J. Phys. Chem. 1994,98, 70367040
7036
13C NMR Spin-Lattice Relaxation Study of Carbonyl Carbon-Water Interaction in the Aerosol OT Reversed Micelles
Akihiro Yoshino, Hirofumi Okabayashi,' and Tadayoshi Yoshida Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan Received: October 29, 1993; I n Final Form: March 15, 1994"
For the aerosol OT reversed micelle systems in n-dodecane, n-heptane, and benzene, the effect of solubilized
HzOand DzO on the I3C NMR spin-lattice relaxation for the two carbonyl carbons of an AOT molecule has been investigated in connection with the AOT-water interaction. The contribution of water molecules to the observed spin-lattice relaxation rate was calculated. The results lead to the conclusion that the carbonyl group of the a-chain is more tightly bound to water molecules than that of the @-chain. It has also been discussed that the difference in an interaction between the carbonyl group and water molecule is reflected in the 13C chemical shift variation of the two carbonyl carbons.
Introduction
Experimental Section
A reversed micelle is an aggregate of surfactant molecules formed in organic solvents. In particular, for a reversed micelle of aerosol OT (AOT, sodium 1,2-bis(2-ethylhexyl)sulfosuccinate), its size, shape, and aggregation number have been well defined.' An AOT reversed micelle has the ability to solubilize relatively large amount of water in the polar core, as is well-known. For the aqueous microphase located in the polar core of this micelle, a water-pool model has been presented, and its composition and dimensions have been investigated in detail.2 Many investigations on water molecules incorporated into the AOT reversed micelles have been made by using NMR,'v4 ESR,3954 and vibrational spectroscopy?-12 and the quantity and properties of the solubilized water have been studied. However, little attention has been devoted to the sites of an AOT molecule bound to water molecules. The SOa-Na+ head group could be regarded as a part of water binding sites on an AOT reversed micelle, in which water molecules could be hydrogen-bonded to these oxygen atoms. In order to obtain more insight into such a hydration problem, the twocarbonyl groups of an AOT molecule should also be considered as the sitesfor hydration and the carbonyl carbon-water interaction study becomes highly desirable. Recently, the I3C NMR signals arising from the two (a-and j3-) chains of an AOT molecule in reversed micelles have been successfully assigned to the individual carbon atoms belonging toeach 2-ethylhexyl chain,13and for the 13Cspin-lattice relaxation rates of the a- and &chains, it has been found that there is a marked difference between the a-and /3-chains.14 This difference is expected to reflect the interaction between an AOT molecule and solubilized water. This paper reports an investigation focusing on a carbonyl carbon-water interaction in the AOT reversed micelles. The effect of solubilized H2O and D20 on the 13C NMR spin-lattice relaxation for the two carbonyl carbons has been discussed in detail by making a relaxation data analysis. Henriksson et al.15 have investigated the replacement effect of HzO by D20 on the 19FNMR T Ivalue of some 19F-containing surfactants in aqueous solutions and discussed the exposure of the fluorocarbon chains in the micelles to water. Ulmis et a1.I6 have also discussed the contact between the hydrocarbon chain and water on the surface of a micelle and water penetration into the micelles by the use of the l9F T I values of fluorinated amphiphiles in HzO and D2O.
Sodium 1,2-bis(2-ethylhexyl)sulfosuccinate (aerosol OT, AOT) (pure grade) was purchased from Tokyo Kasei Kogyo Co., Ltd., and was purified according to the method described in the literature." The purified AOT was stored in a vacuum desiccator over P203. Extrapure grade benzene, n-heptane, and n-dodecane (Tokyo Kasei Kogyo Co., Ltd.) was used after being dried and distilled. Benzene-d6 was obtained from Merk Sharp & Dohme Canada, Ltd., and its minimum isotopic purity was 99.6 atom % D. The concentration of AOT in benzene, n-heptane, and n-dodecane was maintained at 40 wt 5% throughout the work. Water was added to these surfactant solutions by microsyringe, and the water content (wt %) was obtained by weighing. I3CNMR chemical shifts (internal reference, tetramethylsilane (TMS)) and spin-lattice relaxation times (TI)were measured by JEOL-FX-90Q Fourier transform spectrometers (22.5 MHz) at 25 OC,locking on deuterium. A time-domain of 32 768 points and a spectral width of 18 000 Hz were used for each 13Cchemical shift. The TI values were measured by the inversion-recovery Fourier transform method (?r-~-u/2sequence), with a spectral widthof 5000Hzandwith8192datapoints. The pulserepetition time (t,) was chosen so as to satisfy the relation t, 1 5Tl. Numbering Scheme of an AOT Molecule. The numbering scheme of carbon atoms for an AOT molecule (I) adopted by Ueno et a1.18 is used, and that of the protons is also represented by the number of the proton-attached carbon.
