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The orientation of adsorbed furan molecules
The Orientation of Adsorbed Furan Molecules H. K. L I V I N G S T O N AND R A Y M O N D S E N K U S
Deparlment of Chemistry, Wayne State University, Detroit, Michigan 48202 Received December 7, 1967 The room temperature adsorption of furan vapors by graphitized carbon black was studied by determining adsorbent weight increase at equilibrium as a function of the temperature of the liquid or solid furan that was the source of the vapors. By converting furan temperatures to vapor pressures, the data could be used to solve the Brunauer-Emmett-Teller adsorption equation. The area per furan molecule at the monolayer point was 2.18 times the monolayer area of a nitrogen molecule. Comparison with molecular models indicates that the furan molecules are oriented with the ring parallel to the adsorbing surface. This is the same conclusion that had been previously indicated by the BET equation for the orientation of benzene on graphitized carbon black. However, Ross and Olivier (1) have concluded from their theory of adsorption on homogeneous surfaces that benzene adsorbs on graphitized carbon black with the ring perpendicular to the adsorbing surface. Their theory, applied to the present data for furan adsorption, predicts that the furan rings are perpendicular to the surface. More data will be required to determine which theory is correct. Molecules that are not spherically symmetrical may assume preferred orientations in the adsorbed state. Molecules that are markedly unsymmetrical may also have many points of free rotation within the molecule and therefore have a random distribution of conformations that makes studies of orientation difficult. Platelike molecules like benzene and its simple heterocyclic analogs can exist in only a single conformation and present a possible choice between orientations in which the plane through the centers of the ring atoms is either parallel to the adsorbing surface ("plates on the table") or perpendicular to that surface ("plates in the drainer"). With the widespread adoption of the Brunauer-Emmett-Teller (BET) theory of adsorption, data for the apparent crosssectional area of adsorbed molecules became available for comparison with models of orientation. The adsorption of benzene on wellcharacterized solid surfaces has been extensively studied and provides experimental data from which it might be possible to reach
a clear-cut decision as to orientation. However, there is disagreement as to which orientation exists for benzene molecules adsorbed on a relatively homogeneous surface such as graphitized carbon black. Pierce and coworkers (2, 3) used the B E T treatment and concluded by comparison with models that the benzene rings adsorbed with their planes parallel to the carbon black surface (i.e., with the 6-fold axis of molecular symmetry perpendicular to the surface). Ross and Olivier (1), recognizing that the theoretical basis of the B E T theory is particularly unsatisfactory for homogeneous surfaces, have developed a complete (and completely different) theory of adsorption. This theory predicts that benzene molecules on relatively homogeneous graphitized carbon black have their 6-fold axis parallel to the surface. The problem is important, both because the understanding of surface processes such as catalysis requires an understanding of orientation and because it provides a test between a relatively new, theoretically attractive theory and a familiar, empirically accepted theory.
Journal of Colloid and Interface Science, VoI. 30, No. 3, J u l y 1969
312
ORIENTATION OF ADSORBED FURAN MOLECULES I. ADSORPTION OF PLATELIKE MOLECULES An appropriate experiment to attempt to settle the problem involves extension of the two methods of determining orientation to another platelike molecule, furan. As will be shown in the present paper, when this is done it is possible to conclude that the B E T method indicates the "plates on the table" orientation is taken for furan on carbon black, as it is for benzene. For furan the Ross-Olivier theory is equivocal because it can not be decided with certainty from adsorption experiments alone whether the adsorbed furan molecules are fixed or freely rotating on the adsorbing surface. Further experiments to settle this point are indicated. Before considering the new experimental data on furan, it is desirable to review briefly the published data with which they will be compared. Few data are available on platelike molecules other than benzene. McClelland and Harnsberger (4) in their master summary of adsorptiort data list four studies of cyclopentane. The only adsorption studies on furan have used poorly characterized adsorbents. Pierce and Ewing (5) have published adsorption data for thiophene on graphitized carbon black. These indicate that thiophene has about the same BET molecular area as benzene, presumably because it also has the same orientation. This coincides with the conclusion of Avgul, Kiselev, and Lygina (6), who compared benzene and pyridine adsorption on graphitized T I thermal carbon black. They treated their data by the B E T method and concluded that both these ring compounds adsorbed with the ring parallel to the solid surface. The most familiar platelike organic molecule is benzene. Experiments with StuartBriegleb molecular models show that there is a large difference between the area per molecule depending on whether the 6-fold axis is perpendicular or parallel to the adsorbing surface. Several authors have used the B E T method to make careful comparisons of the cross-sectional areas of benzene and nitrogen, using graphitized carbon adsorbents with
313
TABLE I RATIOS OF CROss-SECTIONAL AREAS ON ]LIoMOGENEOIJS GRAPHITE SURFACES,
C~H6 vs. N~ Ratio
Carbon
P-33 Sterling MT Avg. of several MTg Graphon Spheron 6 (= Graphon?)
2.34 2.36 or 2.44 2.47 2.48 2.53 2.87
Reference
7 8; 5 9 3 2 10
well-characterized surfaces. Their results are summarized in Table I. I t is possible to define two-dimensional figures whose close packing will indieate the area which must be available for each molecule. Packings of such figures are shown in Fig. 1 (6-fold axis perpendicular) and Fig. 2 (parallel). Then, the areas can be converted to ratios of C6H6 to N2 areas, comparable to the experimental ratios given in Table I. If we take ~N2 as 13.S-16.2 A 2 molecule, the ratio is 2.41-2.05 if Call6 is fiat and close packed, or 1.65-1.41 if C6H6 is edgewise and close packed. These results clearly support the idea that the 6-fold axis is perpendicular to the surface. Pierce and Ewing (3, 5) have recently accepted the idea that the benzene moleeules sit on sites on the graphite and show that if this is the ease, the 6-fold axis must be perpendicular to the surface, since the effective molecular area is of the order of 47 A 2. However, Ross and Olivier (1) have arrived at the opposite conclusion. The Ross-Olivier treatment of nonlocalized adsorption employs the two-dimensional vau der Waals equation to describe the adsorption process. On the assumption of a homotatic adsorbent the equation actually used to describe the experimental isotherm is
0
p = K~exp
( 0
1 - 0
~T~]"
[1]
Here a and ;3 are the two-dimensional analogs of the three-dimensional van der Waals constants a and b. Ross and Olivier show that the ratio 2 a / ~ represents the lateral interaction between adsorbate molecules. K is a constant taking into account the adJournal of Colloid and Interface Science. Vo]. 30, No. 3, July 1969
314
LIVINGSTON AND SENKUS
FIG. 1. Two-dimensional close packing of areas obtained by projection of Stuart-Briegleb models of benzene against a parallel surface. A regular hexagon 6.20 A on a side contains the equivalent of three such "molecules," using the Stuart-Briegleb scale factor.
FIG. 2. Two-dimensional close packing of areas obtained by projection of Stuart-Briegleb models of benzene against a perpendicular surface. The indicated irregular hexagon contains the equivalent of three such molecules. s o r b a t e - a d s o r b e n t i n t e r a c t i o n ; 0 is t h e fract i o n of t h e surface covered; k is t h e B o l t z m a n n c o n s t a n t ; a n d T is t h e a b s o l u t e temperature. Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
For tains value These
e x p e r i m e n t a l use this e q u a t i o n cont h r e e u n k n o w n s : 2a/5, K, and t h e for t h e surface a r e a of t h e a d s o r b e n t . u n k n o w n s are d e t e r m i n e d b y p l o t t i n g
ORIENTATION OF ADSORBED FURAN MOLECULES the equation in its rectilinear form: W -- In
(o0)
- lnp+
0
1 - 0
[2] 2~0
-
in K.
