J. Phys. Chem. 1993, 97, 5430-5434
5430
IR Investigations of Water Structure in Aerosol OT Reverse Micellar Aggregates Giuseppe Onori' and Ado Santucci Dipartimento di Fisica, Universita' di Perugia, V. A. Pascoli, I-061 00 Perugia, Italy Received: January 4, 1993 The structure of water in bis(2-ethylhexyl) sodium sulfosuccinate (AOT) micelles has been studied as a function of the [H*O]/[AOT] ratio ( W)by using the absorption IR due to 0-H stretching modes in the 3800-3000-~m-~ range and the 1928-nm absorption of water usually assigned to a combination mode of asymmetric stretching and bending vibrations. The results show that the IR spectra can be expressed as sum of contributions from interfacial and bulklike water. The fraction of water in the two "regions" within the water pool was evaluated 6 and then, at higher as a function of W. On increasing W, the IR spectra change significantly up to W water contents, gradually approach those of bulk water.
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1. Introduction
A reversed micelle is an aggregate of surfactant formed in a nonpolar solvent. The main interest on the reversed micelles is based on their ability of dissolving water in their core. This provides a unique opportunity to study the properties of water aggregates close to the ionic center (head polar groups and the corresponding counterions of the surfactant) without the interference due to large quantities of bulk water. Among the surfactants capable to form reversed micelles, bis(2-ethylhexyl) sodium sulfosuccinate (AOT) has received particular attention in recent years. Thermodynamic and spectroscopic properties of water in reversed AOT micelles have been studied by means of a large variety of experimental These studies indicate that the properties of surfactant-trapped water pools strongly depend on the water to surfactant molar ratio W = [HzO]/[AOT], suggesting the following general picture of micelle hydration as a function of W, At low Wvalues ( W C 2-8) the solubilized water exists mainly as H20-bonded molecules whose static and dynamic properties are determined by the local interactions with the Na+ counterions and the strong dipole of the AOT polar group. Additional water molecules occupy the core of the micelles and their properties resemblebulkwater. 'Bonded"and"free" water moleculescoexist and exchange quickly. Even though the above qualitative picture of the micellar core is generally accepted, literature data show disagreement on a more quantitativedescription,and several featuresof these systems remain to be explained. One of these concerns the determination of the relative amounts of free and bonded water within the water pool as a function of W. In this regard the analysis is often devoted to the determination of a critical value (W,) of W for which water with properties of bulk water appears in the micelles. The estimated values of W, are in the range 2-8 and depend on the technique employed and change from author to author."IO To gain more insight into these problems, we present in this paper an IR investigation of water-AOT-carbon tetrachloride reversed micelle in the region of fundamental stretchingvibrations of HzO (3800-3000-cm-I) and in the 1850-2050-nm range where water exhibits a strong absorption band due to a combination mode of stretching ( 4 and bending (YZ) vibrations.Il Although most of the properties of water in reversed micelles have been closely investigated, little attention has been paid to their spectra of vibrational bands.l2-Is Infrared spectroscopy is a noninvasive technique particularly suitable to detect hydrogen bonds and has been often employed to study solvent modifications in aqueous solutions. 6 To investigateAOT and other micellar systems with isooctane, heptane, chloroform, or benzene as nonpolar component, some authors14J5have adopted the infrared spectroscopy technique, and some others the near-infrared.12J3 In these solvents a part 0022-3654/93/2097-5430$04.00/0
of water is dispersed in the bulk organic phase by making more complex data analysis. In CC4 solvent almost all the water is solubilized in the reversed micelle," and therefore the data analysis is more direct. The interpretation of the infrared spectrum of water not being straightforward,16J7the use of two independent spectral methods seems suitable to interpret the experimental data. 2. Experimental Section
