Dielectrics

DIELECTRICS, non-conductors, or insulators - bodies that poorly conduct or completely do not conduct electricity. Such bodies are, for example, glass, mica, sulfur, paraffin, ebonite, porcelain, etc. For a long time, in the study of electricity, attention was paid exclusively to bodies that carry or conduct electric charge. Faraday, who introduced the name dielectrics, showed that in electrical phenomena, the surrounding dielectric medium plays no less a role than the charged body itself. Dielectric constant. According to the law established by Coulomb, two bodies, separated by a distance r and carrying charges e₁ and e₂, act on each other with a force f₀ of electrostatic repulsion or attraction (depending on whether the sign of both charges is the same or opposite), equal to f = k·e₁·e₂/r² (1). If the same bodies are not in air, but in some other D., then the force will be different, smaller than in the first case. It can be expressed by the general formula: f = k·e₁·e₂/D·r² (2). The coefficient D entering this formula is called the dielectric constant; it represents one of the most important constants characterizing the physical properties of different bodies. The dielectric constant of air is taken as unity. It would be more correct to attribute this value to a vacuum; however, the dielectric constants in air and in a vacuum differ so little that this difference has no practical significance. For other bodies, the dielectric constant is greater than unity, and the electrical forces are correspondingly smaller. The influence of the dielectric constant can be easily verified, for example, by replacing the air layer in a capacitor with a layer of some D. The measurement of the changes occurring in this process forms the basis of the methods usually used to determine the dielectric constant. The values of the latter are given for several substances (at 18°) in the following table. Substance D Substance Benzene ...... Xylene ...... Toluene ...... Carbon disulfide . . . Ethyl ether . . . Chloroform . . . 2,26 2,35 2,34 2,64 4,3. 5,2 7,2 Propyl alcohol . . . » ethyl. . . . » methyl . . . Nitrobenzene . . . Hydrocyanic acid . 20,7 22,2 25,8 31,2 34,0 81,196 The weakening of electrical forces produced by D. can be understood if one imagines D. as consisting of a multitude of the smallest, electrically charged, and mutually isolated particles. The modern theory of the electrical structure of matter gives concrete and real content to this hypothetical structure of D. The weakening of the external electric field depends on the displacement of electrons produced by it or on the rotation of D. molecules, which represent dipoles. Influence on electrolytic dissociation. As first indicated by J. J. Thomson and Nernst, the magnitude of the dielectric constant of the solvent should have a strong influence on the degree of dissociation of the electrolyte. Between its constituent ions act electrostatic forces following Coulomb's law. These forces, combining oppositely charged ions into electrically neutral molecules, are weakened all the more, the higher the dielectric constant. An increase in the latter therefore increases the degree of dissociation. Quantitative measurements made by Walden gave excellent experimental confirmation of this theoretical conclusion. The same electrolyte turns out to be barely dissociated in benzene (D=2.3), very weakly in ether (D=4.3), significantly more in ethyl alcohol (D=26), and almost completely in water (D=81). The exceptionally high dielectric constant of water explains its well-known "dissociating action", thanks to which many electrolytes show very significant dissociation in aqueous solutions. The usual ideas about the "strength" of various electrolytes are based precisely on the fact that ordinary, most widespread solvent is water. In other media, the same electrolytes can be incomparably less dissociated. However, the figure given for the dielectric constant characterizes water as a pure solvent. Dissolved substances (see below) can themselves change the dielectric constant of the solution. No less influence than on the degree of dissociation of electrolytes, the dielectric constant has on the activity of ions. The presence of a number of features common to all independent particles in solution (both ions and molecules) for a long time kept in the shade the peculiarities of ions depending on their electric charge. Between ions, as between any charged particles, act electrostatic forces of attraction and repulsion. These electrostatic interionic forces, obeying Coulomb's law, reduce the mobility of ions, their kinetic energy, etc., compared to neutral molecules. The electrical conductivity of ions, the osmotic pressure they produce, their chemical activity are functions of these electrostatic forces and, like the latter, depend on the dielectric constant of the solution. Dielectric constant in biology. This influence on the degree of dissociation and on the activity of electrolyte ions determines the enormous importance that the study of the dielectric constant of various organic media has. Keller first drew attention to it. As measurements by Fürth, given in the following table, show, in various organic substances, vital media (blood) and individual tissues, the dielectric constant has very different values. Substance Substance Casein .... Hemoglobin . Starch . . . Dextrin . . . Cholesterol . Lecithin . . . 8,0 14,2 11,6 8,0 6,4 13,0 | 82,8 1 Blood serum Milk (human) . . . » (cow's) . . . Brain (white matter) » (gray matter) . . . Optic nerve . . . 85,5 85,5 75,0 66,0 90,0 85,0 89,0 The extremely high values of the dielectric constant observed in blood and especially in nervous tissue deserve attention. For the study of the physiological action of alcohol, it is also interesting that (according to measurements by Fürth, Keller and others) the addition of it in small amounts (up to 0.6%) to blood serum noticeably increases its dielectric constant (up to 93), while larger amounts of alcohol have the opposite effect. However, the total figures given do not at all convey the enormous local differences that must exist between different parts of a living cell. In this respect, the cell wall is of particular interest. The participation of lipoids in its construction should give it (as well as generally to lipoid phases of the cell) a very low dielectric constant. In the same direction should act other capillary-active substances located in a thin layer on the surface of the protoplasm. It is interesting to note that even water retains its high dielectric constant only in the bulk of the liquid. According to Blum, its dielectric constant drops to an unusually low value (approaching unity) if instead of its usual random distribution, water dipoles take a regular orientation, arranging themselves around a "hydrated" ion. If the hydration water bound by swelling colloids takes a similar orientation, then its physical properties (and along with them the properties of substances dissolved in it) should undergo profound changes. Very low dielectric constant, as is generally characteristic for capillary-active substances, have narcotics. In addition to other known effects of narcotics (blocking active surfaces, displacing other substances from them, etc.), their accumulation can create purely physical conditions that suppress dissociation and ion activity, delay biochemical processes. These few, still very fragmentary data and considerations show what enormous importance for the life activity of the cell the dielectric constant of its constituent parts has. The study of the dielectric constant of a living organism and in particular of the individual microstructures of a living cell should be recognized at present as one of the most urgent tasks of biophysics.