M. BLANK AND R. H. OTTEWILL
2206
The Adsorption of Aromatic Vapors on Water Surfaced&
by M. Blanklb Department of Physiology, College of Physicians a n 8 Surgeons, Columbia University, N e w Y o r k , X e w Y o r k
and R. H. Ottewill Department of Physical Chemistry, University of Bristol, England
(Receiued M a r c h 8,1964)
The adsorption of benzene, toluene, o-xylene, and chlorobenzene on the surface of a 0.1 M sodium chloride solution was studied as a function of the partial vapor pressure. The surface tensions were measured by the dipping plate method and the surface potentials were measured with a Radium 226 air electrode and an electrometer. (The surface potentials of adsorbed films of cumene were also examined.) By applying Gibbs’ adsorption equation to the surface pressure-vapor pressure data, the adsorption isotherms were calculated. The dependence of the surface pressure on the molecular area indicates that the monolayers are gaseous. The free energies, enthalpies, and entropies of adsorption were calculated from measurements a t different temperatures, and the entropies were consistent with the loss of one degree of translational freedom on adsorption. The surface potentials for all adsorbates correlated with the surface tensions and indicated that a t low surface coverages the surface potential depended largely upon the interaction between the water and the phenyl ring. h’ear saturation pressure, when the film molecules were close packed and oriented, the surface potential also depended upon the permanent dipole moment of the adsorbed vapor.
Introduction Since the surfaces of liquids are homogeneous, they are ideally suited for studying adsorption. I n recent years, the adsorption of a number of vapors on water surfaces has been studied by measuring the variations in surface tension with partial pressure of organic vapor. 2--7 The surface pressure-area characteristics of several adsorbed films and the changes in the thermodynamic functions upon adsorption have been determined. The related problem of the adsorption of vapors on insoluble monolayers (on water) has also received attention, largely because of the dependence of monolayer properties on the spreading solvent.8-10 These studies have found that the interaction of vapors with monolayers depends upon the manolayer concentration as well as the vapor pressure. Another liquid surface that has been used to study the adsorption of vapors is niercury11t12; and in this case, the surface potential changes have been measured. Surface potentials indicate changes in the orientation of dipoles at the surface and therefore provide additional information about the adsorption process. In the present work, the T h e Journal of Physical Chemistry
change in surface potential (as well as the surface pressure) was investigated as a function of the vapor pressure for benzene and several substituted benzenes. The (1) (a) An account of this work was presented before Dhe Division of Colloid and Surface Chemistry a t the 144th National Meeting of the American Chemical Society, Los Angeles, Calif., April, 1963; (b) Supported by a Research Career Development Bward (GM-K38158) and a Research Grant (G11-10101) from the U. S. Public Health Service. (2) (a) L. I. -4.Micheli, P h i l . M a g . , 3, 895 (1927); (h) H. Cassel and F. Formstecher, Kolloid-Z., 61, 18 (1932). (3) R. 1%. Ottewill and D. C. Jones, Aratu7e, 166, 687 (1950). (4) C. L. Cutting and D. C. Jones, J . Cham. Soc., 4067 (1955). (5) D. C. Jones and R. H . Ottewill, ibid., 4076 (1955). (6) D. C. Jones, R. H. Ottewill, and A. P. J. Chater, Proceedings of the Second International Congress of Surface Activity, Vol. 1, Butterworth and Co. Ltd., London, 1957, p. 188. (7) R. H. Ottewill, Ph.D. Thesis, Queen Mary College, London, 1951. (8) K. E. Hayes and R. B. Dean, J . P h y s . Chem., 57, 80 (1953). (9) H. D. Cook and H. E. Ries, Jr., ibid.,60, 1533 (1956). (10) R’I. L. Rohhins and V. K. La Mer, J . Colloid Sci., 15, 123 (1960). (11) C . Kemball and E. K. Rideal, Proc. R o y . SOC.(London), A187, 53 (1946). (12) C. Kemhall, ibid., A201, 377 (1950).
