The adsorption of benzene on homogeneous substrates

3OURNAL OF COLLOID AND INTERFACE SCIENCE 9-2, 4 6 9 - 4 8 1

(1966)

The Adsorption of Benzene on Homogeneous SubstratesI R. A. P I E R O T T I XND R. E. SMALLWOOD

School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 303S2 Received February 3, 1966 Adsorption isotherms for benzene adsorbed on P33 (2700°) and on hexagonal boron nitride are reported at five temperatures in the range 0°C. to 50°C. The data span the submonolayer and multilayer regions. Isosteric heats of adsorption and equilibrium heats and entropies of adsorption are presented as a function of surface coverage. An estimate of the heat capacity change on adsorption is given. The data are discussed in relationship to previously reported data. The variation of the lateral interactions with respect to the adsorption potential is discussed. INTRODUCTION During the past twenty years, much has been written concerning the physical adsorption of gases on solids (1). Statistical thermodynamic treatments of both mobile and localized layers have been developed for a number of specific models. Attempts to apply the models to experimental systems have usually been indecisive as to which of several models is most appropriate (1). Until the production of the highly graphitized carbon blacks, it was clear that heterogeneity accounted for most of the difficulty in comparing theory and experiment (2). Even now, however, it is almost fruitless to expect more than semiquantitarive agreement between theory and experiment unless the theory has one or more adjustable parameters. P a r t of the difficulty stems from the complexity of the phenomena involved and part from the unavailability of adequate data for relatively simple adsorbates adsorbed on well-characterized substrates over a range of temperatures. The most extensively studied homogeneous adsorbent has been the graphitized carbon black designated P33 (2700) (3-5). Most of the work on this solid has been in the submonolayer region, but a notable 1 This work received support from a grant from the Petroleum Research Fund and from grant NsG-657 from the National Aeronautics and Space Administration.

exception to this is t h e investigation by Prenzlow and Halsey o f the multilayer adsorption of argon on P33 (6). I t is desirable to study the adsorption of gases on other homogeneous substrates. A substance of particular interest along this line is the hexagonal modification of boron nitride. Graphite and B N are isoelectronic and structurally very similar. Previous adsorption studies on this solid indicate that the surface is quite homogeneous and that the dispersion interaction of a particular gas with graphite is somewhat larger than with B N (5, 7). This smaller interaction energy coupled with the structural similarity of the two solids makes it possible to investigate the effect of the adsorption potential on the properties of the adsorbed layers. The present paper presents data for the adsorption of benzene on P33 (2700) and on hexagonal BN. The data include both monolayer and multilayer adsorption on these solids at five temperatures in the range 0°C. to 50°C. The thermodynamic properties of the adsorbed layers are calculated and presented as a function of the amount adsorbed. EXPERIMENTAL THB ADSORPTION SYSTEM A Calm R G Automatic Eleetrobalance (Cahn Instrument Co., Paramount, California) was used to weigh directly the

469

470

PIEROTTI AND SMALLWOOD

amount of benzene adsorbed by the solid, The balance was used inside a Calm Glass Vacuum Bottle. The ground-glass seal of this container was lightly greased with Apiezon T and then the outer ring of the seal was covered with Apiezon W wax. This seal did not develop leaks under the normal operating conditions of this work. The output of the balance was displayed on a 1 my. recorder. Only one range of the balance was used--the 0-10 rag. mass range. With this arrangement, a mass measurement could be made to 4-0.001 rag. throughout the 0-10 rag. range. Zero drift and other noise resulted in an uncertainty of :t:0.005 rag. per reading. The balance and recorder were calibrated in accordance with the Cahn instrument manual. The adsorbent was contained in a thinwalled glass bucket which was suspended from the balance with 0.002 inch diameter Nichrome wire. The samples were lightly covered with a layer of glass wool. The total weight of the bucket and sample was made up as follows: 0.5 g. sample, 0.3 g. bucket, and 0.1 g. g]ass woo]. Blank isotherms were determined on the bucket and glass wool. Low-pressure measurements were made with a Granville-Phillips Series 212 Model B capacitance manometer. This manometer was used as a direcVreading instrument and was calibrated against a precision triple range McLeod gauge which was read with a Gaertner M-911 cathetometer. The McLeod gauge was constructed and calibrated in these laboratories and also compared with a calibrated CVC McLeod gauge. Above 1 torr the capacitance manometer was used as a null instrument and the benzene pressure in the adsorption system was balanced against air in the McLeod gauge. The pressure was then read directly with the McLeod gauge. Above 9.6 torr, the pressure was measured using a U-tube manometer (16 ram. tubing) in conjunction with the capacitance manometer which was used as a null instrument. At no time was the mercury in the manometers exposed to the benzene or the adsorption system exposed to mercury vapor. The U-tube manometer was read with a Gaertner M-911 cathetometer. The pressure measurements were accurate to better than 1%

