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Notes CfkSC Folklore. 5. Gravitational Stability of Sols. From Perrin to Usher Karol J. Mysels Chemistry Department, University of California San Diego, La Jolla, California 92093-031 7 Received May 12,1992. In Final Form: July 24, 1992
Introduction Jean Perrin’s work on Brownian motion, first published in 1908,l was recognized by a Nobel prize in 1923. Its importance stems from its role in the establishment of the reality of molecules. This has been reviewed in a book by Nyee2The work on Brownian motion that preceded Perrin has also been described in an article by Kerker.3 Thus the mainstream is well-known. This note deals with a sidestream, a little known sequel of special interest to those concerned with colloids but with more general lessonstoo. Perrin showed that a dilute suspension of fine particles in water behaves like an ideal gas of enormous molecular In particular the concentration decreases exponentially with height. He determined the size of his particles by severalmethods: one of them was macroscopic measurement over a string of particles after they settled into compact monolayer clusters; another was by the rate of fall through quiescent water and application of Stokes’ law. His main tool was the microscope and his cells were 0.1 mm in depth, but he had no doubts that his theory applied to other dimensions as well. This seemed to fly in the face of some everyday experience as was well stated by Emil Hatschek at the 1922 Discussion of the Faraday SocietyS8His remarks to the audience are reported as follows: “Hitherto they had all been giving Perrin’s results in books and lectures, although it was obvious to all of them, and equally so to their students, that a gold sol in a 100 cc cylinder showed exactly the same concentration at the bottom as at the top as far as could be judged visually. It was some relief to him that students have generally been tactful enough not to inquire what happened beyond the fraction of a millimeter covered by Perrin’s formula, but it was highly gratifying to have now a complete answer to possible questions.”
A Porter Layer? The “complete answer” was a papergjust presented at the Discussion by the President of the Faraday Society Alfred W. Porter, DSc., FRS, Professor at the University of London, past President of the Rontgen Society, and future president of Section A of the British Association. Porter was certainly an impressive figure.1° His paper (1) Perrin, J. B. C. R. Hebd. Seances Acad. Sci. 1908,146, 967; 147, 475, 530. (2) Nye, M. J.MolecularReality;American Elsevier: New York,1972. (3) Kerker, M. J. Chem. Educ. 1976,67, 190. (4) Perrin, J. B. Ann. Chim. Phys. 1909, 18, 5. (5) Perrin, J. B. J. Phys. (Paris) 1910, 9, 5. (6) Perrin, J. B. J. Chim. Phys. 1908, 8, 57. (7) Perrin, J. B. Les Atomes; F. Alcan: Paris, 1913. (8) Hatschek, E. Trans. Faraday SOC.1922,18, 99. (9) Porter, A. W.; Hedges, J. J. Tram. Faraday SOC.1922,18,91; also 1923, 19, 1. (10) Trans. Faraday SOC.1939,35, 302.
presented experiments which disagreed greatly with Perrin’s law of sedimentation equilibrium. He also had a theory which assumed very long range repulsion due to solvation to explain his results. He presented a graph, Figure 1,showing a theoretical line and nicely fitting pointa not only from his recent experiments but also from a previouslypublished theory of Burton,” which the latter used to explain his data. Hatschek is reported to have opened the discussion by saying that the reading of this paper would be a “historical occasion”. Porter’s views are best summarized in this paragraph9 “...There are in reality three regions in a suspension to which particular names might be given ...a very thin layer close to any surface in which there may be a special value for the concentration. This is the layer discussed by Willard Gibbs and might be called after him. Its thickness is of the order of the range of intermolecular forces. Secondly,there is the layer studied by Perrin, “inferieure au dixieme de millimetre” [less than a tenth of a millimeter], in which the change of concentration of a suspensoid can be calculated from an application of the laws of gases This is the Perrin layer. Third; there is a layer of 1 to 2 millimeters thickness (in the particular cases studied in this paper) in which further change of concentration occurs which cannot be calculated in the way adopted by Perrin. This gradually merges into the main body of the suspension in which the concentration is sensibly uniform.” Thus new long range forces, peculiar to colloids, and keeping the particles apart had to be postulated. One may also note that three layers seem eligible for eponymy but only two are named! It is hard to escape the impression that Porter hoped that his name would be attached to the third one. It was not to be so, and the person who did the decisive experiment showing that the “Porter layer” was an artifact, F. L. Usher, was very different from Porter.
...
Francis Lawry Usher There is no obituary indexed for Usher. There is a brief biographical note on the occasion of his retirement12and I am indebted to Professor P. G. Laye, Head of the Chemistry School at the University of Leeds, for making it available to me with his comments. Usher was born probably in 1885,started his scientific career in Bristol, spent some time at the German University in Prague, then moved to University College, London where he worked with Wm. Ramsey, and later became Professor of Chemistry at the Central College, Bangalore, India. In 1926 he became Reader in Colloid Chemistry at Leeds,a post which he held until his retirement in 1950. He died on January 24,1969. He was an enthusiastic rock climber, a conscientious teacher, and “... won universal esteem by his shy friendliness, his kindness and his integrity.” His publications are few in number but cover a remarkable range of topics including photosynthesis, solubility of gases, the decomposition of ammonia, atomic weight determination, atmospheric contamination, radiochemistry,dying, and the stability of colloids. The papers ”... are characterized by high standards of accuracy and experimental skill and real originality of thought.“ It was (11) Burton, E. F.; Bishop, E. S.Proc. R. Soc.,London 1922,10OA,414. (12) Cox, E. G. Uniu. Leeds Rev. 1950-51,2, 167.
