CHAPTER XV
THE COLLOIDAL CONDITION
Reference has frequently been made, in preceding chapters, to the fact that proteins, enzymes, lipoids, etc., exist in the protoplasm of plants and animals in the colloidal condition. The properties and uses of these compounds by plants depend so much upon this fact that, before proceeding to the consideration of the actual physical chemistry of protoplasm itself, it will be appropriate and profitable to give some attention to the nature and significance of the colloidal condition of matter and of some of the phenomena which grow out of it.
Every discussion of the colloidal condition in general properly begins with reference to the work of the English physicist, Thomas Graham, who carried on his investigations of the so-called "colloids" through a period of forty years, beginning with 1851. His most important results were published, however, from 1861 to 1864. Graham studied the diffusibility of substances in solution through the parchment membrane of a simple dialyzer. As a result of his earlier investigations, he divided all the chemical compounds which were known to him into two groups, which he called "crystalloids" and "colloids," respectively, the first including those substances which readily diffused through the parchment membrane and the second those which diffused only very slowly or not at all. He at first thought that crystalloids are always inorganic compounds, while colloids are of organic origin. He soon learned, however, that this distinction in behavior is not always related to the organic or inorganic nature of the compound. He further discovered that the same individual chemical element or compound may exist sometimes in crystalloidal, and sometimes in colloidal, form. This latter discovery led to the conclusion that diffusibility depends upon the _condition_, rather than upon the _nature_, of the material under observation.
As a result of the long series of investigations which were stimulated by Graham's work, the modern conception is that diffusibility is a _condition_ of matter when in minute subdivision, or in solution, in some liquid, as contrasted with its _state_, or condition, when existing alone. That is, the _state_ of a substance may be either gaseous, liquid, or solid; and its _condition_ when in solution may be either crystalloidal or colloidal. Substances which are in crystalloidal form, in true solution, exist there in molecular or ionized condition; but, as will be pointed out below, when in the colloidal condition they exist in aggregates which are somewhat larger than molecules, but not large enough to be visible as individual particles under the ordinary microscope, even under the highest magnification which has yet been obtained. Colloidal particles are, however, generally visible under the Zigmondy "ultramicroscope." (See below.)
The use of the word "colloid" as a noun, or as the name for a substance which is in the colloidal condition, is of the same nature as the use of the words "gas," "liquid," and "solid," in such statements as "ice is a solid," "water is a liquid," or "steam is a gas," etc.; i.e., the noun represents a state or condition rather than an actual object or thing. Hence, the expression "enzymes are colloids," means only that enzymes exist in the colloidal condition, and not that enzymes represent a definite type of substances having the group name "colloids."
THE COLLOIDAL CONDITION A DISPERSION PHENOMENON
When one substance is distributed through the mass of another substance, the mixture is said to be a "two-phase system," composed of the _dispersed phase_, or substance, and the _dispersion medium_, or _continuous_ phase, through which the other substance is distributed. The following examples illustrate the possibilities of such two-phase systems:
(1) Dispersion medium a gas. (_a_) Disperse phase a liquid--mist in the air. (_b_) Disperse phase a solid--smoke or dust in air.
(2) Dispersion medium a liquid. (_a_) Disperse phase a gas--foams. (_b_) Disperse phase a liquid--emulsions. (_c_) Disperse phase a solid--suspensions.
(3) Dispersion medium a solid. (_a_) Disperse phase a gas--solid foams, pumice stone, etc. (_b_) Disperse phase a liquid--liquid inclusions in minerals. (_c_) Disperse phase a solid--alloys, colored glass, etc.
Although the same general principles of physical chemistry apply to all two-phase systems, the term "colloidal condition" is commonly used only in connection with a particular type of dispersions, in which the dispersion medium is a liquid and the dispersed material is either a solid or a liquid.
Thorough and careful studies have shown that when a solid or a liquid is introduced into another liquid, and becomes dispersed or distributed through it, the mixture may be either a true solution, a colloidal solution, or a mechanical suspension. The characteristic differences between these three conditions may be tabulated as follows: although the significance of some of the phrases used will not be apparent until the phenomena in question have been considered in some detail.