throughout the entire range, of pressures measured. Benzene vapor entered the adsorption system through a 4 ram. vacuum stopcock (lightly greased with Apiezon T) from a benzene reservoir. After a dose of benzene was introduced the stopcock was closed and the system allowed to equilibrate. The approach to equilibrium was followed directly by observing the recorder. Equilibrium was usually achieved quickly (within 20 min.). Desorption was carried out by first placing a cold bath around the benzene reservoir and then opening the stopcock to the system. Desorption was also found to be quite rapid. The balance hangdown tube (a goldplated 25 ram. glass tube) containing the adsorbent was immersed in a constant-temperature water bath which could be both heated and cooled as required. The bath was controlled to =t=0.05°C. at all times with a Yellow Springs Thermistor Controller Model 71. A magnetic stirrer was used to agitate the bath. An immersion heater was used to heat the bath. Cold methanol circulated through copper coiks immersed in the bath was used for cooling. Both the heater and the circulating pump for the methanol system were operated by the Thermistor Controller. The zero degree isotherms were determined using an ice-water slush bath for temperature control. MATERIALS

The graphite used in this work was obtained from Dr. Carl Prenzlow. It was prepared by Dr. W. Smith of Godfrey Cabot Co. and designated Sterling FT (2700°). It is the same as P-33 (2700°). Boron nitride powder, 325 mesh, was obtained from the Carborundum Co. Spectrographic analysis showed this sample to be greater than 99.92% pure. X-ray diffraction showed it to be highly crystalline, having a crystal structure very similar to that of graphite (7). This sample came from the same lot as the sample used by Pierotti for other studies (7). Baker analyzed reagent grade (thiophenefree) benzene was used. Vapor phase chromatography indicated that the benzene used was greater than 99.98 % pure. The

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES 5 -

-

T

T........

I

I . . . .

,

471

T

Benzene on G r a p h i t e

0,00"

O

IEl.O O"

0

5

W~ mqm gm

2 28,00" ~0.00 C

5o.oo" o

0.2

0,4

0.6 P

0,8

1.0

1.2

1.4

mm of Hg

FIG. 1. Isotherms for the adsorption of benzene on graphite in the low-pressure region.

impurity was probably toluene. The benzene was distilled into the adsorption system and then further purified b y three separate vacuum sublimations in which the first distillate and the residue were removed.

OUTGASSING PROCEDUI~E The adsorbent was outgassed at least 8 hours at 510 ° ± 20°C. under high vacuum conditions (10-6 torr or better). Graphite was easily out gassed by this method, and a new sample of graphite lost about 3 rag. per gram of sample during such treatment. A sample of B N previously exposed to air was much more difficult to outgas. A fresh sample of B N lost around 15 rag. per gram of sample when heated for 24 hours at 525°C.

and about 1 rag. more after heating for an additional two days. This treatment was necessary in order to obtain reproducible results. Even a very small quantity of air leakage was enough to necessitate the above treatment. Boron nitride and graphite were outgassed overnight at 510°C. after exposure to benzene. RESULTS

BENZENE-GRAPHITE SYSTEM Figures 1, 2, and 3 display the isotherms for benzene adsorbed on graphite at various temperatures. Each isotherm is a composite of many individual runs and contains hundreds of points (both adsorption and desorption are included in the same curve since no

472

PIEROTTI AND SMALLWOOD I

Benzene

I

on

I

I

|

I

(i)-

o.oo ° ¢

L2"I-

IS.00

°

C

(3) - 2 6 . 0 0 "

C

(4) - 3 0 . 0 0 "

Graphite

(5)

-

50.00

6

(I) " ~

W.

