J . Phys. Chem. 1991, 95,4551-4563
4551
Localization of an a-Enone In Organized Media by Fourier Transform Infrared Spectroscopy. 1. Micellar Solutions H.Gonplves* and A. Lattes Laboratoire des IMRCP. UA au CNRS No. 470, UniversitC Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cgdex, France (Received: July 9, 1990; In Final Form: November 28, 1990)
Solubilization of 3,5,5-trimethyl-2-cyclohexenone(isophorone) in cationic (CPC, CPB, CTAB, DTAB) and anionic (SDS, LDS)micellar media was investigated by Fourier transform infrared spectroscopy. The distribution of the solute in the micelles as a function of solute (0.03-0.10 M) and surfactant concentration (from values slightly above the cmc to 0.1 M (CPC, CPB, CTAB) or 0.25 M (DTAB, SDS, LDS)) was evaluated. With the determination of the influence of hydrogen bonding on the carbonyl stretching vibration, this technique provides information on the environment of the probe in the micellar medium. Isophorone is situated mainly in the polar medium. Four sites were observed in the cationic micellar solutions, and three in the anionic ones. The distribution between these sites (aqueous phase, diffuse layer, external and internal layer of the palisade) as a function of the [surfactant]/[solute] ratio is discussed. The capacity of cationic micelles to solubilize isophorone increases with increasing chain length of the surfactant. CPC micelles take up more isophorone than the CPB micelles. The pyridinium ion favors interaction of isophoronewith polar heads, and in cationic micelles, isophoroneis preferentially localized in a more hydrophobic environment than in the anionic micelles. Determination of the sites of solubilization by FTIR spectroscopy should further understanding of, and enable prediction of, reactions occurring in micellar solutions.
Introduction
The nature of the surfactant (sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB)) has been shown to influence both the products and yield in the reactions of 3,5,5-trimethyl-2-cyclohexenone(isophorone) in micellar solution~.'-~Opposite regioselectivitiesare observed for the reduction of isophorone in these two media? and at surfactant concentrations above 0.03 M, different amounts of isophorone are photodimerized in anionic and cationic micellar solution^.^ Fourier transform IR spectroscopic analysis of the C=O group of partially deuterated i s o p h o r ~ n ecan ~ . ~provide information on the localization of this probe in micellar solutions of heavy water. This information should help account for the results of the reduction and photodimerization of isophorone in micellar mediae4 Isophorone was selected because of its high polarity ( p = 3.99 D),'which confers adequate solubility in both water-4 and organic media, and because its chemical reactivity had been investigated in previous e~periments.'?~ The conjugated nature of the carbonyl group makes it polarizable and thus susceptible to influences from reactants and polar environments. Furthermore, in a previous study,' we showed that micellization depended on the nature of the surfactant, and we thus pursued our investigations with other surfactants used in the laboratory8 with different counter ions, chain lengths, and polar heads.'J Six different surfactants were employed in the present study: 2 anionic, lithium and sodium dodecyl sulfate (LDSand SDS), and 4 cationic, cetyl- and dodecyltrimethylammonium bromide (CTAB and DTAB) and cetylpyridinium chloride and bromide (CPC and CPB). The localization of solutes in micelles was initially studied by N M R spectro~copy,~ and various methods have been employed subsequently such as X-ray diffraction, UV and fluorescence spectroscopy, or neutron scattering, etc.'*I3 Only recently have a few studies using IR spectroscopytet6 been reported. An essential characteristic of micelles is the lability of the chains in the core region. The solute is thus mobile in this environment, and solubilized molecules can be exchanged rapidly." The results obtained depend somewhat on the method used and the time span of the measurements. For example, Casal and Martin'6b detected two environments for a long-chain ketone in a micellar solution using IR spectroscopy, whereas only an average location was observed by using N M R spectroscopy.18 The shorter interval of measurement of vibrational spectroscopy enables detection of various environments of the probe molecule. For ketone probes, the stretching frequency of the C=O bond is decreased if this Author to whom the correspondence should be addressed.
group is hydrogen bonded,19and IR absorption bands are highly sensitive to the microenvironment of the relevant group. Changes in frequencies and intensities of the bands can thus provide information on the structures of the different microphases of the system. The concentrations of surfactant and solute used in this study were based on values used in investigations of the reactions of isophorone. The [surfactant]/[solute] ratio ranged from 8 to 0.3. Under these conditions, we detected various sites of solubilization of isophorone in micellar solutions, three in the anionic micelles and four in the cationic micelles.
