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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
(18) F. Accascina and F. Schiavo, "Chemical Physics of Ionic Solutions", B. E. Conway and R. G. Barradas, Ed., Wiley, New York, 1966, pp 515-520. (19) S. Katayama, Bull. Chem. SOC.Jpn., 46, 106 (1973). (20) (a) K. Miyoshi, J. Phys. Chem., 76, 3029 (1972); (b) R. J. P. Williams, J. Chem. SOC.,457 (1958). (21) G. Atkinson and C. J. Hallada, J . Am. Chem. SOC.,84, 721 (1962); M. Linard, Z. Electrochem., 30, 225 (1944).
C. A. Bunton and L. Sepulveda (22) H. Yokoyama and H. Vamatera, Bull. Chem. SOC. Jpn., 48, 2708 (1975). (23) E, Grunwald, G. Baughman, and G. Kohnstarn, J . Am. Chem. Soc., 82, 5801 (1960). (24) E. Gordon, ref 8, p 259. (25) J. B. Hyne, J . Am. Chem. Soc., 85, 304 (1963). (26) J. 8. Hyne, J . Am. Chem. Soc., 82, 5129 (1960). (27) A. Diamond, A. Fanelli, and S. Petrucci, Inorg. Chem., 12, 611 (1973).
Hydrophobic and Coulombic Interactions in the Micellar Binding of Phenols and Phenoxide Ions Cllfford A. Bunton* Department of Chemistry, University of California, Santa Barbara, California 93 106
and Luis Sepulveda" Department of Physical Chemistry, Facuitad de Ciencias, Universidad de Chile, Santiago, Chile (Received October 23, 1978)
The transfer free energies of a series of substituted phenols and phenoxide ions from water to micelles of cetyltrimethylammonium bromide (CTABr) have been measured at high and low pH. The contribution of each methylene (or methyl) group to the free energy is ca. -300 cal mol-' for both phenols and phenoxide ions, and the transfer free energies of phenol and its phenoxide ion are respectively -5680 and -6870 cal mol-l, whereas that of %naphthol is -6660 cal mol-'. The contribution of the Coulombic component of binding of the anions is consistent with a micellar surface potential of 50 mV, and there appears to be a major contribution from ion-dipole interactions between the cationic head groups of the surfactant and the polarizable aryl groups.
The effect of micelles upon overall reaction rates and equilibria depends upon the incorporation of solutes into the micellar pseudophase and the micellar effects therefore typically increase with increasing hydrophobicity of the There are now a number of bimolecular reactions for which the rate-surfactant profiles have been interpreted largely in terms of the distribution of both reactants between the aqueous and micellar pseudop h a ~ e s . ~The - ~ aim of the present work was to examine systematically the effect of increasing hydrophobicity upon the binding constants of weak acids and their conjugate bases to cationic micelles of cetyltrimethylammonium bromide (CTABr), cf. ref 10. Phenols are very convenient solutes for this purpose because the binding constants to the micelle can be determined spectrophotometrically (cf. ref 11). A number of estimates have been made of the contributions per methylene group to the free energy of transfer from water to the micellar pseudophase of a solute which - l ~ our aim was to find out contains an n-alkyl g r ~ u p , ~ ' and whether the incremental free energy of transfer per methylene group depended on the charge of the solute. For example, phenol and its phenoxide ion might have different locations in the micelle and the hydrophobic interactions might then be different. Experimental Section Materials. The phenols were purified by distillation under vacuum or by recrystallization, and CTABr was purified by recrystallization.s Binding Studies. Incorporation of a phenol or phenM) in CTABr shifts its absorption oxide ion (2 X spectrum, and wavelengths were chosen a t which the change in absorbance was greatest. The wavelengths, nm, used for phenol and phenoxide ion (in parentheses) were 0022-3654/79/2O83-0680$0 1.0010
