1434
J. Phys. Chem. 1081, 85, 1434-1439 Polarizabilities -A.
10
40
30
20
s,-
*' IO
I
'
30
70 Equivalent weights 8
50
I
Figure 0. Transfer total free energies Aboc as a function of the polarizabilities (A) and of the equivalent weights (0)for different anions.
plained in terms of the electric polarizabilities of the anions in the sense that higher polarizabilities are reflected directly in a more negative value for Ap0c, as is shown in Figure 9, according to the values of polarizabilities reported by Harber and Porter16 for F-, C1-, and Br-. In order to include all anions in this correlation, we have assumed that polarizability is proportional to the number of electrons carried by each anion and therefore proportional to their equivalent weights (EWs). The Apoc values as a function of EW are also shown in Figure 9. Roughly speaking, (16) Harvey, K. B.; Porter, G. B. "Introduction to Physical Inorganic Chemistry"; Addison-Wesley: New York, 1963; p 251.
Figure 9 shows a similar tendency for both EW and polarizabilities, which supports the proposed hypothesis. In this sense the last correlation is similar to the classification into soft and hard anions given by Pearson1'J8 and recently proposed to explain the specificity of anion binding to CTA mice1les.l In conclusion,we accept that both enthalpic (eletrostatic and dehydration energies) and entropic (dehydration) factors are responsible for the specificity of binding and that the electrostatic contribution may be due to a different location of the anion in the Stern layer and to its polarizability expressed in terms of soft and hard properties. All of the experimental data presented in this work may in principle be treated by using a Langmuir isotherm, but we believe that that kind of approach is only correct for uncharged molecules or amphiphilic ions when they are present in small concentrations and when their binding constants are large. Acknowledgment. Support of this work by the Programa Regional de Entrenamiento de Postgrado en Ciencias Bioldgicas (PNUD/UNESCO RLA 78/_024)and the Servicio de Desarrollo Cientifico, Artistic0 y de Cooperacidn Internacional de la Universidad de Chile, is gratefully acknowledged. We also thank Dr. H. Chaimovich and Dr. F. Quina for helpful discussions. (17)Pearson, R. G. J. Am. Chem. SOC.1963,85,3533. (18) Pearon, R. G. Science, 1966,151, 172.
Bindlng of Phenols to Inverted Micelles and Microemulsion Aggregates Linda J. Magld," Kljlro Kon-no,t and Craig A. Martin Depadment of Chemistry, University of Tennessee, Knoxville, Tennessee 379 16 (Received: November 10, 1980; In Final Form: February 11, 198 1)
Binding constants (Ks)for a series of phenols to Aerosol OT [sodium bis(2-ethylhexyl)sulfosuccinate] inverted micelles in 2,2,4-trimethylpentanevary from 87.0 M-' for o-tert-butylphenol to 7510 M-l for p-nitrophenol. The other phenols (XCBH,OH)studied were p-t-Bu, 0-and p-CH3, p-CHBO, pC1, and H. The strength of binding is determined by the phenol's hydrogen-bonding ability and by ita solubility in 2,2,4-trimethylpentane (or equivalently, by its hydrocarbon-water partition coefficient). The binding of phenol to Aerosol OT w/o microemulsion aggregates in 2,2,44rimethylpentaneshows a decrease in K followed by a plateau value as the water-to-AerosolOT ratio is increased. This effect is discussed in terms of phenol-water competition for interfacial binding sites.
