JOURNAL OF CHEMICAL EDUCATION
W H A T A N INTRODUCTORY COURSE AND TEXTBOOK ON COLLOID CHEMISTRY SHOULD OFFER1 ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Massachusetts
AN INTRODUCTORY
textbook on, and an introductory course in, colloid chemistry must first of all explain to the reader or student, respectively, what the term actually implies. This is considered necessary because when one speaks of colloid chemistry today, one is talking of branch of natural science far more embracing than when Thomas Graham coined this word. It is particularly important because the original meaning has too frequently been misunderstood or misinterpreted. T~ accomplish this it seems mostlogical to start by offering an historical review of publications whish appeared before Graham introduced the term colloid into scientific literature. Such an approach would also serve another purpose, namely, to prove $0 the reader or how important a solid foundation in humanities, and especially history, is for anyone who wants to become a progressive scientist ( I ) . In the case of colloid chemistry, it will also serve the purpose of proving how correctthe ~ , . i t i ~physician, h sir william osier (18491910), was in his following statement: "In science the credit goes to the man who convinces the world, not to the man to whom the idea first occurs." To discover how true these y ~ are,~ it is~onlydneces~ the most pertinent passages of paper sary to in 1847 (4) and those to be written 1 This paper is based o n the forthcoming second edition of the author's book, "Colloidal Phenomena," published by MeGraw-Hill Book Co., Inc.. New York.
found in Thomas Graham's classical publication of 1861 (8). Selmi had this to say: Speaking of solution I have noted that it occurs whenever a hody, hard, soft, or liquid, unites withaliquidsubstsncewithout strictly combining with it, and diffuses in it uniformly . In the emulsions, on the other hand, I have shown that the hody dispersed in the vehicle is less divided and comminuted than it is in solutions, since it exists there in the form of globules or fragments of sufficientsize to he visible in a powerful microscope. There is, however, a special type of association which stands ?idyay between solution and emulsion, because the hody dwellmg in the vehicle by its complete transparency would induce one it is dissolved, whereas, on the contrary, it is disto persed therein in flakes, vesicles, or other shapes. Alumina. and silica offerus remarkable examples of this, since they, when hydrated and of recent origin, remain expanded in the liquid in which the reaction t w k place which separated them from the other hody with which they were chemically combined; meanwhile they do not deprive it of transparence nor render it vist some time, not many days, cous or mucilaginous. ~ u after they contract and separate gradually, with the appearance of insoluble precipitate-an effect which shows itself more rapidly on addition of s. soluble salt in certain quantities, or of otherreagents. Tho and silica from the separation of medium, the precipitation induced by salts prove clearly that they do not exist thereinin a state of truesolution. But how is it that alumina and silica, while remaining incorporated with the liquid, do not render it opaque and turbid, or a t least impart to i t some slight degree of opalescenoei The reason lies in the peculiar qualities of finely divided alumina, and of swollen and gelatinous silica, because, when they are a t once examined in the light, it may he observed that they do not allow the entire pencil of rays to pass, without abstracting part or decomppsing it, as do transparent bodies.
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JANUARY. 1949
27
Pseudc-solution, or false solution, then, is that special mode of combination between a bard or soft body and any liquid, in which the former expands in the form of minute flakes without affecting the transparence, and can be easily separated by soluble foreign matter introduced into the liquid afterwards.
Fourteen years later, Thomas Graham read a paper on "Liquid Diffusion Applied to Analysis" before the Royal Institute and therewith started that branch of natural science today known as Colloid Chemistry. The following sentences are taken from the publication of his address (g) : The comparatively ''fixed" class, as regards diffusio& is represented by a different order of chemical substances, marked out by the absence of the power to crystallize, which are slow in the extreme. Among the latter are hydrated silieic acid, hydrated alumiha. and other metallic peroxides of the alnminous class, wben they exist in the soluble form. Although often largely soluble in water, they are held in solution by a most feeble force. The plastic elements of the animal body are found in this class. As gelstine appears to be its type, it is proposed to designate substances of the class as colloids, and to speak of their peculiar form of aggregation as the colloidal condition of matter. Opposed to the colloidd is the crystalline condition. Substances affectingthelattef form will beclassedas crystalloids..