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, June 1, 1994.
/Ichain
Results and Discussion For the l3C NMR spectra of AOT in organic solvents, it has been reported that the two 13C signals arising from carbonyl carbons 2 and 2' are observed in the regions of 169.2-170.4 and 171.6-172.4 ppm.18J9 However, the 13C signalscoming from the two carbons have been reassigned by Heatley,zo based on selective 1H-decoupling experiments. Recently our two-dimensional
0022-3654/94/2098-7036$04.50/0 0 1994 American Chemical Society
AOT-Water Interaction in Reversed Micelles
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7037
TABLE 1: IJC Chemical Shifts‘ (b/ppm) and Spin-Lattice Relaxation Timesb ( T I / s )for AOT in Organic Solvents carbonyl carbon AOT samples solvent water content 2‘ 2 n-dodecane Hz0 6 171.60 169.90 0 wt % TI 0.85 1.08 Hz0 TI 1.71 2.55 20 wt %
n-heptane
Dz0
TI
20 wt %
Ac 6
HzO Owt%
Hz0
TI TI
2.61 0.90 171.85 0.95 1.84
2.88 0.33 170.09 1.16 2.84 3.01 0.17 169.76 1.11 2.87 3.15 0.28
, --+
20 wt 56
Dz0 Hz0
Ti Ac 6
0 wt %
TI
Hz0
TI
2.29 0.45 171.89 0.88 2.20
TI Ac
2.81 0.61
20 wt %
benzene
2
20 wt %
Dz0 20 wt %
MHz. Reference: TMS. Errors: kO.01 ppm. Errors: hO.05 A = ( T I ) ~ -~ OTI)^^^.
a 22.5
S.
INADEQUATE experiments13 for the AOT-benzene-water system have provided direct evidence that the I3Csignal at higher field is due to carbonyl carbon 2 and that a t lower field to carbonyl carbon 2’. For the AOT reversed micellar systems in n-dodecane, n-heptane, and benzene, t h e 13C chemical shifts of carbonyl carbons 2 and 2’ are listed in Table 1. In the present study, in order to obtain information on hydration sites in the AOT reversed micelles, our attention is focused on the 13Cspin-lattice relaxation times and l3C chemical shifts of carbonyl carbons 2 and 2’. 1% Spin-Lattice Relaxation of Carbonyl Carbons and Its Solubilized Water Dependence. The 13C N M R spin-lattice relaxation times (TI) have been successfully used to discuss the dynamic structure of an AOT molecule in the W/O microemulsions,I*J9 Martin et al.21have studied the additive water-induced conformational and dynamic changes of the CH-CH2 segment adjacent to the SO3- group and of the 2-ethylhexyl segment for an AOT molecule in the microemulsions, using the 13CTIvalues. However, spin-lattice relaxation times of carbonyl carbons 2 and 2’ have not yet been used to discuss the microstructure of AOT in the aggregated state. For carbonyl carbons 2 and 2’, we may expect that the 13Cspin-lattice relaxation rates are sensitive to neighboring protons in the AOT reversed micelle because of the absence of carbon-proton direct bonding. The 2’ carbonyl carbon is located adjacent to the methylene group ( 1’) and another carbonyl carbon 2 is beside the methine group (1). When we consider the 1H-13C dipolar coupling between the adjacent protons and carbonyl carbons, the observed TIvalues of carbonyl carbon 2 must be larger than those of carbonyl carbon 2’. In fact, when water is absent in the AOT-organic solvent systems, the observed 13C T I values of carbonyl carbon 2 are slightly larger than those of carbonyl carbon 2’ (Table 1). Theeffect of solubilized water on the I3Cspin-lattice relaxation times of carbonyl carbons 2 and 2’ has also been measured. Table 1 also shows the influence of solubilized water on the I3C spinlattice relaxation times of the carbonyl carbons in the AOT reversed micelles. As is seen in Figure 1, the observed 13C T I values of the two carbonyl carbons tend to increase with an increase in water content. It should be noted that the relaxation rate of carbon 2’ is faster than that of carbon 2 a t higher water content, indicating that the interaction between carbonyl carbon 2‘ and water becomes stronger than that between another carbonyl carbon and water. In particular, the difference in the observed 13CTI values between carbonyl carbons 2 and 2’ is very small at
c.2
10
20 wt
Water Content
Figure 1. Solubilized H20 effecton the I3C TIvalues of carbonylcarbons 2 and 2’ for the three AOT reversed micelle systems at 25 OC; (a) AOTn-dodecane, (b) AOT-n-heptane, and (c) AOT-benzene.
