2788
DONALD GRAHAM
Physical Adsorption on Low Energy Solids.
11.
Adsorption of Nitrogen,
Argon, Carbon Tetrafluoride, and Ethane on Polypropylene’
by Donald Graham Research and Development Division, Organic Chemicals Department, Jackson Laboratory. E . I . d u P o n t de Nemours and C o m p a n y , Wilmington, Delaware (Receiued February 26#1964)
Nitrogen and argon are adsorbed on a solid hydrocarbon surface (polypropylene) as twodimensional gases, nitrogen failing t o show the supermobility previously observed on polytetrafluoroethylene (‘l’eflon). Carbon tetrafluoride and ethane are adsorbed with some restriction of mobility, possibly due to their greater energies of self-interaction, the entropy changes in adsorption being intermediate between those associated with localization and with free inobility in two dimensions. Film pressures a t B coverage representing one statistical monolayer are 12 to 14 ergs/cm.2 for nitrogen, argon, and ethane! but only 9 ergs/cmS2 for carbon tetrafluoride, reflecting its appreciably lower free energy of adsorption.
Introduction An earlier paper2reported thc adsorption of nonpolar gases on polytetrafluorocthylene, a solid characterized by the lowest surface energy of any solid readily available. The present investigation is concerned with the properties of nonpolar adsorbates on polypropylene -more particularly, Kith the adsorbate mobilities and the film pressures. The hydrocarbon polymers, of which polypropylene is one, are “low energy solids” but are more strongly bonded than the perfluorocarbon polymers. The surface energy of polytetrafluoroethylene has been estimated earlier as 56-69 ergs/ cm.2,3but more recent extrapolation of measurements on related liquids indicates a value closer to 40 ergs/ cm. and similarly, for the hydrocarbon plastics, about 50 e r g s / c ~ n . ~ , ~ An isotherm for the adsorption of nitrogen on polyethylene reported earlier5 yielded a linear I3.E.T. plot and a reasonable value for the adsorbent surface area. This isotherm is essentially duplicated in the present study by the isotherm for adsorption of nitrogen on polypropylene, supporting consideration of the adsorbent properties of polypropylene as representative of those of the gcneral class of hydrocarbon polymers.
Experimental The polypropylene used as thc adsorbent was a pon-dercd sample supplied by Dr. L. F. Bestc (Textile T h e .loiirnnl of I’hz/sicnl Chemistry
Fibers Dcpartmcnt, E. I. du Pont de Nemours arid Co., Tnc., Wilmington, Del.), prepared by cracking a Hercules “Profax” resin in a hot mill to a wcightaverage molecular weight of 150,000 and a melt index of 24.1, precipitation from a solvent, and drying under vacuum. It is realized that thc cracking process may have involved some oxidation, but this was not expected to have any appreciable effect on the adsorption of nonpolar molecules, an assumption supported by the similar behavior of the polyethylene adsorbent reported by Zettlemoyer, et aL5 The ethane used was obtained as Phillips research grade and was freed of noncondensible impurities by stripping under vacuum at liquid oxygen temperature. All other materials were the same as used in the carlier paper2 and the same equipment and rriethods of operations were employed.
Results and Discussion Adsorption Data. As in the preceding paper, adsorption isotherms were obtained a t two temperatures (1) Contribution No. 308 from the Research and Development Division, Organic Chemicals Department, Jackson Laboratory, E. I. du I’ont de Nemours and Co., M‘ilmington, Del. ( 2 ) D. Graham, .I. Phus. Chem., 6 6 , 1815 (1962). (3) It. J. Good, L. A. Girifalco, and G. Kraus, ibid., 6 2 , 1418 (1958). (4) Private communications from R. E , Johnson and 11. 11. 1)ettre. (5) A, C. Zettlenioyer, A. Chand, and E. Gamble. J . Am. Chem. Soc.,
72, 2752 (1950).
Table I : Physical Constants of Adsorption Systems Na
Latetit heat of vaporization a t I., i n cal./mole
1300
T I ,O K . T2, O K . Yay, OK. T'tLpor pressures of adsorptive n1ateri:ils at t h e adsorption temperatures, in mm. POC,) (ttt TI) P O ( 2 ) (at 7'2) Capacity of rnonolayer ( Vm) in ml. at S'1'P/g. of aclsorbent Cross;sertional iireit per molecule in A . 2 ( a t 7'",) Adsorbent surface area. i n ni.?/g.
I
I
500
1000
PRESSURE IN mm. Hg
Ar
1580
CF,
3005
CIlrCIJJ
3487
77.4 90.2 83.8
77.5 90.2 83.9
139.5 144.9 142.2
184 6 189.8 187.2
762 2740 23.30
210 1024 27.40
500 745 17.65
761 1001 16.45
16.6
14.1
21.9
23.5
103.4
103.4
103.4
103 4
I5(
3
tlic 1)oiliiig point of t h e :tdsorptive iiiaterial for c w h systvin. . \ l b o , as bvforc,, values of u (crossioiiul a i ~ aof a11 adsorptiw ~iiolcculc)aiid 8 (fract ional c o v c ~ a g cof t h r adsorbeiit siwfacc) i*cprcsmting an a v t ~ a g otcmpcr:iturc w r e uscd i i i thc heat aiid c>nt,ropy calculations. 'The physical constaiits of the systciiis stiidicd are given in Table I. iicx
Figure 2 . ,4dsorpt ion of argon on polypropylene at 90.2 and 77.5"K.