Experimental Section Isophorone (Aldrich) was redistilled, and its purity was checked (1) Fargues, R.; Maurette, M. T.; Oliveros, E.; Rivitre, M.; Lattes, A. Nouv. J . Chim. 1919, 3, 487. (2) Fargues-Sakellariou, R.; Rivitre, M.; Lattes, A. Nouv. J . Chim. 1985, 9, 95. (3) Delpech, V.; Fargues-Sakellariou, R.; Rivitre, M.; Latts, A. Bull. Soc. Chim. Fr. 1989, 3, 49. (4) Gonwlves, H.; Lattes, A. C.R. Acad. Sci., Ser. 2 1990, 310, 1179. (5) Noack, K. Spectrochim. Acta 1962. 18, 697. (6) Gonplves, H.; Coronel, M. D.; Fargues-Sakellariou, R.; Rivitre, M.; Lattes, A. C. R . Acad. Sci., Ser. 2 1987, 305, 851 and references therein. (7) Chapman, 0. L.; Nelson, P. J.; King, R. W.; Trecker, D. J.; Griswold, A. A. Rec. Chem. Prog. 1967, 28, 167. (8) (a) Coronel, M. D.; Gonpalves, H.; Gas, G.; Rivitre, M. In Science et Industrie du Bois; Lor, A. R. Bo., Ed.; Actes du 2tme Colloque Nancy, France, 1988; Vol. 11, p 329. (b) Coronel, M. D. ThZse, Universiti Paul Sabatier, Toulouse, France, 1988. (9) Eriksson, J. C.; Gilberg, G . Acta Chem. Scand. 1966, 20, 2019. (10) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macro-molecular Systems; Academic Press: New York, 1975; Chapters 2 and 3. ( 1 1 ) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100. (1 2) Hayter, J. B.; Hayoun, M.; Zemb, T. Colloid Polym. Sci. 1984, 262, 798. (13) Attwood, D.; Florence, A. T. Surfacrant Sysfems,Chapman and Hall Ltd.: London, 1983; (a) Chapter 5, p 229, (b) Chapter 3, p 91. (14) (a) Prieto, M. J. E.; Villalain, J.; Gomez-Fernandez, J. C. Spectrosc. Biol. Mol., Proc. Eur. ConJ, Ist, 1985, 296. (b) Villalain, J.; Gomez-Fernandez. J. C.; Prieto, M. J. E. J . Colloid Interface Sci. 1988, 124, 233. (15) Hayakawa, S.;Matsui, T.; Tanaka, S.Appl. Spectrosc. 1987, 41, 1438. (16) (a) Casal, H. L.; J . Am. Chem. Soc. 1988,110,5203. (b) Casal, H. L.; Martin, A. Can. J . Chem. 1989, 67, 1554. (c) Casal, H. L.; Wong, P. T. T. J . Phys. Chem. 1990, 94, 717. (17) Zana, R. J . Chim. Phys. 1986,83, 603. (18) Menger, F. M.; Jerkunica, J. M.; Johnston, J. C. J . Am. Chem. Soc. 1918, 100, 4676. (19) Bellamy, L. J. Advances in Infrared Group Frequencies; Methuen and Co.: London, 1968.
0022-3654/91/2095-4557%02.50/00 1991 American Chemical Society
4558 The Journal of Physical Chemistry, Vol. 95, No. 11, 199'I
Gontplves and Lattes I._
d 0.04
*
'
16bOc
1700
Figure 1. Spectra of 0.03 M isophorone D in aqueous solution (-, a) and in micellar solutions of CTAB (0.01 M), b; - (0.02 M),c). (e-
--
by GC and proson NMR methods. partially deuterated isophorone (isophorone D)M was purified by distillation. LDS (Fluka), DTAB and CTAB (Aldrich), and CPC*HzO(Janssen) were usad without further purification. SDS (Prolabo) and CPB.HzO (Fluka) were recrystallized in ethanol. The solvents for spectroscopy CCl,, CSz (Fluka), cyclohexane and methanol (Merck), and DzO (99.8% Spin et Techniques) were used without further purification. The samples were examined at a thickness of 100 Fm in the aqueous and micellar solutions and 50 pm (CaF2 window) in the various solvents at an isophorone concentration of 0.1 M in a Bruker FTIR spectrometer (IFS 110) equipped with a mercury cadmium telluride (MCT) detector. The instrument was continuously swept through with dry air, and the room temperature was maintained at 25 OC. The use of a linear collector (shuttle) allows the alternate gathering of data on the solvents and on the isophorone solutions; the experimental curves of the solute are therefore free of water vapor. The experimental resolution betwec 1710 and 1580 cm-' was 2 cm-'. The second and fourth derivatives were determined on the experimental curve without smoothing. The spectra were recorded and frequencies determined with a resolution of 0.5 c d . An accumulation of 96 interferograms were apodized by using the Happe Genzel function, and resolution was enhanced by deconvolution with the Dcconv-Bruker program,which is based on the algorithm described by Kaupinnen et aLzo The bandwidths (15 and 6 cm-') and reduction factors (2 and 1.2) were set to give a slightly negative absorbance at the bottom of the band. The critical micellar concentrations both with and without solute were determined by conductimetry.
Results In the absence of solutmlvent interaction (Hor D bonding), the solvent has little effect on the C - 0 and C - C stretching frequencies of isophorone D. The frequencies in CSz and CC14 ( C 4 , 1667-1668 cm-'; C = C , 1617 cm-I) are close to those determined in the condensed state (1664 and 1615 an-'),but lower than those measured in cyclohexane (1679 and 1619 an-'). The lowering of the frequency observed in alcohols (H or D n-butanol, 1655-1614 cm-'; methanol, 1652-1613 cm-I) and in DzO (1628-161 1 cm-'; Figure 1, curve a) that concerns mainly the C 4 absorption bands is proportional to the polarity of the solvent. The decrease in the strength of the hydrogen bond as we go from D20 to the alcohols induces an increase in the frequency of roughly 25 cm-' as well as a lowering of the intensity (of roughly 40% when compared to n-butanol, for instance). If surfactant is added at a concentration below the cmc to an aqueous solution of isophorone, the solubility of isophorone increases from 0.066 to 0.080 M?J1 The relative absorbance of the C 4 band is little affected by the presence of DTAB for isophorone concentrations between 0.03 and 0.08 M. The second derivative of the experimental curve displays a maximum at 1630 cm-' (Figure 2, curve a), and the fourth derivative a second maximum at 1622 Appl. Spectiarc. 1981. 35, 271. (21) Due to the presence of hydrophobic group, addition ofO.01 M DTAB makes the a a u w solution more lipophilic and enables solubilization of more .. isophorone.