0

C ° C

2

4

P

6 8 mm of Hg

I0

12

14

Fro. 2. Isotherms for the adsorption of benzene on graphite in the intermediate-pressure region.

hysteresis was observed). The data from p/pO of 0.05 to 0.2 are linearized by the BET equation, and B E T areas based on a benzene cross section of 40 A 2 (8) are given in Table I. The weight of a monolayer wm per gram of sample is also given in Table I along with the BET constant c. It is noteworthy that wm is virtually temperature independent. Isosteric heats of adsorption at 298°K. are given as a function of the fractional surface coverage 0 in Fig. 7; equilibrium heats and entropies of adsorption at 298°K. are given in Figs. 8 and 9 as a function of the weight adsorbed. The isosteric heat is defined by the equation

0 In p \

k ]o°

= - - q , ~ = /ira -- Hg,

[1]

where p is the equilibrium pressure of the adsorbate, T is the absolute temperature, q,t is the isosteric heat for the process of desorption, Hg is the enthalpy of the gas phase (assumed ideal), and /t~ is the differential enthalpy of the adsorbed phase at a coverage corresponding to w~, the weight of the adsorbed phase. In the present work a plot of In p versus 1IT showed a slight curvature indicating a temperature dependence for q,~. We thus can write the integrated form of Eq. [1] assuming q~t to be a linear function of T as

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES

°1 / 0.00" 0

/]

/

15.00 ° C

9-

473

~'5.00° C

30.00 °

6

Wo gm 50.00

Benzene

I

o

;o

I

4o P

mm of

FIG. 3. Isotherms for the adsorption of benzene ]n p = -- q°~/RT

[2]

+ (AC~t/R) In T + C, where q°t, AC~t, and C are constants, and q~t = q°t + TACit, The constants were evaluated using the least squares procedure on a Burroughs B220 computer. The heat capacity change, AC~t = Cg - Ca, is found to be a slight function of the coverage, but is approximately 5.5 4- 0.5 cal./deg.-mole over most of the range. The equilibrium heat (molar integral enthalpy change) AHad~. is defined by the equation

0 R-T]¢

on

C

Graphite

I

I

50

60

70

Hg on

graphite in the high-pressure region.

where ¢ is the spreading pressure of the adsorbed film and H~ is the molar enthalpy of the adsorbed phase at ¢. In order to evaluate these heats it is necessary to determine the spreading pressure as a function of p. This was done as described by Hill (9) using the equation

¢ = RT

F d (ln p),

[41

where F is the number of moles of gas adsorbed per unit surface area of adsorbent. The technique used for evaluating the integral in Eq. [41 was to expand very small portions of the weight adsorbed versus pressure data in a polynomial in p. The constants in the polynomial were determined using

474

PIEROTTI AND SMALLWOOD TABLE I BET PARAMETERSFOR BENZENE ON GRAPHITE AND BN

gas was chosen to be 1 atmosphere pressure in these calculations. BENZENE-BoRoN ~ITRIDE

wm (reg./gin.)

0°C. 15°C. 25°C. 30°C. 50°C. Average 0°C. 15°C. 25°C. 30°C. 50°C. Average

area a (m2/g.)

Graphite 3.80 3.78 3.76 3.76 3.74 3.77

11.8 11.7 11.6 11.6 11.6 11.7c

Boron Nitride 6.18 19.1 6.12 18.9 5.83 18.0 5.73 17.7 5.60 17.3 5.93 18.3~

cb

153 132 116 93 72

12.7 12.8 12.9 12.2 10.8

° Based upon benzene cross section of 40 A~ (ref. 8). b C, the energy parameter in the BET equation. c The area of this black has been estimated to be 10 =t= 2 m.2/g, by Halsey et al. (3); Stepheight measurement of Ar yields 12.5 (6); BET areas using N~ are about 12.2 m.2/g,; Kiselev (10) reports the BET area for FT (2800) as 12.2 m.2/g. using benzene. The BET v~ of this sample has been found to be quite temperature dependent for a number of gases including argon, nitrogen, and krypton. Areas from about 18 to 23 m.~/g, have been found depending upon the temperature. the computer. The small portions were chosen so that they overlapped one another. The first portion included the origin and a large number of very low coverage points. Once the coefficients of the polynomials were obtained, the value of the integral was computed. The above method was used to determine spreading pressures at each ternperature. After having obtained curves of ¢ versus w~ at various temperatures, the equilibrium heats were evaluated in the same manner as described for the isosteric heat except at constant ¢. The molar integral heat capacity change was found to be approximately 5.2 ± 0.5 cal./mole-deg. The molar integral entropy change ASad,. was calculated from the equilibrium heat in the usual manner. The standard state of the