AOT 99% (Alfa Product), purified by recrystallization from methanol and drying in vacuum, was stored in vacuum over PzOS. Bidistilled water and CC14 (purity >99.5%) were used without additional purification. Some residual water molecules remain bound to the AOT molecules after the drying process of the surfactant. Analysis of the water content of purified AOT and CC14mixtures with a Karl Fisher titrator revealed the presence of 0.2 mol of water residual/mol of AOT. Such a small residual of water was considered as a part of the total water in the mixtures under study. The AOT/H2O/CCL mixtures for IR measurements were prepared by weight. IR spectra were recorded by means of a Shimadzu Model 470 infrared spectrophotometer equipped with a variable path-length cell and CaFz windows. Typical path lengthsemployed were 50-800 pm for AOT/H2O/CCh mixtures. Pure water spectra were taken with shorter path lengths. Near-infrared (NIR) spectra were recorded by means of a Shimdazu Model 360 spectrophotometer equipped with a thermostated cell compartment by using quartz IR cells 1-cm path length. All the spectra were recorded at 20.0 & 0.1 OC and for W = [H20]/[AOT] ranging among 0 to 12. Water was added to AOT solutions in CC14 with a microsyringe directly into the sample cuvette. Good mixing was obtained by handshaking the cuvette. The molar extinction coefficient of water was calculated by using the expression e = A/(cd),where A is the absorbance, c the water concentrationin mol L-l and d the cell depth in centimeters. Gaussian curve fitting was achieved with a computer application of the Marquardt algorithm. 3. Results and Discussion 3.1. Infrared Spectra. Figure 1 shows the IR spectrum of the H20/AOT/CC14system at two values of the molar ratio W (W = 0.2 and 5.2). Thespectra wererecordedin the400&2500-cm-1 range, where the absorptions due to the 0-H stretchings of H20 and to the C-H stretchings of AOT are present. The AOT concentration was the same in both solutions ([AOT] = 0.15 mol L-I). The contributions to the spectrum of the absorptions due to the vibrationalmodes of water and AOT are well characterized, because they appear in distinct spectral ranges. In the 4000-3000-~m-~range appear the absorptions due to stretching 0 1993 American Chemical Society
Aerosol OT Reverse Micellar Aggregates
7‘he Journal of Physical Chemistry, Vol. 97, No. 20, 1993 5431
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v)
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decreases while that of the linear H bonds increases with W.The fraction of peak area attributed to the H bonds structures with unfavored angles increases initially with Wreaching a maximum at W 3. The different fractions of peak area change significantly up to W 6;then at higher water contents they level up but remaining appreciably different from those of bulk water. For example, the free -OH groups’ fraction is near 40% of the total at low Wvalues and it approaches 11%at high W values; in bulk water the same fraction is 7% only. It is notable that the value of 7% agrees with the estimation of the content of non-Hi-bondedOH groups in pure water given by other authors employing different techniques.~ ~ 2 5 - 2 7 As previously noted, the IR spectrum of water solubilized in the micellar aggregates remains appreciably different from that of bulk water, also for very large Wvalues (- 12). On the other hand, for W >6,the difference spectra (not shown) at two water concentrations are similar to the “bulk one”. This is consistent with previous reports’-3 that a pool of more bulklikewater develops in the reversed micelle as hydration of surfactant becomes complete and it suggests the copresence in the reversed micellar cores of two water regions having different structural and dynamical properties. With this in mind, we have subtracted to the spectra of surfactant-entrapped water a contributiondue to “bulklikewater” spectrum obtained from the first Gaussian component peaked at 3300 cm-I. So, any IR spectrum can be partitioned in two contributions linked to the “bonded” and “bulk” water. The concentration of bonded water per AOT molecules, Wb,,nded, was then evaluated as a function of Wand plotted in Figure 6. The data show that on increasing W,Wbndd gradually increases reaching a value of -3 at W > 6. These values compare well with the data reported in literature relative to hydration number for this system. One can try to represent the water pool formation in the AOT reversed micelle by assuming the existence of a continuous equilibrium between bonded and bulk water:
3500
3000
2500
v (cm-’) Figure 1. Infrared spectrum of HZO/AOT/CCl4 system. [AOT]= 0.15 mol L-I; (-) W = 0.2;(- - -) W = 5.2.
