4212
J. Phys. Chem. 1982, 86, 4212-4216
Size of Sodlum Dodecyl Sulfate Mlcelles in the Presence of Addltives. 2. Aromatic and Saturated Hydrocarbons Mats Almgren' and Shantl Swarup Physical Chemlstfy I, Chemical Center, University of Lund, S-220 07 Lund, Sweden (Received: February 23, 1982; In Final Form: June 18, 1982)
The steady-state fluorescence quenching method has been used to investigate how the size of sodium dodecyl sulfate (SDS)micelles changes on increasing the volume of the hydrophobic micelle constituents, either by addition of hydrocarbonsor by addition of sodium tetradecyl sulfate. It is found that the micelle sue is controlled mainly by the surface charge density: on addition of hydrocarbon the micelle responds by growing until the number of ionic heads per surface area is about the same as in the original micelle. This is so both if the hydrocarbon is incorporated by solubilization and by the addition of a long-chain homologue. The data give no evidence that the normal SDS micelle is in a constrained state, in spite of the fact that its hydrophobic radius is longer than a C12 alkyl chain.
Introduction The new techniques of fluorescence quenching aggregation number determinati~nl-~ and Fourier transform NMR pulsed-gradient spin-echo self-diffusion measurement4.5make possible a detailed study of micelle size and composition in the presence of various additives. In a previous article6we have presented results for SDS micelles with polar additives, in particular alcohols. The main conclusion was that the surface charge density is the most important controlling factor for micelle size. Moreover, all n-alkanols from C4 to Cl0, and in addition tert-butyl alcohol- and benzyl alcohol, give approximately the same surface charge density a l e (or surface area per charged head group) at the same mole fraction, X 2 ,of alcohol in the micelle. The variation of a l e with X 2 was close to linear over a wide composition range, 0.25 5 X 2 5 0.75. A linear relationship of this kind can be only a rough first approximation in these very complicated system^.^^^ It is nevertheless tempting to relate the deviation from linearity at low mole fractions with the fact that the unperturbed SDS micelle is larger than what would be allowed if the shape were spherical with a hydrophobic radius shorter than the length of a C12alkyl chain. One question that we want to address is this: how important is the length of the alkyl chain as a constraint on micelle size and shape? Already Hartley@clearly recognized that this was an important factor. Tartarlo and later Tanford" stressed such constraints in their discussions of micelle size and shape, and more general packing constraints were used exhaustively in the theories of amphiphile aggregation by Israelachvilii, Mitchell, and Ninhaml2-I4and by Jons~on.'~~ (1) Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978,100,5951. (2) (a) Almgren, M.; Lofroth, J.-E. J.Colloid Interface Sci. 1981,81, 486-99. (b) Almgren, M.; Lofroth, J.-E. J. Chem. Phys. 1982, 76,2734. (3). (a) Atik, S.; Nam, M.; Singer, L. Chem. Phys. Lett. 1979, 67, 75. (b) Llanos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100. (4) Stjlbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176, 215. (5) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (6) Almgren, M.; Swarup, S. J. Colloid Interface Sci., in press. (7) Jonsson, B. Ph.D. Thesis, University of Lund, Lund, Sweden, 1981. (8) Jonsson, B.; Wennerstrom, H. J. Colloid Interface Sci. 1981, 80, 482. (9) Hartley, G . S. Kolloid-2. 1939,88, 22. (10) Tartar, H. V. J.Phys. Chem. 1965,59, 1195. (11) Tanford, C. "The Hydrophobic Effect"; Wiley: New York, 1973. (12) Israelachvilii, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1976, 72, 1525. 0022-3654/82/2086-4212$01.25/0