Introduction Inverted micelles and water-in-oil (w/o) microemulsions have been used by several groups of workers to provide for the catalysis of chemical reactions in microenvironments which are thought to mimic in certain respects the active sites of enzymes. Reactions investigated have included ester hydrolyses and aminolyses,l-1° mutarotation of 2,3,4,6-tetramethyl-cu-~-glucose,'~ aquation of chromium and cobalt c ~ m p l e x e s , etc.15-16 ~ ~ - ~ ~ A number of factors are important in producing the observed catalysis: (1) participation of unhydrated surfactant head groups (of 'Science University of Tokyo, Kagurazaka, Shinjuku-Ku, Tokyo, Japan. 0022-3654/81/2085-1434$01.25/0
appropriate functionality) as very reactive nucelophiles or activity of proton transfer a g e n t ~ ; ~ *(2) ~ J enhanced ~J~ (1) Friberg, S.; Ahmad, I. J. Phys. Chem. 1971, 76, 2001-2004. (2) Menger, F. M.; Donohue, J. A.; Williams, R. F. J . Am. Chem. SOC. 1973,95, 286-288. (3) Menger, F. M.; Vitale, A. C. J. Am. Chem. SOC. 1973,95,4931-4934. (4) Kondo, H.; Fujiki, K.; Sunamoto, J. J. O g . Chem. 1978, 43, 3584-3588. (5) El Seoud, 0. A.; PivGtta, F.; El Seoud, M. I.; Farah, J. P. S.; Martins, A. J. Org. Chem. 1979,44,4832-4836. (6) Sunamoto,3.; Kondo, H.; Akimari, K. Chem. Lett. 1978,821-824. (7) Kon-no, K.; Kitahaa, A.; Fugiwara, M. Bull. Chern. SOC.Jpn. 1978, 51, 3165-3169. (8) Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1978, 67, 477-482. (9) Kitahara, A. Adu. Colloid Interface Sci. 1980, 12, 109-140.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 10, 1981 1435
Binding of Phenols to Inverted Micelles
solubilized water when it is p r e ~ e n t ; ~ ~(3) ~ 'substrate ~'~ binding in favorable orientations for the attack described in (1). We have been particularly interested in micellar solubilization sites and thermodynamics of binding for organic molecules which are substrates or reaction products of catalysis by inverted micelles. We report here on the binding of a series of substituted phenols to the inverted micelles and w/o microemulsion aggregates formed by the surfactant Aerosol OT [sodium bis(2-ethylhexy1)sulfosuccinate] in organic solvents. When surfactants aggregate in nonpolar solvents, their polar or charged groups are located in the interior, or core, of the aggregate, while their hydrocarbon tails extend into the bulk solvent. Depending on the surfactant, these aggregates are capable of solubilizing modest (sufficient for hydration of the head groups) to substantial (molar ratios up to 60 or so) amounts of water. The resulting aqueous core has been referred to as a water pool by Menger.l9 Even in nominally binary surfactant-nonpolar solvent systems, some residual water is present and participating in the aggregation.20 Although there are a wide variety of definitions of the term w/o microemulsion, there is fairly compelling experimental and theoretical evidence for the following picture.2"-22 Up to a water-to-surfactant molar ratio ( R ) equal to the surfactant's hydration requirements, one is dealing wtih inverted micelles. The water in the aggregates is highly structured by hydrogen bonds stabilized by the strong dipole moments of the head groups; aggregate size does not vary with temperature;21there is insufficient space for all head groups to be a t a well-defined water-hydrocarbon interface. Once the R value modestly exceeds the hydration requirement, free water starts to appear in the water p001;23124 a well-defined surfactant monolayer can form at the water-hydrocarbon interface. The microenvironments of solubilized molecules as a function of R in these aggregates have been studied by using a variety of t e c h n i q u e ~ , ~ for ~?~ example ~ @ by the use of a solubilizate which has a fluorescence or absorption spectrum which is sensitive to solvent polarity.2630 Sol(10)O'Connor, C. J.; Ramage, R. E. A u t . J.Chem. 1980,33,757-770, 771-777.779-784. (11)Fendler, J. H.; Fendler, E. J.; Medary, R. T.; Woods, V. A. J.Am. Chem. SOC.1972,94,7288-7295. (12) O'Connor, C. J.; Fendler, E. J.; Fendler, J. H. J . Am. Chem. SOC. 1973,95,600-602. (13)O'Connor. C. J.: Fendler. E. J.: Fendler, J. H. J.Am. Chem. SOC. 1974.96.370-375. (14)OConnor, C. J.; Fendler, E. J.; Fendler, J. H. J. Chem. SOC., Dalton Trans. 1974,625-631. (15)See Fendler, J. H. Acc. Chem. Res. 1976,9,153-161,and references cited therein. (16)See Menger, F. M. Pure Appl. Chem. 1979,51, 999-1007,and references cited therein. (17)Nome, F.; Fendler, J. H. J.Am. Chem. SOC.1977,99,1557-1564. (18)Robinson, B. H.; Steytler, D. Ber. Bunsenges Phys. Chem. 1978, 82,1012. (19)Menger, F. M.; Saito, G. J.Am. Chem. SOC.1978,100,4376-4379. (20)Eicke, H.-F. Topics Curr. Chem. 1980,87,85-145. (21)Zulaf, M.; Eicke, H.-F. J. Phys. Chem. 1979,83,480-486. (22)Prince, L.M. J. Colloid Interface Sci. 1975,52,182-188. (23)Zinsli, P. E. J.Phys. Chem. 1979,83,3223-3231. (24)Wong, M.; Thomas, J. K.; Griitzel, M. J. Am. Chem. SOC.,1976, 98,2391-2397. (25)Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC.1977,99, 2039-2044. (26)Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977,81, 2175-2180. (27)Correll, G. D.; Cheser, 111, R. N.; Nome, F.; Fendler, J. H. J. Am. Chem. SOC.1978,100,1254-1262. (28)Miller, D. J.; Klein, U. K. A.; Hauser, M. J. Chem. SOC.Faraday Trans. 1 , 1977,73, 1654-1656. (29)Hunter, T. F.; Younis, A. I. J.Chem. SOC.,Faraday Trans 1 1979, 75,550-560.
