POLARIZATION

From Big Medical Encyclopedia

POLARIZATION — the collective concept combining a number of the phenomena, various by the nature.

In an electrophysiology use this term for the characteristic of potential difference on both sides of the membrane dividing external environment and contents of a cell, and their change under the influence of various irritants and also during generation of bioelectric potential (see. Bioelectric potential , Excitement , Depolarization ). Use the term «polarization of light» for the description of dimensional orientation of magnetic and electric components of natural (unpolarized) light (see. Polarization of light ). The term «polarization» is applied also to the characteristic of the so-called passive electric phenomena — the phenomena connected with passing through biol, systems of direct or alternating electric current (see. Conductivity of biological systems ). Below two types of P. — the polarization of dielectrics and electrochemical polarization important for understanding of the mechanism P. biol, systems are considered.

Polarization of dielectrics — emergence on the surface of substance (dielectric) of the connected charges and the return field corresponding to them weakening external electric field; it consists of the sum of so-called dipole moments of the molecules which are in unit volume of substance. Dielectrics call the substances incapable to carry electric current in usual conditions. According to classical representations it is connected with restriction of mobility of charges, rather initial positions which only are displaced in atom or a molecule of substance under the influence of electric field. According to quantum submissions of the modern zonal theory of a structure of a solid it is connected with need of a big metabolic cost on creating a stock of free electrons (carriers of current) in a so-called power zone of conductivity. Dielectrics are such structural components biol, fabrics and cells as proteins, lipids and other macromolecules. Distinguish the so-called elastic P. of dielectrics consisting in shift it is elastic the connected charges forming a molecule (atom or an ion) of dielectric and relaxation, or orientation, P. of dielectric consisting in emergence of orderliness in chaotic thermal agitation of poorly connected dipole molecules (ions) under the influence of electric field.

Depending on what particles participate in elastic P., distinguish electronic (shift of electrons concerning kernels in atoms or ions), atomic (shift of various atoms forming a molecule from each other and asymmetrical redistribution of valence electrons), ionic (shift of opposite loaded ions in a molecule, an ionic crystal from their situation balances). Elastic P. of dielectrics is established almost instantly (10 - 15 — 10 - 12 sec.).

For understanding of mechanisms of a number of the phenomena proceeding in biol, systems, it is important relaxation the Item. The essential contribution to development of the theory of P. of this type was made by P. Debye. Relaxation P. arises in polar dielectrics owing to preferential orientation of poorly connected dipole molecules in electric field. Time of establishment such P. (time of a relaxation) for the majority of dielectrics lies within 10 - 10 — 10 - 2 sec. also depends on temperature and force of interaction between molecules (see. Relaxation phenomena ). In complicated polar molecules (e.g., molecules of protein and other large organic molecules) also intramolecular rotation can be observed: incomplete or full rotation of various parts of a molecule. Also dielectrics with heterogeneous structure are of special interest, separate parts a cut have various dielectric properties that leads to formation of interfaces (layers). During the room of such system in electric field on interfaces electric charges will collect. The suspension of the spherical particles surrounded with the membrane which is badly carrying electric current can serve as such system (e.g., a suspension of erythrocytes). The big contribution to development of the theory of this type P. was made by J. Maxwell and Wagner (To. Wagner). In variation electric field of P. of dielectrics develops as well as in constant electric field; i.e. until dipoles manage to follow change of the field, the frequency of change of the field is lower, than own frequency of fluctuations is elastic bound particles or the frequency of a relaxation of poorly bound particles. At increase in frequency of fluctuations of electric field are observed the expressed dependence of dielectric properties of substance on frequency accompanied with considerable dielectric loss and calorification (see. Dispersion ).

Nek-ry substances in usual conditions have areas with a certain orientation of polar molecules called by domains. Set of the domains oriented in various directions forms structure of the dielectrics carrying the name a ferroelectric material (seignette salt, barium titanate, etc.). In electric field domains of a ferroelectric material are guided in the direction of the field, providing greatest possible the Item. Polarization of molecules or the structural groups formed by them in nek-ry crystals (quartz, a ferroelectric material) can result from mechanical deformation under the influence of external forces. At the same time on the surfaces limiting this body there are electric charges and there is a potential difference (see. Piezoelectric phenomena ).

Polarization electrochemical — the change of potential on the phase boundary connected with structural change of the interfacial layer or change of potential of an electrode from its equilibrium state at course on it electrochemical reaction.

On the interphase boundary, each of to-rykh contains charged particles or dipole molecules owing to course of various fiziko-chemical processes, there is a formation of a peculiar condenser — the double electric layer creating potential difference on the phase boundary (see. Electrodes , Electrokinetic phenomena ).