The importance of such constraints is thus well settled for the gross features of the aggregation behavior: if small micelles, long rods, vesicular or liquid crystalline lamellae, etc., will form. The importance and significance of the constraint for the shape and size of small globular micelles is less evident. As pointed out by Hartley15 several mechanisms could lessen the restrictions. Firstly, the micelles are not in reality smooth spheres under any conditions. They are made up of monomers with varying protrusions and in liquidlike dynamic motion.16 This fluctuating, rough surface could remain spherical in the mean and yet envelope a hydrophobic volume substantially larger than that of a sphere with radius equal to the length of the monomer alkyl chain. Secondly, further growth can be explained by transient formation of a cavity in the center of the sphere. The free-energy cost of formation of a cavity with 1-8, radius in a bulk hydrocarbon could be in the order of 1-2kT. The gain by allowing the micelle radius to grow could be bigger.17 Another possibility is that one of the monomers gets its head group partly buried in the hydrophobic parts of the micelle. This is a clear possibility for SDS, where the uncharged ester oxygen atom could be withdrawn from water contacts without prohibitively high free-energy costs. This is less demanding than a similar proposal by Lianos and Zana3bwho recently suggested that the ester oxygen should be included in the hydrophobic core, in order to explain why the normal SDS micelle has an aggregation number substantially higher than that of micelles formed by most other C12 surfactants. Implicit in the above reasoning is that the spherical shape would be most favorable in the absence of geometrical constraints. There are two reasons for that. Firstly, the sphere gives the smallest interface for a given volume. If a certain interfacial area per volume unit (or area per monomer) is aimed at, the spherical shape will allow the smallest particles which are entropically favorable. Secondly, it is a well-known consequence of the laws of elec(13) Israelachvilii, J. N.; Mitchell, D. J. Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (14) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (15) Hartley, G. S. In 'Micellization, Solubilization, and Microemulsions";Mittal, K. L., Ed.;Plenum Press: New York, 1977; Vol. I, p 23. (16) Aniansson, E. A. G . J. Phys. Chem. 1978,82, 2805. (17) Jonsson, B., private communication.
0 1982 American Chemical Society
Size of SDS Micelles in the Presence of Additives
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4213
TABLE I: Size and Composition of SDS Micelles in the Presence of Hydrocarbons at 22 "C [RH], [micellel-', [SDSI, mM mM-' NSDS NRH R, A M 0.138
0.512
69.6
area/NSDS, A 2
18.1
59.2
20 43.6
20.2 22.6
60.3 58.1
8.6 19.4 27.5
19.8 21.1 21.9
57.0 57.6 57.6
11.1 23.5 37.5 40.8
19.7 20.6 21.4 21.1
57.6 58.9 59.7 62.3
Hexane 0.138 0.138
32 53.6
0.625 0.813
0.135 0.135 0.135
13.3 26.6 35.0
0.65 0.73 0.788
0.135 0.135 0.135 0.135
19.0 37.6 56.4 66.0
0.637 0.681 0.725 0.675
85.0 110.5
Heptane 86.4 97.1 104.7
Toluene
tricity that charges tend to accumulate on highly curved surfaces. On an ellipsoidal or spherocylindrical micelle there will be an electrostatic striving toward an uneven charge distribution, with higher charge density at the curved ends. This is counteracted by the finite sizes of the monomer: indeed, the packing constraints would suggest a lower head-group density at the high-curvature ends of the micelle. The spherical shape would be much more favorable electrostatically. A second issue which is brought up by the present results is the site of solubilization. Hartley18 measured the solubilization of azobenzene in various micelles in order to show that the micelle interior behaves like a liquid. He also found clear evidence for solubilization at the surface and rationalized differences in the solubilization capacity between alkylpyridinium surfactants with different counterions in this way. The NMR studies of Eriksson and Gillberglg showed a difference between saturated and aromatic hydrocarbons. Benzene but not cyclohexane was solubilized preferentially at the surface of CTAB micelles. Some controversy over the solubilization site in the case of