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Adsorption studies on polystyrene
Adsorption Studies on Polystyrene ''2 ROGER F. HOBURG, 3 GEORGE S. HANDLER, ~ AND JOHN J. SCttOLZ Avery Laboratory of Chemistry, University of Nebraska, Lincoln, Nebraska 68508
Received February 1, 1968 The interaction of nitrogen and argon with samples of highly dispersed polystyrene in the temperature range 65-75°K was investigated. Hysteresis was encountered when desorption was attempted. The hysteresis is attributed to penetration of gas molecules into the polymer structure. Reversible isotherms, suitable for thermodynamic analysis, were obtained after the polymer was equilibrated with argon for 1 week at 75-77°K. Clustering functions were calculated from the experimental isotherms. Surface area determinations, thermodynamic analysis, and the clustering functions all indicate that the adsorbent is very heterogeneous energetically.
The growing interest in surface properties of polymers and other organic substances makes it important to have available as much basic information as possible on the interaction of molecules with these solids (1-7). Experiments on the physical adsorption of gases appear to offer a promising source of such information. Studies of gas adsorption by polymers have been reported by Zettlemoyer and co-workers (8-9), Thompson (10), Graham (11), Lohr and Seholz (12), and Hightower and Emmett (13). Except for cases in which hydrocarbon vapors were adsorbates, there was no hysteresis and adsorption equilibrium was reached in a short time. This behavior has been found even for highly dispersed materials such as polypropylene with a surface 1 Taken in part from a thesis submitted by George S. Handler in partial fulfilhnent of the requirement for the Ph.D. degree, University of Nebraska, 1963. 2 Taken in part from a thesis submitted by Roger F. I~loburg in partial fulfillment of the requirement for the Ph.D. degree, University of Nebraska, 1967. Present address: Department of Chemistry, University of Omaha, Omaha, Nebraska, 68132. 4Present address: Naval Weapons Center, China Lake, California 93555. Journal of Colloidand Interface ~cience, Vol. 27, No. 4, August 1968
area of approximately 100 m~/gm (11) and for poly(methylmethacrylate) with an area near 150 m2/gm (12). Materials with high specific surface areas possess several advantages for adsorption studies and we have found that the method used in the earlier work to obtain these highly dispersed materials are applicable to a wide variety of polymers. These circumstances make it desirable to extend this work to systems displaying a wide range of chemical constitution. This report describes experiments on two different samples of high area polystyrene in which hysteresis was found when N2 and Ar were adsorbed. However, procedures have been developed by which reversible adsorption can be obtained. EXPERIMENTAL
METHODS
Materials Two samples of high area adsorbent were prepared from Dew Chemical Company Styron polystyrene by methods described previously. An account has also been given of the sample cleaning procedure (12, 14). The argon and nitrogen adsorbates were supplied by 2\~atheson Company with guaranteed minimum purites of 99.990 % and 99.996 %, respectively. 642
ADSORPTION STUDIES ON POLYSTYRENE
Apparatus and Methods Only minor changes were made in the volumetric adsorption apparatus used previously (12). Hysteresis was observed when the usual procedure was used: the isotherm was traced by adding successive doses of gas to the system and then retraced by removing increments of gas (procedure 1). Reversible isotherms were obtained using procedure 2: the adsorbent was subjected to an extended equilibration with argon. A carefully measured quantity of gas was introduced into the sample bulb at a relative pressure of 0.95 at 75°K. The adsorbent was maintained at 75°K during the day, but was allowed to drift up to 77°K at night. The polymer sorbed argon slowly for nearly a week, although the uptake was 90 % complete within 2 days. Measurements were started after the polystyrene was equilibrated with argon for 8 days. The first set of measurements extended over a period of 6 weeks, during which time the adsorbent was never warmed above 77°K. In the course of these experiments, the sample had stood overnight at 77°K at various argon pressures between 5 and 140 Tort. Adsorption and desorption points at 75°K were determined over a range of pressures from 0.1 to 140 Torr. The entire isotherm was traced in both directions, and except as noted below, it was completely reproducible. No departures from the isotherm were ever observed when the argon pressure was increased, but large sudden decreases in adsorbate pressure could cause the measured adsorption to fall a few percent below the isotherm defined by gradual desorption and subsequent adsorption. It was also observed that, at a pressure of 0.5 Torr, allowing the sample to stand for 1-2 hours at 77°K caused the amount adsorbed to fall slightly below the previously determined isotherm. A return to the reversible isotherm was readily effected in each case by bringing the argon pressure to near saturation either by compressing the gas or by lowering the tempera-
643
ture. After these observations were made, the conditions producing them were avoided in subsequent work. The equilibration procedure was repeated twice during the course of the experiments, each time after the adsorbent had been warmed to room temperature and the system had been evacuated at 10-6 Torr for 48 hours. Both times portions of the 75°K isotherm were repeated to establish that the equilibration procedure led to a reproducible state. Isotherms at 65 and 70°K were determined by the same procedures as those used at 75°K. It was also found possible to pass between the three isotherms merely by changing the temperature and allowing equilibrium to be reestablished. RESULTS
Isotherms The first polystyrene sample, the more highly dispersed of the two, was investigated by procedure 1 and exhibited hysteresis in all experiments. A B E T plot made from the adsorption branch of a 77°X nitrogen isotherm yielded a monolayer volume of 55.3 m//gm at 8TP, which corresponds to a surface area of 241 m2/gm. Some typical results, those for nitrogen at 77°K, appear in Fig. 1. Only the lower pressure region of the isotherm is given so that some of the details are shown more clearly. As their origin is moved farther up the isotherm, the desorption curves deviate more and more from the adsorption curve. The abrupt drop in the upper desorption curve was caused by the sample being warmed to near room temperature. When the sample was cooled again to 77°K the nitrogen readsorbed to a point on the original ascending curve as shown in the Fig. 1, but further desorption from this point again showed hysteresis. The upper desorption curve exhibits at least limited reversibility as shown by the group of points that are numbered in the order in which they were determined. Journal of Colloid and £nterface Science, Vo]. 27, No. 4, August 1968
644
HOBUP~G, HANDLER, AND SCHOLZ
Iio I00
82 -g
90
3 7 5
80
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tO
N
a£ 70 P2
tn 60
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• ADSORPTIONPOINTS n DESORPTIONFROM 0.89P/Po a DESORPTIONFROM 0.77P/Po v DESORPTIONFROM 0.26P/Po
50
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1
I
I
I
0.I
0.2
0.5
0.4
RELATIVE PRESSURE (PI%)
Fro. 1. t t y s t e r e s i s in nitrogen adsorption on polystyrene at 77°K.
