Incidentally, a phase transition introduces an “allor-none” type of behavior‘ not otherwise present. The titration curve for the In y = 2.25 case is given in Fig. 2. Although no experimental curve is available for bovine or human serum albumin at zero ionic strength, the theoretical curve in Fig. 2 is almost certainly not steep enough. It would become more steep (and eventually vertical) at higher values of In y. We discuss only first order electrolyte effects here. Suppose, following Eq. (19), we write in general *A8 = (___ N n)* [du) ZO@(V) . . . I (32) kT
+
+
40. 20 I
1
1
I
-7
-6
-5
+ +
In f c = In 7 ( N n)[Xv) B - N
+ so@(v) + . . . I (33)
and Eq. (27) as
We then find, on expanding in powers of xo In f c = In fco
+ Zoe(v) + . . .
(35)
and
e(v) = (No
(36)
+ n ) [@ - 2 b@/bV bp/av p
--
BO 2No(B0- N O )b d h V
The subscript zero on ca, u0 and No refers to the limit xo = 0. I n general we expect, on the right hand side of Eq. (37), 9 to be negative and the other terms to be positive, with 8 also positive. Thus v would increase with xo a t constant c in Eq. (36)-that is, the protein molecule would decrease in volume when electrolyte is first added ( c constant). This behavior is found experimentally by Yang and Foster. Unfortunately, the particular
I
I
-3
-4 -LOG
Then Eq. (21) appears as
where
361
THEDENSITY A N D STRUCTURE OF CHRYSOTILE
Mar., 1956
I
I
-2
-1
I
0
fc
curve of protein.
Fig. 2.-Titration
combination of model and approximations used in the electrostatic calculation here happens, by cancellation, to give independent of V (compare Eqs. (21) and (33)), and hence d@/dV = 0 and 0 < 0 in Eq. (37). This behavior is opposite to that just described (but it is reversed by the second order term at rather low electrolyte concentrationof the order of 10-3 M ) . Any refinements will undoubtedly make 9 dependent on V as expected in general. Conclusion The preliminary calculations reported here indicate that the suggestion of Yang and Foster is a reasonable possibility from a theoretical point of view. Of course the calculations do not provide any direct evidence that this suggestion is actually correct. The discrepancy between theory and exrJeriment on the direction of the salt effect is presumably due to the approximate nature of -the electrostatic part of the theory. Other expansion mechanisms might also prove to be “reasonable possibilities” in the above sense. We are, in fact, examining Tanford’s2 proposal (swelling caused by breakage of cross-links) in an analogous fashion. The direction of the salt effect will be the same in this case, incidentally, unless the electrostatic theory is modified.
THE PROPERTIES OF ASBESTOS. 11. THE DENSITY AND STRUCTURE
OF CHRYSOTILE’ BY FRED L.PUNDSACK Contribution from the Johns-Manville Research Center, Manville, New Jersey Received August 26, 1066
It is shown that the tubular structure h othesized for chrysotile asbestos is not compatible with experinientally determined density values for sealed, solid blocgof asbestos fiber. The data indirate that the fundamcntal fibers are packed together efficiently with very little void space. The only void space observed appears to he associated with the volume occupied by sorbed water. Sorption and desomtion of water probably causes reversible swelling of the fiber bundles. It is hypothesized that the fundamental fibers may exist as sheet- or ribbon-like structures possessing a certain degree of distortion due to limited curvature about the fiber axis,
The unusual nature of serpentine minerals (3Mg0.2SiO2.2H20) which occur in massive, pseudo-fibrous and fibrous (chrysotile) forms has led numerous investigators t o attempt a structural (1) Preceding paper in
69, 892 (1955).
this series, F. L. Pundsack, THIU JOUBNAL,
analysis of the various species. Chrysotile was classified first as an amphibole structure with a repeating Si4OI1-6unit,2 but this was revised later to a layer- or sheet-type structure with a repeating (2) B. E. Warren and (1930).
W.
L.
