J . Phys. Chem. 1989, 93,1409-7416 7409

Structural Studies of Aerosol OT Reverse Mfcellar Aggregates by FT-IR Spectroscopy

Tanoj Kumar Jain, Manoj Varshney, and Amarnath Maitra* Department of Chemistry, University of Delhi, Delhi- 110 007, India (Received: September 6, 1988; In Final Form: May 8, 1989)

The microstructural characteristics of water/AOT/isooctane microemulsions have been investigated by FT-IR spectroscopic technique. The broad peaks obtained for water OH and carbonyl bands have been resolved by Gaussian curve fitting, and the vibrational characteristics, particularly the peak intensity and peak area corresponding to each peak, have been analyzed. It has been observed that the aqueous core of the microemulsion droplet is composed of bound and free water while a small amount of water remains trapped in the interface. The maximum hydration number of AOT was found to be 12. The phenomena of rotational isomerism of the AOT molecule reported earlier with other techniques have also been revisited from the deconvoluted carbonyl bands of AOT molecule.

Introduction Aerosol OT or AOT, i.e., sodium bis(2-ethylhexyl) sulfo-

succinate, forms inverted micelles when dissolved in nonpolar solvents in which the sulfonate and ester head groups are pointed toward the polar side constituted by the aqueous core and the hydrocarbon chains are extended outside toward the continuous oil phase.'%2 These reverse micellar aggregates exhibit the re- markable ability to solubilize large amounts of water, resulting in the formation of water-in-oil microemulsions. The extent of water solubilization can be changed by adding organic or inorganic a d d i t i ~ e s . ~ . ~ The micellization of AOT in nonpolar solvents and the influence of various additives on it were the subject of intense studies both from theoretical and experimental viewpoint^.^.^ The maximum water uptake by reverse micellar systems has been calculated by Mitchell and Ninham6 on the basis of free energy variation of the total system' due to changes of the molecular property of the surfactant. Conformational changes in the sur- factant molecule have a strong influence on these molecular properties. An observation on rotational isomerism of the AOT molecule and its effect on the amphiphilicity in various systems have shown that water binding of the surfactant depends on the composition of the rotamers of AOT existing in the eq~i l ibr ium. '~

Attempts were made to correlate the liquid microstructure of AOT microemulsions in oil with their various colloidal behaviors from NMR,'*12 neutron scattering measurements,I3-l5 photon correlation spectroscopy,l6I8 etc. But, in concentrated solutions,

(1) Luisi, P. L., Straub, B., Eds. Reverse Micelles; Plenum Press: New

(2) Eicke, H. F., Parfitt, G. D., Eds. Interfacial Phenomena in Apolar

(3) Kunieda, K.; Shinoda, K. J. Colloid Interface Sei. 1977, 70, 577. (4) Maitra, A. N. In Surfactants in Solution; Mittal, K. L., Bothorel, P.,

(5) Vos, K.; Laane, C.; Visser, A. J. W. G. Photochem. Photobiol. 1987,

(6) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981,

(7) Maitra, A. N.; Eicke, H. F. J . Phys. Chem. 1981, 85, 2687. (8) Magid, L. J.; Martin, C. A. In Reverse Micelles; Luisi, P. L., Straub,

(9) Maitra, A. N.; Dinah; Varshney, M. Colloids Surf. 1987, 24, 119. (IO) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977,99,

(11) Martin, C. A.; Magid, L. J. J . Phys. Chem. 1981, 85, 3938. (12) Llor, A.; Rigny, P. J. Am. Chem. Soc. 1986, 108, 7533. (13) Robinson, B. H.; Toprakcioglu, C.; Dore, J. C.; Chieux, P. J . Chem.

York, 1984.

Media; Dekker: New York, 1987.

Eds.; Plenum Press: New York, 1986; Vol. 5, p 591.

45, 863.

77, 601.

B., Eds.; Plenum Press: New York, 1984, p 181.

4730.

Soc.. Faradav Trans I 1984. 80. 130. (14) Fletcher, P. D. I.; Robin&, B. H.; Tabony, J. J. Chem. Soc., Faraday

Trans 1 1986. 82. 23 11. - - . - ~ 1 ~ - ~ ~ . .. .~ ~ ~

(15) Howe, A. M.; Toprakcioglu, C.; Dore, J. C.; Robinson, B. H.; J. Chem. Soc., Faraday Trans. 1 1986,82, 2411.

(16) Hilfiker, R.; Eicke, H. F.; Geiger, S.; Furler, G. J. Colloid Interface Sci. 1985, 105, 378.

(17) Chatenay, D.; Urbach, W.; Cazabat, A. M.; Vacher, N.; Waks, M. Biophys. J . 1985, 48, 893.

(18) Nicholson, J. D.; Clarke, J. H. R. In Surfactants in Solution, Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1663.