At this point it seemed worthwhile to assess the generality of these observations, so another sample of polystyrene was prepared by similar methods. No detectable hysteresis was found in a procedure 1 nitrogen isotherm at 75°K. A BET calculation produced a surface area of 150 m~/gm, indicating that this adsorbent was considerably less dispersed than the first sample. However, after the adsorbent had been flushed with argon, some hysteresis was detected in an argon isotherm at 75°K. It was found only at higher pressures and manifested itself as a slow uptake of adsorbate following the initial rapid adsorption. The initial rapid adsorption was similar in every way to the earlier experience with nitrogen and the isotherm yields exactly the same BET surface area. An experiment with nitrogen as adsorbate, which followed the argon work, showed that hysteresis was also observable with the nitrogen. The uptake of gas which followed the rapid adsorption was much slower for nitrogen than for argon but the magnitude of the effect was such that it certainly would have been detected with earlier experiments had hysteresis been present. These results indicated that hysteresis can be expected with high area freeze dried polystyrene adsorbents, so subsequent work was directed toward investigating the reversibility noted in the upper isotherm of Fig. 1. In Fig. 2 the argon isotherm obtained Journal of Colloid and Interface Science, ¥ o L 27, N o . 4, A u g u s t 1968
Pressure {MM of Hg}
FI~, 2. Effect of e q u i l i b r a t i o n on argon a n d sorption on polystyrene at 75°K. (O) = d e s o r p t i o n points.
by procedure i is compared to the reversible isotherm determined by procedure 2. Although at some pressures there are differences in slopes and curvatures, in the range from 10 to 60 Tort, the differences in volume adsorbed remain within 10 % of 9 ml/gra. Above 60 Tort where hysteresis was observed by procedure 1, the two curves come together slightly. Figure 3 presents the equilibrium isotherms for argon on polystyrene determined at 65, 70, and 75°K. None of the isotherms has a sharp knee but all of them have two reproducible linear segments: one separating a region with negative curvature from a region with positive curvature and another, at lower pressure, separating two regions with negative curvature. The beginning of the upper linear region thus corresponds to the conventional Point B (15); however, the other linear segment can be used to locate a point which will be referred to as a lower Point B.
ADSORPTION STUDIES ON POLYSTYRENE
were assigned to nitrogen and argon. In the case of the reversible procedure 2 isotherms, there is one rather short linear section in each of the BET plots and the corresponding V~,~values coincide with the lower Point B's. Probably the best approximation to a conventional surface area is obtained from the procedure I nitrogen isotherm. Although the BET and lower Point B areas from the procedure 2 argon isotherm agree with this estimate, the monolayer points occur at considerably lower relative pressures. On the other hand, the upper Points B of the procedure 2 isotherms occur at appreciably higher relative pressures than does the Point B of the procedure 1 isotherm. While the difference in volume adsorbed at the same pressure between the two 75°K isotherms is approximately 9 ml/gm, the apparent monolayer volumes differ by nearly twice this amount. Thus the significance of points selected by conventional methods used for determining surface areas is different for the two types of isotherms. Differential enthalpies and entropies of adsorption were calculated from the reversible procedure 2 isotherms by the methods of Hill et aI. (16). The saturated vapor pressure was chosen as the standard state for the adsorbate and all results refer to the process: Gas on Solid ~- Gas -~ Solid.
The results of surface area determination are given in Table I. Both the BET and Point B methods could be applied to the procedure 1 nitrogen isotherms with consistent results. There was no linear segment in the BET plots for the procedure 1 argon isotherm, probably due to the onset of hysteresis. The area determined from Point B agreed with the nitrogen area if the cross sections used earlier (12) of 16.2 and 13.8 ,~2 Z'K
g~ 70"1(
6
u
o
< ®
•
Adsorption
o
Desorptlon
645
Pressure (MM of H9}
Fro. 3. Argon isotherms on polystyrene.
