Oct., 1954
SURFACE PROPERTIES OF CHRYSOTILE ASBESTOS
885
THE SURFACE PROPERTIES OF CHRYSOTILE ASBESTOS BY F. H . HEALEY AND G. J. YOUNG Surface Chemistry Laboratory, Lehigh University, Bethlehem, Penna. Received March 6 , 1064
Thermodynamic functions for the adsorption of gases on unactivated and activated chrysotile asbestos indicate that the unactivated asbestos surface is homogeneous to water vapor adsorption but is heterogeneous to argon adsorption. Distribution curves for the adsorption site energies of the activated and unactivated asbestos surface were calculated from the isosteric heats for argon adsorption. These curves indicate that the surface of the capillary structure made available on activation is of lower energy in regard to non-polar gas adsorption than the external surface of the fibers. Heats of immersion for the asbestos-water system support the conclusion that the unactivated asbestos surface is homogeneous for polar gas adsorption.
Introduction The previous paper' presented evidence which suggested that chrysotile asbestos consists of hollow tubular fibrils bound together in fiber bundles. It was hypothesized that the internal capillaries were blocked by plugs of sorbed water. The water plugs could be removed by activation a t 425" and restored by saturation with water vapor. Nonpolar gases were unable to penetrate the plugged capillaries and were adsorbed only on the external surface of the unactivated asbestos. It was thus possible to study separately the surface properties of the two types of surfaces. The present paper presents some thermodynamic functions obtained for the adsorption of argon and water vapor on activated and unactivated chrysotile asbestos. Experimental.-The asbestos used in this investigation was a sample of Canadian Johns Manville grade 7-R, the same as in the previous paper.' An "activated" sample is one that has been heated to 425" for one hour under a vacuum of 10-6 mm'., giving a maximum surface area. "Unactivated" refers to samples evacuated a t 25' for 24 hours. A standard BET-type volumetric apparatus was used for argon adsorption. The purification of gases has been reported previously.1
Results and Discussion.-The isosteric and equilibrium heats of adsorption for water vapor on unactivated asbestos, determined from adsorption isotherms a t 15 and 23", have been published.2 These curves are nearly linear a t the lower surface coverages and exhibit maxima in the vicinity of V m . The absolute entropy of the adsorbed state, S,, and the corresponding entropy values calculated from the isosteric heats, Sa,are presented in Fig. 1. The general shape of the enthalpy and entropy curves is indicative of a fairly homogeneous surface. This conclusion is confirmed further by determination of the equilibrium oonstant, K = 8/[(l 8 ) p / p 0 ] ,as suggested by Grahame3 Values for K were constant a t 66 f 8, though exhibiting a slight downward trend, over the relative pressure range of 0.001 to 0.07 (ie., from 0 = 0.07 t o 0 = 0.8) thus indicating a surface relatively homogeneous to water vapor adsorption. The values of the equilibrium constant did not give evidence of interactions between adsorbed molecules at as low coverages as did the isosteric heat curves. The maximum in the isosteric heat curve is more pronounced than generally encountered. It appears reasonable that the first molecules adsorbed -T
(1) G. J. Young and F. H. Healey, THIEJOURNAL, 58, 81 (1954). (2) A. C. Zettlemoyer, G . J. Young, J. J. Chessick and F. H. Healey. ibid., 67, 649 (1953). (3) D. Graham, ibid., 67, 665 (1953).
on the surface are separated by a sufficient distance so that there is little nearest neighbor interaction. The isosteric heat at' low coverages, therefore, will result only from surface forces and will be constant if the surface is homogeneous. At higher coverages the molecules are in closer proximity and hydrogenbonding results. Since hydrogen-bond formation represents considerably more energy than the usual molecular interactions, a substantial contribution to the heat of adsorption is to be expected. The isosteric heat values therefore are greater than the initial values (at low coverages) by ca. 4 kcal. a t the maximum, a reasonable value for hydrogen-bond formation in two dimensions.
