The electrophoretic mobility of large colloidal particles

Analytical approximation formulae linking ζ potential and electrophoretic mobility have been derived for a variety of limiting conditions. In this paper we present a simplified derivation of an equation first established by Dukhin and his collaborators for a large spherical particle with a thin double layer. Although potentially a very useful expression it has been little used to date, partly because of the complicated nature of Dukhin's derivation and partly because of the lack of a reliable method of testing its validity. The expression compares very favourably with the computer calculations of O'Brien and White, provided κa is sufficiently large.

Motion Picture Study of Balata and Hevea Latices. With Observations on Buna-S and Neoprene Latices

In dispersions of Hevea or balata latex, motion of the particles appears to be controlled by forces acting between particles. The particles are known to have negative charges. It is believed that these charges may not be uniformly distributed over the surface of the particles, and that they fluctuate in intensity. The charges tend to keep the particles separated. Under normal conditions of motion there is no evidence that the particles collide. They approach and recede from one another. Particles differing in size are found associated. A common unit is the doublet, composed of a large particle and an associated small particle acting as a satellite. Particles of varying size seem to have a tendency to form a group or constellation. The motions of a constellation appear to be centered about the largest particle. This particle may have several satellites. Each satellite may have one or more satellites. A large particle is never seen to gyrate about a smaller particle. If the negative charges are nullified, agglomeration takes place. There seems to be some force acting in opposition to that of electrical repulsion. Isolated single particles or units (doublets, triplets, and quads) seem to manifest very little motion. As the concentration of the dispersed particles is increased, the motions appear to be greatly increased. Mathematical analyses based on measurements of the films should provide data which will determine whether this assumption is valid. In dilute dispersions, particle relations can be readily demonstrated, and the particles seem to assume a state of colloidal equilibrium. It appears that the particles were mutually in a state of tension. Motions are slight. Rather concentrated dispersions form chains, groups, and dark interspaces or “holes”. No evidence has been deduced that magnetism plays any part in this grouping arrangement. It is believed that the phenomenon may be fully accounted for on the basis that it is the resultant of forces acting between particles. Time studies show that merging of particles occurs. A simple case may be described as two spherical particles, one larger than the other, in doublet arrangement. The smaller particle may lose its charge, or through some other cause is attracted by the larger particle to which it becomes attached. Through the action of other forces, it appears to be pulled into the larger particle, resulting in an irregularly shaped particle. Ultimately, under favorable conditions, the larger particle completely absorbs the smaller particle and then becomes spherical. During the merging process, however, other particles may be attracted and become attached to this irregular-shaped particle. A common arrangement seems to be a tapering chain of particles, consisting of a large bulbous particle at one end to which have become attached several particles, each succeeding particle somewhat smaller in diameter than its neighbor. In process of merging, these particles are gradually pulled into the large bulbous particle, which ultimately may become a large spherical particle.