TABLE I SURFACE AREAS Adsorbate N2 N2 N2 Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar
Method BET BET Point B Point B P o i n t B (upper) P o i n t B (lower) BET P o i n t B (upper) P o i n t B (lower) BET P o i u t B (upper) P o i n t B (lower) B E T (lower)
Procedure 1 1 1 1 2 3 2 2 2 2 2 2 2
Temp 77.1 75 75 75 75 75 75 70 70 70 65 65 65
Vm(ml/(gin) 55.3 35.0 34.43 40.0 56.36 40.5 40.16 57.30 39.5 39.82 58.50 40.0 40.34
(M~/gra)
241 154 150 150 209 150 149 210 14~ 146 211 144 145
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
646
HOBURG, HANDLER, A N D SCHOLZ
2-
~
_ ®
\
~
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l
-20
-10
%, :,o
i
,~o-"
~
Vods~grorn
e'o
1
- -
($.T.P.)
F~G. 4. Enthalpies and entropies of adsorption of argon on polystyrene.
In Fig. 4, H~' and S~' are the standard molar enthalpy and entropy of the unadsorbed gas at the average temperature of the experiments. The /I~ and ~ are the partial molar enthalpy and entropy of the adsorbed phase at the average temperature of the experiment. The data in Fig. 4 represent the average of values calculated from the 75-70, 75-65, and 70-65°K isotherm pairs. The average deviation among the three values is ±0.i kcal/mol. The shape of the enthalpy curve is similar to those reported by Graham for the adsorption of argon on polypropylene (II) and by Lohr and Seholz for the adsorption of argon on poly(methylmethacrylate) (12). At a surface coverage of 0.9 of the upper Point B monolayer volume, the enthalpy of adsorption calculated in this work was 1.79 kcal/mol. This value can be compared to 1.56, 1..52, and 2.01 kcal/mol at similar coverage which have been reported for the adsorption of argon on teflon (II), polypropy]ene (II), and poly(methylmethaJournal of Colloid and Interface Science, ¥ol. 27~ 1~o. 4, August 1968
crylate) (12), respectively. Our result thus falls into the order that would be predicted by comparing critical surface tension for spreading (1) although it is possibly somewhat high. In view of the complexity of the process studied here which involves sorption as well as adsorption, the agreement seems to be very good. The principle features of the entropy and enthalpy curves are their fairly rapid decrease at low coverages and the occurrence of two minima; one at an adsorbed volume near the monolayer volume determined by the lower Point B and the other at a volume which is close to that of the upper Point B. At coverages somewhat beyond the upper Point B, the enthalpy decreases monotonically and approaches the heat of vaporization of argon. DISCUSSION Adsorption hysteresis has been thoroughly reviewed recently by Everett (18). He points out that the only satisfactory theories of hysteresis which persists to very low pressures invoke penetration of the adsorbent into the molecular structure of the solid. Solid polystyrene, when prepared from solution in a high area form, can be expected to have a particularly open structure due to the presence of the bulky phenyl substituents on the polymer chains. This provides the most plausible explanation for the occurrence of hysteresis here in contrast to its absence in earlier work (8-13). The rate of penetration of argon into the polystyrene was so slow as to be undetectable at moderate pressures but was much more rapid at pressures near saturation. This suggests that it took place by a cooperative mechanism in which molecules, once they had entered the polymer matrix, helped stabilize configurations which allowed other molecules to enter the polymer more readily. When desorption is begun and the concentration of molecules at the polymer surface is reduced, most of the molecules that had penetrated into the adsorbent remain trapped, accounting for the remarkable stability of the equilibrated system and for
ADSORPTION STUDIES ON POLYSTYRENE the large amount of argon retained by the polymer even at very low pressures. The experimental results certainly do not imply that the adsorbate can be strictly classified as either interior bound molecules or surface desorbable molecules. Rather, it seems likely that the number of interior molecules that can be desorbed varies with the surface coverage. This gives rise to differences in shape between the procedure 1 and proeedure 2 isotherms and also accounts for the fact that sudden large decreases in pressure could bring about desorption to points below the equilibrium isotherm. Clustering Functions
Further insight into the nature of the adsorbed phase can be gained from the clustering function, 4)iGn/vl which was introduced by Zimm (19) and first applied to sorption data by Zimm and Lundberg (20). This function can be interpreted in molecular terms as the mean number of type one molecules in excess of random expectation in the neighborhood of a given type one molecule. If the compressibility of the condensed phase can be neglected, 4)~gn/vl is related to experimental quantities by 4)~a~/~
=
- (i -
{(1
as 4),q,~/~
=
- (~ -
oi)
(1 - ~1) (2 - 4)~)
4)~)
L 4)i (i
[2]
0 In 4)k1 ~ V n K d + -I - -4)i 4)i}.
It can be seen that, to good approximation, the clustering function for a heterogeneous surface is the sum of additive contribution from all of the patches, each weighed by fk¢~/¢1 • When Eq. [1] is applied to the type of isotherm found on energetically homogeneous surfaces, the clustering function is zero in the Henry's law region at low coverage, rises sharply during the formation of the first monolayer, and then drops rapidly toward a value of minus one as the monolayer is completed, i t is reasonable to assume that this is also its behavior on the individual patches making up an energetically heterogeneous surface.
