Variations in the Cataphoretic Mobilities of Oil Drops in Water

Abstract

Cataphoretic mobilities of oil drops in water and weak electrolytes.---The cell used was a thin walled glass tube 20 cm in length and 6 mm in diam., with gold electrodes sealed into the ends, held in a vertical position. By means of a microscope, velocity measurements were made on suspended drops both along the axis and in the stationary layer $\frac{R}{\sqrt{2}}$ from the axis, where $R$ is the inner radius. (1) Variation with diameter of drop. For emulsions of ten oils in distilled water, the mobility for a field of 10 volts/cm, was found to increase with the diameter $D$ for the range from.0005 to.04 mm. This is also true for emulsions of Stanolind in weak electrolytes, but for.0008 normal CuS${\mathrm{O}}_{4}$ the mobility is constant. In all cases the drops were negatively charged. (2) Variation with the potential gradient. For the smaller drops the mobility is constant, but larger drops, above.02 mm, show a slight increase for fields of 10 volts/cm. (3) Initial time lag. Observation on a large drop in a capillary tube open at both ends and immersed in a dish filled with.0001 normal NaOH, indicates that the mobility increases with time during the first 5 or 10 sec. after the electric field is applied. (4) Theory. It is suggested that the electric double layer around a drop is considerably distorted by an external electric field, so that equilibrium is established rather slowly.Endosmotic flow of water in a closed cylindrical tube.---(1) Hydrodynamic theory gives the velocity as a linear function of the square of the distance from the axis, with a zero value for $r=\frac{R}{\sqrt{2}}$. This was experimentally verified. (2) Mobility past an oil surface was measured after coating the inside surface of the cell with oil, and was found approximately equal to the cataphoretic mobility of large drops of oil in water. This is also true for.0004 molar CuS${\mathrm{O}}_{4}$ solution.