J ~ J ADE N D ~ o LOPEZ-GONZALEZ, s FUNKG. CARPEXTER AND VICTOR R. DEITZ
1112
T'ol. 6,5
ADSORPTION OF NITROGEN AND ARGON ON MINERALOGIC4L GRAPHITE ASD DIAMOIVD AT 77 A S D 90°1, Academic Press, Inc., New Tork, N Y..1955, pp. 118-124.
(1957).
July, 1961
Fig. 3.-Adsorption
ADSORPTION OF KITROGEX AND ARGOXON CARBON
1115
isotherms a t low relative pressures. The ordinate scales are logarithmic in order t o cover the range of measurements.
indication that :a filial Henry’s law region was approached in these measurements. The data for this sample of industrial diamond gave a close approximation to a straight line over the entire range of observed pressures in Fig. 3 below about
p / p o = 0.05 and, therefore, this sample should have a very heterogeneous surface. The low-pressure isotherms for mineralogical graphite include relatively sharp breaks in slope, even on the logarithmic scale of Fig. 3, which give
1110
JUAN D E DIOSLOPEZ-GONZALEZ, F R A N K G. CARPENTER
AND VICTOR
R. DEITZ
Vol. 65
preted in terms of a van der Waal two-dimensional gas. Estimation of V,.-The B.E.T. plots for graphite were not linear as shown in Fig. 4. Values of Vm, corresponding to monolayer coverage, were estimated from the point B intercepts and these are listed in Table I. These values correspond to a B.E.T. slope approximated by the dotted lines of Fig. 4 and are listed in Table I together with values corresponding to point B. The B.E.T. plots for the diamond were more linear, but the values derived for Vm differed appreciably from point B. The point B values were used in the following calculations of surface coverage. It may be significant that the crossover points shown in Fig. 2 occur very close to unit coverage as determined by the point B intercept. TABLE I" ESTIMATE OF Vm (ml., S.T.P.) -Graphite----. B.E.T. (dotted line Point B Fig. 4)
b--(
05 '
'
'
'
'
' .IO
'
'
'
'
RELATIVE PRESSURE.
'
.I5
'
'
'
'
'
20
Fig. 4.-B.E.T. plots for the adsorption data (closed circles 77.2'K.; opened circles 9O.O"K.). The dotted lines correspond to the value of V , estimated from point B of the isotherms.
these isotherms a stepwise appearance. The step occurs a t a relative pressure between 10-3 and 10-4. The scatter in the results below p / p o < lod8 make these data of value only in pointing out the general trend of the adsorption isotherms. The step between lov8 and 10-4 occurs a t far lower values of relative pressure than those originally reported by Polley, Schaeffer, and Smith4 and subsequently studied by Singleton, Champion and Halsey13 for the case of graphitized carbon blacks. It will be shown that the steps in Fig. 3 correspond to surface ('overages much less than 1. In the isotherm of argon on graphite (see Fig. 3) the slope in the step a t about p/po = is very nearly unity. This particular behavior corresponds to Henry's law, and if data had not been obtained a t lower pressures, one might have surmised that a Henry's law region was valid a t yet lower pressures, as the simplified models of physical adsorption dictate. While this undoubtedly is true, the region of Henry's law has not as yet been reached in the present work. The step in the isotherm cannot be interpreted as phase changes,I4 since this concept is not plausible a t low coverages. Moreover, as already indicated, the isotherms were determined a t temperatures above the two-dimensional critical temperature and thus must be inter(13) J. H. Singleton and G. D. Halsey, Jr., J . Phys. Chem., 68, 1011 (1954); W. M. Chirmpion and G. D. Halsey, J . Am. Ckem. Soc., 76, 974 (1954). (14) 9. Ross and W. Winkler, ibid., 76,2637 (1954).
