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IMPORTANCE OF THE IMPLICATIONS OF COLLOID CHEMISTRY IN SCIENCE COURSES1 ERNST A. HAUSER Massachusetts Institute of Technology and Worcester Polytechnic Institute, Cambridge, Massachusetts
INTHE preface to a high-school textbook on chemistry written by the late head of the science department of a high school in one of the Eastern states2 one finds the following statements: "A textbook in chemistry for use today, whether designed for those who are heading toward college or for those who are planning a different course, must contain the real kernel of the science, if it is to proye to be a textbook of chemistry and not merely a book about chenzistry. . . .If his pupils are preparing for entrance or for state examinations [the teacher1 will find in this book all the material needed by those pupils. . . . An effort has been made to make this book so readable and it will be unnecessary for the comprehensible time in explaining and Vend translating." The author of this textbook has an admirable ambition in wanting to offer a book fulfillingespecially the objective last quoted. even the readable textbook on chemistrj. cannot alone do justice to colloid chemistry unless it is supplemented by the instructors' demonstrations and detailed explanations of the phenomena. Although it must be admitted that the high-schoo1 textbook is of an elementary nature and the colloidal aspects of chemistry are only treated comparatively briefly, whereas the college texts are naturally of a nature and much more advanced in their terminology, still this does not absolve the author of a high-school text from the responsibility of offering an up-to-date definition of matter in the colloidal state. will be obvious from the following quotations that can only lead to a falling On this
serious confusion in the mind of the student who has supposedly learned the elementary basis of colloid chemistry in high school, when he turns to more advanced texts a t the college level. To give specific examples, I should now like to quote again from the high-school textbook in comparison with three widely accepted textbooks on colloid chemistry of college grade. From the high-school text:
A neurer field of ehomistry deals what ?hemists loidal suspensions. They are not true solutions, and they have little effectupon tho freezing point of the medium in which they are suspended. Thcy do not pass through membranes by asmosii" the way truc solutcs do. Thesuspended material in a. colloidal suspension has particles large enough to diffuse light, but too small to he separated from the suspending medium by filtration. They are so tiny that they do not settle when the suspension stands, but they are capable of adsorbing other substances upon their surfaces. The tiny particles in a colloidal suspension are electrically charged. Glue, gelslcine, white of egg, and the tissues of which our nerves are mat+ are examplcs of colloidal su,pensions. How do colloidal suspensions differ from ordinary suspensions? If we stir some line sand with water, a suspemion is formed temporarily. The sand soon settlos to the bottom when tho stirring is stopped. The very finest particles settle the most slowly of all. The sand particles can be removed from the suspension, too, by fittratim. By adding a small quantity of gelatine to some boiling wstor, a. colloidal suspension is-formed. The gelatine does not. settle when such a suspension stands, even for a long time, and the gelatine cannot be removed by filtration. Such behavior is characteristio of oalloidsl suspensions. [Colloidal] particles average about O.M)O,WI mm. indiameter. .. . . Colloidal particles are found to be electrically charged; some of them carry positive charges, and others carry negative. charges. Suppose we are given a colloidal suspension whose particles are positively charged. If we add ta it a colloidal suspension which has negatively charged particles, the colloid mill be. preoipitated. Presented boforc thc Eleventh Summer Conference of the How are colloidsl suspeuaions made?. . . There are several New England Association of Chemistry Teachers, University of methods of accomplishing such a result.. . . [One of these is] New Hampshin?, Durham, New Hampshire, August 22 to 27, by using a. large excess of one reagent. If u-e add 10 em.' of a 1949. molar solution of silver nitrate to 10 om.= of a molar solution of 1 DULL, CIIARLESE., "Modern Chemistry," Henry Halt and potassium bromide, complete precipitation of the silver bromideocours, as represented by the equation Co., New Ymk, 1942. 566
OCTOBER, 1949 AgNO.
+ KBr
-
567
AgBr
1 + KNOi
?
TABLE 1 Characteristics of Disperaa Systems (Ostwald) If, Irowrwr, r e add a l a r g ~ercess of rilwr trirrate to a solution c ~ f p o r ~ ~ ~ iI,ronlidv, um ~olloidd~ i l w lrrumidr r is forned. Rnnnr? nf How van cdloidr bc stabilircd? Somt.tinles it is i~trcmrlvimportant to prevent the precipitation of colloids. For example, Characteristics ". the blood stream and the nerve tissues are colloidal. The preCoarse >0,5 w Particles do not run through Dr. Fischer, of the cipitation of either one would cause death. dispersions (>5 X cm.) a papcr filter; do not difUniversity of Cincinnati, stated it was his opinion that the bad fuse; do not pass through effect of alcohol upon the nervous system is due to its tendency sdialyzing membrane; are to precipitate colloids. microscopicdly visible.
Now let me quote from some of the textbooks used in colloid chemistry courses in colleges throughout the United States? Although Graham's observations led him to distinguish two classes of substances, he recognized that suoh a classification was arbitrary since "in nature there are no abrupt transitions," and crystalline colloids exist. Moreover, with respect to colloids, Graham was the first "to speak of their peculiar form of aggregation as the colloidal condition of matter." Although the limitations of Graham's classification were recognized by Graham himself, the important investigations of von Weimarn with a definitely crystalline inorganic salt, barium sulfate, established the view that the distinction between crystalloids and colloids is not tenable and that we should speak of the colloidal state of matter just as we speak of the liquid, solid, and gaseous states of matter. Van Weimarn generalized, further, that any crystalline material can be made to assume the colloidal state under suitable conditions. , A phase is said to be colloidal when it is sufficiently finely divided in a t least one dimension. Strictly speaking, therefore, the term colloid should be used only as an adjective to define a physical system of matter usually made up of more than one substance. But, for convenience, we frequently refer to a finely divided phase as a. colloid. This is particularly true if we are dealing with colloidal organic materials suoh as gelatine, agar, and mbber whioh are either noncrystalline or submicroscopically orystalline. Certain substances take up liquids strongly, swell in them, and become peptized to form sols. Gelatine swells in water and is peptized by water to give a hydrophilic sol. Wolfgang Ostwald first classified dispersed systems on the basis of the sire of the particles of the dispersed phase and set the limits of the calloidd zone. His most recent classification is given in Table 1. The colloidal zone which Ostwald onoe referred to as "the land of neglected dimensions" is thus set between two arbitrarily chosen limits of particle size: 0.5 p, whioh is near the lower limit of the resolving power of an ordinary microscope, and 1.0 mp, whioh is somewhat greater than the diameters of ordinary moleculcs and ions. The arbitrary nature of he h m t s is evi' denced by the fact that certain substances sue as egg albumin and hemoglobin may be molecularly dispersed in water, but the moleculcs of the com~oundsare so laree that thev come well virlrin 1 I w voll,,idaI rang?. For t1.r most part, honww, pnrrirlcs in rlw wlluirlul state r
