2127
THESURFACE AREAOF SILICATE MINERALS
The Surface Area of Silicate Minerals by the BET Method Using the Adsorption of Xenon at -78"
by K. A. Kini, R. M. Manser, and A. S. Joy Warren Spring Laboratory, Stevenage, Herts, England Accepted and Transmitted by The Faraday Society
(Received November 17, 1968)
The surface areas of graphon, graphite, beryl, chrysotile, muscovite, and talc have been measured by the adsorption of xenon at -78" and argon and nitrogen at -196". The results suggest that for nonporous or close-packed layer solids, argon and xenon give similar values for the calculated surface areas, which are smaller than the nitrogen values. For the open-layered minerals (muscovite and talc) and the open ring structure beryl, the xenon values are about twice the argon values, nitrogen giving intermediate values. The relevance of xenon areas to the froth flotation process, viz., in comparing the amounts of collector needed to float different minerals, is briefly discussed.
Introduction For solids with ultrafine structure, such as coals,' molecular sieves, etc., it is known that the BET method using the adsorption of nitrogen or argon a t liquidnitrogen temperatures gives values strongly dependent on the temperature of measurement. When the rate of adsorption is measurably slow2J an activated diffusion process, with activation energies of the order of 5 kcal/mol, is usually operative. If the activation energy is higher, say, 10 kcal/mol, the adsorption at low temperatures may be immeasurably slow and an apparent equilibrium may be obtained. Experimentally, the problem may be overcome by using molecules of very small cross-sectional area or by working with gases which have convenient vapor pressures at higher temperatures. The cross-sectional area of xenon, calculated by the method of Emmett and B r ~ n a u e r ,is~ 19.5 A2 at -78" (a recent survey of cross-sectional areas of molecules5 appears to suggest that the xenon molecular area might be considerably higheor), whereas for nitrogen and argon it is 16.2 and 13.7 A2, respectively, at - 196". Though the larger molecular cross section of xenon is admittedly a disadvantage, this may be more than compensated for by the higher rate of penetration at the higher temperature. A serious practical consequence of the uncertainty in the determination of the surface area of penetrable solids arises when the surface, as measured by gas adsorption, is used for the calculation of the adsorption density of small ions and surfactant molecules from solution. These reagents may have cross-sectional areas comparable with those of the gases used for surface area determination. Under these circumstances, the area accessible to the solute may be greater than the gas-determined area, owing to the fact that measurements are carried out at room temperature. Cal-
culated coverages may be grossly in error. For instance, in the case of the adsorption at pH 8.5 of sodium oleate on muscovite used in the present study, work at this laboratory indicates a coverage of about 90% at a concentration of 40 mg/l. This is based on a carboxyl end group area6 of 20.5 k2and an argon area at -196" of 1.7 m2/g. The mineral, however, shows hardly any sign of flotation at this concentration. I n the present study, the adsorption of xenon at -78" has been compared with the adsorption of nitrogen and argon a t - 196" on a number of common minerals.
Experimental Section The particulars regarding source, particle size, etc. of the minerals are given in Table I. Graphon is a nonporous graphitized carbon black. Details of preparation of synthetic graphite are not available. Beryl is an orthosilicate, while muscovite and talc are layer silicates. Chrysotile is basically a helix of tightly bonded brucite and silica sheets.' The adsorption measurements were made in a volumetric gas adsorption apparatus, degassing being mm. carried out at 400" under a reduced pressure of An equilibrium time of about 1.5 hr was allowed between readings. A computer program was prepared for the calculations, and the linearity of the BET plots was (1) K. A. Kini, Fuel, 43, 173 (1964). (2) R. M. Barrer, Trans. Faraday Soc., 45, 358 (1949). (3) A. S.Joy, Proceedings of the Conference on Science in the Use of Coal, Institute of Fuel, London, 1958. (4) P. H. Emmett and S . Brunauer, J. Amer. Chem. SOC.,59, 1553 (1937). (5) A. L. McClellan and H. F. Hoynsberger, J . Colloid Sci., 23, 577 (1967). (6) N. K. Adam, "The Physics and Chemistry of Surfaces," Clarendon Press, Oxford, 1930. (7) A. A. Hodgson, Roy. Inst. Chem. (London), Lectures, Monographs, Rept., No. 4, 1 (1965).