From a plot of W vs. 0, the surface area V~ is determined by varying the value V¢ until one finds the smallest least-squares deviation from a straight line in the above plot (V~ = V/O). There has been, however, one factor which has not been accounted for in this treatment, and that is surface heterogeneity. The Ross and Olivier adsorption theory takes into account the surface heterogeneity of the adsorbent. Their model is a surface made up of an infinite number of patches of varying degrees of energy with a Gaussian distribution. Their equation is 0 = 1_ f 0~- exp [--~,(U n
1
U') 2] dU,
[3]
where n is a normalizing constant, U' is the average adsorptive energy about which the Gaussian distribution is centered, ~/ determines the width of the distribution, U is the energy of adsorption of the ith patch, and 0~ is derived from the equation of state pertaining to each patch. Ross and Olivier have calculated theoretical isotherms for the above equation for various values of 2a/¢~ and ~. The value of 2a/3 obtained previously for the homotatie surface may be used as an approximation for the 2a/~ for the heterogeneous surface. Then using this value of 2a/¢~ one superimposes the experimental adsorption curve over theoretical curves for various values of ~/. When a fit is found ~, is known, and in addition K and Va may be determined. If the adsorbed molecules are not oriented or polarized by the substrate one may obtain ideal two-dimensional van der Waals constants from their three-dimensional counterparts by means of the following equations: ( 9 7 r ~ ~/3
~~
=
a \2-g~/
;
~i~ = 2 b \ ~ /
[4] .
315
Here a and b are the three-dimensional van der Waals constants. Using this approach, Ross and Olivier consider free rotation and fixed molecules, with and without surface electric fields, with axis parallel or perpendicular to the surfaee. They eonclude that the a, ¢~, and polarizability data support the conclusion t h a t the orientatioil of benzene molecules is one of free rotation with the 6-fold axis parallel to the carbon black surfaee. Free rotation of adsorbed moleeules is a critical concept. It was unimportant in the early BET consideration with molecules that were perfectly spherically symmetrical (e.g., argon), or nearly so (e.g., nitrogen). For such molecules, the liquid or solid density could be used to ealeulate the eross-sectional area of a molecule. That this is not the ease for benzene can be recognized by considering the crystallographic data for benzene. As determined by Cox, Cruiekshank, and Smith (11), the CGH6unit cell at - 3 ° C consists of 4 molecules fitting together like two pairs of beveled 6-toothed gears. A given pair are close fitting with axes of the "gears" at 90°22 '. The closest approach puts hydrogens within 2.64 and 2.78A of each other. It is conceivable that in molecular rotations originating in the thermal energy of the crystal the molecules in a "corrugated sheet" of such 90 ° pairs could rotate together as a cluster of "intermeshed gears." The other pair of molecules in a unit cell are also intermeshed and have axes at 90 °. However, between pairs there is no intermeshing. The large eoe~cient of thermal expansion of C6H6 crystals along the c axis results from the fact that intermeshing is in the ab plane only, so expansion along the c axis involves much less energy. These data show how unreliable it is to predict two-dimensional packings from solid densities. Various types of graphitized carbon have been frequently used for adsorption studies of many adsorbates. The advantage of graphitized earbon is that the surface is one of the most homoenergetie ones available for study. In our experiments, Graphon (also known as Spheron 6) was used. The fact that Karnaukhov, t{iselev, and Khrapova (10) obtained a different result from that of Smith, Pierce, and Cordes (2) for benzene Journal of Colloid and Interface Science, Vol. 30, No., 3 July 1969
316
LIVINGSTON AND SENKUS
adsorption on this carbon black (see Table I) m a y reflect international confusion in labeling• All work in the United States on Graphon seems to have given consistent results. The Graphon was purchased from Cabot Corporation, Boston, Massachusetts. The manufacturer gives a value of 87.4 m2/gm for the surface area of the Graphon. Our own measurements, based on a value of 16.2 A 2 for the effective area of a rlitrogen mo]ecule, give 84.7 m2/gm as the surface area of Graphon.
II. EXPERIMENTAL
45
2 ••.."
4o
.°.-" °-
~ 35 ~=
, ,,,..•' .,• •,..'"'"•
30
~< •° cr 2 5 ill
o-
The general setup of the apparatus is essentially that of MeBain and Bakr (12). Our only modification has been to place the liquid furan, the temperature of which is controlled in order to control the furan pressure over the adsorbate, in a side arm. The Graphon
Z <
cc 20 2 15- . 10
f
J
"
Po=6OOmmHg T = 25°C
.
5-
45 01 Z 01"
40
35
02
03
04
05
06
%
J
FIG. 4. The data of Fig. 5 replotted as furan adsorption as a function of the partial pressure of furan.
30 rr uJ Q_ r~ 2 5 uJ m tw O
.."
~2o < Z
(.9 10
2'
.•"
5 ,, . . " •
I
I
I
-70 -50 -30 TEMPERATURE °C
I -10
I
+10
+30
To=25°C
Fro. 3. Experimental data for the adsorption of furan on Graphon as a function of the temperature of the liquid (or solid) furan in equilibrium with the Graphon. Journal of Cdloid a~d Interface Science, ¥ o l . 30, N o . 3, J u l y 1969
was hung in a platinum bucket from a 2-gm capacity quartz spring obtained from Mieroehemieal Specialties Co., Berkeley, California. The Graphon was degassed for at least 12 hours at 200°C and at 5 × 10.4 torr. After degassing the Graphon was allowed to cool to 25 ~ 0.2°C and was kept at this t e m p e r a t u r e during the run. The furan used was "ehromatoquality reagent" grade obtained from Matheson, Colemen, and Bell. I t was used with no further purifieation. The liquid furan was cooled to dry ice temperature and allowed to w a r m up slowly as adsorption took place• The warming from dry ice temperature to 0°C took approximately 10 hours. The temperature change was followed with an ironeonstantan thermoeouple and potentiometer. The spring extension was followed with a eathetometer.
ORIENTATION OF ADSORBED FURAN MOLECULES
317
2
E)
1--
0.9-0.8-0.7-0-6-0.5-0.4--
0,3--
0,2--
0.1
I
I
I
I
I
I
10
20
30
40
50
60
TI M E ( M in.)
TEMPERATURE=25°C
FIG. 5. Adsorption as a function of time for furan olt Graphon at 25°C. and a maximum 0 of 1.3. III. RESULTS Figure 3 gives the raw data in the form of adsorption vs. furan temperature. The more familiar adsorption isotherm of furan oil Graphon is Fig. 4. The results gave a B E T monolayer value of 27.2 mg furan per gram Graphon. This corresponds to a ratio of furan area to nitrogen area of 2.18. Some difficulty was encountered in these adsorption runs in attempting to obtain reproducible data. Some of the runs indicated a greater amount adsorbed than the one shown, especially from p / p o = 0.1 to p / p o = 0.4. As much as 2 mg greater adsorption was obtained in this interval. Several of the runs indicated "steps" at low pressures. So far no satisfactory explanation has been found for this behavior. Desorption immediately after adsorption or 24 hours after adsorption showed no hysteresis. Adsorption was fairly rapid, as shown by a typical rate curve (Fig. 5). The data are plotted for a single run; a duplicate rate measurement was in good agreement. Since 10 hours elapsed during the measurement of the adsorption isotherms (as in Figs. 3 and 4), the isotherms represent equilibrium measurements. The only previous work on the adsorption
of furan to be published is described by Fejes, Kiraly, and Schay (13), who adsorbed furan oll aluminum oxides. They obtained a ratio of furan area to nitrogen area of 2.47. Models for the orientation furan molecules were made by the same methods that had been used for benzene (see Figs. 1 and 2). The figures representing the area occupied by furan molecules were more irregular than for benzene, but as Kitaigorodskii has shown (14) closest packing in two dimensions always requires six nearest neighbors. Packings are reproduced in Figs. 6 and 7. The areas indicated for the models correspond to 17.8 A2/molecule if the furan pentagons are close packed on edge on the surface and 29.0 AS/molecule if they are flat. If the nitrogen area is taken as being in the range 13.8-16.2 A2/molecule, this gives a furan nitrogen ratio of 1.29-1.10/1 in the first ease and 2.10-1.79/1 in the second. Since our own work indicates that the ratio of the B E T area of furan to the B E T area of nitrogen is 2.18/1 on Graphon, the B E T method definitely indicates that the molecules are lying flat on the surface. When Hill (15) originally proposed the use of a two-dimensional van der Waals equation Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
318
LIVINGSTON AND SENKUS
]PIG. 6. The closest two-dimensional packing found for areas obtained by projection of Stuart-Briegleb models of furan against a paralle] surface.
