Vol. 62
NORMAN HACKERMAN AND ARTHUR C. HALL
1212
THE ADSORPTION OF WATER VAPOR ON QUARTZ AND CALCITE BY NORMAN HACKERMAN AND ARTHUR C. HALL The University of Texas, Austin, Texas Received March S. 1968
Adsorption isotherms of water vapor on quartz and calcite powders of known specific surface area have been established and used to calculate the isosteric heat of adsor tion, the two-dimensional s reading pressure of the adsorbed films, free energies of immersion and work of adhesion. T f e effects of thermal recrysta%zation on the adsorption characteristics of quartz were investigated and are attributed to the replacement of a superficial amorphous layer by a crystalline surface.
Introduction Adsorption of vapor on a solid surface is accompanied by a decrease in surface free energy which is commonly defined to be the two-dimensional spreading pressure a, of the adsorbed film. If the variation with pressure of the quantity of vapor adsorbed on a solid surface is known a may be calculated froni the Gibbs equation
==
Ye0
- Yev
=
sp'==op r d In P
(I)
where ysois the surface free energy of the clean solid, ysvthat of the vapor covered solid, and r is the surface density of the adsorbate.
- 1.6 -
is known to wet the solid in question it is possible to compute two other quantities. The free energy of immersion in liquid YSO
- YSl
=
?SO
-
YBV'
+
(2)
YlV
where y s V o refers to the vapor covered surface at saturation pressure and ylv is the surface tension of the liquid. The work of adhesion W
=
+
~ a o
~
l
- Ysl
(3)
v
ysl is
the free energy of the solid-liquid interface. Isosteric heats of adsorption are calculated by application of the Clausius-Clapeyron equation to isotherms a t neighboring temperatures. r
I
I
I
I
I
- 1.6
- 1.4 1.4
- 1.2 - 1.0 - 0.8 - 0.6
- 1.2
- 1.0 - 0.8
-
0.6
- 0.4 -0.4
- 0.2 I
0
I
I
0.2
0.4
I
0.6
I
0.6
Io
1.0
P/E. Fig. 1.-Adsorption of water vapor on annealed quartz silica: 0 , 15" (left ordinate); 0, 25' (right ordinate).
When an experimental isotherm is available the two-dimensional spreading pressure is easily calculated by graphical integration of the V I P versus P plot, V being the amount of adsorbate per unit surface of adsorbent at pressure P. By extrapolation to Po, the vapor pressure of liquid adsorbate, a value of the free energy of immersion of clean solid in saturated vapor is obtained. For a liquid which (1) D. H. Bangham, Trans. Faraday Soc.. 33, 805 (1937). (2) D. H. Banghem and R. I. Razouk, ibid., 3 3 , 1463 (1937).
- 0.2
1213
ADSORPTION OF WATERVAPORON QUARTZ AND CALCITE
Oct., 1958
I
1.2
I
I
I
1.c n
+ v)
v
s (D
>" 0.
0.
1.0
0
2.0
4.0
3.0
5.0
v/vM*
Fig, 5.-Isosteric heat of adsorption of water vapor on calcite as a function of surface coverage.
0
0.8 I
I
0.2
0.4
I
0.6
0
I
0.8
1.0
PIP.. Fig. 3.-Adsorption of water vapor on calcite: e, 15' (left ordinate); 0 , 25' (right ordinate).
0.7
0.6
Y 0.5 Q
I
G >
&
0.3
0.2
______________.____ _____________________ 0.1
10
20
30
40
50
v/vm. Fig. 4.-Isosteric heat of adsorption of water vapor as a function of surface coverage: 0,annealed quartz silica; e, unannealed quartz silica. The adsorption apparatus was based on a design of Harkins and Jura.3 Differences in level of the mercury in the manometer arms were measured by a cathetometer calibrated t o 0.05 mm. For readings below 4 mm. pressure the cathetometer microscope was equipped with a moving reticle and scale calibrated to 0.001 mm. A light source similar to that of Freeman4 was used to illuminate the menisci for observation. The system was evacuated by a air of two stage oil diffusion pumps in series with a We/& Duo Seal fore ump Final vacuum pressure was measured by a CVC PEG-09 Philips gauge. Outgassing of samples was done a t 4 X mm.; for 24 hours a t 125" for silica, for 1 hour a t 100" for calcite. The temperature of the adsorbent bulb was held well within 0.1" by immersion in a controlled temperature bath equipped with a bimetallic regulator and electronic relay. Isotherms were measured a t 15 and 25' and tested for adsorption reversibility, to establish the applicability of the (3) W. D. Harkins and G. Jura, J . A m . Chcm. S o c . , 66, 1356 (1914). (4) M. P. Freeman, Rev. Sci. Inslr., 28,59 (1957).
I
0
0
0.1
I
I
I
I
0.2
0.3
0.4
0.5
.
P I P. Fig. &--BET plot for adso:ption of yater vapor on calcite: 0, 15 ; a, 25 . Clausius-Clapeyron equation. Integration of the Gibbs equation was done graphically, by planimeter.