TABLE I: B i n d i n g Constants a n d M o l a r Free Energies of Transfer phenols
substituent 1
2, 3, 4, 5, 6, 7, 8, 9,
p-Me
p-Et p-nPr P-S-BU
P-t-Bu m-t-Bu
p-t-BuO p-t-Am
phenoxide ions
KS > M"
P"M -
Ks
P3W
M"
263 485 794 1350 1900 1700 1700 1630 4330
-5680 -6040 -6330 -6650 -6850 -6780 -6780 -6760 -7340
P O M-
I
POW
1980 3350 5320 10800 14000 11650 9860 7000 26600
- 6870
-7180 -7460 -7880 -8030 -7920 - 7820 -7620 -8410
as follows: 272 (300); for substituted phenols p-Me, 280 (302);p-Et, 288 (310); p-n-Pr, 288 (308);p-s-Bu; 287 (308); p-t-Bu, 378 (305); m-t-BuO, 280 (300); p-t-BuO, 300 (320); p-t-Am, 285 (308). The absorbances were measured over a range of CTABr up to 3 X M with CTABr in the reference cell, and the binding constants, K,, were calculated following methods already described.l' The binding constant is given by'O
K , = [sMl/([sWl([CTABrl - cmc - [SMl)) (1) where the subscripts W and M denote solute in the aqueous and micellar pseudophases, cmc is the critical micelle concentration, and concentrations are expressed as molarities in terms of the total volume of solution. In most of our experiments [CTABr] was large enough for changes in the cmc due to the solute to be neglected and [CTiBr] >> [SM]. For experiments with phenols we added 0.02 M HC1 and with phenoxide ions 0.02 M NaOH. Experiments in alkali I
1979 American Chemical Society
The Journal of Physical Chemistty, Vol.
Micellar Binding of Phenols and Phenoxide Ions
were done rapidly to avoid oxidation and with both phenols and phenoxide ions the absorbances reached constant values a t high CTARr where the solutes were fully bound to the micelles. All measurements were a t 25.0 "C.
Results and Discussion The standard free energy," AG,", is related to K, (Table 1) by (2) AG," = -RT In K, where the standard state is taken to be a dilute solution of the solute in aqueous 0.02 M electrolyte, and we will assume that it depends on two contributions, one, AG"R, of the substituent and the other, AGOPh, of the phenol or phenoxide moiety. Each of the n methylene or methyl moieties of the substituent alkyl group is assumed to contribute independently (and equally) t,o AG'R: AGOR
= nAG"c
The transfer free energy from water to the micelle is given by p o M - pow = RT In (X,/XM) (5) where the subscripts denote the aqueous and micellar pseudophases, and the mole fractions Xw and XMare given by =e
[Swl [Sw]+ 55.5 + cmc
and
where AD,] denotes the concentration of micellized surfactant. The values of p " M - pow cal rnol-l, for phenols and phenoxide ions are in Table I. In eq 5a 55.5 >> [Sw] cmc, and [D,] >> [SM] so that
+
XW/XM = (1/55.5)K8
(6)
and
( p " -.~p o w ) 2 nApoc 4- &"ph = -RT In 55.5 - RT In K,
8000
/
-'t
0 8
3
4
n
03
3-
1
2
5
n Figure 1. Free energies of transfer of phenols, 0, and phenoxide ions, 0 , from micelles of CTABr. (For symbols see Table I.)
