COLLOIDS

From Big Medical Encyclopedia

COLLOIDS (Greek kolla glue + eidos a look; synonym colloid systems) — disperse systems with particles, rather large in comparison with molecules of gases and usual liquids, with a radius of 10 - 9 — 10 - 7 M, or 0,001 — 0,1 microns. To. can represent the colloid solutions called sols (see), or the gelatinous (structured) systems — gels (see) and jellies. Solutions of proteins, polysaccharides, nucleinic to - t and other biologically active agents represent To., combined under the general name of biocolloids. Colloid systems make that basis, without cut it is impossible to imagine existence of all live.

To. are eurysynusic in the nature — organisms of plants, animals and the person consist of them, they are in the atmosphere, in oceans, in soils and some minerals. Receiving a number of pharmaceuticals, production and processing of many minerals, production of food, clothes and footwear, construction and other materials are connected with creation and change of colloid systems. Knowledge of properties K. important for understanding of many biochemical, and biophysical, the processes including happening and in a human body. Many methods developed for a research K., find broad application in problem solving, facing medicine.

So-called dispersoids, or micellar To., which particles are insoluble in the environment surrounding them called by a dispersion medium form a dispersed phase. Micellar To. call also lyophobic, emphasizing with that lack of direct interaction between substance of particles and the dispersion medium surrounding them. Examples micellar To. the sols of silver used in medical practice as antiseptic agents under the names «colloid silver» and «protargol» in which the most fine particles of silver — the micelles consisting of several hundred atoms everyone are weighed in an aqueous medium can serve.

So-called molecular (macromolecular) colloids represent solutions (or jellies) high-molecular compounds, a pier. weight (weight) of which more than 10 000. These are proteins, nucleinic to - you, polysaccharides, and also numerous synthetic polymers. The diluted solutions molecular To. contain the particles representing separate big (sometimes curtailed into balls — a globule) molecules — macromolecules; solutions of higher concentration contain so-called swarms, or the associates consisting of rather small amount of macromolecules. Molecular To. are, as a rule, single (homogeneous) mixing systems, and, therefore, the names «dispersed phase» and «dispersion medium» can be applied to them only conditionally. Earlier molecular To. called lyophilic, assuming that their degree solvations (see) it is very big (i.e. solvation shells represent multimolecular layers of solvent), however measurements of degree of solvation showed that high-molecular compounds of a solvatirovana in the same measure, as low-molecular weight compounds. Thus, the terms «lyophobic» and «lyophilic» underlining existence of solvation molecular To. and absence it at micellar To., can be recognized unsuccessful since interaction with molecules of a dispersion medium on a surface of particles micellar To. occurs sometimes even more than at molecular To.

Except the described two types K., there are systems called by hemicolloids or seven-colloids in which it is possible to observe equilibrium transitions: true solution ⇄ sol ⇄ gel, i.e. true solutions representing in one conditions, and in others sols, the structured liquids, gels. These transitions can happen reversibly at change of temperature, concentration, the size pH, ionic strength of solution (i.e. at addition of electrolytes). Solutions of soaps and other surfactants, tanning matter (tanning agents of a plant origin), some organic dyes can be examples of hemicolloids (benzopurpurine, a geranin, etc.). The colloid surfactants (CS) are most studied. They are applied as emulsifiers of slightly soluble liquids in water, napr, in production cosmetic and pharm, drugs, as the means facilitating formation of dispersions of some solid matters, napr at floatation benefication, to processing of fabrics in the textile industry as frothers in the conditions of an ognetusheniye and as flocculants at water treatment (see. Flocculation ).

Distinguish the following types of KPAV: ionogenic (anionic, cation-active, ampholytic), nonionic.

Anionic KPAV are bile acids (see) and their salts — the dressing gowns acting as emulsifiers at absorption of fats in intestines, salt (preferential sodium) the highest fat to - t — stearin, palmitic, etc., long since used as [[ | washed SOAP ]] (see), sodium salts of organic sulfonic acids — alkyl - and alkyl aryl sulphonates, and also salts of ether acids a chamois to - you and the higher fatty alcohols which are among economic and effective detergents (see).

From cation-active KPAV the organic salts of quarternary ammonium which are N-alkyl-replaced pyridine and other its derivatives having bactericidal properties are most often applied; they are used in medicine, and also in the equipment as inhibitors of acid corrosion of metals.

