ELECTRON MICKOSCOPY 



Concentration 



o( 



Otspcrtc PhQtc 



in 



Solution. 



Fig. 10. Schematic diagram illustrating the 

 mode of production of monodisperse sols. (After 

 La Mer). 



tion A and exceeds at B the level at which 

 the rate of niicleation becomes appreciable. 

 When the rate of production of molecules is 

 slow, however, the sudden appearance of 

 nuclei relieves the supersaturation so rapidly 

 and effectively that the region of nucleation 

 (II) is restricted in time and no nuclei are 

 formed after the initial outburst. Hence the 

 nuclei produced grow uniformly by a dif- 

 fusion controlled process (region III) and a 

 sol of monodisperse particles is obtained. 



If the initial solutions used are not very 

 dilute, then the rate of production of mole- 

 cules becomes so rapid that their concentra- 

 tion in solution continually exceeds the 

 saturation concentration (Co) and continuous 

 creation of nuclei in addition to growth oc- 

 curs. Thus a polydisperse sol is formed since 



the size of any particle depends upon the 

 stage at which it was formed. 



A luimber of monodisperse sols have been 

 prepared and investigated by electron mi- 

 croscopy and other methods. The formation 

 and properties of monodisperse sulfur sols 

 were investigated by LaMer and collabora- 

 tors (20) apparently without detailed exami- 

 nation by electron microscopy. The mono- 

 disperse sols most widely investigated by 

 electron microscopy are undoubtedly those of 

 polystyrene latex (21, 22) and sized samples 

 of these particles are now widely used for the 

 magnification calibration of electron micro- 

 scopes. 



Watillon, Grunderbeeck and Hautecer (23) 

 by reducing selenium oxide with hydrazine 

 in the presence of amicronic gold particles 

 produced monodisperse sols of selenium. Ex- 

 amination by electron microscopy showed 

 the particles to be almost perfectly spherical 

 with a deviation from perfect sphericity of 

 about ±2%; the spherical shape was con- 

 firmed by shadowing experiments (see Fig. 

 11). Selected area micro-diffraction showed 

 the particles to be essentially amorphous. 



^Monodisperse barium sulfate sols have 

 been prepared by Takiyama (24) by decom- 

 posing the barium-EDTA complex with hy- 

 drogen peroxide in the presence of am- 

 monium sulfate. The particles were shown 

 by electron microscopy to be spindle 



6 



Fig. 11. Monodisperse sols (a) electron micrograph of selenium sol particles {bij courtesy 

 of Dr. A. Watillon), (b) carbon replica of silver bromide sol particles shadowed with chromium 

 at 60°, (c) carbon replica of silver iodide sol particles shadowed with chromium at 60°. 



134 



