COLLOIDS, LYOPHOBIC 



K* 

 i 





Fig. 13. Electron micrographs of varioiLS types of flocculation, a) side to side ad- 

 hesion of hexagonal plates, b) formation of chains, c) formation of clumps, d) "lace- 

 like" aggregates of particles embedded in added surface active agent. 



be considered, since the section of the 

 former by the latter can be regarded, ap- 

 proximately, as the electron diffraction pat- 

 tern obtained. 



Apart from the direct use of diffraction 

 patterns to obtain the structm-e of the par- 

 ticles, it is very useful in the colloid chemical 

 field to use the "x-ray" structure of the bulk 

 material, if known, to identify the colloidal 

 particle, or confirm its identity, and to de- 

 termine the orientation of the particle; 

 moreover, the indices of the crystal faces ex- 

 posed can be obtained from the disposition 

 of the spots. Diffraction analysis can also be 

 used to give an indication of the imperfec- 

 tions present in the particle. The main 

 limitation to the use of this technique on 

 crystalline colloidal particles is the thick- 

 ness of the particles since for more than a 

 certain thickness of the particle, which varies 

 according to the nature of the material, too 

 much of the beam is scattered, or absorbed, 

 for a distinct pattern to be obtained. 



In Fig. lob is shown a selected area micro- 

 diffraction pattern from a thin colloidal par- 

 ticle of silver iodide (thickness ca. 100-200 

 A) and in Fig. 15a a selected area micrograph 



of the portion of the particle from which the 

 diffraction pattern was obtained. The pat- 

 tern is that expected for a hexagonal crystal 

 of silver iodide of a = 4.59 A and c = 7.49 

 A resting on the OOOi plane. The clarity of 

 the diffraction pattern demonstrates clearly 

 the almost perfect crystalline nature of 

 colloidal particles of this type. 



If instead of a single particle, a field is 

 taken containing a number of small particles 

 then a ring pattern is obtained (see Fig. 14b). 

 Only those planes which satisfy the Bragg 

 equation contribute to the pattern and thus 

 a series of discrete rings are obtained; if only 

 a small number of particles are present the 

 rings are broken up into spots. A typical 

 ring pattern, obtained from a group of silver 

 iodide particles of particle size 300-400 A, is 

 shown in Fig. IG. The angular breadth of 

 the diffraction line depends upon the diame- 

 ter of the particle D and the wavelength of 

 the incident radiation X, the quantity \/D 

 usually being termed the Scherrer breadth. 

 Thus theoretically an estimate of particle 

 size can be obtained from the breadth of the 

 diffraction lines (2, 34). However, other facts 



137 