Ampholytic KPAV are polypeptides and to a certain extent proteins (see).

Many nonionic non-toxic KPAV apply in the food industry (e.g., in bread baking, confectionery production, production of ice cream, margarine) as flocculants, frothers or emulsifiers.

The most important characteristic of solutions of hemicolloids is critical concentration of a mitselloobrazovaniye, i.e. the minimum concentration of solute { mol/l, %), at a cut it is possible to find a colloid phase experimentally. Obraaovaniye of micelles in solutions of hemicolloids can be observed on increase in intensity of scattered light; the size of critical concentration is determined, measuring change of surface intention, electrowater content and other parameters connected with change of concentration of hemicolloid.

The big sizes of micelles of KPAV and their special structure explain the phenomenon solubilizations (see), hydrophobic binding or colloid dissolution (usually in water in the presence in it KPAV) substances which without them are almost water-insoluble.

Preparation of solutions molecular To. from high-molecular compounds usually comes easy therefore all considered methods of preparation belong only to micellar To.

Disperse systems, including and To., receive in two opposite ways: 1) smashing, or dispersion, rough, rather coarse particles on smaller; these are methods dispersions (see), or dispersions; 2) by aggregation of molecules in more coarse particles (see. Aggregation ); these are the methods of condensation called so by analogy with the processes of condensation coming, e.g., at formation of droplets of fog from steam.

Use methods of dispersion usually for receiving coarse-dispersion systems — dispersions, i.e. suspensions, emulsions and powders. Practically it is methods of mechanical and ultrasonic crushing of mother substances, and for aerosols — their spraying. Receiving sols electric spraying in a voltaic arch of the metal electrodes placed in a dispersion medium can only conditionally be considered as dispersion since at high temperature of an arch metal of electrodes at first evaporates, and then it is condensed with formation of colloid particles, at the same time there can be also a separation of particles directly from electrodes and their hit in a dispersion medium. Peptization (see), i.e. formation of sols from gels or friable rainfall at effect of some substances — peptizators which, being adsorbed on a surface of colloid particles, tell them affinity to a dispersion medium, it is also impossible to refer only to methods of dispersion since at peptization there is not a change of a degree of dispersion of the particles forming gel (or a deposit) but only separation of already available particles.

The choice like crushing of hard materials depends on their properties: brittle crush blow, viscous — attrition. In spherical mills — homogenizers (see. Homogenates ) a certain type — it is possible to receive particles about 2 — 3 microns in the diameter, especially if to use vibration. More pulverizing is reached in so-called colloid mills which provide a possibility of receiving suspensions with average particle sizes of 1 micron (i.e. receiving To. with their help it is not reached yet).

Ultrasonic dispersion happens at the expense of the cavitational ultrasonic waves destroying material. The degree of dispersion increases with increase in frequency of ultrasonic fluctuations. Aerosols can be received by means of ultrasound by spraying of liquids or liquid solutions. The method allowing to receive To. with high concentration of a dispersed phase, it is used in medical practice for receiving aerosols of water solutions of antibiotics.

Spraying of solution compressed air by means of a spray is long since used for receiving aerosols, including and in medicine. If to connect a spray to a pole of voltage, then in electric field steadier aerosols turn out. The industry devices for receiving medicinal substances in the form of such aerosols are issued.

Formation of solutions molecular To. also sometimes consider as dispersion. However at the same time there can be an unlimited swelling which is coming to an end with dissolution, the new (disperse) phase does not arise or the bulked-up jellies which are in balance with the solution containing more low-polymeric fractions of high-molecular compounds are formed. There are solutions K., in which big molecules of high-molecular compounds, being in «bad solvent», are curtailed into compact skein — a globule, having an accurate interface with the environment surrounding it; such a globule are available in solutions of proteins. Are similar to such systems natural latex of many plants (e.g., rubber-bearing plants), and also synthetic latexes in which the curtailed particles (usually larger, than colloid) are in an aqueous medium, and their stability is defined by special substance — the stabilizer.

Receiving disperse systems by condensation can be carried out actually condensation, evaporating substance and creating then suitable conditions for formation of fine particles; environmental change or other conditions of receiving the condensed substance so that substance from soluble became insoluble, napr, changing composition of solvent, entering into it the liquid incapable to dissolve this substance; carrying out the chemical reaction which is followed by formation of almost insoluble substances.