low water content for an AOT molecule in n-dodecane and n-heptane. However, this difference increases with further increase in water content. This may indicate that formation of a water pool accompanied by an increase in water content is responsible for such a marked difference in observed TI values between the two carbonyl carbons. For further clarification of the mechanism of this difference in the carbonyl groupwater interaction, we have also measured the I3C spin-lattice relaxation times of carbons 2 and 2’ in the presence of D2O instead of H20. In the case of carbon atoms without directly bonded protons, 13Cspin-lattice relaxation is mainly determined by intermolecular dipole-dipole interaction. Therefore, in the presence of ordinary water the dipole-dipole interaction between the carbonyl carbons and the HzO protons becomes predominant. However, when the HzO molecules are replaced by D20, the dipole-dipole interaction becomes quite small for the following reason. The observed 13C spin-lattice relaxation rate ([ 1 / T l l 0 ~ = [ R l l o ~is) expressed in terms of inter- and intramolecular dipole-dipole (DD) interaction terms ([RIDDlinter and [RlDDlintra) and another interaction term ([Rllother) and is given by eq 1.22
When ordinary water (HzO) is incorporated into an AOT reversed micelle, the resulting expression for the observed 13C spin-lattice relaxation rate of carbonyl carbon is
where
The [RlDD]Awjntraand [Rllothcr,inuaterms represent the relaxation rates due to the AOT intramolecular DD interaction and other intramolecular interaction mechanisms, respectively. The term ([RIDD]AOT-H.@)interdenotes the relaxation rate due to only an intermolecular DD interaction between an AOT molecule and
~~
n-dodecane
n-heptane benzene
2' 2 2' 2 2' 2
1.176 0.926 1.053 0.862 1.136 0.901
0.585 0.392 0.544 0.352 0.454 0.348
0.370 0.344 0.430 0.331 0.349 0.3 15
water. Accordingly, the [RIB] term implies the contribution of water to the observed 13C T1 values. As has already been reported?' addition of water to a dried micelle in benzene induces conformational changes throughout the molecule, resulting in a open, extended structure, and the extended conformation depends upon the water content in the reversed micelle. Therefore, the values of the [&A] and [RlB] terms strongly depend upon the amount of solubilized water. In this article, since the 20 wt 5% water was solubilized in the AOT reversed micelles, the [RIA] and [RIB]values are characteristic of this water content. On the other hand, in the presence of heavy water instead of ordinary water the observed I3C spin-lattice relaxation rate is expressed by eq 3,22assuming that there is no perturbation of the system followed by the use of D20 instead of H2O.