SYSTEM Figures 4, 5, and 6 display the isotherms for benzene adsorbed on boron nitride at various temperatures. These data are also a composite of m a n y adsorption-desorption isotherms. No hysteresis was observed when the B N sample was outgassed as discussed earlier. The B E T theory linearizes these data from p/pO of 0.5 to about 0.15. B E T areas are reported in Table I. The thermodynamic quantities q,t, AHad~., and AS,~,. at 298°K. are shown as a function of coverage in Figs. 7, 8 and 9. These quantities were computed as mentioned above. The heat capacity changes are equal to those reported for the graphite system to within the uncertainties given there.

ERRORS The pressures were determined to within 0.5 % throughout the entire range above 1 torr. Below this pressure the error was q-0.002 torr. The weight adsorbed has an error of q-0.005 mg./g, throughout the entire range. The temperature was maintained to within 0.05°C. at each temperature. These errors give rise to errors in the isosterie heats for the graphite system of approximately q-800 cal./mole at 0 = 0.i; ±300 cal./mole at 0 = 0.25; ±175 cal./ mole at 0 -- 0.4; ±125 cal./mole at 0 -0.5; ±75 cal./mole at 0 -- 0.7; and -~50 cal./mole at 0 = 1 and above. For the BN system the errors are approximately ±300 cal./mole at 0 = 0.i; ±I00 cal./mole at 0 = 0.2; and ±50 cal./mole at 0 -- 0.3 and above. Because of the computations involved in determining the equilibrium heats, it is difficult to assign uncertainties to the reported values, but we believe them to be approximately the same as the uncertainties in the isosteric heats. DISCUSSION

ISOTHERMS The only isotherm data for benzene on well-characterized homogeneous substrates are those of Isirikyan and Kiselev (10),

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES 2.5

j

Benzene

I

I

I

475

/

on Boron N i t r i d e

2.0

1.5

wa

~

gm

0 0e

c

1.0

0.5

0.2

0.4

0.6 0.8 P mm o f Hg

I.O

1.2

1.4

FIG. 4. Isotherms for the adsorption of benzene on BN in the low-pressure region. Ross and Olivier (5), and Pierce and Davis (11). These references are for benzene adsorbed on graphitized carbon blacks. There are no reported data for benzene adsorbed on boron nitride. The data of Isirikyan and Kiselev (10) are only for one temperature (293°K.) and include data for the adsorption of benzene on several carbon blacks including F T (2800), which is very similar to the black used in this work. Their isotherms fall somewhat higher than ours at a given relative pressure. Ross and Olivier (5) report isotherms for benzene on P-33 (2700) at 0°C., 35°C., and 50°C. The pressure range is from 0 to 10 ram. and the coverage is only in the submonolayer region. Their data appear to be in

fairly good agreement with ours at low coverages, but there is no indication given as to the precision of their data. Pierce and Davis (11) have studied ben: zene adsorption on Sterling M T (3100) at 238°K. and 251°K. Their results at these low temperatures are in qualitative agreement with ours at the higher temperatures. The isotherms on both the graphite and the B N are concave throughout the monolayer region. This fact might lead one to suspect that benzene is localized on these surfaces, but other evidence strongly indicates t h a t the adsorption is nonlocalized. Relative pressure plots of the graphite data indicate that although multilayers form they are not stable at low temperatures--a point first made by Pierce and Davis (11). This

476

PIEROTTI AND SMALLWOOD I01

l

l

I

I

Benzene

on

I

l

Nitride

Boron

0.00 e

(:

6

Wa 15.00 ~ O

30.oo °

r

i

l

I

i

I,

0

2

4

6

8

I0

P

mm

of

Hg

I

12

C

t 14

Fro. 5. Isotherms for the adsorption of benzene on BN in the intermediate-pressure region. latter effect is not observed for benzene on BN. The very low coverage data (below 0 = 0.1) for the B N system appear to have an excessive amomat of curvature and are indicative of some high-energy sites. K r y p t o n adsorption on this same solid shows the same effect. The hot spots are apparently located in patches since they do not prevent a vertical discontinuity due to two-dimensional condensation of the krypton on this surface. This same effect has been observed by Ross (5). H~ATS OF ADSOa~TiON The isosterie heat curves have the characteristics found for the adsorption of other