ISOOCTANE
-f\
w 0 2
.
4
m (L
0
m
0.4-
265
2 75
A, n m
Figure 1. Absorption spectra ( I = 10 cm) for 3.55 X M phenol in isooctane and in 0.0024, 0.014, and 0.026 M AOT at 25 OC. (As [AOT] increases, the absorbance at 271 nm decreases).
ubilizates which have a very low solubility in the organic solvent are generally localized in the water pools, but there are notable exceptions. Thus p-nitrophenol is solubilized in the interfacial region of w/o microemulsions formed by AOT in heptane, despite favoring water approximately 100-fold when partitioned between heptane and aqueous acid.Ig There have been no studies, prior to the present one, on the effect of R value on the binding constant of a solubilizate to a w/o microemulsion. Some sol~bilizates~~ are known to shift from interfacial sites to solubilization within the water pools as R increases. Alternatively, the solubilization site may stay substantially the same with changes in apparent polarity of the site (caused by hydration, for example). Will there be a resulting change in binding constant for that solubilizate? This work addresses that question.
Experimental Section Aerosol OT.' The surfactant supplied by Aldrich or Fluka was used; purification procedures (to remove inorganic salts and unsulfonated bis(2-ethylhexy1)maleate) followed were those of FendleP and Menger.lB The resulting AOT showed good elemental analysis. (Calcd C, 54.03;H, 8.39. Found: C, 53.99; H, 8.39.) Because it is so hygroscopic, solid AOT was always handled in a glove bag. Phenols. Reagent-grade materials were sublimed, distilled, or recrystallized as appropriate and dried over PzOb Dryness was evaluated by comparing the observed vibrational fine structure in the absorption spectrum with literature data. Solvents. Isooctane (2,2,4-trimethylpentane, Aldrich, spectrophotometric quality) and carbon tetrachloride (Fisher, ACS reagent grade) were dried over sodium wire or molecular sieves before use. ~~
(30)Wong, M.; Griitzel, M.; Thomas, J. K. Chem. Phys. Lett. 1975,30, 329-333. (31)Menger, F. M.; Saito, G.; Sanzero, G. V.; Dodd, J. R. J.Am. Chem. SOC.1976,97,909-911. (32)El Seoud, 0.A,; Fendler, J. H. J.Chem. SOC.Faraday Trans. 1 1975,71,452-460.
f436
The Journal of Physical Chemistty, Vol. 85, No. 10, 1981
Magid et ai.
TABLE I: Spectral Features Employed in the Calculation of Binding Constants for the Phenols XC,H,OH P,, nm V , , nm P,, nm 278.9 275.0 271.0 O-t-Bu p-t-Bu 283.9 280.2 276.5 0-Me 278.2 275.6 272.0 279.2 pMe 285.7 283.3 p-Me0 300.8 29 8.9 294.0 H 277.5 275.5 270.8 287.0 282.5 289.5 P-(J NAP! P.NO 1 Absorbances, rather than absorbance ratios, were used here. For p-nitrophenol in isooctane the wavelengths monitored were 305, 310, 315,320, 325,and 330 nm; for CCI,: 315,320,325,330,and 335 nm.
