Stability of Dispersions-An
Analogy
The theory of stability of hydrophobic sols in terms of the electric double-layer repulsion has been adequately established in general, even though details remain unresolved. The instruction of this topic is hampered by a lack of suitable analoeies or demonstrations. even at the advancedlevel. Somewhat accidentauv. an analomof this somewhat complex phen&nenon has been discdvered. The device wmmonly used to show the kinetic-molecular properties of a. gas [J. CHEM.EDUC.,30,68 (1963)l is quitesuitable for this purpose, but glass beads in aplasticcontainerrather thanglass works better. A devicevery much like theone cited here was built with Plexiglas (polymethacrylate) walls. Upon operating this device at room conditions at low humidity, a charge is developed on the glass beads sufficient to cause them to repel each other. When the vibration is stopped the glass beads remain almost uniformly dispersed as the result of their electrid charge. This can be verified by tipping the box a few degrees and passing a Tesla coil near the bottom of the box, whereupon the glass beads roll to the edge, or "coagulate," as shown in the figure. Starting again with the dispersed, charged glass beads, the box is slowly tipped to sucli an angle that the gravity vector is just sufficient to cause collapse or "coagulation" of the dispersed state. At this point, the angleis measured and the force of electrostatic repulsion can be equated to the gravitational force. As an example of the quantitative nature of this analogy, glass beads having s. diameter of 4.0 mm and density of 3.0 glcma were dispersed st a distance of 0.5 cm between their surfaces. At an angle of 25' to horizontal, the beads began to collapse. The gravitational force is mg sin 25', or 41.6 dynes. Assuming a, dielectric constant for the medium of unity, the charge on each bead is 3.22 esu, which is equivalent to about 1.3 X 10" electronic charges per cma. For real dispersions of glass and quartz hydrosols, the charge density is in the range of 108-10'a electrons per cm' for dispersions in indifferent electrolytes such as KNOI and KC!. Consequently, this analogy gives a magnitude of "charge stabiliaation" wmparable to the charge density in real dispersions.
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W. H. SLABAUQH O R E Q ~STATE N UNIVERSITY 97331 CORVALLIS,
Volume 47, Number 7, July 1970
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