FIG. 7. The closest two-dimensional packing found for areas obtained by projection of Stuart-Briegleb models of furan against a perpendicular surface. to fit adsorption data, he recognized the imp o r t a n c e of the two-dimensionM v a n der Waals constants a and ~. I n a later publication (16) Hill showed t h a t a and/~ could be evaluated b y the equations mentioned above Journal of Colloid and lnterfoce Science, Vo]. 30, No. 3, July 1969
(see Eq. [4]) and pointed out t h a t fl h a d a value v e r y near t h a t of the molecular crosssectional areas being used at t h a t time in B E T adsorptior~ studies. Subsequently orte of us (17) evaluated 5 for a n u m b e r of adsorb-
ORIENTATION OF ADSORBED FURAN MOLECULES
319
0.@
O.5
0.4 @ 0.3 SOLID CURVE THEORETICAL DOTS EXPERIMENTAL
Q2
O3
I
I
I
1
10 20 30
I
I
40 50 60
I
1
70 80
I
I
1
I
I
I
I
1
I
I
I
90 100 110 120 130 140 150 160 170 180 190
PRESSURE mm Hg
TEMPE RATURE=25°C
Fro. 8. The experimental adsorption for furan on Graphon compared with the "best fit" isotherm obtained from the theory of Ross and Olivier. ents including benzene. I t is of interest that 33.0 A2/moleeule, obtained for benzene, corresponds almost exactly to the area obrained from our model studies with the flat benzene orientation. I n the case of furan the experimental three-dimensional van der Waals constants could not be found in the literature. They were calculated from critical data (18) as a = 7.5 arm liter 2 mole-2 and b = 72.7 em "3 mole-1. This resulted in a id = 2.04 X 10-2s erg em 2 and flid = 23.4 X 10-16 cm ~. Thus for furan is precisely the average of the two furan areas calculated above from model studies, suggesting that furan molecules can assume either orientation. I t will be worth noting that one must concern himself with dimensions in caiculating a and fl, the two-dimensional van der Waals constants. Equation [4] is valid if the three-dimensional van der Waals equation is used in the form
(p + a N 2 n 2 / V 2 ) ( V / n N -
b) = R T ,
[5],
where n is the number of moles and N is Avogadro's number. I n such a case if p is in atmospheres and V is in liters, a has the units arm liter 2 and cm 3 and b has the units of liters. I t is then possible to convert a and b to erg em 3, and em a respectively, and obtain directly a in erg em 2 and ~ in em 2 for the molecule of interest. Tabulations such as
those given by DeBoer (19(a)) and by Ross and Olivier (l(a)) are misleading in their implication that a and b in molar units can be raised to the two-thirds power to yield, with suR~ble factors, a and ¢/. Ross and Olivier believe that benzene on graphitized carbon black is oriented with the benzene hexagons perpendicular to the solid surface. I t is desirable to apply their treatment to our furan data. With reference to the basic Ross-Olivier equation [2], which assumes a completely homotatic surface, V~ = 48.2 m g / g m in the case of furan on Graphon. At this value of V~, 2 a / ~ and K are determined from the slope and intercept of the graph. I n our ease 2a/~ = 1.60 kcal/ mole and K = 0.103 atm. Figure 8 shows both the experimental isotherm for furan on Graphon and the theoretical one calculated with these values of the constants. When our data were applied to Eq. [3], which assumes a heterogeneous surface, v = 1000 and the values for K and Ve were the same as those obtained for the assumed homotatic surface. This is as it should be, for according to Ross and Olivier a surface with v = 1000 is hardly to be distinguished from a completely homotatic surface (V = m). One may determine the v at 77.5°K, which Ross and Olivier use as a standard, or at any other temperature b y means of the following equation Journal of Colloid and Interface Science, VoI. 30, No. 3, July 1969
320
LIVINGSTON AND SENKUS 7T = q'vz.~ ~ 7 ~ . 5 ~ 2 •
For Graphon this gives a "Y77.5 value of 68. A comparison of the adsorptions for a monolayer (0 = 1) as indicated by the Brunauer-Emmett-Teller theory (V~) and the Ross-Olivier theory (V~) is given in Table II. The differences are not systematic and reflect the fact t h a t the two theories are completely different in their postulates and approaches. I t is not advisable to a t t e m p t to use V~ to determine the speeifie surface of solids. If one is interested in the ratio between the cross-sectional areas of adsorbed molecules, it is B t h a t should be compared, not V~ (see Hill (16)). With our v a n der Waals data indicating t h a t 2aid/~ ie = 2.49 keal/mole it is possible to compare several models for the orientation of the adsorbed molecules to see which model corresponds most closely to the experimental data. IV. FREE ROTATION AND INSIGNIFICANT SURFACE ELECTRIC FIELD MODEL This model implies t h a t the adsorbed molecule is essentially in the same condition as in the gas phase; hence the value of 2a/fl observed should equal the value of 2aid/2ie (2.49 keal/mole). But 2a/fl = 1.60 keal/ mole. The observed value is only 64 % of the ideal. Thus this model must be rejected. V. FREE ROTATION AND LARGE SURFACE ELECTRIC FIELD MODEL I n this ease one might assume t h a t the decrease in the value of 2 a / ~ as compared to the ideal value is entirely due to a lowering in a caused b y the mutual repulsion of dipoles induced b y the surface electric field. The value of ~ should remain unchanged. DeBoer (19) has derived an expression for the reduction of the two-dimensional v a n der Waals constant a resulting from this effect:
x=-a;
~
[7]
where X is the change in a, a ~ is the total effective dipole moment of the adsorbed molecules normal to the surface, and d °~ is the diameter of the oriented molecule as Journal of Colloid and Interface Science,
TABLE
[6]
Vol. 30, No. 3, July 1969
Furan (25°C) Benzene (28.6°C) Nitrogen (-182.8oc)
II
v~
g,m (BET)
(R - - O)
27.2 6.5 16.6
48.2 11.6 19.6
Reference
This work 2 20
calculated from ¢~ = 7r (d°r)2/2. In the case of furan d °r = 3.86 X 10-s em and a = 1.30 X 10-~s erg emL This results in a calculated value of 0.947D for the total effective dipole m o m e n t of the molecule. Using the literature value for the dieleetric constant and dipole moment one m a y calculate the polarizability of furan from the equation
e--lM_4~rN[ ~+2
p
~--
tL~; ~ + ~
,
[8]
where e is the dielectric constant, 3 t the molecular weight, p the density, N Avogadro's number, ~ the polarizability, and the dipole moment. Using values of 0.72D for the dipole m o m e n t (21) and 2.95 for the dielectric constant one obtains ~ = 7.14 X 10-24 em 3. Knowing the induced dipole moment and polarizability of furan one m a y calculate the strength of the surface electric field needed to produce such an induced dipole moment from ~i~e = F~, where F is the surface electric field. The field needed is 1.33 X 105 esu-cm-2. But this is a contradiction since it is easy to show t h a t a surface electric field of such magnitude will effectively prevent the molecules from rotating. The fraction of the molecules able to rotate is given b y exp (-2t~F//cT). One finds t h a t at 25°C only 0.2 % of the adsorbed molecules is able to rotate. Therefore this model cannot be considered applicable. VI. NEGLIGIBLE SURFACE ELECTRIC FIELD MODEL Since w e assume t h a t there is no surface electric field the lowering of a id should be caused b y the alignment of the permanent dipoles of furan plus the anisotropy of the polarizability. I n order to investigate this model fully
ORIENTATION OF ADSORBED FURAN MOLECULES one would need to know the polarizability of furan in each of the three directions. This d a t u m could not be found in the literature so an approximation has been made. I t has been assumed that the polarizabilities of furan are in the same ratios as the polarizabilities of pyridine. Ross and Olivier (1) note that the ratios (1: ~e2:~a are 11.88: 5.78:10.84, where the subscripts 2 and 3 refer to polarizability perpendicular to the plane of the ring and along the axis through the nitrogen atom, respectively. This approximation results in the following values for the polarizabilities of furan: 8.1 X 10-0"4 cm a in the direction of the C2 axis, 4.3 X 10-24 em a perpendicular to the plane of the molecule, and 8.9 X 10-2~ em a in the remaining direction. First let us assume that the adsorbed furan is oriented with the oxygen pointing toward the Graphon surface. Once again we set = 5i~ since the spinning molecule should have more or less the same effective diameter as the free molecule. The average polarizability of the adsorbed furan is 1/{ (4.3 q8.9) X 10-24 = 6.6 X 10-2~ cm a. The correction of oe'd for the effect of orientation may be made by means of a factor defined a s co = ( d 4 / ~ 2 ) i d ( ~ 2 / d 4 ) ° p , where the superscripts i d and o p refer to the ideal and operative two-dimensional values of d. Then a = con% For this model d °~ = d fd, resulting in co = 0.86. Since we have assumed a negligible electric field, we solve Eq. [7]. The result is X = - 0 . 4 1 X 10-28 erg-em2. Then substituting these values of co and X into the equation a = coc~ie q- X, where conid corrects for orientation of the adsorbed molecules and X for parallel dipole repulsioa, one obtains a = 1.33 X 10-28 erg-cme. If we assume a negligible surface field but with furan adsorbed fiat, then in this case there is no permanent dipole oriented toward the surface so the decrease in a nmst be due to the factor co defined above. =
(~.~)o~/(~)i~ =
(8.5 ×
10-~)~/
(7.1 X 10--°4)5 = 1.4. Then a = con'e = (1.4) (2.04 X 10-~s) = 2.90 X 10-~s. But experimentally a = 1.30 X 10-~s so this model is rejected.