Figure 2. Second derivatives of spectra of 0.03 M isophorone D in aqueous solution (-, a) and in micellar solutions of CTAB ( - - -(0.02 M), b; (0.03M),C; -.-(0.04M), d). TABLE I: Vibration Frequencies ( c d ) of the V h y >M B.ad of 0.03 M Imphone D in Micellar sdutions for Different Concentratioas of Surfactants concn,M 0.001
0.01 0.02 0.03 0.04 0.05 0.08 0.1 0.10
CPC 1628 1629
CPB
1628 1628
1630
1629
1632
1631
1642 1640
1640 1638
0.15 0.2 0.25
surfactant CTAB DTAB
1628 1628 1629 1629 1632 1632 1638 1640 1636
1628 1628 1630 1631 1634 1635 1634 1639 1640 1641
SDS
LDS
1629 1630 1630 1632 1633 1634 1635 1634 1635 1636 1636
1629 1630 1633 1634 1634 1635
[isophorone D] = 0.1 M.
cm-I. This indicates that the C 4 band is the sum of two elementary bandszzcorresponding to isophorone D complexed with one or two molecules of water.u The relative proportions of these two forms depend on the concentration of isophorone D in the aqueous solution. The fall in intensity of the C - 0 band and its increase in frequency with increasing surfactant concentration indicate that there is a change in solvation of isophorone D in micellar solutions that is not observed in aqueous solution (Figures 1, 3-6; Table I). The peaks in the second derivative spectra provide an indication of the type of d a t i o n between isophorone D and DzO, although maxima due to harmonics and combinations cannot be discounted (deformation bands of the different functional groups and of the skeleton in the region 1200-400cm-l). By comparison with the second derivative peak at 1630 cm-' in aqueous solutions (Figure 2, curve a), we attributed the lowestfrequency (1630cm-') to isophorone D in the aqueous phase of the micellar solutions, while the higher frequencies were attributed to isophorone D localized in less polar environments (the frequencies being inversely proportional to the polarity; Figure 2, curves c,d). In fact, the highest frequency observed (1653 cm-') was close to that measured in methanol (1652 cm-l). For us, the different localizations of the solute can be described in terms of the structure of m i ~ e l l e s which , ~ ~ ~includes ~~ (1) a continuous aqueous phase, (2) a diffuse, essentially aqueous layer (22) Maddams, W. F.; Mead, W. L. The Measurement of Derivative IR. Part I. Spectrochim. Acra 1982,38A, 437-444. Hawkes, S.; Maddams, W. F.; Mead, W. L.; Southon, M. J. The Measurement of Derivative IR. Part 11. Spctrochim. Acra 1982,38A, 445-457. Maddams, W.F.;Southon, M. J. The Measurement of Derivative IR. Part 111. Spectrmhim. Ada 1982, 38A, 459-466. (23) Combelas. P.; Garrigou-Lagrange, C.; Lascombe, J. Ann. Chim. (Paris) 1970, 5, 315. (24)Mittal. K.; Mukerjtc, P. In Micellizarion, Solubilizotion and Microemulsions: Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. l , p 1.
(25) Mukerjee. P. In Solution Chemistry ofSurJactants; Mittal, K. L., Ed.;Plenum Press: New York, 1979; Vol. 1, p 153.
The Journal of Physical Chemistry, Vol. 95, No. 1 I , 1991 4559
Localization of an a-Enone in Organized Media 0.2 ~
CPB
I
1760 16bOc rfl Figure 3. Spectra (s) and their second derivatives (d) for 0.03 M iso-
phorone D in micellar solutions for different CPB concentrations: -, 0.03 M; 0.05 M;---, 0.1 M. *.a,
025
CTAB
Figure 4. Spectra (s) and their second derivatives (d) for 0.03 M iso-
phorone D in micellar solutions for different CTAB concentrations: -, 0.03 M; 0.05 M;---, 0.1 M. *a*,
5ec.der. 1700
Figure 5. Spectra (s) and their second derivatives (d) for 0.03 M isophorone D in micellar solutions for different DTAB concentrations: -, 0.03 M; 0.05 M;---,0.1 M. .*a,
containing 60-80% counter ions and some polar heads of surfactants, (3) the palisade region, which comprises the polar heads of the surfactant, counter ions, water, and short sections of the alkyl chains (1-3 carbons). This can be divided into two regions: the external zone (ionic heads) and an internal zone (alkyl chain part), and (4) the hydrophobic core. This arrangement of sites is consistent with currently accepted models, such as the early description of Hartley26and the more recent proposals of Zemb et aLna and Berr et aLB" The maxima close to 1650 cm-' were attributed to isophorone D solvated in the palisade region and the 1640-cm-' peak to isophorone D in the polar head region, while the 1636-cm-' peak was attributed to the presence of the ketone in the diffuse layer (Figure 2). As surfactant concentration increases from the cmc to 0.25 M (Figures 1-8; Table I), the peak shift, the fall in intensity of the C=O band, and the change in second derivative were attributed to an increase of isophorone in the organic pool and interfacial zone, which would favor displacement of the ketone to less polar zones. In this situation, the (26) Hartley, G. S. Aqueous Solutionr of Paraffin Chain Salts, Herman and CIC: Paris. 1936. (27) Cabanc, B.;Duplcssix. R.:Zemb, T.J. Phys. (Paris) 1985,16,2161. (28) Zemb, T. Thtse, Universit6 de Paris Sud (Orsay), France, 1985. (29) &rr, S. S.; Caponetti. E.;Johnson, J. S.,Jr.; Jones, R. R. M.;Magid, L. J. J. Phys. Chem. 1986, 90,5766. (30)Berr, S . S . J. Phys. Chem. 1987, 91, 4760.
