COLLOIDS, LYOPHOBIC 



a set of dark-field images obtained which 

 correspond to the sections of the crystal con- 

 taining the diffracting lattice planes. Com- 

 parison of the dark-field images with the 

 bright -field images reveals that each black 

 line in the latter becomes a bright line in 

 the former, and thus the Miller indices of 

 the lattice planes giving rise to the bright 

 lines in dark field can be identified. An in- 

 teresting study using this technique has been 

 carried out by Suito and Uyeda (33) on 

 lamellar colloidal gold crystals, in which they 

 were able to identify the crystal planes giv- 

 ing rise to the striped patterns often observed 

 on thin gold crystals (see Fig. 18). 



Studies of Internal Structure in Thin 

 Colloidal Particles. Many colloidal parti- 

 cles, when studied by direct electron micros- 

 copy, appear completely opaque, and there- 

 fore any interior details which might be 

 expected to be visible, such as grain bound- 

 aries from mosaic growth, dislocation lines, 

 etc., are completely obscured. Dislocation 

 lines usually appear dark on micrographs, 

 an increase of contrast which is thought to 

 be due to the increased Bragg reflection from 

 the strained region around the dislocation 

 line. Moreover, studies on thin metal foils of 

 aluminum, stainless steel, etc., have shown 

 that such imperfections are readily visible in 

 the electron microscope (38, 39). Very few 

 direct observations have been recorded on 

 the presence of these defects in colloidal par- 

 ticles, although such defects may play an 



Fig. 16. Micro-diffraction pattern from a group 

 of colloidal silver iodide particles, a) selected area 

 of particles, b) micro-diffraction pattern. 



important role in the stability of colloidal 

 systems. In fact, observation of these defects 

 in colloidal particles does not appear to be 

 an easy problem and may well mean that the 

 particles are very close to perfect crystals. 

 For observations of interior structure it 

 would appear necessary for the thickness of 

 the particles to be of the order of onlj^ a few 

 hundred angstrom units. 



Thin crystals may often be prepared by 

 the controlled ageing of nuclear sols; this 

 method is particularly successful in the case 

 of silver iodide and some detailed studies on 

 thin hexagonal plates of this material have 

 been carried out by Home and Ottewill (40, 

 41). Some crystals were found to exhibit a 

 mosaic appearance, sections of different con- 

 trast, which would correspond to different 

 crystal orientations, being clearly visible 

 (Fig. 17a); extinction contours were also 

 clearly visible. In other crystals dark bands 

 were observed (Fig. 17b) which appeared to 

 be due to the presence of dislocations or 

 stacking faults; these were often seen to 

 migrate across the crystal under the influ- 

 ence of the beam. 



In the case of thin lamellar particles, such 

 as those of gold, striped or spotted patterns 

 are often obser\-ed; a typical example is 

 shown in Fig. 18. These patterns are thought 

 to arise from diffraction effects due to local 

 curvature of the crystal, that is the sub- 

 strate with which the crystals are in close 

 contact undergoes local curvature in the 

 form of a valley; the crystals follow this 

 curvature and have a common axis of bend- 

 ing along the \'alley. The thin strips which 

 accompany the central one can be considered 

 to arise from reflections which correspond to 

 the subsidiary maxima which surround the 

 main diffraction spots (33). The crystal 

 planes giving rise to these diffraction effects 

 can be identified by dark-field image analy- 

 sis. The displacement of the subsidiary 

 maxima from the main spot, which precisely 

 satisfies the Bragg condition, is closely re- 

 lated to the thickness of the crystalline 



139 