Bragg, Z. Kriat., 76, 201
362
FFCED I,. PUNDSACK
Si4Ol0-~unitb3 Although there appears t o be general agreement that chrysotile has a sheet- or layertype structure, many workers have pointed out that the fundamental sheet structure must be distorted in some way to account for the X-ray diffraction patterns which are 0bserved.4-~ I n recent years interest in the structural characteristics of chrysotile has been stimulated by the almost simultaneous publication by Bates, Sand and Mink’ in this country and No11 and Kirchefl in Germany of electron photomicrographs of chrysotile which appear to show the fibers in the form of hollow tubes. Hillier and Turkevichg had previously noted this phenomenon in passing. Recently a number of attempts have been made to correlate X-ray diffraction data from chrysotile with the hypothesized tubular structure. lo--l* It is apparent that a tubular structure of the type hypothesized for chrysotile will contain considerable void volume (i-e.,space not occupied by chrysotile) when the fibers are packed into solid bundles of the type found in nature. Thus, if the density of naturally occurring blocks of fiber is determined in a liquid which does not penetrate the fiber blocks, the magnitude of the void volume can be ascertained, and the existence of a tubular structure can be shown to be either possible or highly improbable. Experimental Materials.-The chrysotile fiber used for the density determinations in water was selected from a supply of No. l Danville Crude from Canada. The analysis of this material has been re orted previously .1 Portions of the selected solid fiber blocEs were broken off into unopened bundles about 2.4 cm. long and 0.4-0.5 cm. in diameter. The samples were examined carefully for flaws and imperfections. Density determinations on these specimene are referred to as density values for “blocks of fiber.” After the experimental runs had been completed on these blocks of fiber, they were opened carefully by hand and found to be free of any foreign inclusions which might otherwise invalidate the measuremen ts The density determination in air was carried out with an essentially flawless, translucent specimen of chrysotile from Arizona. This sample was in the form of a slightly irregular block about 5 cm. X 3 cm. X 1.6 cm. with a mass of approximately 58 g. Distilled water with a specific resistance >5 X 106 ohms was used for density determinations in an aqueous medium. Density Measurements.-Two 2 5 4 . capacity pycnometers fitted with thermometers and side-arm capillary tubes were calibrated to zkO.002 cc. with the liquid being used in the density measurements. Samples were weighed out into the pycnometer flasks, and then they were outgamed in accordance with a method described by Tschapek18 and later used successfully by Culbertson and Weber.“ The procedure consists of adding enough liquid to the flask to cover the sample and then evacuating the system in a vacuum desiccator to just below the vapor pressure of the liquid.
.
W.
(3) B. E. Warren and K. Hering, Phys. Rev.,69, 925 (1941). (4) V. A. Foak and V. A. Kolpinsky. J . Phye. U.5.S.R.. 8 , 125 (1940). (5) B. E. Warren, Am. Mineralogist. 47, 235 (1942). (6)E. Aruja, Minerdog. Mas., 41, 66 (1944). (7) T. F. Bates, L. R. Sand and J. F. Mink, Science, 111, 512 (1950). (8) No11 and R. Kircher, Naturw., 81, 540 (1950). (9) J. Hillier and J. Turkevich, And. Chum., 31, 475 (1949). (10) E. J. W. Whittaker. Acto Cwat., 1, 827 (1954) (11) E. J. W. Whittaker, ibid., 8, 261, 265 (1955). (12) H.Jagodzinski and G. Kunse, Ncuee Jahrb. Mineral. Mondsh., 95, 113, 137 (1954). (13) M. W. Tschspek. Kolloid-2.. 68, 343 (1933). (14) J. L. Culbertson and M. IC. Weber, J . Am. Chow, ,?OF., 60, 2695 (1938).
W.
