COLLOIDS, LYOPHOBIC 



ABC 



Fig. 9. Sequence of electron micrographs of carbon replicas showing Ostwald ripening of 

 silver iodobromide emulsion crystals in a solution containing gelatin and ammonium bromide 

 at 50°C. a) immediately b) 5 minutes, and c) 20 minutes after mixing. (By courtesy Messrs 

 Ilford Ltd.) 



ageing process is very important industrially, 

 principally in the production of photographic 

 emulsions, where the nuclear sols produced in 

 the presence of gelatin are allowed to "Ost- 

 wald ripen" before coating onto plates or film 

 base. 



The ageing process in poh^disperse systems 

 is usually considered to involve the smaller 

 particles going into solution and the larger 

 particles growing at their expense. An alter- 

 native explanation is that coagulation of the 

 small particles occurs followed by recrystal- 

 lization of the coagula to form regular 

 particles. Most evidence would appear to 

 favor the former mechanism but in some 

 cases mosaic crystals have been found (see 

 for example Fig. 17a) which would tend to 

 favor the latter. It is possible that in prac- 

 tice both mechanisms occiu' with the former 

 usually being the predominant one. 



Electron microscopy forms a suitable 

 method for the examination of the ageing 

 process since both the average size of the 

 particles and the number present per unit 

 volume of sol may be evaluated at a given 

 time. Moreover, from the shape of the par- 

 ticles formed during the ageing process, it is 

 possible to tell whether growth of the parti- 

 cles occurs preferentially in certain direc- 

 tions. 



A detailed study of the ageing of silver 



bromide sols has been carried out by Kolthoff 

 and his collaborators (16). 



An interesting example of the ageing proc- 

 ess is found in the case of vanadium pentox- 

 ide sols. The sol particles formed in the initial 

 sol, e. g., in a Biltz sol (17) have been found 

 to be small needles several hundred ang- 

 stroms long and 140 A thick. On ageing these 

 are transformed into fibrous crystals several 

 microns in length. In detailed studies on the 

 ageing process (18, 19) it was found that the 

 large numbers of small needle-like particles 

 were redistributed to give smaller nmnbers 

 of large filaments; growth was attributed to 

 recrystallization of the fibrils. 



Formation of Monodisperse Sols. Inti- 

 mately connected with the study of nu- 

 cleation and growth is the problem of pro- 

 ducing monodisperse sols. The latter may be 

 defined as sols in w^hich all the particles con- 

 tained therein have exactly the same size and 

 shape. The conditions for the preparation of 

 monodisperse sols, which are verj^ important 

 from the viewpoint of colloid chemistry, 

 have been investigated in detail by LaMer 

 and his collaborators (20), and may be il- 

 lustrated by consideration of Fig. 10. Thus 

 if a slow chemical reaction occurs which con- 

 tinuously generates molecules of a disperse 

 phase, the concentration of these molecules 

 increases steadily, passes the point of satura- 



133 