-Diamond--
Point B
B.E.T. (dotted line Fig. 4)
Argon a t 77" K. 0.19 0.19 0.30 0.33 Argon at 90' K. .18 .18 .28 .42 Nitrogenat77'K. .21 .l7 .30 .35 Nitrogmat,9OoK. .20 .28 .44 .I8 NOTE: The surface area may be calculated from the following constants derived from the densities of the liquid: argon, 3.66 m.2/ml. a t 77.2'K. and 3.86 90.0'; nitrogen, 4.37 m.2/ml. a t 77.2OK. and 4.62 a t 90.0 . The values of p a were those of the liquid except argon a t 77.2"K. for which the vapor pressure of the solid was used.
tt
Determination of Spreading Pressure.-In order to calculate15 the heat and entropy quantities a t constant spreading pressure, @) it is necessary to know the values of @ as a function of the equilibrium pressure a t the two different temperatures. For this purpose, the Gibbs equation was used
sop
= absolute temperature, r = RT
r d In p
= 0.371
where T = surface concentration of the adsorbed gas ( N s / A ) , N , = the number of adsorbed molecules, and A = surface area of the adsorbent. If A is expressed in m.*/g. and V in ml. (S.T.P.)/g., then @ is given in erg/ cm.2. The integration was performed graphically for each isotherm down to the lowest experimental point and a suitable extrapolation made to .2: = 0. The extrapolation to zero pressure was made in all cases from the best line throughout the last few experimental points. The values obtained for @ at x = are given in Table 11. Because data were available a t lorn pressures, the uncertainty involved in this extrapolation is small. This is illustrated in Table I1 in which the uncertainty estimated from the upper and lower limits of the scatter of the last few points may be compared with the value of the spreading pressure a t x = 10-6. The adsorption data for diamond show slightly more scatter a t the lower pressures (see Fig. 3) which lead to a slightly higher uncertainty. (15) T. L. Hill, P. H. Emmett and L. G
(1951).
T w n e r , t b d . , 73, 3102
ADSORPTION OF NITROGEN A N D ARGON ON CARBON
July, 1961
1117
TABLE I1 ESTIMATE OF THE ~JNCERTAINTIES IN
SPREADINQ
PRESSURE,
(erglcm.2) Aip Estd. from dif10-6 ference Based on t h e extrapn. between of best line A+ Estd. % of @ Henr through from scatter a t z = 10-6 a d . lowest exptl. of d a t a Freundlich points extrapn.
0 at z =
OK.
77 90
0 28 .:17
.'16
77 90
.:22
77 90
.057 .046
77 90
,137
Nitrogen and diamond 0 053 19 0 032 23 .021 .062 Nitrogen and graphite 5 .030 ,023 4 ,023 .009 Argon and diamond 018 31 45 .021
% of
Q
at z =
10 3
11 8
7 10
,0059 .0073
10 16
,045 .016
33 23
Argon and graphite
,008 ,009
,071
6 13
The difference between a Henry's law and a Freundlich extra,polation to zero pressure was estimated. When 17 = kx is introduced into equation 3, @J = 0.371(T/A)V. When V = k ~ l /@J~=, 0.371 (T/A)nV, The difference between the two extrapolations is At@ = 0.371
T (n A
- 1)v
(4)
where V in this instance is the volume adsorbed a t the lowest obs,erved point. The results in Table I1 show that i t is small, being of the same order of magnitude as that due to the scatter of the data. The errors in the argon isotherms are larger because these were not obtained a t as low relative pressures as nitrogen and the extrapolation was somewhat longer. At higher relative pressures, however, this error in @ becomes entirely negligible. Heat and Entropy Quantities.-The heats of adsorption a t constant spreading pressure for mineralogical graphite and for the diamond sample were calculated by the method of Hill, Emmett and Joyner.15 Smooth plots of 9 as a function of relative pressure were drawn on a large scale and the corresponding vitlues of relative pressure a t constant @ a t the two temperatures were obtained. The average values for relative pressure, z, and for the temperature, T , were, respectively In x = 51 [In s i
+ In 24 and T1
-1[L
-2
TI +
The isosteric heats and entropies also have been calculated from the isotherms and one of these, the isosteric heat of argon on graphite, is plotted against rovcrage in Fig. 5 . There is a sharp maximum a t about e = 0.9 that is almost identical with the dorimetrie behavior reported b i' Beebe and TounglG for argon on a Spheron carbon black that had been heated to 2700'. These authors interpreted the maxiinum as due to lateral interaction on a graphite-lik e surface, an acceptable explanation in view of the high coverage. The isosteric lieat plotted in Fig. 5 increases with decrease in (11,)
R.4. Beebe a n d D. AI. Y o u n g . J
f h y s . Chem.. 6 8 , 93 (1954).