Vol. 60
This method effectively removes adsorbed air from the system. After the system had been outgassed a sufficient length of time (determined by repeated runs until a constant density was attained) the vacuum was broken and the flask filled with li uid and allowed to come to room temperature, 25 i 1”. %he pycnometer unit was assembled and the density determined. The actual temperature of the system was read to within 0.1” from the thermometer in the pycnometer h k . After the density values had been obtained for the unsealed specimens the samples were collected, dried and the density calculated on both an “as-is” (Le., original sample weight which includes sorbed water) and a dry basis. For the “sealed block” density determinations unopened blocks of chrysotile were weighed and then dipped in molten paraffin a t about 78”. This procedure formed a relatively smooth, even coating of solidified araffin over the entire block of fiber. Subsequent gas A o r p t i o n measurements showed that the coating was impervious. The density of the “sealed blocks” was determined in water, and then the value was corrected for the density of the par& present. Sorbed water content of the blocks was determined by dehydration runs on untreated fiber blocks from the same source. The untreated blacks were weighed under the same conditions of relative humidity as the blocks used in the density determinations. T h e density of the para&, 0.901 ./cc., was determined independently by coating glass rods o f known volume with the molten p a r 5 . The density determination in air on the specimen of fiber from Arizona was carried out aimply by mapping the contours and profile of the block on graph paper and calculating the volume on the basis of the mapped dimensions. The block waa weighed and then the sorbed water content was determined on a small portion of the block by the procedure described below. Water Determinations.-Experiments showed that reversibly sorbed water could be removed from chrysotile by drying the material a t temperatures between 175 and 240’. At temperatures much above 200” a slight, non-reversible water loss occurs in chrysotile. Therefore, for purposes of dculating the density of the fiber eamples on a dry sample basis the water loss up to 175” was considered sorbed water. As an approximation the sorbed water was assumed to occupy the same volume as an equal quantity of free water, and this correction was used to obtain the denRity of the solid fiber. Although the sorbed water may actually OCcupy a volume slightly less than that of free water, this assumption is not critical to the conclusions reached in this
work.
Theoretical When hollow cylindrical tubes with an outer radius rl and an inner radius r2 are placed together in hexagonal close-packing the ratio of the gross volume of the bundle of tubes, VG,t o the volume of the solid, V S ,is Equation 1 may be rearranged and reciprocal density values D substituted for corresponding volumes to give where DS = absolute density of the solid DO = observed gross density of the fiber bundle
Equation 2 reflects the relationship of the density of the solid to the observed bundle density when the liquid fails to penetrate the void volume. If the fibers axe solid (Le., r2 = 0), or the liquid penetrates the intrafibril pores but not the interfibril pores, equation 2 reduces t o . Using the aveFage values u = 5.33 A.,2, = 9.24 A., and c = 7.33 A. the absolute density of chrysotile is 2.56 g./cc. Since the samples used in this work contained about 2% FeO FezOs isomorphously substituted for magnesium, ' the theoretical density of 2.56 can be corrected to 2.58 g./cc. to take this into account. This correction is incidental since it is within the range of experimental error.
+
TABLEI UNITCELL( Mg,(OH&3LOlP)DATAFOR CHRYSOTILE Axis
a
b C
Warren and Bragg'
Warren and Hering'
5.33 9.25 7.33
5.33 9.24 7.33
5.32 9.2 7.31
Whittakers
Padurow"
5.33 9.2 7.33
5.33 9.26 7.36
The magnitude of DG t o be expected for hollow tubes of the dimgnsions which gave been suggested,'? r1 = 175 A. and r2 = 75 A., can be calculated from equation 2 by using Ds = 2.58. In this case DG = 1.91 g./cc. assuming a sealed block in which no penetration of liquid into the void volume occurs. Results and Discussion The data for a seriea of density measurements on both sealed and unsealed blocks of fiber are summarized in Table 11. It.is clearly evident from the data on the paraffi-sealed blocks that they do not contain the void space required by a hollow tube structure. In fact, the results strongly suggest that the only appreciable void space in the solid blocks of fiber is that occupied by sorbed water. Furthermore, the void space is not constant, but increases as the sorbed water content increases. That is, the blocks of fiber must swell as water is sorbed. The only exception among the sealed blocks is sample TABLE I1 DENSITYOF SEALED AND UNSEALED SOLIDBLOCKSOF FIBER Sample
Sorbed HIO, Gross % g./cc.
Cliryaotile, b g./cc.
