Examination of Freshly Prepared Silver Iodide Sols 



103 



Fig. 2. Electron micrograph ofsilversol. Inslrumenlal magni- 

 fication 40,000. Final magnification 400,000. 



that the state observed here is that of a large number 

 of small nuclei which may eventually grow to larger 

 particles. 



It is of interest to determine whether such particles 

 disappear during the aging process or to some extent 

 remain in "equilibrium" with the larger particles. 

 The irregularity of many of the larger particles may 

 to some extent be attributed to the angle at which 

 the particle is resting on the supporting film and, or 

 alternatively, to irregular growth during formation. 



A serious limiting factor in determining the size 

 and shape of particles below 10 A is the presence of 

 background eflfects due to supporting films and also 

 optical effects caused by focusing. 



The relationship between the true crystal structure 

 and the form of particles observed in the electron 

 microscope is not easy to interpret because of the 

 "shadowing" eff"ect which occurs. However, it is of 

 interest that J. J. Trillat and A. Laloeuf (3), in the 

 electron diffraction study of silver iodide smokes, 

 found that the particles present existed in both 

 hexagonal and cubic crystal forms. The unit cell of 

 the hexagonal form had the dimensions, r 7.94 A. 

 a = 4.58 A, and the unit cell of the cubic form had 

 a = 6.47 A. 



It thus appears possible that colloidal silver iodide 

 exists in both crystal forms, a fact which is being 



further investigated by micro-diffraction techniques 

 where it is possible to select areas down to 5 /< 

 diameter. 



The presence of very small particles in the sol 

 shows clearly the fact that colloidal particles may 

 exist in size almost down to the range of atomic 

 dimensions, and indicates that there is no strict 

 boundary between atomic and colU)idal particles. 

 The distribution of particles of very small size also 

 appears to extend much further than had been rec- 

 ognised hitherto using electron microscopes with 

 a resolution of the order of 30 A. Comparison of 

 silver with silver iodide sols strongly suggests that 

 the dense scattering material is probably silver. 



The mean diameter of the particles in a colloidal 

 solution is an important factor in colloid chemistry, 

 and its accurate evaluation essential for correlation 

 between experiment and theory. The presence of 

 such a magnitude of small particles may mean that 

 hitherto, where the electron microscope has been 

 used for the evaluation of size, the mean value has 

 been too high because the smaller particles have 

 been unresolved. The theory of Verwey and Over- 

 beek (4) predicts that small particles of diameter 

 ca. 20 A should have a very low stability, since 

 they would require very high electric potentials 

 (H'*,,), and have low absolute values of repulsive poten- 

 tial energy. The latter is directly proportional to 

 particle radius. The fact that particles of this size 

 have been shown to exist in such profusion in the 

 present work does indicate, however, that the prepara- 

 tion of very small colloid particles in a stable form 

 is by no means impossible. 



References 



1. DE Bruyn, H. and Troklsira, S. A., Kolloiil-Z. 84 192, 



(1938). 



2. Harmsen, G. J., VAN ScHOOTEN, J., and Overbeek, J. In. 



C, J. Colloid Sci. 8, 64 (1953). 



3. Trillat, J. J. and Laloeuf, A., /. Chim. P/nw. 46, 168 



(1949). 



4. Verwey, E. J. W. and Onerhlik, J. Th. G., Thoor> of 



the Stability of Lyophobic Colloids. Elsevier, 1948, 

 170-178. 