-----------------------+------------------------+------------------------ True Solutions. | Colloidal Solutions. | Suspensions. -----------------------+------------------------+------------------------ | | (_a_) Particles of the | | disperse phase are: | | | | In molecular | In colloidal | In mechanical subdivision | subdivision | subdivision | | Invisible | Visible under | Visible under | "ultrascope" | microscope or to naked | | eye | | Less than 1µµ | 1µµ to 1µ in diameter | Greater than 1µ in in diameter[6] | | diameter | | Pass through filters | Pass through filters | Do not pass through and parchment membranes | but not through | filters or parchment | parchment | | | In molecular motion | In Brownian movement | In gravitational | | movement | | (_b_) The system | | exhibits: | | | | High osmotic pressure | Low osmotic pressure | No osmotic pressure | | Transparency | "Tyndall phenomenon" | Is generally opaque | | No gel-formation | Forms gels | No gel-formation | | -----------------------+------------------------+------------------------
FOOTNOTES:
[6] 1µ is one-thousandth of a millimeter; 1µµ is one-thousandth of a µ, or one millionth of a millimeter.
It is recognized by all students of these matters that it is not possible to draw a sharp dividing line between these three types of conditions, and that they shade into each other, in many cases; but in general it may be said that a colloidal solution is one in which the dispersed particles are usually between 5µµ and 200µµ in diameter, are difficultly or not at all diffusible through the membrane of a simple dialyzer, cannot be filtered out of solution, do not settle out under the action of gravitation, and are visible only under the "ultramicroscope"; and one which has certain peculiar optical, osmotic, and other physical and chemical properties. Since colloidal particles are very minute in size, they possess very large relative surface areas as compared with their total mass or volume, very high surface tension, and a relatively high surface energy as compared with their total, or molecular, energy. These properties bring into play, in a substance which is in the colloidal condition, in a remarkable degree, all the phenomena which are associated with surface boundaries between solids and liquids, liquids and gases, etc.
The properties arising out of the colloidal condition are of such tremendous importance in connection with the vital phenomena exhibited by cell protoplasm that it is necessary to give some detailed consideration to them here. Many large volumes dealing with this condition of matter have been written, and it is very difficult to condense even the most important facts concerning it into a few pages, but an attempt has been made to present in this brief summary the most essential facts and principles involved in the colloidal phenomena.
NOMENCLATURE AND CLASSIFICATION
Colloidal mixtures may exist in two different forms: one, in which the mixture is fluid and mobile, like a true solution, is known as a "sol"; and the other, which is a semi-solid, or jelly-like, form, is known as a "gel." Sols may be easily converted (or "set") into gels, by changes of temperature or of the electrolyte content, or by changes in the concentration of the mixture, etc., and in most cases gels can be converted again into sols. In some cases, however, gel-formation is irreversible, the gels are permanent and cannot be changed back again into sols by any known change in environmental conditions.
Depending upon whether the liquid dispersion medium is water, alcohol, ether, etc., sols are known as "hydrosols," "alcosols," "ethersols," etc.; and gels as "hydrogels," "alcogels," etc.
Sols in which the disperse phase is a solid are known as "suspensoids"; while those in which it is a liquid are "emulsoids." Thus, sols of most inorganic compounds, of dextrin, gelatin, and (probably) of casein, etc., are suspensoids; while sols of egg-albumin, of oils, etc., are emulsoids. The classification of these substances into suspensoids and emulsoids is, however, more a matter of convenience than of real difference in composition, since it is practically impossible to say whether many of the organic substances which normally exist in colloidal form are themselves liquids or solids, when in the non-dispersed form.
CONDITIONS NECESSARY TO THE FORMATION OF SOLS
Suspensoids differ from mechanical suspension of solids in a liquid in that in the latter the solid particles settle toward the bottom of the mixture, because of the effect of the attraction of gravity upon them. The rate at which such particles settle depends upon the size and density of the particle and the viscosity of the liquid, and can be roughly calculated from the formula for Stokes' law for the rate of falling of a spherical body in a liquid. This formula is
_V_ = 2_r_^2(_s_ - _s_´)_g_ / 9_n_;
_V_ = velocity of the falling body, in millimeters per second;
_r_ = radius of the particle, in millimeters;
_s_ = specific gravity of the solid;
_s_´ = specific gravity of the liquid;
_g_ = the attraction of gravity, in dynes;
_n_ = the viscosity of the liquid.