4)1)
0,3-
O in 4)1) 0ln~
647
4)1 } +1
[1]
¢--~
where v~, 4)1, and a~ are the volume per molecule, volume fraction and activity of component one (adsorbate). Under the conditions of most adsorption experiments, the volume fraction of the absorbed phase, ¢1 is much less than 1. Considering an energeticMly heterogeneous surface to be made up of a large number of homogeneous patches, it is convenient to define f~ as the fraction of the surface comprising the/cth patch and 4)k, the volume fraction of component one which is adsorbed on this patch. Then 2kfk = 1, and 2Jk[¢~/(1 -- ¢k)] = ¢1/(1 -- ¢1). Using these relationships, Eq. [1] can be rewritten
0.1---
O--
,.D-
-Q3-(25O
- 0.7-
-0.9-
Vm fEE.T) Vm (Poinl B) I
50
L
--T
60
90
Vads./gram (S.T.R)
FIG. 5. Clustering
of argon
oF polystyrene
at
75°K. Journal of Colloid and Interface Science,
¥o]. 27, No. 4, August1968
648
HOBURG, HANDLER, AND SCHOLZ
The clustering function determined for the 75°K argon isotherm1 is shown in Fig. 5. Its low value at low adsorbed volumes predicts that most of the adsorbate is concentrated in isolated, filled patches. The subsequent increase in the clustering function indicates that a new group of patches is being filled. This is followed by a minimum, which occurs near the monolayer volume predicted by the BET equation and also the first minimum in the differential entropy curve. The clustering function then rises to a second maximum and goes through another minimum. This second minimum agrees with the upper Point B monolayer volume and the second minimum in the entropy curve. All three lines of evidence suggest that these are points about which the nature of the adsorption process is undergoing the type of change which accompanied the completion of a monolayer and a homogeneous surface. ACKNOWLEDGMENTS G.S.H. gratefully acknowledges the financial support given him by the U.S. Naval Ordnance Test Station under its Off Station Advanced Study Program. I~.F..H was the recipient of a Summer Research Fellowship from the 1V[onsanto Chemical Corporation. Dr. James E. Lohr prepared the second polystyrene sample. REFERENCES 1. ZIS~AN, W. A., Ind. Eng. Chem. 55, 19 (1963); Advan. Chem. Set. 43, 1 (1964). 2. GInI~ALCo, L. A., AND Goon, R. J., J. Phys. Chem. 6l, 904 (1957); J. Phys. Chem. 64, 561 (1960).
3. GOOD, R. J., GmlFALeo, L. A., ANl)KR_~US,G., J. Phys. Chem. 62, 1418 (1958); GOOD,I:L if., Advan. Chem. Ser. 43, 74 (1964). 4. FOWKES, F. M., J. Phys. Chem. 67, 2538 (1063); Advan. Chem. Set. 43, 99 (1964); "Chemistry and Physics of Interfaces," p. 1. American Chemical Society Publication, Washington, D.C. (1965). 5. MELROSE, J. C., J. Colloid Sei. 20,801 (1965); Advan. Chem. Ser. 43, 158 (1964). 6. SC~ONI~O~N, H., J. Phys. Chem. 70, 4086 (1966); J. Phys. Chem. 69, 1084 (1965); Sharpe, L. H., J. Polymer Sei. 132, 719 (1964). 7. S ~ m , E , L. H., ANn SC~ON~{ORN, H., Advan. Chem. Set. 43, ]89 (1964). 8, ZETTLEMOYER, A. C,, CHAN, A., AND GAMBLE, E., J. Am. Chem. Soc. 72, 2752 (1950). 9. CHESSICK, J. J., HEALY, F. H., AND ZETTLEMOVER, A. C., J. Phys. Chem. 60, 1345 (1956). 10. T~O~PSON, W., Physiea 9.6,890 (1960). 11. GRAHAM, D., J. Phys. Chem. 66, 1815 (1962); J. Phys. Chem. 68, 2788 (1964); J. Phys. Chem. 69, 4387 (1965). 12. LoHa, J. E., AN]) SCHOLZ,J. J., J. Colloid Sei. 20,846 (1965). 13. HIGHTOWEn, J. W., AND EMMETT, P. H., J. Polymer Sei., Pt. A 2, 1647 (1964). 14. HANDLES, G. S., Ph.D. Thesis, University of Nebraska (1963). 15. CROWELL,D . M., A N D YOUNG, A. D., "Physical Adsorption of Gases," Butterworths, London (1962). 16, HILL, T. L., EMMETT, P. H., AND JOYNER, L. G., Y. Am. Chem. Soc. 73, 5102 (1952). 17. LoHR, J. E., Ph.D. Thesis, University of Nebraska (1965). 18. D. H. EVERETT,in "The Solid Gas Interface," (A. E. Flood, ed.), VoI. 2. Marcel Dekker, New York (1967). 19. ZI~M, G. H., J. Chem. Phys. 21,934 (1953). 20. Z1MM, B. H., AND LUNDBERG, J. L., J. Phys. Chem. 60,425 (1956).