benzene, caused by the interpretation of other spectroscopic evidence, was largely cleared out when Murkerjee et aL2OS1gave quantitative evidence for a "two-state model" in the case of benzene and some derivatives in various micelles. Almgren, Grieser, and Thomas22found from solubilization data a clear difference between alkyltrimethylammonium surfactants and alkyl sulfates in their solubilization capacity for aromatic hydrocarbons. Pyrene was thus found to have a greater solubility in CTAB, and also in dodecyltrimethylammonium bromide than in SDS. In SDS the solubility was close to that expected for droplets of pure hydrocarbon solvent. It was concluded, therefore, that at least the "extra" amount solubilized in CI2TABwas at the surface. The difference between CTAB and SDS is noteworthy. As has been pointed out before,z2 there are also other facts that indicate a specific interaction between aromatic molecules and the alkyltrimethylammonium head group. Many aromatic molecules induce a sphere-to-rod transition in CTAB, and indications have been found for the formation of a weak complex between pyrene and tetramethylammonium ions in a homogenous ethanol solution.22 Molecules of similar size but solubilized in different regions of the micelle would be expected to affect the (18) Hartley, G. S.J. Chem. SOC.1938, 1968. (19) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966,20, 2019. (20) Murkerjee, P.;Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620. (21) Cardinal, J. R.;Murkerjee, P. J. Phys. Chem. 1978, 82, 1614. (22) Almgren, M.; Grieser, F.; Thomas, J. K. J.Am. Chem. SOC.1979, 101, 279.
84.7 90.5 96.4 89.0
micelle size differently. We have therefore compared the effects of toluene and saturated hydrocarbons on SDS micelle size.
Experimental Section Materials. Sodium dodecyl sulfate (SDS), BDH specially purified grade, was used as supplied. Samples from two batches gave similar results, and samples of the same quality have been shown to be of acceptable Sodium tetradecyl sulfate (STDS, Eastman), 9-methylanthracene (9-MeA, Aldrich), and 2,2'-rutheniumbipyridyl perchlorate (Ru(bpy)t+,G. Fredrick Smith) were all used as supplied. Hydrocarbons and alcohols were of highest commercial qualities. Measurements. All luminescence measurements were made with an Aminco SPF-500 spectrofluorimeter, using a cell in a thermostated holder at 22 OC. The concentration of 9-MeA in the micelle solution was assessed by absorption measurement at 388 nm using a molar absorptivity of t = 7500 M-' cm-l. Calculations. The reciprocal micelle concentration was obtained from the slope of plots of In (Zo/l) vs. [Q]'p2 where Zo and Z are the luminescence intensities without and with quencher, respectively. The aggregation number of surfactant and additive were calculated from their concentrations in the micelles.
NZ= [Almice~~e/[micellel
(1)
The concentrations of the additive in the micelles was calculated from Stilbs' results5 or for the most insoluble species taken as the total concentration. The concentration of surfactant in micelle form was not taken as the total concentration minus the cmc, but the concentration of the free surfactant was instead estimated from published data showing the decrease above the c ~ c . ~ ~ The hydrophobic volume of the micelle was taken as the sum of contributions from the components; these contributions were in turn calculated either from group contributions (49 A3 per CH3, 28 A3 per CH,)' or from the molar volumes of the pure liquids, or by a combination. The radius and the area per surfactant monomer were then obtained by assuming a spherical shape. As discussed earlier2*25 the static fluorescence quenching method gives too small aggregation numbers for large micelles. As a rule of thumb, SDS micelles with a radius (calculated as above) (23) Missel, P.J.; Mazer, N. A,; Benedek, G. B.; Young, C. Y.; Carey, M. C. J . Phys. Chem. 1980,84, 1044. (24) Gunnarsson, G.; Josson, B.; Wennerstrom, H. J. Phys. Chem. 1980,84, 3114. (25) Infelta, P.P.Chem. Phys. Lett. 1979, 61, 88.