For example, if this formula be applied to determine the rate at which the particles of gold of the size of those in a red gold sol would settle, if they were in mechanical suspension in water (_r_ = 10µµ, or one-ten-thousandth of a millimeter; _s_ = 19.3; _s_´ = 1; _g_ = 980, and _n_ = 0.01), it will be found that such particles will settle at the rate of approximately 0.0146 millimeter per hour, or a little over 10 mm. (0.4 inch) per month. Hence, the settling of such particles, if in mechanical suspension, would be measurable, although very slow. Shaking up the _suspension_ would cause the particles to rise through the liquid again. But in a gold sol, or _suspensoid_, which contains particles of gold of the size used in this calculation, the gold particles do not settle, even at the slow rate as calculated above. They remain uniformly distributed throughout the liquid for an indefinite period or time. The reason for this phenomenon undoubtedly lies in the fact that these minute particles carry an electric charge, which, is of the same sign for all of the particles and results in a repellent action which keeps the particles in constant motion. This constant motion may easily be conceived to keep the particles uniformly distributed throughout the liquid, just as constant shaking would keep those of a mechanical suspension uniformly distributed through the mixture.
The sign of the electric charge on the particles of a sol may be either negative or positive, depending upon the chemical nature and dielectric constants of the two phases of the system. The proportion of the total electric charge of the system which is of the opposite sign to that borne by the dispersed particles is, of course, borne by the liquid which constitutes the other phase. The origin of this electric charge on the colloidal particles is, as yet, not known with certainty; but it seems probable that it is due to a partial ionization of these small particles, similar to, but not so complete as, that which takes place when compounds which are soluble go into true solution in water, or other solvents which bring about the dissociation of dissolved substances.
The conditions necessary to bring a solid substance into a colloidal mixture with some liquid, or, in other words, to produce a suspensoid sol, require that the proportion of liquid to solid shall be large and some means of disintegrating the material which is to be dispersed into very fine particles. Many common chemical reactions, if carried out in very dilute solutions, result in the production of sols, especially if a small amount of some emulsoid is present in the reacting mixture; sols produced in this way are very stable, and the emulsoid which is used in stabilizing the sol is known as a "protective colloid." Direct methods of disintegration; such as reduction by chemical agents, discharge of a strong electrical current through the substance which is to be dispersed while it is submerged in the liquid, alternate treatment of finely ground material with alkali and acid so as to frequently change the electric charge, etc., are utilized for bringing inorganic compounds into the colloidal state.
Suspensoids usually contain less than 1 per cent of the solid dispersed through the liquid. In fact, extreme dilution is one of the necessary conditions for suspensoid-formation.
Emulsoids are much more easily produced than are suspensoids. The property of forming an emulsoid seems to be much more definitely a characteristic of the substance in question than does the formation of sols from solids which, under other conditions, may form true solutions. This difference may be due to the fact that the liquids which easily form emulsoids (usually those of organic origin) have very large molecules, so that the transfer from molecular to colloidal condition involves much less change in such cases than it does in the case of solid (inorganic) substances of relatively low molecular weight. This view of the matter is further borne out by the fact that solids which have very large molecules (generally of organic origin) take on the colloidal form much more readily than do those of small molecular size.
At the same time, a given liquid may form a true emulsoid when introduced into one other liquid and a true solution when introduced into another. Thus, soaps form emulsoids with water (true hydrosols); but dissolve in alcohol to true solutions, in which they affect the osmotic pressure, the boiling point of the liquid, etc., in exactly the same way that the dissolving of other crystalloids in water affects the properties of true aqueous solutions. Again, ordinary "tannin," when dissolved in water, produces a sol, which froths easily, is non-diffusible, etc.; but when dissolved in glacial acetic acid, it produces a true solution.
The concentration of the disperse phase may be much greater in the case of emulsoids than it can be in suspensoids. This is probably because the dispersed particles do not carry so large an electric charge and are not in such violent motion.