Volume 7.8, Number 6 J u n e 1968
K. A. KINI, R. M. MANSER,AND A, S. JOY
2128 Table I : Details of Minerals Mineral
Graphon Artificial graphite Beryl Chrysotile Muscovite Talc
Source and supplier
Cabot Co., Boston, Mass. K. S. Paul Products, London, England a b a C
Size
27.6 mp -44 P (85%) -200 BSS f24 /A Short fiber - 100 +150 BSS -100 +la0 1355
= S. Rhodesia; United Kingdom Atomic Energy Authority, Harwell, Berkshire, England. * Canada; Turner Bros. Asbestos , Isles; R. F. D. Go. Ltd., Manchester, England. ~ U i s t Shetland Parkinson and Co. Ltd., Shepton Mallet, Somerset, England.
verified by actual plotting. The reproducibility of the surface area value was .t5% for areas up to 1 m2/g, &2% for those between 1 and 20 m2/g, and k l % at higher values. The results are given in Table 11.
-5
-4
-3
-2
-I
0
I
DISTANCE FROM RING,
System
Xenon at -78" Nitrogen at -196' Argonat -196'
77 91 79
146 157 153
Beryl
Chrys- Muscootile vite
Talc
0.39 0.28 0.19
19.6 21.7 19.2
0.73 0.42 0.37
3.5
2.3 1.7
Results and Discussion It is seen that xenon gives values for surface area which are lower than those measured by nitrogen in the case of graphon, synthetic graphite, and chrysotile and vice versa with beryl, talc, and muscovite. The lower values may be due either to the use of too small a value of the cross-sectional area of the xenon molecule in the calculations or to the absence of complications arising from the quadrupole moment of the nitrogen molecule.* Support for the latter explanation is given by the fact that the surface areas obtained with argon and xenon for graphon and chrysotile are nearly the same. The values for the surface area of chrysotile using nitrogen at -196" agree with those of Young and Healy,g .uvhere the temperature of degassing is about the same, vix., 400". With respect to the data obtained by Young and Healy on a sample degassed at a lower temperature, viz., 25") there is, nevertheless, some similarity to the present data, in that the present values with nitrogen at -196" and xenon at -78" are reasonably close, while the values of Young and Healy with nitrogen a t -195" and acetylene at -78" are identical. The values for surface area using xenon are about The Journal of Physical Chemistry
3
4
5
Figure 1. Potential energy for passage of xenon through hexagonal channels of beryl.
Table 11: Surface Area (mz/g) of Graphon, Synthetic Graphite, and Silicates by the Adsorption of Xenon a t -78", Nitrogen at - 196', and Argon at - 196' Synthetic Graph- graphon ite
2
A
twice those using argon in the case of talc, muscovite, and beryl. The higher values for talc and muscovite are considered to be due to penetration of xenon between the sheet layers of these minerals. Whitelo has calculated that the interlamellar surface area of omuscovite, onDthebasis of a unit cell having a = 5.18 A and b = 9.02 A, could be as high as 707 m2/g. It is difficult to calculate the energy required for penetration of sheet structures, but an approximate value should be calculable for beryl. In beryl, silicon and oxygen atoms form rings of the composition SieOu, stacked on each other along the hexagonal axes, forming a series of open channels of radius approximately 2.55 A. Neglecting thermal contraction effects, it is possible to calculate, by the method of Kington and Laing," the energy of activation required to enable a xenon molecule to move from its potential minimum on one side of the six-membered silicate ring to its corresponding minimum on the other side. The potential energy as a function of the distance from the ring, using the values for polarizabilities, diamagnetic susceptibilities, etc., given in the Appendix, is shown in Figure 1. The activation energy is found to be 149 kcal/mol. Thus the result appears to contradict the experimental observation, since this magnitude of activation (8) B. G. Aristov and A. V. Kiselev, Russ. J . Phya. Chem., 11, 1359 (1963). (9) G.J. Young and F. H. Healy, J . Phya. Chem., 58, 881 (1964). (10) J. L.White, Nat. Acad. Sci.-Nat. Res. Council, Publ., No. 566, 289 (1958). (11) G. L. Kington and W. Laing, Trans. Faraday SOC., 51, 287 (1955).
