INTERACTION OF WATERAND VARIOUS SILICAPOWDERS
relationship expressed by eq 13 is not generally true. We have confirmed this conclusion from our own measurements. Thus for protoacetic acid in water and in deuterium oxide at 25",ApK = 0.556,but for the second stage in the dissociation of phosphoric acid, ApK =
0.580. If, however, HA1 and HA2 are acids of nearly equal strength, we would expect eq 13 to be closely valid. When AI is CHICOO and A2 is CDaCOO, for example, it might be anticipated that eq 13 would be a very good approximation, though not necessarily exactly true. We have shown in an earlier paper3 that ApK
2077
-
(=pK in DzO pK in HzO) for protoacetic acid ranges from 0.577at 5" to 0.537at 50". It is now found (Table 11) that ApK for deuterioacetic acid in these two solvents ranges from 0.576 at 5" to 0.538 at 50". Although eq 13 holds to a very good approximation in this instance, there is no apparent reason to expect it to be exactly valid. Acknowledgment. The authors acknowledge gratefully the assistance of E. E. Hughes, who performed the mass spectrometric and gas chromatographic analyses, and of Dr. R. T. Leslie, who purified the deuterioacetic acid.
Adsorption Thermodynamics of the Interaction of Water and Various Silica Powders
by Donald E. Meyer and Norman Hackerman Department of Chemistry, The University of T e w s , Austin, T e w s (Received February 8, 1966)
-
Free energies, heats and integral entropies, and enthalpies of adsorption are presented for powdered fused silica (0.056to 13.58m2/g) and powdered crystalline silica (0.11to 5.65 mz/g). A volumetric adsorption system and a microcalorimeter were used. Both crystalline and amorphous silica were mechanically ground and separated by water sedimentation into several particle-size distributions. In addition, one sample of the ground crystalline silica was etched with varying amounts of dilute H F to yield a particle-size distribution. The effect of grinding and etching on the thermodynamic values of adsorption are reported, and the results are discussed on the basis of surface structure related to particle size and etching. Electron micrographs are also presented.
Introduction Many investigators have considered the interaction of water with silica surfaces. Collectively, precision immersion calorimetry,l-3 adsorption st~dies,4$and infrared studies6'7 have indicated that the energetics of water-silica interactions depend upon the presence and density of surface hydroxyl groups. Additional experimental evidence indicated that these surface OH groups participate in still another manner. Immersional heatss-I0 and adsorption free energies'O
normalized to unit surface area were found to vary with particle size for silica powders. Of the explana(1) A. C. Makrides and N. Hackerman, J . Phy3. Chem., 63, 594 (1959). (2) J. W. Whalen, Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961,p 281. (3) M. M. Egorov, V. G. Krasilnikov, and E. A. Sysoev, Dokl, Akad. Nauk SSSR, 108, 103 (1956). (4) N. Hackerman and A. C. Hall, J . Phys. C h m . , 62, 1212 (1958). (5) M.M.Egorov, V. F. Kiselev, and K. G. Krasil'nikov, Rues. J . mys.ma., 33, NO.io,323 (1959).