GEL-FORMATION
The one property which most sharply distinguishes sols from true solutions is their ability to "set" into a jelly-like, or gelatinous semi-solid, mass, known as a "gel," without any change in chemical composition, or proportions, of the two components of the system. In the gel, the two components are still present in the same proportions as in the original sol; but the mixture becomes semi-solid instead of fluid in character. Thus, an agar-agar sol containing 98 per cent of water sets into a stiff gel; while many other gels which contain 90 to 95 per cent of water can be cut into chunks with a knife and no water will ooze from them. The water is not in chemical union with the solid matter in the form of definite chemical hydration, however, as the same gel is formed with all possible variations in the water content.
Gels may be either rigid, as in the case of those of silicic acid, etc., or elastic, as are those of gelatin, egg-albumin, agar-agar, etc. The latter are the common type of gels among organic colloids. They can be easily changed in shape, or form, without any change in total volume.
In gel-formation, the two phases of the system take a different relationship to each other. The disperse, or solid, phase becomes associated into a membrane-like, or film, structure, surrounding the liquid phase in a cell-like arrangement. That is, the whole mass takes on a structure similar to a honeycomb except that the cells are roughly dodecahedral in shape, instead of the hexagonal cylinders in which the bees arrange their comb cells, in which the original disperse phase constitutes the cell-walls and the original liquid, or continuous phase, represents the cell-contents. The cells of an elastic gel resemble closely the cells of a plant tissue in many of their physical properties. They are roughly twelve-sided in shape, as this is the form into which elastic spherical bodies are shaped when they are compressed into the least possible space.
=Imbibition and Swelling of Gels.=--When substances which are natural gels, such as gelatin, agar-agar, various gums, etc., are submerged in water, they imbibe considerable quantities of the liquid and the cells become distended so that the mass of the material swells up very considerably. This swelling will take place even against enormous pressures. For example, it has been found that the dry gel from sea-weeds will swell to 330 per cent of its dry volume, if immersed in water under ordinary atmospheric pressure; but that it will increase by 16 per cent of its own volume when moistened, if under a pressure of 42 atmospheres.
During the swelling of gels by imbibition of water, the total volume of the system (i.e., that of the original dry gel plus that of the water absorbed) becomes less. For example, a mixture of gelatin and water will, after the gelatin has swelled to its utmost limit, occupy 2 per cent less space than the total volume of the original gelatin and water. It has been computed that a pressure equivalent to that of 400 atmospheres would be necessary to compress the water to an extent representing this shrinkage in volume.
On the other hand, gels when exposed to the air lose water by evaporation, shrink in volume, and finally become hard inelastic solids, as in the case of the familiar forms of glue, gelatin, agar-agar, gum arabic, etc.
The difference in the relation of gels and that of non-colloidal solids to water may be illustrated by the different action of peas, beans, etc., and of a common brick, when immersed in water. Each of these substances, under these conditions, absorbs, or "imbibes," water; but the peas and beans swell to more than twice their original size and become soft and elastic, while the brick undergoes no change in size, elasticity, or ductility. In all cases of colloidal swelling, the swollen body possesses much less cohesion, and greater ductility, than it had before swelling. The essential difference in the two types of imbibition is that in the case of the non-swelling substances the cohesion, or internal attraction of the molecules of the material, is too great to permit them to be forced apart by the water; while in colloidal swelling, the particles are forced apart to such an extent as to make the tissue soft and elastic. It is possible, of course, to make this separation go still further, until there is an actual segregation of the molecules, when a true solution is produced; for example, gum arabic when first treated with water swells into a stiff gel, then into a soft gel, and finally completely dissolves into a true solution.
=Reversibility of Gel-formation.=--In some cases, the change of a sol to a gel is an easily reversible one. Glue, gelatin, various fruit jellies, etc., "melt" to a fluid sol at slightly increased temperatures and "set" again to a gel on cooling, and the change can be repeated an indefinite number of times. On the other hand, many gels cannot be reconverted into sols; that is, the "gelation" process is irreversible. For example, egg-albumin which has been coagulated by heat cannot be reconverted into a sol; casein of milk when once "clotted" by acid cannot again be converted into its former condition, etc. Irreversible gelation is usually spoken of as "coagulation." Some coagulated gels, by proper treatment with various electrolytes, etc., can be converted into sols, the process being known as "peptization"; but in such "peptized" hydrosols, the material usually exists in a different form than originally, having undergone some chemical change during the peptization, and the coagulation and peptization cannot be repeated, that is, the process is not a definitely reversible one.
