Aggregation of colloidal suspensions by fibrous surfaces: II. The effects of added salts and soaps

Abstract

The addition of soaps or simple salts changes the stability of the D.D.T. suspension and also the rate of aggregation by fibrous surfaces. Low salt concentrations cause an immediate partial coagulation of the suspension, but further coagulation is generally very slow. The same salt concentrations produce very marked increases in the rate of aggregation of D.D.T. by hydrophobic fiber surfaces. This is attributed to decreases in the electrical repulsive forces around the fibers and the particles. Added soaps usually lower the stability of the suspension (without added fibers), the lowering being much more marked at soap concentrations above the c.m.c. This observation and the formation of large D.D.T. crystals suggest a process of crystal growth by transport of D.D.T. in the soap micelles. When fibers are also present the soaps are shown to be adsorbed on the fibers, rendering them more hydrophobic, and hence increasing the rate of aggregation of the D.D.T. by the fiber surfaces.