321
The third orientation possible with this model is that of furan adsorbed with a double bond toward the surface. I n this case co = 0.77, which would make the value for predicted for this model 1.55 X 10.58 ergem ~. This value is reasonably dose to the experimental value of 1.30 X 10-28. I t would appear that of the models investigated so far the ones corresponding most closely to experimental data are those with the plane of the molecule perpendicular to the adsorbing surface. VII. ORIENTED MOLECULES WITIcI A SURFACE ELECTRIC FIELD First let us investigate the model of vertically adsorbed furan with the oxygen oriented toward the surface. We will assume in this and the following cases that ¢~ = 5% Previously co was calculated to be 0.86 for furaa oriented this way. Then ), = a-conic=-0.45 X 10-28 erg cm 2. We can then find the total effective dipole moment from Eq. [7]. With the use of the value for X, the total effective dipole tooment is found to be 0.75 D. This dipole moment is a sum of the induced dipole moment and the component of the permaneat dipole oriented towards the surface. The component t* of the permanent dipole in the direction of the electric field may be
found by means of the Langevin function: #=
u [ c o t h ttF
kT]
The induced dipole moment, ~ d , is given by ~,.d = F ~ , where Fi is the electric field and (~ the polarizability in that direction. After a little trial and error one may find the strength of the electric field needed to give a total effective dipole moment of 0.75D. If the field is 0.62 X 10 '~ esu-cm-2, = 0.24D and ~,,d = 0.51D. The value which Ross and Olivier obtained for the surface field of P-33 (2700 °) is 1.12 × 10 '~ esu-cm--~. The value just obtained for the surface field of Graphon is sufficiently close to the P-33 value so that this orientation cannot be disregarded. The second orientation possible with this model is that of furan adsorbed flat. If = t5id then co = 1.4, X = a -- codd = Journal of Colloid o,nd Interface Sc~'ence, VoI. 30, No. 3, July 1969
322
LIVINGSTON AND SENKUS
--1.6 X 10-2s erg-cm 2. Then from Eq. [7] we find t h a t a" = l A D . Since the furan is fiat it has no permanent dipole moment in the direction of the surface so the entire effective dipole moment must be induced. This would require an electric field of 3.2 X 105 esu - c m -2, which is roughly three times the value for the surface field of P-33 (2700°). In a similar manner to that above one may calculate the surface field for the model of furan adsorbed on edge with a double bond pointing downwards. The result is a value for the surface field of 0.63 esu-cm -2. Thus if it is assumed that the surface field of Graphon is roughly the same as that of P-33 (2700°), once again the orientation of adsorbed furan that agrees most closely with experiment is that with the plane of the molecules perpendicular to the surface.
VIII. CONCLUSIONS A great many approximations had to be made in order to carry out the calculations originating from the Ross-Olivier theory. Because of this none of the calculations " p r o v e s " any single model. We must recognize t h a t the ratio of the B E T molecular area for furan to t h a t for nitrogen suggests a flat orientation, just as the B E T data do for benzene. The v a n der Waals constant lies halfway between the "fiat" and "erect" values for cross-sectional area. The Ross-01ivier t r e a t m e n t indicates the plane of the ring is perpendicular to the surface. Further experimental work must be done to obtain more information if one is to establish any model as representing the actual orientation of adsorbed furan on Graphon. One method of experimentation would be to study adsorption polymerization. For a molecule such as furan, this almost certainly involves addition polymerization at the double bond. I n preliminary experiments we have determined t h a t furan molecules t h a t are physically adsorbed (i.e., readily desorbable) on Graphon become fixed on the solid surface b y irradiation with highenergy electrons. The change from desorbable furan to fixed furan m a y involve polymerization. The effect of variations in surface concentration of furan on polymerization kinetics and polymer structure will Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
depend on the mobility and orientation of the adsorbed furan molecules. I t should therefore be possible to choose between various possibilities based on the results of adsorption polymerization studies. ACKNOWLEDGMENT The authors are pleased to acknowledge the financial support of the U.S. Atomic Energy Commission. REFERENCES 1. Ross, S., AND OLIVIE~, J. P., "On Physical Adsorption." Interseienee, New York, 1964; (a) ibid., pp. 172, 173. 2. SMITH, I~. N., PIERCE, C., AND CORDES, H., J. Am. Chem. Soc. 72, 5595 (1950). 3. PIERCE, C., AND EWING, B., J. Phys. Chem.
68, 2562 (1964). 4. McCLELLAN, A. L., AND HARNSBERGER,H. F., J. Colloid and Interface Sci. 23,577 (1967). 5. PIERCE, C., AND EWING, B., J. Phys. Chem.
71, 3408 (1967). 6. AVGUL, N. N., KISELEV, A. V., AND LYGINA, I. A., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk 1962, 32. 7. PIEROTTI, R. A., AND SMALL~VOOD,R. E., J. Colloid and Interface Sci. 22, 469 (1966). 8. DAvis, B. W., ANDPIERCE, C., J. Phys. Chem.
70, 1051 (1966). 9. ISIRIKYAN, A. A., AND KISiELEV, A. V., J. Phys. Chem. 65,601 (1961). 10. KARNAUKHOV, A. P., KISELEV, A. V., AND KgRAPov~, E. V., Dokl. Akad. Nauk SSSR,
92,361 (1593). 11. Cox, E. G., CI~UICKSHANK, D. W. J., AND SMITH, J. A. S., Prec. Roy. See. (London) A247, 1 (1958). 12. MCBAIN, J. W., AND BAKR, A. M., J. A m . Chem. Soc. 48,690 (1926). 13. FEJES, P., KIRALY, J., AND SCHAY, G. Z. Anorg. Allgem. Chem. 300, 72 (1959).
14. KITAIGORODSKII, A., "Organic Chemical Crystallography." Consultants Bureau, New York, (1961). 15. HILL, T. L., J. Chem. Phys. 14,441 (1946). 16. HILL, W. L., J. Chem. Phys. 16, 1811 (1948). 17. LIVINGSTON, i . K., J. Colloid Sci. 4, 447 (1949). 18. KOBE, K. A., Ind. Eng. Chem. Data Set. 1, No. 1, 50 (1956). 19. EEBoER, J. H., "The Dynamical Character of Adsorption." Clarendon Press, Oxford (1953); (a) ibid., p. 148. 20. JOYNER, L. G., AND EMMETT, P. I-I., J. Am. Chem. Soc. 70, 2353 (1948). 21. HA~IS, B., J. Chem. Soc. 1953, 1622.