Volume 70, Number 7 July 1966
2078
DONALD E. MEYERAND NORMAN HACKERMAN
tions for this particle size effect, the most generally Electron micrographs of particle surfaces were obtained accepted are based on evidence that structural order from preshadowed carbon replicas. A suspension of the powder in deionized water was spread over a glass in the surface region of ground quartz powders decreases with decreasing particle size. microscope slide and allowed to dry. The particleCrushing and grinding quartz crystals produces a covered slide was placed in a vacuum evaporator and less crystalline,11J2 more easily s o l ~ b l esurface ~ ~ ~ ~ ~shadowed a t 18" with a 80% Pt-20% Pd alloy. The region. Continued grinding produces smaller particles slide was then coated with carbon a t normal incidence. with a disturbed region descending deeper into the The evaporated film was cut into 1/8-in. squares bulk. One explanation suggests that a less ordered and floated free of the slide by immersion in HF. (more amorphous) surface has a lower density of surThe films were then washed in water and mounted on face OH groups and thus exhibits a lower heat of wetting 200-mesh electron microscope grids. per unit area. The other explanation pictures the Materials. A billet of fused quartz, 99.97% pure, Si4+02- structure and the SiOH3+grouping producing was obtained from General Electric Co., Willoughby a surface electrostatic force field which interacts with Quartz Plant. Particles in the 100- to 300-p range the sorbate molecules. A more ordered Si02 surface were produced using an iron jaw crusher and were structure would present a more ordered array of treated with HC1 to remove traces of iron. The fused surface OH groups resulting in a stronger force field quartz was further ground for 5 days in a water slurry and greater interaction energies with water. using a motor driven agate mortar and pestle. The The object of this investigation was to obtain a ground material was fractioned into particle size dissystematic and detailed correlation between different tributions by water sedimentation ranging from 5 min surface structures (determined by type of silica, to 48 hr and then dried a t 110". mechanical grinding, and chemical etching) and Ground 99.9% pure crystalline silica was supplied by surface thermodynamics. Charles A. Wagner Co. of Philadelphia. The powder was washed in HCl and several times in distilled water Experimental Section and then stored in distilled water for 9 months. As a Equipment. Specific surface areas ( 2 ) were degiven amount was needed, it was taken from the bulk, termined by the BET method using either krypton, fractioned into particle-size ranges, and air-dried a t argon, or nitrogen. On samples where two or more 110". of the gases were used, results were in good agreement. A solution of 3% by weight H F in water was used to Excess surface free energies of adsorption (two-dimenetch the ground crystalline silica. The extent of etching sional spreading pressures, T ) were obtained from water was controlled by the ratio of H F to silica. After allowing the powder to settle and decanting the liquid, adsorption isotherm data a t 25 f 0.01" and 15 f 0.02". H F was removed by NaOH washes of decreasing conThe volumetric adsorption apparatus was similar to centrations down to 0.001 N . This method of rethe system previously reportedlo except for two major modifications. Tandem bulb trains, allowing a selecmoval is similar to that reported by van Lier, et a1.13 Extensive washings in boiling and cold distilled water tion of 35 volume combinations, were used which decreased the pressure surge when volume changes were used to minimize the H F and KaOH contaminawere made. A micrometer-U-tube assembly15 was tion. found to be superior to cathetometric methods for measuring pressure changes. The adsorption system (6) R. S. McDonald, J . Am. Chem. Soc., 7 9 , 850 (1957); J . Phys. could be evacuated to torr provided liquid nitroChem., 6 2 , 1169 (1958). gen trapping was used. All samples were outgassed ( 7 ) A. V. Kiselev and V. I. Lygin, Kolloidn. Zh., 21, 581 (1959). a t 150 f 5" for 48 hr a t pressures below torr. (8) G . J. Young and T. P. Brush, J . CoEEoid Sci., 15, 361 (1960). Heats of immersion (AHi) for certain of the SiOz (9) W. H. Wade, R. L. Every, and N. Hackerman, J . Phys. Chem., 6 4 , 355 (1960). samples were determined using the microcalorimeter (10) R. L. Every, W. H. Wade, and N. Hackerman, ibid., 65, 25 and general procedures described previously. lo All (1961). measurements were at 25 f 0.1", and the samples were (11) D. D'Eustachio and S.Greenwalk, Phys. Rev., 6 9 , 532 (1946); torr. outgassed at 150 f 5" for 48 hr a t 70, 522 (1946). Using electron diffraction and electron microscopy, (12) D. W. Clelland and P. D. Ritchie, J . A p p l . CPem.,2, 42 (1952). (13) J. A. van Lier, P. L. de Bruyn, and J. Th. G. Overbeek, J . Phys. particle surface structures were obtained for many of Chem., 64, 1675 (1960). the samples. The samples were pressed into a 200-mesh (14) 1. Bergman and M. S.Paterson, J . A p p l . Chem., 11, 369 (1961). screen, and diffraction patterns were obtained by trans(15) D. E. Meyer and W. H. Wade, Reu. Sei. Instr., 33, 1283 mission a t 50 kv (through edges of larger particles). (1962). The Journal of Physical Chemistry
INTERACTION OF WATERAND VARIOUS SILICAPOWDERS
2079
Calculations
Results
Each isotherm was composed of from 30 to 50 points, usually half within the first monolayer. The free energy of adsorption ( T ) was calculated from a modification of the Gibbs equation by Bangham
Table I gives a comprehensive report of all the silica samples studied. Specific surface areas were averages of several runs usually differing less than 1%. Values of the heat of adsorption (AH,) were averages of three or more measurements. All adsorption isotherms were duplicated a t least twice and were found to be reproducible at relative pressures below 0.6. Small variances in isotherms of a given sample a t higher relative pressures caused differences of less than 2% in calculated values of X .