In nature fogs and clouds, i.e. aerosols, are formed at steam condensation of water in the atmosphere. By the method actually condensation for receiving lyophilic sols (lyosols) developed in the USSR by C.3. Roginsky and A. I. Shalnikov, the substances forming a dispersed phase evaporate in vacuum and couples condense on the surface cooled by a liquid air.

The method of replacement of solvent is convenient for receiving many lyosols. For receiving sol substance is dissolved in suitable solvent and solution is poured out in the large volume of another, the liquid which is not dissolving this substance, about a cut the first solvent beyond all bounds mixes up. The greatest distribution was gained by methods where almost insoluble substance is formed as a result of the chemical reaction happening in that liquid, edges serves further as a dispersion medium. Quite so prepare lyosols of such metals as silver and gold.

Colloid solutions of metals in a solid dispersion medium, napr, in glass, are called pyrosols. They can have the same coloring, as lyosols. For the first time M. V. Lomonosov gave the correct explanation of the nature of the ruby glasses representing colloid solutions of gold in glass.

In order that in system there could be a condensation of this substance, it shall be peresyshchenny in relation to this substance, in it conditions for emergence of germs of a new phase, napr, nucleuses of crystal shall be created; the substance forming a dispersed phase shall be slightly soluble in surrounding liquid and, at last, the conditions interfering aggregation of colloid particles with each other shall be created. Since usually nucleating and their growth happen at the same time, unequigranular sols, i.e. the sols containing particles of the different sizes turn out. The monodisperse sols containing particles of almost identical size receive by means of special methods. E.g., for receiving monodisperse sol of silver (or gold) at first prepare so-called germinal, i.e. very high-disperse sol, and then enter a certain amount of such sol into solution of salt of this metal and make its recovery. The particle size of the monodisperse sol received at the same time will be that more, than less germs were entered into solution before its recovery.

For purification of sols of low-molecular impurity, their concoction, for fractionation of colloid particles by the sizes and, at last, for separation of a dispersed phase from a dispersion medium methods are often used ultrafiltrations (see) and dialysis (see). Some sols steady at elevated temperature, it is possible to concentrate evaporation, others — partial removal of a dispersion medium ultrafiltration.

Many To. are brightly painted and, like all painted systems, can absorb light. Besides, To. scatter light falling on them, and thanks to it the beam of light passing through a colloid system perpendicular to an axis of sight of the observer forms the shining cone called by Faraday's cone — Tyndall.

Light scattering micellar To., where particle sizes there is less than a half of wavelength of visible light, occurs only due to diffraction, i.e. each particle, being lit, does not reflect light, and disseminates it in all directions. Such particles are as if light sources, and it gives the chance to observe them in ultramicroscope (see) as the separate shining and randomly moving points. By calculation of particles in the known microscopic volume of the diluted sol, knowing its concentration, it is possible to determine the sizes of colloid particles, assuming that they have the form of cubes or balls. If the particle shape strongly differs from spherical (rhabdoid or plastinchatovidny particles), then it can be noticed on blinking of particles in sight of an ultramicroscope. The form of colloid particles and their sizes can be defined also with the help submicroscopy (see).

As J. Rayleigh, for the spherical particles which are not carrying electric current showed, intensity of scattered light depends on a particle size of a dispersed phase, their concentration, a difference of indices of refraction of a dispersed phase and a dispersion medium and is especially strong — from the wavelength of incident light, and short waves most intensively dissipate. So, sols of uncolored substances seem bluish in reflected and yellowish — in a transmitted light because the blue beams having shorter wavelength dissipate particles stronger. Such distinction of coloring by consideration To. in the passing and reflected light, caused by light scattering, received the name opalescences (see). Molecular To. in much smaller degree scatter light passing through them, for this reason of a macromolecule cannot be observed in the form of separate particles in an ultramicroscope. Light scattering in such solutions occurs for the same reason that in gases, liquids and usual solutions where molecules owing to the chaotic movement form micro and ultramikrooblasta (fluctuations) with the increased concentration of molecules. These fluctuations exist only very short time terms, continuously arising and dissipating. So, in the atmosphere due to fluctuations of molecules of gases blue color of the sky is explained by dispersion of a sunlight; light scattering in sea water caused color of the sea. Measurements of intensity of scattered light or the return to it sizes — coefficient (t) — are carried out by means of nephelometers (see. Nefelometriya ) or photometers (see. Photometry ) also are used for determination of the molecular weight (weight) To. and their concentration in sols.