+..a
-u 01
0.1
v/vm. Fig. 5.--Isosteric heats for argon on mineralogical graphite.
coverage, beginning a t a coverage of about 0.70 and continuing to about 0.02. Below 0.02 there is an indication that it might drop again, but it is difficult to attribute much certainty to this trend in view of the scatter of the isotherm data a t coverages less than 0.02 (i.e., relative pressures less than 10-6). The heats and entropies a t three values of fractional coverage are given in Table 111 for the different adsorbate-solid systems. At a designated coverage, there are important differences in these quantities, but in the case of argon on diamond, the values at constant spreading pressure agree with those calculated a t constant volume adsorbed. TABLE 111 HEATAND ENTROPY QUANTITIES AT DESIGNATED COVFRACTI': Coverage
-1sostericAH
AS
Free energy AF
Constant spreading -pressureAH AS
0.02 0.1 1.0
Nitrogen on diamond ( Vm = 0.29) 100 9 -2200 -1470 -1250 1.7 -1400 -1000 - 90 f 4.2 440 300
25 6 2
0.02 0.1 1.0
Argon on diamond (T', = 0.29) 5.0 -1650 -1200 -1200 5.0 - 720 -1150 - 750 - 410 - 80 - 85 3.9
6 5 4
0.02 0.1 1.0
Nitrogen on graphite ( V , = 0.20) -1550 1 -2300 -2200 980 7.0 -1550 - 800 650 +13.8 - 500 0
9 !I 5
0.02 0.1 1.o
Argon on graphite ( V , = 0.19) -1700 5 -2100 -1050 -1200 2.2 -1400 - 850 700 - 2 580 - 400
13 7 2
+ +
-
-
+ + +
-
+
-
+ + + +
-
The variation of the heats of adsorption a t POI:stant spreading pressure with coverage is given i l l I;ig. 6. One interesting aspect of these curves is the leveling off at coverages bet'ween0.1 and 1. 7 k ' s is followed in three cases out of four by n stead? I * : e to higher values a t lower coverages. The heat of adsorption per niolecule might be expected t'o increase with decrease in coverage, perhaps leveling off a t some particular finite value as zero coverage is approached. 111 the case of nitrogen on diamond, however, there is a pronounced maximum
JUANDE DIOS LOPEZ-GONZALEZ, FRANK G. CARPENTER AND VICTORR. DEITZ
1118
Vol. 65
The entropies at constant spreading pressure are given in Fig. 7. There are minima in the neighborhood of monolayer coverage in all cases. Graphite shows an additional minimum at about e = 0.03 for nitrogen and a plateau for argon in the same region. It is interesting to note that there are maxima in the corresponding heat curves of Fig. 6 a t about the same values of coverage. It is tempting to suggest that the two minima of the entropy curves on mineralogical graphite are due to the completion of .+0.5 3 .! monolayers on two types of sites. At 0 = 1.0 the GRAPHITE c monolayer is completed on the more abundant ' ""' z :-I 5 basal planes and at 0 = 0.05 the layer is complete z u on the prism faces. This is consistent with the + estimate that about 5% of the total area is in the -1 0 t prism faces. This behavior of the entropy curve': ' 0 is not seen in diamond because of the difference in e 6 -0.5 crystal structure, the diamond being isotropic. There is a trend in the molar entropy of the 0 adsorbate to very high values a t the lowest cover0 c age. Hill19predicted that this must approach m I w at zero coverage on the basis of the model of statis+o 5 tical mechanics employed to yield the B.E.T. isotherm. The steady increase in entropy is in agreeFig. 6.--Calculated heats a t constant spreading pressure ment with the behavior of an adsorbed two-dimen(integral heats). sional gas film which must be present under the conditions of these measurements. The entropy of nitrogen on diamond is above that a t argon to i 3 O I DIAMOND the lowest coverage, in agreement with the order z of the two dimensional critical temperatures (63O K . for nitrogen and 75OK. for argon). Concluding Remarks ARGON The mounting interest in low energy surfaces brings into a sharp focus the problem of initial - sample preparation. The concept of an inert adsorbent surface has been helpful for high energy surfaces in the region of large adsorption. However, it has been shown that the particular topography of the outgassed surface makes a significant contribution and, in fact, it may even make itself felt through several statistical layers of adsorbate as indicated by the work of Singleton and Halsey2O with xenon multilayers. The location of adsorbed argon atoms or nitrogen molecules on a carbon surface above the critical temperature is indefinite. The use of the van Fig. 7.