where y s and SA are the surface tensions of a vacuumbaked surface and a surface following adsorption. Also, V = molar volume of the gas, f~ = area for 1 g of solid, v = volume of sorbed gas per square meter of surface, and p = equilibrium pressure of the sorbing gas. Graphical integration of eq 1on the basis of Simpson's trapezoidal using a computer gave the integral free energy of adsorption as well as B as a function of coverage (e). By applying Simpson's rule to isotherm data at 25 and 15', the integral (constant free energy) enthalpies [(Ho - Hs),] and entropies [(SG- Ss),] of adsorption were obtained using the equations of Hi11I6
( b In p / b T ) , = (SG - Ss)/RT
(2)
and
( b In p / b T ) , = (HG - Hs)/RT
(3)
where G corresponds to the gaseous state and S the sorbed state. These thermodynamic values were calculated by computer and expressed as a function of coverage (e). Absolute entropies (8s) were evaluated using water vapor at 25' and 1 atm as the standard state. Heats of immersion (AHi) yielded adsorption heats (AH,) according to
AHa = AHc
- 118.5 (ergs/cmz)
(4)
where 118.5 is the surface enthalpy of water. Heats of adsorption and corresponding values of T gave entropies of adsorption (ASS) from
Table I Sample designation, description
m2/g
Fused silica Sample A Sample B Sample C Sample CLT* Sample D Sample E Sample ELT
0.056 0.603 1.81 1.81 7.46 13.58 13.58
Crystalline silica Sample F Sample G Sample GLT Sample H Sample HLT Sample I Sample J Sample JLT
0.112 0.27 0.27 1.34 1.34 3.2 5.65 5.85
Etched silica (crystalline) Sample K 0.136 Sample M 0.164 Sample h h 0.164 0.364 Sample N Sample NLT 0.364 Sample 0 1.76 Sample P 0.185 0.194 Sample Q Sample R 1.57 Sample S 1.70 a
a modification of the Hill-Jura equation." Values of X , AH,, and AS, for only the amorphous samples were reported previously.1s Also, surface areas ( 2 ) used to calculate T values in this reference were in slight error. That there was no effect of particle size on x as reportedl8 for ground amorphous silica is in fact an error. The present work gives corrected x values and therefore corrected AS. values (eq 5 ) .
2,
See ref 18.
AHS,
*,
ergs/ cm2
ergs/ cmp
372" 362" 378" 345" 359"
290
385 300 175 120
* LT corresponds
139 143 151 155 176 202 217 48.6 113.5 121.1 122.4 127 127 122.5 127.4 135.5 105.1 110 86.8 99.3 73.8 101
98 75 72
0.78 0.65 0.84
17.6 14.3 18.4
0.57 0.53
14.4 11.6
0.65
48 20.7 20.6 18.5 18.0 14.6 20.6 20.4 15.9 22.4 19.7 24.8 24.4 31.2
0.95 0.67 0.33 0.16
t o 15" isotherm.
Absolute isotherms (amount adsorbed normalized to unit area) for various particle size distributions for the three types of silica studied are presented in Figures (16) T.L. Hill, A d v ~ n Catalysis, . 4,242 (1952). (17) G.Jura and T. L. Hill, J . Am. Chem. SOC.,74, 1598 (1952). (18) W. H. Wade, H. D. Cole, D. E. Meyer, and N. Hackerman, ref 2, p 35.