Deparlment of Chemistry, Wayne State University, Detroit, Michigan 48202 Received December 7, 1967 The room temperature adsorption of furan vapors by graphitized carbon black was studied by determining adsorbent weight increase at equilibrium as a function of the temperature of the liquid or solid furan that was the source of the vapors. By converting furan temperatures to vapor pressures, the data could be used to solve the Brunauer-Emmett-Teller adsorption equation. The area per furan molecule at the monolayer point was 2.18 times the monolayer area of a nitrogen molecule. Comparison with molecular models indicates that the furan molecules are oriented with the ring parallel to the adsorbing surface. This is the same conclusion that had been previously indicated by the BET equation for the orientation of benzene on graphitized carbon black. However, Ross and Olivier (1) have concluded from their theory of adsorption on homogeneous surfaces that benzene adsorbs on graphitized carbon black with the ring perpendicular to the adsorbing surface. Their theory, applied to the present data for furan adsorption, predicts that the furan rings are perpendicular to the surface. More data will be required to determine which theory is correct. Molecules that are not spherically symmetrical may assume preferred orientations in the adsorbed state. Molecules that are markedly unsymmetrical may also have many points of free rotation within the molecule and therefore have a random distribution of conformations that makes studies of orientation difficult. Platelike molecules like benzene and its simple heterocyclic analogs can exist in only a single conformation and present a possible choice between orientations in which the plane through the centers of the ring atoms is either parallel to the adsorbing surface ("plates on the table") or perpendicular to that surface ("plates in the drainer"). With the widespread adoption of the Brunauer-Emmett-Teller (BET) theory of adsorption, data for the apparent crosssectional area of adsorbed molecules became available for comparison with models of orientation. The adsorption of benzene on wellcharacterized solid surfaces has been extensively studied and provides experimental data from which it might be possible to reach
a clear-cut decision as to orientation. However, there is disagreement as to which orientation exists for benzene molecules adsorbed on a relatively homogeneous surface such as graphitized carbon black. Pierce and coworkers (2, 3) used the B E T treatment and concluded by comparison with models that the benzene rings adsorbed with their planes parallel to the carbon black surface (i.e., with the 6-fold axis of molecular symmetry perpendicular to the surface). Ross and Olivier (1), recognizing that the theoretical basis of the B E T theory is particularly unsatisfactory for homogeneous surfaces, have developed a complete (and completely different) theory of adsorption. This theory predicts that benzene molecules on relatively homogeneous graphitized carbon black have their 6-fold axis parallel to the surface. The problem is important, both because the understanding of surface processes such as catalysis requires an understanding of orientation and because it provides a test between a relatively new, theoretically attractive theory and a familiar, empirically accepted theory.
Journal of Colloid and Interface Science, VoI. 30, No. 3, J u l y 1969
312
ORIENTATION OF ADSORBED FURAN MOLECULES I. ADSORPTION OF PLATELIKE MOLECULES An appropriate experiment to attempt to settle the problem involves extension of the two methods of determining orientation to another platelike molecule, furan. As will be shown in the present paper, when this is done it is possible to conclude that the B E T method indicates the "plates on the table" orientation is taken for furan on carbon black, as it is for benzene. For furan the Ross-Olivier theory is equivocal because it can not be decided with certainty from adsorption experiments alone whether the adsorbed furan molecules are fixed or freely rotating on the adsorbing surface. Further experiments to settle this point are indicated. Before considering the new experimental data on furan, it is desirable to review briefly the published data with which they will be compared. Few data are available on platelike molecules other than benzene. McClelland and Harnsberger (4) in their master summary of adsorptiort data list four studies of cyclopentane. The only adsorption studies on furan have used poorly characterized adsorbents. Pierce and Ewing (5) have published adsorption data for thiophene on graphitized carbon black. These indicate that thiophene has about the same BET molecular area as benzene, presumably because it also has the same orientation. This coincides with the conclusion of Avgul, Kiselev, and Lygina (6), who compared benzene and pyridine adsorption on graphitized T I thermal carbon black. They treated their data by the B E T method and concluded that both these ring compounds adsorbed with the ring parallel to the solid surface. The most familiar platelike organic molecule is benzene. Experiments with StuartBriegleb molecular models show that there is a large difference between the area per molecule depending on whether the 6-fold axis is perpendicular or parallel to the adsorbing surface. Several authors have used the B E T method to make careful comparisons of the cross-sectional areas of benzene and nitrogen, using graphitized carbon adsorbents with
313
TABLE I RATIOS OF CROss-SECTIONAL AREAS ON ]LIoMOGENEOIJS GRAPHITE SURFACES,
C~H6 vs. N~ Ratio
Carbon
P-33 Sterling MT Avg. of several MTg Graphon Spheron 6 (= Graphon?)
2.34 2.36 or 2.44 2.47 2.48 2.53 2.87
Reference
7 8; 5 9 3 2 10
well-characterized surfaces. Their results are summarized in Table I. I t is possible to define two-dimensional figures whose close packing will indieate the area which must be available for each molecule. Packings of such figures are shown in Fig. 1 (6-fold axis perpendicular) and Fig. 2 (parallel). Then, the areas can be converted to ratios of C6H6 to N2 areas, comparable to the experimental ratios given in Table I. If we take ~N2 as 13.S-16.2 A 2 molecule, the ratio is 2.41-2.05 if Call6 is fiat and close packed, or 1.65-1.41 if C6H6 is edgewise and close packed. These results clearly support the idea that the 6-fold axis is perpendicular to the surface. Pierce and Ewing (3, 5) have recently accepted the idea that the benzene moleeules sit on sites on the graphite and show that if this is the ease, the 6-fold axis must be perpendicular to the surface, since the effective molecular area is of the order of 47 A 2. However, Ross and Olivier (1) have arrived at the opposite conclusion. The Ross-Olivier treatment of nonlocalized adsorption employs the two-dimensional vau der Waals equation to describe the adsorption process. On the assumption of a homotatic adsorbent the equation actually used to describe the experimental isotherm is
0
p = K~exp
( 0
1 - 0
~T~]"
[1]
Here a and ;3 are the two-dimensional analogs of the three-dimensional van der Waals constants a and b. Ross and Olivier show that the ratio 2 a / ~ represents the lateral interaction between adsorbate molecules. K is a constant taking into account the adJournal of Colloid and Interface Science. Vo]. 30, No. 3, July 1969
314
LIVINGSTON AND SENKUS
FIG. 1. Two-dimensional close packing of areas obtained by projection of Stuart-Briegleb models of benzene against a parallel surface. A regular hexagon 6.20 A on a side contains the equivalent of three such "molecules," using the Stuart-Briegleb scale factor.
FIG. 2. Two-dimensional close packing of areas obtained by projection of Stuart-Briegleb models of benzene against a perpendicular surface. The indicated irregular hexagon contains the equivalent of three such molecules. s o r b a t e - a d s o r b e n t i n t e r a c t i o n ; 0 is t h e fract i o n of t h e surface covered; k is t h e B o l t z m a n n c o n s t a n t ; a n d T is t h e a b s o l u t e temperature. Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
For tains value These
e x p e r i m e n t a l use this e q u a t i o n cont h r e e u n k n o w n s : 2a/5, K, and t h e for t h e surface a r e a of t h e a d s o r b e n t . u n k n o w n s are d e t e r m i n e d b y p l o t t i n g
ORIENTATION OF ADSORBED FURAN MOLECULES the equation in its rectilinear form: W -- In
(o0)
- lnp+
0
1 - 0
[2] 2~0
-
in K.
