COLLOIDS, LYOPHOBIC 



planes of platinum phlhalocj-aninc can be 

 resolved in the electron microscope and are 

 found to be 11.97 A apart (6). It is also pos- 

 sible to check resolution by means of Fresnel 

 fringes which are visible when' the objective 

 lens is slightly off -focus (7, 8). The use of 

 image intensifiers with the electron micro- 

 scope may well prove useful in the future in 

 the field of high-resolution work. 



Shadow Casting. Normally, only a two- 

 dimensional aspect of the colloidal particle 

 on the grid can be obtained. This is insuf- 

 ficient for many purposes particularly if the 

 3-dimensional shape of the particle is re- 

 quired. In order to enhance contrast and 

 obtain an approximate idea of the vertical 

 height and shape of the particle, shadowing 

 with a heavy-metal vapor such as that of 

 gold, platinum, chromium or uranium may 

 be employed. The metal is evaporated onto 

 the sample in a high vacuum at a suitable 

 angle; usually a special evaporator unit is 

 needed to meet the strongest requirements of 

 high resolution work (1). From the known 

 angle of shadowing and the length of the 

 shadows, a three-dimensional picture of the 

 particle shape can be built up. These shapes 

 can be checked by constructing models and 

 shadowing with a beam of light. IMore reli- 

 able information can be obtained if the ma- 

 terial is shadowed in two directions at right 



Ofn 



Fig. 3. Colloidal silver iodide particles sha- 

 dowed with uranium at 50°, a) and c) in one direc- 

 tion, b) and d) in two directions at right angles. 



Fig. 4. Diagram of particle models based upon 

 shadowed micrographs of the type illustrated in 

 Fig. 3. 



angles, but it is essential that the shadowing 

 be very light; overdeposition of metal com- 

 pletely obliterates the first shadow. In Fig. 3 

 micrographs of specimens shadowed with 

 uraniimi at 50° in one direction only and 

 with uranium at 50° in two directions at 

 right angles are shown. From these micro- 

 graphs particles were found to have shapes 

 such as those shown diagrammatic ally in 

 Fig. 4. 



Replication. One of the most useful tech- 

 niques for determining the exact shape of 

 particles and also their surface structure is 

 replication; this technique is, moreover, in- 

 valuable for the study of specimens which 

 under normal conditions are decomposed by 

 the action of the electron beam. The tech- 

 nique is carried out by depositing samples of 

 the sols either on clean glass slides or freshly 

 cleaved mica and allowing them to dry. A 

 thin film of carbon is then evaporated on to 

 the slide; normally it is advantageous at this 

 stage to shadow very slightly with a heavy 

 metal vapor such as chromium. The carbon 

 film is then removed from the slide using as 

 a hquid substrate a solvent for the embedded 

 particles. When this solvent is a concentrated 

 salt solution it is advisable to follow by wash- 

 ing with more dilute solutions of the salt 

 and then to give a final rinse in distilled^ wa- 

 ter. For the best results from the replication 

 technique it is essential that the films em- 

 ployed should be extremely thin {ca. 100-300 

 A). 



127 