T * Y b 3 0
, 1600cm' I
Figure 6. Spectra (s) and their second derivatives (d) for 0.03 M im-
phorone D in micellar solutions for different SDS concentrations: -, 0.03 M; 0.05 M;- - -, 0.1 M.
...,
micellar phase appears to extract isophorone from the aqueous phase. At the maximum surfactant concentration employed (0.25 M;[surfactant]/[solute] -8), isophorone appeared to be situated mainly in the palisade of the cationic micelles (DTAB (Figure 7, curve d3), 1653 and 1640 cm-') and in the diffuse layer of the anionic micelles (SDS (Figure 8, curve d3), 1636 cm-I). As surfactant concentration was reduced, the reverse behavior confirmed that solubilization depends on surfactant concentration. The relatively high polarity of isophorone ( p = 3.99 D) and its solubility in water (0.066 M,close to that of hexanol, 0.073 M) mean that a minimum concentration of surfactant is required before the micelles take up a significant amount of this probe. At an isophorone concentration of 0.03 M, this minimum concentration appears to lie between 0.02 and 0.03 M. We thus focused our attention on a value of 0.03 M. In comparison to the case for the aqueous phase, up to surfactant concentration of 0.03M, all the second derivative spectra exhibited two other peaks (1636, 1653 cm-'; Figure 2, curve c). Beyond this concentration, the behavior depended on the nature of the surfactant. A further peak at 1640 cm-' was observed for the cationic surfactants (Figure 2, curve d). Micellar Solutions of CPB and CPC. Variation in Surfactant Concentration. The changes in intensity and frequency of the C=O band as a function of surfactant concentration were comparable in these two micellar solutions. The second derivative
Gonqalves and Lattes
4560 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 0.25
0.2 DTA B
1700
Seeder. -
1
,r
"
sDs
I
1600 ~
,
v.
Secder.
I
'li
16cOcm-
Figure 7. Spectra (s) and their second derivatives (d) for 0.03 M isophorone D in micellar solutions for different DTAB concentrations: -,
0.1 M;
*.e,
0.2 M; ---,0.25 M.
peak at 1636 cm-' (Figure 3, curve d,) was not observed for the 0.05 M and 0.1 M surfactant concentrations (Figure 3, curves d2and d3). Four peaks could be distinguished at 1653, 1647, 1640 and 1630 cm-I. Self-deconvolutionmcarried out under identical experimental conditions showed the presence of three elementary bands at 1651, 1640, and 1630 cm-' (Figure 9, curve sd,). The intensity ratios listed in Table I1 are not affected by deconvolution. The intensity of the spectral envelope for the carbonyl band falls as surfactant concentration increases. Without knowledge of the integrated molar absorption coefficients, the intensity of the elementary bands determined from the deconvoluted spectra is not a true reflection of the probe concentration. We therefore compared the relative percentages at a given frequency. For an increase in surfactant concentration from 0.05 to 0.1 M, the Cgo frequency increased by 9-10 cm-' (Table I), and the relative intensity of the elementary band a t 1630 cm-' attributed to isophorone in the aqueous phase fell (Table 11). This is further evidence that probe solubility depends on surfactant concentration. Variation in Solute Concentration. For a surfactant concentration of 0.1 M, we examined the effect of isophorone concentration (0.03,0.06, and 0.1 M) on the distribution of isophorone in the micellar solution (Figure 9, Tables I and 11). The rise in probe concentration was accompanied by a increase in the relative intensities of the low-frequency components of the CEO band and a decrease in frequency of the maximum. At equal isophorone and surfactant concentrations (0.1 M), the second derivatives display a peak at 1636 cm-' (Figure 9, curve d,). Self-deconvolution still shows three bands (Figure 9, curve sd3; Table 11): one a t 1651 cm-' that enveloped the bands a t 1653 and 1647 cm-'," a band at 1639 cm-' (wider than the one a t 1640 cm-' recorded at an isophorone concentration of 0.03 M, Figure 9, curve sd,) that enveloped the bands at 1640 and 1636 cm-I, and a band at 1630 cm-I. The appearance of a peak at 1636 cm-' was indicative of isophorone in the diffuse layer, suggesting that the palisade had become saturated (Figure 9, curve d3). Micellar Solutions of C T A B . Variation in Surfactant Concentration. Comparison of the micellar solutions of CTAB and CPB demonstrates the effect of the nature of the polar head (Figures 3 and 4). Although the changes in frequency of the C = O band with change in surfactant concentration were almost identical (Table I), the second derivatives indicated a different distribution of solute. At the maximum surfactant concentration employed (31) The peak at 1647 cm-' was oberved in all the micellar solutions. It was assumed to correspond to either the elementary band of isophorone (with respect to 1653 cm-I) situated at varying depths in the internal region of the palisade, or to a nonelementary band. In the latter case, the intensity of the band at 1651 cm-' overestimates the proportion of isophorone in the internal region of the palisade.
TABLE II: Relative Intensities (96) of the Three Band8 of the Deconvoluted Spectra of Isophorone D around 1640 cm-' for Micellar Solutions of CPB and CPC concn, 1639 1651 surfactant M 1630 (1636 + 16401 1640 (1653 + 1647) CPB 0.05" 49 30 21 34 27 0.05" 39 CPC CPB 0.1" 31 34 35 25 CPC 0.1' 38 37 CPB 0.lb 31 39 30 28 41 CPC O.lb 31
" [isophorone D]
= 0.03 M. [isophorone D] = 0.1 M.
