanation of this 

 curious phenomenon. When increasing amounts of 

 the iron oxide hydrosol are added to the oil emulsion, 

 it is found that the interfacial P.D. falls to zero, and 

 then reverses its sign, becoming increasingly positive 

 — an action which is due to the adsorption of the 

 positively charged micelles at the oil-water interface 

 When the P.D. is above a certain value (positive or 

 negative) the system is stable. But within the critical 

 zone a rapid and relatively complete mutual coagulation 

 takes place. 



NO. 2825, VOL. I 12] 



These studies of oil emulsions (and of the glass-water 

 interface), by means of the micro-cataphoresis method, 

 have thrown a great deal of light on many previously 

 ill-understood points in the theory' of colloids. The 

 following table contains the concentrations (in millimols 

 per litre) of certain electrolytes required to reduce the 

 potential of a certain hydrocarbon oil emulsion from 

 its " natural " value (against pure water) of o'o46 volt 

 to the critical value, 0*03 volt : 



These results show the enormous influence of the valency 

 of the cation in a series of salts with the same univalent 

 anion, and explain in a striking manner the analogous 

 effects in the coagulation of lyophobic hydrosols. The 

 exact value of the critical potential and the range of 

 the critical zone will depend, of course, on the experi- 

 mental definition of " rapid coagulation," and on the 

 concentration, nature, and degree of dispersion of the 

 hydrosol. It is not to be supposed, therefore, that 

 these critical values are constants except under very 

 definite conditions. The fundamental fact is that 

 under given conditions the rate of coagulation of the 

 particles of an oil suspension or of a lyophobic hydrosol 

 undergoes a relatively sudden and very great increase 

 when the interfacial P.D. falls below a certain finite 

 value (positive or negative). 



In discussing the " stabilities " of hydrocarbon oil 

 emulsions, it must not be forgotten that I was dealing 

 with very dilute suspensions of oil in water, produced 

 by mechanical agitation without the addition of any 

 " emulsifier." I pointed out that in the emulsification 

 of oils in water by means of soap, the soap lowers the 

 interfacial tension and concentrates at the interface. 

 When we wish to produce oil emulsions in the ordinary 

 sense of the term, we must use some such emulsifying 

 agent, and for this purpose many substances are 

 employed, such as soap, gum acacia, gelatin, casein, 

 starch, etc., etc. All these substances concentrate or 

 condense on the surfaces of the oil globules. If we may 

 regard these surface films as very mobile from the 

 molecular-kinetic point of vdew, it is clear that they 

 will confer an increased degree of stability on the 

 emulsion. 



It is probable, however, that the stability of the 

 emulsion is in many cases due to the fact that the surface 

 films possess a very viscous, quasi-rigid, or gel-like 

 character, so that a more mechanical explanation is 

 necessary. As S. U. Pickering showed, oils may be 

 emulsified in water by the gels of certain basic salts ; 

 and A. U. M. Schlaepfer has shown that emulsions of 

 water in kerosene oil may be obtained by means of 

 finely divided " carbon." Nevertheless, even in cases 

 where an emulsifier is used, we may hope to succeed 

 in obtaining a more precise physical analysis of the 

 system. It is interesting in this connexion to note 

 that Mr. W. Pohl has recently found in my laboratory 

 that when a neutral hydrocarbon oil is emulsified in 



