Hydrosols diffusion

From what has been said it is. evident that the question, whether hydrosols are solutions or suspensions, is purposeless. As a matter of fact they occupy an intermediate position. They exhibit properties that resemble crystalloidal solutions or suspensions depending upon the kind of hydrosol and upon the fineness of division. As Nernst f has often pointed out, the general behavior of most colloidal solutions would place them with the crystalloidal rather than with suspensions. The fact that hydrosols diffuse, and that they possess osmotic pressure would tend to justify this point of view. 

For a hydrosol at room temperature, the time ty2 in which the number of particles is halved by diffusion-controlled coagulation is calculated from the above equations to be of the order of 10u//io seconds, if n0 is expressed in the unit, particles cm-3. In a typical dilute hydrosol, the number of particles per cm3 may be about 10lw-10u, and so, on this basis, tm should be of the order of a few seconds. 

The nature of stability of disperse systems with solid dispersed phase and liquid continuous phase against coagulation is determined by phase composition, particle size and particle concentration. The stability of hydrosols at low electrolyte concentrations is usually related to the electrostatic component of disjoining pressure (Chapter VII), arising from the overlapping diffuse parts of electrical double layers. 

The ultramicroscope affords an opportunity for the closer examination of the nature of the diffusion of light rays by hydrosols, and gives a glimpse into the spacial discontinuity in these systems. The mere presence of this discontinuity could be foretold by the Tyndall effect, but it remained for the ultramicroscope to reveal the fact that colloidal solutions contain small individual particles in rapid motion, the size of which particles is somewhat larger than the molecules in crystal-loidal solutions. The investigation of hydrosols with this instrument has made possible much more definite ideas with regard to the size, color, polarization, motion, etc., of these particles, and permitted the scientist to enter a field that had been practically closed to him. However, some of the particles present in the hydrosols are so small that they cannot be detected even by the ultramicroscope, and this led to a further subdivision of particles according to whether or not they could be seen by this means. 

Direct measurements of diffusion of hydrosols were made by Graham and later by Arrhenius f and Herzog.J The diffusion constants, cm... 

Optically those gold solutions with small amicrons behave exactly as homogeneous solutions of dyestuffs. A part of the absorption is to be attributed to diffusion of light only when the subdivision of the gold is much coarser. The diffusion in the larger subdivisions has an appreciable influence on the color. In the case of the finer subdivisions of gold hydrosols the color is not a question of the color of a... 

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