4214
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
Almgren and Swarup
TABLE 11: Size and Compositions of SDS Micelles with Normal Alcohols and Hydrocarbons at 22 "C [SDSI, [ROHI, [ RH], [micelle] -', M M M mM -' NSDS NROH NRH R, A 0.132 0.132 0.1 27 0.127 0.108 0.108
0.326 0.326 0.654 0.654 2.182 2.182
0.133 0.133 0.1 29 0.129 0.122 0.122
0.1 84 0.1 84 0.461 0.461 0.923 0.923
0.133 0.133 0.1 29 0.129 0.1 22 0.1 22
0.184 0.184 0.461 0.461 0.923 0.923
less than about 21
A
0.0766 0.1 53
0.183
0.0766 0.229 0.229
0.0051 0.0615 0.123
Butanol 0.375 0.644 0.242 0.525 0.637 0.863
area/iVSDS, A '
+ Hexane 48.7 83.6 30.2 65.6 67.6 91.4
37.7 64.7 47.5 103.0 371 502
49.3 80.4 158
Pentanol + Hexane 0.437 57.2 52.8 0.613 80.2 74.0 0.275 34.9 82.5 190.2 0.634 80.5 323 0.55 66.0 448 0.763 91.5
47.0 145 175
Pentanol + Decane 0.437 57.2 52.8 0.513 67.1 62.0 0.275 34.9 82.5 0.500 63.5 150 0.55 66.0 323 0.725 87.0 426
2.6 30.8 89.2
17.5 22.6 16.0 23.4 25.9 31.6
79.0 76.8 106.5 104.9 124.7 137.2
19.0 22.9 18.3 27.8 26.2 3 2.3
79.3 82.2 120.6 120.0 130.7 143.2
19.0 20.2 18.3 23.8 26.2 31.3
79.3 76.4 120.6 112.1 130.7 141.5
should be correct within
Results and Discussion The results, summarized in Tables 1-111 and Figures 1-3, will be discussed from the two aspects mentioned in the Introduction. Site of Solubilization. The results point to a clear distinction between the polar compounds and the hydrocarbons. When hydrocarbons are solubilized, the micelle responds by growing until the surface area per head group is the same as in the undisturbed micelle. This holds both for pure SDS micelles (Table I) and for SDS-alcohol micelles (Table 11). On the other hand, addition of alcohols induces an increase of surface area per head group as discussed elsewhere! In Figure 1 the difference between polar and nonpolar additives is very clear. The results for the hydrocarbon additives are close to the fully drawn curves, representing radius-composition values under the assumption that the area per surfactant (calculated at the surface of the hydrophobic core) remains constant at A. = 59 A2. The radius R at a mole fraction X2of an additive with volume V , is given by
where Ro is the radius without additive. The curves shown are for V2= 246 A3, representing heptane (and close to the hydrophobic volume of octanoic acid and octanol) and for V2= 177 A3 calculated for toluene. Octanoic acid and in particular octanol give a much smaller increase in micelle radius. The big difference between these two shows the importance of the head group: whereas benzyl alcohol and pentanol, which chemically are very different but have the same head group and hydrophobic volume, show a remarkably close agreement in the radius-composition curve. A close examination of the results in Table I reveals a small trend in the area/ head group values with toluene as additive. The variations observed are within the error limits. It can only be concluded, therefore, that the differences in the behavior of the saturated and aromatic hydrocarbons are very small although the latter may show a slight preference for the micelle surface. It should be understood that the observed approximate constancy of the area per monomer head group at the micelle surface does not imply that the solubilized mole-
A /
/
16 0
03
0.2
0.3
0.4
0.5
0.6
Flgure 1. SDS-micelle growth with some additives. The fully drawn curves represent the predicted behavior on addition of a mole fraction X , in the micelle of a hydrocarbon with molecular volume V,. For comparison the behavior with some polar additlves is also shown: (A) n-heptane ( V , = 246 A3), (A)toluene ( V , = 177 A3), (0) n-hexane ( v , = 218 A3), (a)natanoic acu, (v)l-octanol, (v)1-pentanol, (0) benzyl alcohol.