0022-365418912093-1409$01.50/0

the effect of interdroplet interactions and polydispersity make the interpretation of the results comp1i~ated.I~ In order to have a clear picture about the structure of various microphases in the systems, use of vibrational spectroscopy like FT-IR and Raman seems to be more fruitful. Both techniques are noninvasive, functional group selective, sensitive to chemical environments, and free of problems arising out of scattering experiments.

The structure of the surfactant aggregates can be characterized by a property characteristic to the surfactant molecule and is known as the packing parameter.6 Any change in physical property or in composition of the surfactant monolayer of the droplet or of the aqueous core would cause a change in the packing parameter of the surfactant. The change of the packing parameter of the AOT molecule in different systems is a direct consequence of the change in composition of rotamers in the equilibrium mixture and the change of water structure of the aqueous core, as well as bending and kinking of the hydrocarbon chains of the surfactant molecule. Investigations by vibrational spectroscopy of the AOT microemulsion systems are thought to be useful tools in observing the above-mentioned conformational changes and water structures.20 This paper is composed of three parts: (i) assignment of various vibrational bands from IR spectra, (ii) deconvolution of water bands and quantitative estimation of various kinds of water present in the aqueous core, and (iii) deconvolution of the carbonyl stretching bands and analysis of rotational isomerism of AOT in the aggregated systems.

Experimental Section Materials. Aerosol OT (AOT, sodium bis(2-ethylhexyl) sul-

fosuccinate) was obtained from Fluka AG and was purified as described elsewhere.' Isooctane was purchased from Fluka AG and was dried before use. Lithium chloride (Fluka AG), potassium iodide (BDH), and cholesterol (Sigma) were used as additives without any further purification. A 0.1 M AOT solution in isooctane was used in all studies. Triple-distilled water was used in preparing the ternary solution of water/AOT/isooctane. A 0.5% solution of lithium chloride and potassium iodide in tri- ple-distilled water was used in place of pure water t6 study the effect of electrolytes on the water structure of the aqueous core of reverse micellar aggregates. Cholesterol was added directly to the AOT solution in isooctane as per their molar ratio. The molar ratios of added water to AOT and added cholesterol to AOT were represented by W , and R, respectively.

Methods. IT-IR spectra of all the samples were taken on a Perkin-Elmer Model 1710 ET-IR spectrometer using Irtran cells of a fixed path length of 0.053 mm. All spectra were recorded at room temperature (25 "C). Every sample has been given 100 scans at a resolution of 2 cm-' before recording. No smoothening

(19) Cazabat, A. M.; Chatenay, D.; Guering, P.; Langevin, D.; Meunier, J.; Sorba, 0. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; p 1737.

(20) MacDonald, H.; Bedwell, B.; Gulari, E. Langmuir 1986, 2, 704.

0 1989 American Chemical Society

7410

TABLE I: Infrared Frequencies of Solid AOT and Related Compound Systems and Their Assignments

The Journal of Physical Chemistry, Vol. 93, No. 21, I989 Jain et al.

water/AOT/isooctane AOT isooctane 2-ethylhexanol wo = 0 Wo = 30 assignment

980 960 980 980 CH, rock

1050 1051 1048 sym S=O str of sulfonation 1093 (b)" 1118 CH2 wag, CH, twist

1035 1039

1169 1169 1169 1206

1248

1366 1380 1380

1393 1416 1464 1471 1463

1736 2861 2874 293 1 2947 (b) 2928 2959 2959

1218 (b)

3332 (b)

b = broad.

CH, twist, SO3 asym band, C-0 str of ester linkage I 1207 1207

1248 1248

1366 1353 1366

was done due to the clarity of the spectra. The hydroxyl stretching region (3 100-3700 cm-') and carbonyl stretching region (1650-1750 cm-') were curve fitted with three and two Gaussians, respectively, with the help of a Gaussian curve fitting program in order to study the water structure in reverse micellar droplets and rotational isomerism in the AOT molecule.

Results and Discussion I . Assignments of Vibrational Bands. The IR spectra of AOT