Solution Preparation. All solutions were prepared on a weight/weight basis; molalities were converted to molarities by using densities determined pycnometrically at 25
0.
W
2
m 0
m
0.
“C.
Spectral Acquisition. All samples were thermostatted for 20 min at 25.00 f 0.05 “C in the sample compartment of a Cary 17 spectrophotometer. Both 10- and 1-cm length cells were used. Reference solutions contained all components, at appropriate concentrations, except the phenol of interest. Careful attention was given to spectral bandwidth (0.4 nm or less) and rate of scanning, since vibrational fine structure was being investigated; pen fidelities of 2.0 or greater were employed.
Results and Discussion Spectral Features of the Phenols Used. The absorption spectra of benzene and benzene derivatives show a blurring of vibrational fine structure of the lBzu lAlg transition in polar solvents, both hydrogen and non-hydrogen bondingqm7 For benzene itself, the electronic origin (0,O) in the band system does not appear in the vapor-phase spectrum. It is the first member of a progression of vibrational peaks known as Ham bands, which are displaced 520 cm-l to lower energies from the relatively strong e2g progression of vibrational peaks (e 200) starting at 261 nm (38300 cm-1).38t39 The Ham bands’ intensities are enhanced by solute-solvent interactions; because of their spectral position, this has the effect of filling in the valleys between the peaks of the e%progression. With substituted benzenes, the lBzu lAIgtransition is no longer strictly forbidden@and the 0 0 transition is observed. However, some vibrational fine structure is still present, since the molar absorptivities of the two progressions are not equal (see Figure 1for the spectrum of phenol in isooctane and at various AOT concentrations). The Ham effect has been used by several groups to monitor solubilization sites of aromatic molecules in both normal and inverted miA molar absorptivity ratio employing a peak celles.25*26@*3e and adjacent valley, or two different peaks, is observed as a function of solvent dielectric constant, dipole moment, 2 value, etc. in order to construct an effective polarity scale
290
270
270
290
A, nrn
Figure 2. Absorption spectra I = 10 cm) for 5.4 X M pchlorophenol (left) and 2.7 X IO-‘ M pcresol (right) in isooctane, with
the spectral features used in the binding constant calculations labeled.
-
-
-
+
(33)Cundall, R. B.; Ogilvie, S. M. “Organic Molecular Photophysics”; Birks, J. B., Ed.; Wiley: London, 1975; Vol. 2,pp 33-93. (34)Gerrard, D. L.;Maddams, W. F. Spectrochim. Acta, Part A 1978, 34,1205-1211, 1213-1218,1219-1223. (35)Gerrard, D.L.; Maddams, W. F.; Tucker, P. J. Spectrochim Acta, Part A 1978,34,1225-1230. (36)Cardinal, J . R.;Mukerjee, P. J. Phys. Chem. 1978,82,1614-1620, 1620-1627. (37)Nbmethy, G.; Ray, A. J. Phys. Chem. 1973,77,64-68. (38)Platt, J. R.J. Mol. Spectrosc. 1962,9,288-309. (39)Ham J. S.J. Chem. Phys. 1953,21,756-756. (40)The fB, lAl, transition appears for benzene because there is some degree of mixing with a higher, allowed excited state, lE1,,. This occurs via a distortion of the benzene ring due to a bending vibration.8*
-
If
““pi
0.5
[Aoflx
IO2,
1.3
M
1
Figure 3. X, for p-nitrophenol as a function of [AOT] In isooctane.
