Adsorptive Behavior of Fused Quartz Powders W. H. WADE, H. D. COLE, D. E. MEYER, and NORMAN
HACKERMAN
Department of Chemistry, The University of Texas, Austin, Tex.
The integral heats and entropies of adsorption for water on fused quartz powder have been obtained
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as a function of particle size.
In the light of pre
vious studies, this is indicative of identical surface structure for all samples studied. The results fur ther indicate that many crystalline quartz powders are amorphous in their surface layers.
Q u r i n g the past few years there has been a continued and probably intensified interest in the adsorptive properties of quartz and amorphous silica. This is in a large part attributable to a more detailed knowledge of the surface structure of SiOo as obtained from infrared measurements (17, 18, 20). By using these techniques it has been possible to show unequivocally for the first time the exist ence of surface hydroxyl groups on silica and how their population density varies with outgassing pretreatment. On the basis of this information it has been pos sible to clarify the variation of immersional heats ( Δ ί ^ ) of quartz and silica in water with outgassing temperature. A number of studies (8, 21, 22, 27) for a wide variety of quartz and silica gel samples with different modes of production show that the ΔΗ/s increase with increasing outgassing temperature, pass through a maximum at 2 0 0 ° to 3 0 0 ° C , and then decrease at higher outgassing temperatures. This is usually interpreted as an initial loss of reversibly adsorbed water, followed by gradual loss of surface hydroxyl groups. This leaves a rather stable siloxane ring structure and provides only a diminished opportunity for hydrogen bonding during the subsequent im mersion process; consequently a diminished Δ//* is observed. Although the AH vs. outgassing temperature behavior is fairly well under stood, another aspect of the behavior of quartz and amorphous S i 0 appears to be somewhat anomalous— the remarkable variation of AHi with particle size. A survey of the older literature (1, 2, 4, 14, 15, 19, 28) showed a disparity in ΔΗ/s obtained at various laboratories. The only apparent correlation (21) was that the AH (per unit area) seemed to decrease with increasing area of the sample under investigation. Several studies initiated in this laboratory to check the validity of this observation (21, 22) show the effect to be real and that AHι can vary from 100 to 900 ergs per sq. cm., depending on the sample and the outgassing temperature chosen. The variation of AH with specific area of the sample is not restricted to the S i 0 - H 0 system but is prominent in the A 1 0 - H 0 (23), T i 0 H 0 (24), and to a lesser extent A l 0 - M e O H (25) and TiOo-MeOH (25) syst
2
i
{
2
2
2
2
2
3
2
8
35
Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
2
36
ADVANCES IN CHEMISTRY SERIES
terns. It thus appears to be a general phenomenon, except possibly for nonpolar absorbâtes as discussed later. For the S i 0 - H 0 system the possibility of impurity contents being the determining factor has been eliminated (21). It was also shown (11, 22) that the AH variation with specific area can be generated from an initially coarse quartz powder by extensive grinding and subsequent fractionation into a series of particle size samples. The various AH 's obtained from AH^s (see Equation 1) for S i 0 samples outgassed at 1 6 0 ° C. and 10~ mm. of H g are listed in Table I. 2
2
i
2
a
6
Table I.
Integral Heats of Adsorption for S i 0 Samples Outgassed at 160° C. 2
Surface Area, Sq. M./G.
Sample Quartz
AH , Ergs/Sq. Cm. a
0.070 0.138 0.541 0.910 8.12 162.0 188.0
Gristobalite Quartz Amorphous
729 546 531 388 334 —29 43
The adsorption isotherms for many of the samples in Table I have been measured (11), allowing the integral entropies of adsorption to be evaluated according to the prescription of Jura and Hill (16). It was found (11) that the entropy of the adsorbed species relative to that of liquid water decreases regularly with decreasing specific surface area. This would be expected for a gradual decrease in periodicity in the underlying structure and thus in the adsorbed H 0 . Recent independent measurements from two other laboratories (26, 27) also show the decrease of Δ ί ^ with increasing specific area. This number of inde pendent studies should confirm the validity of the effect, even in the field of surface chemistry. However, the explanation of this effect remains largely be clouded by the complexities of the system. Our hypotheses are that there is a direct correlation between particle size and the crystalline-amorphous character of the substrate surface. Specifically, quartz powders of low area and large particle size present a crystalline array at the interface, whereas powders of high area and small particle size are amorphous in the surface region. This certainly holds for many quartz powders prepared by grinding processes, which are known to introduce amorphicity in the surface region, and several studies (5, 6, 7, 9, 10, 12) show its existence. Moreover, S i 0 samples of very high area showed low Δ Η / S (21, 22) and since these were prepared by flame hydrolysis they are thus, undoubtedly, amorphous. Once the correlation between AH and the crystalline-amorphous surface character is accepted, there still remains the need to explain why the interaction energy of a water molecule with the surface Si+ , O , and hydroxyl groups should depend on the crystalline character. As was discussed above, experi mental results show that the AH decreases with decreasing surface O H concen tration for a given quartz or silica sample. The correlation between AH and crystalline-amorphous character could be by a diminution of surface O H concen tration for higher area samples. Unfortunately, there are no direct measurements of the extent of surface O H coverage as a function of particle size, since infrared measurements are restricted to samples of high specific area (>50 sq. meters per gram) which are probably completely amorphous. Surface O H concentration can be inferred from weight-loss measurements made on samples with surface areas as low as 1 sq. meter per gram. However, the 2
2
i
4
- 2
i
t
Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
WADE ET AL.
