ORGANIZATION OF SOLS 



17 



which can be measured by X-ray methods (Scherrer, 1920), 200 of 

 the 380 Au-atoms, i.e., more than half the total number, lie at the sur- 

 face of the crystals. With decreasing particle size, of course, an even 

 higher percentage of atoms lies at the surface, until, with 14 or still less, 

 all the atoms lie at the surface (face-centred cube. Fig. 25, p .27). Thus 

 one can no longer speak of a homogeneous phase in the case of atoms 



Phase I 



Phase E 



Fig. 14 



Fig. 15 



Fig. 16 



Fig. 14. Colloid chemical concept of phase. I Dispersed phase (colloid portion), 11 

 dispersing medium. The inhomogeneity of boundaries reigns throughout the system - 

 Fig. 15. Thermodynamical concept of phase. The homogeneity of the phases reigns 

 throughout the system. - Fig. i6. Inbomogeneous surroundings of the lattice points of 

 boundary' planes (face-centered cubic lattice). 



that are not similarly surrounded on all sides. It is only by a still 

 further increase in dispersion that finally a homogeneous, molecularly 

 dispersed solution of Au-ions is obtained. 



On the other hand, it has been ascertained (Zsigmondy, 1925, p. 39) 

 that the homogeneous primary particles of suspensoids can cluster 

 together to form bigger heterogeneous secondary particles (compare 

 Fig. 73, p. 104) without any fundamental change in the properties of 

 such sols. This strengthened the opinion that the properties of sols 

 and other colloids were not decided by the inner structure of the 

 particles. Since with increasing dispersion the surface of the particles 

 increases considerably in proportion to their mass, colloid chemistry has 

 developed much more into a science of surfaces. The properties and 

 reactions of colloids have been elucidated to a great extent by the study 

 of surface reactions. Whereas the phase theory is concerned with the 

 equilibrium between different phases and is able to predict under what 

 conditions phases cease to exist (dissolution) or new phases appear 