(0.1 M), the peak a t 1636 cm-' attributed to isophorone in the diffuse layer was well-defined for the CTAB micelles (Figure 4, curve d,) but undetectable for the CPB micelles (Figure 3, curve d,). Under conditions similar to those used for the pyridinium surfactants, self-deconvolution showed the presence of elementary bands corresponding to the second derivative peaks (1653, 1640, 1636, and 1630 cm-'), but we were unable to determine their relative intensities because they were so close together. Variation in Solute Concentration. As isophorone concentration was raised from 0.02 M to the surfactant concentration (0.1 M) (Figure lo), the distribution of isophorone in the micelle could be assessed from the second derivative peaks. At an isophorone concentration of 0.02 M, the peak at 1640 cm-' was well-defined (Figure 10, curve dl). At 0.1 M, second derivative peaks were observed at 1636 and 1630 cm-'(Figure 10, curve d3). The 4cm-' decrease in frequency of the C=O peak (Table I) indicated that the proportion of isophorone in the polar medium increased as its concentration in the micellar solution increased. Micellar Solutions of D T A B . Variation in Surfactant Concentration. As surfactant concentration increased, the intensity of the C=O band was always higher in DTAB than in CTAB (curve s, Figures 4 and 5), although the frequency shift was more gradual in DTAB (Table I). Variation in Solute Concentration. At a surfactant concentration of 0.1 M, there was only a small change in frequency of the C=O band (-1 cm-') as isophorone concentration was increased from 0.03 M to 0.1 M. The peaks at 1630 and 1636 cm-' were well-defined at the low isophorone concentration (Figure 11, curve dl). These observations can be accounted for by the presence of a significant proportion of isophorone in the aqueous and diffuse phases, and so there is less reduction in the proportion of isophorone in the palisade with increase in isophorone concentration in the micellar solution. Micellar Solutions of SDS and LDS. Variation in Surfactant
Localization of an a-Enone in Organized Media
Figure 9. Spectra (s) and their second derivatives (d) including selfdmnvolution (sd)for different concentrations of isophorone D in 0.1 M micellar CPB solutions.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4561
Figure 10. Spectra (s) and their second derivatives (d) for different concentrations of isophorone D in 0.1 M micellar CTAB solutions.
m Figure 11. Spectra (s) and their second derivatives (d) for different concentrations of isophorone D in 0.1 M micellar DTAB solutions.
Concentration. For surfactant concentrations between 0.01 and 0.1 M, the valency vibration bands of the C=O group of isophorone D in micellar solutions of SDS and LDS were identical (Table I). Curves ~ 1 - 3 of Figure 6 represent the change in intensity with change in SDS concentration. The second derivative spectra display four peaks at 1653, 1647, 1636,and 1630 cm-l (Figure 6,curve di). For surfactant concentrations of 0.1 and 0.2 M, the intensities and frequencies of the C=O bands were only slightly different in the two surfactants (Table I). Variation in Solute Concentration. Altering the concentration of isophorone (0.03to 0.1 M) at a surfactant concentration of 0.1 M had little (SDS) or no effect (LDS) on the peak frequencies (Table I), although there were increases in relative absorbances of the low frequency components (Figure 12, curves sI-s.~). Self-deconvolution could not determine the change in relative intensities of the 1653,1636,and 1630 cm-' bands, as 1636 and 1630 cm-'peaks were not well separated. However, we noted that the relative intensity of the elementary band at 1653 cm-' fell by around 10% as the isophorone concentration increased from 0.03 to 0.1 M, indicating that the proportion of isophorone in the polar medium increased. In summary, the localization of isophorone essentially in polar media agrees with the results of previous studies using compounds of similar polarity in different micellar solution^.^"^^*^^^^^ The influence of solute concentration on its distribution in the micelle has been studied by numerous w o r k e r ~ ? J 2 ~ ~ItJ ~is~established * that the extent of solubilization of the solute depends greatly on surfactant concentration, and the transition point for solute distribution has been identified, which is thought to reflect saturation of the interface or palisade zone?J2M This agrees with the results of our studies: the solute fraction remaining in the micelles
Figure 12. Spectra (s) and their second derivatives (d) for different concentrations of isophorone D in 0.1 M micellar S D S solutions.
decreases as its concentration in the solution increases. Micellar solutions have definite solubility power: at constant concentration in cationic micelles, the isophorone solubilizaton in the palisade is limited. When saturation occurs, the excess of isophorone is solubilized in the bulk phase.
Discussion Influence of Surfactant on Solute Behavior in Micellar Solutions. Differences in solubilization and localization as a function of the nature of the surfactant have been studied by numerous workers, some suggesting that addition of solute affects micelle size3wzor that there are differences in the capacity of the micellar medium to solubilize solute. Chaiko et ala3' reported that CPC micelles solubilize more benzene than SDS micelles, and Nagarajan et al.43concluded that there was a different distribution of solute in surface and interior regions between the CPC and SDS micelles. Aizawa et al." reported that DPB micelles solubilized 2,2,6,6-tetramethyl-4-benzoyloxypiperidine1-oxy1 radical in a more hydrophobic site than did SDS micelles. The lowering of the cmc of DTAB (2 mM) by 0.03 M isophorone indicates that this solute participates in micellization and probably affects micelle s t r ~ c t u r e . ' This ~ ~ ~effect ~ ~ ~will ~ clearly differ between anionic and cationic micelles, as solute distributes differently in these two types of micelle. In agreement with other workers, we found that isophorone was situated preferentially in the palisade zone of the cationic micelles, as this represents a more hydrophobic site than the diffuse layer of the anionic micelles. However, modifications of the C=O band by the different surfactants cannot be solely attributed to differences in integrated molar absorption coefficients. In a relative way, the characteristics of the C-0 bands can be compared, and we show that IR spectroscopy can provide more detailed information on the
(32) Bunton, C. A.; Cowell, C . P. J. Colloid Interface Sci. 1988,122, 154.
