ELECTRON M l( .IU)S( .( )I»Y 



of the particles originally formed. Sols of 

 primary particles are often stable for long 

 periods of time, but addition of electrolyte 

 beyond a certain concentration, or the addi- 

 tion of strongly adsorbed organic ions, causes 

 the particles to clump together (coagula- 

 tion); this process may, or may not, be ac- 

 companied by recrystallization to form even 

 larger particles, depending on the system. 

 The process of sol formation and destruc- 

 tion of the sol either by coagulation or 

 recrystallization can be represented sche- 

 matically in the following manner: 



Ions 



Nuclei 



growth 

 (ageing) 



Large crystals 



(ageing)/^ J 



Primary particles 



\ 



Coagula 



Most of the stages represented in this 

 scheme can be investigated by electron 

 microscopy and will therefore be considered 

 separately. 



Nucleation. Nucleation can be defined 

 as the formation of a discrete particle of a 

 new phase in a previously homogeneous solu- 

 tion. Nuclear or amicronic sols contain par- 

 ticles which cannot be resolved as discrete 

 entities in the ultramicroscope. They can be 

 prepared from many materials, e.g., gold, 

 silver, silver iodide etc.. Electron micro- 

 scopic examination of these sols shows par- 

 ticles down to 10 A or less (see Fig. 2) which 

 can be clearly resolved. The resolution of 

 these particles constitutes one of the highest 

 resolutions so far achieved with the electron 

 microscope. From the point of view of col- 

 loid chemistry this illustrates the small size 

 range in which colloidal particles can exist 

 and it is of considerable interest that the 

 regular shapes of many of the particles ap- 

 pear to be maintained down to the limits of 

 resolution. Thermodynamically, the smaller 

 particles would be expected to have a larger 

 solubility than the larger ones and thus 

 would be expected to go into solution as ions 



and deposit on the larger particles to increase 

 their size. Charge would be expected to in- 

 fluence this process (12), but in view of the 

 large value of the free energy of most solid- 

 liquid interfaces it is doubtful whether this 

 does in fact play a significant role. 



Several theories have been proposed for the 

 mechanism of nuclei formation in dilute solu- 

 tion, of which the impurity, organizer and 

 fluctuation mechanisms appear to have re- 

 ceived the most attention. The impurity 

 theory is based on the idea that nuclei are 

 introduced into the system as foreign bodies, 

 e.g., dust particles; it has been found, for 

 example, that in the preparation of colloidal 

 gold different sols are obtained according to 

 the state of the glass vessel used. However, 

 it was concluded by Turkevich, Stevenson 

 and Hillier (13), who prepared gold sols un- 

 der many different conditions, that impuri- 

 ties were not a variable in their investigation. 

 These authors proposed the organizer mecha- 

 nism to account for the formation of nuclei 

 in gold sols. Their suggestion was that the 

 nucleating agent, e.g., hydroxylamine, grad- 

 ually built up a complex between the gold 

 ions, chemically binding a large number of 

 gold ions and reducing agent molecules into 

 large macromolecules. It was suggested that 

 the latter underwent a molecular rearrange-i 

 ment to give metallic gold and oxidation 

 products of the reducing agent. Some sup- 

 port was lent to this hypothesis by the na- 

 ture of the reducing agents, but there are 

 clearly many conditions under which such a 

 mechanism cannot apply. 



The fluctuation theory of nucleation is 

 probably that most widely accepted. It is 

 based on the hypothesis that the formation 

 of a nucleus occurs only when a statistical 

 fluctuation of the ionic (atomic or molecu- 

 lar) concentration brings a sufficiently large 

 number of ions together to form a particle 

 of thermod3niamically stable size. This 

 theory has been shown to apply to the forma- 

 tion of colloidal sulfur (14). 



Studies of Nucleation bv Electron 



130 



