Electrophoretic properties of AgBr dispersions

Abstract

A procedure was developed for obtaining concentrated suspensions of crystallographically defined, homodisperse AgBr crystals which are suitable for both electro-kinetic and adsorption studies. A zero point of charge (zpc) at pAg 5.3 ± 0.2 was found for all the 10 −3 M AgBr sols examined. This zpe was independent of ( a) crystal habit, ( b) crystal size from 0.4 to 1.6 μ edge-length, ( c) crystal size distribution, and ( d) incorporation of 4 mole % iodide in the crystals. However, mere dilution of these sols to 10 −5 M AgBr lowered the zpe to pAg 2–4. Independent electrophoresis and adsorption measurements were carried out on the same cubic 10 −3 M AgBr dispersion at pBr 3 to which varying amounts of a cationic cyanine dye were added. A simple linear relation was established between electrophoretic mobility and adsorbed dye ions, from which it was determined that adsorption of 10 14 dye ions/cm. 2 was just sufficient to reduce the initial negative mobility of AgBr to zero. This amount of cyanine dye coincides with the concentration of Br − ions adsorbed to cubic AgBr crystals at pBr 3, suggesting simple charge neutralization at the surface.