COLLOIDS, LYOPIIOBIC 



shaped, and the mole number x per particle 

 was calculated from the equation 



X = ^7r(a/2)(6/2)2p/M 



where a and h were the mean length of the 

 long and short axes, respectively, and p and 

 M were the density and molecular weight of 

 barium sulfate. The size of the particles was 

 found to increase with the concentration of 

 the reagents used and the relation between 

 the mole number of the particle and the 

 concentration of the reagents (C) was given 

 by the expression 



xC" = K 



where a and K were constants. 



]\Ionodisperse gold sols have also been 

 prepared by Takiyama (15) using the reduc- 

 tion of chlorauric acid by sodium citrate. 

 The sol particles thus prepared were found 

 to be almost spherical, the average diameter 

 being 172 A with a standard deviation of 13 

 A. 



Silver bromide and silver iodide sols have 

 been prepared in a monodisperse form by 

 Ottewill and Woodbridge (25). The silver 

 bromide sols were prepared by slow cooling 

 of a hot solution of silver bromide, which was 

 slightly supersaturated at room temperature, 

 and the silver iodide sols by dilution of the 

 potassium iodide complex with water. The 

 silver bromide particles were found to be 

 cubes and the silver iodide particles rhom- 

 boids; typical micrographs of both types of 

 sol particles are given in Fig. 11. 



Effect of Additives on Sol Formation. 

 It is well known that molecules of dyes or 

 surface-active agents often show a preference 

 for adsorption onto particular crystal faces 

 (26) and thus can exert profound influences 

 on crystal shape (27). Support for the idea 

 of preferential adsorption on certain crystal 

 faces has been found in electron microscope 

 studies on the coagulation of sols. For exam- 

 ple, in the coagulation of hexagonal plates of 

 silver iodide by dodecylpyridinium ions (28) 

 it was found that the plates were joined by 



, .^. 



a . 



^a^u. 





;:C 



,»Jb 



Fig. 12. Electron micrographs illustrating the 

 injfluence of additives on the formation of silver 

 iodide sol particles, a) sol formed in the presence 

 of 2.2 X 10"'' M mercaptotriazole, b) sol formed in 

 the presence of 4 X 10" M dodecylpyridinium io- 

 dide, c) sol formed in the presence of 7.74 X 10~^ M 

 dodecylpyridinium iodide. 



edge to edge adhesion suggesting a preference 

 for adsorption of the ion on the 0110, 1010, 

 1100, Olio, lOlO and IlOO faces. 



The influence of different media on the 

 shape of sol particles can conveniently be 

 investigated by electron microscopy. In Fig. 

 12a are shown particles of silver iodide 

 formed in the presence of 2.2 X 10~^ M 

 mercaptotriazole; comparison with Fig. 12b 

 shows that the crystal form has been altered 

 from predominantly flat plates or tetrahedral 

 particles to rod-like particles. It is advisable 

 when such changes are noticed to carry out 

 micro-diffraction experiments on the parti- 

 cles to determine whether any of the additive 

 has been incorporated into the crystal. 



High concentrations of surface-active 

 agents, particularly those above the critical 

 micelle concentration, often have a consider- 

 able influence on the sol formation process 

 (28). In the case of silver iodide particles, it 

 appears that a twofold adsorbed layer of sur- 

 face-active agent ions is formed on the 

 particles at the nucleus stage; the particles 

 formed are finely dispersed with many of 

 diameter less than 25 A (Fig. 12c). Owing to 

 the strongly adsorbed layer the ageing proc- 

 ess appears to be retarded and a protected 



135 