(33) Fendler, J. H.; Fendler, E. J.; Infante, G. A.; Smith, P. S.;Patterson. L. K. J . Am. Chem. Soc. 1975, 97,89. (34) Mitra, P.;Ganesh, K. N.; Balasubramanian. D. J. Phys. Chem. 1984, 88, 318. (35) Nagarajan, R.; Chaiko, M. A.; Rukenstein, E. J . Phys. Chem. 1984, 88, 2916. (36) Kandori, K.; McGrcevy. R. J.; Schcchter, R. S.J. Phys. Chem. 1989, 93, 1506. (37) Chaiko, M. A.; Nagarajan, R.; Rukenstein, E. J . Colloid Interface Sci. 1983, 99, 168. (38) Mahmoud, F. Z.; Christian, S.D.; Tucker, E. E.;Taha, A. A.; Scamchorn, J. F. J . Phys. Chem. 1989, 93, 5903.
(39) Jean, Y.C.; Ache, H. J. J . Phys. Chem. 1978,82, 811. (40) Wolff, T. Ber. Bunsen-Ges. Phys. Chem. 1981, 85. 145. (41) Hirsch, E.; Candau, S.;Zana, R. J . Colloid Interface Sci. 1984, 97. 318. (42) Das Gupta, P. K.; Moulik, S.P. Colloid Polym. Sci. 1989,267,246. (43) Nagarajan. R.;Chaiko, M. A.; Rukenstein. E. J . Phys. Chem. 1984, 88, 2916. (44) Aizawa, M.; Komatsu, T.; Nagawa, T. Bull. Chem. Soc. Jpn. 1977, 50, 3107. (45) Emerson, M. F.; Holtzer, A. J . Phys. Chem. 1967, 71. 3320. (46) Malliaris, A. Ado. Colloid Interface Sci. 1987, 27, 153.
4562 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
localization of solutes in the micellar solutions. Micellar Solutions of CPC and CPB. These two surfactants differ in counter ion. The chloride ion is more hydrated than bromide?' and bromide is more strongly bound to the micelle." One would thus expect that adsorption (according to the definition of Cardinal and M~kerjee'~)would be favored in the chlorinated surfactant. The relative intensity of the elementary band at 1630 cm-' corresponding to isophorone in the aqueous phase was higher in the CPB micelles (Table 11). This indicated the CPC micelles have a greater solubilizing capacity for isophorone than the CPB micelles. This can be observed even when there is little micellization (Table I). The C - 0 stretching frequency was higher in the CPC micellar solution. These difference (between CPC and CPB) diminish with increasing surfactant or isophorone concentration (Table 11). Although we could not rule out a change in structure, the difference in spectral behavior was attributed to a difference in the size of the micelles. The bromide ion is a better charge screen than chloride, and so the bromide micelles can grow more rapidly in D 2 0 as surfactant concentration is increased. For the chloride micelles, there is an increase in number rather than in size.5o The effect of a polar additive, which would reduce the size of the SDS micelles (reduction in charge density on the micellar surface thus destabilizing aggregates"), would therefore be smaller in the CPB than in the CPC micelles. If the CPB mixed micelles (surfactant and solute) are larger than the CPC mixed micelles, they have larger areas and volumes, and this hides their differences. Micellar Solutions of CPB and CTAB. The pyridinium ion has a wider charge distribution and less steric hindrance than the trimethylammonium ion. Thus the polar pyridinium head should interact better with isophorone. For surfactant concentrations close to the CMC, the aggregation number and the effective charge of CPB micelles are lower than those of CTAB.51 Since bromide screens charge better in CPB than in CTAB, the CPB micelles will grow faster with increase in surfactant Concentration. This in turn will facilitate localization of solute in the polar head region. The modification of the C - 0 band and the second derivative spectra as a function of surfactant or isophorone concentration was interpreted as an enhanced interaction between isophorone and the pyridinium ion (Figures 3 and 9). At an isophorone concentration of 0.03 M and low surfactant concentrations (50.03 M), there is little micellization and an insufficient number of surfactant molecules, and so isophorone is found in the diffuse layer of the CPB micelles (Figure 3, curve dl, 1636 cm-I). At 0.05 M surfactant, the [surfactant]/[solute] ratio is now high enough for the solute to interact significantly with the pyridinium ion (Figure 3, curve d2, 1640 cm-l). At a surfactant concentration of 0.1 M, even if the micelles of CTAB are larger than those of CPB, saturation of the external part of the palisade (polar head region) happens at a lower isophorone concentration in the CTAB micelles than in the CPB micelles (curve d3, Figures 3 and 4). Micellar Solutions of CTAB and DTAB. The effects on the structure of micelles by replacing H 2 0 with D 2 0 have been investigated by Berr.% Provided the fractional charge of the micelle is not affected by the change in isotope, micelles are larger in D 2 0 than in H20, and the isotopic effect increases with increasing chain length.? The aggregation number for DTAB increased from 47 to 55, and for CTAB from 104 to 195, as surfactant concentration was increased from 0.05 M to 0.2 M. At the maximum concentration used in our experiments (0.1 M), the aggregation number for CTAB was 160, but only 51 for DTAB. The micelles (47) Choux, G.; Benoit, R. L. J . Am. Chem. Soc. 1969, 91,6221. (48) (a) Patterson, L. K.; Vieil, E. J. Phys. Chem. 1973, 77, 1192. (b) Hautala, R. R.; Schore, N. E.; Turro, N. I. J . Am. Chem. Soc. 1973,95,5508. (c) Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1973,95,6885. (d) Fabrr, H.; Kamenka, N.; Kan, A.; Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J . Phys. Chem. 1980.84.3428. (49) (a) Cardinal, J. R.; Mukerjee, P. J . Phys. Chem. 1978,82, 1614. (b) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978.82, 1620. (50) Berr, S.S.Ph.D. Thesis, Wake Forest University, Winston Salem, North Carolina, 1986. (51) Czerniawski, M. Rocr. Chem. 1967, 41, 119.