which can be used to interpret the micellar results. In using changes in vibrational fine structure to follow the binding of phenols to AOT inverted micelles, we have chosen to use peak-to-valley molar absorptivity ratios (or equivalently, absorbance ratios, since we hold the phenol concentration constant while varying [AOT]). The same binding constants are obtained when ratios at two constant wavelengths are used, however. This occurs because the spectral shifts induced by binding to the AOT micelles are small ( 5 2 nm) in the cases studied. Figure 2 shows the absorption spectra for p-cresol and p-chlorophenol; the ratios A(Pl)/A(Vl) and A(P2)/A(Vl)were employed in determining the binding constants. Spectral features used for the other phenols studied are collected in Table I. In the case of p-nitrophenol, the lBlu ‘Alg transition submerges the ‘Bzu lAlg transition. However, in this case ,A, is strongly solvent sensitive (see Figure 3), and the change in molar absorptivity as a function of [AOT] at a series of wavelengths can be used. The phenol concentrations used in this work are low enough so that phenol self-association is negligible.4144
-
-
(41)Baba, H.; Suzuki, S. J. Chem. Phys. 1961,35,1118-1127. Christian, S. D.; Tucker, E. E. J. Phys. Chem. 1978, (42)Lin, L.-N.;
~ 2 . 1--~ 9.7 ---1 -c -m. i . --,
(43)El Seoud, 0.A.; Fendler, E. J.; Fendler, J. H. J. Chem. SOC., Faraday Trans. 1 1974,70,450-456,459-470. (44)El Seoud, 0.A.; Ribeiro, F. P. J. Org. Chem. 1976,41,1365-1369.
The Journal of Physical Chemistry, Vol, 85, No. 10, 1987 1437
Binding of Phenols to Inverted Mlcelles
0.10 M AOT
\
0.a
W 0 2
m a
:: 4
0.1
n
4.0 x 10-5
2
~
I
--------
260
4.0 [AOT]
280
h , nm
Flgure 4. A comparison of the absorption spectra ( I = 10 cm) for p-chlorophenol (left) and p-cresol (rlght) In water vs. bound to AOT inverted micelles (phenol concentrations same as in Figure 2, except where noted).
08 [AOT]
1.6 1
IO3, M
Flgure 5. Critical micelle concentration for AOT In Isooctane, determined by using the absorption spectrum of 4.5 X 10" M acridine orange as a function of [AOT].
Solubilization Sites in the Aggregates. Menger's datal9 on the pK, of p-nitrophenol solubilized in AOT w/o microemulsions clearly establishes it as being located in the surfactant monolayer, with the hydroxyl group hydrogen bonded to the sulfonate head groups of the surfactant. Our UV data confirm this, since the addition of water (Rfrom 9.3 through 43.3) to a 0.01 M AOT solution in isooctane containing p-nitrophenol does not alter A,, (307.3 nm). As Figure 4 indicates, 307.3 nm is the A, displayed by bound p-nitrophenol in this system without added water. The observed A,, for p-nitrophenol in water is 318 nm. Other data, for p-nitrophenol in AOT inverted micelles in benzenet5 puts this solubilizate in a still less polar environment, namely, out among the surfactant's hydrocarbon tails. The loss of vibrational fine structure as the other phenols are transferred from a nonpolar to a polar solvent is due to solute-solvent interactions which include hydrogen bonding in addition to dispersion forces and dipole-dipole interactions. When phenols participate only as hydrogen bond donors, a red shift of the spectral bands is observed134*35p37*41 while blue shifts are observed when the ~~
~~
(45) Jean, Y.; Ache, H. J. J . Am. Chem. SOC. 1978,100,6320-6327.
8.0 X
IO4,
M
Flgure 6. [S,]/[S,] for 8.6 X
M p-nltrophenol as a function of [AOT] in Isooctane, determined at 320 nm.
phenols are hydrogen bond acceptor^.^^*^^ Inspection of the spectra in Figure 5 shows that neither p-cresol nor p-chlorophenol are in a highly aqueous environment in AOT inverted micelles. The spectrum in water is shifted a few nanometers to the blue in both cases, suggesting that the phenols function as both H-bond donors and acceptors in water. When the bound phenols' spectra are compared with the free phenols' spectra in Figure 4 and Table I, one can see that binding to the AOT micelles shifts Pz to the red by less than 1 nm. Calculation of the Phenol-AOT Binding Constants (Ks). We have chosen to use eq 1,which was derived by [sfl K[DTI (1) Turro4' and Singer4@withthe assumption that the mean number of substrate molecules per micelle is described by the Poisson distribution function; the overall K equala nK,, where K,, is the constant for each association step. The terms [sb] and [Sf]refer to the concentration of bound and free solubilizate, respectively; [DT]is the total surfactant concentration, not the concentration of aggregates (vide infra). The ratio [sb]/[sf] is calculated for each AOT concentration by use of eq 2. The A values are of course
[sbl/[sfl = f / ( l f = (Aobsd - Asolv)/(Am
-f)
- Asolv)
(2)
actually absorbance ratios for all phenols except p-nitrophenol. The A& values are determined from the spectrum of the phenol in the solvent without added AOT. The A, values are determined directly at high AOT concentrations for those phenols having large Ks, or by extrapolation when the concentration of AOT required for -100% binding was experimentally unattainable. AOT concentrations were chosen to give several f values (8-15) between 0.2 and 0.8 for each phenol studied. The [Sb]/[Sf]vs. [AOT] plots gave very good straight lines, with r = 0.995 or better. (46) Yekta, A.; Aikawa, M.; Turro, N. J. Chem. Phys. Lett. 1979,63, 543-548. (47) Kwan, C. L.; Atik, S.; Singer, L. A. J. Am. Chem. SOC.1978,100, 4783-4786. (48) Herrmann, U.; Schelly, Z. A. J. Am. Chem. SOC.1979, 101, 2665-2669.