(solid) and structurally related compounds like isooctane and 2-ethylhexanol as well as water/AOT/isooctane reverse micelles (W, = 0 and 30) have been recorded as follows. Solid AOT has been dissolved in methanol, and a film was developed in a KBr disk by spreading the methanol solution and drying. The I R spectra of this film was taken. The spectra of 2-ethylhexanol was recorded with its film on a potassium bromide disk while those of isooctane and reverse micellar systems were taken in an Irtran cell with fixed path length of 0.053 mm. The characteristic peaks of these compounds lie in the region 800-3800 em-'. The im- portant peaks with their probable assignments of various group frequencies are shown in Table I. The band in the region 3200-3700 cm-' is due to 0 - H stretching vibrations.21 A broad band near 3330 cm-' has been observed for 2-ethylhexanol due to the hydroxyl group present in the molecule that is evident from the disappearance of the band in this region in the case of solid AOT, isooctane, and dry micelle. A broad band around 3420 cm-' has been observed due to the 0 - H stretch of the water molecule in the case of the microemulsion system (W, = 30). The de- formation band of water around 1650 cm-' has been found in the case of this microemulsion system, which is absent in the spectra of AOT, 2-ethylhexanol, and dry micelles. In the C-H stretching region, the important bands appear in the region 2840-3000 cm-l. These bands contain symmetrical (us) and asymmetrical (v,) C-H stretching of methyl groups and methylene g r o ~ p s * ~ * ~ ~ that have been observed for all the systems under investigations and are shown in Table I. The bending motion of hydrogen in a C-H bond give rise to bands in the region 1300-1500 cm-1.22923 The methyl group exhibits two types of bending vibrations: one is symmetrical [6,(CH3)] around 1375 cm-' and other is asymmetrical [6,(CH3)] near 1450 cm-' while the methylene groups give rise to four types of bending vibrations, viz., scissoring, rocking, wagging, and twisting.a23 The band at 1465 cm-' is due to methylene scissoring [6,(CH2)]. The twisting and wagging modes are observed in the

1385 CHI wag 1393 1393 sym CHI bend

1471 1471 asym C-H bend of CHI, CH, sciss

1737 (b) 1735 (b) C=O str 1650 (b) H 2 0 deform

CH, sym str CH, antisym str

2939 (b) 2934 (b) termin CH, assym str 3420 (b) 0 - H str

(21) Stuart, A. V.; Sutherland, G. B. B. M. J. Chem. Phys. 1956,24,559. (22) Hartwell, E. J.; Richards, R. E.; Thompson, H. W. J . Chem. Soc.

(23) Sheppard, N. Trans. Faraday SOC. 1955, 55, 1465. 1948, 1436.

1150-1350-cm-' range as shown in the Table I. A strong band near 1737 cm-' has been observed in the case of solid AOT and in reverse micellar aggregates that may be assigned to be due to carbonyl groups present in the molecule.23 The band due to C-O of ester linkage has been observed in the region 1150-1300 cm-'.% The absorption bands of the sulfonate group also appear in this region as well as in the 1025-1080-cm-' region due to an asym- metric and a symmetric SO3 stretch, respectively.

The water bands and bands corresponding to carbonyl stretching vibrations were found to change both in intensity and position when the composition of the microemulsion is changing. The bandwidth and peak frequencies corresponding to sulfonate bands were also found to change significantly as a function of the water content of the microemulsion system. The C-H stretching bands at various positions in the spectra were, however, found to remain unchanged in all the microemulsion systems.

2. Water Structure in the Aqueous Core of Water/AOT/ Isooctane Microemulsions. It has already been reported by many author^^'-^^ that the water close to the biological membranes or protein interface exhibits behavior markedly different from that of bulk water. The water solubilized in reverse micelles, in any respects, is similar to the interfacial water present near the bio- logical membranes or a t the protein surfaces.28~ It has restricted mobility and a depressed freezing point and lacks the normal hydrogen-bonded structure present in the bulk ~ a t e r . ~ " ~ In the case of water/AOT/oil microemulsions, the structure and prop- erties of solubilized water have been studied by a number of techniques.3639 Some investigators looked at the properties of

(24) Fowler, R. G.; Smith, R. M. J. Opt. SOC. Am. 1953,43, 1054. (25) Halle, B.; Anderson, T.; Forsen, S.; Lindman, B. J. Am. Chem. Soc.

1981, 103, 500.

?Ad8 (26) Thompson, K. F.; Gierasch, L. M. J . Am. Chem. SOC. 1984, 106,

_- (27) Ah, S.; Bettelheim, F. A. Colloid Polym. Sci. 1985, 263, 396. (28) Grigolini, P.; Maestro, M. Chem. Phys. Lett. 1986, 127, 248. (29) Kim, Vi.; Frolov, G. Yu.; Ermakov, V. I.; Pak, S . E. Kolloidn. Zh.

(30) Wells, M. A. Biochemistry 1974, 13, 4937. (31) Wong, M.; Thomas, J. K.; Gratzel, M. J . Am. Chem. SOC. 1976, 98,

(32) Day, R. A,; Robinson, B. H.; Clarke, J. H. R.; Doherty, J. V. J .

(33) Douzou, P.; Keh, E.; Balny, C. Proc. Natl. Acad. Sci. U.S.A. 1979,

(34) Seno, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H . J . Colloid

(35) Keh, E.; Valeur, B. J. Colloid Interface Sei 1981, 79, 465. (36) Maitra, A. N. J. Phys. Chem. 1984, 88, 5122. (37) Politi, M. J.; Chaimovich, H. J. Phys. Chem. 1986, 90, 282. (38) Quist, P.; Halle, B. J . Chem. Soc., Faraday Trans. 2 1988,84, 1033.

