An Electron Microscope Examination of Freshly Prepared 



Silver Iodide Sols 



R. Ottewill and R. W. Horne 



Dept. of Colloid Science and Cavendish Laboratory, University of Cambridge 



Although silver iodide, which forms a typically 

 hydrophobic sol. has been extensively investigated 

 in the field of colloid chemistry, very little research 

 has been devoted to an examination of this material 

 in the electron microscope. Of the numerous hydro- 

 phobic sols which can be prepared the gold sols 

 are the only ones to have received any considerable 

 attention by this method, primarily as standards of 

 resolution. In connexion with other physicochemical 

 work on silver iodide sols, this material has been 

 examined in a high resolution electron microscope. 

 Numerous small particles of the order of 10 to 25 A 

 have been resolved and particles of below 10 A 

 have been detected. Since the resolution of such 

 particles is of extreme interest in the field of electron 

 microscopy and colloid science and probably con- 

 stitutes the first resolution of colloidal particles ap- 

 proaching atomicdimensions, it is thought worthwhile 

 to submit a preliminary report of the work at this 

 stage, although a more extensive investigation is 

 still in progress. 



100 ml of a solution of silver nitrate (1.25 mM) 

 were added with continuous stirring to 100 ml of a 

 solution of potassium iodide (1.38 mM). The silver 

 iodide sol formed under these conditions is a nega- 

 tively charged sol, stabilized by the adsorption of 

 I~ ions. The sol was freed from potassium nitrate 

 formed during the reaction and from excess potassium 

 iodide by continuous electrodialysis against distilled 

 water. During this process the K^ gegenions are 

 removed to give a sol which may be represented as 

 (Agl) I H^ (1). The sol obtained after electrodialysis 

 was diluted with an equal volume of distilled water 

 to give a concentration of ca. 0.3 mmoles of silver 

 iodide, and used at this strength for the preparation 

 of the electron microscope grids. Silver iodide sols 

 are reasonably stable to daylight, but specimens 

 were always stored in the dark, as a precautionary 

 measure. 



Distilled water was always treated before use with 

 an ion-exchange column to remove any remaining 

 ions, and both reagents were recrystallised from this 

 water. All glass ware was steamed for half an hour 

 before use to remove any adhering grease film. 



The sol was freshly prepared in order that aging 

 phenomena, a process which includes possible chan- 

 ges in the form of the particles and changes in the 

 size of particles owing to recrystallisation or slow 

 flocculation, should not become important. How- 

 ever, only a small change in the mean diameter of a 

 silver iodide sol after aging is found (2). 



An examination of silver and silver iodide sols in 



the electron microscope at instrumental magnifica- 

 tions of 40,000 and 80,000 revealed the presence of 

 a large range of particle sizes from ca. 180 A down 

 to particles which were only just resolved by the 

 microscope in the range of 6-10 A. Only such 

 particles were measured which were clearly resolved 

 from the background structure and capable of being 

 produced on separate electron micrographs. A 

 preliminary analysis of diameters of 300 particles 

 from a 400,000 times enlargement gave a distribu- 

 tion curve giving a mean particle diameter of 

 25 r; 5 A. 



The fact that the solubility of silver iodide is very 

 small, 5 10"' mmoles litre compared with 0.3 

 mmoles litre present in the sol form, suggests that 

 the small particles are colloidal in nature and are 

 not formed during the drying down process by 

 crystallisation of the silver iodide in molecular solu- 

 tion. 



The shape of the silver and silver iodide particles 

 in the electron micrographs appear to take many 

 different forms. In the present work particles having 

 a cubic face of side of 25 A have been clearly resolved, 

 also hexagonal particles are visible in the 50 A region, 

 whilst the larger particles (125 A upwards) appear to 

 be either approximately spherical or have irregular 

 shapes. 



The formation of a colloidal particle is almost 

 certainly that of rapid growth from a nucleus, the 

 latter consisting of only a few atoms, and it is possible 



Fig. 1. Electron micrograph of silver iodide sol. Instrumental 

 magnification 80.000. Final magnification 500,000. 