0143-7463/92/24Q8-3191$Q3.QQ~Q 0 1992 American Chemical Society
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Notes
9
1
8
x '
j6
I' a'
-~
'1
...
% 2
1
O
-
Figure 3. Westgren'slo cell as shown in Svedberg's b00k.l'
*ox
am .03
*q '05
'06
....... Perfect gas law.
*q
*io
q
-11
*I*
'13
ai4
Depth in centimetrcr.
-15
0 0 Bunon'i equation.
+ + + Experimental points.
Port:r and Hed:es.
+#-
Figure 1. Porter and hedge^'^ figure showing the transition from a top depleted by sedimentation in accord with Perrin's work to uniform bulk concentration according to their experimenta and two theories.
I
microscope du
I
ii (1
em o / s i o n
disgua
Glace
parto
-
4
evide
iH
abJer
Figure 2. Perrin's original schematicof his cell. H is the height
of the cuvette (0.1 mm). "disc BvidB" is a flat ring of glass; "emulsion"is his term for suspension or sol.
said of him that he "is too much of the real scientist to go for pot-boiling."12 The Early Experiments Perrin was not concerned with colloid stability, He was after a proof that molecules exist by careful determination of Avogadro's number in a variety of more or less independent ways. Hence he chose for his experiments a suspensionof the gum gambodgewhich could be rendered quite monodisperse by fractional centrifuging, scattered light strongly, and sedimented readily. He did not need large volumes of solution and did most of his measurement in an early version of a hemacytometer,Figure 2, a glass cell 0.1 mm deep and used for counting the number of red corpuscles per unit volume of blood. Perrin's experiments were extended to gold sols and other inorganic colloids by Arne Westgren,13a student of Svedberg,who also showed that sedimentationequilibrium could be approached from both sides. His cell (Figure 3) is described as follows "A cover glass was cemented on three sides to a microscope slide with picein. By using drawn out threads of picein of uniform thickness one could obtain very straight and even boundaries of the microscopic cell. Then a drop of the solution studied was brought to the opening of the capillary cell thus formed. Capillary forces immediately sucked the drop into the cell. The (13) Westgren, A. 2.Anorg. Algem. Chem. 1916,93, 281.
opening was then tightly sealed with some vaseline The thickness of the cuvette was 20-60 pm." Westgren extended the distance over which sedimentation equilibrium was observed to slightly more than 1 mm. (Incidentally, the picein used is not the water-soluble glucoside of that name but a black, insoluble, low vapor pressure thermoplastic cement which is no longer obtainable in the
USA.) The Article and the Book Perrin's work was rapidly recognized in France and in 1909 he summarized it in a loo+ page article4Brownian Motion and Molecular Reality, with slightly shorter versions published in two other leading jo~~nals.5~6 Soon thereafter, in 1913,Perrin published his book7The A t o m , which went through many editions and translations. It was a broad and influential popularization of the kineticmolecular theory with emphasis on Brownian motion. At first glance the whole article seems to have been incorporated into the book, but on closer examinationone finds many changes, some significant. For example the cell shown in the book is quite different from Figure 2. In the article, Perrin used a finely tuned stability control to get his particles to settle in tight monolayer clusters. In the book he depends on capillary pressure of the evaporating liquid to do this. More importantly for our story, the book discusses only briefly Perrin's Stokes' law work without any experimental details. In contrast, the article has Figure 4 and states: "It is enough to place the sol in a capillary tubing to a height of a few centimeters, to seal the two ends, and to place it vertically in a thermostat, and one can see the sol leave graduallay the upper layers of the liquid, falling like a cloud with a quite well defined surface and descending each day by the sameamount. The figure (Figure4) shows the observed appearance. One must use capillary tubing to avoid convection currents which would muddle the surface of the cloud. These currents are produced with extreme ease in large tubes". The Controversy While the kinetic-molecular theory became widely accepted, the pesky colloids refused to settle in their laboratory bottles and many wondered why. "This fact was pointed out by the writer in an article in Stahler's handbuch 1916 together with the suggestion that the discrepancy was probably due to the mixing effect of convection currents", wrote Svedberg.I4 But many did not accept his views. Some, including Wilder D. Bancroft,'sthe editor of The Journal of Physical Chemistry, believed that somehow if particles were small enough, Brownian motion overcame their tendency to sediment. (14)Svedberg, T. Colloid Chemirtry, 2nd ed.; Chemical Cataloque Co.: New York, 1928; p 102; let ed., 1924; p 101. (15) Bancroft, W. D. J . Phya. Chem. 1914, 18, 549.