Fused Quartz Powders
37
results are ambiguous, since the physical adsorption energies for low area powders ( as mirrored in the ΔΗ/s ) as low coverages do not differ greatly from the hydroxyl group bond energies. Also, it is difficult to separate the weight loss vs. outgassing temperature curve into sections corresponding to the loss of physically adsorbed water and the loss of surface hydroxyl groups. This is illustrated by measure ments on quartz samples with surface areas of 1.28 (22) and 7.5 sq. meters per gram (26), where the nominal O H surface concentrations were 20 and 9 OH's per 100 sq. Α., as calculated from the weight loss from 1 1 0 ° to 4 5 0 ° C. The theoretical upper limit is 8 OH's per 100 sq. A. (15). Though definite information on O H coverage for low area quartz samples is lacking, it is nevertheless a reasonable guess that any variation in O H coverage cannot explain the major portion of the variation of ΔΗ^ with particle size for two reasons: First, many high area gels with AHi < 200 ergs per sq. cm. are com pletely covered with O H groups (15), and, second, the AH at its maximum for a given sample is at most 150 ergs per sq. cm. higher than at 4 5 0 ° C. where the O H concentration is low (22). Thus, differences of as much of 800 ergs per sq. cm. between low area quartz and high area silica must be explained in another way. It has been suggested (22) that, qualitatively, the difference can be ex plained by a detailed consideration of the difference in binding forces of a water molecule in the force field of two limiting types of ionic lattices: (1) The water dipole interacts (vertical to the surface) with alternating layers of O ions ( O H groups in the outer layer) and Si+ ions, and (2) the water dipole interacts with layers each of which consists of random Si+ and O ions with the only ordered structure being the S i O tetrahedra. The latter structure in its bulk is the usual model for fused quartz, whereas the former model is that for several crystal planes of crystalline quartz. The summation of all interaction energies of a water dipole with lattice ions in alternating positive and negative sheets is greater than the summation where positive and negative ions alternate within a given layer (3). In essence the amorphous structure more resembles a covalent structure such as graphite, for which the interaction energies with water are very low (28). Various aspects of the preceding proposal have recently been discussed (27). A pertinent analogy can be drawn to calculated electrostatic field strengths (3) at the surface of NaCl lattices. The field strength (which is a direct measure of interaction energy for polar molecules) over a Cl~ ion in a 110 plane surface is three times that over a Cl~ ion in a 100 plane surface. For the reason cited above for a dipole adsorbed on a Cl~ ion in the 110 plane, there is an excess of Cl~ ions in the immediate vicinity and thus less cancellation of ion-dipole contributions to the total interaction energy. i
- 2
4
4
2
- 4
To check on the dependence of AH on crystallinity, a cristobalite sample was prepared at temperatures high enough to cause a phase transformation (22), and it was assumed that at this temperature any amorphous surface layer would also crystallize. As predicted, the ΔΗ/s increased (approximately 200 ergs per sq. cm.) above the original quartz values. Difficulties of interpretation were not completely obviated, since the surface areas differed by a factor of 2. Also, the ΔΗ/s were for two different crystalline modifications and thus would not be expected to display exactly the same interaction energies with water. The present study was initiated to eliminate the difficulties inherent in the previous experiments. It consists of measurements of the integral enthalpies (via immersional heat measurements) and entropies (via adsorption isotherms) for ground and fractionated powders of fused quartz with a wide spectrum of specific surface areas. i
Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
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ADVANCES I N CHEMISTRY SERIES
The commonly accepted structure of fused quartz is one of randomly bonded SiO~ tetrahedra. Relying on the high bond strength within these tetrahedra, grinding of a coarse fused quartz should have little effect on its surface structure. Therefore, it is possible to predict that All and AS for fused quartz should be independent of particle size. 4
(l
a
Experimental The fused quartz was obtained as a single billet from The General Electric Co., Willoughby Quartz Plant, and had a stated purity of 99.97%. The original billet was crushed to a particle size of approximately 100 microns in an iron jaw crusher, washed with HC1, and then ground under water in an agate mortar for approximately 5 days. The resulting material was fractionated by sedimentation in water to give the series of nine samples listed in Table II. The surface areas were obtained by the standard B E T analyses of Kr adsorption isotherms taken over a relative pressure range of 0.05 to 0.30 as previously described (22). The heats of immersion were measured at 2 5 . 0 ° C . by techniques already described (21, 22). Samples were run in duplicate, and these agreed to 2 to 3%. Pretreatment consisted of outgassing at 1 6 0 ° C. and 1 0 mm. of H g for 4 days. e
Table II.