=Importance of Gel-formation.=--From the physiological point of view, gel-formation is undoubtedly the most important aspect of colloidal phenomena. In the first place, the ability to absorb and hold as much as 80 to 90 per cent of water in a semi-solid structure is of immense physiological importance. In no other condition can so large a proportion of water, with its consequent effect upon chemical reactivity, be held in a structural, or semi-solid, mass. But a vastly more significant feature of the conditions supplied by the gel lies in the fact that the non-water phase, or phases, of the system are spread out in a thin film, or membrane, thus giving it enormous surface as compared with its total volume. This effect is easily apparent if one thinks of the enormous surface which is exposed when a tiny portion of colloidal soap is blown out into a "soap-bubble" several inches in diameter. This condition brings into play all the phenomena resulting from surface boundaries between solids and liquids, liquids and liquids, liquids and gases, etc., from surface tension, surface energy, etc. Among these effects may be cited those of adsorption, increased chemical reactivity due to enlarged areas of contact, permeability and diffusion, etc., the importance of which in the vital phenomena of cell-protoplasm will be discussed in detail in the following chapter.
GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS
=Non-diffusibility.=--The most characteristic property of all sols is the failure of the suspended particles to pass through a parchment, or any similar dialyzing membrane.
=Visibility under the "Ultramicroscope."=--The particles of a sol, in contrast with the molecules of a true solution, are visible as bright scintillating points under the ultramicroscope. This is a modification of the type of dark-field illumination of the ordinary microscope, as applied to microscopic studies, in which the solution to be studied is contained in a small tube or box of clear glass which is mounted on the stage of an ordinary microscope and instead of being illuminated from below by transmitted light is illuminated by focusing upon it the image of the sun, or of some other brilliant source of light such as an electric arc, by passing the rays from the source of light through a series of condensing lenses which are adjusted at the proper distance and angles to bring the image of the illuminating body within the tube containing the substance which is to be examined and in the line of vision of the microscope. Obviously, this results in intense illumination of any particles in the solution which come within this brilliant image of the sun, or arc, and therefore renders visible particles which are of less diameter than the wave-length of ordinary light (450µµ to 760µµ for the visible spectrum) and, hence, are not visible by the ordinary means of illumination in the direct line of vision. It will be apparent that what is seen in the field of the ultramicroscope is not the particles themselves, but rather the image of the sun (or other illuminating body) falling upon the particles which come within the image, just as one does not see the paper but only the image of the sun when the rays from the sun are brought to a focus upon a sheet of paper through any ordinary convex lens, or "burning glass." Hence, the ultramicroscope gives no idea of the shape, color, or size of the particles upon which the image falls; but it does permit the counting of the number of particles within a given area, and a study of their movements, from which it is possible, by mathematical computations, to calculate the relative size of the particles themselves. Repeated studies have shown that particles of the sizes between 5µµ and 250µµ in diameter, which are visible under the ultramicroscope, are sufficiently small to bring about the surface phenomena which are known as properties of colloidal solutions. Further, the ultramicroscope permits the observation of the growth, or disintegration, under various chemical reagents, of the individual colloidal particles, which appear as scintillating points in the field of the microscope; and the study of changes in relationships during gel-formation, peptization, etc.
=The "Tyndall Phenomenon."=--Colloidal solutions exhibit this phenomenon; that is, if a bright beam of light be passed through a sol which is contained in a clear glass vessel having parallel vertical sides, and the solution be viewed from the side, it appears turbid and often has a more or less bluish sheen. This effect is due to the small particles in the sol, of polarizing the light which is reflected from them, the blue rays being bent more than are those in the other part of the spectrum. The Tyndall phenomenon is similar in its effect in making the tiny particles of the sol visible to the illumination of the dust particles in the air of a darkened room when a ray or narrow beam of light passes through it. In a true molecular solution, the particles are too small to be visible by this mode of illumination.
=Other Optical Properties.=--Sols are generally translucent and opalescent; many of them are highly colored, some of the sols of gold, platinum and other heavy metals possessing particularly brilliant colors. In general, metallic suspensoids are red, violet, or some other brilliant color; while inorganic suspensoids are bluish white, and emulsoids generally blue to bluish white.