Gongalves and Lattes formed by these two surfactants are not spherical. The CTAB micelles are more ellipsoid than those of DTABnSOThe change in isotope does not affect the thickness of the interface, nor the depth of penetration of water into the micelle. The CTAB micelles appear to be "drier" than the DTAB ones. The superficial layer is "thinner", and so less methylene groups are The results obtained can be interpreted in the light of these considerations. Although numbers and frequencies of the elementary bands of isophorone were the same in CTAB and DTAB (curve d3, Figures 4 and 5), the differences in intensities, peak shifts, and second derivatives indicated that there were differences in micellar size between CTAB and DTAB. The CTAB micelles extract more isophorone from the aqueous phase than do the DTAB micelles. The peak frequency shift (1640 cm-I) was observed at 0.1 M CTAB and 0.2 M DTAB (Table I). Micellar Solutions of SDS and LDS. The micelles of anionic surfactants are relatively insensitive to the nature of the alkali metal. Mysels and PrincenS2reported a small effect on micellar weight by replacing lithium with sodium, and Kamenka et aLs3 noted a slight increase in degree of association for the series Li' < Na+ < Rb+ < Cs+. There is a less than 10%difference between the cmcs of LDS and SDS, and they have similar aggregation numbers ( - 5 9 , although lithium is more hydrated than sodium (Li, 22; Na, 13).50*54The difference in hydration between these two ions means that the sodium ion acts as a more effective charge screen between both the polar heads of a micelle and the intermicellar coulombic repulsive forces than l i t h i ~ m . ~ ~LDS -~~ micelles are thus less compact and more hydrated then SDS micelles. At surfactant concentrations 3 0.1 M ([surfactant]/[solute] I 3), differences in the IR spectra of the probe were observed between these two surfactants. The C=O stretching frequency was lower in the LDS micellar solution (Table I). The LDS micelles present a larger interface to the solute, and so more isophorone is found in the diffuse layer of the LDS micelles. At a surfactant concentration of 0.1 M, the frequency of the C = O band in LDS micellar solution was unaltered as isophorone concentration was increased from 0.03 M to 0.1 M (Table I). A difference in definition of the peak at 1630 cm-' was observed between SDS and LDS micellar solutions (Figure 12, curve d3). These comments suggest that there were differences in the characteristics of the elementary bands. The diffuse layers of SDS and LDS micelles have different thicknessess4,and the increase in isophorone concentration in the diffuse layer makes the thinner SDS layer more hydrophobic, with a concomitant influence on the carbonyl hydrogen bonds. Micellar Solutions of DTAB and SDS. These two surfactants have the same chain length and similar aggregation numbers, although their polar heads have opposite charges, which will give rise to differences in polar head hydration and separation. Increasing concentration of surfactant in D 2 0 has little effect on the aggregation number and the structure of DTAB micelles'O and has no effect on SDS micelle^.^^*^^ In a determination of the polarity of the interface of various micellar solutions (DTAB, 33; SDS, 5 3 , Zachariasse et aLs*reported that the probe, phenol betaine (ET30) was situated further away from the core in the anionic micelles than in the cationic ones. They suggested that this probe was preferentially localized in the aqueous phase. Hayter and Penfoldsgalso reported differences between SDS and DTAB micelles, and suggested that anionic micelles had a less (52) Mysels. K. J.; Princen, L. H. J . Phys. Chem. 1959, 63, 1696. (53) Kamenka, N.; Chorro, M.; Fabre, H.; Lindman. B.; Rouvisre. J.; Cabos, C. Colloid Polym. Sci. 1979, 257, 757. (54) Ben, S. S.;Jones. R. R. M. hngmuir 1988.4, 1247. (55) Jones. R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kcvan, L. J . Phys. Chem. 1986, 90,1126. (56) Bcndedouch, D.; Chen, S.H.; Koehler, W. C. J . Phys. Chem. 1983, 87, 153. (57) Candau, S.;Hirsch, E.; Zana, R. J . Colloid Interface Sci. 1982.88, 428. ( 5 8 ) Zachariassc, K. A.; Phuc, N. V.;Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (59) Hayter, J. B.; Penfold. J. J . Colloid InterfaceSci. 1983, 261, 1022.
