Colloids & The Tyndall Effect from General Chemistry by N. Glinka, Peace Publishers, Moscow, circa 1960s
(continued from part 1) 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 colouring 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 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. [end] -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: [email protected] Silver-list archive: http://escribe.com/health/thesilverlist/index.html List maintainer: Mike Devour <[email protected]>