Sealed 1 0 2.57 2.57 Sealed 2 0 2.55 2.55 Sealed 3 0 2.56 2.56 Sealed 4 0 2.50 2.56 Sealed 5 2.2 2.48 2.57 Sealed 6 1.3 2.48 2.53 Sealed 7 0.8 2.53 2.56 Unsealed 8 1 .o 2.51 2.55 Unsealed 9 1.0 2.53 2.57 Unsealed 10 0.7 2.56 2.58 Arizona" 2.0 2.45 2.53 a Density of gross sample including sorbed water but excluding paraffin. Calculated on the assumption that The density #orbed water occupies a volume of 1 cc./g. of this sample was determined in air. (15) E. J. W. Whittaker, Acta Crnat.. 6, 143 (1952). (16) N. N. Padurow, ibid.. 3, 204 (1950). (17) G. J. Young and F. H. Healey, T ~ i JOURNAL., s 68,881 (1954).
would expect. In view of the consistent behavior of the other values it seems likely that the value for no. 6 may have been low because of a slight imperfection in the otherwise solid block of fiber. The fact that the sealed .samples which contain essentially no sorbed water have density values closely approximating the theoretical absolute density of chrysotile indicates that the fibers in a desorbed condition are packed together with very little void space. This behavior is not compatible with a hollow tube structure. I n order to account for the close-packing of the fibers it seems more plausible to view them as strip or ribbon-like structures which may be distorted by limited curvature about the fiber axis.18 This type of structure awaits confirmation by X-ray diffraction studies. The density values obtained for the unsealed blocks of fiber in water indicate the consistent nature of the measurements. 111the unsealed blocks the only appreciable void space measured is also that occupied by sorbed water. Taken alone the values for the unsealed blocks could riot conclusively establish the improbability of a tubular structure. However, the results on sealed blocks indicate that when unsealed blocks of fiber are placed in water, the blocks probably sorb water and swell although initially the only appreciable void space present is that occupied by water sorbed prior to immersion. Measurement of the density of t.he block of Arizona chrysotile in air furnishes additional confirmation of the improbability of the existence of a tubular structure in chrysotile. Considering the crudeness of the method, the results are in good agreement with the premise that the only significant void space in bundles of fiber is that occupied by sorbed water. This void space is much less thaii would be required by a tubular structure. A series of studies of the amount of water sorbed by chrysotile as a function of relative humidity indicates that the maximum amount of sorbed water retained by blocks of fiber at 100 per cent. relative humidity and 25" is approximately 2.5 per cent. If, as seems probable, sorbed water causes the fiber bundles to swell, the maximum degree of swelling can be calculated using the sorbed water value at 100 per cent. relative humidity. (hie gram of desorbed fiber occupies 2.58
=
0.388 cc./g.
When 1 g. of chrysotile sorbs the maximum amount of water the gross sample will weigh 1.026 g. and occupy a volume of 0.388
+ 0.026 = 0.414 cc./g. chrysotile
The apparent change in volume of the block of fiber is 0.026 cc./g. chrysotile. The percentage increase in volume over the range of complete desorption to complete sorption would be 0.026
X 1 0 0 = 6.7%
Experimental work is now in progress to observe directly the swelling of fiber bundles and to establish. (18) Such a limited curvature has been suggest.ed for antigorite, a so. called massive variety of serpentine. by J. Ziissnian, hfineralog. hfog., SO, 498 (1954).
364
P. L. WALKER,JR., AND EMILERAATS
whether the swelling is of the predicted order of magnitude. No satisfactory explanation of the hollow-tube appearance of chrysotile fibers when viewed in the electron r n i c r o s ~ o p e ~can - ~ ~be~ ~offered other than to suggest the obvious: the sample viewed in the electroil microscope no longer bears a one to one relationship with the iiative fiber. Whether this is the result of the treatment the fibers have received (19) R. K. Iler. “The Colloid Chemistry of Silica and Silicates,” Cornell University Press. Ithaca, N. Y., 1955, p. 208.
Vol. 60
during the preparation or of the exposure t o the electron beam in a high vacuum remains to be determined. Acknowledgment.-The successful execution of this work is due in large part to Mr. George Reimschussel who made many of the density measurements reported here. Mr. Marion Badollet and Mr. William Streib made available certain excellent specimens of chrysotile which were measured in the course of this investigation.