(3) where
Thus, comparison of the intermolecular DD interactions in the presence of H20 and D2O provides important information about the environment around the carbonyl groups. The 13C T Ivalues of carbonyl carbons 2 and 2' for the 20 wt % D2O-solubilized AOT reversed micelles are also listed in Table 1. It is evident that the replacement effect of deuterium oxide on the TIvalue of carbon 2' is greater than that observed for carbon 2. This fact reveals that the 2' carbonyl group interacts strongly with water molecules in the aqueous microphase compared to the 2 carbonyl group. One can calculate the contribution terms ([RlA] = ( ~ / T I A ) and [RIB] = ( l / T l ~ ) from ) the observed relaxation rates by the use of eqs 2 and 3. For the AOT-n-dodecane-water system, the contribution ( 1 0 0 R l ~ / ( R 1 ~RIB))of water to theobserved l3C spin-lattice relaxation rate for carbonyl carbon 2' is 37%, and that for carbonyl carbon 2 is 12%. For the AOT-n-heptanewater and AOT-benzene-water systems,the water contributions for carbon 2' are 21-23% and those for carbon 2 are 69%. It must be emphasized that the contribution of water molecules to the observed relaxation rates for carbonyl carbon 2' is greater than that for carbon 2. This result provides ample evidence that the 2' carbonyl group is more tightly bound to water molecules than the 2 carbonyl group. When we compare the R I Avalues between carbons 2 and 2' for the three AOT-organic solvent systems, the RIAvalue for carbonyl carbon 2' is found to be larger than that for carbonyl carbon 2. This finding can be mainly explained by the dipolar interactionbetween the adjacent protons and thecarbonyl carbons. Moreover, the contribution of such an intramolecule interaction to the observed spin-lattice relaxation rate may be related to the different dynamic structure between the a- and @-chains. We have investigated the IH-lH internuclear correlation between the 2-ethylhexyl protons and the polar segment protons by two-dimensional NMR methods (NOESY and ROESY t e ~ h n q i u e s ) .The ~ ~ results have indicated that there are dipoledipole connectivitiesbetween the polar segment protons and the
+
63 88 79 94 17 91
0.215 0.048 0.114 0.021 0.105 0.033
37 12 21 6 23 9
~
3.178 2.691 2.448 2.604 3.255 2.860
terminal CH3 protons. In particular, it has been suggested that the terminal CH3 protons of the @-chainhave spatial proximity with the H1 and H3 protons while spatial proximity between the terminal CH3protonsof the a-chain and the polar segment protons is not so large. Such a difference in the terminal CH3-polar segment proximity between the a-and &chains may result in the larger value of RIAfor carbonyl carbon 2'. The [RJZ' values, listed in Table 2, mainly represent the relaxation rates of an AOT molecule in a dried micelle. On the other hand, the [&A] values, obtained from the water-solubilized AOT reversed micellar system, involve the contribution of the relaxation rate variation, which was caused by the structural change of an AOT micelle due to the water solubilization. Therefore, the resulting [RIA] values are not consistent with the [ R l ] r values. When we compare the [Rl]Z'/[RlA] ratios between two carbonyl carbons 2 and 2', it is found that the ratios are 2.5-3.3. We may assume that the differences in the ratios come from an increase in segmental mobility resulting from the water solubilization. We shall assume the following equation:
where [ R I ~denotes ] the effect of micellar structural variation due to water solubilizationon the relaxation rates. The [ R l ~ l h ~ terms reflects the segmental mobility of a polar core due to the structural change, and the [Rl~linerterm implies the effect on the relaxation rate caused by the AOT-AOT intermolecular distance variation. Considering the [R1]ZW,'"/[R1A] ratios of 2.5-3.3 in the AOT reversed micelle system containing 20 wt 8 H20, we may assume that the contribution of the [RlE] terms is almost the same as or larger than that of the [RIA]term. At the present stage, it is difficult to discuss the intra- ( [ R I & ~ ) and intermolecular ( [RI~]intn) terms separately. It should be emphasized, however, that the higher value of this ratio indicates an increase in the segmental mobility caused by formation of a hydrated shell. For the water-solubilized AOT reversed micellar systems in ndodecane and benzene, the additional effect of water on the ratio for carbon 2' is obviously larger than that for carbon 2, indicating that the 2' carbonyl group is strongly hydrated compared with the 2 carbonyl group. This explanation is consistent with the quantitative results discussed above using the 1 0 0 R l ~ / ( R 1+~ RIB)percentage. On theother hand, for the reversed micellar systemin n-heptane there is no marked difference in the ratios between carbons 2 and 2', and moreover, these ratios are smaller than those for the systems in n-dodecane and benzene. This may indicate that the extent of hydration of 2 and 2' carbonyl groups is not so large. That is, water penetrationis small for the system in n-heptane, compared with the cases in n-dodecane and benzene. The extent of water penetration into AOT reversed micelles + seems to depend on the solvent properties. The 100Rl~/(Rl~ R l ~percentage ) is useful in discussing the penetrating ability of water quantitatively. Judging from the R I Bpercentages (Table
AOT-Water Interaction in Reversed Micelles
I A l
173*2k 173.0
L
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7039
\.;a
c-2' 0 . __./
11
0-R
Figure 3. Schematic hydration model in an AOT reversed micelle. The extent of hydration increases in the order [A] < [B] < [C] < [D].