vibrations of water. The C-H stretching vibrations show absorptionsin the 3000-2700-~m-~ range. No perturbativeeffects on the peaks’ shapes of C-H stretching modes due to the presence of water are observed. Parts a 4 of Figure 2 show the 0-H stretch region for pure water and water in H20/AOT/CC14 system at selected values of W. The spectrum of pure water (Figure 2a) can be fitted very well in terms of three Gaussian components centered at 3603 f 6 cm-I (bandwidth = 74 f 7 cm-I) at 3465 f 5 cm-I (bandwidth = 130 f 10 cm-I), and 3330 f 20 cm-I (bandwidth = 206 f 8 cm-I). These values are in good agreement with those reported in literature.14Js The hydrogen bonding in water has been extensively studied,18J9 and there are recent numerous theoretical and experimental reports.11-17-20-23 Matrix spectra of HzO exhibit only few discrete H bond bands showing that certain H-bond structures are strongly preferred.24 The highest frequency component at 3603 cm-I is usually assigned to the non-H-bonded or weakly H-bonded 0-H group.16.23 The hydrogen bond formation causes for homologous series a decrease in the 0-H stretch frequency proportional to the hydrogen bond energy.I6 Thecomponent at 3330cm-I shifted down by 300 cm-I from the free 0-H groups has been related to molecules in more regular structures with unstrained H bonds, and the component at 3465 cm-I to molecules in more distorted structures (as cyclic dimers or higher aggregates in cyclic forms) with energetically unfavored H bond~.I”16.20-~4 The IR spectrum of surfactant-entrapped water is significantly different from that of pure water, indicating that the water solubilized in the reversed micelles lacks the normal hydrogenbonded structure present in the bulk water. However, the total peak area, A, of the 0-H stretching band of water has been found to increase linearly with water content, as it is predicted by Beer’s law with E = 37 000 L mol-km-l (Figure 3). To quantify the changes in the 0-H stretching region, we fitted each spectrum as a sum of three Gaussian components (see parts b 4 of Figure 2). The fitted curves are virtually indistinguishablefrom the measured ones. The parameterscharacterizing the Gaussian components (peak frequency and bandwidth) differ just a little from those of pure water. They depend on the waterto-surfactant ratio, gradually changing with Wtoward the values characteristic of pure water (parts a and b of Figure 4). Figure.5 shows the variation of the ratio between the area of any Gaussian component (A,) to the total peak area (A) as a function of W ,The observed behavior could be interpreted in terms of distinct types of bounded and free water molecules, whose relative concentrations change with W.Due to the independence of e on W, it is reasonable to assume that the curves in Figure 5 are representative of the variations of the different fractions of -OH groups assigned to each Gaussian component. It is seen from the figure that, as expected, the fraction of free -OH groups
N
+ H20
N*H,O
(1) where N is an unoccupied site in the hydration zone and NmH2O the occupied one. On assuming [N.H20]
i~
+ [N] = A*[AOT]
(2)
Le., the total number of sites in the hydration region proportional to the AOT concentration, the association constant can be evaluated as Wbndcd
K=
( A- Wbndd)(WFrom eq 3 follows
wbndd)
[AOT1
+ A + W)[(K-’[AOT]-’ + A + W2- 4AW]’’*)/2
(3)
Wbndd = ((K-’[AOT]-’
(4)
Fitting our data with eq 4 gives for the parameters A and k thevalues A = 3.5 f 0.1 and k = 6.4 f 0.7L mol-’, respectively. The fitting result is shown by the continuous curve in Figure 6. In spite of the simplicity of the model, the data are reproduced within the experimental errors. It is to note that from the data it appears a continuous variation in the water properties inside micellar cores rather than a two-step hydration mechanism. 3.2. Near-Infrared Spectra. Differences in the vibrational properties of pure water with respect to the water in the micellar core are also evidenced in the NIR region where strong combination and overtone bands appear. These absorptions are better separated than in the fundamental region andcells of more convenient and reproducible thickness can be used. Figure 7 shows the NIR spectrum of bulk water and water inside the micellar core in the range 2100-1 800 nm. In this spectral range,
Onori and Santucci
5432 The Journal of Physical Chemistry, Vol. 97, No. 20, I993 120
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Figure 2. (-) 0-H stretching band for pure water (a) and water in H#/AOT/CCL components from least-squares fitting.