Notes
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Figure 5. Elaborate thermostating of McDowell and Usher.16
M indicates 1 to 2 cm high cells in which sedimentation is observed and measured.
Figure 4. Perrin’s picture of a gamboge sol sedimenting in a thermostated sealed capillary, from the article,’ it is not in the book.‘
Still others led by Porter and Burton tried to measure the deviations from Perrins predictions and explain them by long range repulsions due to hydration. However their experiments used very poor or no thermostating and veaseb whose smallest dimension was several millimeters or centimeters! Such completedisregard for Perrin’s warning is probably explained by the fact that they never read his article but only his book! Burton makes reference to the English translation whereas Porter refers to the french original. It may be noticed that his quote “moins d‘un dixieme ...” given above, appears in the book but is absent from the article. In 1927 Elmer 0. Kraemer wrote a 30-page, very complete review of this situation16pointing out the logical inconsistencies and experimental failings of those who thought that the uniformity of sols in macroscopic vessels was intrinsic. He concludes that “...isolated cases of discrepancy between observations and theory may be suspected to be due to spurious effects of an external character”. Nevertheless the title of Kraemer’s review is “Some Unsolved Problems ...”. Thus simplistic theories as well as experimentsshowing “discrepancies”could be criticized,but convincing evidence was lacking that colloidal particles would settle according to Perrin’s law over macroscopic distances and at higher concentrations.
I 02
04
06
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og
18
IB
20
cnt.
Figure 6. Results of McDowel and Usher15 showing that by eliminating extraneous disturbances an exponential decrease of concentration with height can be obtained over large distances and for large particle concentrations.
The Light at the End of the Tunnel Substantial progress was made by Johnston and Howell17 who used a thermostat drifting by less than 0.005 “C to obtain exponential settling over some 0.05 cm in a cell 8 mm in diameter, larger than that used by Porter. However their measured concentrations were limited to about 1OE4 of relatively large particles per cubic centimeter because, like most of their predecessors, they determined their concentrations by counting the particles within a small volume defined by a diaphragm in the microscope and its depth of field.
McDowell and Usher Usher and McDowell (the latter presumably a graduate student about whom I could find no information) were apparently unaware of the work of Johnstone and Howell which was published two years before their own. Their paper18 describes first qualitative experiments showing the importance of avoidingconvection due to temperature gradients. They then proceed to describe careful experiments in a specially designed thermostat shown in Figure 5. The two specks in the middle are two wedge-shaped cells containing the solution and a standard. They are in the center of a 3-L water tank with optical windows. This
(16) Kraemer,E. 0.In ColloidSymposiumMonograph;Weiser, H. B., Ed.; Chemical Catalogue Co.: New York, 1928; Vol. 5. (17) Johnstone, N.; Howell, L. G. Phys. Reu. 1930, 35, 274.
(18)McDowell, C. M.; Usher, F. L. R o c . R. SOC.London I932,138A, 133.
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is within, but separated by a layer of air from, a double box whose walls are packed with 2 in. of goose down. This thermal resistancecapacitance package is within the air thermostat proper which is heavily insulated. The air of the thermostat is circulated by a fan supported from outside the thermostat to avoid vibrations. The concentrations were obtained by photometric measurement of photographs taken against a white background which permitted measurement of about 10E12 of relatively small (15 nm radius) particles per cubic centimeter. The wedge-shaped cells mounted with one 45O edge at the bottom and one face vertical, extended the range of concentrations that could be measured. Sedimentation equilibrium could be followed over some 8 mm as shown in Figure 6. The cella were 1cm high and equally deep at the top. Their width is not given. About 4 weeks was allowed to reach equilibrium. Thereafter, upon exposure to laboratory temperature fluctuations, the cell contents soon became homogeneous. Usher's Legacy These careful experiments extended Perrin's and Westgren's results to distancesand concentrationswhich clearly showed that there was no need to assume long range forces peculiar to colloids. Whereas Perrin showed that "spurious effects" can be made negligible by drastically reducing
the size of the sample studied, McDowell and Usher demonstrated that these effects can be eliminated by proper experimental procedures and thus brought a long controversy to an end. Their verdict was, I think, accepted in the colloid science community but spread only slowly to the chemical one. In 1955 Bancroft's explanation was still popular with textbook writers and a Textbook Errors column1gwas devoted to the subject. Today a survey of 10 freshman texts showed that they all duck any explanation of the gravitational stability of colloids! Conclusion This brief history omits many interesting facets such as the rate of approachto equilibrium and the real deviations from Perrin's law when the volume fraction of the particles becomes significant. Nevertheless it may remind us of some old precepts: High office and position is no guaranty of scientific insight. A curve may fit points for wrong reasons. Much can be gained sometimesby consultingthe original references. Convection and diffusion are very different though they both cause mass transfer. (19) Mysels, K. J. J. Chem. Educ. 1955, 32,319.