1987, 49, 1067.

2391.

Chem. Soc., Faraday Trans. 1 1979, 75, 132.

76, 681.

Interface Sei. 1980, 78, 57.

Structural Studies of AOT Reverse Micellar Aggregates

Figure 1. Schematic representation of reverse micelles of aerosol OT, showing the different regions of the micellar interior. (Reprinted with permission from ref 40. Copyright 1986 Plenum.)

water directly while others have drawn inferences about its properties from its effects on spectra (absorption, fluorescence, NMR, etc.) of the solubilized species. The unusual behavior of this water has been attributed to its strong interaction with the ionic head groups of the surfactant molecule as well as to an overall disruption of the three-dimensional hydrogen-bonded network usually present in bulk water. As seen from Figure 1, there are at least three different distinct solubilizate environments available, viz., in the interfacial environment, in the bound water layer, and in the water p d 4 0 However, the exchange among these different types of water is very fast and occurs on an even less than na- nosecond time scale.41 Using N M R and IR investigations of water-in-oil microemulsion droplets of AOT and related systems, many people have reported three types of water existing in the water core of the droplets.424s

The operational definition of the bound water species specifies that it is the water possessing motional properties that is detectively different from those of pure bulk water. The characteristic features of these bound water species includes the extent of hydrogen bonding, effective dielectric constant, viscosity, mobility (trans- lational and rotational), etc. It can reasonably be assumed that bound water is composed of water molecules bonded to negatively charged polar head groups of AOT through sodium ions that are in the vicinity of the interfa~e. '~ The water molecules of hydrated sodium ions are hydrogen bonded with the polar groups of AOT and, therefore, constitute the bound water layer. The interfacially trapped water can be considered as those water molecules that are located at the interface and are not bound to any other molecule of group but are trapped between the polar head groups of surfactant molecules a t the interface. These water molecules should behave either as monomers or dimers and are considered to have penetrated the interfacial layer. Thus, they should behave like water molecules in a matrix. Although the existence of water

(39) Dazzo, G. M. Dim. Abstr. Int. B 1986, 46, 2677. (40) El Seoud, 0. A. In Reverse Micelles; Luisi, P. L., Straub, B., Eds.;

(41) Hertz, G. In Watet-A Comprehensive Treatise; Franks, F., Ed.;

(42) Eicke, H. F. Private communication. (43) de Marco, A,; Menegatti, E.; Luisi, P. L. J . Biochem. Biophys.

(44) Eicke, H. F. In Microemulsions; Robb, I. D., Ed.; Plenum Press: New

(45) Boicelli, C. A.; Giomini, M., Giuliani, A. M. Appl. Spectrosc. 1984,

Plenum Press: New York, 1986; p 81.

Plenum Press: New York, 1975; Vol. 3, Chapter 7.

Methods 1986, 12, 325.

York, 1982.

38, 537.

The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 741 1

molecules in the bulk oil phase is theoretically possible, the extent to which they are distributed is too insignificant to be taken into account for this case.

Parts A-C of Figure 2 depict the shape of water band in the OH stretching region (3100-3700 cm-I) of the IR spectrum of water/AOT/isooctane microemulsion systems at three different water contents, Le., Wo = 7, 15, and 40, respectively. It can be seen from the figure that the bands are very broad and have asymmetric shapes. The vibrational characteristics of bound water molecules are different from those of trapped ones as well as those of bulk water molecules. The hydrogen bonding broadens the OH stretch bands.46 In the bound water layer, the water molecules are hydrogen bonded with the polar head groups of AOT, which results in absorption in the low-frequency region of the IR spectrum. The trapped water molecules are matrix isolated and monomeric in nature, So, they absorb in the high-frequency region. The free water molecules, however, have strong inter- molecular hydrogen bonding, leading to an array of water mol- ecules that further shifts the band frequency to the lower side of the spectrum. As there is overlapping of the OH stretching bands of the water molecules present in different states, a single broad and asymmetric band in the O H stretching region was observed in the spectra.