--Calculated entropies a t constant spreading pres- derWaals two-dimensionalequation of state attempts sure (integral entropies). to take into account the attractive forces between a t about 7y7 coverage. Orrl7 made calculations the adsorbate and the surface atoms. The dimenof the differential heats of adsorption from iso- sions of the adsorbate atoms and the separation therms of argon and nitrogen on potassium chlo- of the adsorbent surface atoms are generally not ride and cesium iodide. He reported maxima in the commensurable and the interaction results in the neighborhood of monolayer coverage and explained formation of clusters, or patches, the sizes of n hich these in ternis of mutual van der Waals attraction enlarge with increase in relative pressure. In the formation of the statistical multilayer, the various of the adsorbed molecules. Drain and calculated contributions of the dispersive forces clusters approach but in all likelihood do not doveto the isosteric heats of adsorption of argon on tail neatly with each other. This introduces a rutile, but these indicate a similar behavior with the noli-uniformity in the over-all multilayer that can maximum heat in the neighborhood of 0 = 0.5. influence the adsorption a t individual sites. Some The maxima indicated a t still lower coverage in of the peculiarities in the multilayer region Fig. 6 remain an unexplained but reproducible might be explicable on such a model. From the point of view of a would-be adsorbate atom or anomaly. molecule, there are many location^ having rather 4
t
' ' l ' ' " ' !
'+
0
4
4 0
U.
+
0
\
i
(17) m-.J. C . Orr, "The .Idsorption of Non-Polar Gases on Alkali Halide Crlstals," Proc. R o y . Soc.. AlTa, 349 (1939). (18) I,. E. Dram and J. A. Morrison, Trans. Faraday Soc., 48, J l b , 840 (1952).
(19) T L. Hill, J . Chem Phys., 17, 520 (1949) (20) J. H. Singleton and G. D. Halse,, Jr., J . Piii/s Chem., 68, 330 (1954).
July, 1961
OPTICaL
ROTATORY DISPERSION O F P-FORM OF P O L Y P E P T I D E
shallow minimum potentid energy wells in the region where the clusters merge. I n fact i t has been suggestedz1that entropy differences in cluster formation relative to cluster decomposition could possibly account for hysteresis. Although high surface mobility would tend to favor a uniform packing, the residual int'eraction due to the presence of clusters would tend to count'eract this. There is a possible analogy with the various arrangements realized in packing mono-sized spheres on a macro scale. When spheres are packed experimentally by clumping t'hem into a box, regular packings are never realized. Herdan2zfound t'hat upon prolonged vibration the voids are about 39.5y0 and the corresponding packing esseiitmiallyorthorhombic. Graton and FrazerZ3concluded that the usual packing of mono-sized spheres roiisists of colonies of rhornbohedral packing with random packing ill bet'ween. Smithz4 measured both porosity and the number of points of contact'. For porosit'ies of about 44% there was a maximum a,t 7 or 8 points of coiltact, indicating predominantly orthorhombic packing. At porosit'ies of about 3 i % (achieved by sha.king) maxima occurred a t bot,h 8 (21) L. F. Glrydeen rind
a,.as, ss,285
(i~m.
V. R. Deita, J . Research S a t l . Bur. Stand-
(22) G. Herdnn. "sniitll Particle Statistics," Elsevier Publishing Co.. Amsterdam, 1953, 520 pp. (23) L. C. Gratonnnc H. J. Frazer, J . Geol., 43,785 (1935). (21) W.0. Smith, Phys. Rei,., 34, 1271 (1929).