CHANGES IN PHYSICAL PROPERTIES OF GRAPHITIZED CARBON RODS UPON GASIFICATION WITH CARBON DIOXIDE’J BY P. L. WALKER,JR., AND EMILERAATS Department of Fuel Technology, The Pennsylvania State University, University Park, Penna~lvania Received September 1 , 1066
Changes in the physical properties of graphitized carbon rods after gasification t o different burn-offs at 1000”and to 11% burn-off at temperatures between 970 and 1372’ have been determined. The physical roperties investigated were surface area, total and incremental pore volume, average ore radius, macropore surface area anispecific reaction rate. The results can be explained if the carbon rods are consideref to be composed of relatively homogeneous, non-porous particles of petroleum coke bound together by a thin shell of coal tar pitch coke, which essentially consists of a condensed “bubble type” structure. Reaction Rate Apparatus.-The reactor was the same as Introduction that described in a recent gasification study.‘ Two methods The widespread importance of the heterogene- were used to suspend the samples in the reactor. For reacous gasification reactions of carbons in present-day tion studies to 1 g. weight loss (11% burn-off) at different industry necessitates a complete understanding of temperatures, a hole l/n-inch in diameter and ‘/*-inch deep drilled into the top of the sample. Into this hole was the mechanism of these reactions. On the one was cemented a ‘/e-inch diameter ceramic rod which connected hand, the primary interest in gasification is related to the balance. In studies to carbon burn-offs greater than to the conversion of carbonaceous materials to 1 g., it was found that reaction weakened the bond between either gaseous fuels or synthesis gas. Here of pri- the rod and sample sufficiently to result in the inability of the rod to support the sample during weighing. For thia mary concern is the attainment of high gasification work, a l/s-inch hole was drilled through the center of the rates. On the other hand, the primary interest is samples. The ceramic support in this case consisted of a connected with the lack of gasification of carbons base plate ‘/*-inch in diameter coiinected directly to the ‘/aaiid graphites when used as electrodes, structural inch rod. The carbon sample was placed over the l/n-inch and sat on the base plate with no cement being used. carbons or moderators in atomic reactors. I n rodMercury Porosimeter.-A description of the design and either case, a basic understanding of the relation- operation of the mercury porosimeter used has been given ship between reactivity of carbon to gases and the recently.‘ Low Temperature Gas Adsorption Apparatus.-A standphysical properties of the carbons is essential. gas adsorption ap aratus was employed and has been I n the present work, the reactivity of a highly ard described recently .6 iurface areas and micropore volume graphitized carbon with carbon dioxide, as a func- distributions were determined from the adsorption and capiltion of burn-off and temperature, has been com- lary condensation of nitrogen using the BET equation’ and pared with changes in the physical structure of the Pierce technique,n respectively. carbon. This work is an extension of the finding Results of Walker and Rusinko,s who investigated the reEffect of Gasification of Carbon Rods to Difactivities of six different carbons. ferent Burn-Offs at 1000° on their Physical Properties.-In studying the effect of gasification of carExperimental bon rods to different burn-offs on their physical Carbon .-The samples used were National Carbon Com- properties, it was desirable to work a t a sufficiently pany AGKSP special graphit.e spectrosco ic electrodes containing an ash content less than 0.Olk. They were 5 low temperature so that the gasification process cm. long by 1.3 cm. in diameter. A more detailed de- proceeded uniformly through the sample. It was scription of the manufacture of graphitized carbon can be found from the uniformity of the bulk density found el~ewhere.~ profile data, a technique previously described16 (1) Based on a Ph.D. thesis submitted by Emile Raats to the that a gasification temperature of 1000° fulfilled Graduato School of The Pennsylvania State Univenity, June, 1955. this reqilirement. (2) This paper presents the results of one phase of research carried The reaction rate curves for four carbon rods out under Contract No. AT(30-1)-1710, sponsored by the Atomic Energy Commission. (3) P. L. Walker, Jr., and F. Rusinko, Jr., THISJOURNAL, 69, 241 (1955).
(4) H. W. Abbott, “Encyclopedia of Chemical Technology.”
Vol.
3, The Interscience Encyclopedia Inc., New York. N. Y., 1949, pp.
1-23.
(5) P. L. Walker, Jr., R. J. Foreati. Jr.. and C. C. Wright, Ind. Eng. Chem.. 46, 1703 (1953). (6) P. L. Walker, Jr., F. Rusinko, Jr., and E. Raats, THISJOURNAL, 69, 245 (1955). (7) P. H. Emmett, A . S . T . M . Tech. Publ., 61, 95 (1941). (8) C. Pierce, THIBJOURNAL, 57, 129 (1953).