cules are confined to the center of the micelle. The area per monomer at the surface of the hydrophobic volume is about 60 A2, which is more than twice the cross-sectional area of hydrocarbon chain. In a pure micelle the excess interfacial area is taken by other monomer tails. Hydrophobic guest molecules may and do replace monomer hydrocarbon chains also in this part of the micelle. It is this water contact-and not the very unfavorable penetration of water molecules into the micelle-that is the reason that spectroscopic and kinetic evidence quite unequivocally shows that hydrophobic solubilizates experience a "wet" environment in micelles. Monomer Length as a Constraint of Micelle Growth. The length of a fully extended CI2alkyl chain is about 16.7 A. The hydrophobic radius of the SDS micelle is larger, 18.1 A according to the present results and 17.0 A if the aggregation number is 58, obtained by Huisman26from light scattering and thus representing a value at very low micelle concentration. Lianos and recently reported SDS aggregation numbers obtained by a time-resolved fluorescence technique utilizing pyrene excimer forma(26) Huisman, H. F.R o c . K. Ned. Akad. Wet.,Ser. E , Phys. Sci. 1964, 67, 367.
The Journal of Physical Chemisrry, Vol. 86, No. 21, 1982 4215
Size of SDS Micelles in the Presence of Additives
TABLE 111: Size and Composition o f Mixed Micelles o f Sodium Dodecyl Sulfate and Sodium Tetradecyl Sulfate at 22 "C [SDSI,
mM
60.0 56.0 52.0 48.0 44.0 40.0 30.0 20.0 10.0
60.0 58.0 56.0 52.0 48.0 40.0 30.0
[STDSI,
[micelle ]
mM
mM-'
N,
4.0 8.0 12.0 16.0 20.0 30.0 40.0 50.0 60.0
1.25 1.28 1.31 1.45 1.52 1.55 1.50 1.80 1.93 1.78
70.0 66.3 62.8 63.8 62.1 58.9 42.0 32.4 17.3 0
2.0 4.0 8.0 12.0 20.0 30.0
1.55 1.58 1.68 1.53 1.63 1.74 1.85
With 0,100 M NaCl 86.8 85 87.3 73.2 71.5 66.1 51.8
R, A
areal(Nl t N 2 ) , A z
5.1 10.5 17.4 25.2 31.0 45.0 72.0 94.3 104.7
18.1 18.3 18.5 19.2 19.8 20.1 20.0 21.4 22.1 21.8
58.8 59.0 58.7 57.1 56.5 56.3 57.9 55.3 54.9 56.9
3.2 6.7 12.2 19.5 34.8 55.5
19.4 19.6 20.0 19.5 20.0 20.8 21.4
54.5 54.7 53.5 55.9 55.2 53.9 53.6
N 2
n I " ' " ' ' ' ' 6NO p 2
R/il
22 20
.
0.1 M NaCl
50
0
18 16 0
Flgwe 2. Slvface area per SDS monomer (upper curves) and micelle radius (lower curves) for SDS-1-butanol micelles at various mole fractions X 2 of butanol in the micelles. Filled circles represent values with 0.1 M NaCi.