carbon and solubilized water, resulting in the decrease in polarity of the C-0 bond, which causes the upfield shift of the carbonyl carbons. Water Content 1 wt%
Figure 2. Observed 13Cchemical shifts (reference: TMS) of the 2 and 2'carbon-13 signals as a function of water content in the AOT-n-heptane system at 25 "C.
2), it may be concluded that the extent of water penetration decreases in the order n-dodecane > benzene > n-heptane. ljC Chemical Shifts of Carbonyl Carbons and the Solubilized Water Effect. Addition of water has no effect on the 13Cchemical shift of carbons 4, 4' through 10, 10'. However, significant variation of the chemical shift occurs in the head group carbons involving C-3 and (2-3'. In particular, the largest changes occur in the 13C chemical shifts of carbonyl carbons, which are the focus of discussion in the present study. Figure 2 shows the observed chemical shifts of the 13CNMR signals for carbonyl carbons 2 and 2' as a function of the content of water incorporated into the AOT-n-heptanesystem. It is found that the 13C resonance signals of carbonyl carbons 2 and 2' are shifted upfield with an increase in water content. The 13Csignal of carbonyl carbon 2 is asymptoticallyshifted upfield on addition of water. However, the resonance position of carbonyl carbon 2' definitely converges to a limiting value of ca. 173.0 ppm at 7.5 wt %I water content, and above this water content there is little further change in the chemical shift. Similar observationswere made in the l3C signals of carbonyl carbons 2 and 2' for the AOT-n-dodecane and AOT-benzene systems. Such a different behavior in the upfield shift between the two carbonyl carbons may be related to the difference in the sizes of hydration shells around the carbonyl groups. Probably, the numbers of water moleculesbound to carbonyl group 2'are limited, since this brings about such a convergence of the chemical shift. On the other hand, the carbonyl group 2 can be surrounded by a greater number of water molecules, resulting in a larger upfield shift of carbonyl carbon 2 on addition of water. Martin et a1.21have pointed out for the same AOT-benzene system that the l3C chemical shifts of carbonyl carbons vary with an increase in water content. However, they have not discussed the different behavior in the upfield shifts for the two carbonyl carbons. Furthermore, we may estimate from the l3C spin-lattice relaxationdata that water addition to the AOT reversed micelles brings about an increase in the degree of freedom of the polar segmentcontaining the ester groups.24 This may lead to a decrease in the interaction by hydrogen bonding between the carbonyl
Conclusions For the three AOT-organic solvent systems, the solubilized water effect on 13Cspin-lattice relaxationrates of the two carbonyl carbons has been investigated. We have found that the observed 13C spin-lattice relaxation rate of carbonyl carbon 2' becomes faster than that of carbonylcarbon 2 with an increasein solubilized H20 content. For further elucidationof this mechanism, the l3C spin-lattice relaxation rates of carbonyl carbons 2 and 2' have also been measured in the presence of D2O instead of H20, and the water contribution to the observed relaxation rate was calculated, providing fruitful evidence that the 2' carbonyl group is more tightly bound to water molecules than another carbonyl group. We have observed that the 13C signal of carbonyl carbon 2 is asymptotically shifted upfield with an increase in water content while the resonance position of carbonyl carbon 2' converges to a limiting value