gases on homogeneous substrates, except for the high initial heats. These high initiM heats were observed by Kiselev and Serdoboy (12) and ascribed by them to the strong preferential adsorption of benzene on edges and cracks. The same effect is observed for the B N system. After the initialhigh heats, the heats rise from a low at about 0 = 0.3 to a maximum at ~ = 0.85 for graphite and 0 = 0.7 for BN. A sharp drop in the heat is observed for the graphite amounting to 2300 cal./mole. This drop occurs between 0 = 0.85 and 1.5. After this drop the heats again rise and go through a maximum corresponding to the filSng of the second layer. Beyond 6 -- 2.2 the heats again fall and approach the heat of vaporization of benzene.

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES

'°l '..l' 0.0

15.00" (

25.00"

G

477

30,0

9

6 Wo qlm

on

Benzene

4

|.

0

I

~

I0

20

I

P

50

mm

of

Boron

Nitride

I

I

I

40 Hg

50

60

YO

FIG. 6. Isotherms for the adsorption of benzene on BN in the high-pressure region. The drop in heats beyond the maximum at 0 = 0.7 for the B N is not so sharp and the structure of the heat curve is washed out beyond this point showing primarily a slow decline towards the heat of vaporization of benzene. The lack of structure for the B N heat curve as compared to the graphite curve is as would be expected considering the lower adsorption potential of BN. This lower potential results in an increase in the distribution of molecules in the higher layers prior to the filling of the lower layers. The only reported isosteric heats for benzene on graphite in the vicinity of room temperature are those calorimetrically determined by Kiselev and Serdobov (12) at 293°K. These heats are presented in Fig. 7.

The precision of their data is reported to be about ± 1 5 0 cal./mole below 0 = 0.8 and about ± 5 0 0 cal./mole above O = 0.8. The agreement between their heats and the present results is excellent considering the different experimental techniques used. Although Kiselev reports virtually zero (at most 300 cal./mole) slope for the q~t versus 8 curve in the submonolayer region, we estimate the slope to be about 700 ~= 150 cal./ mole using Kiselev's data at low coverage and our data at the higher coverage. No previous heats for the adsorption of benzene on B N have been reported. The slope of the isosteric heat vs. coverage curve for the submonolayer region is about 1680 2= 300 cal./mole or more than twice the slope

478

P I E R O T T I A N D SMALLWOOD I

I

|

I

Z98°K • Kiselev

/"

.

o

CH-C

•

CH-BN

/

S

EIO

/

"

.}

v

=

/

}

//

}

ot

I

}i

~[}

I

0.5

1.0

}

}

I

i

1.5

2.0

8 Fro. 7. Isosteric h e a t s of adsorption as a f u n c t i o n of t h e fractional surface coverage.

I

I

l

I

I

I

I

I

I

298 ° K

I0

0

v

"-r"

<,8

I

I

I

I

2

5

,

I

I

4 Wo

I

I

I

I

5 6 (mg/gm)

7

8

9

FIG. 8. Molar integral heats of adsorption as a f u n c t i o n of t h e weight of benzene adsorbed.

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES

2~

E k:

5O

I

2

3

4

5

6

7

8

9

W. (mg/gin) FIG. 9. Molar integral entropies of adsorption as a f u n c t i o n of t h e weight of benzene adsorbed.

for the graphite system. This represents a clear-cut case of the effect of adsorption potential on the lateral interaction. The greater adsorption potential for the graphite system results in a smaller net lateral interaction energy. The zero coverage value of q~ for graphite is estimated to be 10,000 -4- 100 cal./raole, whereas the corresponding value for the BN is 8720 ± 150 cal./mole. The value of q~t at zero coverage for graphite is in agreement with Kiselev's value of 10,200 cal./mole, but is in disagreement with Ross's value of 9250 cM./mole (5). The equilibrium heat curve is found to be quite structureless displaying a monotonic increase (becoming less negative) in the heat with increasing coverage. This increase amounts to about 2000 cM./mole for the graphite and only about 200 cal./mole in the case of BN. ENTROPIES OF ADSORPTIO1Nv