t i ~ n . ~Their * results show a very weak concentration dependence, from 60 at the cmc to about 64 at 0.1-0.3 M SDS. Since the calculations were based on [SDS]& = cmc, the true aggregation numbers should be somewhat greater, around 68 at 0.1 M SDS, and are thus in very good agreement with the present results. Ionic micelles grow when salt is added to the solution. For SDS the growth is slow and gradual up to about 0.4 M of NaC1, but very rapid at higher salt concentrations1A2.23y2B*n where rodlike micelles start to form.23There is nothing in the initial gradual growth that suggests an important change of shape, or onset of the geometrical constraint due to the finite length of the monomer. Figure 2 displays another facet of the effect of added salt. The radius of SDS-1-butanol micelles goes through a minimum well below the critical 16.7 A on increasing 1-butanol content.6 Addition of NaCl induces a growth that is stronger at higher alcohol content, but not particularly strong for compositions around the minimum. (The results for the three solutions with the highest alcohol content are not quite reliable, as discussed elsewhere.6) The reasons that a spherical shape is favorable were discussed in the Introduction. When the micelle has grown so far that the geometrical limitations prohibit further growth as a sphere, the micelle will adopt a state which is a compromise between an unfavorably small spherical micelle and a big but nonspherical one. Henceforth, a (27) Kratohvil, J. P.J. Colloid Interface Sci. 1980, 75,271.
I
,
I
I
,
0.5
I
1
t
0x2
1.0
Flgure 3. Surface area per head group and radius of SDS-STDS mixed micelles at various mole fractions X , of STDS.
micelle in such a state will be referred to as being constrained. It would be expected that, if the constraint is released, the micelle would respond by quickly changing into an unconstrained spherical shape. For an initially nonspherical micelle, the release of the constraint could result in a decrease of aggregation number (or in a smaller growth than what would otherwise have been expected). A micelle that is spherical but constrained to be small would grow. One way to release the geometrical constraint would be by solubilizing a hydrocarbon which could fii the center of the micelle. The results in Tables I and I1 and in Figure 1 show only a smooth and gradual increase in size, in the way that is expected for an unconstrained sphere with the area per head group as regulating factor. Another way to release the constraint is by the addition of a surfactant homologue with a longer chain. Mixed micelles of sodium dodecyl sulfate and sodium tetradecyl sulfate were studied, therefore, with results shown in Table I11 and Figure 3. If release of the constraint were important, a rapid growth of the micelles would have been expected on the first additions of the longer homologue. The observed behavior is a gradual change at about the same rate over the whole composition range, also in the presence of added NaC1. The radius increases from a value about 1.5 A larger than the extended Clz chain for SDS to a value about 2 A larger than the C14chain of STDS. The area per head group decreases slightly, from about 59 A2 for SDS to 57 A2 for STDS. These small changes may be artifacts; they are otherwise difficult to comprehend. Thus, there is nothing in the present results that supports the assumption that the SDS micelle in aqueous
4218
J. Phys. Chem. 1982, 86, 4216-4221
solution or at low salt concentrations is in a constrained state. A t any moment there will certainly be micelles present with widely different shapes and of very different sizes; however, it does not appear as if the deviations from spherical shape for the bigger micelles are important for their properties.
Conclusions The surface area per free head group seems to be the most important regulating factor for micelle growth. For pure SDS this area is about 59 A2 counted at the surfaces of the hydrophobic volume; it rises to 75 A2 at the surface
through the center of charge of the head groups. On solubilization of nonpolar compounds this area remains constant, whereas it increases on solubilizing alcohoh6 The experimental results give no evidence that the pure SDS micelle is in a constrained state, in spite of the fact that the hydrophobic core-if spherical-has a radius almost 2 A bigger than that of the extended C12alkyl chain.
Acknowledgment. We thank Dr. B. Jonsson for many helpful discussions. Financial support from the Swedish Natural Sciences Research Council is gratefully acknowledged.