(cm-')
system at selected values of W (b), (c), (d); (- - -) Gaussian
E* is the molar extinction coefficient obtained from the spectra recorded at low W values (W < 0.5). The experimental spectra turn out to be reproduced very accurately by eq 1 and the residuals are randomly distributed. The x values calculated by the fitting are plotted as a function of Win Figure 9. From the analysis of IR spectra previously reported (see section 3.1) it appears that at low water contents not all the water is bonded but w b n d d / W tends to an a value of about 0.77 as W 0. A similar result is attended from the eq 4 being
-
a =: lim-
Wbndtd
=
w - ~W 1.7
KA [AOTI l+KA[AOT]
(6)
Consistently, one can assume 0
3
9
6
12
15
w
e*
= a d b n d t d + (1 - aY)Ebulk
(7)
Figure 3. Molar extinction coefficient of 0-H stretching band of microemulsion water as a function of W.
where bulk water exhibits a strong absorption band at 1928 nm due to a combinationmode of asymmetric stretching and bending vibrations,Il noabsorptiondue to AOTvibrationmodesispresent. When W < 1, a nearly symmetric band at 1924 nm, with an enhanced molar extinction coefficient (E), dominates. On increasing W,water displays more marked "bulk" properties and the absorption band approaches that of the bulk water. Our data are consistent with the existence of two types of water in the water pool of reversed micelles. In fact at any W value the molar extinction coefficient of the water in the micellar cores can be accurately expressed (see Figure 8 ) as where Cbulk is the molar extinction coefficient of pure water and
Substituting for
wbndd
in eq 4 and resolving for x, one obtains
+ W ) - [(K-'[AOT]-l+ A + W)'4AW]"2][2W]-' {I + KA[AOT])/{KA[AOT]] (9)
x = [(K-'[AOT]-' + A
From a best-fit procedure of x (eq 9) to the experimental data, wehaveobtainedA = 3.4f0.1andK= 7f 1 Lmol-l (continuous curve in Figure 9) in good agreement with the results obtained from the IR spectra analysis. From eq 6 one obtains for u the value0.78 f 0.04; Figure loshows thequantity Wb,nddcalculated by eq 8 after inserting the a value obtained from the fitting. For comparison, in the same figure are reported the values of w b n d d from the IR spectra analysis. The agreement between IR and NIR data is evident from the Figure 10. The solid line in Figure
Aerosol OT Reverse Micellar Aggregates 3650
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Figure 4. Peak frequency (a) and bandwidth (b) of the Gaussian components of the water spectrum in H20/AOT/CCL system as a function of W Component centered at -3603 (0),-3465 ( O ) , and -3330 cm-' (A). Plain symbols refers to pure water. 0.8
1800
I
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1900
2000
X
(nm)
2100
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Figure 7. Near-infrared spectra of water at 20 OC; (- -) water; (-) water in H20/AOT/CC4 system; [AOT] = 0.150 mol L-I; W = 0.5.
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0.7 -
H20
06 -
A
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00
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With this in mind, we have explored one simple alternative to eq 1 assuming three different binding sites according to
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02 1
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~
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I
2
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4
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W Figure 5. Ratio of the area of the ith Gaussian component (Ai) to the total peak area (A) vs W. Symbols are the same as in Figure 4. The continuous line serves as a guide to the eyes.