Volume 70, Number 7 July 1966
DONALD E. MEYERAND NORMAN HACKERMAN
2080
Pi
1.6
0
-qo.0 -'**
'*O
ETCHED CRYSTALLINE A SAMPLE K 0.136 m% o SAMPLE M -0,164 m2/g OSAMPLE N-0.364m2/S
-
A
SILICA
U 0
*SAMPLE 0-1.76m4g
A
UNETCHED
.SAMPLE
G-O.ZT~%
(SOURCE OF ETCHED SAMPLES)
U
hoe
N
0
fJ+
t;;c 0
0
-
g0.6 Y
(u
I
A
-
3
0.4
d
-
A
A FUSED [SAMPLE E) D
A
ETCHED CRYSTALLINE (SAMPLE K)
0
0
O.
a
D
h
o
b
' * *
t
0
+
0 CRYSTALLINE ISAMPLE DI
O 0
0.2
0.1
0.3 d
P/P*
Figure 1. BET plots for three types of silica studied: 3.7-m2/g crystalline sample I, 0.136-ma/g etched sample K, 13.5&mZ/g fused sample E ( p = equilibrium pressure, po = saturation pressure, and u = volume of gas adsorbed (cc a t STP)).
P/ P"
Figure 3. Absolute adsorption isotherms for several samples of etched crystalline silica and the parent sample, unetched sample G.
180 170 160 150 140 -
200
190
SAMPLE E- 1358m2/g A SAMPLE C- 1.81 m2/ SAMPLE A 0.056 me/g CRISTALLINE SILICA 0 SAMPLE 0 0.27 mqg +SAMPLE I 3.2 m2/g A SAMPLE J 5.65m2/a
-
--
3
A e
FUSED SILICAS
/
CRYSTALLINE SILICAS
ETCHED CRYST4LLINE
E 130-
c
5
E
120-
Is
110-
100
-
70 -
90 80
P/PO
'
'
I I .o r t r r 10.0 t' SURFACE AREA ( ~ M * / G
'
Figure 2. Absolute adsorption isotherms for fused and crystalline silicas of various specific surface areas.
Figure 4. The free energy of adsorption (7)as a function of specific surface area for the three types of silicas studied.
1 and 2. The vertical displacement of any given isotherm is seen to depend upon the specific surface area (2) of the sample and the type of silica. In general, the relative vertical position of the isotherm determines T and w . In determining the excess surface free energy using the Gibbs equation, extrapolations were made
from relative pressures ( p / p o ) of 0.005 to 0 and above 0.96 to 1.0. The BET equation was also applied to the water adsorption data. Figure 3 gives representative linear BET plots for the three types of silica studied. Over the applicable pressure ranges, the water-silica adsorption data produced a straight-line
The Journal of Phgsical Chemistry
INTERACTION OF WATERAND VARIOUS SILICA POWDERS
2081
ETCH RATIO
Figure 5. The change in specific surface area of sample G as a function of the etch ratio (volume of 3% IIF tn weight of SOX; sample 1 arbitrarily taken as unity). Incressing sample number correqponds to increased etching. Sample 2 is identical with sample K, Table I.
.t
.(
.
Figure G . ' h i i s m i s s i o n clectroii-dilir:iclii,ir pattern for crystalline silica before etching.
fit from which values for the effective area of an adsorbed water molecule ( w ) were obtained. I n Figure 4 the dependence of T on Z is demonstrated for all three types of silica samples. For the ground crystalline and particularly for the amorphous samples, T increased with 2. I n contrast, for the etched samples r decreased with increasing 2 . I n Figure 4, squares numbered 1 through 4 correspond to samples from increased etching of crystalline sample G . The general effect of etching on 2 is shown in Figure 5. There is a rapid initial drop in 2. This corresponds to the initial rise in T going from source sample (circle 2) to first etched sample (square 1) in Figure 4. Thereafter, Z rises with increased etching and T drops. Time was
not a factor in controlling etching, but the volume of 3% H F to weight of sample was. The ratio used to obtain the first sample (circle 1, Figure 5) was assigned a value of unity. Both grinding and etching changed not only the surface structure but the particle-size distribution as well. Electron-diffraction patterns clearly indicated the presence of many crystallites on the ground crystalline particles before etching. The number of crystallites also appeared to vary with particle size. A diffraction pattern for sample G is shown in Figure 6. The dotted rings of the diffraction pattern apparently originates from the submicron particles on the surface of the larger particles (Figure 7). After slight etching, these particles practically disappear (Figure 8) and it Volrrme 70, Number 7 Julv 1966
DONALD E. MEYERAND NORMAN HACKERMAN
2082
. .
Figure 8s. Electron micrograph (IR,OlIOX ) ,,I m etched crystalline particle (sample 11).
