66 



NATURE 



[March 28, 19 18 



engineering science to provide the material and 

 shape it. But lead and direction are required of 

 theory by preliminary calculation to show how to 

 make use of our resources to the best advantage. 

 As Bacon said : *' Experiment not directed by 

 Theory is blind. And Theory unsupported by Ex- 

 periment is uncertain and misleading." 



G. Greenhill. 



COLLOIDS AND CHEMICAL INDUSTRY.^ 



COLLOID chemistry, in its widest sense, deals 

 with chemical processes which occur in the 

 immediate neighbourhood of surfaces — that is, 

 chemical effects which are brought about as a 

 result of capillary and electrocapillary forces. 

 Such effects are necessarily limited to a small 

 range, the thickness of the capillary layer being 

 of the order lo"^ to 10-'^ cm. It is obvious that 

 these effects can become of importance only if the 

 surface area itself is very large. Under ordinary 

 conditions, in which two fluid masses in bulk are 

 separated by a definite surface — as in distribution 

 phenomena — the capillary effects are too small to 

 be observed. To magnify the effect it is usually 

 necessary to realise a state of affairs in which 

 one phase is distributed in a state of fine sub- 

 division or "dispersed" through the other phase 

 or medium. In these circumstances the total inter- 

 facial area is enormously great. We find such 

 conditions in the case of fine suspensions (diameter 

 of particle 10-* cm. approx.), emulsions (diameter 

 of particle lo-^ cm.), and colloid solutions 

 (diameter of particle lO"^ cm. approx.). Colloidal 

 solutions are systems in which the solute in- 

 dividuals or solSy though apparently soluble, have 

 not broken down to the 'molecular limit, but consist 

 instead of aggregates, composed roughly of 

 several hundred molecules or atoms. Such 

 soluble aggregates or sols will not diffuse through 

 membranes (as Graham showed in his original 

 work on the colloidal state), and thus differ 

 markedly from the behaviour of dissolved crystal- 

 loids, e.g. salts. 



The most fundamental problem in connection 

 with such disperse systems is the problem of thei'- 

 stability. It is evident that uniformity in size of 

 the particles plays an important part in this con- 

 nection, as do also the electric charge and the 

 Brownian movement which each particle possesses. 

 The methods whereby the equilibrium is disturbed 

 are equally remarkable and characteristic. A very 

 minute amount of electrolyte added to a stable col- 

 loidal solution may bring about complete precipi- 

 tation or flocculation of the sol, the sol separating 

 out in a gelatinous form known as a gel. In some 

 cases, and possibly in all — though this is a dis- 

 puted point — such precipitation may be reversed. 

 A closely allied phenomenon is that known as 

 "peptisation," in which a substance, normally 

 insoluble in a solvent, may be made to dissolve 

 by the addition of a peptiser. This is illustrated 



1 First Report of the British Association Committee on Colloid Chemistry 

 and its Industrial Applications. (1917.) 



NO. 2526, VOL. lOl] 



by the stabilising or protective effect produced by 

 a small quantity of gelatine (itself a colloid) upon 

 solutions of colloidal metals, and also by the well- 

 known phenomenon met with in the case of the 

 hydroxides of zinc and aluriiinium which "dis- 

 solve" in excess alkali. Experiment has shown 

 that the alkali may be dialysed away and the pep- 

 tised colloidal hydroxide reprecipitated. Such 

 phenomena depend essentially upon selective ad- 

 sorption or surface condensation of certain parts 

 of the peptiser (usually the hydroxyl ion) upon the 

 suspension or colloid. Gibbs showed, many years 

 ago, that, as a thermodynamic necessity, any sub- 

 stance (solute) which lowers the surface tension 

 of the solvent is positively adsorbed at the surface 

 — ^that is, the concentration of the solute is greater 

 in the surface layer than it is in the bulk of the 

 solution. This phenomenon lies at the basis of 

 many technical operations, such as dyeing and 

 tanning, though, of course, other effects of an 

 irreversible character enter later. 



Another important surface phenomenon is that 

 known as electrical endosmose. If a liquid be 

 divided into two parts by means of a porous par- 

 tition or membrane, and an electromotive force 

 be applied across the partition, the liquid will be 

 found to pass through the membrane, the direction 

 of motion depending upon the electrical state of 

 the partition in relation to the liquid and its con- 

 stituents. By a suitable choice of membrane and 

 solution certain constituents may be separated 

 from others, e.g. crystalloids from colloids, or 

 certain colloids may be precipitated and others 

 left in solution. 



Surface effects, the realisation of colloid equi- 

 librium, electrical neutralisation, preferential ad- 

 sorption, peptisation, colloid precipitation, imbi- 

 bition or swelling of gels, electrical endosmose, 

 and other phenomena of a similar nature might 

 at first sight appear to have little significance for 

 industrial operations and processes, although their 

 importance has already been recognised to a 

 certain extent in other directions, e.g. in agri- 

 cultural processes (quality of soils, retention of 

 salts, emulsions for crop spraying, etc.), in geo- 

 logical formations, and in biological problems (cell 

 contents, nature and permeability of cell-walls, dis- 

 tribution of electrolytes, blood serum, coagulation 

 of proteins, enzyme action, etc.). That colloidal 

 phenomena enter into numerous technical pro- 

 cesses may be demonstrated by a brief enumera- 

 tion of some industrial operations which depend 

 fundamentally upon what we may call the prin- 

 ciples of colloid chemistry. 



We have already instanced dyeing and tanning. 

 We find further that colloid chemistry plays a 

 fundamental part in certain stages of soap manu- 

 facture ; in washing and scouring processes, in 

 connection with textile fabrics, hides, skins, and 

 in fur dressing; in mercerlsation and finishing; 

 in the manufacture of photographic materials; in 

 the treatment of cellulose and wood pulp in paper 

 manufacture ; in paper sizing and colouring (car- 

 bon and other copying papers); in the treatment 