Many To. have high kinetic and small aggregate stability. At observation in an ultramicroscope of colloid solutions it is possible to see that colloid particles are in a condition of heavy and random motion, a cut received the name of Brownian motion by the name of English the botanist R. Brown for the first time observing such movement of microscopic particles of flower pollen in 1827. Motion of the molecules of a dispersion medium, surrounding colloid particles is the reason of Brownian motion.

Osmotic pressure (see) unstable micellar lyosols have not enough and changeably because of aggregation of their particles, and at solutions molecular To., napr, at solutions of protein which can be prepared in rather big concentration measurement of osmotic pressure gave the chance to determine the size of the molecular weight (weight) of solute.

The provision of colloid particles in the environment is defined by their Brownian motion aiming to distribute them evenly and gravity. When between these two factors balance is reached, colloid particles are definitely distributed on height, i.e. concerning the Earth's surface. To. are kinetic steady in gravitational field, and establishment of so-called sedimentation equilibrium it is necessary to use ultracentrifuges, i.e. the devices allowing to receive centrifugal force to their sedimentation or overseeing, on several orders (almost by 105 times) exceeding acceleration of the gravity (g). The ultracentrifuge was for the first time created in 1923 the Swede, the scientist T. Svedberg who with its help determined sizes of the molecular weight (weight) of many proteins and characterized also a form of their molecules, having proved that particles of proteins are separate large molecules (macromolecules). Results of such measurements for proteins led to a conclusion that their solutions as well as solutions of other polymers, it is impossible to consider as disperse heterogeneous systems. Further researches of many scientists (in the USSR — hl. obr. V. A. Kargina and it sotr.) it was shown that solutions of polymers are thermodynamic reversible homogeneous (single-phase) systems. Thus, colloid systems were more or less accurately divided into the specified two types: micellar To. and molecular To., being true solutions, the molecular weight (weight) of solute in which reaches big sizes owing to what such solutions show many properties of colloid systems.

Aggregate stability of many To. is explained by the fact that their particles have electric charge which can be caused by preferential adsorption (see) ions of a certain sign from a dispersion medium, dissociation of the molecules forming colloid particles, or both that and another (see. Adsorption , Dissociation ). The charge can be found in colloid particles, placing solution in constant electric field where charged particles move to one of electrodes (see. Electrophoresis ). Sign of a charge of particles To. it is easy to determine by to what electrode they move. Since colloid solutions in general of an elektroneytralna, in the dispersion medium surrounding colloid particles shall be electric charges (ions) of an opposite sign in accuracy compensating charges of particles.

Studying of electric properties K., characterized by existence of potential jump on border of a dispersed phase and dispersion medium, led to a thought of existence around colloid particles of a double electric layer from charges of an opposite sign. G. Helmholtz in 1879 suggested that the double electric layer represents a peculiar condenser, inner lining to-rogo is on a surface of a colloid particle, and external — in a fluid dispersion medium. L. G. Gouy and D. Chapman created the theory of mobility of charges of an external facing — ions owing to what distribution of these ions is diffusion, and their concentration decreases with removal from a surface of a particle. Such arrangement of ions — carriers of charges leads to education around each colloid particle of a peculiar ionic sphere. However the distribution of charges in an external facing constructed according to this theory not quite precisely corresponded to their true arrangement since charges of ions were considered as dot. The accounting of volume of ions, and also their specific, i.e. not depending on their size and a sign of a charge, interaction with a surface, allowed Stern (O. of Stern) to give more correct picture of a structure of a double electric layer. The inner layer of the ions which received the name potentsialoobrazuyushchy is densely adjoined nek-paradise by a part of opposite loaded ions called by antiions. This part of antiions moves together with a particle and forms the layer called by the adsorptive. Other antiions are diffuzno distributed in a dispersion medium.

According to the rule Panetta and Faience, as potentsialoobrazuyushchy ions the ions capable to complete a crystal lattice of the unit or forming with the ions which are its part, almost insoluble connections usually act.