ADSORPTION OF AROMATIC VAPORSON WATERSURFACES
study of this group of related compounds has enabled further information to be obtained about the nature of the adsorbed film and about the interaction brtween the adsorbate aiid the water surfaccl.
Experimental Materials. Water was doubly distilled froin an allglass apparatus. h 0.1 M sodium chloride solution was made up using this water aiid reagent grade KaC1 which had been roasted for 5 hr. a t 700'. The vapors wercl fornied by bubbling water pumped nitrogen through the> pure liquids of reagent grade or better (purchased from Matheson, Coleman and Bell). The boiling ranges of the liquids were : benzene 80.080.3', o-xylcne (1,2-dimethylbenzene) 14.3-144.5', cuiixnc (isopropylbenzene) 131- 133', and chlorobenzene 130-132'. The toluene, which was used to standardize t h t technique (gas flow rate, electrode position, etc.), was a Chromatoquality Reagent 99+ mole yo. Liquids of reagent grade (from British Drug Ilousts, Ltd.) with similar specifications were used for thc work done in England. Vapor J'ressurcs. Vapor prcssurcs of benzene and toluenc. w c w calculated from the formula log p,,,
=
0.05223 A+B T
(1)
For benzene A = 44,222 and B = 9.846 and for toluene A = 34,172 arid H = 7.966. The vapor prtwures of oxylcnc wcre interpolated from data given in the International Critical Tables. For curnene, the vapor pressures were calculated from a forrnula, similar to eq. 1, given in thc A.P.1. Research Project 44 of the Kational Hurcau of Standards. The vapor pressures of chlorobenzene were Calculated from an equation (containing five constants) givcn by hliindel. l 3 Measurement of Adsorption and Suyface I'otmtial. Thc apparatus was csscntially the same as that used by Jones and Ottewill.6 The adsorption vessel, however, has a deeper inner trough aiid the nitrogen-vapor niixture splits into two streams which enter the cell in two tangential jets. This achieves good mixing with a minimum of disturbance to the surface. ,4 0.1 M sodium chloride solution was used in the trough. Surface pressure measurements were made using a torsion balance in conjunction with a roughened mica (or platinum) plate. The surface potential measurements were madc using a scaled Radium 226 air electrode (purchased from U. S. Radium Corp.) in the air just above the trough and a silver-silver chloride clectrodc in the sodium chloride solution. The air electrode was connected to a Keithley electrometer (Model 610A). The best position for the electrode appeared
2207
to be about 2-5 mm. above the surface near the center of the trough. The potential readings were quite stable but it was found helpful to surround the electrode with a grounded cage. The surface in the trough was cleaned by overflowing and the potential of the clean surface read off (Vo). At the same time, the scale reading obtained with the mica plate in the surface was taken as corresponding to the surface tension of the clean surface (yo). Sitrogcnorganic vapor mixture was then admitted to the cell at a rate of 650 cc./min.; this corresponded to a complete change of atmosphere in the cell every half minute. Readings were usually constant after about 2 min., and were normally recorded after d niiii. Icroni the reading of the potential (V), the surface potential was calculated as A V = V o - 1'. The surface pressure, A = yo - y, the difference bc>twecnthe surface tension of pure water (yo) arid a film-covered surface (7). The partial vapor pressure, p , of the organic liquid was increased gradually until the saturation pressure, po, was reached. Experiments were carried out at 15' unless otherwise indicated.
Results and Discussion The curves of A V and A us. partial pressure of vapor, p , for benzene, toluene, o-xylene, and chlorobenzene arc presented in Fig. 1 and 2. The AT7 us. p curve for cumene is given iii Fig. 3.
t
.
d'
10
c
L
o
z
4
7 0
s 2 v)
.iP
2
10
20
30
40 80 4 Vapor pressure, mm.
