15 Thermodynamics of Wetting of Solid Oxides
WILLIAM H. WADE and NORMAN H A C K E R M A N
Downloaded by MONASH UNIV on July 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch015
Department of Chemistry The University of Texas Austin, Tex.
This paper surveys the results of a long series of adsorption experiments designed to determine the reasons why experiments of this kind generally have not given reliable information in regard to the obviously r e l e vant surface thermodynamic parameters.
Few thermodynamic adsorption parameters are listed in the International Critical Tables, and rightly so. The literature commonly r e veals considerable disagreement between investigators for these p a rameters, even for surfaces which should be structurally simple. To understand better the sources of this disagreement, a series of investigations was begun in this laboratory some years ago. Several representative samples of metal oxides were to be used as the adsorbents and the adsorbates were to consist of simple molecular species such as water, alcohols, and hydrocarbons. The object was a broad survey of the thermodynamic adsorption parameters for the various combinations. The integral heats of adsorption and the adsorption isotherms were to be measured, and the integral entropies calculated therefrom. It was hoped to provide an inclusive picture of many of the variables which affect these adsorption parameters. T o cite a specific case showing that these studies were necessary, literature data for the heats of immersion of S i 0 in H 0 [1,2,4,18] show values ranging from 200 to 800 ergs per sq. c m . Since all of these investigators are reputable, a reasonable conclusion is that the surface properties of the a d sorbents used differed sufficiently to evoke this spectrum of values. It was decided to measure the integral heats of adsorption m i c r o calorimetrically, free energies of adsorption by a Gibbs integration of the adsorption isotherms, and integral entropies by numerical difference. Though the results of this survey may be incomplete, they have provided some insight into the specificity of physical adsorption processes. Several other investigators have been working along similar lines. Whalen has clearly shown that a variation of adsorption heats is to be found in the S i 0 - H 0 system [ l ô ] . In looking at the data available in the literature, the only correlation one finds in the variation of heats of 2
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In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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adsorption from sample to sample for a particular substrate material— e.g., S i 0 — is a trend showing decreased heats of adsorption with decreased particle size (increased specific surface area). Along with thermodynamic data acquired from various sources, other recent types of experimental measurement have done much to elucidate the chemical structure of the surface of inorganic metal oxides. The most conclusive measurements have been the infrared studies of these surfaces [3,8,17]. These studies show that surfaces of oxides exposed to atmospheric conditions differ chemically from that of bulk phase. In particular, the surface is covered with strongly bonded hydroxyl groups. Although these groups were thought to exist prior to the infrared measurements [7], these measurements offered the first direct proof of this fact. These OH groups would be expected to modify the surface of an inorganic metal oxide chemically, and promote phenomena such as hydrogen bonding. Downloaded by MONASH UNIV on July 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch015
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Experimental The samples of S i 0 , A 1 0 ,and T i 0 studied have been previously characterized [12,13,14] with regard to impurity and specific surface area. F o r purposes of uniformity, the surface areas of all the samples studied were measured by K r adsorption at 77°K. Gel samples were not included, in order to minimize hysteresis effects which would i n validate thermodynamic arguments. In general, the purity of the samples was 99.9%, some samples were less pure, and a few were even purer. The adsorbates—water, methanol, and hexane—were highly pure and, in the case of the organic adsorbents, freed from trace quantities of water by storing over Molecular Sieves. The calorimeter employed [9] is a twin differential semiadiabatic calorimeter with thermistor temperature-sensing elements. The thermistors are stable to the equivalent of ± 2° x 1 0 " ° C . over 30-minute periods. The temperature rise noted on sample breakage varied from 10~ ° to 1 0 " ° C . The reproducibility of immersional heats was approximately 2 to 3%. Adsorption isotherms were measured with volumetric adsorption apparatus, previously described [5,6] and the Gibbs integrations to give the integral free energies of adsorption at a r e l a tive pressure of 1.00 were done graphically. Heats of adsorption were obtained from the immersional heats by subtraction of the surface enthalpy of the appropriate liquid. 2