From a plot of W vs. 0, the surface area V~ is determined by varying the value V¢ until one finds the smallest least-squares deviation from a straight line in the above plot (V~ = V/O). There has been, however, one factor which has not been accounted for in this treatment, and that is surface heterogeneity. The Ross and Olivier adsorption theory takes into account the surface heterogeneity of the adsorbent. Their model is a surface made up of an infinite number of patches of varying degrees of energy with a Gaussian distribution. Their equation is 0 = 1_ f 0~- exp [--~,(U n
1
U') 2] dU,
[3]
where n is a normalizing constant, U' is the average adsorptive energy about which the Gaussian distribution is centered, ~/ determines the width of the distribution, U is the energy of adsorption of the ith patch, and 0~ is derived from the equation of state pertaining to each patch. Ross and Olivier have calculated theoretical isotherms for the above equation for various values of 2a/¢~ and ~. The value of 2a/3 obtained previously for the homotatie surface may be used as an approximation for the 2a/~ for the heterogeneous surface. Then using this value of 2a/¢~ one superimposes the experimental adsorption curve over theoretical curves for various values of ~/. When a fit is found ~, is known, and in addition K and Va may be determined. If the adsorbed molecules are not oriented or polarized by the substrate one may obtain ideal two-dimensional van der Waals constants from their three-dimensional counterparts by means of the following equations: ( 9 7 r ~ ~/3
~~
=
a \2-g~/
;
~i~ = 2 b \ ~ /
[4] .
315
Here a and b are the three-dimensional van der Waals constants. Using this approach, Ross and Olivier consider free rotation and fixed molecules, with and without surface electric fields, with axis parallel or perpendicular to the surfaee. They eonclude that the a, ¢~, and polarizability data support the conclusion t h a t the orientatioil of benzene molecules is one of free rotation with the 6-fold axis parallel to the carbon black surfaee. Free rotation of adsorbed moleeules is a critical concept. It was unimportant in the early BET consideration with molecules that were perfectly spherically symmetrical (e.g., argon), or nearly so (e.g., nitrogen). For such molecules, the liquid or solid density could be used to ealeulate the eross-sectional area of a molecule. That this is not the ease for benzene can be recognized by considering the crystallographic data for benzene. As determined by Cox, Cruiekshank, and Smith (11), the CGH6unit cell at - 3 ° C consists of 4 molecules fitting together like two pairs of beveled 6-toothed gears. A given pair are close fitting with axes of the "gears" at 90°22 '. The closest approach puts hydrogens within 2.64 and 2.78A of each other. It is conceivable that in molecular rotations originating in the thermal energy of the crystal the molecules in a "corrugated sheet" of such 90 ° pairs could rotate together as a cluster of "intermeshed gears." The other pair of molecules in a unit cell are also intermeshed and have axes at 90 °. However, between pairs there is no intermeshing. The large eoe~cient of thermal expansion of C6H6 crystals along the c axis results from the fact that intermeshing is in the ab plane only, so expansion along the c axis involves much less energy. These data show how unreliable it is to predict two-dimensional packings from solid densities. Various types of graphitized carbon have been frequently used for adsorption studies of many adsorbates. The advantage of graphitized earbon is that the surface is one of the most homoenergetie ones available for study. In our experiments, Graphon (also known as Spheron 6) was used. The fact that Karnaukhov, t{iselev, and Khrapova (10) obtained a different result from that of Smith, Pierce, and Cordes (2) for benzene Journal of Colloid and Interface Science, Vol. 30, No., 3 July 1969
316
LIVINGSTON AND SENKUS
adsorption on this carbon black (see Table I) m a y reflect international confusion in labeling• All work in the United States on Graphon seems to have given consistent results. The Graphon was purchased from Cabot Corporation, Boston, Massachusetts. The manufacturer gives a value of 87.4 m2/gm for the surface area of the Graphon. Our own measurements, based on a value of 16.2 A 2 for the effective area of a rlitrogen mo]ecule, give 84.7 m2/gm as the surface area of Graphon.
II. EXPERIMENTAL
45
2 ••.."
4o
.°.-" °-
~ 35 ~=
, ,,,..•' .,• •,..'"'"•
30
~< •° cr 2 5 ill
o-
The general setup of the apparatus is essentially that of MeBain and Bakr (12). Our only modification has been to place the liquid furan, the temperature of which is controlled in order to control the furan pressure over the adsorbate, in a side arm. The Graphon
Z <
cc 20 2 15- . 10
f
J
"
Po=6OOmmHg T = 25°C
.
5-
45 01 Z 01"
40
35
02
03
04
05
06
%
J
FIG. 4. The data of Fig. 5 replotted as furan adsorption as a function of the partial pressure of furan.
30 rr uJ Q_ r~ 2 5 uJ m tw O
.."
~2o < Z
(.9 10
2'
.•"
5 ,, . . " •
I
I
I
-70 -50 -30 TEMPERATURE °C
I -10
I
+10
+30
To=25°C
Fro. 3. Experimental data for the adsorption of furan on Graphon as a function of the temperature of the liquid (or solid) furan in equilibrium with the Graphon. Journal of Cdloid a~d Interface Science, ¥ o l . 30, N o . 3, J u l y 1969
was hung in a platinum bucket from a 2-gm capacity quartz spring obtained from Mieroehemieal Specialties Co., Berkeley, California. The Graphon was degassed for at least 12 hours at 200°C and at 5 × 10.4 torr. After degassing the Graphon was allowed to cool to 25 ~ 0.2°C and was kept at this t e m p e r a t u r e during the run. The furan used was "ehromatoquality reagent" grade obtained from Matheson, Colemen, and Bell. I t was used with no further purifieation. The liquid furan was cooled to dry ice temperature and allowed to w a r m up slowly as adsorption took place• The warming from dry ice temperature to 0°C took approximately 10 hours. The temperature change was followed with an ironeonstantan thermoeouple and potentiometer. The spring extension was followed with a eathetometer.
ORIENTATION OF ADSORBED FURAN MOLECULES
317
2
E)
1--
0.9-0.8-0.7-0-6-0.5-0.4--
0,3--
0,2--
0.1
I
I
I
I
I
I
10
20
30
40
50
60
TI M E ( M in.)
TEMPERATURE=25°C
FIG. 5. Adsorption as a function of time for furan olt Graphon at 25°C. and a maximum 0 of 1.3. III. RESULTS Figure 3 gives the raw data in the form of adsorption vs. furan temperature. The more familiar adsorption isotherm of furan oil Graphon is Fig. 4. The results gave a B E T monolayer value of 27.2 mg furan per gram Graphon. This corresponds to a ratio of furan area to nitrogen area of 2.18. Some difficulty was encountered in these adsorption runs in attempting to obtain reproducible data. Some of the runs indicated a greater amount adsorbed than the one shown, especially from p / p o = 0.1 to p / p o = 0.4. As much as 2 mg greater adsorption was obtained in this interval. Several of the runs indicated "steps" at low pressures. So far no satisfactory explanation has been found for this behavior. Desorption immediately after adsorption or 24 hours after adsorption showed no hysteresis. Adsorption was fairly rapid, as shown by a typical rate curve (Fig. 5). The data are plotted for a single run; a duplicate rate measurement was in good agreement. Since 10 hours elapsed during the measurement of the adsorption isotherms (as in Figs. 3 and 4), the isotherms represent equilibrium measurements. The only previous work on the adsorption
of furan to be published is described by Fejes, Kiraly, and Schay (13), who adsorbed furan oll aluminum oxides. They obtained a ratio of furan area to nitrogen area of 2.47. Models for the orientation furan molecules were made by the same methods that had been used for benzene (see Figs. 1 and 2). The figures representing the area occupied by furan molecules were more irregular than for benzene, but as Kitaigorodskii has shown (14) closest packing in two dimensions always requires six nearest neighbors. Packings are reproduced in Figs. 6 and 7. The areas indicated for the models correspond to 17.8 A2/molecule if the furan pentagons are close packed on edge on the surface and 29.0 AS/molecule if they are flat. If the nitrogen area is taken as being in the range 13.8-16.2 A2/molecule, this gives a furan nitrogen ratio of 1.29-1.10/1 in the first ease and 2.10-1.79/1 in the second. Since our own work indicates that the ratio of the B E T area of furan to the B E T area of nitrogen is 2.18/1 on Graphon, the B E T method definitely indicates that the molecules are lying flat on the surface. When Hill (15) originally proposed the use of a two-dimensional van der Waals equation Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
318
LIVINGSTON AND SENKUS
]PIG. 6. The closest two-dimensional packing found for areas obtained by projection of Stuart-Briegleb models of furan against a paralle] surface.
