ELECTRON MICROSCOPY 



Fig. 5. Carbon replicas of silver iodide parti- 

 cles shadowed with chromium at 50°. 



Results obtained by the replica technique 

 with subsequent shadowing are illustrated 

 in Fig. 5. 



Distribution Curves. It is possible from 

 electron micrographs of a field containing a 

 large number of particles to determine not 

 only their shapes but also the frequency 

 distribution of diameters, or appropriate di- 

 mension. Once these factors are established 

 the surface area of a sample may be obtained, 

 and from a knowledge of the physical density, 

 the weight of the particles. Thus a distribu- 

 tion curve can be made by plotting the fre- 

 quency of appearance of a certain diameter 

 against that diameter. A typical example is 

 given in Fig. 6. Strictly, a number of photo- 

 graphic plates should be taken of different 

 fields and several thousand particles meas- 

 ured in order to obtain a truly representative 

 curve. In practice, however, counts are usu- 

 ally made on 300 to 400 particles; for reason- 

 able representation it is essential to take 

 several micrographs of different parts of the 

 field. 



Such a curve enables information to be 

 obtained on the degree of polydispersity of 

 the system with respect to size and shape 

 and can be of great assistance in following 

 rate processes such as nucleation, particle 

 growth and coagulation. The shape of the 



size distribution curve depends on the rates 

 of the nucleation and growth processes (see 

 later). In general there is good agreement be- 

 Iween the size of particles determined by 

 electron microscopy and those determined 

 by other methods. 



Determination of Absolute Particle 

 Number per unit Volume. Two procedures 

 can be employed for this determination. In 

 the first the sol is sprayed on to the grid in 

 the form of fine droplets using a high-pres- 

 sure nebuUzer (9, 10). Under favorable con- 

 ditions the drops are clearly visible and 

 assuming the diameter of the dried drop to 

 be the same as that in the spray, the drop 

 volume can be estimated. A typical drop 

 formed by spraying a polystyrene latex sus- 

 pension is shown in Fig. 7. Thence from a 

 count of the number of particles contained 

 in the drop the number of particles per unit 

 volume of the original sol can be calculated. 



In the second method it is essential for 

 accurate results that a monodisperse sol 

 should be used. Electron microscopy can be 

 used to determine the diameter, or in the 

 case of non-spherical particles the appropri- 

 ate dimension, and the volume of a particle 



40 



30 



% 

 PARTICLES 



20 



lO - 



300 600 900 I200 I5CXD 

 PARTICLE DIAMETER IN A° 



Fig. 6. Particle size distribution curve for a sol 

 of silver iodide. 



128 



