December 2^, 1920J 



NATURE 



547 



Colloid Chemistry. 

 By I'KOK. W. C. McC. Lewis. 



COLLOID chemistry — the science which deals with 

 the phenomena occurring at the interfaces which 

 separate two contiguous phases, such interlaces being 

 very large in extent relatively to the actual masses ot 

 the phases themselves — stands in a rather peculiar 

 position. Numerous colloid problems are encountered 

 Dy workers who in many cases fail to realise the 

 significance of their observations from the point of 

 view of colloid chemistry itself. The scope of the 

 subject is so wide that it is not surprising to find 

 the literature exceedingly scattered, and part of it 

 not easily accessible. Alany of the results and ob- 

 servations are rendered difficult to interpret, or even 

 lose the greater part of their value, through insuffi- 

 cient attention having been paid to considerations 

 which a general knowledge of colloid chemistry as a 

 science would have suggested. For similar reasons 

 there has been a considerable amount of overlapping 

 and, in some cases, a distinct lack of agreement in 

 results, which probably has its origin in unsuspected 

 dissimilarity in the conditions employed. With the 

 literature, scientific and technical, in this state it is 

 evident that considerable good might be done by 

 attempting to correlate such observations and results 

 with the object of impressing the essential unity of 

 the subject upon those engaged in problems apparently 

 diverse. Such an attempt at co-ordination has been 

 made by the Colloid Committee of the British Asso- 

 ciation, which has just issued its third report.' 



This report, following the plan already adopted in 

 the two previous reports, contains the subject-matter 

 arranged under two heads : (i) classification accord- 

 ing to scientific subject, and (2) classification accord- 

 ing to industrial process. Under the first head the 

 following subjects have been dealt with : Colloid 

 Chemistry of Soap Solutions; Ultra-microscopy; 

 Solubility of Gases in Colloidal Solutions ; Electrical 

 Charge on Colloids; and Imbibition of Gels. Under 

 the second head we find : Industrial Applications of 

 the Imbibition of Gels ; Colloid Problems in Bread- 

 making; Colloid Problems in Photography; Cellu- 

 lose Esters ; Colloid Chemistry of Petroleum and 

 of Asphalt ; Varnishes, Paints, and Pigments ; and 

 Clays and Clay Products. 



Merely to read over the list of industrial subjects 

 dealt with in this and in the two previous reports is 

 sufficient to indicate the extraordinarily wide technical 

 field in which colloid considerations play a decisive 

 part. .At the same time one cannot but be struck 

 by the predominance of empiricism in almost all 

 cases, and, consequently, the urgent need there is 

 for each industry to investigate its own colloid 

 problems on scientific lines. The Committee will 

 nave done something if it succeeds in emphasising 

 this fact. 



Unfortunately, at the present time the retort may 



"11 be made that broad scientific principles in col- 



. id operations and processes are few, and that even 



'he simplest colloid phenomenon is bv no means com- 



:!rtely understood. By way of illustration, let us 



Itc the case of the stability of a colloidal metal in 



dispersing medium such as water. .Are we to take 



it as true that small particles consisting of pure 



metal only arc capable of perm-inent distribution in 



a pure solvent, or is it always necessary to have some 



" impurity " present to stabilise the system ? We know 



that a finelv divided metal such as that con.sidered 



r)«p«r1m«nt nf >< 

 for th« Ad Tanc 

 l|« C«n«rlil An 



on«ry OffiLc, .,. 



nl Rm«arch. RHihh Amotiallon 



■siwrt an Colloid ChomUtrr •nd 



Pp. U + 154. (txiiKlon: MM. 



NO. 2669, VOL. 106] 



carries an electric charge, but we have not any notion 

 of its magnitude; and although we know that it can 

 be discharged, the mechanism is not altogether clear. 

 Ooes the stability of the colloid system depend on 

 the e.vistence of the charge alone or upon the magni- 

 tude of the interfacial tension round each individual 

 particle, or upon both etfects; and, if so, in what 

 manner? Is there a real limiting size to such par- 

 ticles, and, if so, what opposing agencies determine 

 the size and how do they operate? In the peptisation 

 of such particles, say by the addition of gelatin, does 

 the peptiser form an actual coating round each 

 individual? If so, what are its properties, and what 

 relation does the amount of bound peptiser bear to 

 that still left free in the liquid? We know that 

 peptising effects are highly specific. What deter- 

 mines the specific nature of the effect? .Again, what 

 is the mechanism of coagulation, and whv are 

 certain coagulations reversible and others not? 

 Numerous queries of this kind naturally occur even 

 in connection w ith a problem which is' onlv one of 

 many included in the term "colloid chemistry," and it 

 may be said with confidence that to answer at all 

 adequately some of these queries will require many 

 years of intensive scientific investigation. Further, 

 it must be borne in mind that several technical pro- 

 cesses in active operation at thfe present time depend 

 directly upon such phenomena as stability of dis- 

 persed systems, peptisation, and coagulation. It is 

 obvious that effective control and ultimate extension 

 of such industries must involve a scientific knowledge 

 of what is taking place. 



Let us turn for a moment to some of those aspects 

 of colloid phenomena upon which we may be said 

 to possess some real qtwntitative , though necessarily 

 incomplete, knowledge. First of all we have Willard 

 Gibbs's theoretical investigation of the influence of an 

 interface upon tlie concentration of gaseous and dis- 

 solved substances. .A small section of Gibbs's work 

 has led to the onlv quantitative law of adsorption by 

 capillary forces which we possess, and it seems not 

 improbable _ that other generalisations still lie un- 

 heeded in his famous essay. Next we have Donnan's 

 quantitative investigation of the effect of a chemicallv 

 inert membrane upon the distribution of electrolvtos 

 when electrolytic colloidal material is present on one 

 side of the membrane, leading to the phenomena of 

 membrane hydrolysis and membrane potentials. To 

 this we must add the application of Donnan's method 

 of treatment in the hands of Procter and of Wilson 

 to the imbibition of gels and the process of vegetable 

 tanning. Next we have the Quantitative investiga- 

 tions and generalisations of McBain. of Bavliss. and 

 of Pauli upon the composition and behaviour of col- 

 loidal electrolytes, such as the soaps and the proteins, 

 as a function of the dilution of the svstem. We have, 

 too, the work of Perrin. of Gee, and of Bancroft on 

 the mechanism of electrical endosmoso. We have 

 the beginnings of a science of electro-capillarity in 

 the work of Lipomnnn and of firlmholtz. Our know- 

 ledge of the molecular and atomic mechanism of con- 

 densation upon surfaces h.is been greatlv increased 

 by the novel ideas of Langmuir and of Hnrklns — a 

 subjert of the utmost importance in connection with 

 heterogeneous catalysis generally. We know some- 

 thing, though not a great deal, about such phenomena 

 as the viscosity of colloidal solutions, the rigidity of 

 surface films, the mechanism of luhrirnfion, the 

 Liesepang phenomenon, and other tvpirallv colloid, 

 chemical prohlrms. The fact is that an enormous 

 field rxicts the •srope of which is hv no means realised 