The integral entropy vs. coverage curve is also quite structureless. It is found to monotonically decrease (become more negative) with increasing coverage approaching the entropy of vaporization at high coverage. The integral entropy of the adsorbed phase, Sa, is given byA Sad~. + Sg°, where S~° is the absolute molar entropy of the adsorbate in the gas phase at 1 atmosphere pressure. At 298°K., S~° for benzene is 64.34 cal./mole(leg.; hence S~ for benzene on graphite varies from 44.8 cal./mole-deg, at 0 = 0.15 to 40.7 cal./mole-deg at ~ = 2.4. For the BN sys-

479

tern, Sa varies from 49.3 cal./mole-deg, at 0 = 0.15 to 41.6 cal./mole-deg, at 0 = 2.0. The loss of entropy upon adsorption can be related to information concerning the modes of molecular motion available to the adsorbed molecules. Kiselev and co-workers have made calculations of the expected entropy loss associated with the adsorption of benzene on graphite (13) and on other solids (14) considering several possible combinations of molecular motions on the surface (15). Without making a detailed comparison of their calculated values and our experimental values, let it suffice to say that our experimental results for the graphite system are in accord with their theoretical calculations for a two-dimensional mobile layer the molecules of which can rotate freely in the plane of the surface, vibrate normal to the surface~ and have two degrees of librational freedom (13). Although one cannot on the basis of such calculations rule out hindered translations and rotations, it is certain that the observed entropy losses cannot be reconciled with the localized adsorption of the benzene. The entropy losses for the benzene adsorption on BN are qualitatively in accord with the weaker adsorption potential of the BN; this results ill lower vibrational frequencies relative to the surface and hence in a smaller entropy loss upon going from the gas to the adsorbed phase. TWO-DIMENSIONAL EQUATION OF STATE

The spreading pressures calculated as described earlier were used to determine the equation of state of the adsorbed phase over a large portion of the submonolayer region. It was found that for benzene adsorbed on both solids the equation of state was given by the expression 6A = kT -4- B~,

[5]

where ¢ is the spreading pressure, A is the area per molecule, and B is the effective coarea of the adsorbate at a given temperature. The quantity B is in fact equal to the twodimensional second virial coefficient. Equation [5] is of the form of the Volmer equation if one keeps in mind that B is not simply the excluded area of the adsorbate, but is

480

PIEROTTI AND SMALLWOOD

rather a quantity dependent upon the potential function for the lateral interactions between the molecules and upon the temperature. A plot of CA versus ~ yields a straight line for the benzene-graphite data from ¢ = 3 dynes/cm, up to about 15 dynes/era, corresponding to the region from 0 = 0.25 to 0 = 0.85. The slope of this line is the value of B and was found to be 20.1 ± 0.5 A~/ molecule over the fifty degree temperature range. The data are not sufficiently precise to determine the effect of temperature on B in this temperature interval. Kemball and Rideal (16) found B for benzene on mercury to be approximately 34 A~/molecule in this temperature range. .The boron nitride data yield a straight line from ~ = 3 dynes/era, to ¢ = 11 dynes/ cm. corresponding to the region from 0 = 0.15 to 0 = 0.7. The value of B determined for this system is 0 ± 3 A2/molecule over the temperature range studied. Thus on B N benzene very nearly obeys the equation of state of an ideal two-dimensional gas. The adsorption isotherm corresponding to a twodimensional ideal gas should be linear and it should be noted that the isotherms shown in Figs. 4, 5, and 6 deviate only slightly from linearity in the regions being considered here. The fact that the isotherms are not exactly linear favors the nonzero limits given above. The data are again not sufficiently precise to ascribe a temperature dependence to B. These equations of state are consistent with the AH~,. data. For a two-dimensional gas obeying the equation of state CA = /~T, H , is independent of the coverage and hence AHead. should be independent of the coverage (assuming the gas phase to be ideal). We note in Fig. 8 that this is very nearly the case for the B N system. For a gas obeying the Volmer equation of state, H~ should increase linearly with ¢ at a rate determined by B if B is not sensibly temperature dependent. Hence, for such a gas AH~d,. should increase (become less negative) linearly with respect to ~. With the use of the present data a plot of AH~d,. vs. ~ yields a fairly good straight line whose slope B is equal to 16 A~/molecule in fairly good agreement with

TABLE II ADSORPTION POTENTIALSAND CO-AREAS ~OR B ~ Z ~ E ON P-33, BN, ANI) tIg Substrate

Uo (cul/mote)

B (AS/ molecule)

P-33 BN Hg

9840~ 8720b 10,200~, 18,400d, 10,600e

2W 0~ 34o

Estimated by Kiselev and Poshkus (13). b Estimated from ratio of dispersion interaction of argon on the two surfaces (P-33 and BN). ° Estimated by Kemball (15) from Margenau and l~ollard equation. dEstimated by Kemball (15)from LennardJones equation. Calculated by use of the equation B = 17.7 Uo

-- 154.

s Present work. o Kemball and P~ideal (16). the value

obtained

from

the

force-area

curves.