(3)
This assumption is supported by the results so that -RT In K, = nAGoc + AGOPh (4)
xw
83,No. 6, 1979 681
(7) (7a)
The data obey eq 7 surprisingly well (Figure 1). A plot of Ap"M - Apow against the number of carbon atoms in the alkyl group is approximately linear for both phenols and phenoxide ions, even though in some cases meta rather than para substituents are considered, and the points for n-butoxyphenol are close to those for butyl phenols. Substituent Effects. The slopes of the plots in Figure 1 are very similar, suggesting that the hydrophobic interactions of the alkyl groups are the same for phenols and phenoxide ions. This result suggests that phenols and phenoxide ions have rsimilar locations in the micelle, and is consistent with spectroscopic evidence that hydrophobic polar and ionic solutes are located on the average a t the micelle-water interface (cf. ref 2-5, 15-17). Neither branching of the alkyl chain in the butyl and amyl phenols nor the position of the tert-butyl group has a large effect upon the transfer free energies (Figure 1). The transfer free energy for p-tert-butoxyphenol is also very similar to that of p-tert-butylphenol, but slightly lower
for tbe phenoxide ion (Figure 1). The ethereal oxygen atom is probably less hydrophobic than a methylene group.18 For both phenols and phenoxide ions the incremental free energy of transfer per methylene (or methyl) group is approximately -310 cal mol-l. It is useful to compare these values with those for other micellar systems. The values of transfer free energies for a variety of nonionic and zwitterionic surfactants into their micelles are approximately -700 cal mol-l,' and similar values are found for ionic surfactants in solutions of relatively high ionic ~ t r e n g t h . ' ~These ~ J ~ values are similar to the incremental free energy of transfer of an alkane to anionic micelles of sodium lauryl sulfate in 0.1 M NaC1, and to the incremental free energy of vaporization of alkanes. The similarity of free energies of transfer for a number of surface processes has been discussed by Mukerjee.lQ Another example comes from the work of Gitler and Ochoa-Solano who showed that the incremental free energies of transfer of esters containing n-alkyl groups to cationic micelles was -630 cal mol-l per methylene group.2o Again these solutions contained added electrolyte. The transfer free energies for surfactant monomer to ionic micelles in the absence of added salt is often much smaller than -700 cal mol-l, being approximately -400 cal mol-l, but increasing steadily with added salt.14 These salt effects might suggest that added salts bring the ionic head groups in a micelle closer together, as suggested from surface tension data,21and thus squeeze out water from between surfactants in the micelle which would therefore become more hydrophobic. However, Tanford has pointed out that the ionic strength in solutions of surfactants is given by the cmc, so that the ionic strength changes markedly with surfactant chain length unless it is maintained by relatively high concentrations of adlded electr01yte.l~~ These differences in the free energies of transfer suggest that in our system the alkyl groups of the solutes are in a medium which is far from hydrocarbon-like, i.e., that the micellar surface is very rough, with water penetrating between the n-alkyl groups in the micelle. This supposition is consistent with observations on micellar structure which Professor Menger has made on the basis of model building. His inspection of a molecular model of a cationic micelle suggests that there are relatively deep clefts in its surface into which water may penetrate.22 It is therefore not surprising that our incremental transfer free energies are so much lower than those es-
682
The Journal of Physical Chemistry, Vol. 83, No.
6, 1979
C. A. Bunton and L. Sepulveda