FIG. 7. The closest two-dimensional packing found for areas obtained by projection of Stuart-Briegleb models of furan against a perpendicular surface. to fit adsorption data, he recognized the imp o r t a n c e of the two-dimensionM v a n der Waals constants a and ~. I n a later publication (16) Hill showed t h a t a and/~ could be evaluated b y the equations mentioned above Journal of Colloid and lnterfoce Science, Vo]. 30, No. 3, July 1969
(see Eq. [4]) and pointed out t h a t fl h a d a value v e r y near t h a t of the molecular crosssectional areas being used at t h a t time in B E T adsorptior~ studies. Subsequently orte of us (17) evaluated 5 for a n u m b e r of adsorb-
ORIENTATION OF ADSORBED FURAN MOLECULES
319
0.@
O.5
0.4 @ 0.3 SOLID CURVE THEORETICAL DOTS EXPERIMENTAL
Q2
O3
I
I
I
1
10 20 30
I
I
40 50 60
I
1
70 80
I
I
1
I
I
I
I
1
I
I
I
90 100 110 120 130 140 150 160 170 180 190
PRESSURE mm Hg
TEMPE RATURE=25°C
Fro. 8. The experimental adsorption for furan on Graphon compared with the "best fit" isotherm obtained from the theory of Ross and Olivier. ents including benzene. I t is of interest that 33.0 A2/moleeule, obtained for benzene, corresponds almost exactly to the area obrained from our model studies with the flat benzene orientation. I n the case of furan the experimental three-dimensional van der Waals constants could not be found in the literature. They were calculated from critical data (18) as a = 7.5 arm liter 2 mole-2 and b = 72.7 em "3 mole-1. This resulted in a id = 2.04 X 10-2s erg em 2 and flid = 23.4 X 10-16 cm ~. Thus for furan is precisely the average of the two furan areas calculated above from model studies, suggesting that furan molecules can assume either orientation. I t will be worth noting that one must concern himself with dimensions in caiculating a and fl, the two-dimensional van der Waals constants. Equation [4] is valid if the three-dimensional van der Waals equation is used in the form
(p + a N 2 n 2 / V 2 ) ( V / n N -
b) = R T ,
[5],
where n is the number of moles and N is Avogadro's number. I n such a case if p is in atmospheres and V is in liters, a has the units arm liter 2 and cm 3 and b has the units of liters. I t is then possible to convert a and b to erg em 3, and em a respectively, and obtain directly a in erg em 2 and ~ in em 2 for the molecule of interest. Tabulations such as
those given by DeBoer (19(a)) and by Ross and Olivier (l(a)) are misleading in their implication that a and b in molar units can be raised to the two-thirds power to yield, with suR~ble factors, a and ¢/. Ross and Olivier believe that benzene on graphitized carbon black is oriented with the benzene hexagons perpendicular to the solid surface. I t is desirable to apply their treatment to our furan data. With reference to the basic Ross-Olivier equation [2], which assumes a completely homotatic surface, V~ = 48.2 m g / g m in the case of furan on Graphon. At this value of V~, 2 a / ~ and K are determined from the slope and intercept of the graph. I n our ease 2a/~ = 1.60 kcal/ mole and K = 0.103 atm. Figure 8 shows both the experimental isotherm for furan on Graphon and the theoretical one calculated with these values of the constants. When our data were applied to Eq. [3], which assumes a heterogeneous surface, v = 1000 and the values for K and Ve were the same as those obtained for the assumed homotatic surface. This is as it should be, for according to Ross and Olivier a surface with v = 1000 is hardly to be distinguished from a completely homotatic surface (V = m). One may determine the v at 77.5°K, which Ross and Olivier use as a standard, or at any other temperature b y means of the following equation Journal of Colloid and Interface Science, VoI. 30, No. 3, July 1969
320
LIVINGSTON AND SENKUS 7T = q'vz.~ ~ 7 ~ . 5 ~ 2 •
For Graphon this gives a "Y77.5 value of 68. A comparison of the adsorptions for a monolayer (0 = 1) as indicated by the Brunauer-Emmett-Teller theory (V~) and the Ross-Olivier theory (V~) is given in Table II. The differences are not systematic and reflect the fact t h a t the two theories are completely different in their postulates and approaches. I t is not advisable to a t t e m p t to use V~ to determine the speeifie surface of solids. If one is interested in the ratio between the cross-sectional areas of adsorbed molecules, it is B t h a t should be compared, not V~ (see Hill (16)). With our v a n der Waals data indicating t h a t 2aid/~ ie = 2.49 keal/mole it is possible to compare several models for the orientation of the adsorbed molecules to see which model corresponds most closely to the experimental data. IV. FREE ROTATION AND INSIGNIFICANT SURFACE ELECTRIC FIELD MODEL This model implies t h a t the adsorbed molecule is essentially in the same condition as in the gas phase; hence the value of 2a/fl observed should equal the value of 2aid/2ie (2.49 keal/mole). But 2a/fl = 1.60 keal/ mole. The observed value is only 64 % of the ideal. Thus this model must be rejected. V. FREE ROTATION AND LARGE SURFACE ELECTRIC FIELD MODEL I n this ease one might assume t h a t the decrease in the value of 2 a / ~ as compared to the ideal value is entirely due to a lowering in a caused b y the mutual repulsion of dipoles induced b y the surface electric field. The value of ~ should remain unchanged. DeBoer (19) has derived an expression for the reduction of the two-dimensional v a n der Waals constant a resulting from this effect:
x=-a;
~
[7]
where X is the change in a, a ~ is the total effective dipole moment of the adsorbed molecules normal to the surface, and d °~ is the diameter of the oriented molecule as Journal of Colloid and Interface Science,
TABLE
[6]
Vol. 30, No. 3, July 1969
Furan (25°C) Benzene (28.6°C) Nitrogen (-182.8oc)
II
v~
g,m (BET)
(R - - O)
27.2 6.5 16.6
48.2 11.6 19.6
Reference
This work 2 20
calculated from ¢~ = 7r (d°r)2/2. In the case of furan d °r = 3.86 X 10-s em and a = 1.30 X 10-~s erg emL This results in a calculated value of 0.947D for the total effective dipole m o m e n t of the molecule. Using the literature value for the dieleetric constant and dipole moment one m a y calculate the polarizability of furan from the equation
e--lM_4~rN[ ~+2
p
~--
tL~; ~ + ~
,
[8]
where e is the dielectric constant, 3 t the molecular weight, p the density, N Avogadro's number, ~ the polarizability, and the dipole moment. Using values of 0.72D for the dipole m o m e n t (21) and 2.95 for the dielectric constant one obtains ~ = 7.14 X 10-24 em 3. Knowing the induced dipole moment and polarizability of furan one m a y calculate the strength of the surface electric field needed to produce such an induced dipole moment from ~i~e = F~, where F is the surface electric field. The field needed is 1.33 X 105 esu-cm-2. But this is a contradiction since it is easy to show t h a t a surface electric field of such magnitude will effectively prevent the molecules from rotating. The fraction of the molecules able to rotate is given b y exp (-2t~F//cT). One finds t h a t at 25°C only 0.2 % of the adsorbed molecules is able to rotate. Therefore this model cannot be considered applicable. VI. NEGLIGIBLE SURFACE ELECTRIC FIELD MODEL Since w e assume t h a t there is no surface electric field the lowering of a id should be caused b y the alignment of the permanent dipoles of furan plus the anisotropy of the polarizability. I n order to investigate this model fully
ORIENTATION OF ADSORBED FURAN MOLECULES one would need to know the polarizability of furan in each of the three directions. This d a t u m could not be found in the literature so an approximation has been made. I t has been assumed that the polarizabilities of furan are in the same ratios as the polarizabilities of pyridine. Ross and Olivier (1) note that the ratios (1: ~e2:~a are 11.88: 5.78:10.84, where the subscripts 2 and 3 refer to polarizability perpendicular to the plane of the ring and along the axis through the nitrogen atom, respectively. This approximation results in the following values for the polarizabilities of furan: 8.1 X 10-0"4 cm a in the direction of the C2 axis, 4.3 X 10-24 em a perpendicular to the plane of the molecule, and 8.9 X 10-2~ em a in the remaining direction. First let us assume that the adsorbed furan is oriented with the oxygen pointing toward the Graphon surface. Once again we set = 5i~ since the spinning molecule should have more or less the same effective diameter as the free molecule. The average polarizability of the adsorbed furan is 1/{ (4.3 q8.9) X 10-24 = 6.6 X 10-2~ cm a. The correction of oe'd for the effect of orientation may be made by means of a factor defined a s co = ( d 4 / ~ 2 ) i d ( ~ 2 / d 4 ) ° p , where the superscripts i d and o p refer to the ideal and operative two-dimensional values of d. Then a = con% For this model d °~ = d fd, resulting in co = 0.86. Since we have assumed a negligible electric field, we solve Eq. [7]. The result is X = - 0 . 4 1 X 10-28 erg-em2. Then substituting these values of co and X into the equation a = coc~ie q- X, where conid corrects for orientation of the adsorbed molecules and X for parallel dipole repulsioa, one obtains a = 1.33 X 10-28 erg-cme. If we assume a negligible surface field but with furan adsorbed fiat, then in this case there is no permanent dipole oriented toward the surface so the decrease in a nmst be due to the factor co defined above. =
(~.~)o~/(~)i~ =
(8.5 ×
10-~)~/
(7.1 X 10--°4)5 = 1.4. Then a = con'e = (1.4) (2.04 X 10-~s) = 2.90 X 10-~s. But experimentally a = 1.30 X 10-~s so this model is rejected.