12
8
10
Figure 1. Surface pressure us. vapor pressure and surface pot~entialus. vapor pressure curves for the adsorption of benzene and toluene on water a t 15": -0-, benzene; -0-, toluene. ~~
(13) C. F. MUndel, 2 . p h y s i k . Chcm., 8 5 , 435 (1913).
Volume 68, Sumber 8 Airgust, i , W 4
2208
M. BLANKAND R. H. OTTEWILL
20
16
$
1a
101
al
s
3
3 r n
51
2
4
6
8
2
4
6
8
Vapor pressure, mm.
Figure 2. Surface pressure us. vapor pressure and surface potential us. vapor pressure for the adsorption of o-xylene o-xylene; and chlorobenzene on water a t 15": -0-, chlorobenzene.
I
1 ba RTblnp
= -~
The curves of r us. p were all of the same general form and obeyed the general relationship
r=----U P
1 - bp
(3)
where a and b are constants. A typical example, the curve for benzene at 1 5 O , is given in Fig. 4. Comparison of the adsorption isotherm with the AV us. p curve T h e Journal of Physical Chemistry
2.0
2.5
Figure 3. Surface potential us. vapor pressure for the adsorption of cumene on water at 15".
The 7~ us. p curves are similar to those obtained in earlier work with aliphatic hydrocarbons, anisole, phenetole, carbon tetrachloride, fluorobenzene, and chl0roform.~~6The AV us. p curves have the same general form for all the materials investigated in the present work. In the low vapor presure region AV changes very little, then a t the point where a starts to increase rapidly AV also starts to increase rapidly. As saturation pressure is approached, AV appears to level off. The adsorption excess of the organic vapor a t the interface, with the adsorption of water taken as zero, was derived from the a us. p curves using Gibbs' adsorption equation
r
1.0 1.5 Vapor pressure, mm.
0.5
shows that both AV and a start to change rapidly in the same p region, i.e., p > 30 mm. The surface area per molecule can readily be derived from the surface excess using the relationship
(4) where N = Avogadro's number. The a us. A and AV us. A curves for benzene at 15' are shown in Fig. 5. It is clear from the a us. A curves that the films are gaseous. From plots of aA us. a, it appears that the attractive term predominates and that the aA values fall increasingly below the ideal value as a increases. Adsorption Measurements. The Gibbs' free energy of adsorption, -AG, can be calculated6 from the intercept at T = 0 o n a plot of log ~ / us.p a. The values calculated from the data presented here and from some earlier work of Ottewill' are given in Table I. The enthalpies and entropies of adsorption were also calculated, where possible, from the variation of the free energy of adsorption with temperature using the Gibbs-Helmholtz equation AG
-
AH
=
T(bAG/dT)
=
-TAS
(5)
ADsoriPTroN O F
AROMATIC VAPORS
2209
ON W A T E R S U R F A C E S
Table I : Free Energy, Enthalpy, and Entropy of Adsorption on Water Temp., "C.
Material
0.0
Benzene
7.5
15.0 5.0
Toluene
15.0 &Xylene
15.0
AH! oal.
A Q,
cal.
4037 4022 3968 4569 4467 4452
AS, B.U.
6174
7.2
7681
10.2
The translational entropy of a molecule which has three-dimensional freedom is given by the Sackur-Tetrode equation 10
20 30 Vapor preaeure, mm.
40
60
Figure 4. The surface excess of vapor us. vapor pressure curve for the adsorption of benzene on water a t 15".
aStrane = 2.303R log (M"'T"/') - 2.30
(6)
where M is the molecular weight and the entropy is in cal. deg.-l for a pressure of 1 atm. An expression for the two-dimensional translational entropy has been given by Kernball" as 2Stranl
=
2.303R log ( M T u )
+ 65.80
(7)
where a is the area per molecule, defined in the standard state as equal to (22.531') A.z If it assumed that adsorption involves only the loss of one degree of translational freedom, the entropy of adsorption, AS, is given by eq. 6 minus eq. 7. In Table I1 a comparison
Table 11: Theoretical and Observed Entropy Changes
Figure 5. The surface pressure and surface potential us. the area occupied per molecule for the adsorption of benzene on water a t 15".