In order to quantify the changes in the OH stretching region, the spectrum has been fitted as a sum of Gaussian peaks with the help of a Gaussian curve fitting program. It was found that when the 0-H stretch peak was fitted with two Gaussians, the error between the experimentally observed curve and the fitted one was increased by nearly 4 times while when the fitting was done with four Gaussians, the solution either contained negative values for peak area and peak intensity that do not have any physical sig- nificance or gave a high degree of error between the experimental one and the observed one. Of course, a curve fitting with three Gaussians only yields a solution with a minimum amount of error, and it yields three peaks centered at 3290 f 20, 3490 f 20, and 3610 f 10 cm-' with half line widths of 120 f 20,90 f 5, and 46 f 5 cm-l, respectively. The Gaussian curve fitted OH stretch bands for three microemulsion systems, namely, with Wo = 7, 15, and 40, are shown in Figure 3A-C, respectively. In all the parts of the figure, peak a corresponds to 3290 f 20 cm-', peak b to 3490 f 20 cm-', and peak c to 3610 f 10 cm-'. The resultant of the three peaks, viz., a-c, is calculated from the curve-fitting program and is shown by the resultant peak d, which is found to be indistinguishable from the experimental curve. These values are in good agreement with those reported by MacDonald et aLZ0 for the water/AOT/heptane system.

It has been concluded from the results that the peak at 3290 f 20 cm-I is due to the O H stretch in hydrogen-bonded-associated chains of water molecules in bulk water.20 The 3490 f 20-cm-I peak is assigned to the hydrogen-bonded dimers bound between the interface and the bulk The high-frequency peak at 3610 f 10 cm-l has been assigned to monomeric water or matrix-isolated dimer^.^',^*

The total peak area of the O H stretching band of water has been found to increase with Wo (Figure 2), which is evident from Beer's law. As water is composed of three different states of water in the system, viz., bound water, trapped water, and free water, it is reasonable to assume that the total peak area corresponding to the water band is the sum of the peak areas of the different states of water.20 Thus, if the area of peak at 3290 f 20 cm-I is GI, that a t 3490 f 20 cm-I is G2, and that at 3610 f 10 cm-I is G3, then the total peak area is given by the expression

Gtotal = GI + Gz + G3

If PF is the fraction of free water corresponding to the 3290 f 20-cm-' peak, PB is the fraction of bound water corresponding

(46) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S . J . J .

(47) Tso, T. L.; Lee, E. K. C. J . Phys. Chem. 1985, 89, 1612. (48) Walrafen, G. E. In Hydrogen Bonded Soluent Systems; Covington,

Colloid Interface Sci. 1985, 103, 56.

A. K., Jones, P., Eds.; Taylor and Francis: London, 1968.

7412 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 Jain et al.

0,9991

0.0992

0.79 93

0.6994

0.59 95

c)

2 0.4996 m

a

:: 6

0.3997

0.2998

0.1 999

0.1000

0.0001 31

4

i

I I I 0 3600 3400 3200 3(

- F R E Q U E N C Y (cm-')

1.50

1.39

1.28

1.17

1 1.06 2 2 0.95 m

w

2 0.84

0 73

0 62

0.5 I

B

P

2.9991 I

I 2.6992

03000

0.0001

10 -FREQUENCY (cm-')

Figure 2. Representative infrared spectrum of microemulsion water in the water/AOT/isooctane system: (A) Wo = 7 ; (B) Wo = 15; (C) Wo = 40.

t W 0

2 w a m =x

0 : F R E Q U E N C Y (cm-') -

B I

3300 3500 3700 FREQUENCY (cm') - FREQUENCY (c m-' ) --L

Figure 3. Deconvoluted infrared water spectrum for the water/AOT/isooctane system: (A) W, = 7 ; (B) Wo = 15; (C) W, = 40.

to the 3490 f 20-cm-' peak, and PT is the fraction of trapped water corresponding to the 3610 f 10-cm-' peak, then their respective values can be calculated from the following relations

G3 PT = - G2

Gtotal GtotaI Gtotal PB = - G I PF = -

where PF + PB + Pr = 1. A provision has been made in the Gaussian curve fitting pro-

gram to give the area of different peaks arising from the curve- fitting procedure. Figure 4 depicts the variation of different fractions of water, viz., PF, PB, and PT, with water to AOT molar ratio ( Wo). It has been noted from the figure that when the water content of the water/AOT/isooctane system is increased, the bound water and trapped water fractions remain practically constant up to Wo = 10 and then gradually increases until Wo = 18, above which the values for both the fractions decrease. For the free water fraction (PF), however, it exhibits no significant

change up to Wo = 10 but decreases with an increase in W, value until Wo = 18 and then it increases. The results are interesting when one considers the various microstructural changes taking place within the isotropic domain of microemulsion. We can assume that, up to Wo = 10, there exists an equilibrium between the AOT monomers and aggregated systems,49 and hence, with the gradual addition of water, some water molecules are used up in hydrating the monomers while the others remain as free water inside in the aqueous cores in addition to the micellar-bound water. In the range 10 < Wo < 18, all the surfactant molecules are known to form micellar aggregates, leaving practically no monomer in the bulk solvent. The added water goes inside the aqueous core of the droplets, and the droplets grow in size. After Wo = 18, the water that is added to the microemulsion system mainly

(49) Eicke, H. F.; Denss, A. In Solution Chemisrry ofSurfacrants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 699.