timated for transfer of monomeric surfactant where the Y O M - FOW = e40 (8) incremental effect depends on transfer of an additional where e is the charge on the electron (eu), $o is the surface methylene group into the hydrocarbon-like micellar potential (stat volts), and the free energy is expressed in ~ore.'~J~ erg molecule-l. Thus for an incremental free energy of 1150 The linearity of the plots in Figure 1, despite branching cal mol-I $o = 50 mV. This value of the surface potential of the alkyl groups and the inclusion of a meta isomer, is considerably lower than those usually cited for ionic suggests that all the alkyl groups are in a similar envi~~ micelles, e.g., it is -185 mV for sodium lauryl ~ u f a t e .(It ronment. is, however, of similar magnitude to the experimentally Contribution of t h e Aromatic Moeity. From the results determined [ potentials.) shown in Figure 1 we estimate -5720 and -6870 cal mol-l, Surface potentials have been calculated from variations respectively, for the transfer energies of a phenoxy and a of pK, of hydrophobic indicators fully bound to nonionic, phenoxide moiety from water into the micelle. The difanionic, and cationic micelles. These potentials are -134 ference between these two values presumably represents mV for NaLS and +148 mV for CTABr,26based on a the Coulombic interactions between the negative charge relation between equilibrium constant and surface poand the cationic head groups of the micelle. tential, which should depend, according to the model, on the interactions between a charge and the micellar surface. Considering first the transfer free energy of the phenoxy It is not obvious why our apparent value of the surface moiety we note that it is considerably larger per carbon potential is so much lower than the generally cited values, atom than expected from the value of ca. -300 cal mol-l because it is based on direct measurement of solute disper alkyl carbon atom, even though the phenoxy group is tribution. Phenoxide ions are not point charges, but in close to the micelle-water interface in a relatively aqueous nearly all indicators charges are also extensively deloenvironment. These differences in transfer free energies calized. A major problem with the interpretation of almost centainly arise from the interactions between the micellar effects upon acid-base dissociation is that alcationic head groups of the micelle and the polarizable though the use of hydrophobic indicators eliminates phenyl groups. There is evidence from NMR and elecdue to incomplete solute binding to the micelle tronic spectroscopy for these favorable i n t e r a c t i o n ~ . ~ ~ J ~ - ~problems ' the treatments do not consider the distribution of hyThe transfer free energy of 2-naphthol from water to drogen (or hydroxide) ions between the aqueous and micellized CTABr is -6680 cal mol-l and K , = 1390 M-l. micellar pseudophases.'B8 It seems to us that this problem This introduction of the four additional carbon atoms in should be considered; for example, the effect of added salt going from a phenyl to a naphthyl group has approximately upon indicator equilibria in micelles can be considered the same effect as introduction of a butyl group (Table I). qualitatively in terms of a competition of, say, a hydrogen These transfer free energies for aromatic moieties are ion and an added cation for an anionic micelle rather than also considerably larger than expected solely from conin terms of effects on surface potential and its role in sideration of hydrophobicities. Based on effects on critical determining the relative interactions between a micelle and micelle concentrations Tanford estimates that the h y an indicator and its conjugate acid or base.27 drophobic contribution of a phenyl group is equivalent to Acknowledgment. Support of this work by the National that of three methylene Le., it is similar to the Science Foundation (Chemical Dynamics and International difference between a naphthyl and a phenyl group. These Programs) is gratefully acknowledged. The collaboration comparisons strengthen our conclusion that dipole-ion was developed with the support of the University of interactions between a polarizable phenyl group and a Chile-University of California Cooperative Program. cationic head group are of considerable importance. E f f e c t of Negative Charge o n T r a n s f e r Free Energy. References and Notes The intercepts of the plots for phenols and phenoxide ions For discussions of rate effects see ref 2-5. differ by 1150 cal mol-' (Figure l),showing that, as exJ. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975. pected, the negative charge of the phenoxide favors E. H.Cordes, Ed., "Reaction Kinetics in Micelles", Plenum Press, New transfer from water to the cationic micelle. However, the York, 1973. effect of the negative charge is not particularly large, e.g., E. H. Cordes and C. Gitler, Prog. Bioorg. Chem., 2, 1 (1973). C. A. Bunton in "Application of Biomedical Systems in Chemistry", it contributes little more to the binding to the micelle than Part 11, J. E. Jones, Ed., Wiley, New York, Chapter I V , p 731;Pure do three saturated carbon atoms of an alkyl group. This Appl. Chem., 49,969 (1977). modest effect is perhaps not surprising because although K. Martinek, A. K. Yatsimirski, A. V. Levashov, and I. V. Berezin in "Micellization, Solubilization and Microemulsions", Vol. 2,K. L. Mittal, Coulombic interactions assist binding of the phenoxide ion Ed., Plenum Press, New York, 1977,p 489. to the micelle, as compared with a phenol, there is an L. S. Romsted in "Micellization, Solubilization and Microemulsions", opposing effect due to the greater solvation by water of Vol: 2, K. L. Mittal, Ed., Plenum Press, New York, 1977,p 409. C. A. Bunton, F. Ramirez, and L. Sepulveda, J . Org. Chem., 43, a phenoxide ion as compared with a phenol. In addition, 1166 (1978);C. A. Bunton, L. S. Romsted, and H. J. Smith, bid., the head groups of ionic micelles are extensively screened 43,4299 (1978);C. A. Bunton, N. Carrasco, S . K. Huang, C. H. Paik, by counter ion^,^^^^ and the dielectric constant at the miand L. S. Romsted, J . Am. Chem. Soc., 100,5420 (1978). I.M. Cuccovia, E. M. Schroter, P. M. Monteiro. and H. Chaimovich, cellar surface is relatively high, although less than that of J . Org. Chem., 43,2248 (1978). water, e.g., values of ca. 40 have been estimated for micelles F. M. Menger and C. E. Portnoy, J. Am. Chem. Soc., 89,4698(1967). of several ionic surfactant^.^^^^ L. Sepulveda, J . Colloid Interface Sci., 46,372 (1974). For discussions of the thermodynamics of micellar processes see The effect of the negative charge could be exerted diref 13 and 14. rectly through a Coulombic attraction between anionic (a) C. Tanford, "The Hydrophobic Effect", Wiley-Interscience, New oxygen and a cationic quaternary ammonium ion, or inYork, 1973,Chapters 6 and 7; (b) C. Tanford, Science, 200, 1012 (1978). directly because of electron release into the phenyl group K. S.Birdi in "Micellization, Solubilization and Microemulsions", Vol. and therefore a more effective interaction between it and 1, K. L. Mittal, Ed., Plenum Press, New York, 1977,p 151. a cationic head group.16 J. C. Erickson and G. Gillberg, Acta Chem. Scand., 20,2019 (1966). C. A. Bunton, M. J. Minch, L. Sepukeda, and J. Hidalgo, J. Am. Chem. The binding of counterions to ionic micelles is often Soc., 95,3622 (1973);C. A. Bunton and M. J. Minch, J. Phys. Chem., treated in terms of the electrostatic effect upon a point 78, 1490 (1974). Reference 2, Chapter 2. charge, e.g., for a univalent ion:25
Surface Reactions of Oxygen Ions
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
(18) For a discussion of this point see ref 13a, Chapter 7. (19) P. Mukerjee in "Micellization, Solubilization and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 171. (20) C. Gitler and A. Ochoa-Solano, J. Am. Chem. SOC.,90, 5004 (1968). (21) R. L. Venable and R. V. Nauman, J. Phys. Chem., 68, 3498 (1964). (22) F. M. Menger, J. M. Jerkunica, and J. C . Johnston, J . Am. Chem. Soc., 100, 4676 (1978); F. M. Menger, Acc. Chem. Res., in press. (23) D. Stigter, J . Phys. Chem., 68, 3603 (1964); 70, 1323 (1966).
683
(24) P. Mukerjee, J. R. Cardinal, and N. R. Desai in "Micellization, Solubilization, and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 241. (25) J. T. Davies and E. K. Rideal, "Interfacial Phenomena", Academic Press, New York, 1961, p 159. (26) M. S. Fernandez and P. Fromherz, J. Phys. Chem., 81, 1755 (1977). (27) C. A. Bunton in ref 3, p 73; C. A. Bunton and L. Robinson, J . Phys. Chem., 73, 4237 (1969); 74, 1062 (1970).