The values of A H and A S are also given in Table I. The A H of adsorption is comparable to the A H of vaporization. The A S values are discussed below.
1Stnna
&trans
&ana ast.all6
ASexp f
Materia
Benzene Toluene
38.83 39.33
29.95 30.28
8.88
9.05
7.2 10.2
20%
is made between the values calculated theoretically and those obtained experimentally for benzene and toluenc. The agreement, allowing for experimental error, is sufficiently close to suggest that adsorption involves essentially only the loss of one translational degree of freedom. Therefore, the adsorbed film must be a mobile one. The curves of r us. p show a continuous increase of I? up to po. Calculations of the percentage monolayer, d = lOOr'/r,,,, where rrn= the surface excess for a vertically oriented monolayer, indicate that in sonw cases niultilayers were formed. Since water has a homogeneous surface, multilayer formation cannot occur as a consequcnce of capillary condensation but can be accounted for by thc concept of a disjoining Volume 68,;Vumber 8 August, 1964
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M. BLANK AKD R. H. OTTEWILL
pressure. In the case of condensation on glass, Derjaguin and Zorin14 found that niultilayers were usually formed when p / p o was between 0.95 and 1.0. In the case of the benzene-water system, the surface tension of benzene against air a t 15' is 29.4 dynes/cm. and the interfacial tension of water-benzene is 34.5 dyneslcm., a total of 63.9 dyiies/cm. From the present work, the surface tension of water in equilibrium with benzene vapor was found to be 63.6 dynes/cm. or 0.3 dyne/cm. lower. As pointed out by Kitchener,16 the surface free energy in the adsorbed layer of benzene must increase with the thickness of the layer. This is equivalent to a negative disjoining pressure in the multilayer. In the case of benzene, on the basis of vertically oriented molecules the monolayer was completed a t a p/po of about 0.96. This would be consistent with a small negative disjoining pressure in the multilayer. Surface Potential Measurements. There is a difficulty in interpreting changes in surface potential since adsorption inevitably occurs on the electrode as well as on the surface. To assess the importance of adsorption on the air electrode two mater surfaces were used; one was kept clean while toluene vapor, at various p's, was circulated over the other. The radium electrode was then passed many times between the two surfaces and the AV readings, obtained from 15 to 20 sec. after transfer, mere as if adsorption had not occurred on the electrode surface. In another test, three different air electrodes were used to measure the AV due to an adsorbed film of toluene and the values agreed to within the experimental error, even though the absolute potential values differed by over 100 1 1 1 ~ . The AV obtained with an electrode that has been immersed in water is the same during the several days it takes for the absolute potential to return to its original value. Therefore, it appears safe to conclude that adsorption on the air electrode did not affect the general nature of the results. The change in the potential across an interface, AV, caused by the adsorption of a molecule may be written as
AV
=
4Hnp
4- $0
(8)
where p = the surface dipole moment, n = the number of dipoles per cni.,2 and $o = the potential drop due to the formation of an electrical double layer a t the interface. (In the present work all the adsorbates were nonionic and $o = 0.) Although p is usually ascribed to the dipole moment of the adsorbed molecule, it also iiicludes the effects of the reorientation of water dipoles in the surface. Table I11 gives a suinniary of the maxiinuin values of p , II,and AV for the various vapors. (The value for T h e Journal of Physical Chemistry
Table 111: Properties of Vapors and Adsorbed Films -Vapor--Pressure, mm., Substance
Benzene Toluene o-Xylene Cumene Chlorobenzene
15'
58.5 16.8 8.10 2.40 6.70
Dipole moment, e.8.u.
0 +0.36 $0.62 +0.79 -1.70
----Adsorbed Maximum surface pressure, dynes/om.
