438
J ~ ~ NOFACHEMICAL L EDUCATION
APRIL,1926
HOW MANY COLLOIDS HAVE YOU USED TODAY? V. N. Monms, UNIVERSITY o*. MINNESOTA, MINNEAPOLIS, MINN. Increasing Interest in Colloids In a recent issue of THISJOURNAL,^ the editor took occasion to call attention to the steadily increasing interest which is being manifested in the study of colloids. A good indication of this interest is given by an analysis of the registration a t the Third National Colloid Symposium held a t the University of Minnesota during the past summer. That the symposium attracted widespread attention, considered from the geographical standpoint, is evidenced by the fact that visitors were registered from Canada, South Africa, and some of the European countries, as well as from nearly two-thirds of the states of the union. Further proof that interest in colloids is not a highly specialized one is offered by the fact that the symposium not only drew chemists, chemical engineers, and teachers of chemistry in large numbers, but also representatives of such widely diversified sciences as physics, horticulture, pharmacology, bacteriology, metallurgy, biology, pharmacy, and botany. At the conclusion of the editorial mentioned above the hope was expressed that the teachers of chemistry would not be the last to recognize the "handwriting on the wall," and would allot to the consideration of colloids an appropriate amount of time. While it is realized that courses discussing in some detail a t least the theoretical aspects of colloids are now given a t a number of institutions, there are, however, undoubtedly many other courses in elementary physical, general, or other branches of chemistry which have become so well standardized that opposition is encountered in any attempt to take time away from any of the old orthodox subjects and give it to colloids. I t seems to the author, however, that, if judiciously expended, any time devoted to a consideration of colloids in such courses will be anything but wasted. Suggestions for the Presentation of the Subject In taking up the subject of colloids, some time must necessarily be applied to a brief discussion of the theoretical considerations. The student should be given a brief introduction to the nature, classes, methods of preparation, and some of the peculiar properties, such as adsorption, flocculation, protective action, hydration, etc. Lacking time to go farther into the subject, the instructor can furnish inspiration and arouse considerable interest by a final lecture depicting the contacts which the student has with colloids during the course of his ordinary daily life. There follows a description of a number of such contacts, presented in the manner which the author has found will stimulate interest among both students and faculty members. 2, 518 (1925). Tars JOURNAL,
VOL.3. No. 4
How MAXY COLLOIDS HAVEYOU USED TODAY?
439
Colloids in W a s h i i The student begins his day, after delaying it as long as possible in order to recuperate from the burning of midnight oil (or other midnight activities) by dragging himself out of bed, gathering up his towel and wash cloth, and starting for the bathroom. There he picks up a bar of soap, without being aware that he has a handful of colloids of the emulsoid type, as demonstrated by their tendency to give viscous solutions, their power of gelation, etc. Using these colloids, he proceeds to carry out a colloidal process, that of washing, during which adsorption and other colloidal phenomena come into play. If the hour of his first class is not too imminent, he may make a thorough job of i t by jumping into the bathtub, little realizing what knowledge of colloid chemistry may have been applied during the manufacture of that tub. In the older method of making tubs, it was necessary to mould them, and as a consequence cracks and deformations were common. A study of the clay from which the tubs are made revealed it to be a colloid carrying a negative charge, and consequently capable of being peptized by alkalies. It was later found that alkalies a t higher concentrations tended also to so peptize the tannins and other protective colloids present that eventually a molecular condition is reached and the protective action lost. Silicate of soda was also discovered to have a deflocculating effect, so that the former stiff, plastic mass could be made so fluid that a "pouring" process could replace the earlier one of moulding. Although more fluid, this material really has less water. By the application of the principles of colloid chemistry it was thus possible to obtain a faster drying and better product. Colloids in Clothing Having completed his ablutions, our hero dashes back to his room only to encounter another group of colloids as he picks up his clothes. The wool in his trousers and the cotton of his shirt are both largely made up of colloidal particles, as are also his socks, whether real silk or only of the artificial variety. The cellulose of the cotton fiber, for example, can readily be shown to be colloidal in nature, on account of its electrical charge, its tendency to become hydrated, and its ability to adsorb acids and bases from solution. Depending upon the taste of the owner, his clothing may be of various colors, but whether in the case of the black of his socks or the brown of his coat, the dyestuff used was probably colloidal, and adsorption may have played a role in its application. Not content with being wrapped in colloids, he puts some more on under foot, as he laces up his No. 11's. The skin, from which his shoes originally came, was so colloidal in nature that i t might have been used as a source of that very popular colloid, gelatin. Moreover, the tannins and many
440
.TOURN&
OF
CRBMICAL EDUCATION
APRIL,1926