10 represents the best-fit curve of both sets of data by eq 4. The values of the fit parameters are R = 3.4 i 0.1 and K = 7 f 1 L mol-' and the standard deviationof the fit u = 0.05 is comparable with the precision of the experimental data. So, in spite of its simplicity, the model leads to calculated values of Wbondcd as a function of K that are in excellent accord with the observations. One disturbing feature is the occurrence of a non integral value of A. Equation 4 hold for one class of A identical independent binding sites. It is desirable to find whether or not other assumptions generate models which fit the data equally well.
where Kl, K2, and K3 are the equilibrium constants for the consecutive reactions. A reasonably good computationalfit, represented by the dashed line in Figure 10,is produced with two classes of different binding sites characterized by the following Ki values K I= 18 f 4 L mol-', KZ = K3 = 6 f 1 L mol-'. In spite of a greater value of u = 0.10 obtained by this model, it seems to be preferred on physical grounds.
Onori and Santucci
5434 The Journal of Physical Chemistry, Vol. 97, No. 20, 1993
0.4
reversed micelles. The data indicate that as Wincreases, Wbndd and Wbulk gradually increase, and it has been possible to explain such a growth in terms of an equilibriumbetween water molecules in two different regions. The bound water region seems to hold three water molecules per AOT molecule and its formation is nearly complete at W > 6. This behavior is qualitatively in agreement with data in the literature referring to several physicochemicalproperties of water solubilized in reversed AOT micelles.'-3 One can tentatively attribute the three binding sites on AOT molecules to the three oxygen atoms of the S - 0 3 - head group to which water molecules could be hydrogen bonded. The environments of these molecules could be different from that of bulk water and would to be complete for W 6. In a recent paper Hauser et al.,10on studying the hydration of water/AOT/isooctane system by three independent methods, found hydration numbers dependent on the method employed. From both calorimetry and 2H NMR authors conclude that two water molecules are tightly bound to one AOT-Na+ molecule, whereas ESR spin labeling identifies four strongly bound water molecules and calorimetry reveals that six water molecules are unfreezable. As noted from these authors, "this variation from method to method is not too surprising considering the difference in the basic principles involved, in the sensitivity, and in the assumptions made in the evaluation of the primary data". Really different methods measure different properties. Since IR experiments involve a measurement on a time scale (- 1&l4 s) shorter than the residence time of water molecules in the bound region, distinct bands can be assigned to the two populations of water and so the IR measurements identify bonded water as vibrationally different from the bulk one.
0.2
Acknowledgment. This work was supported by the Consorzio Interuniversitario Nazionale per la Fisica della Materia, Italy.
-
1800
2000
1900
X
2100
(nm)
Figure 8. Experimental (-) and calculated (O), according to eq 5; NIR
spectrum of water in water in H2O/AOT/CCl4 system ([AOT] = 0.150 mol L-I; W =7.5). Also reported a r t the two contributions xes (A)and (1 - X)ebulk (0)(see 5).
1.o
0.8 X 0.6
0.0
Refereaces and Notes 0
2
4
6
8
10
12
W Figure 9. x values (see eq 5) as a function of W (0)experimental values obtained from NIR spectrum; (-) fitting according to eq 9. 4
3
3! 2 1
0
0
2
4
6
8
10
12
W 10. Concentration of 'bonded" (&ndd) water in HzO/AOT/ CCld system vs the total water concentration. Experimental values obtained from IR ( 0 )and NIR (0)spectra; (-) fitting according to eq 4; (- -) fitting according to multiple equilibrium (10).
-re
-
4. Concldoas
Aconsistentpictureisemergingfrom thcuseoftwoindependent spectral methods to the description of the state of water in the AOT reversed micelles. Our observations agree with previous reports that two types of water, "bonded" and "bulk", coexist in the water pools of
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