Emergence of jump of potentials can come owing to charge transfer through the phase boundary, nonequivalent anion adsorbtion and cations, adsorption and orientation of dipole molecules, deformation and polarization of atoms and molecules in heterogeneous force field of the interfacial layer, a supply of charges from an external source of current etc. The potential difference can arise also owing to distinctions in permeability of the ions through membranes of the different nature dividing solutions (see. Membrane equilibrium ). Various processes proceeding in live organisms at any levels of the structural organization (molecular, subcellular, cellular, fabric and organ), are also followed by emergence of potential difference in these systems that is one of the most characteristic features of all live.

The system consisting of the metal shipped in solution of its salt can be a private, simplest example of formation of potential difference on border of two phases. At contact of phases reactions of recovery of ions of metal and ionization of metal proceed. At balance when speeds of both processes are equal, on border metal — solution is established the potential difference called by equilibrium potential. During establishment of equilibrium potential and at its achievement there is an exchange of ions between metal and solution. As charged particles participate in exchange, express its intensity in current units and characterize by current of exchange. Currents of exchange carry to 1 cm 2 interfaces electrode solution. Intensity of exchange depends by nature an electrode and changes over a wide range: from 10 - 15 to 1 and/cm 2 . Electrodes in size of current of exchange can be divided on reversible, or ideally not polarizable, and on irreversible, or ideally polarizable. The big size of current of exchange is characteristic of reversible electrodes that provides constancy of potential of an electrode at a transmission of current from an external source. The small size of current of exchange is characteristic of irreversible electrodes owing to what all current from an external source goes for change of a structure of a double electric layer and change of capacity of the interface.

Electrodes find broad application in biological and medical a lab. to practice. Reversible electrodes use as reference electrodes of sources of electrical energy, and irreversible — for a research of the phase boundary and other electrochemical reactions (e.g., receiving oxygen, hydrogen, chlorine etc.).

Course on electrodes of electrochemical reactions is followed by change of potential of an electrode from its equilibrium value. Speed of response is defined by density of current, or the relation of the general current flowing on an electrode to the area of an electrode. Dependence of size of potential on density of current is called a polarizing curve. It is characteristic of this electrode reaction.

Electrochemical reactions are fullestly investigated for system metal — electrolyte. Main stages of reaction: a supply of the reacting particles to an electrode, a stage of electron transfer or ions through the interphase boundary, removal of reaction products from a surface of an electrode. The first and third stages have identical patterns and are called stages of a mass transfer. Speed of the process consisting of a number of consecutive stages is defined by the speed of the slowest stage, and P. is a consequence of slowness of one or several stages of electrochemical reaction. Depending on the nature of these stages distinguish concentration, electrochemical, chemical P., etc.

Concentration P. is connected with change of concentration of potentsialoobrazuyushchy substances in at - an electrode layer that is connected with a slowness of giving to an electrode or removal from it of the substances participating in reaction.

Electrochemical P. is caused by slowness of transfer of an electron, and this slowness is connected with the fact that electrochemical reaction demands a certain activation energy, edges depends on the potential of an electrode. The analysis of the polarizing phenomena in live systems is complicated owing to their complex organization. Data for nek-ry model systems are obtained: borders of two immiscible liquids, borders of a bisloyny lipidic membrane and water solution of electrolyte.

P.'s phenomenon in biol, systems is found during the passing through them direct or alternating electric current. At the same time the size of a direct current gradually decreases, and at shutdown of an external source of current the electromotive force directed opposite to the electromotive force of an external source is registered. The similar phenomena practically disappear at dying off of fabric that is explained with P.'s change cellular structures. It is used for the analysis of safety of structures of a cell during the development of methods of preservation, storage conditions of fabrics etc. During the carrying out various medico-biol. researches, in laboratory and physiotherapeutic practice it is necessary to consider existence of polarizing effects, in particular P. of electrodes, with the help to-rykh electric current is brought to biol, fabrics. P.'s phenomenon is considered at selection of the corresponding electrodes and conditions of carrying out measurements (see. Galvanization , Polyarografiya , Potentiometric titration , Elektrolecheniye ). P.'s phenomenon accompanying transformation of mechanical energy in electric and back (piezoelectric effect), apply in medical practice during the receiving ultrasound in supersonic generators and as the converter in various medical sensors (see. Hearing aids , Sfigmografiya , Ultrasound ).



Bibliography: Andreyev of V. S. Konduktometricheskiye methods and devices in biology and medicine, M., 1973; Boguslavsky L. I. Bioelectrochemical phenomena and interphase boundary, M., 1978; Damaskin B. B. and Petry O. A. Introduction to electrochemical kinetics, M., 1975; Troughs And., Dvorak I. and Bogachkova V. Electrochemistry, the lane from Czeches., M., 1977; Shvan G. P. Spectroscopy of biological substances in the field of alternating current, in book: Electronics and cybernetics in biol, and medical, the lane with English, under the editorship of P. K. Anokhin, page 71, M., 1963.

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