321
The third orientation possible with this model is that of furan adsorbed with a double bond toward the surface. I n this case co = 0.77, which would make the value for predicted for this model 1.55 X 10.58 ergem ~. This value is reasonably dose to the experimental value of 1.30 X 10-28. I t would appear that of the models investigated so far the ones corresponding most closely to experimental data are those with the plane of the molecule perpendicular to the adsorbing surface. VII. ORIENTED MOLECULES WITIcI A SURFACE ELECTRIC FIELD First let us investigate the model of vertically adsorbed furan with the oxygen oriented toward the surface. We will assume in this and the following cases that ¢~ = 5% Previously co was calculated to be 0.86 for furaa oriented this way. Then ), = a-conic=-0.45 X 10-28 erg cm 2. We can then find the total effective dipole moment from Eq. [7]. With the use of the value for X, the total effective dipole tooment is found to be 0.75 D. This dipole moment is a sum of the induced dipole moment and the component of the permaneat dipole oriented towards the surface. The component t* of the permanent dipole in the direction of the electric field may be
found by means of the Langevin function: #=
u [ c o t h ttF
kT]
The induced dipole moment, ~ d , is given by ~,.d = F ~ , where Fi is the electric field and (~ the polarizability in that direction. After a little trial and error one may find the strength of the electric field needed to give a total effective dipole moment of 0.75D. If the field is 0.62 X 10 '~ esu-cm-2, = 0.24D and ~,,d = 0.51D. The value which Ross and Olivier obtained for the surface field of P-33 (2700 °) is 1.12 × 10 '~ esu-cm--~. The value just obtained for the surface field of Graphon is sufficiently close to the P-33 value so that this orientation cannot be disregarded. The second orientation possible with this model is that of furan adsorbed flat. If = t5id then co = 1.4, X = a -- codd = Journal of Colloid o,nd Interface Sc~'ence, VoI. 30, No. 3, July 1969
322
LIVINGSTON AND SENKUS
--1.6 X 10-2s erg-cm 2. Then from Eq. [7] we find t h a t a" = l A D . Since the furan is fiat it has no permanent dipole moment in the direction of the surface so the entire effective dipole moment must be induced. This would require an electric field of 3.2 X 105 esu - c m -2, which is roughly three times the value for the surface field of P-33 (2700°). In a similar manner to that above one may calculate the surface field for the model of furan adsorbed on edge with a double bond pointing downwards. The result is a value for the surface field of 0.63 esu-cm -2. Thus if it is assumed that the surface field of Graphon is roughly the same as that of P-33 (2700°), once again the orientation of adsorbed furan that agrees most closely with experiment is that with the plane of the molecules perpendicular to the surface.
VIII. CONCLUSIONS A great many approximations had to be made in order to carry out the calculations originating from the Ross-Olivier theory. Because of this none of the calculations " p r o v e s " any single model. We must recognize t h a t the ratio of the B E T molecular area for furan to t h a t for nitrogen suggests a flat orientation, just as the B E T data do for benzene. The v a n der Waals constant lies halfway between the "fiat" and "erect" values for cross-sectional area. The Ross-01ivier t r e a t m e n t indicates the plane of the ring is perpendicular to the surface. Further experimental work must be done to obtain more information if one is to establish any model as representing the actual orientation of adsorbed furan on Graphon. One method of experimentation would be to study adsorption polymerization. For a molecule such as furan, this almost certainly involves addition polymerization at the double bond. I n preliminary experiments we have determined t h a t furan molecules t h a t are physically adsorbed (i.e., readily desorbable) on Graphon become fixed on the solid surface b y irradiation with highenergy electrons. The change from desorbable furan to fixed furan m a y involve polymerization. The effect of variations in surface concentration of furan on polymerization kinetics and polymer structure will Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969
depend on the mobility and orientation of the adsorbed furan molecules. I t should therefore be possible to choose between various possibilities based on the results of adsorption polymerization studies. ACKNOWLEDGMENT The authors are pleased to acknowledge the financial support of the U.S. Atomic Energy Commission. REFERENCES 1. Ross, S., AND OLIVIE~, J. P., "On Physical Adsorption." Interseienee, New York, 1964; (a) ibid., pp. 172, 173. 2. SMITH, I~. N., PIERCE, C., AND CORDES, H., J. Am. Chem. Soc. 72, 5595 (1950). 3. PIERCE, C., AND EWING, B., J. Phys. Chem.
68, 2562 (1964). 4. McCLELLAN, A. L., AND HARNSBERGER,H. F., J. Colloid and Interface Sci. 23,577 (1967). 5. PIERCE, C., AND EWING, B., J. Phys. Chem.
71, 3408 (1967). 6. AVGUL, N. N., KISELEV, A. V., AND LYGINA, I. A., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk 1962, 32. 7. PIEROTTI, R. A., AND SMALL~VOOD,R. E., J. Colloid and Interface Sci. 22, 469 (1966). 8. DAvis, B. W., ANDPIERCE, C., J. Phys. Chem.
70, 1051 (1966). 9. ISIRIKYAN, A. A., AND KISiELEV, A. V., J. Phys. Chem. 65,601 (1961). 10. KARNAUKHOV, A. P., KISELEV, A. V., AND KgRAPov~, E. V., Dokl. Akad. Nauk SSSR,
92,361 (1593). 11. Cox, E. G., CI~UICKSHANK, D. W. J., AND SMITH, J. A. S., Prec. Roy. See. (London) A247, 1 (1958). 12. MCBAIN, J. W., AND BAKR, A. M., J. A m . Chem. Soc. 48,690 (1926). 13. FEJES, P., KIRALY, J., AND SCHAY, G. Z. Anorg. Allgem. Chem. 300, 72 (1959).
14. KITAIGORODSKII, A., "Organic Chemical Crystallography." Consultants Bureau, New York, (1961). 15. HILL, T. L., J. Chem. Phys. 14,441 (1946). 16. HILL, W. L., J. Chem. Phys. 16, 1811 (1948). 17. LIVINGSTON, i . K., J. Colloid Sci. 4, 447 (1949). 18. KOBE, K. A., Ind. Eng. Chem. Data Set. 1, No. 1, 50 (1956). 19. EEBoER, J. H., "The Dynamical Character of Adsorption." Clarendon Press, Oxford (1953); (a) ibid., p. 148. 20. JOYNER, L. G., AND EMMETT, P. I-I., J. Am. Chem. Soc. 70, 2353 (1948). 21. HA~IS, B., J. Chem. Soc. 1953, 1622.