Journal of Colloidandlnterface Science,Vol. 27, No. 4, August 1968
Received February 1, 1968 The interaction of nitrogen and argon with samples of highly dispersed polystyrene in the temperature range 65-75°K was investigated. Hysteresis was encountered when desorption was attempted. The hysteresis is attributed to penetration of gas molecules into the polymer structure. Reversible isotherms, suitable for thermodynamic analysis, were obtained after the polymer was equilibrated with argon for 1 week at 75-77°K. Clustering functions were calculated from the experimental isotherms. Surface area determinations, thermodynamic analysis, and the clustering functions all indicate that the adsorbent is very heterogeneous energetically.
The growing interest in surface properties of polymers and other organic substances makes it important to have available as much basic information as possible on the interaction of molecules with these solids (1-7). Experiments on the physical adsorption of gases appear to offer a promising source of such information. Studies of gas adsorption by polymers have been reported by Zettlemoyer and co-workers (8-9), Thompson (10), Graham (11), Lohr and Seholz (12), and Hightower and Emmett (13). Except for cases in which hydrocarbon vapors were adsorbates, there was no hysteresis and adsorption equilibrium was reached in a short time. This behavior has been found even for highly dispersed materials such as polypropylene with a surface 1 Taken in part from a thesis submitted by George S. Handler in partial fulfilhnent of the requirement for the Ph.D. degree, University of Nebraska, 1963. 2 Taken in part from a thesis submitted by Roger F. I~loburg in partial fulfillment of the requirement for the Ph.D. degree, University of Nebraska, 1967. Present address: Department of Chemistry, University of Omaha, Omaha, Nebraska, 68132. 4Present address: Naval Weapons Center, China Lake, California 93555. Journal of Colloidand Interface ~cience, Vol. 27, No. 4, August 1968
area of approximately 100 m~/gm (11) and for poly(methylmethacrylate) with an area near 150 m2/gm (12). Materials with high specific surface areas possess several advantages for adsorption studies and we have found that the method used in the earlier work to obtain these highly dispersed materials are applicable to a wide variety of polymers. These circumstances make it desirable to extend this work to systems displaying a wide range of chemical constitution. This report describes experiments on two different samples of high area polystyrene in which hysteresis was found when N2 and Ar were adsorbed. However, procedures have been developed by which reversible adsorption can be obtained. EXPERIMENTAL
METHODS
Materials Two samples of high area adsorbent were prepared from Dew Chemical Company Styron polystyrene by methods described previously. An account has also been given of the sample cleaning procedure (12, 14). The argon and nitrogen adsorbates were supplied by 2\~atheson Company with guaranteed minimum purites of 99.990 % and 99.996 %, respectively. 642
ADSORPTION STUDIES ON POLYSTYRENE
Apparatus and Methods Only minor changes were made in the volumetric adsorption apparatus used previously (12). Hysteresis was observed when the usual procedure was used: the isotherm was traced by adding successive doses of gas to the system and then retraced by removing increments of gas (procedure 1). Reversible isotherms were obtained using procedure 2: the adsorbent was subjected to an extended equilibration with argon. A carefully measured quantity of gas was introduced into the sample bulb at a relative pressure of 0.95 at 75°K. The adsorbent was maintained at 75°K during the day, but was allowed to drift up to 77°K at night. The polymer sorbed argon slowly for nearly a week, although the uptake was 90 % complete within 2 days. Measurements were started after the polystyrene was equilibrated with argon for 8 days. The first set of measurements extended over a period of 6 weeks, during which time the adsorbent was never warmed above 77°K. In the course of these experiments, the sample had stood overnight at 77°K at various argon pressures between 5 and 140 Tort. Adsorption and desorption points at 75°K were determined over a range of pressures from 0.1 to 140 Torr. The entire isotherm was traced in both directions, and except as noted below, it was completely reproducible. No departures from the isotherm were ever observed when the argon pressure was increased, but large sudden decreases in adsorbate pressure could cause the measured adsorption to fall a few percent below the isotherm defined by gradual desorption and subsequent adsorption. It was also observed that, at a pressure of 0.5 Torr, allowing the sample to stand for 1-2 hours at 77°K caused the amount adsorbed to fall slightly below the previously determined isotherm. A return to the reversible isotherm was readily effected in each case by bringing the argon pressure to near saturation either by compressing the gas or by lowering the tempera-
643
ture. After these observations were made, the conditions producing them were avoided in subsequent work. The equilibration procedure was repeated twice during the course of the experiments, each time after the adsorbent had been warmed to room temperature and the system had been evacuated at 10-6 Torr for 48 hours. Both times portions of the 75°K isotherm were repeated to establish that the equilibration procedure led to a reproducible state. Isotherms at 65 and 70°K were determined by the same procedures as those used at 75°K. It was also found possible to pass between the three isotherms merely by changing the temperature and allowing equilibrium to be reestablished. RESULTS
Isotherms The first polystyrene sample, the more highly dispersed of the two, was investigated by procedure 1 and exhibited hysteresis in all experiments. A B E T plot made from the adsorption branch of a 77°X nitrogen isotherm yielded a monolayer volume of 55.3 m//gm at 8TP, which corresponds to a surface area of 241 m2/gm. Some typical results, those for nitrogen at 77°K, appear in Fig. 1. Only the lower pressure region of the isotherm is given so that some of the details are shown more clearly. As their origin is moved farther up the isotherm, the desorption curves deviate more and more from the adsorption curve. The abrupt drop in the upper desorption curve was caused by the sample being warmed to near room temperature. When the sample was cooled again to 77°K the nitrogen readsorbed to a point on the original ascending curve as shown in the Fig. 1, but further desorption from this point again showed hysteresis. The upper desorption curve exhibits at least limited reversibility as shown by the group of points that are numbered in the order in which they were determined. Journal of Colloid and £nterface Science, Vo]. 27, No. 4, August 1968
644
HOBUP~G, HANDLER, AND SCHOLZ
Iio I00
82 -g
90
3 7 5
80
946
tO
N
a£ 70 P2
tn 60
f
• ADSORPTIONPOINTS n DESORPTIONFROM 0.89P/Po a DESORPTIONFROM 0.77P/Po v DESORPTIONFROM 0.26P/Po
50
~4o 30 20
1
I
I
I
0.I
0.2
0.5
0.4
RELATIVE PRESSURE (PI%)
Fro. 1. t t y s t e r e s i s in nitrogen adsorption on polystyrene at 77°K.
