Cataphoresis

CATAPHORESIS, the movement of suspended particles under the influence of an external electric field. - Electrokinetic phenomena. If particles carrying an electric charge on their surface are suspended in a liquid, then when an electric current is passed, the suspension will be transferred to the oppositely charged pole. This phenomenon, already observed at the beginning of the last century by Reuss, was named C. or electrophoresis. The direction of C. is determined by the sign of the electric charge of the suspended particles: positive particles are transferred to the cathode, negative ones to the anode. The speed of C. depends on the magnitude of the boundary potential difference. If a solid body is fixed immovably in the form of a porous plug or diaphragm dividing the vessel between the electrodes, then when current is passed, the liquid moves: instead of C., electroosmosis occurs. The study of electrokinetic phenomena (the movement of liquid or particles suspended in it in an electric field) allows one to even in the smallest particles to investigate the electrical properties of their boundary surface, the magnitude and sign of the boundary potential difference. - The theory of electrokinetic phenomena was quantitatively developed mainly by Helmholtz, and then by Smoluchowski and Perrin. The speed of C. is expressed by the formula: where E is the potential difference (in volts per cm) caused by the external electromotive force, ζ is the potential difference at the boundary of the solid particle and liquid, η is the viscosity and D is the dielectric constant of the liquid. In this formula there is not a single quantity depending on the size of the particle. This is explained by the fact that as the surface of the particle increases, its charge (and consequently the force acting on it) increases to the same extent as the resistance it encounters when moving. Thus, the speed of C. is directly proportional to the boundary potential and does not depend on the size of the particles. With the same value of the boundary potential, all particles suspended in a given liquid, regardless of their size, move in a constant electric field with equal speed. Experience shows that in water, bodies of very different chemical nature (cells of bacteria and protozoa, microscopic particles of suspensions and ultramicroscopic, colloidal particles) have approximately the same speed of C. Obviously, very different bodies become charged in water to approximately the same potential, which, as calculations show, is equal to several tens of millivolts. In an external field giving a potential difference of 1 volt per 1 cm, the speed of movement of suspended particles is on average 2-4 μ/sec. It is interesting to note that the speed of electrical transfer of most ions has approximately the same magnitude, which thus remains unchanged with an increase in size by several thousand times. The absolute magnitude of the electric charge at this time, of course, increases many times over. The charge of a colloidal particle can exceed the charge of a monovalent ion by several thousand times. According to Helmholtz's theory, the boundary potential difference depends on the uneven distribution of ions at the phase boundary. Ions of the same sign predominate near the surface of the solid body, creating an electric charge here. The forces of electrostatic attraction cause an accumulation of an identical excess of oppositely charged ions in the immediately adjacent layer of liquid. Helmholtz called such a distribution of ions an electric "double layer" (see). One half of the double layer directly adjoins the surface of the solid body, the other lies in a freely movable layer of water. If the solid body is immobile, the outer ionic layer carries the adjacent liquid particles with it in its movement. The potential difference between the inner and outer parts of the double layer represents the electrokinetic potential difference, from which the phenomena of C. and electroosmosis depend. Thus, the electrokinetic potential difference is connected with a narrow zone of the double layer and, as Freundlich showed, can differ greatly from the measurable by other methods total difference between the potentials prevailing in the middle of each phase. Methods of measurement. To measure the speed of C., the suspension under investigation is placed in a U-shaped tube and pure water is carefully layered over it, into which both electrodes are immersed. A sharp boundary is maintained between the suspension and the water, the speed of movement of which is easy to measure. The measurement is significantly complicated in cases when not homogeneous suspensions as a whole, but individual microscopic objects, for example isolated cells or even the smallest microscopic or ultramicroscopic particles, are subjected to investigation. For the microscopic study of such small particles, special chambers have been constructed, for example Northrop's chamber. However, when using such microscopic chambers, it is necessary to take into account one serious source of errors: at different distances from the glass bottom, the suspended particles move with different speeds. In negatively charged particles, it reaches its maximum value at the average depth of the chamber and gradually decreases as it approaches both glass plates. In the immediate vicinity of the latter, the movement of particles can even occur in the opposite direction. This heterogeneity is due to the superposition of electroosmosis phenomena on C. The water at the boundary with the glass becomes positively charged and, moving toward the cathode, carries negatively charged particles with it. In the middle of the closed chamber, the water transferred along the walls flows back to the anode, here adding its speed to the speed of C. itself (for positively charged particles, on the contrary, the speed is increased at the boundary with the glass, decreased in the middle of the chamber). The true speed of the latter is equal to the average of the speeds in all successive layers. In practice, one can limit oneself to measuring the speed in two layers - at 1/4 and 3/4 of the height of the chamber and with sufficient approximation take for the speed of C. the value: U = 3/4 U1 + 1/4 U2. In some cases, when the main interest is not in the absolute magnitude, but only in the sign of the charge, one can limit oneself to a rapid determination of the direction of C. (without measuring its speed). For this, it is desirable to accelerate it as much as possible. As the above formula shows, C. proceeds with the greater speed, the greater the potential difference caused by the external electric field. At the same time, the electric current passing through the solution under investigation should be as weak as possible to avoid secondary changes occurring during electrolysis. This combination of a large potential difference and a weak current is achieved by the use of so-called "semiconductors", which give high resistance. As electrodes in the solution, for example, two strips of parchment paper or two sticks from unfired clay, impregnated with distilled water, are immersed. If the substance under investigation is some colloidal paint, then after several minutes or even seconds, its precipitation on the corresponding electrode can be seen under the influence of the sharp potential difference established at its boundary with the solution (Fürth). Influence of electrolytes. As indicated above, the speed of C. is determined by the magnitude of the boundary potential. Therefore, all factors that change the magnitude of the latter accordingly also affect C., and not only the speed of C. can change, but also its direction itself. Here the relationship of the dielectric constants (see Dielectrics) of the suspended particle and the liquid and the nature of the dissociation of the suspended body are also of importance; however, the adsorption by the particle of certain ions is of greatest importance. Even if the dielectric constant or the chemical composition of the suspended body causes the appearance of a certain surface charge, the presence in the solution of a sufficient amount of strongly adsorbed ions can change its absolute magnitude or even its sign. Numerous observations have shown that to reduce the existing electrokinetic potential or even, with a sufficiently strong action of the electrolyte, to reverse its sign (for "recharging"), the adsorption of an ion opposite in sign to the charge of the surface of the solid body is of decisive importance. In the case of a negatively charged surface, the change in the sign of the surface charge is produced by cations, with a positively charged surface by anions. Among monovalent ions, H+ and OH- ions have high activity. Therefore, in the case of a negatively charged surface, alkalis strengthen the charge, while acids reduce it, and with a higher concentration even recharge the surface, giving it a negative charge. On a surface charged positively, acids and alkalis have the opposite effect. The addition of acids or alkalis can therefore change not only the speed but also the direction of C. For more about the change in surface charge - see Potentials.