COMPLEXES OF IODINE WITH TETRAMETHYLUREA AND TETRAMETHYLTHIOUREA energy prohibits the penetration of xenon into the channels. Since the xenon atom is spherical, there is no possibility of passage with a preferred orientation. However, the above theoretical calculation, as admitted by Kington and Laing, assumes the silicate rings to be rigid and takes no account of the amplitude of vibration, concerning which there appears to be no information available at yesent. Moreover, in this system an increase of 0.1 A in the radius of the ring can decrease the value of the activation energy by 67 kcal/mol and the value of 2.55 used in the calculation is the minimum distance from the hexagonal axis. Nevertheless, the explanation offered for the higher value of the surface area of beryl using xenon at -78” compared with that using argon at -196” is to be considered at present hypothetical. I t is tentatively suggested that an explanation similar to the one advanced in the case of muscovite and talc is valid for beryl also.
Appendix I n the calcuolations of activation energy, values of 3.92 X lO-Z4 A3 and 20.92 X 10-30 cgs unit for the
2129
polarizabilities and diamagnetic susceptibilities, respecda and tively, of the oxygen ion12 and 4.01 X 71.5 X 10-30 cgs unit for the corresponding quantities of the xenon a t 0 m ~ ~ swere ~ 4 used. The internuclear separation re was assumed, as advocated by MacLeod and Kington, 15 to be given by the sum of half the kinetic diameter of xenon, r0/2”’, where ro is the ecpilibrium separation of two xenon atoms,16viz., 4.46 A, and the (Goldionic radius of oxygen is taken to be 1.32 Schmidt). The values of r were obtained from the square root of the sum of the squares of the radius of the ring and of the distance along a line perpendicular to the plane of the ring and passing through its center. (12) L. Pauling, Proc. Roy. Soc., A114, 181 (1927). (13) J. A. Beattie and W. H. Stockrnayer, “Treatise on Physical Chemistry,” Vol. 11, D. Van Nostrand Co., Inc., Princeton, N. J., 1951,p 297. (14) P. W.Selwood, “Magnetochemistry,” Interscience Publishers, Inc., London, 1956. (15)A. C. MacLeod and G . L. Kington, Trans. Faraday SOC.,55, 1799 (1959). (16) E.A. Mason and W. E. Rice, J. Chem. Phys., 22, 843 (1954).
Molecular Complexes of Iodine with Tetramethylurea and Tetramethylthioureal by Robert P. Lang Department of Chemistry, Quincy College, Quincy, Illinois
62601 (Received November $0, 196’7)
Absorption spectrophotometric studies in the near-ultraviolet and visible spectral regions have been made on the iodine complexes of tetramethylurea and tetramethylthiourea, in both n-heptane and dichloromethane. KO,AH”, and AS” have been determined for both complexes in both solvents. A comparison is made of the K Ovalues ,determined for both the uv and the visible spectral regions for the tetramethylurea-iodine complex. Thermodynamic evidence for the location of the donor site, in both the tetramethylurea and the tetramethylthiourea complexes with iodine, is discussed. The relative effect of a polar and a nonpolar solvent on the complex-formation equilibria and thermodynamic data is considered and rationalized in terms of Mulliken’s charge-transfer (CT) theory. The spectral characteristics of the C T band have also been determined for both complexes. The effect of solvent on the CT-band position is considered and also rationalized in terms of Mulliken’s C T theory. Data on the “blue-shifted” visible iodine band are also reported.
Introduction Although extensive spectrophotometric studies have been made on molecular complexes of iodine with various amides, in dichloromethane2“ and carbon tetrachloride,Zb complete information is still lacking on the spectral characteristics of the charge-transfer band of an amide-iodine complex. Information on the long-wavelength side of the chargeetransfer band in the ultraviolet spectral region, along with thermodynamic data, has been reported for theiodine complexes with acetamide and N,N-dimethylformamide, in dichloro-
methane. 2s Since the spectral characteristics for both the charge-transfer band and the “blue-shifted” visible iodine band have been determined for a great number of iodine complexes covering a relatively large range of donor strength3 it seemed worthwhile to obtain this infOi-mation for an amkb-iodine complex. (1) Presented before the Division of Physical Chemistry at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968.
@)(a) H. Tsubomura and R. P. Lang, J . Amer. Chem. SOC.,83, 2085 (1961); (b) R. S. Drago, D. A. Wens, and R. L. Carlson, ibid., 84, 1106 (1962).
Volume ‘78, Number 6 June 1968
