- from General Chemistry
by N. Glinka,
Peace Publishers, Moscow, circa 1960s
188. Crystalline and Colloid States of Substances. If a solution of sodium silicate is added to concentrated hydrochloric acid, the resulting silicic acid does not separate out as precipitate but remains in solution together with the sodium chloride formed during the reaction. The hydrochloric acid and sodium chloride can be removed from the solution in the following way.
The solution is placed in a bottomless cylinder with a membrane of parchment paper or an animal bladder bound over its end. The cylinder is submerged in a wider vessel containing water which is continuously renewed. The sodium chloride and hydrochloric acid diffuse freely through the membrane into the outer vessel, but the silicic acid cannot penetrate the membrane and remains in solution. As a result, the cylinder will be found after some time to contain a pure solution of silicic acid
The method of separating dissolved substances based on the fact that one of them will not diffuse through a membrane is known as dialysis, and the apparatus described is called a dialyzer.
Many other dissolved substances, besides silicic acid, viz., glue, gelatin, egg albumen, etc., cannot penetrate a membrane of parchment paper or bladder.
In the sixties of the last century, the diffusion of dissolved substances through vegetable and animal membranes was studied in detail by the English chemist Graham. Graham found that all substances capable of diffusing in solution are crystalline in the solid-state. On the contrary, substances which could not diffuse through membranes were found to be amorphous and formed shapeless (and to a certain degree plastic) masses when isolated from solution. On this basis Graham called the former crystalloids and the latter colloids (from the Greek “colla”– glue).
However, as early as 1869, the Russian botanist I. Borshchov put forth the assumption that the particles of certain colloids may also be of crystalline structure. Further investigations confirmed this assumption and led to the conclusion that Graham’s division of substances into crystalloids and colloids should be rejected, as not only such typical colloids as albumen could be obtained in the form of crystals but many indisputable crystalloids, such as common salt, could be obtained in the form of colloids.
Finally, it was proved that the same substance could behave like a colloid in some solvents and like a crystalloid in others. For instances ordinary soap dissolved in water diffuses very slowly and cannot penetrate a membrane, showing it to be a colloid; but in alcohol solution, the same soap possesses the properties of a crystalloid.
Thus, the sharp demarcation line between crystalloids and colloids gradually disappeared, and at present, we can speak only of the crystalloid or colloid states of substances, just as we have spoken above of their solid and liquid states.
The colloid state of substances plays a very important part not only in chemistry, but also in biology, medicine, technology, and agriculture, and therefore we shall dwell on it in some detail.
189. Dispersed Systems. If a fine powder of any insoluble substance, say clay, is shaken with water, the larger particles will soon settle at the bottom while the finest will remain in a “suspended” state in the water for a considerable length of time, so that the liquid may remain turbid sometimes for weeks.
Liquids with particles of a solid substance suspended in them are called suspensions. If minute drops of a liquid are suspended in another liquid the system is called an emulsion. An emulsion can easily be obtained by shaking an oil vigorously with water in the presence of substances capable of lowering the surface tension of the oil. Ordinary milk is an emulsion of minute drops of butterfat in water.
Particles suspended in liquids can be separated from them by filtration. Ordinary filter paper will detain particles down to 5 microns, i.e., 0.005 mm., in diameter, specially prepared filter paper down to 1 micron, while clay filters detain particles as small as 0.2 microns.
As long as particles above 0.1 micron in diameter are present in a liquid, it will not seem quite transparent, and the suspended particles can be detected in a drop of the liquid with the aid of an ordinary microscope.
A substance can be divided artificially into such tiny particles, that the liquid containing them will seem quite transparent and homogeneous, although actually it is not homogeneous. For instance, if we dip two silver wires into distilled water, connect them to a sufficiently powerful source of electric current and bring their ends together under the water, an electric arc will be struck and a brownish cloud will appear. Soon the entire liquid will turn brown, though remaining quite transparent. This coloring is due to minute particles of silver sent into the water by the electric arc. If gold wires are used instead of the silver ones, the liquid will turn purple and will contain minute particles of gold. The particles obtained in this manner cannot be detected even with the most powerful magnification possible in an ordinary microscope, but their presence can be revealed by means of the so-called Tyndall effect.