Application of the Poianyi Adsorption Potential Theory to Adsorption from Solution on Activated Carbon. 12. Adsorption of Organic Liquids from Binary Liquid-Solid Mixtures in Water Mlck Greenbank and Milton Manes' Chemistry Depaflment, Kent State University, Kent, Ohio 44242 (Received: February 16, 1982; I n Final Form: July 6, 1982)
An earlier study of the adsorption of multiple liquids from water solution onto activated carbon has been extended
to the adsorption on the same carbon of organic liquids from fixed concentrations (largely at saturation) of dissolved solids, the solids representing a wide range of displacing power (adsorption potential per unit volume) relative to the liquids. The systems studied comprise the following: ethyl acetate from methionine (MET), phthalic acid (OPA), benzamide (BZD),and coumarin (COU); diethyl.ether from MET, OPA, thiourea (THI), and BZD; dichloromethanefrom MET,THI, OPA, and BZD; and 1-pentanolfrom MET, OPA, BZD, phthalide (PHL), COU,and p-nitrophenol. The data were fitted to the predictions of three models: (a) a Polanyi-based model (previously used for liquids) with the assumption of an ideally miscible (but nonuniform) adsorbate; (b) a Polanyi-based model with the assumption of similar properties for bulk solute and adsorbate; and (c) a slightly modified version of the ideal adsorbed solute (IAS) model of Radke and Prausnitz. Model b failed badly; model c was out of bounds more often than not; and model a worked best, but not without some anomalies. For the most part, the isotherms showed linear dependence (Henry's law) at low concentrations; for liquids in more strongly displacing solids the Henry's law dependence extended from traces to saturation.
This communication,' which follows an earlier one on the adsorption of mixed organic liquids from water onto activated carbon,2 deals with the adsorption at 25 "C of binary mixtures of a liquid and a solid, where the concentration of liquid is systematically varied in the presence of a fixed (frequently saturated) concentration of solid, and where the liquid is usually a trace or minor component in the adsorbate. A subsequent communication w ill deal with the converse case in which the solid is the minor component in a fixed concentration of liquid. The systems to be described comprise the following: ethyl acetate from (individual) solutions of methionine, phthalic acid, benzamide, and coumarin; diethyl ether from methionine, phthalic acid, thiourea, and benzamide; dichloromethane from methionine, thiourea, phthalic acid, and benzamide; and 1-pentanol from methionine, phthalic acid, benzamide, phthalide, coumarin, and p-nitrophenol. They were chosen to encompass a wide range of relative adsorbabilities (or adsorption potential densities) of the liquid and solid components. The continued use of the same carbon as in the preceding 11 articles of this series facilitates comparison of the new and previously existing data. (1) This and t h e immediately preceding article2are baaed on the Ph.D. dissertation of Mick Greenbank.4 (2) Greenbank, M.; Manes, M. J . Phys. Chem. 1981, 85, 3050-9. 0022-385418212086-4216$01.25/0
The components and their concentrations were chosen to provide severe tests of alternative adsorption models rather than for relevance to specific practical systems. The alternative models originally comprised (a) the RadkePrausnitz3 (IAS) model and (b) a Polanyi-based model ("immiscible adsorbate model")4that WEIS a straightforward extension to liquid-solid adsorbates of the earlier models of Rosene and Manes5" (multiple solids) and of Greenbank and Manes4 (multiple liquids). This model, like its predecessors, incorporated the assumption that the properties of all adsorbate components are identical with the corresponding bulk properties, except for reduced bulk densities for some adsorbates; for solid-liquid systems (as well as for multiple solids) this assumption is the direct opposite of the Radke-Prausnitz postulate that the adsorbate behaves as an ideal solution regardless of the state of the (liquid or solid) components. For a trace component in a saturated solution of a more strongly competing major component the immiscible adsorbate model is highly sensitive, and the IAS model completely insensitive, to whether the major component is a liquid or a solid. For (3) Radke, C. F.; Prausnitz, 3. M. AIChE J . 1972, 18, 761. (4) Greenbank, Mick, Ph.D. Dissertation, Kent State Univenity, Kent, OH, May 1981. (5) Rosene, M. R.; Manes, M. J . Phys. Chem. 1976,80,953-9. (6) Rosene, M. R.; Manes, M. J. Phys. Chem. 1977,81, 1646-50.
0 1982 American Chemical Society