J. Phys. Chem. 1991, 95,4563-4568 well-defined core/polar interface than the alkyl trimethylammonium micelles. Unfortunately, it has not been found p i b l e to quantitate the water in the vicinity of methyl benzoate at the interface of SDS micelles due to difficulty in mimicking the water/hydrocarbon interface.*4bThis is a further demonstration of the illdefined nature of the polar region and water/hydrocarbon interface. As surfactant concentration was raised from 0.02 to 0.25 M, the peak of the C = O band shifted by 6 cm-' in SDS micellar solutions and by 13 cm-l in DTAB micellar solutions (Table I). The area of the C 4 ) peak in the DTAB micelles fell progressively (curve s, Figures 5 and 7), whereas in SDS it passed through a minimum at [surfactant] = 0.1 M (Figure 8, curve sl),@' which supports a preferential localization of isophorone in the palisade region of DTAB micelles (1653, 1640 cm-', Figure 7, curve d3), and in the polar region of SDS micelles (Figure 8, curve d3). In anionic surfactants, the significant hydration of the polar heads may produce a rather ill-defined interface between the external part of the palisade and the diffuse la er which agrees with previous observations and
hypo these^.^^^^^^^^^
Remarks and Conclusions With the [surfactant]/[solute] ratios employed in this study, we did not reach saturation of solute, and so we did not detect isophorone either in the core or aggregated on the surface of the micelles (absence of elementary band above 1660 cm-l). Our results for a concentration of isophorone of 0.03 M demonstrated the importance of this ratio. The relative solubility of isophorone (60)The area of the C 4 peak in micellar solutions of CTAB, DTAB, SDS (e.g. 0.1 M in Figures 4-6) falls in the order DTAB > SDS > CTAB, Casal and Martin (ref 16b) observed a similar order for strength of hydrogen bond between the >c--O group of 6-undtcanoneand D20 in DTAC, SDS, and CTAB micelles. In our case, the order of sequence was thought to reflect the proportion of isophorone in the aqueous phase.
4563
in water requires a minimum surfactant/solute ratio to produce sufficient micellization for observation of differences in behavior between the anionic and cationic surfactants (-0.03 M) and between different anionic surfactants (-0.1 M). Micellar solutions have definite solubility power: at constant concentration in cationic micelles, the isophorone solubilization in the palisade is limited. When saturation occurs, the excess of isophorone is solubilized in the bulk phase. The capacity of cationic micelles to solubilize isophorone increased with increasing surfactant chain length. The CPC micelles solubilized more probe than the CPB micelles did. The pyridinium ion favors interactions between the polar head and isophorone, which is preferentially situated in the more hydrophobic regions (internal and external parts of palisade rather than the diffuse layer and aqueous phases) of the cationic than the anionic micelles. Thus, it can be seen that IR spectroscopy cannot give a quantitative evaluation without knowledge of the integrated molar absorption coefficients, but its ability to detect the microenvironment of the solute can provide useful information on its distribution in the various sites we identified. It thus appears that the existence of a transition point is quite general, and IR study of dilute solutions of solute can give information about site saturation at the interface. Various transition points may be identified, especially in cationic micelles where the various sites are better defined. Vibrational spectroscopy (IR or Raman) can thus be employed to localize solutes exhibiting a resonance that is altered to different extents in the various microenvironments. Definition of these sites should further understandingof, and enable prediction of, reactions occurring in micellar solution^.^
Acknowledgment. We thank Th. Bouissou (Head of Infrared Spectroscopyin the Groupement Rtgional de Mesures Physiques, Universitd Paul Sabatier) for letting us use the Bruker IFS 110 FTIR instrument.
Nonradlatlve Energy Transfer In Block Copolymer Micelles K. ProcbPzka,' B. Bed&,* E.Mukhtar,' P.Svoboda, J. Tmhii, and M. Almgred Department of Polymers, Prague Institute of Chemical Technology, Technickd 5. 166 28 Prague 6, Czechoslovakia (Received: September 7. 1990; In Final Form: January 2, 1991) Nonradiative energy transfer (NET) was studied in micellar systems of Kraton G-1701 (a diblock copolymer containing polystyrene and hydrogenated polyisoprene blocks) with a polystyrene block tagged with a fluorescence donor (carbazole) and a fluorescence acceptor (anthracene). On addition of selective precipitants for either block of the copolymer, the donor-acceptor pair was located either in the compact micellar core, or in the diffuse solvated shell. It is shown by steady-state and time-resolved fluorescence measurements that the NET is highly efficient in the micellar core and much less efficient in the shell. By measurement of the time-dependent increase in anthracene emission at 413 nm (due to energy transfer, when carbazole is excited at 295 nm) and the concomitant decrease of carbazole emission at 364 nm after mixing two solutions of micelles, labeled by carbazole and anthracene, respectively, the rates of unimer =micelle i exchange under equilibrium conditions were obtained in several selective precipitants. By comparison with the existing data on the unimer ?T micelle mass exchange rate when relatively large perturbations of the equilibrium conditions are induced, the mass exchange rates under the nonperturbed equilibrium conditions are slower by several orders of magnitude, indicating that a limited segment mobility in micellar cores leads to a slow micelle dissociation.
Introduction Nonradiative energy transfer (NET) can be observed in systems containing two fluorescent species, such that the emission spectrum of the donor overlaps the absorption spectrum of the acceptor. The excitation energy of the donor may thus be transferred by dipoltdipole interaction to the acceptor molecules. The efficiency ( I ) Department of Physical Chemistry, Charles University in Prague, Albcrtov 2030, 12840 Prague 2, Czechoslovakia. (2) Department of Physical Chemistry, Uppsala University, Uppsala, Sweden.
0022-3654/91/2095-4563$02.50/0
of NET is, according to F B r ~ t e r , ~expressed -~ by E = (1 + (r/R#)-'
R$ = (8.8 X 10-25)Jn4+D0~2 (1) where r is the distance between donor and acceptor, J is overlap integral between the emission spectrum of the donor and the absorption spectrum of the acceptor, n is the dielectric constant, (3) Fijrster, T. Ann. Phys. ( N . U.)1948, 2, 55. (4) F6rster, T. Z . Naturforsch. 1949. A4, 321. ( 5 ) Farster, T. Discuss. Faraday Soc. 1959. No. 27.1.
0 1991 American Chemical Society