At this point it seemed worthwhile to assess the generality of these observations, so another sample of polystyrene was prepared by similar methods. No detectable hysteresis was found in a procedure 1 nitrogen isotherm at 75°K. A BET calculation produced a surface area of 150 m~/gm, indicating that this adsorbent was considerably less dispersed than the first sample. However, after the adsorbent had been flushed with argon, some hysteresis was detected in an argon isotherm at 75°K. It was found only at higher pressures and manifested itself as a slow uptake of adsorbate following the initial rapid adsorption. The initial rapid adsorption was similar in every way to the earlier experience with nitrogen and the isotherm yields exactly the same BET surface area. An experiment with nitrogen as adsorbate, which followed the argon work, showed that hysteresis was also observable with the nitrogen. The uptake of gas which followed the rapid adsorption was much slower for nitrogen than for argon but the magnitude of the effect was such that it certainly would have been detected with earlier experiments had hysteresis been present. These results indicated that hysteresis can be expected with high area freeze dried polystyrene adsorbents, so subsequent work was directed toward investigating the reversibility noted in the upper isotherm of Fig. 1. In Fig. 2 the argon isotherm obtained Journal of Colloid and Interface Science, ¥ o L 27, N o . 4, A u g u s t 1968
Pressure {MM of Hg}
FI~, 2. Effect of e q u i l i b r a t i o n on argon a n d sorption on polystyrene at 75°K. (O) = d e s o r p t i o n points.
by procedure i is compared to the reversible isotherm determined by procedure 2. Although at some pressures there are differences in slopes and curvatures, in the range from 10 to 60 Tort, the differences in volume adsorbed remain within 10 % of 9 ml/gra. Above 60 Tort where hysteresis was observed by procedure 1, the two curves come together slightly. Figure 3 presents the equilibrium isotherms for argon on polystyrene determined at 65, 70, and 75°K. None of the isotherms has a sharp knee but all of them have two reproducible linear segments: one separating a region with negative curvature from a region with positive curvature and another, at lower pressure, separating two regions with negative curvature. The beginning of the upper linear region thus corresponds to the conventional Point B (15); however, the other linear segment can be used to locate a point which will be referred to as a lower Point B.
ADSORPTION STUDIES ON POLYSTYRENE
were assigned to nitrogen and argon. In the case of the reversible procedure 2 isotherms, there is one rather short linear section in each of the BET plots and the corresponding V~,~values coincide with the lower Point B's. Probably the best approximation to a conventional surface area is obtained from the procedure I nitrogen isotherm. Although the BET and lower Point B areas from the procedure 2 argon isotherm agree with this estimate, the monolayer points occur at considerably lower relative pressures. On the other hand, the upper Points B of the procedure 2 isotherms occur at appreciably higher relative pressures than does the Point B of the procedure 1 isotherm. While the difference in volume adsorbed at the same pressure between the two 75°K isotherms is approximately 9 ml/gm, the apparent monolayer volumes differ by nearly twice this amount. Thus the significance of points selected by conventional methods used for determining surface areas is different for the two types of isotherms. Differential enthalpies and entropies of adsorption were calculated from the reversible procedure 2 isotherms by the methods of Hill et aI. (16). The saturated vapor pressure was chosen as the standard state for the adsorbate and all results refer to the process: Gas on Solid ~- Gas -~ Solid.
The results of surface area determination are given in Table I. Both the BET and Point B methods could be applied to the procedure 1 nitrogen isotherms with consistent results. There was no linear segment in the BET plots for the procedure 1 argon isotherm, probably due to the onset of hysteresis. The area determined from Point B agreed with the nitrogen area if the cross sections used earlier (12) of 16.2 and 13.8 ,~2 Z'K
g~ 70"1(
6
u
o
< ®
•
Adsorption
o
Desorptlon
645
Pressure (MM of H9}
Fro. 3. Argon isotherms on polystyrene.