of the chemical shift at lower water content and becomes constant. Such a behavior in the upfield shifts was due to the differencein the amount of water bound to the two carbonyl groups and that in the carbonyl group-water interaction. We may speculate on the following hydration model (Figure 3). When the amount of water incorporatedinto an AOT reversed micelle is small, water molecules are probably predominantly bound to the SO3-Na+ head group at first. As the water content increases,however, water molecules will be partially incorporated into the ester regions of the micelle. Thus, in addition to the S03-Na+ region, the hydration shells may also be formed in the 2C-0 and 2'C-0 regions. The hydration shell of the 2 C = O group may be larger than that of the 2'C-0 group. With a further increase in water content, a water pool is formed in the polar cavity, and the SO3-Na+and 2C-0 regions may be soaked in a water pool. However, another smaller hydration shell around the 2'C=O group remains separated. The hydrophobic 1'methylene group may interupt the fusion of this small hydration shell together with the larger hydration shell. In this hydration model, the hydrated water may be weakly hydrogen-bonded to carbonyl group 2, and the water exchange between the hydration shell and a water pool may occur rapidly. On the other hand, in the hydration shell which belongs tocarbonyl group 2', water molecules are probably stronglyhydrogen-bonded to the carbonyl group, and the water exchange between the hydration shell and a water pool may be very slow owing to localization of the hydration shell.
7040 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
References and Notes (1) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sei. 1970, 33, 215. (2) Wong, M.; Thomas, J. K.; Gratzcll, M. J. Am. Chem. Soc. 1976,98, 2391. (3) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93,7869. (4) Maitra, A. N.;Eicke. H. F. J. Phys. Chem. 1981,85, 2687. ( 5 ) Haering, G.; Luisi, P. L.; Hauser, H. J . Phys. Chem. 1988,92,3574. (6) Yoshioka, H.J. Colloid Interface Sci. 1981,83, 214. (7) Yoshioka, H. J . Colloid Interface Sci. 1983,95,81. (8) Kotake, Y.; Janzen, E. G. J. Phys. Chem. 1988, 92,6357. (9) Kotlarchyk, M.; Huang, J. S.;Chen, S.H. J . Phys. Chem. 198$,89, 4382. (10) MacDonald, H.; Bedwell, 9.; Gulari, E. bngmuir 1986, 2, 704. (1 1) D’Aprano, A.;Lizzio, A,; Liveri, V. T.; Aliotta, F.; Vasi, C.; Migliardo, P. J. Phys. Chem. 1988,92,4436. (12) Onori, G.; Santucci, A. J. Phys. Chem. 1993,97,5430.
Yoshino et al. (13) Yoshino, A.; Sugiyama, N.;Okabayashi, H.; Taga, K.; Yoshida, T.; Kamo, 0. Colloids Surf 1992,67,67. (14) Yoshino, A.; Okabayashi, H.; Yoshida,T.; Kushida, K. Unpublished data (to be submitted). (15) H e n r i h n , U.; Odberg, L. J. ColloidIntcrfaee Sci. 1974,46,212. (16) Ulmis, J.; Lindman, 9.1.Phys. Chem. 1981,85,4131. (17) Kunicda, H.; Shinoda, K. 1.Colloid Interfie Sci. 1979, 70, 577. (18) Ueno, M.; Kishimoto, H.; Kyogoku, Y. Bull. Chem. Soc. Jpn. 1976, 49, 1776. (19) Ueno, M.; Kishimoto, H.; Kyogoku, Y. J. ColloidInterfaceSci. 1978, 63, 113. (20) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1987,83,517. (21) Martin, C. A.; Magid, L. J. J. Phys. Chem. 1981,85,3938. (22) Wehrli, F. W.;Wirthlin, T. Interpretation of Carbon-13 NMR Spectra; Heyden & Son: London, 1978;Chapter 4. (23) Yoshino, A.; Okabayashi, H.; Yoshida, T. The 45th Symposium on Colloid and Interface Chemistry, Fukuoka, Japan, Abstract p 34, 1992. (24) Okabayashi, H.; Taga, K.; Tsukamoto, K.; Matsushita, K.; Kamo, 0.; Yoshikawa, K. Colloids Surf 1987,24,337.