Structural Studies of AOT Reverse Micellar Aggregates

0.9

0.8

The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7413

- b

I .o

0 ..)

0 . 8

0 . 7

0 .6 I 0.5 c

0.4

0.3

0.2

0.1

0.0

0.6

t 0'5

h 0.4

0.3

1 0'09

-

-

-

-

io,o' Figure 4. Variation of different fractions of water PF (-O-), PB (-0-), and PT (-i3-) with water to AOT molar ratio (Wo).

/ I

0 0 4 0 I2 I6 20 24 28 32 36 40 44 4a

we - Figure 5. Variation of the number of water molecules per AOT nF (+), ne (-0-), nT (-X-) with water to AOT molar ratio (Wo).

behaves as free water, and thus, the PF was found to increase with the resultant decrease of PB.

The situation can be best understood if one calculates the number of each type of water per AOT molecule in the droplet using the relation

n, = WoPi

where n, is the average number of the ith type of water molecule per AOT and Pi is the fraction of water in the ith state. The computed values have been plotted against Wo as shown in Figure 5 . It is seen from the figure that the number of bound water per AOT molecule increases initially with Wo and reaches a maximum value of about 12 at Wo = 18, above which it remains constant. The number of free water, nF, also increases up to Wo = 10 and remains constant between Wo = 10 and 18. Beyond Wo = 18, nF starts increasing sharply and reaches, for example, 27 at Wo = 40. Any water added in the microemulsion system between Wo = 10 and 18 hydrates AOT molecules so that an enhancement of bound water fraction and a decrease of free water fraction was observed. This region has been explained by many authors as a micellar-swollen region.4e52 The number of trapped water per

(50) Sjoblam, E.; Friberg, S. J. Colloid Interface Sci. 1978, 67, 16. (51) Bellccq, A. M.; Gowche, G. J . Colloid Interface Sci. 1980, 78, 275.

09

0.6

0 7

0 5

0 5

0 4

0 3

0 2

0 1 I I I I I I I I L 5 10 15 20 2 5 3 0 31 40 45

WO -

0.7 1

O 2 I 01

I I I I I I I I I 0 5 10 I5 2 0 25 30 35 40 45

w. - 00 I

Figure 6. (a) Variation of PB as a function of Wo in the presence of electrolytes: LiCl (-El-); KI (-O-), reference system (a-). (b) Var- iation of PF as a function of Wo in the presence of electrolytes: LiCl (-X-); KI (-0-); reference system (-O-).

AOT molecule, nT, is much smaller than the above two values, and its increases with Wo up to Wo = 18, above which it remains practically constant.

2.1. Effect of Structure-Making and Structure-Breaking Ions on the Water Structure of Water/AOT/Isooctane Microemulsion Systems. The infrared spectroscopic studies of aqueous ionic solution were found to be a useful tool in the characterization of structural changes in the solvent.5g Parts a and b of Figure 6 depict the effect of structure-maker ions such as Li+ and structure breaker-ions such as I-41 on the bound and free water fractions, respectively. The addition of lithium chloride to the water/ AOT/isooctane micellar system increases the bound water fraction and decreases the free water fraction while that of potassium iodide has opposite effect. Calculation of the numbers of free and bound water per AOT molecule as shown in Table I1 for these micro- emulsion systems reveals that they are significantly influenced by the addition of these electrolytes. In case of lithium chloride, the bound water molecules, nB, increase with the increase of Wo up to Wo = 25 above which the solution becomes turbid with phase

(52) Lagues, M.; Sauterey, C. J . Phys. G e m . 1980, 84, 3505. (53) Walrafen, G. E. In Water-A Comprehenriue Treatise; Franks, F.,

Ed.; Plenum Press: New York, 1972; Vol. 1 , p 151.

7414 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989

0 7 -

0 6

8 1 0 5 L

0 4

0 3

Jain et al.

-

-

-

TABLE 11: Number of Free and Bound Water Molecules per AOT at Different W , for tbe Water/AOT/Isooctane System and in the Presence of LiCl and KI