Figure 9. I:leclron micrograph (I:~,llllllX)of surface of a h r g e amorphous silien pnrticle.
I
Figure Sb. Elect.ron micrograph C2:3,lllNlX) id c r p t a l l i w parbiele more extensively etched (snmple 9).
was impossible to obtain this crystallite diffraction pattern. Ground amorphous samples also showed submicron particles attached to the surface of larger particles (Figure 9); however, no diffraction patterns were observable. Electron diffraction studies gave no indication of devitrification as a result of prolonged grinding. Close observation of Figure 9, particularly the right side, reveals surface irregularities other than attached particles. This micrograph shows a particle (sample F, 0.11 m*/g) greater than 50 fi in size. A representative micrograph of particles of sample A, 13.58 m2/g, is presented in Figure 10a and a closeup of an individual particle in Figure lob. The Journal OJ Phyaical Chemialrv
Using adsorption data from isotherms at 25 and 15", adsorption thermodynamic values were obtained as a function of coverage (e). As defined by Hill," the integral heats (HG - Hs),,integral entropies (SG &), and the absolute entropies are presented as a function of coverage in Figures 11 and 12. All three types of silica studied are represented, and there are significant variations related to particle size and type of silica at lower coverages. Even at higher coverages, several layers, there appears to he a particle size effect as the integral heats approach the heat of condensation ingoing from gas to liquid (Figure 11). The absolute entropy of the sorbed water on various silica samples as a function of coverage is presented in Figure 12. At lower coverages the order of condensed
2083
INTERACTION OF WATERA N D VARIOUS SILICAPOWDERS
FUSED SILICA SAMPLE E-l3.58m% CRYSTALLINE UNETCHED OSAMPLE G-0.27m2/g A SAMPLE J - 3.65m2/g SAMPLE n- 1.3.?& CRYSTALLINE ETCHED m SAMPLE M- 0.164m2fg SAMPLE N 0.36m2fg
-
+
'*L
A
0
I
2
3
4
5
e Figure 12. Absolute entropy of adsorption BF a function of coverage. Lower dashed line corresponds to solid KO, upper dashed line to liquid H20.
Discussion It has been shown that heats of immersion and free energies and entropies of adsorption for the watersilica powder system are dependent upon the specific surface area. One explanation suggests a correlation between particle size and crystalline surface order and between surface order and an electrostatic force field 0 : I' ' e' 3 ' ' 4' ' 5' causing a greater polarization of surface hydroxyl e groups. As a result, a ground crystalline silica sample Figure 11. Integral heats of absorption as 8 function of small specific surface area (2) and large particle of coverage for silica samples of diRering histories and size would possess a greater normalized energy of adspecific surface area-. Ihshed line eorre3ponds to sorption for water vapor. As this powder is ground, the heat of condensstion of water (HG, enthalpy in gaseous state; Hs, enthalpy in sorbed state). 2 increases, the heat of adsorption decreases, and the excess surface free energy increases. Amorphous Silica. Amorphous silica ground into water on the largest crystalline particle sample (C) powder should not vary in crystallinity with particle and an etched sample (N) was quite high. At higher size, and the normalized heats of immersion or free coverages, Saapproached a value closer to that for the energies of adsorption should remain constant with solid state than for the liquid state. For calculations changing 2 . As expected, AHa was practically indeof Ss, So was expressed in tcrms of S"tss.,~ = 45.106 pendent of 2 (Table I), which supports the particle eu for water vapor a t 1atm. size effect-crystallinity postulate. However, Figure
'
Volume 70. Number 7 Julv 1966
2084
DONALD
4 shows a to be dependent on 2. It would seem that a is not influenced by the electrostatic force field and the polarity of the surface hydroxyl to the extent AH, is. The change in surface free energy for a silica particle interacting with water vapor appears to depend on the size of the particle. For 2 of 0.056 to 13.58 mz/g, a varied from 139 to 202 ergs/cm2 a t 25". Since average particle size varied from approximately 50 to 1 p, this apparent surface effect cannot be explained as directly as surface effects of particles 0.01 p or less. Several factors should be considered. Electron micrographs, Figures 7-10, showed submicron particles attached to the surfaces of larger particles. These smaller particles, together with other surface irregularities resulting from grinding damage, could affect the surface tension both before (rs)and after (ysa)adsorption and thus i? = -
(7s
-
YWJ