1438
The Journal of Physical Chemistry, Vol. 85, No. IO, 1981
Magld et al.
TABLE 11: Binding of Phenols to AOT Inverted Micelles at 25 "C A G ", b
AG~~,~c,H,,-+H,o,~
K,aM-1 XC,H40H [ArOH], M kJ mol-' kJ mol" solvent 0-t-BU 4.29 x 10-5 isooctane 87.0 i 29.0 -11.1 f 0.9 p-t-BU 4.11 x 10-5 isooctane 187 t 9 -13.0 * 0.1 p-t-Bu 3.52 x 10-4 isooctane 172 3 -12.8 t 0.1 +l0.8 o-Me 4.63 x 10-5 isooctane 1175 16 -11.8 f 0.3 +5.11 isooctane 206 5 -13.2 f 0.1 + 3.62 P-CH, 2.74 x 10-5 isooctane 175 f 6 -12.8 t 0.1 P-CH, 3.59 x 10-4 2.79 x 10-5 isooctane p-CH,O 246 t 30 -13.6 0.3 -1.72 H 3.55 x 10-5 isooctane 350 t 6 -14.5 i 0.1 -0.417 H 3.36 x 10-4 isooctane 327 f 12 -14.4 f 0.2 p-c1 5.39 x 10-5 isooctane 1310 f 66 -17.8 0.2 + 0.433 -22.1 f 0.2 -6.56 isooctane 7510 t 350 8.64 X lo+ PNO, isooctane 3670t 220 -20.4 * 0.1 9.78 x 10-5 P-NO, 1080 c 16 -17.3 * 0.1 ca4 P-NO, 8.64 X -17.4 j: 0.1 ca4 1110 f 23 P-NO, 7.80 x 10-5 a Calculated by using eq 1. A G O = - RT In K (K E K,;the standard state is a dilute solution of the phenol in pure solvent). AGtr = - RT In (XH,O/X~-C,H,,).
In eq 1,the quantity [DT] conventionally refers to surfactant in aggregated form. Although there is some dispute as to whether all surfactants which aggregate in organic solvents display critical micelle concentrations (crnc),AOT is one which does. Values in isooctane depend on the technique used to measure the cmc (and on the residual water content of the solvent); we have found a value of 4.0 X M using Schelly's procedurea with acridine orange (see Figure 5 ) , and others have reported values from 5 X M.20p46For normal micelles in aqueous to 9 X solutions which incorporate solubilizates, there is frequently no pre-cmc association. However, phenols are known to form strong hydrogen-bonded complexes with proton acceptors in isooctane, CC4, et^.,^*^^^^^^^^^^ and we in fact see pre-cmc binding. Thus in Figure 6 one sees no discontinuity in the [Sb]/[Sf]vs. [AOT] plot for p-nitrophenol through the cmc region. The same behavior is observed with other phenols having large K values. For this reason we have not used the concentration of micellized surfactant in eq 1. It could be argued that the phenols depress the cmc of AOT, but we consider this unlikely since we are dealing with such low concentrations of the solubilizate and would need such large depressions of the cmc to explain the discontinuity's absence. Interpretation of the Observed Binding Constants. Table I1 shows the inverted micellar systems studied and the K values obtained. In the case of p-nitrophenol, the quoted error limits arise because K was evaluated at several wavelengths. For the remaining phenols, they reflect disagreement between the values obtained by using P1/ Vl vs. Pz/ Vl ratios. The binding constants vary over two orders of magnitude. Having a substituent ortho rather than para to the hydroxyl group causes only a modest decrease in K, with the bulky tert-butyl group showing a larger effect than a methyl group. This result is consistent with other work which shows that a single ortho substituent has relatively little effect on phenol's hydrogen-bonding ability.35 What causes the observed variation of K with substituent? Two factors come to mind (1)the solubility of the phenol in the bulk organic solvent (or its partition coefficient between organic solvent and water) and (2) the substituent's effect on the proton-donating ability of the phenol. We have determined the solubility of three of the phenols at 25 "C in isooctane spectrophotometrically; estimates are available from the literature for some of the (49)Mizushima, S.;Tsuboi, M.; Shimanouchi, T.; Tsuda, Y. Spectrochirn. Acta 1955,7, 100-107. (50)Ito, M. J . Mol. Spectrosc. 1960,4 , 125-143.