Results All of the isotherms are typically sigmoid and all reversible over the entire range of adsorption. The irreversibility encountered by some investigators5 probably is due to pretreatment of the surface which results in its partial dehydration. BET plots are linear in the relative pressure range 0.05 to 0.4 and permit calculation of the area per niolecule of water, by comparison with the krypton area. The absolute isotherms are shown in Figs. 1-3, the variation of isosteric heat of adsorption (5) A. I. Sarakhov, Izvest. Akad. Nauk S.S.S.R.,Otdel. Khim. Nauk, No. 2, 150 (1956).
JOHN G. Foss AND LLOYDH. REYERSON
1214
with surface coverage in Figs. 4 and 5 and a sample BET plot in Fig. 6. Free energy changes and work of adhesion are shown in Tables I and 11. TABLE I 15’ Ya0
System
- YWO
ergs/crn.s’
HzO-annealed Quartz HAO-unannealed Quartz HZO-calcite
yao
- Yell
w,
ergs/cm. 2
ergs/cm. *
318 283 281
391 357 355
244 209 208
TABLE I1 25 O %O
-
yavo,
System
ergdcm.2
HzO-annealed Quartz HzO-unannealed Quartz HZO-calcite
231 203 186
yao
- YSl.
w,
ergdcm.2
ergs/cm.l
303 275 258
375 347 330
Discussion The areas per water molecule on the adsorbent surface are: 13.72 A.z on unannealed quartz silica, 11.73 on annealed quartz silica and 14.30 A.2 on calcite. The difference between the values for annealed and unannealed quartz may be explained
Vol. 62
as follows. Hydroxyl groups, bound to silicon atoms are known to be the species active in the physical adsorption of water vapor on quartz.6 When quartz is crushed Si+4ions are replaced, in the surface, by the readily polarizable 0- ions and an amorphous layer is produced.’ Annealing, which restores the crystal structure,* results in an increase in the number of silicon atoms, and therefore in an increased number of adsorption sites per unit surface. The values calculated and shown in Tables I and I1 tehd to confirm the validity of this explanation. Similar work was done on the system waterkaolinite but adsorption hysteresis precluded an accurate interpretation of the data in the foregoing terms. Acknowledgment.-The authors are pleased to express their appreciation to the American Petroleum Institute for its financial support of this work. (6) V. A. Dzis’ko, A. A. Vishneyskaya and V. A. Chesalova, Zhur. F i z . Rhim., 2 4 , 1416 (1950). (7) W. A. Weyl, “Structure and Properties of Solid Surfaces,” edited by R. Gomer and C. S. Smith, The University of Chicago Press, 1953. (8) D. D’Eustachio and 8. Greenwald, P h y s . Rev., 69,532 (1946).
T H E SORPTION OF WATER VAPOR BY LYOPHILIZED RIBONUCLEASE1 JOHN G. Foss AND LLOYDH. REYERBON The School of Chemistry, University of Minnesota, Minneapolis 14, Minnesota Received March 3, lot8
Isotherm data for the sorption of water on ribonuclease are reported. Heats and entropies of adsorption are calculated and discussed briefly.
Because of the important role played by water in determining the properties of proteins it would be desirable to learn more about the binding forces and mechanism of binding between these molecules. Since the biologically important properties of proteins manifest themselves in solution the water-protein interactions in solution are of greatest interest. However, there are also good reasons for studying the very much simpler case of water sorption on dry proteins. Amberg2 has commented on some of these in a paper published while the present investigation was being carried out. This study was initiated for several reasons. First it was hoped that entropy changes on sorption might be used together with model calculations to determine any marked structural changes which might take place on hydrating the protein. If such entropy changes could be measured starting with the dry protein and progressing till the protein was in solution, something might be learned of what important changes occur on going into solution. A second general goal of this work was to determine experimentally whether any real confidence can be placed in using isotherm data to determine heats of sorption when compared with calorimetrically determined values. Finally it was ’
(1) This work was supported in part by a Grant-in-Aid from the E. I. du Pout de Nemours Company and from the National Institutes of Health. (2) C. H. Amberg, J . A m . Chsm. S o c . , 79, 3980 (1957).
hoped to learn whether sorption isokherms or calorimetric heats are affected by marked changes such as denaturation. Amberg’s2 recent paper on the calorimetrically determined heat of sorption of bovine serum albumin already has provided a partial answer t o the second question as will be discussed below. However, calorimetric studies will be continued using a diphenyl ether phase change calorimeter similar to that described by G i g ~ e r e . ~This paper will deal only with the isotherm data obtained for ribonuclease. Experimental Sorption isotherms were measured gravimetrically using a quartz spring helix having a sensitivity of 11.9 mg./cm. Displacements were measured with a traveling microscope to +0.001 cm. The protein was held in a light glass or aluminum bucket and thermostated with a water jacket. A silicone oil manometer was used to measure the lower y s u r e s (up to 7 mm.) while higher values were determined y equilibrating a thermostated water sample with the protein. This method was almost essential a t pressures close to saturation for room temperature since temperature fluctuations would then cause marked changes in the pressure. The water used for the adsorbate was distilled and deionized. It was outgassed by a series of freezings, thawings and evacuations. The ribonuclease was purchased from the Armour Company (Lot 381-059) and lyophilized from a one per cent. water solution. The protein was outgassed by pumping (3) P. A. Giguere, B. G. Morissette and A. W. Olmos, Can. J . Chem., 33, 657 (1955).