=Formation of Froth, or Foam.=--Colloidal solutions, especially those of the natural proteins, fats, glucosides, gums, and the artificial soaps, have a strong tendency to produce froth, or foam, when shaken; this being due to the enormous surface tension resulting from the finely divided condition of the dispersed material.
=Low Osmotic Pressure.=--All colloidal solutions exhibit a very low osmotic pressure; the freezing point of the dispersion medium is lowered only very slightly and its boiling point is only very slightly raised by the presence of the dispersed particles in it.
=Precipitation by Electrolytes.=--Sols of all kinds are precipitated, or caused to form gels, by the addition of electrolytes, since these cause a disturbance of the electric charge on the dispersed particles, to which the colloidal condition is due. In the case of most emulsoids and of a few of the suspensoids, this change converts the mass into a stiff gel; but in that of many of the metallic suspensoids, the dispersed particles are gathered together into larger aggregates, which settle out of the liquid in the form of a gelatinous precipitate. In the latter case, the effect is usually spoken of as "precipitation" by electrolytes; while in the former, it is called "coagulation," or "gelation."
The effectiveness of the various electrolytes in bringing about this change is proportional to their valency; bivalent ions are from 70 to 80 times, and trivalent ions about 600 times as effective as monovalent ions.
Further, all sols in which the dispersed particles carry a charge of the opposite sign likewise precipitate both suspensoids and emulsoids.
A demonstration of the presence of an electric charge on the particles of a sol and a determination of its sign can be made by placing the solution in a U tube, with a layer of distilled water above the sol in each arm of the tube, and then passing an electric current through the contents of the tube, keeping the electrodes in the distilled water, so that the migration of the particles toward one pole or the other can be observed by their appearance in the clear water at that end of the tube; or by passing an electric current through the observation chamber of an ultramicroscope, in which the solution under examination has been placed, and observing the migration of the particles across the field toward either one or the other (positive or negative) electrode.
_Emulsoids and suspensoids_ differ in their properties in the following respects. Suspensoids are always very dilute, containing less than 1 per cent of the dispersed solid; while emulsoids may be prepared with widely varying proportions of the two component liquids. Suspensoids have a viscosity which is only slightly greater than that of the liquid phase when it exists alone, and their viscosity varies with the proportion of dispersed solid which is present in the sol; while emulsoids have a very high viscosity in all cases. Emulsoids usually form stiff gels when treated with electrolytes; while suspensoids more commonly yield gelatinous precipitates under the same conditions.
Suspensoids and emulsoids which carry electric charges of opposite sign mutually precipitate each other. But emulsoids often protect suspensoids from precipitation by electrolytes, by forming a protective film around the particles of the suspensoids, which prevents the aggregation of the particles into the precipitate form.
ADSORPTION
If a sol be precipitated or coagulated by the action of an electrolyte, substances which may be present in solution in the liquid of the sol are carried out of solution and appear in the gel or precipitate. This phenomenon is known as "adsorption," which means the accumulation of one substance or body upon the surface of another body, as contrasted with "absorption," which means the accumulation of one substance within the interior of another. Since substances which are in the colloidal form have very large relative surface areas, it follows that the opportunity for surface adsorption on colloidal materials is very great.
Surface adsorption is a common phenomenon. It was extensively studied by the physicist, Willard Gibbs, who showed that adsorption will take place whenever the surface tension of the adsorbing body will be lowered by the concentration in its surface layer of the material which is available in the solution or other surrounding medium.
As applied to colloidal phenomena, adsorption may be exhibited in either one of four different ways, as follows: (1) A crystalloidal substance which is in solution may be adsorbed on the colloidal particles of a hydrosol, so that if the mixture be dialyzed, or filtered through a so-called "ultrafilter" (i.e., a filter with pores so small that it will retain colloidal particles) the dissolved crystalloid will remain with the separated colloidal particles, or the dissolved crystalloid will not react chemically as it would in a free solution. For example, if to a solution of methylene blue, which dyes wool readily, there be added a small quantity of albumin (a colloidal substance), the dye is adsorbed by the albumin and will no longer color wool with anything like the same readiness. (2) During gel-formation, electrolytes and other soluble substances which may be present in solution in the liquid may adsorbed out of the solution and appear in the gel. For example, a precipitate of aluminium hydroxide, or of silicic acid, is nearly always contaminated with the soluble salts which are present in the solution, and can be prepared in pure form only by repeated filtering, redissolving, and reprecipitating. (3) Colloidal substances may be removed from sols by being adsorbed upon porous materials like charcoal, fuller's earth, hydrated silicates, etc. For example, animal charcoal (or bone black) is used commercially for the clarification of sugar solutions, because it adsorbs out of these solutions the colloidal proteins, coloring matters, etc., with which they are contaminated. (4) Finally, colloids mutually adsorb each other, as in the case of the "protective colloids" previously referred to.