of the other substances used in making leather from that skin are also colloids. Incidentally, if his shoes are well shined, as we trust is the case, he has them covered with a thin layer of a non-aqueous colloidal solution. Before leaving the room, he combs his hair with a celluloid comb, in the manufacture of which camphor and moist pyroxylin are ground together, dehydrated by pressure, sprayed with alcohol, and given such subsequent treatment that the final product is a colloidal gel. While he may allow his thoughts to dwell with some uneasiness on any gray hairs he combs out, our student will, of course, have no thought of the possibility that the color of those very hairs may be attributed to the presence of air bubbles of colloidal size in the hairs. Colloids at the Table With his personal appearance entirely to his satisfaction, he is now ready to present himself for breakfast. Were he one of those hearty eaters, who crowd a lot of heavy proteins down their throats a t the first meal of the day, his stomach would soon be filled with colloids. Just because he prefers a cereal instead is no reason for thinking that he can escape them. Whether he chooses Shredded Wheat, Quaker Oats, or even Grape-Nuts, "there's a reason" for classing his breakfast among the colloids. Over his cereal he pours milk-or cream perhaps, if he happens to be in a state of affluence-but in either case he pours colloids. As a beverage he may have milk, tea, or coffee, all of which swarm with colloids. (For those who are skeptical a few experiments in dialysis or electrophoresis of coffee are suggested.) Even if he is a member of that large class of citizens who consider the Volstead Act as no act a t all, and has beer as a beverage, he is still not avoiding colloids. Beer would not froth so well, nor be so turbid when nice and cold, were i t not for its colloidal content. Who can say but that the reason the manufacturers of nearbeer have been called such poor judges of distance is because they have not made proper use of the chemistry of colloids? If his choice has fallen on coffee or tea, he will probably also want toast, and where else could be found a better illustration of some of the colloidal phenomena? The making of bread is, in the first place, a process of the formation of a gluten emulsion which contains suspensoid starch. Upon becoming stale the colloidal character is less pronounced, as the degree of dispersion has probably decreased, and a loss of water has occurred. In toasting, this process of dehydration has been unduly hastened, so that the resulting product not only tastes different, but is structurally different from stale bread. Colloids in Books, etc. Having completed breakfast, he hurries back to his room to get his books together, and once again comes in contact with these omnipresent
colloids, which are to be found in the paper, the ink, the cloth, and the glue. One of the functions of the heater in paper making is to hydrate the cellulose, the hydration resulting not only in a swelling but prohably an increase in the degree of subdivision as well. The paper was prohably sized by allowing aluminum sulfate and silicate of soda to react, the resulting gelatinous precipitate being adsorbed by the paper to give the latter a better finish. As for the glue, it need merely be mentioned that the properties of this substance were so well-known by the father of colloid chemistry, Graham, that he named this new branch of the science after it. Even the pictures in the books have had intimate contact with colloids from the very moment of their origin. The medium of the so-called photographic emulsions is usually gelatin' or collodion, while suspended therein are the silver halide sols. Gelatin, if used, functions not only as a binder and dispersion medium, but also retards reduction of the suspended silver salt in the absence of light. It now appears probable that both the latent and visible images represent dispersions of a small amount of colloidal silver in a relatively large amount of silver halide. The work of several investigators, who have prepared photohalides synthetically, starting with sols of silver and of silver halides, has helped to discredit the older theory of the existence of silver subhalides. In the process of development adsorption comes into play in the first stage, while other colloidal phenomena follow in the subsequent phases. Moreover, in the final process of fixing, i. e., removal of the residual halide by "hypo," colloidal considerations are still involved, since the hydration of the gelatin must be so maintained as to allow proper diffusion of the "hypo." But to get some more action out of our hero, we will say that he takes out a cigarette and lights it, failing, of course, to see any connection between the luminosity of the flame from his match and any carbon particles of colloidal size that may be present. He then picks up his books and his fountain pen, which is quite probably filled with colloidal ink, and starts for his first class. Incidentally, we cannot let that fountain pen be mentioned without calling attention to the fact that it is made of hard rubber, and as such merits discussion with the colloids. Rubber, among other things, illustrates very nicely that property of some colloids which is manifested by their ability to take up liquids and swell. Although raw rubber best illustrates this property, even a vulcanized rubber band, when dropped into benzene or other organic liquids, will swell to several times its original size. Colloids in the Open
It might seem reasonable to expect a t least a temporary freedom from colloids during the short trip to the campus. The insidious thing about these little fellows, however, is that, like halitosis, "you rarely know
442
JOURNAL OP
CHEMICAL EDUCATION
APRIL,1926
when you have them and even your closest friends won't tell you." The air which surrounds our student as he walks along is filled with colloidal moisture and dust particles, whose deflecting power on that mixture of variously colored rays whose composite effect is that of white light, accounts for both the blue of the ordinary sky and the red of the sunset. Incidentally, a close observation of the cloud of tobacco smoke which he is puffingout will show i t to be blue by reflected light and red by transmitted, just as is the atmosphere. The only hope of escape now is under foot, but that hope is soon lost, as we find him traversing a concrete walk. I n the process of the setting of that walk, a calcium silicate gel has formed in the interstices between particles and as the mixture hardens, water leaves this gel and enters the cement particles. Even if he happens to he passing a group of co-eds, in which case, of course, he would be crowded off the walk entirely, he will be right on top of the biggest mass of colloids of all. There is generally an intimate relation between a soil's content in colloids and its fertility. Farther down below the surface are the stones and particles of very coarse dispersion, whose disintegration helps to produce soils. Nature has provided some very beautiful mineral formations, some of which, such as the banded agates, may he closely imitated in the laboratory. It would obviously be too long a process t o follow our friend through his entire day, so we will leave him in peace as he enters his first class, sits down, opens up his note-book, and takes on a bored expression, which is decidedly indicative of his blissful ignorance that he, personally, has ever had anything to do with those tiresome substances, colloids, which the professor is again starting t o discuss.