others.% The following order obtains: p-nitrophenol(4.6 X M) < p-methoxyphenol (1.2 X M) < pchlorophenol (9.5 X M) < phenol < p-cresol (at least 3 Mm).Similarly, partition coefficient data51give, in order of increasing preference for water: o-cresol < p-cresol < p-chlorophenol < phenol < p-methoxyphenol < p-nitrophenol. Table I1 shows the relevant free energies of transfer when the hydrocarbon is cyclohexane. They do not correlate with the free energies of binding, which establishes that factor (1)operating alone cannot explain the observed variation in the binding constants. Phenol-acetophenone hydrogen-bonded complexes in CC1t2have K values which correlate with the pK, values of the phenols and hence also follow the Hammett equation (log K = 0.77 0.96a). The Ks therefore increase in the following order: p-methoxyphenol < p-cresol < phenol < p-chlorophenol < p-nitrophenol. In binding to AOT inverted micelles p-nitrophenol displays the highest K, which both solubility and hydrogen bonding ability predict. p-Cresol (and by implication the other alkylphenols) displays a low K, as both factors predict. pMethoxyphenol, despite a fairly low solubility in isooctane, has a low K, demonstrating the contribution made by hydrogen-bonding ability. The importance of hydrogen bonding to strong solubilizate-inverted micelle interactions has been noted previously, for example in imidazolelNmethyliiidazole binding to dodecylammonium propionate micelles.44 As Table I1 indicates, binding behavior for several phenols was investigated as a function of phenol concentration. The motivation for this was the observation in several normal micellar systems that solubilizate binding was described by a Langmuir adsorption i s ~ t h e r m . ~ ~ ~ ~ For the AOT inverted micelles, only p-nitrophenol, whose binding constant is very large, shows binding which depends on [phenol]. Furthermore, this dependence is observed in isooctane, but not in CC14; p-nitrophenol is much more soluble in the latter solvent. Effect of the Inverted Micelle w / o Microemulsion Transition on the Binding Constants. When water is added to AOT inverted micelles in isooctane where [AOT] produces an intermediate f value for a particular phenol, a spectral change occurs which might be interpreted as a
+
-
(51)Leo, A,; Hansch, C.; Elkins, D. Chern. Rev. 1971,71, 525-616. (52)Thijs, C.; Zeegere-Huyskens, Th. Spectrosc. Lett. 1977, 10, 593-602. (53)Dougherty, S.J.; Berg, J. C. J. Colloid Interface Sci. 1974,48, 110-121. (54)Azaz, E.; Donbrow, M. J. Colloid Interface Sci. 1976,57,11-19. (55)Azaz, E.; Donbrow, M. J.Phys. Chern. 1977,81,1636-1638.