The surface energy or surface tension is reportedly related directly to the tensile strength.lg Though it is somewhat more difficult to envision tensile strengths of various sized silica particles compared to various sized silica fibers or rods, there is apparently a difference in the change in tensile strength with adsorption on a larger particle (50 p) compared to a smaller particle (1 p). Stranski20found the tensile strength of ground rock salt to be a function of particle size, increasing as the dimensions decreased. The tensile strength and, therefore, the surface energy depend upon the presence of surface cracks and fissures. Grinding is an important factor in affecting the tensile strength, and larger particles apparently have more surface damage of a nature affecting the tensile strength. Large particles would preferentially split into smaller particles along surface cracks and fissures. Crystalline Silica. Everylo reported a particle size effect for both T and AH, using the same crystalline powder from Charles A. Wagner Co. used in this work. No immersional heats were obtained on crystalline samples reported here, but the crystalline a values (Table I) increased with decreasing particle size just as Every observed. The change in a with 2 differed for amorphous and crystalline silica, Figure 4, in an inverse fashion. For crystalline samples the greatest change occurred a t low Z and for amorphous samples a t high Z. There is an apparent correlation between these differences of a with Z and the population of attached particles which decreased as the parent particles grew smaller (increased grinding, larger 2 ) . For crystalline powder, The Journal of Physical Chemistry
E. MEYERAND NORMAN HACKERMAN
electron micrographs showed that the greatest change in population took place between sample F (Figure 7a) and sample G (Figure 7b). This corresponds to the greatest change in a. Likewise for amorphous powder, there was a greater change in particle population between sample C (1.81 m2/g) and sample E (13.58 m2/g) than between sample C and sample A (0.056 m2/g). This was also true for the change in a for samples C, E, and A which tends to relate changes in a to these attached particles and thus suggesting their effect on the tensile strength and surface energy of the particles before and after adsorption of water vapor. Cracks in the surface and smaller particles aggregated together on the surface of larger particles have been reported to effect adsorption properties of silica. Zhdanov21 suggested fissures in quartz produce an excess surface area available to an H20 molecule but not to the larger Nz molecule. Normalizing water isotherms for different samples with Ne BET 2 values would lead to error. Egorov, et ~ l .found , ~ that grinding silica in air produced aggregations of particles which grinding in water does not. Again, crevices between particles would decrease the adsorptive capacity of Nz relative to HzO. Kiselev, et a1.,22 provided further evidence supporting the theory that dry grinding results in particle aggregation and low Nz specific surface areas. They pointed out that cracks or fissures, being the most probable place for cracking, should decrease with grinding. Thus particle aggregation rather than microfissures21are responsible for the difference in areas "seen" by adsorbing Nz compared to HzO. Several observations do not agree with the explanations given above. The amorphous samples were ground in a water slurry yet showed aggregation. Also, AH, was constant with 2 , Table I, while a varied (Figure 4). If a difference existed between the total surface seen by the Nz molecule (used to normalize AH, and a) and between the total surface seen by the HzO molecule, then AH, should vary as did K. For the crystalline samples AH,'O decreased with increasing 2 while a increased. Particle aggregation must play some other role. StranskiZ0observed that when ground rock salt was placed in a saturated solution small crystals formed on the surface of the larger particles accompanied by a lowering of their tensile strength. After the crystal(19) W. D. Harkins, "The Physical Chemistry of Surface Films," Reinhold Publishing Corp., New York, N. Y., 1952. (20) I. N. Stranski, Ber., 75b, 1667 (1942). (21) S. P. Zhdanov, Dokl. Akad. Nauk SSSR, 115, 938 (1957). (22) V. F. Kiselev, K. G . Krasil'nikov, and G. S. Khodakov, ibid., 130, 1273 (1960).
INTERACTION OF WATERAND VARIOUS SILICAPOWDERS
line silica powder used by Everylo was placed in water for several months, the values of a for the soaked samples (Table I and Figure 4) were lower by a factor of 2 to 3 over the a values for unsoaked samples.1o This suggests that the surface of the silica powder was altered in a manner that changed the tensile strength and surface energy. Griot and I