Surface Reactions of Oxygen Ions. 3. Oxidation of Alkanes by Of on MgO Yusaku Taklta and Jack H. Lunsford" Department of Chemistry, Texas A&M University, Coiiege Station, Texas 77843 (Received August 14, 1978) Publication costs assisted by the National Science Foundation
Ozonide ions on MgO reacted with C1 to C4 alkanes at 25 "C with half-lives between 1.7 and 5.2 min. A stoichiometry of one alkane molecule reacted per one 03-ion was determined. Following the reactions, only ethane gave an appreciable amount of gas phase product (ethylene) at 25 "C; however, upon heating the samples to elevated temperatures, other hydrocarbons and C 0 2 were obtained. At intermediate temperatures the corresponding C2 to C4 alkenes were the principal products, although the yields were less than observed in the corresponding reactions with 0-. The presence of gas phase oxygen during the initial reaction suppressed the formation of these alkenes. Infrared spectra of the surface complexes indicate that the reaction of 03-with ethane at 25 "C resulted in the formation of carbonate and ethoxide ions. At 150 "C the ethoxide ions decreased in concentration and acetate ions were formed. The acetate ions, as well as the carbonate ions, decomposed at elevated temperatures. The initial step in the reaction of 03- with alkanes is believed to be hydrogen atom abstraction, with the alkyl radical either forming an alkoxide ion or an alkyl peroxy radical. The alkoxide ion is largely responsible for the formation of the corresponding alkene, and the peroxy radical, as well as other oxidation products, result in the formation of C02 and CHI.
Introduction The role of' oxygen ions in promoting oxygen addition and oxidative dehydrogenation reactions is of interest both in homogeneous systems and in heterogeneous catalysis. Surface reactions of 0- ions with simple alkanes112and alkenes3are facile, and the initial step probably involves hydrogen atom abstraction from the organic molcule. In this paper we describe the reactions of 03-on magnesium oxide with alkanes. The identification of 03-on metal oxides by electron paramagnetic resonance (EPR) spectroscopy was closely associated with the discovery of 0-, since the reaction 0- + O2 P 03-
(1)
occurs when small amounts of molecular oxygen are p r e ~ e n t . ~The ? ~ ozonide ion has now been identified on Mg0,495Ti02/Si02,6and V206/Si02.7 From oxygen-17 hyperfine data it has been shown that the ion on MgO is bonded to the surface by a terminal oxygen atom.5 Gas phase studies carried out by Parkes* demonstrate that the rate of reaction of 03-with hydrocarbons has an upper limit which is more than two orders of magnitude lower than that of 0-. Similarly, Naccache and Cheg have reported that 03-on MgO did not react a t -196 O C with molecules such as carbon monoxide, ethylene, or propylene. The 0; ion, however, has been proposed as an intermediate in the homomolecular oxygen isotope exchange reaction over Vz05/Si02.7 It has been suggested that catalytic oxidative dehydrogenation reactions on metal oxides may involve 0- ions, but since the oxidant is usually 02,one might expect that Os-would also be present on the surface. The extent of 0022-3654/79/2083-0683$0 1.OO/O
03-formation, its relative reactivity, and the products formed are thus important in understanding selective oxidation reactions. The MgO surface is well suited for such a study since eq 1 is shifted far in favor of the 03ion, which is not the case, for example, with V205/Si02. Nevertheless, one should recognize that the results reported here represent stoichiometric and not catalytic reactions. Experimental Section The magnesium oxide was prepared according to the method reported previously.1° Samples were evacuated a t 530 "C overnight before each experiment in order to eliminate hydrocarbons, carbon dioxide, and water. 'The ozonide ion was produced on the MgO surface by UV irradiation (254 nm) of the sample in the presence of l V 2 0 at 25 "C for 10 min-3 h. The concentration of 0, on MgO was controlled by varying the N 2 0 pressure and the duration of UV irradiation. The EPR spectra of 0; were measured using the Varian E-6S, X-band spectrometer with the sample either a t - 196 or 25 "C. The g values were evaluated relative to a phosphorus-doped silicon standard having a g value of 1.9987. The concentration of 03-was determined by comparing the second integral of the EPR spectrum with that of the standard. Infrared spectra of self-supporling MgO wafers with adsorbed molecules were obtained in a manner similar to that described elsewhere,'l using an Beckman IR-9 spectrophotometer. The alkanes were purified in the same manner as reported previous1y.l Hydrogen (Matheson 99.999% purity) and helium (Airco 99.999% purity) were used without further purification. 0 1979 American Chemical Society