Certain characteristics of adsorption phenomena are of interest and importance from both the physiological and the industrial point of view. The following may be mentioned: (_a_) _Amount of adsorption._ Relatively more material is adsorbed out of dilute solutions than out of more concentrated ones. An increase of ten times in the concentration of the dissolved material results in only four times as much adsorption by the colloidal substance which may be introduced into the two solutions. In this, adsorption differs from chemical action, as the latter is proportional to the concentration of the reacting material which is present in the solution. (_b_) _Adsorption out of different liquids_, by the same adsorbing body, is different in amount. It is usually greatest out of water. Hence, many dyes may be adsorbed out of water by charcoal, porous clay, etc., and if the latter be then introduced into alcohol, or ether, the dye goes back into solution in these latter liquids. This process is often used industrially and in the laboratory for the purification of such substances when they are present in impure form in aqueous solutions. (_c_) _Selective adsorption._ Different substances are not adsorbed out of the same solvent to the same extent by the same adsorbing agent. Advantage is taken of this fact when filter paper is used in the so-called "capillary analysis" to separate different dyes, or other colloidal materials which have been stained different colors, into alternate layer by reason of the different rate at which the paper adsorbs the different materials out of the solution in which they are present together. (_d_) _Similar relative adsorption by different adsorbing agents._ Although different adsorbing agents may possess varying active surfaces and hence, variable adsorbing power, or rates of adsorption, they adsorb the same relative amounts of different materials; i.e., if substance _A_ adsorbs more of _X_ than it does of _Z_ out of any given solution, substance _B_ will likewise adsorb more of _X_ than of _Z_ out of the same solution; although the actual amounts adsorbed by _A_ may be quite different from those adsorbed by _B_.
CATALYSIS AFFECTED BY THE COLLOIDAL CONDITION
The velocity of a chemical reaction is the net result of opposing influences. It is directly proportional to the chemical affinity of the reacting bodies and inversely proportional to the so-called "chemical resistance." The first factor, chemical affinity, is not easily measured, as it depends upon both the mass of the reacting molecules, atoms, or ions, and their attraction for each other. But if, as the result of chemical affinity, a reaction takes place, it is evident that the time required for its completion (which measures the velocity of the reaction) is made up of two separate periods. The first is the time required for the reacting molecules to come into contact; and the second is that required for the molecular rearrangement which constitutes the reaction. Clearly, the time required for the substances to come into molecular contact will be greatly diminished if they are mutually adsorbed in large quantities on the extended surface area of some colloidal catalyst which is present in the mixture rather than scattered throughout its entire volume. The application of this principle to the catalysis of hydrolytic reactions is not apparent, if it is considered that the H_{2}O molecules which cause the hydrolysis are those of the solvent itself; but is clear on the assumption (which is discussed in the following chapter) that the water which enters into a colloidal complex is in multimolecular form, represented by the formula (H_{2}O)_{_n_}, in which the oxygen atoms are quadrivalent and, hence, much more active chemically than as illustrated in the simple solvent action of water.
Hence, the surface adsorption of reacting bodies by a colloidal catalyst may have a very important influence in decreasing the time required to bring the reacting molecules into intimate contact, and so increasing the velocity of the reaction.
But the colloidal condition of the catalyst may also aid in decreasing the "chemical resistance" which tends to slow up the reaction. Chemical resistance may be understood to be the internal molecular friction of the densely packed atoms within the reacting molecule, which tends to prevent the molecular rearrangement and so to prolong the second period of the reaction time. To overcome this friction and so decrease the reaction time, some form of energy is necessary. If there be present in the solution in which the reaction is taking place some colloidal catalyst, and if the reacting bodies are concentrated at the surface boundaries between the two phases of the colloidal system, they may be conceived to be within the sphere of influence of the surface energy of the dispersed particles of the catalyst, so that this may furnish the energy necessary to overcome the chemical resistance of the reacting bodies, and so to speed up the second portion of the reaction time.