electrolyte

LiCl KI water/ s o h / AOT/isooctane

~- AOT/isooctane

WO nB nF nB nF nF

5 10 13 15 18 20 25 30 40

2 5 7 9

12 12 12 12 12

3 2 3 2 3 5 5 5 5 5 5 8 4 6 6 5 10 4 8 6 5 12 5 9 8 6 13 5 10 9

11 16 8 12 12 16 12 16 26 12 26

separation. In the potassium iodide containing system, the rate of increase of bound water per Wo is less than that in the reference system. AOT molecules attain saturation hydration (nB = 12) at W, = 25. An AOT molecule was found to bind a maximum of 12 water molecules in its hydration sphere. It can be presumed that out of them some water molecules are strongly bound to polar groups of AOT molecules, and we call it a primary hydration layer. The rest of the water molecules are bound to the secondary hy- dration layer, and the structure of this secondary bound water layer can be modulated by changing the composition of the electrolytes in the aqueous core. The hydration number of AOT reported in the l i t e r a t ~ r e ~ ~ , ~ * - ~ ~ lies between 6 and 12. It is pre- sumed that some of the water molecules are strongly perturbed by surface-water interaction from the primary hydration layer while the rest remain associated with the secondary hydration layer. Lithium being a structure-maker ion helps the binding of water molecules, and the secondary bound water layer becomes more structured. As a result, the maximum of ne exceeds from 12 to a value 16 at Wo = 25. In the case of iodide ion, the hydrogen-bond-associated structure of water is disrupted because of its unconventional large size. This hinders the formation of the secondary bound water layer. The saturation hydration of AOT (nB = 12) is, therefore, attained only at higher Wo values.

2.2. Effect of Cholesterol on the Water Structure of Water/ AOTlIsooctane Microemulsion System. Cholesterol is an important constituent in all membranes of mammalian cells. It is an amphiphilic molecule, and structurally, it is characterized by a relatively small hydrophilic 3/3-OH group and a bulky fused-ring system that is hydrophobic and stereochemically rigid and flattened. Some work has been reported on the interaction between the 3/3 -OH group of cholesterol and the polar head groups of lipid^,^^-^^ and it has been established that lecithin becomes apathetic to hydration in the presence of cholesterol.m Recent study from water proton NMR relaxation measurements has established that the intermolecular hydrogen bonding between the 3/3 -OH group of cholesterol and the carbonyl ester of AOT is responsible for the decrease in hydrophilicity of the surfactant molecule.54 The interfacial water structure can be best studied from the deconvoluted IR spectra of water bands of the water/ AOT/choiesterol/isooctane system. The variation of free water fraction with the amount of added water for the above system is shown in Figure 7. In the range between Wo = 10 and 30, the free water fraction, PF, in the cholesterol-containing system was found to remain, unlike the reference system, constant and no

(54) Maitra, A. N.; Patanjali, P. K. Colloids Surf. 1987, 27, 271. (55) Yeagle, P. L.; Hutton, W. C.; Huang, C. H.; Martin, R. B. Proc.

(56 ) Kruft, B. de.; Damel, R. A.; Van Deenen, L. L. M. Biochim. Biophys.

(57) Bittman, R.; Blau, L. Biochemistry 1972, 2 2 , 4831. (58) Keough, K. M.; Oldfield, E.; Chapman, D.; Baynon, P. Chem. Phys.

(59) Maitra, A. N.; Dinesh; Patangali, P. K.; Varshney, M. Colloids Surf

(60) Phillips, M. C.; Finer, E. G. Biochim. Biophys. Actu 1974, 356, 199.

Natl. Acad. Sei. U.S.A. 1975, 72, 3471.

Acta 1972, 255, 331.

Lipids 1973, 10, 37.

1986, 20, 2 1 1.

0.1

o-2 I I I I I I I I I L

0 5 10 I S 2 0 2 5 30 35 40 45 0 0 I

w.a - Figure 7. Variation of PF as a function of Wo in presence of cholesterol: (-e-) R = 0; (+-) R = 0.1; (-0-) R = 0.2.

TABLE III: Number of Water Molecules per AOT (np nh nT) at Different W o Values for Water/AOT/Isooctane and Cholesterol Containing Systems

water /AOT/cholesterol/isooctane R = O R = 0.1 R = 0.2

Wo n~ nr ns n~ n~ n~ n~ 5 2 3 0 2 3 0 2 3 0

1 0 5 5 0 5 5 0 5 5 0 1 3 7 5 1 6 6 1 6 7 0 1 5 9 5 1 7 7 1 7 8 0 1 8 1 2 5 1 8 9 1 9 9 0 2 0 1 2 6 2 9 1 0 1 9 1 0 1 25 12 11 2 11 13 1 11 13 1 30 12 16 2 12 17 1 12 17 1 40 12 26 2 12 27 1 12 27 1

significant increase of PF was observed when the cholesterol content in the systems was increased from R = 0.1 to 0.2. Table I11 shows the hydration number per AOT molecule a t different Wo values in both the reference- and cholesterol-containing sys- tems. It is observed in all cases that nB gradually increases with Wo and attains a maximum value of 12 a t a certain Wo, above which the hydration number remains constant. It is interesting to note that while in the reference system, the saturation hydration of AOT is attained at Wo = 18; the same is observed in the cholesterol-containing system at Wo = 30. The rate of increase of hydration with W, is much less in the cholesterol-containing system. The AOT head group is composed of three polar units that form hydrogen bonds with water molecules. In the presence of cholesterol, one of the polar groups, Le., the ester carbonyl group that is vicinal to the sulfonic group, is hydrogen bonded with cholesterol through the 3p-H of the latter.% Since the amount of cholesterol is much less than that of AOT, a large fraction of AOT molecules remain free from cholesterol bonding. Never- theless, the saturation hydration of AOT takes place only at a larger water content. Because of rotational isomerism, the AOT molecules in the microemulsion droplets remain predominantly populated by gauche conformers with the concerned ester group away from the polar side of the interface and toward the hy- drocarbon monolayer of the surfactants. We assume that in the presence of cholesterol, the hydrocarbon monolayer of the droplets becomes more rigid due to condensation so that penetration of water molecules into the monolayer and hydration of the ester group become difficult. As a result of this condensation, the number of trapped water molecules (nT) is also decreased con- siderably.