Adsorption Parameters for Fused Quartz Samples of Various Specific Surface Areas AS , Ergs/Sq. Cm.° C.
ω, Sq. A.
128
0.82
17.1
143 159 126
0.74 0.77 0.84
19.9 14.6 19.5
163
0.62
14.7
a
Sample A Β G D Ε F G H I
Surface Area, Sq. M./G. 0.054 0.297 0.562 1.04 1.92 3.62 7.47 13.58 25.55
AH , Ergs/Sq. Cm. a
7Γ,
Ergs/Sq. Cm.
372 393 362 391 378 379 345 359 347
The adsorption isotherms for water with five of the samples were obtained at 2 5 ° C . by using a volumetric adsorption apparatus described previously Prior to adsorption measurements these samples were also outgassed at 1 6 0 ° and 1 0 mm. Hg. Each adsorption isotherm consisted of 20 to 40 experimental points covering the relative pressure range 0.01 to 0.95. 6
Results and Discussion The integral heats of adsorption in Tables I and II were calculated from the heats of immersion by AH
a
= AHi -
H8.5
(1)
where 118.5 is the surface enthalpy of water. Both AH and AH are heats per unit surface area and are referred to the standard states as liquid water. The free energies of adsorption were obtained from the adsorption isotherms by graphical integration of the Gibbs equation a
χ = RT
C
t
Vd\n P/P°
(2)
JO
where V is the volume of gas adsorbed per unit area, P/P° is the relative pressure, Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
WADE ET AL.
Fused Quartz Powders
39
and the other symbols have their usual significance. adsorption per unit area were obtained from AS
=
a
AH
The integral entropies of
-
a
(3)
which is the Jura and Hill equation (16) modified to unit area for the problem at hand. Table II lists ΔΗ , ττ, AS , and , which is the area per water molecule obtained from B E T analyses of the adsorption isotherms. As is common practice in surface studies, ΔΗ , ττ, and AS„ are opposite in sign when compared with standard thermodynamic conventions. The AH 's in Table II show no definite trend and have an average value of 370 ergs per sq. cm. with an average deviation of ± 1 4 ergs per sq. cm. or approximately ±4 /c This uncertainty would be ex pected from a combination of uncertainties in measurements of Δί/j and of surface area. The adsorption isotherms are shown in Figure 1. There are some differences in shape, although they are all examples of the classic Type II adsorption isotherm. fl
η
ω
α
a
r
Adsorption Isotherm for Fused Quartz Samples Θ • Ο Δ •
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Figure 1.
Adsorption isotherm
Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
10
40
ADVANCES I N CHEMISTRY SERIES
The integrated free energies of adsorption are listed in Table II. There is no definite trend in these either. The average of all five π values is 140 ergs per sq. cm., with an average deviation of ± 1 4 ergs per sq. cm. or ± 1 0 % . The dis agreement in ττ is somewhat greater than in Δ Η . The entropy values being ob tained from both ΔΗ (or ΔΗ^) and ττ show their corresponding uncertainties. Considering the limitations of the experiment, the Δ Η , ττ, and AS values are independent of particle size over a range in which both the surface areas and AH 's of crystalline quartz samples varied by a factor of 2 (see Table I). In comparing Tables I and II, it is observed that the heats of adsorption of the fused quartz are about the same as for the 8 sq. meters per gram "crystalline" quartz and much higher than those for high area amorphous silicas. The entropies of adsorption for the fused quartz samples are between those for 1 and 8 sq. meters per gram crystalline quartz. This indicates that the surface structure of fused quartz and a crystalline quartz of moderate surface area (5 to 10 sq. meters per gram) are probably very similar. In addition, the surfaces of very high area amorphous gels and flame-hydrolyzed silicas must be basically different from fused quartz, since the ΔΗ/s differ so drastically. An additional bit of information obtained from the experiment indicated that the fused quartz surface is extensively populated with surface O H groups. Sample A (Table II) was inadvertently heated to 4 0 0 ° C. prior to adsorption measure ments. The