CHAINS
1119
and 12 points of contact, indicating colonies of rhombohedral interspersed with colonies of orthorhombic. It is likely, therefore, that the arrangement of adsorbate molecules in parts of the monolayer and in the statistical multilayer adsorbed phase can lead to signihcant entropy changes aiid, for the case of a lorn energy surface, these can make a sigiiificaiit contribution to the free energy change of the adsorption process. It may be concluded that there are niaiiy similarities and some differences in the behaviors of mineralogical graphite and heat-treated carbon blacks. The similarity is closest when the carbon black is heated to 2700". The isosteric heats of adsorption are in close agreement in the neighborhood of monolayer coverage and the partial molar free energies of adsorption a t different temperature, Le., the isotherms, show the same ability to crossover after monolayer coverage. The behavior a t low relative pressures, i.e., the steps in the isotherms, suggests that the surface of graphite may be more homogeneous than a heat-treated carbon black, and the recent work by Ross aiid Olivierzj provides definite indication that this is more a question of degree than a fundamental difference in the adsorbing surface. (23) S. Ross and J. P. Olirier J P h y s Chem., 66, 608 (1961).
OPTICAL ROTATORY DISPERSIOK OF THE p-FORM OF THE POLYPEPTIDE cHAIs BY
XKIYOSHIWADA,~ ~ A S A X I C HTST'BOI' I i 1 3 D EMIKO KONISHI
Department of Cheinzstt y, Facidty of Sczence, Ochanomzcc Cnicersity, Bunkyo-ku, Tokyo, Japan Recewed Octobri 7 1 , 1960
The @-formof poly--,-benzyl-L-glutamate (1'BIG) v a s studied by measurement of the optical rotatory dispersion and infrared absorption spectrum. The fraction xp of this intermolecular hydrogen-bonded configuration of the polypeptide chain increases with the concentration. The relation between xp and the concentration was obtained by comparing the intensities of the 1660 and 1630 cm.-l absorption bands. By this relation to analyze the optical dispersion curve, we estimated two unknown constants characteristics of the p-form. It was also confirmed that the hydrogen bond in the 8-form and the a-helix of low-molecular weight PBLG is much more labile than that of the a-helix of high-molecular weight PBLG.
Introduction By physicochemical methods such as X-ray diff raction and infrared absorption spectrum it' has h e n established that the synthet,ic polypeptide chain exists not only in the a-helix or random coil but also as the &form in s01ution.Z.~ This fact is especially interesting in relation to protein structure since the synthetic polypeptide is a useful model for t'he study of the chemical and biological propert'ies of the natural proteiii. I n poly-y-benzyl-L-glutamate (PBLG), one of the synthetic polypept'ides, the stability of the cyhelix decreases with the molecular weight even in a solvent which stabilizes the a-helix of high molec(1) L)epartment of Chemistry, Faculty of Science. Tokyo Unir p r s i t y , Hongo. Tokyo, Japan. ( 2 ) C. H. Barnford, A. Elliott and JT. E. Ilanhy. "Synthetic Polyl,i.l,tiries," Academic Pxras, New Y o r k , N. Y . , 1956. 6:. R . Blout and -4. Asadourian. J . Am. Chem. S o c . , 73, 955
ular weight PBLG. The critical degree of polymerizat'ion for t'his instability was found to be around Below this point the polypeptide chain form,? an intermolecular rather than an intramolecular hydrogen bond and assumes a pleated sheet configuration which is called the pform. The extent of intermolecular hydrogen bonding in the p-form depends on t'he concentration of PBLG in solution. Yang and Dot,y6 have shown that the optical rotatory dispersion curves also yield useful information for the study of the chain configuration of iiatural protein and synthet'ic polypeptides. This method has been used, for instance, to determine t8he fractions of the a-helix and random coil which comist. in a molecule. Imahori6 has extended his (4) .I. C. Mitchell, A. E. Woodward and P. I l o t y , i b i d . , 79, 3055 (1957). ( 5 ) .J. T. Yane and P. I h t y , ibid., 79, 761 (1957).
