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INDUSTRIAL AND ENGINEERING CHEhlISTRY
VOL. 10. NO. 3
Discussion ELIIER 0. KRAEMER E. I. du Pont de Semours & Co., Inc.. Wilmington, Del.
AS PROFESSOR
Svedberg has in effect pointed out, a complete ultracentrifuge laboratory should possess facilitiee for studying solutions and suspensions a t centrifugal forces from a few times gravity up to the highest attainable centrifugal forces. T o cover such a wide range, Professor Svedberg has found i t practical to employ two machines, one using a direct-connected commercial electric motor for driving the rotor a t speeds up to 18,000to 20,000 r. p. ni.; the other being his famous oil-turbine machine which operates a t the highest attainable speeds. As is vel1 known, the oil-turbine ultracentrifuge is a rather formidable machine, and very few institutions can afford to install it. Owing to the publicity that the high-speed machine has received, there is a widespread impression that ultracentrifuge research is out of the question foi the average chemical laboratory. This is by no means the case, for a 11-idevariety of problems can be successfully attacked with the lo.iT--speed ultracentrifuge. The low-speed machine is relatively simple in construction and operation and of course is much less expensive than tlie high-speed one. The possibilities of the lor-speed machine may be illustrated by some examples from researches carried out a t the du Pont Experimental Station. Using the sedimentation equilibrium method, we hare studied in considerable detail the molecular TTeight and state of aggregation in solution of a number of long-chain, highmolecular-weight materials, particularly synthetic polyhydroxydecanoic acid. cellulose, cellulose derivatives, rubber, and neoprene and have established the correlation between viscosity and molecular weight for such long-chain molecules. I n contrast with the proteins about which Professor Svedberg has told you, all these matelials are mixtures of molecules of many sizes, and a theory has been developed for quantitatively describing the degree of nonuniformity of these high polymers from sedimentation equilibrium data. This method, we feel, is a t the present time the most satisfactory for measuring nonuniformity of materials showing marked diffusivity. The low-speed machine is not limited to the study of high polymers, and under favorable conditions molecular weights less than 2000 may be measured. Thus, a brief study was made of sodium eosinate and sodium erythrosinate, from which i t was concluded that these dyes were associated in solution as double molecules. I n this case, the high density of the dye molecules permitted study a t relatively low centrifugal forces, I n other cases, small molecules may be successfullr studied with the lov-speed machine and high density solvent. Another distinct field for the low-speed machine is in the determination of the particle-size distributions of finely divided materials extending from the finest of inorganic colloidq u p to particles large enough to classify by sieve methods or measure by microscopic means. I n all these cases, the diffusivity is so low that a direct deterrliination of size distriliution is possible from sedimentation velocity data. The first pu1)lislied papers on the ultracentrifuge dealt with this type of work and gave results for materials such as colloidal gold ani1 arsenic trisulfjde, clays, and pigments. The nietliods are applicable to fine pon-ders of any kind, nliich call be su-pentlefl in a liquid, and the data are of obviour importance nlienevei the particle size is an important variable in determining tlic utility and merit of the powdered material.
Out of the study of white powders has developed an understanding of the laws relating particle size and light-scattering efficieixy. This arose because of the fact that the progress of ultracentrifuging is followed by optical means; specifically the changes in concentration in the centrifuge cell are recorded in terms of light absorption. Xow, since the light absorption of suspensions varies with the particle size, a true particlesize distribution cannot be directly obtained from the ultracentrifuge data. It was: therefore necessary, in order to determine true bize distrihtions, to derive from the ultracentrifuge data the relationship betvieen particle size and light absorption, for it is impossible t'o prepare samples of fine powders so homogeneous as to permit a direct measurement of particle size and absorption coefficient. By means of a mechanical Iroduct-int'egraph this problem was solved. The results elucidate such varied questions as the opaqueness of fogs; the covering power of pigments in paper, paints, and lacquers: the transmirsion of opal glass; and numerous others which involve the optical properties of turbid systems. The same methods may be applied to the study of emulsions, using the low-speed machine, and in our publications it is shown how the mechanism of emulsification can be elucidated. I n this connection it was found that the particle size of neoprene latex is extraordinarily small and uniform, compared to emulsions of ordinary oils in water stabilized with conventional emulsifying agents. Closer examination of the neoprene latices revealed the fact that' self-emulsification occurs a t a certain st'age in the polymerization of the chloroprene, probably owing to the heat of polymerization. I n another study, the optical properties of emulsions were worked out in more detail, and jt was found that a single law describes the interrelationship of light absorption of the emulsion &h the concentration and particle size of the emulsified oil, t,herefractive indices of the oil and the suspension medium, and the wave length of light. As another line of study, we may mention the hydrous oxides of the metals and metalloids. Depending upon the concentration and the presence of electrolytes, the condition of the solute in solutions of the alkali silicates, stannates, tungstates, molybdates, manganese, metals of the iron group, aluminum, and many others, is intermediate between that of ivell-defined colloid suspensions and true solutions of single molecules. I n many of these cases, the low-speed centrifuge provides the best method of determining the conditions of the solution. We haT7-e studied in some detail in this way the niechanism of formation of hydrous ferric oxide from ferric chloride, and the same methods could be applied to any number of these systems. These example. should suffice to indicate the wide applicability of the low-speed ultracentrifuge to problems of industrial importance, as well as of purely scient'ific interest. I11 fact, in T-iew of t,he simplicity of the equipment and its operation, I feel that any university making a pretense a t doing thorough work in colloids should by all means possess a lowspeed ultracentrifuge even though it cannot finance an oilturbine machine. I n former years the first piece of equipinent installed in a new colloid laboratory was an ultraniicroscope, but today, it should be an ultracentrifuge.
Further discusion of tlie article by Svedhe1,g fo1lon.s on page 129.