9.9 9.1 5.7 4.9est.
5 8
filmMaximum surface potential, mv.
142 210 232 194 116
the maximum surface pressure of cumene was estimated using a lens of cumene on a 0.1 M SaCl solution a t 1 5 O , a technique that gives the correct value for toluene.) There appears to be no relation between AV and the parameters of the adsorption, PO or II. However, AV appears to be related to a niolecular parameter, the dipole nioment ( p ) , also given in the table. I n Fig. 6 the surface potential values a t po are plotted against the dipole moment of the adsorbate in the vapor phase. The values also have been corrected for the different areas of the adsorbate molecules using: (1) the projections of vertically oriented molecular models onto a surface, (2) the limiting surface areas on surface pressure-area curves (where available), and (3) 300t
6, J *
'bbbi5 J j , 11 .b/,
I
Permanent dipole moment of adsorbate, e.8.u.
Figure 6. The maximum surface potential vs. the dipole moment of the aromatic adsorbate in the vapor phase: -e-, measured values; -(I corrected -, for the different areas per molecule. (14) B. V. Derjaguin and 2. M.Zorin, Proceedings of the 2nd International Congress of Surface Activity, Vol. 2, Butterworth and Co. Ltd., London, 1957, p. 145. (15) J. A. Kitchener, Endeavour, 22, 118 (1963).
ADSORPTION OF ARONMATIC VAPORS ON WATERSURFACES
the molar volumes. This procedure corrects the AV readings to the same concentration of adsorbed dipoles as that of benzene. Although benzene does not have a dipole moment, a AV of 142 mv. was found. This must arise from the interaction between the phenyl ring and the water dipoles a t the surface. The interaction varies as a function of p , as in Fig. 1, presumably because of variations in the concentration and orientation of phenyl rings on the surface. Since a phenyl group was common to all the vapors examined, it is reasonable to assume that deviations from the value of AV obtained for benzene (deviations from thle dashed horizontal line of Fig. 6) arise from the additional interactions due to the permanent dipole moments of the molecules. This suggestion is supported by the fact that the deviations from the AV for benzene vary with the sign and the magnitude of the dipole moments. In studying the adsorption of toluene on mercury, Kemball12 found a sharp change in AV corresponding to the reorientation of the toluene molecules from the horizontal to the vertical. Although the surface isothemns as well as the AV changes indicate that the toluene molecules reoriented a t higher p , the observed variation of AV was gradual. This is probably due to the additional interaction of toluene with the water dipoles. [It may be no coincidence that when this interaction is subtracted (as in Fig. 6)) the AB for toluene on water (68 mv.) agrees approximately with Kemball’s value of 55 mv. on mercury. ]
2211
The initial orientation of water dipoles a t a waterair interface can be inferred from the magnitudes of AV for the different vapors. The data indicate that toluene causes a much greater AV than chlorobenzene, even though it has the smaller dipole moment. This suggests that the toluene molecule adsorbs without causing as great a rearrangement of the surface water dipoles. Therefore, the water dipoles must have the same orientation as the permanent dipole of the adsorbed toluene molecules, namely, with the hydrogens directed toward the vapor phase. In the adsorption of chlorobenzene, where the dipole moment is of opposite sign, there is considerable rearrangement of the water dipoles and the effect of the permanent dipole moment of the adsorbate on AV is obscured. The orientation of the water dipoles with their hydrogen atoms pointing toward the gas phase is in agreement with the findings of other workers.lo
Acknowledgments. Some of the work reported in this paper was done in the Department of Colloid Science, Cambridge, England. We wish to thank the U. S. Public Health Service for supporting M. B. for a visit to the Department of Colloid Science during September to December, 1961. We also wish to thank Dr. D. C. Jones for discussions during the earlier parts of this work.
(16) D. A. Haydon, KoZEoid-Z., 185, 148 (1962).
Volume 68, Number 8 August, 1961,