..
Selmi, however, did not antedate only Graham in describing certain properties of alumina and silica, but also Michael Faraday by pointing out that light passing through such solutions is in part abstracted. Courses in colloid chemistry, and particularly introductory ones, should be illustrated by as many lecture demonstrations as possible. I n the discussion of the history of colloid chemistry, Weimarn's law of precipitation should be demonstrated. Studying the reaction:
+
M ~ S O ,+ B ~ ( C N S ) Z water
=
Mn(CNSh
+ Bas04 + water
he found that when using sufficiently high concentrations the BaSOl was precipitated in the form of a jelly, whereas in great dilutions extremely small insoluble crystals were formed in suspension. In the summary to this first important experimental contribution (5), we find these statements: The so-called colloidal, amo~phou$and crystdloidal states are all together universal (possible) properties of matter. Generally speaking, it follows from these investigations that colloids and crystalloids are by no means two special worlds but that there exist close relations between themselves ss well as between them and the gaseous and liquid states of matter.
...
Here again Sir Osler's words come into their right. The idea that colloids are nothing specific but a state of matter, just as crystals are, was first conceived by Weimarn,but the credithasgonetowo. Ostwald,whoten years later in his book, "The World of Neglected Dimensions" (S), stated that the colloidal condition is a generally possible state of matter, and that any substance can be brought into the colloidal state or condition.
With this as a basis, a modern textbook should also offer as simple an explanation as possible why the following statement, made by another pioneer in Colloid Chemistry, Raphael Eduard Liesegang, is today as true as in the days it was written. Generalization is one of the goals of science. I t can, however, be very dangerous, bemuse it might lead to a feeling of security where it is not warranted. This is especially the case wben dealing with colloids, because their properties cannot be explained by simply applying to them the basic and generally acceptedlaws of chemistry.
This must be done in such a way that the reader comes to realize more and more that colloid chemistry actually is the chemistry and physics of surfaces and their composition and that the properties of matter in the colloidal state and the phenomena characteristic for i t are primarily due to the preponderance of surface over volume. Drawing attention to the chemical unsaturation of ions located in the surface of matter as compared to those located in the interior. will materially assist in explaining why most properties of colloids cannot be harnessed into rigid mathematics. Thereafter, an introductory text should cover in a general way such basic topics as production of colloidal systems, protection, surface and interfacial tension, electrokinetics, dispersion, adsorption, gelation, and coagulation. This should then be followed by a t least one chapter devoted exclusively to a brief, but nevertheless precise, discussion of the most important industrial apphations, as for example catalysis, solvent recovery, paper production, rubber latex, ceramics, soil conservation. naner and-~ rnaint, iudiist,rv. --..-..*, Fina.llv. t,hn ~..-~~-~ ...-tenthook -> r-~.r -should acquaint the reader, preferably in an appendix, with the most important tools used in colloid chemical research, as for example, the various types of ultramicroscopes, electron microscope, dialyzers and electrodialyzers, viscosimeters, surface- and interfacial tensiometers, and x-ray diffraction techniques. Courses based on such a textbook must be so organized that the student is given a cllance actually to observe what he should remember, because colloid chemistryjs that branch of natural science which, more than most others, needs enthusiasm to become truly devoted to it, and generally you will hesitate to believe something you cannot see. ~
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LITERATURE CITED (1) CONANT, J. B., "The Humanities and the Scientist,"lecture presented to the M.I.T. Faculty Club on April 15,1948. ( 2 ) GRAHAM, Th., Phil. Trans. Roy. Soc., 151, 183 (1861). (3) Osrw~~~,Wo.,"DieVeltderVernachlassigtenDimen~ionen," Leipzig, 1918. (4) SELMI,F., "Nuovi Ann. D. Scienze Nrtturali di Bologna," Scries 11,Vol. VIII, 401 (1847). (5) WEIMARN, P. P. YON, J. w a s . Phyaiko-chem. Ges., 37, 949 (1905).