TABLE I SURFACE AREAS Adsorbate N2 N2 N2 Ar Ar Ar Ar Ar Ar Ar Ar Ar Ar
Method BET BET Point B Point B P o i n t B (upper) P o i n t B (lower) BET P o i n t B (upper) P o i n t B (lower) BET P o i u t B (upper) P o i n t B (lower) B E T (lower)
Procedure 1 1 1 1 2 3 2 2 2 2 2 2 2
Temp 77.1 75 75 75 75 75 75 70 70 70 65 65 65
Vm(ml/(gin) 55.3 35.0 34.43 40.0 56.36 40.5 40.16 57.30 39.5 39.82 58.50 40.0 40.34
(M~/gra)
241 154 150 150 209 150 149 210 14~ 146 211 144 145
Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968
646
HOBURG, HANDLER, A N D SCHOLZ
2-
~
_ ®
\
~
-40 zo
l
-20
-10
%, :,o
i
,~o-"
~
Vods~grorn
e'o
1
- -
($.T.P.)
F~G. 4. Enthalpies and entropies of adsorption of argon on polystyrene.
In Fig. 4, H~' and S~' are the standard molar enthalpy and entropy of the unadsorbed gas at the average temperature of the experiments. The /I~ and ~ are the partial molar enthalpy and entropy of the adsorbed phase at the average temperature of the experiment. The data in Fig. 4 represent the average of values calculated from the 75-70, 75-65, and 70-65°K isotherm pairs. The average deviation among the three values is ±0.i kcal/mol. The shape of the enthalpy curve is similar to those reported by Graham for the adsorption of argon on polypropylene (II) and by Lohr and Seholz for the adsorption of argon on poly(methylmethacrylate) (12). At a surface coverage of 0.9 of the upper Point B monolayer volume, the enthalpy of adsorption calculated in this work was 1.79 kcal/mol. This value can be compared to 1.56, 1..52, and 2.01 kcal/mol at similar coverage which have been reported for the adsorption of argon on teflon (II), polypropy]ene (II), and poly(methylmethaJournal of Colloid and Interface Science, ¥ol. 27~ 1~o. 4, August 1968
crylate) (12), respectively. Our result thus falls into the order that would be predicted by comparing critical surface tension for spreading (1) although it is possibly somewhat high. In view of the complexity of the process studied here which involves sorption as well as adsorption, the agreement seems to be very good. The principle features of the entropy and enthalpy curves are their fairly rapid decrease at low coverages and the occurrence of two minima; one at an adsorbed volume near the monolayer volume determined by the lower Point B and the other at a volume which is close to that of the upper Point B. At coverages somewhat beyond the upper Point B, the enthalpy decreases monotonically and approaches the heat of vaporization of argon. DISCUSSION Adsorption hysteresis has been thoroughly reviewed recently by Everett (18). He points out that the only satisfactory theories of hysteresis which persists to very low pressures invoke penetration of the adsorbent into the molecular structure of the solid. Solid polystyrene, when prepared from solution in a high area form, can be expected to have a particularly open structure due to the presence of the bulky phenyl substituents on the polymer chains. This provides the most plausible explanation for the occurrence of hysteresis here in contrast to its absence in earlier work (8-13). The rate of penetration of argon into the polystyrene was so slow as to be undetectable at moderate pressures but was much more rapid at pressures near saturation. This suggests that it took place by a cooperative mechanism in which molecules, once they had entered the polymer matrix, helped stabilize configurations which allowed other molecules to enter the polymer more readily. When desorption is begun and the concentration of molecules at the polymer surface is reduced, most of the molecules that had penetrated into the adsorbent remain trapped, accounting for the remarkable stability of the equilibrated system and for
ADSORPTION STUDIES ON POLYSTYRENE the large amount of argon retained by the polymer even at very low pressures. The experimental results certainly do not imply that the adsorbate can be strictly classified as either interior bound molecules or surface desorbable molecules. Rather, it seems likely that the number of interior molecules that can be desorbed varies with the surface coverage. This gives rise to differences in shape between the procedure 1 and proeedure 2 isotherms and also accounts for the fact that sudden large decreases in pressure could bring about desorption to points below the equilibrium isotherm. Clustering Functions
Further insight into the nature of the adsorbed phase can be gained from the clustering function, 4)iGn/vl which was introduced by Zimm (19) and first applied to sorption data by Zimm and Lundberg (20). This function can be interpreted in molecular terms as the mean number of type one molecules in excess of random expectation in the neighborhood of a given type one molecule. If the compressibility of the condensed phase can be neglected, 4)~gn/vl is related to experimental quantities by 4)~a~/~
=
- (i -
{(1
as 4),q,~/~
=
- (~ -
oi)
(1 - ~1) (2 - 4)~)
4)~)
L 4)i (i
[2]
0 In 4)k1 ~ V n K d + -I - -4)i 4)i}.
It can be seen that, to good approximation, the clustering function for a heterogeneous surface is the sum of additive contribution from all of the patches, each weighed by fk¢~/¢1 • When Eq. [1] is applied to the type of isotherm found on energetically homogeneous surfaces, the clustering function is zero in the Henry's law region at low coverage, rises sharply during the formation of the first monolayer, and then drops rapidly toward a value of minus one as the monolayer is completed, i t is reasonable to assume that this is also its behavior on the individual patches making up an energetically heterogeneous surface.