Binding of Phenols to Inverted Micelles
The Journal of Physical Chemistry, Vol. 85, No. 10, 1987 1439
TABLE 111: Effect of Water on the P,/V, Absorbance Ratio forp-Chlorophenol in the AOT (3.32X M)/ Isooctane System
0.0 8.1 21.4
1.052 1.070 1.076
32.8 35.9
1.090 1.094
TABLE IV: Absorbance Ratios (Fully Bound Phenol) and Binding Constants as a Function of Water-to-AOT Molar Ratio in the AOT/Water/Isooctane System
3.55 x
3.36 x
M phenol
Am
R 0.0 3.0 6.0 12.0 20.0 35.0
Am
P J V , P,/V, K, M-I 0.950 0.962 0.920 0.958
1.100 1.148 1.107 1.118
M phenol
P,/V,P,/V, K, M - 1
350 f 6 0.951 1.134 281 i 8 224 f 4 217* 24 0.963 1.156 0.934 1.129 0.932 1.115
10
R
327
12
227* 2 185 f 3 185f 3
30
Flgure 7. Binding constants for 3.55 X M phenol (X) and 3.36 X lo4 M phenol (0)in AOT/lsooctane as a function of added water ( R = [H,O]/[AOT]).
decrease in f (and hence a decrease in K ) . Table I11 illustrates this effect for p-chlorophenol. However, there is also the possibility that f is not in fact decreasing, but that A , is instead changing, implying a less polar environment for the bound phenol. We investigated this possibility in detail for phenol in AOT/isooctane; as Table IV indicates, A , remains constant and K decreases and then levels off as the R value increases.se Binding studies were done at 3.55 X loT6and 3.36 X 10" M phenol; the Ks obtained do not depend on [phenol], within experimental error. Figure 7 displays this dependence of K upon R; we note that K stops decreasing once the w/o microemulsion regime is entered, Le., once the hydration of the AOT head groups is complete and the well-defined surfactant monolayer is formed.21 We have chosen to use the total surfadant concentration in molarity units when calculating K; one does not see the essential feature of the microemulsion regime's effect if (56) It was important to establish whether the microenvironmentwas changing, since we have '*C NMR chemical shift evidence that one of the ester functionalities, where phenol might hydrogen bond, experiences a change to a less polar environment as R increases in AOT w/o microemulsions.
one attempts to use aggregate concentration. The aggregate concentration of course decreases (at constant [AOT] as R increases.2*21Phenol's association with AOT clearly depends on total AOT concentration, with the aggregated or nonaggregated nature of the AOT making little difference, as discussed earlier. The microenvironment of phenol in the AOT aggregates does not change (at least so far as can be determined by changes in the absorption spectrum) as water is added; how then can one explain the decrease in K? It is best explained as a competition between phenol and water as solvating agents for the AOT head groups (the sulfonate groups plus the oxygens of the ester functions); once there is sufficient water to fulfill (in principle) all the hydration requirements, K stops decreasing. Denss" has proposed that benzene associates with AOT head groups in inverted micelles and can be displaced by the addition of water. We attempted to investigate this by looking at the vibrational fine structure of benzene's lBau IAlg transition in 0.15 M AOT at R = 0 and R = 30. The absorbances at the Ham band wavelengths were the same in pure isooctane and the two AOT-containing solutions, indicating that benzene's binding to the aggregates has a very low K (if any binding occurs). For reactions catalyzed by inverted micelles, reaction rates often decrease as water is added. As this work demonstrates, a decrease in substrate binding will be one cause of this decrease. However, in view of the very large decreases often observed,lSdeactivation of surfactant head groups (by hydration) which act as nucelophiles in the reactions involved, such as ester hydrolysis, will be the major contributing factor.
-
Conclusions A series of ortho- and para-substituted phenols were found to bind to AOT inverted micelles and w/o microemulsion aggregates in isooctane and CC4. The observed binding constants depend on solubilizate concentration only for p-nitrophenol in isooctane. The phenols are located in the interfacial region of the aggregates, with the hydroxyl group hydrogen bonded to the AOT head groups and the aromatic ring penetrating between the AOT's hydrocarbon chains. The strength of the binding is governed by the phenol's relative affinity for polar vs. apolar environments, and by its hydrogen bonding ability (as measured by its pKJ. Addition of water to inverted micelles causes the binding constants to decrease until the w/o microemulsion region is reached, where they level off. Because of this, it is important to proceed with care when using the properties of a solubilizate molecule to determine variations in water pool polarities in these aggregates. An apparent change in microenvironment can in fact be due to a change in the solubilizate's binding constant. Acknowledgment. This work was supported by the National Science Foundation (CHE76-20301) and the Research Corporation; acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support. ~
(57) Denss, A. Ph.D. Thesis, Basel, 1977.