resulting ?r value was 69 ergs per sq. cm. On repeated soaking in boiling water (2 weeks total) ττ gradually rose to a final value of 128 ergs per sq. cm. This is consistent with the relatively irreversible loss of surface OH's at 4 0 0 ° C. Also the extent of initial decrease of ?r indicates an abundant coverage prior to heating to 4 0 0 ° C. The average entropy decrease on adsorption of approximately 0.75 erg per sq. cm. when converted to a molar basis from the volume adsorbed at saturation vapor pressure is 1.8 ± 0.4 e.u. less than liquid water at 2 5 ° C . or about that of liquid water at 0 ° C. The large uncertainty is due to difficulty in extrapolating to P/P° = 1. Since the entropy of adsorbed water molecules in the outermost layer must approximate that of liquid water at 2 5 ° C , then the inner layers, to maintain the average of 1.8 e.u., must be icelike with loss of translational modes. This investigation has offered no firsthand information with regard to the exact mechanism of the crystalline to amorphous conversion in substrates, but it completely supports the previously advanced hypotheses. There yet remains a need for an explanation of the variation of ΔΗ^ with particle size of A 1 0 and T i 0 (23-25). These samples were exposed to no distortional forces of the type dis cussed above following their crystallization at elevated temperatures. {
α
α
a
fl
2
3
2
Conclusions The surface structure of fused quartz powders obtained by grinding is in dependent of specific surface area. The surface structure is similar to a medium area crystalline quartz but differs from that of gels, nonporous silicas of very high area, and coarse crystalline quartz powders. Acknowledgment The authors thank the American Petroleum Institute for continued interest and support of this research under API Project 47D, Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
WADE ET AL.
Fused Quartz Powders
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Literature Cited (1) Armstrong, E. J., Bell System Tech. J. 25, 136 (1956). (2) Bartell, F. E., Suggitt, R. M., J. Phys. Chem. 58, 36 (1954). (3) Boer, J. H. de, Advances in Colloid Sci. 3, 1 (1950). (4) Boyd, G. E., Harkins, W. D., J. Am. Chem. Soc. 64, 1190, 1195 (1942). (5) Clelland, D. W., Ritchie, P. D., J. Appl. Chem. 2, 31 (1952). (6) Ibid., p. 42. (7) Dempsted, P. B., Ritchie, P. D., Ibid., 3, 182 (1953). (8) Egorov, M. M., Krasil'nikov, K. G., Sysoev, Ε. Α., Doklady Akad. Nauk, S.S.S.R. 108, 103 (1956). (9) D'Eustachio, D., Phys. Rev. 70, 522 (1946). (10) D'Eustachio, D., Greenwald, S., Ibid., 69, 532 (1946). (11) Every, R. L., Wade, W. H., Hackerman, N., J. Phys. Chem. 65, 25 (1961). (12) Gibb, J. G., Ritchie, P. D., Sharpe, J. W., J Appl. Chem. 3, 213 (1953). (13) Hackerman, N., Hall, A. C., J. Phys. Chem. 62, 1212 (1958). (14) Howard, F. L., Culbertson, J. L., J. Am. Chem. Soc. 72, 1185 (1950). (15) Iler, R. K., "Colloid Chemistry of Silica and Silicates," pp. 240-2, Cornell Univ. Press, Ithaca, Ν. Y., 1955. (16) Jura, G., Hill, T. L.,J.Am. Chem. Soc. 74, 1598 (1952). (17) Kiselev, Α. V., Lygin, V. I., Kolloid. Zhur. 21, 581 (1959). (18) Kiselev, Α. V., Lygin, V. I., "Proceedings of Second International Congress on Sur face Activity," Vol. II, p. 204, Butterworths Scientific Publications, London, 1957. (19) Kiselev, Α. V., Mikos, N. N., Romanchuck, Μ. Α., Shcherbakova, K. D., Zur Fiz. Khim 21, 1223 (1947). (20) McDonald, R. S., J. Am. Chem. Soc. 79, 850 (1957); J. Phys. Chem. 61, 1168 (1958). (21) Makrides, A. C., Hackerman, N., Ibid., 63, 594 (1959). (22) Wade, W. H., Every, R. L., Hackerman, N., Ibid., 64, 355 (1960). (23) Wade, W. H., Hackerman, N., Ibid., 64, 1196 (1960). (24) Wade, W. H., Hackerman, N., National Colloid Symposium Meeting, 1961. (25) Wade, W. H., Hackerman, N., unpublished data. (26) Whalen, J. W., private communication. (27) Young, G. J., Bursh, T. P., J. Colloid Sci. 15, 361 (1960). (28) Zettlemoyer, A. C., Young, G. J., Chessick, J. J., Healey, F. H., J. Phys. Chem. 57, 649 (1953). RECEIVED May 9,
1961.
Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