4)1)
0,3-
O in 4)1) 0ln~
647
4)1 } +1
[1]
¢--~
where v~, 4)1, and a~ are the volume per molecule, volume fraction and activity of component one (adsorbate). Under the conditions of most adsorption experiments, the volume fraction of the absorbed phase, ¢1 is much less than 1. Considering an energeticMly heterogeneous surface to be made up of a large number of homogeneous patches, it is convenient to define f~ as the fraction of the surface comprising the/cth patch and 4)k, the volume fraction of component one which is adsorbed on this patch. Then 2kfk = 1, and 2Jk[¢~/(1 -- ¢k)] = ¢1/(1 -- ¢1). Using these relationships, Eq. [1] can be rewritten
0.1---
O--
,.D-
-Q3-(25O
- 0.7-
-0.9-
Vm fEE.T) Vm (Poinl B) I
50
L
--T
60
90
Vads./gram (S.T.R)
FIG. 5. Clustering
of argon
oF polystyrene
at
75°K. Journal of Colloid and Interface Science,
¥o]. 27, No. 4, August1968
648
HOBURG, HANDLER, AND SCHOLZ
The clustering function determined for the 75°K argon isotherm1 is shown in Fig. 5. Its low value at low adsorbed volumes predicts that most of the adsorbate is concentrated in isolated, filled patches. The subsequent increase in the clustering function indicates that a new group of patches is being filled. This is followed by a minimum, which occurs near the monolayer volume predicted by the BET equation and also the first minimum in the differential entropy curve. The clustering function then rises to a second maximum and goes through another minimum. This second minimum agrees with the upper Point B monolayer volume and the second minimum in the entropy curve. All three lines of evidence suggest that these are points about which the nature of the adsorption process is undergoing the type of change which accompanied the completion of a monolayer and a homogeneous surface. ACKNOWLEDGMENTS G.S.H. gratefully acknowledges the financial support given him by the U.S. Naval Ordnance Test Station under its Off Station Advanced Study Program. I~.F..H was the recipient of a Summer Research Fellowship from the 1V[onsanto Chemical Corporation. Dr. James E. Lohr prepared the second polystyrene sample. REFERENCES 1. ZIS~AN, W. A., Ind. Eng. Chem. 55, 19 (1963); Advan. Chem. Set. 43, 1 (1964). 2. GInI~ALCo, L. A., AND Goon, R. J., J. Phys. Chem. 6l, 904 (1957); J. Phys. Chem. 64, 561 (1960).
3. GOOD, R. J., GmlFALeo, L. A., ANl)KR_~US,G., J. Phys. Chem. 62, 1418 (1958); GOOD,I:L if., Advan. Chem. Ser. 43, 74 (1964). 4. FOWKES, F. M., J. Phys. Chem. 67, 2538 (1063); Advan. Chem. Set. 43, 99 (1964); "Chemistry and Physics of Interfaces," p. 1. American Chemical Society Publication, Washington, D.C. (1965). 5. MELROSE, J. C., J. Colloid Sei. 20,801 (1965); Advan. Chem. Ser. 43, 158 (1964). 6. SC~ONI~O~N, H., J. Phys. Chem. 70, 4086 (1966); J. Phys. Chem. 69, 1084 (1965); Sharpe, L. H., J. Polymer Sei. 132, 719 (1964). 7. S ~ m , E , L. H., ANn SC~ON~{ORN, H., Advan. Chem. Set. 43, ]89 (1964). 8, ZETTLEMOYER, A. C,, CHAN, A., AND GAMBLE, E., J. Am. Chem. Soc. 72, 2752 (1950). 9. CHESSICK, J. J., HEALY, F. H., AND ZETTLEMOVER, A. C., J. Phys. Chem. 60, 1345 (1956). 10. T~O~PSON, W., Physiea 9.6,890 (1960). 11. GRAHAM, D., J. Phys. Chem. 66, 1815 (1962); J. Phys. Chem. 68, 2788 (1964); J. Phys. Chem. 69, 4387 (1965). 12. LoHa, J. E., AN]) SCHOLZ,J. J., J. Colloid Sei. 20,846 (1965). 13. HIGHTOWEn, J. W., AND EMMETT, P. H., J. Polymer Sei., Pt. A 2, 1647 (1964). 14. HANDLES, G. S., Ph.D. Thesis, University of Nebraska (1963). 15. CROWELL,D . M., A N D YOUNG, A. D., "Physical Adsorption of Gases," Butterworths, London (1962). 16, HILL, T. L., EMMETT, P. H., AND JOYNER, L. G., Y. Am. Chem. Soc. 73, 5102 (1952). 17. LoHR, J. E., Ph.D. Thesis, University of Nebraska (1965). 18. D. H. EVERETT,in "The Solid Gas Interface," (A. E. Flood, ed.), VoI. 2. Marcel Dekker, New York (1967). 19. ZI~M, G. H., J. Chem. Phys. 21,934 (1953). 20. Z1MM, B. H., AND LUNDBERG, J. L., J. Phys. Chem. 60,425 (1956).
Journal of Colloidandlnterface Science,Vol. 27, No. 4, August 1968