CONCLUSION The thermodynamic changes associated with the adsorption of benzene on graphite and boron nitride have been determined from isotherm data. I t is found that benzene on graphite behaves as a two-dimensional fluid obeying the Volmer equation of state, whereas benzene on B N obeys the ideal twodimensional gas equation of state. Although we have not observed any sign of a phase transition from mobile to localized adsorption in the light of the theory of Stebbins and Halsey (17), it is clear that benzene adsorbed on these solids might well display such a transition. A very careful analysis of benzene-graphite data in the temperature range studied by Pierce and Davis (11) could well exhibit such a transition. Table II gives theoretically estimated values of adsorption potentials for benzene on three surfaces, along with values for B from the Volmer equation. It is clear t h a t B increases with increasing adsorption potential. Since B is a measure of lateral interactions (the larger the value of B, the less attractive or the more repulsive the interaction between the molecules), we find that the lateral interactions decrease significantly with increasing adsorption potential.

ADSORPTION OF BENZENE ON HOMOGENEOUS SUBSTRATES A c r u d e a p p r o x i m a t i o n w h i c h fits t h e obs e r v e d d a t a a t 25°C. is t h a t B = 17.7 U0 154, where B is in A2/molecule a n d U0, t h e a d s o r p t i o n p o t e n t i a l , is in k c a l . / m o l e .

9. 10.

REFERENCES

11.

1. YouNG, D. M., AND CnOWELL, A. D., "Physical Adsorption of Gases." Butterworth, Inc., Washington, D. C., 1962. 2. HALSEY, G. D., Advan. Catalysis 4, 259 (1952). 3. SAMS, J. R., CONTAEARIS, G., AND HALSEY, G. D., J. Phys. Chem. 66, 2154 (1962). This paper gives references to a series of papers by these authors. 4. KISELEV, A. V., POSHKUS, D. P., AND AFEEIMOVICH, A. YA., Russ. J. Phys. Chem. English Transl. 38, 821 (1964). This paper contains numerous references to papers by these authors. 5. ROSS, S., AND OLIVIER, J. P., "On Physical Adsorption." Interscience Publishers, New York, 1964. 6. PEENZLOW, C. F., AND HALSEY, G. D., J. Phys. Chem. 61, 1158 (1957). 7. PIEROTTI, R. A., J. Phys. Chem. 66, 1810 (1962). 8. KAnNAVX~OV, A. P., Kinetika i Kataliz 8,

12.

13.

14. 15. 16. 17.

481

583 (1962). This value 40 A~ was originally suggested by Kiselev et al. HILL, T. L., J. Chem. Phys. 17, 520 (1949). ISIRIKYAN, A. A., AND I~ISELEV, A. V., J. Phys. Chem. 65, 601 (1961). PIERCE, C. W., Private communications. DAvIs, B. W., "Adsorption on a Graphitized Carbon Surface." Doctoral Thesis, University of California, Riverside, 1964. •ISELEV, A. V., AND SERDOBOV,M., Russ. J . Phys. Chem. English Transl. 37, 1402 (1963). Kiselev and Isirikyan had measured calorimetric heats earlier with a different calorimeter; their results were in agreement with the more recent measurements. KISELEV, A. V., AND POSHKUS, D. P., Trans. Faraday Soe. 59, 428 (1963); and also Russ. J. Phys. Chem. English Transl. 37, 312 (1963). KISELEV, A. V., AND LYGIN, V. I., Kolloidn. Zh. 23, 574 (1961). KEMBALL, C., Proc. Roy. Soc. (London) A187, 73 (1946). I(EM~ALL, C., AND RIDEAL, E. K., Proc. Roy. Soc. (London) A187, 37 (1946). STEBBINS,J. P., AND HALSEY, G. D., J. Phys. Chem. 68, 3863 (1964).