'I'hc adso~~pt~ion isothcrxiis for nitrogin, argon, clarbon tctmflunridc, and cthaiie 011 polypropylene arc shown in Fig. 1-4. I n geiieral form! tlic isothrriiis S ~ J O Wno
40
h
a'
c.
-
v)
I-
a
\ -
-
CL
5
40-
i
E
n W m
30 &
139.5
(L
8
2 30-
W
f!
8 3
LL
a
20W
I-
1 E
n W m
a
0
g
20
U w
z
a
I IW
IL
0
IO
W
5A
9 0
01
0
I I I I 100 200 300 400 500 600 PRESSURE IN mrn. Hg I
I 100
I I I 400 500 600 7 PRESSURE IN mm. Hg
I I 200 300
1:igin.r 4. Adsorption of etharic on polypropylme :Lt 184.0 and 180.8"K.
I
7y
Figure 3 . Aduorpt ion of carbon tetrafluoride on polypropylene at 144.!1 nntf 130.3"K.
unusual charactrristics, but iii the adsorption of ethane there was a tcridericy for pooi4y rcproduciblc hystercsis in the l o w r part of thc isotherm. This niay be due to niicrocapillai~icsor w e i i to soiiie pmctratiori aiid swelling of the polyiner. 'Hie lower part of the isotherin was not used in the thcrinodynainic calculations, as explained later, so this question was not pursued furthcr. Isosterzr Heats oj Adsorption. T h e isosteric heats of adsorption of each of the four adsorptive niatcrials 0x1 polypropylcnc at tenipcratuim near their respective boiling points werr calculatrd by application of the Clausius Clapcyron cquatiori to the adsorption data. l'hc results arc plotted in F'ig. 5 . In cvciy casc, the initial high hcats (usually ascribed to adsoi~bcnthctcrogmrity) cxtrnd to unusually high cmwagt' (near or ding e = 0.5). The sanir typc of heat curve \vas i*cportcdby Zettlenioyer, et d.,; for the adsoi ptiori of nitrogen on polycthylciicl, suggesting that high surface hctwogcncity niay I)(> cxharactrristic of hydroc:Lrboii plastics. Dur to t h c w high initial hcats, thc
calculation of adsorption cntropivs \vas limited to coverages ot 6 = O..i or grrater. ing to iiotc that the polyatoiiiic molecules, carbon tc4-afluoi idc aiid cfhaiic', arc2 adsorbed ith isosteric h a t s roughly tu I W thosc for nitrogcri arid argon, i n line R itli their btrongrr self-interaction as iiwasured by the latcwt hcats of condensation. AQrgonis the oiily adsorbate yielding isostcric heats close to or loiwr than the latent heat of co~itlensation. Adsorbate dlobzlit!y. As i n the preccding paper, adsorbate mobility is evaluated in tvrrns of standard differential entropies of adsorptioli calculated f'roiii the adsorption data and coiuparcd with thosc froin throretical, entropically idral procmses rcprcwiitiiig adsorption on fixed sites and as a inobilr two-diinensional gas. 'l'hc method 01 calculation and thc notation employed arc thosc. proposed t)y dc I3ocr and l i r u y ( ~ . ? The results ai report (4i n 'l'ablc 11 In each case, the ohsei\-c.d loss of entropy IS lcss than that ivhich \vould occur if the. a d s o h a t c wer(1 loralizcd. That is, - ~ s ' " (a , coinbination of t h c cxpciiinentally obswvcd cwtropy changc with :L configurational tcrrn) (1
(6) ,J H tic Boer alid S K r u \ r r I'IOC ~ 0 1 a pB55, 431 (1032)
lionakl .Ycd Akad T r t f n -
Table I1 : Comparison of Observed Entropy Changes with Those of Idealized iZIodels Representirig Sit(?Adsorption arid 1Iobile Adsorpticm
Nitrogen 0 5 0 7 09
1‘1 2 lcj8 16 4
53 5 41 7 323
1725
I T io 1TOO
14 2 14 1 12 0
20 6 29 0 20 6
0 3 10 2 10 3
10 0 10 0 10 0
31 7 31 7 31 7
x 9 8 7 8 9
10 7 10 7 10 7
3.5 7 33 7 33 7
17 3 16 9
.\rgon
0 5 07
1000 1s40
32h
13 9
13 8
340
12 6
0 (1
1.720
2,iO
14 3 1%; 0
0 5 0 7
3400 3 610
333 224
0 9
3“)
1Ofi
21 f; 23 1 23 2
0 .i
407.3 4110 1110
782 582 487
19 x 21 I 21 x
10 G
Carbon tetrafluoride 21 6 21 4 18 8
16 J
11 7 11 7
I1 7
1q:tharrr 0 7 0 0
50001I
J 0
P
i U
0
-
+ In
I 4
ETHANE
4000
I
T E T R A FLUORIDE
cn
8
2000
0 a ARGON
fn
5: ..