From these considerations, it would appear that the colloidal condition of such catalysts as enzymes, etc., has much to do with their ability to increase reaction velocities, both by reducing the time necessary for the reacting bodies to come into molecular contact and by furnishing the energy to overcome the chemical resistance to the molecular rearrangement which constitutes the reaction itself. Evidence in favor of the accuracy of this view of the nature of the catalytic action of colloidal substances is afforded by the facts that catalysts accelerate the velocity of reversible reactions in either direction and that they do not change the point of final equilibrium, in any case; that is, they do not affect the nature or direction of the reaction, but only accelerate a chemical change which would otherwise take place more slowly because of the stability (or chemical resistance) of the molecules involved, or their inability to come quickly into intimate molecular contact.
These facts and principles have been clearly established in many studies of the nature of enzyme action (enzymes are typical colloidal catalysts) and probably apply equally well to the action of other types of colloidal catalysts. On the other hand, the catalytic action of certain inorganic and non-colloidal substances, such as the action of acids in accelerating the hydrolysis of carbohydrates, etc., may be conceived to be due to chemical influences upon the internal molecular resistance, which are similar in their effects, but entirely different in their mechanism, from the physical effects of the surface boundary phenomena of the colloidal catalysts.
INDUSTRIAL APPLICATIONS OF COLLOIDAL PHENOMENA
Large numbers of industrial processes are based upon colloidal phenomena. Many of these processes were known and practiced long before the nature of the phenomenon itself was understood. But with the coming of the knowledge of the nature, causes, and possibilities of the control, of the colloidal condition of the materials involved, immense improvements in the economy of the process, or the quality of the end-products, have been worked out, in many cases. Many volumes of treatises concerning the industrial applications of colloidal phenomena have been written. Any discussion of these would be out of place here; but the following list of examples will serve to illustrate the immense importance of these matters both in industry and to the needs of everyday life: the tanning of leather; the dyeing of fabrics; vulcanizing rubber; mercerizing cotton; sizing textile fabrics; manufacture of mucilages and glues; manufacture of hardened casein goods; manufacture of celluloid; production of colloidal graphite for lubrication; the prevention of the smoke nuisance by electric deposition; the purification of sewage; the manufacture of soaps; the manufacture of butter, cheese, and ice cream; fruit jellies, salad dressings, etc. This list could be extended to a great length, but is already long enough to emphasize the very great importance and practical value of colloidal phenomena in daily life.
NATURAL COLLOIDAL PHENOMENA
Many of the phenomena of nature are colloidal in character. These may be observed in the mineral, the animal, and the vegetable kingdoms. Here, again, a lengthy discussion of the nature of these phenomena would be out of place in this connection, and a few typical examples will serve to illustrate the general importance in nature of this property of matter.
In the soil, the following properties are easily recognizable as definite colloidal phenomena: water-holding capacity of clays, silts, loams, etc.; adsorption (or "fixation") of soluble plant foods so that they are not readily leached out of the soil by drainage; flocculation and deflocculation of clay, etc.
In the animal body; the contraction of muscles, the conveyance of nerve stimuli, etc., are undoubtedly accomplished by colloidal changes; and the existence of insoluble casein and fat in colloidal form in milk insures the proper nourishment of the young of nearly all species of animals.
In both plants and animals, as will be pointed out in the following chapter, practically all the vital activities of the cell protoplasm are definite manifestations of colloidal phenomena. Enzymes perform their catalytic functions by reason of their colloidal form. Proteins exist in colloidal form and are the seat of all vital functions. The regulation of the passage of materials into and out of the cell is governed by minute changes in the electrolyte concentration, etc., which produce enormous changes in the colloidal character of the protoplasm.
It is apparent, therefore, that the study of the colloidal condition of matter and of the properties arising out of it is of immense importance to the biochemist. No other single field is capable of yielding more fruitful results to the plant physiologist, in his studies of the response of plants to changes in their environment, or of the mechanism by which plants perform their internal functions.
References
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