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Fig. 1.-Entropy
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of water vapor in the adsorbed state on asbestos.
The entropy values S, and S, cross a t a minimum in the S, curve as pointed out by Hill.4 The entropy of the adsorbed state, s,,for water vapor on asbestos a t V m is lower than the entropy of liquid water, SL. This is indicative of strong binding and little mobility. Indeed this is not unreasonable if the adsorbed molecules a t V , are restricted by hydrogen-bond formation as well as surface adsorption forces. (4) T. L. Hill, P. H. Emmett and L. G. Joyner, J . A m . Chem. Soc., 78, 5105 (1951).
F. H. HEALEY AND G. J. YOUNG
886
Isosteric heats of adsorption were determined for argon on both activated and unactivated asbestos from isotherms at 195 and 183". These heat curves were typical of heat curves for a heterogeneous surface in contrast to water vapor adsorption on the unactivated surface which indicated homogeneity. Indeed it appears that 'while the surface force field contributed by polar van der Waals forces is nearly homogeneous for the unactivated asbestos surface, the surface force field contributed by non-polar van der Waals forces is largely heterogeneous. Distribution curves for the adsorption site energies for argon on activated and unactivated asbestos were calculated from the isosteric heat curves and are presented in Fig. 2. The distribu-
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energy distribution for argon adsorption on asbestos: 0 , unactivated; 0 , activated at 425'.
Fig. 2.-Site
tion function, f(E) = dv/dqet, suggested by Morrison and Drain6 was used. These authors assumed that the decrease in the heat of adsorption was due solely to surface heterogeneity. This assumption was found not to be entirely valid for their heat data, which was at O'K., and it is certainly more approximate for our data at a higher temperature where nearest neighbor interactions are more in evidence. However, these curves do give a relative (5) L. E. Drain and (1952).
J. A. Morrison, Trane. Faraday SOC.,48*
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comparison of the potential field suffered by an argon atom on adsorption on the activated and unactivated surfaces. It is seen from Fig. 2 that the adsorption sites available on the unactivated sample are not altered on activation. It is remarkable that the same site energy distribution is followed by the initial portion of the curve for the activated sample as for the unactivated sample. Further, activation appears to create only sites of lower energy. In-the previous paper,l it was suggested that the asbestos fibrils were hollow cylindrical tubes and that the internal capillaries were blocked by sorbed water plugs. It was hypothesized that the interior of the hollow cylinders became available to the adsorption of non-polar gases like argon only by activation which removed the water plugs. Thus the surface of the internal capillary structure is represented by the latter portion of the distribution curve for the activated sample. The surface of the capillaries must therefore exert a lower potential field to a non-polar molecule than the external surface of the fibers. This would imply a structural array for the interior of the capillaries different from that of the exterior of the fibrils. The high heat of wetting by water on the capillary surface1 and the low adsorption energies for argon suggest that the interior of the capillary walls may be composed of highly polar structural groups or atoms. The heats of wetting by water of unactivated 7-R asbestos with varying amounts of adsorbed water vapor on the surface already have been published.2 The heat of wetting is approximately a linear function of V,. Thus, the heat of wetting depends primarily on the amount of bare surface available and not on which portion of the surface is covered. The adsorption energy, UO- U/N,, of the surface is approximately constant. This is additional evidence of surface homogeneity. A slight concavity of the heat of wetting curve toward the axis should be expected if nearest neighbor interactions occur. Calculation shows that the amount of curvature to be expected is small and of the order of magnitude of the experimental error in determining the heats of wetting, That the heat of wetting data are reasonably consistent with adsorption results has been established.2 Acknowledgment.-The authors wish to express their appreciatip to the Armstrong Cork Company for financial support and helpful criticism f;om the research staff. We are also indebted to Professor A. C. Zettlemoyer for helpful discussions in the interpretation of our results.
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