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Results and Discussion Chronologically the S i 0 - H 0 system was studied first. Some of the results of the original calorimetric studies for this system are shown in Figure 1. F i r s t , heats of adsorption decrease with increasing specific surface area and, secondly, differences in behavior due to outgassing temperature are obvious. Low-area quartz samples (1.28 sq. meters per gram) show an initial rise in heats of adsorption with i n creased outgassing temperature, followed by a gradual decrease. This behavior has been noted elsewhere [16]. Here it is attributed to the irreversible loss of surface hydroxyl groups at higher outgassing temperatures; hence to lessened opportunity for hydrogen bonding upon immersion in water. This phenomenon was even more clearly 2
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In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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Downloaded by MONASH UNIV on July 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch015
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Figure 1. Heats of immersion as a function of outgassing temperature for various quartz samples in water demonstrated by means of an adsorption isotherm on a low-area quartz sample accidentally heated at 450°C. The adsorptive capacity at r e l a tive pressures less than 0.5 was reduced by a factor of 4 until the sam ple was exposed to boiling water for a week. The variation of immersional heats of S i 0 in water with particle size seems at first glance to be a very anomalous effect. The only known difference between these samples is that the samples of very low area are crystalline quartz, whereas the samples of high area are un doubtedly amorphous. T o check the importance of the amorphous char acter of the surface, three experiments were performed. A sample of low-area quartz (0.07 sq. meter per gram) was ground extensively and samples of various specific surface areas were ob tained by sedimentation fractionation. The 8.12 sq. meters per gram sample of Figure 1 is one of the higher area samples generated. Once again, the higher the specific surface area the lower was the heat of adsorption. If it is assumed that grinding increases the thickness of amorphous layer, the correlation between this experiment and the earlier ones is good. A sample of quartz of relatively low surface area (0.91 sq. meter per gram) was heated to 1 5 0 0 ° C , converted to β-crystobolite, and quenched in this crystalline modification by rapid immersion in water. The resultant heats of immersion on this sample also shown in Figure 1 are interesting, in that the heats of immersion are higher than those of 2
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
Downloaded by MONASH UNIV on July 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch015
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Oxides
the parent material. In addition, a step in the vicinity of 300°C. indi cates that the rehydration of surface OH groups is rapid and monoenergetic. An ingot of fused quartz was ground to produce a spectrum of p a r ticle sizes which were separated by sedimentation fractionation. Heats of immersion were measured on this series of samples and found [11] to be independent of particle size. The indication is that the grinding process did not notably alter the initially amorphous surface. The heats of immersion of these amorphous samples averaged 370 ergs per sq. c m . , somewhat less than that obtained for "quartz" samples with a specific area of ~ 10 sq. meters per gram. This suggests that the s u r face of quartz samples of higher specific surface area would look com pletely amorphous to the adsorbate molecule. Adsorption isotherms were obtained for most of the samples shown in Figure 1 and the integral free energies of adsorption were calcu lated. The entropies of adsorption were obtained by difference. A l l these thermodynamic parameters are tabulated in Table I. If the amor phous character of the surface has a direct correlation with the parti cle size, the entropies of adsorption of the adsorbate molecules would be expected to bear some relation to the underlying periodic structure of the adsorbent. In particular, large entropies of adsorption would be expected for crystalline samples and relatively small entropies of adsorption for the supposedly amorphous substrates. That this r e l a tionship is observed is clear from the integral entropies of adsorption listed in Table I. Table I. Parameters for Water Adsorption Surface A r e a , Sq. M . / G . Si0
a
2
ΔΗ/
ΔΟ
Α
^s
A
0.070 0.138 0.910 8.12 188
729 547 389 334 44
106 128 227 215 41
2.09 1.41 0.54 0.40 0.01
2.72 65.2 104 221
575 326 322 236
202 171 206 149
1.25 0.52 0.39 0.29
7.65 10.5 188
608 469 231
298 248 179
1.04 0.71 0.17
A H and A G are in units of ergs per sq. c m . ; A S in units of e r g s / ° C . sq. c m . ; Δ Η = ΔΗ. - 118.3. A
A
A
Α
After obtaining the data on quartz it was felt that the observed phenomena should be established as other than a peculiarity of the quartz system alone. Therefore similar measurements on the A 1 0 water [13] and T i 0 - w a t e r [14] systems were carried out. The behav ior of immersional heats with outgassing temperature and particle size for these two substrates is shown in Figures 2 and 3. These systems 2
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In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
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