The Tyndall effect may be explained as follows. If a beam of converging rays, say, from a projection lantern, is passed through a liquid containing minute particles in suspension, each of these particles scatters the light rays that fall on it, becoming, in a sense, a luminous point. Thus, the entire path of the rays through the liquid becomes visible, having the appearance of a bright cone, if viewed in a darkened room.
The Tyndall effect is the underlying principle of the instrument known as the ultra-microscope; with this instrument, particles less than 0.1 micron in diameter, and invisible under an ordinary microscope, can be detected in a liquid. The difference between an ultra-microscope and an ordinary one is that in the former the light falls laterally on the liquid under study, instead of from below. If the liquid is perfectly homogeneous, all the fields of vision will appear dark, as no light rays enter the tube of the microscope.
But if the liquid contains minute suspended particles, say, silver particles formed by an electric arc, the rays scattered by them come to the observer’s eye and the dark background will appear studded with luminous specks in continuous motion (Brownian movement). If the particles are much less than 0.1 micron in size, they may be difficult to discern even with an ultra-microscope, but the beam of rays passing through the liquid will still be observed. Finally, if the particles are as small as 1 millimicron, the light scattering becomes so insignificant that this phenomenon also disappears and the liquid appears quite homogeneous or, as we say, “optically void.” Such, for instance, are ordinary solutions of various substances.
Any system in which one substance is finely divided and distributed as more or less minute particles through another substance is called a dispersed system; the divided substance is known as the dispersed phase of the system while the substance around it is called the dispersion medium. For instance, in the case of a suspension of clay in water, the dispersed phase consists of the clay particles, while the dispersion medium is water.
Dispersed systems, as we have seen, may have different degrees of dispersion. Suspensions and emulsions are classed as coarsely dispersed systems, as the particles of their dispersed phases are comparatively large. On the other hand, ordinary solutions are systems with very high, one may say ultimate, degrees of dispersion, as the distributed substance is broken down into molecules and/or ions. In this limited case there is no dispersed phase to speak of, as the entire solution is one single phase.
An intermediate position is occupied by dispersed systems, in which the size of the dispersed particles is larger than in ordinary solutions, but still so small that they are discernible only with the aid of an ultra-microscope. Such systems are called colloidal solutions or sols. A close study of colloidal solutions shows that no sharp boundary can be drawn between such solutions and ordinary, or, as they are called, “true” solutions, on the one hand, and suspensions or emulsions, on the other. Therefore, the division of dispersed systems with liquid dispersion media into the three above classes is rather conventional. The demarcation line between these classes is determined approximately by the limits of visibility with an ordinary microscope and with an ultra-microscope.
Suspensions and emulsions contain particles visible under an ordinary microscope. Their size exceeds 100 millimicrons (0.1 micron). The heterogeneity of such a system can be detected by the naked eye.
Colloidal solutions. Size of dispersed particles between 100 and l millimicrons. The particles are discernible only under the ultra-microscope; they pass unhindered through the pores of ordinary filters, but can be detained by membranes of parchment paper, bull bladder or special ‘ultra-filters.” In transmitted light colloidal solutions appear quite transparent and homogeneous, in reflected light — slightly turbid, especially if the size of their particles is close to 100 millimicrons. Modern electron microscopes, which give magnifications of tens and hundreds of thousands of times, enable not only detection of colloidal particles, but the determination of their size and shape as well.
True solutions. Size of dispersed particles below 1 millimicron. Such particles cannot be detected by optical means.
Tyndall Effect from McGraw-Hill Concise Encyclopedia of Science and Technology, 1984
Tyndall Effect: Visible scattering of light along the path of a beam of light as it passes through a system containing discontinuities. The luminous path of a beam of light is called a Tyndall Cone. In colloidal systems, the brilliance of the Tyndall Cone is directly dependent on the magnitude of the difference in refractive index between the particle and the medium. For systems with particles with diameters less than one-twentieth the wavelength of light, the light scattered from a polychromatic beam is predominantly blue in color and is polarized to a degree which depends on the angle between the observer and the incident beam.
The blue color of tobacco smoke is an example of Tyndall blue. As particles are increased in size, the blue color of scattered light disappears and the scattered radiation appears white. If this scattered light is received through a Nicol prism which is oriented to extinguish the vertically polarized scattered light, the blue color appears again in increased brilliance. This is called residual blue, and its intensity varies as the inverse eighth power of the wavelength.