0
0
0
1
0
0.5
1.0
FRACTIONAL COVERAGE, 0
33 9
10 % 17 4
53 9 38 (4
14 1 13 0 15 2
10 0 10 0
10 9
is in every case much sinallw than (the translstioiial entropy of the adsorptivcx gas). The test for iiiobile adsorptioii is a coinp:triboii of - AA”,,, (a eoiiibination of thr wpciiiiicntal entropy change with a iiicasure of the arm OVCI’ nliich aii adsorbed niolcculc is free to i n o w a t a spccificd coveragv) with $Sotr - .Sot, (thc diffcwnc.c brtwwii tkic traiislatiorial entropy of thc adsorptiw gas arid that of the adsorbate as art idral, mobilc, tn o-diiiicmsioiitil gas, or the cmtropy change resulting froin loss of on(’ cicgrw of translational frcedotii). Carboii trtrafluoridc arid ethane yield values of - 4S0,, soinwhat greattv than the con-esporidiiig (,Sot, - Jot,) indicating that whilc riot localized, their rnobility is soiiwnhat imtrictcd. This result is in line with thcir largci* hcats of adsorption. 111 thcl cas(’ of iiit~ogcii.t h c ciitcria fov a inol)ilc, two-ditiicnsiona1 gas arc’ closcly iiiatchcd by thv data higoii, h o i w v ~ ~ iyicllds ~, valuc>s of slightly sinallcr thari thc rorwspondiiig valucl of (gSotr - ,,Sot,.). 1 hv loss of cmtmpy in thv adsorption proctx is thus slightly less than that t*c~pivsc~ntiiig 0 1 1 dcyy ~ of truiislatioiial fwdoiii, iridicatirig that argon rcitains sonip “supcriiiol)ility,” possibly as a vibration iiotiiial to t n e adsorbcii’ sui.facc. This i ~ s i ~ ~ ~ c ~ i ~is~ cwn~i~bilit~” sistcwt with the ohwrvatioii, iiic~itioiierl c w ~ l i c ~that , thr isostciric Iioat of adsorptioii of argon on polypropylC I I C is slightly IPSS than its latc~it h w t of coiidcnsal ion. r .
W b-
IO s
I ~ O N A LGRAHAM I)
2792
Film Pressures. Film pressures, or changes in the surface free energy with adsorption, were calculated from the adsorption data for a coverage of one statistical monolayer by graphic integration of thc Gibbs equation as has been described.’ The values obtained for nitrogen, argon, and ethane were 12, 13, and 14 ergs/cm.2, respectively. I n contrast, the film pressure of a statistical monolayer of carbon tetrafluoride was only 9 ergs/cm.2, a value consistent with its lower free energy of adsorption.
Conclusions Polypropylene adsorbs the nonpolar gases, nitrogen, argon, carbon tetrafluoride, and ethane, with small to moderate heats of adsorption, the low coverage results indicating extensive surface heterogcneity. Nitrogen and argon are adsorbed as mobile two-dimensional gascs, argon retaining some “supermobility.” The polyatoniic gascs, carbon tetrafluoride and ethane, were adsorbcd with sornewhat restricted mobility, but the film prcssure of carbon tetrafluoride was lower than those of the other adsorbatcs, reflecting its lower frec energy of adsorption.
Discussion J. H. DE BOER(The Ilague, Netherlantls). I n your introductory remarks you refer to the independent) proof t h a t 16.2 A . 2 is the right value to use for the molec.ular surface area of nitrogen, based on studies of the adsorption of iodine on carbon. It may be interesting to note that our recent work on the adsorption of lauric: acid on aluniina shows t h a t one lauric wid nio1t:cule is adsorbed for every four i2yygen atoms of the surface. This eBec*t for a lauric acid molecule in this parleads to a value of 26.9 ticular type of adsorption and confirms the value of 16.2 ii.z for nit,rogen. P. K . I S A A ~(W. S R. Crace Company, Clarksville). W’oultf a higher energy surface than polypropylene have less tendency to give rise to polyniolecular layers? You seen1 to iriil)ly t h a t polymolecular layers are peculiar to low energy surfacw below the saturation point. I). GRAHAM.Physical adsorption at :my ( w e r a g e is favored by a high adsorbent surface energy.
On a high energy surfaw, the first monolayer may he cwnpleted at a low relative pressurc with rnultilayer deposit,ion becoming appreciable as the vapor pressure of the adsorptive gas is itpproached. On a low energy solid adsorbent the first statistic:tl nionolayer retains wnsiderable mobility and requires a higher relat,ive pressure f o r i t s completion, tending to obscure the usual sigmoid intlct+,iori in the isotherrn, i.e., there is no well defined “point I3”-see Pig. 1-4. (7) W . D. Harkins, “The Physiral Chemistry of Surface Films,” Reinhold Publishing Corp., New York, N. Y . , 1952, p. 211.
