A COMPARISON O F THE ELECTROKINETIC POTENTIALS AT FUSED AND UNFUSED GLASS SURFACES BETTY MONAGHAN AND H. L. WHITE Department of Physiology, Washington University School of Medicine, St. Louis, Missouri ' Received January 6, 1036
It has recently been shown (White, Monaghan, and Urban (6)) that the electrophoretic mobility of microscopic Pyrex particles is much less in dilute solutions than electrosmotic mobility in Pyrex cells. Since, however, the particles (prepared by pulverizing Pyrex tubing in a mortar) had broken surfaces, while the surface of the cell was fused Pyrex, the possibility remained that the observed discrepancies in electrophoretic and electrosmotic mobilities were. due t o differences in the adsorptive properties of fused and unfused surfaces r.ather than to differences in the phenomenon of electrophoresis as compared with electrosmosis. The object of the present paper is (1) to prepare fused Pyrex spheres 3 p or less in diameter and (2) t o compare the electrophoretic mobility of such spheres with (a) the electrophqretic mobility of broken Pyrex particles and (b) electrosmotic mobility a t fused Pyrex surfaces. A method for the preparation of microscopic glass spheres has been described by Sklarew (4). A simpler and much less bulky apparatus for the preparation of small quantities of fused powder was designed with the help of J. H. Zimmer.' The apparatus consists of a glass chamber, A, which holds the powder. A stirrer shaft, B, bearing a stirrer, C, which is beneath the powder, and wings, D, which agitate the air above the powder, is driven by a n air turbine, E. The fine mist of glass powder is carried up through the alundum heating tube, F, and settles in the glass collecting chamber, G, which is covered loosely enough to allow escape of the air. The upward velocity of the particles depends upon the height of the heating tube, the extent to which it is heated, and the size of the air inlet, H. The collecting chamber is supported by a rod, I. A narrow section of the alundum tube is heated to white heat with an oxygen flame, the tube being heated and cooled gradually in order to prevent cracking. When the air intake is so adjusted t h a t the particles move upward with a slow and uniform velocity, all of the particles up to l o p in diameter are fused in passing through the heated 1
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BETTY MONAGHAN AND H. L. WHITE
portion of the tube and on microscopic examination are seen to be perfect spheres. Both electrophoretic and electrosmotic measurements were carried out in a Pyrex electrophoresis cell of the type described by Mattson (1). In such cylindrical cells the water is a t rest at a depth olO.147 diameter from the wall (Mattson (2)) ; consequently the observed velocity of the particles at this depth is true electrophoretic velocity. Electrosmotic velocity of the water in t,he cell may be obtained by subtracting electrophoretic veloc-
OF MICROSCOPIC GLASS SPHERES ( X t ) FIG. 1. APPARATUS FOR PREPARATION
ity from the observed velocity of the particles a t the center of the cell. Both particles and cell were previously cleaned in chromic acid cleaning solution, the acid being removed by repeated washings in distilled water. The electrokinetic potential, 5, was calculated from the well-known formula : 4nVq
{=-
ED
where V represents either electrosmotic or electrophoretic velocity, 17 the viscosity of the medium, E the applied field strength, and D the dielectric constant of the medium.
COMPARISON OF ELECTROKINETIC POTENTIALS
937
As a further check, electrosmotic experiments were also carried out on several Pyrex capillaries by the method described by Quincke (3). This consists in the measurement of the head of pressure developed by electrosmosis in a capillary of known radius under an applied E.M.F. A diagram of the apparatus is shown in figure 2. The Pyrex capi!lary, C , is connected by transparent rubber tubing to the open reservoir, R, and the closed vessel, V. Platinum discs, P, are sealed into the two arms of the apparatus close to the rubber connections so that practically all of the potential drop is across the capillary. Platinization of the electrodes permitted large currents to be used.2 A larger capillary, the climbing tube T (about 1 mm. in diameter), is connected with the closed arm of the apparatus with rubber tubing. When a voltage is applied across the capillary C, the meniscus rises or falls in the climbing tube T according to the direction of the current. Since the cross section of T is very small with respect to that of R, the level of the liquid in R may be considered to
FIQ. 2. APPARATUSFOR ELECTROSMOTIC MEASUREMENTS IN SINQLE GLASS CAPILLARIES
be constant and the entire change in level takes place in T. The horizontal projection of the movement of the meniscus is measured by means of a microscope fitted with a calibrated scale in the eyepiece. The vertical height through which the liquid rises or falls for a given applied E.M.F. is the product of the tangent of the angle, a, which the climbing tube makes with the horizontal and the horizontal projection. The capillary C and the climbing tube T are thoroughly cleaned by heating in chromic acid before each determination. The electrokinetic potential is calculated from the formula:
Pir?
