COLLOIDS, LYOPIIOBIC 



calculated. Then if the particle density is 

 known the weight per particle can be calcu- 

 lated and if a subsidiary determination is 

 made of the weight concentration of the sol 

 used, the number of particles per unit volume 

 can be calculated. 



Both methods have been used extensixely, 

 but in some cases care must be exercised in 

 applying these methods since it is not easy 

 to determine drop diameters accurately, and 

 the second method may yield spurious re- 

 sults, either because of particle shrinkage 

 (beam intensity too high), or because the 

 particles do not possess a well-defined geo- 

 metrical shape. 



A comparison has been made (11) between 

 values for particle numbers per unit volume 

 determined by electron microscopy, and 

 those determined by other methods, such as, 

 direct ultramicroscopic counting using a flow 

 method, turbidity measurements and counts 

 using a haemocytometer cell. The results of 

 the comparison which was carried out using 

 polystyrene latex particles are given in Table 

 1. 



In general it was found that the dry- 

 weight method yielded results very close to 

 those determined by other methods, whereas 

 the spray method tended to give rather high 

 results in terms of absolute numbers. A 



Table 1. A Comparison Between Particle 



Numbers per ml Determined by Electron 



Microscopy and by other Methods 



method for the direct application of micro- 

 drops of reproducible known volume is 

 clearly desirable. 



Processes Involved 

 and Destruction 



in Sol Formation 



Fig. 7. "Droplet" of polj'styrene latex parti- 

 cles obtained by spraying with a Vaponefrin nebu- 

 lizer. 



A variety of methods exist for the prepara- 

 tion of colloidal dispersions of different tj^pes 

 (3) and of these perhaps the most commonly 

 used, and the most studied, is the mixing of 

 two ionic solutions. A typical example is the 

 formation of a sol of silver bromide by mixing 

 silver nitrate and potassium bromide at 

 concentrations sufficiently high to exceed the 

 solubility product ; for a stable sol the stabi- 

 lizing ions Ag+ or Br~ have to be present in 

 certain proportions. In most cases of low- 

 .solubility inorganic materials stable sols can 

 be formed provided that a stabilizing ion is 

 present . 



The nuclei originally formed in the solu- 

 tion, which are usually very small crystals, 

 grow to form the larger sol particles which 

 are usually known as primary particles. This 

 phenomenon is known as ageing, and is usu- 

 ally explained as the growth of extremely 

 small particles to form larger ones either by 

 regular addition to the lattice, i.e., smaller 

 particles going into solution so that the larger 

 ones can grow at their expense, or by a 

 process of ordering of the disordered lattice 



129 