A micelle call set of a colloid particle and the antiions neutralizing its charge in a dispersion medium. The size of potential jump ξ (dzet) on border between a moving colloid particle and a dispersion medium is called electrokinetic potential (see. Electrokinetic phenomena ) Usually its size is less than the size φ — the electrothermodynamic (electrochemical) potential of particles. Also cases when these potentials had various signs are described. Sizes φ and ξ can strongly differ. This results from the fact that when one phase moves concerning another, border of this movement is the surface of the thinnest nappe which is directly connected with a surface of a dispersed phase. For this reason measurement of potentials on the phase boundary, moving (sliding) relatively each other, gives the size of potential jump on a slipping surface. Thus, ξ-potential — is a potential jump between that part of liquid, edges is directly connected with a surface of a particle, and all other liquid. The more thickness of a diffusion layer, the is higher an absolute value of ξ-potential, a charge of colloid particles and forces of pushing away between them. Therefore ξ-potential is often considered as a measure of stability To., their resistance coagulations (see).

The double electric layer is eurysynusic in various biochemical, systems, in particular, it is available on a surface of any biol, membranes.

A condition of colloid solution, at Krom ξ-potential of colloid particles is equal to zero, is called isoelectric. In such state colloid solutions are less steady and, as a rule, coagulations are exposed.

To. usually over time change the properties happening at the same time processes combine under the general name of aging K. Tak, at sols of noble metals aging is expressed in enlargement of particles by their aggregation or recrystallization that can lead to change of a number of properties of colloid solutions: colors, ξ-potential, etc. At molecular To. and hemicolloids of change of viscosity of colloid solutions (η) become the most characteristic eventually; during the aging size η most often increases that can lead to gelation or jelly. This process is called gelatination or jellification and is explained by formation of more or less strong bonds between colloid particles. Increase in value η in the course of aging happens hl. obr. due to so-called structural viscosity (see. Viscosity ). E.g., such viscosity appears at solutions of starch and gelatin, a blood plasma, protoplasm, synovial fluids, at sols silicon to - t. Structural viscosity results from interaction of colloid particles with each other that especially considerably at the systems with rather high concentration containing anizodiametrichesky particles (rigid or flexible sticks, threads, plates); certain sites of a surface of such particles can be protected in various degree from adhesion. Units of particles are so formed, between to-rymi the motionless (immobilized) liquid can be concluded. At small flow rates and weak coupling of particles their units manage to be recovered and the viscosity caused by adhesion of particles remains. At the high speed of a course of communication between the particles forming structure are broken off, and the volume of the immobilized liquid between them and viscosity decrease. If the number of bonds between particles is big, then the current is already impossible, and such structured system behaves as a solid — gel or jelly. Aging can continue and further, and gel, being gradually condensed, decreases in volume, keeping usually the initial form and allocating a part of liquid (dispersion medium). The described phenomenon carries the name of a syneresis. The similar picture of aging can be observed also at hydrosols of many water-insoluble metal hydroxides — iron, aluminum, chrome, etc., and also at sols silicon to - t and at some hemicolloids, napr, at solutions and jellies of dye of a geranin. Aging can be irreversible if process is followed by chemical transformations, reversible or, at last, partially reversible. The phenomena accompanying education and aging To., were studied by means of a submicroscopy by V. A. Kargin and 3. Ya. Berestneva which established that on the first mode of formation colloid particles micellar To. are amorphous units and only later a nek-swarm turn time in the course of aging into crystalline state.

Some gels or jellies under the influence of mechanical influence (stirring, hashing, etc.) are liquefied, but through a nek-swarm the liquids received from them spontaneously gelatinize time (in a quiet state). This phenomenon is called thixotropy (see).

Aging is irreversible if bonds between particles are formed due to their chemical interaction or merging of crystals. E.g., the structures which are formed at a blood coagulation or during the curing of cements when there is a recrystallization are irreversible.

Aging often is reversible process for bodies (e.g., educated high-molecular compounds) which are capable to to swelling (see), i.e. to the absorption of liquid, steam or gas which is followed sometimes by significant increase in volume To.



Bibliography: see bibliogr, to St. Colloid chemistry .


I. N. Putilova.

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