COLLOIDS, LYOPHOBIC 



replicated with silicon monoxide and the rep- 

 licas shadowed with palladium vapor. Dis- 

 location centers were clearly visible, indicat- 

 ing that growth had occurred by a screw dis- 

 location mechanism ; the spiral step heights 

 were found to be of the order of 45 A (see 

 Fig. 20). 



An interesting example of the use of elec- 

 tron microscopy in elucidating the growth 

 mechanism of crystals occurs in the case of 

 silver bromide. It was suggested by Berri- 

 man and Herz (44) that the reason for tabu- 

 lar growth in silver bromide (sodium chlo- 

 ride-type structure) was the occurrence of 

 twinning on the 111 plane; some confirma- 

 tion was found for this hypothesis in that 

 Laue photographs taken by them exhibited 

 six-fold symmetry. Additional support for 

 the mechanism was obtained by Hamilton 

 and Brady (45) in an electron microscope ex- 

 amination of shadowed carbon replicas of 

 tabular silver bromide crystals. The replicas 

 were examined at an angle of 45° to the in- 

 cident beam when the convex and concave 

 intersections at the twin planes were visible 

 on the edges of the crystal. Examples of 

 crystal replicas showing the presence of twin 

 planes are given in Fig. 21. 



The fact that in the immediate vicinity of 

 a crystal imperfection the lattice has a higher 

 chemical potential than in the more perfect 

 parts means that when the crystal is placed 

 in a solvent preferential attack occurs at this 

 point. Thus, provided the reaction is stopped 

 before extensive solution of the crystal oc- 

 curs, etch pits are formed at the sites of 

 preferential attack. In the case of colloidal 

 particles this technique appears to have been 

 employed primarily on silver halide crystals 

 in an attempt to detect dislocation sites. For 

 example, Hamilton, Brady and Hamm (46) 

 carried out chemical etching-studies on large 

 grains of silver bromide and sih^er bromoio- 

 dide emulsions. Etching was carried out by 

 immersion of the particles, for a hmited time, 

 in a silver halide solvent. The particles were 

 then replicated with carbon, and the replicas 



Fig. 20. Silicon monoxide replica of a stearic 

 acid crystal, shadowed with palladium, showing 

 growth from a single dislocation and bimolecular 

 steps. {By courtesy of Dr. I. M. Dawson) 



Fig. 21. Carbon replicas of colloidal particles 

 showing evidence of twinning, a) silver iodide, b) 

 silver bromide. Arrows indicate the position of 

 twin planes. 



examined after shadowing with platinum- 

 palladium at a 5 to 1 angle. The concentra- 

 tion of chemical etch pits and the geometry 

 of the pits were found to be dependent on the 

 solvent used; potassium bromide gave octa- 

 hedral pits and sodium sulfite and potassium 

 cyanide dodecahedral pits. A general increase 

 in etching on certain faces was found after 

 intentionally straining the grains, but the 

 effect was-not sufficiently strong to establish 

 a one-to-one relationship between etch pits 

 and dislocations. The etching experiments 

 did not provide any conclusive e\'idence that 

 normal grains of silver bromide were poly- 

 crystalline in nature. 



A micrograph of a carbon replica of a sil- 

 ver bromide crystal etched with potassium 

 cyanide is shown in Fig. 22. 



141 