Structural Studies of AOT Reverse Micellar Aggregates The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7415

I I

COOR

Figure 8. Stereospecific positions of the three polar groups with respect to the polar/apolar interface for different rotamers of AOT.

1.00

0.94

0.88

0 . 8 2

f 0.76

w V 2

a 0 ln m a 0.64

0 .70

0 . 5 8

0.52

0 46

0.40

&FREQUENCY ( c k ' )

Figure 9. Representative infrared spectrum of carbonyl stretch region for the water/AOT/isooctane system (W, = 20).

2.3. Rotational Isomerism of AOT in Reverse Micellar Ag- gregate. The amphiphilic character of AOT molecules was found to be dependent on various factors like temperature, the nature of the solvent, types of associated counterions, the presence of various other organic compounds, etc., in the aggregated ~ y s t e m . ~ AOT is a diester derivative of succinic acid with two long hy- drocarbon chains from the ester groups. The sulfonate and ester groups remain within the skeleton of the succinic acid part of the molecule. Micellar aggregates are formed through hydrogen bonds between the polar part of the AOT molecule and water. These polar groups are stereochemically directed along the various bonds of the ethane skeleton of the succinic acid part of the molecule and can rotate about the ethane C-C bond.' It has been estab- lished from the spinspin coupling constant of protons that a restricted rotation about the C-C bond of the ethane skeleton of the succinic acid part of the molecule exists that determines the stereospecific position of the above three polar groups relative to one another, a t the interface, and thus governs the amphiphilicity of the molecule.

If the rotational isomerism exists in the molecule, then it can be seen that in the gauche conformation all the polar groups are directed toward the polar side of the interface while in the trans

0 1670 1690 1710 1730 1750 FREQUENCY (cm-' ) -

Figure 10. Deconvoluted carbonyl stretch peak for the water/AOT/ isooctane system (W, = 20).

0 0 @,, ,C-OR I I

\ /C-oR H

-@ = REST PART OF AOT MOLECULE

H

GAUCHE

H

CIS

Figure 11. Assignment of the infrared carbonyl stretches to different conformations about C ( 0 ) C bond. Reprinted with permission from ref 59; p 203. Copyright 1986 Elsevier.

HIGHER FREQUENCY LOWER FREQUENCY

conformation a carbonyl group moves from the polar side toward the hydrocarbon part of the surfactant monolayer (Figure 8). The IR bands are very much sensitive to microenvironments, and therefore, this idea prompted us to study the IR spectra of the carbonyl groups of AOT molecule in the microemulsion system.

A representative IR spectrum of water/AOT/isooctane ( Wo = 20) in the region 1650-1750 cm-I is shown in Figure 9. A broad peak around 1730 cm-' has been observed due to carbonyl groups present in the AOT molecule. From the asymmetric shape of the above-mentioned band, it is clear that the band is a fusion of the sum of the peaks corresponding to carbonyl groups present in different microenvironments.6I Gaussian curve fitting of this broad peak with two Gaussians yields two peaks centered a t 1719 & 5 and 1736 f 5 cm-' as shown in Figure 10. Surprisingly similar data have been reported earlier.20 The contribution due to the bending mode of water in the region has however been subtracted before the Gaussian curve fitting was done in order to study the carbonyl peak only. The fitted curve is found virtually indistinguishable from the measured one. Carbonyl compounds with restricted rotation exhibit two bands: one at high frequency, corresponding to cis configuration, and the other a t lower fre- quency, corresponding to gauche configuration as shown in Figure 1 1. The intensity of the law-frequency band diminishes as the microenvironments of the carbonyl group change from apolar to polar. Martin and Magid" have also reported the two bands, one at 1723 cm-' and the other a t 1740 cm-l, for the carbonyl groups of AOT molecule in the water/AOT/cyclohexane system and observed that the peak intensity changes with the increase of the water content in the system. While the frequency is a mechanical property of the molecule, the intensity depends on the change in dipole moments during intramolecular motion, and as such, the direction and magnitude of the anisotropic solvent effect may not

(61) Rao, C. N. R. In Chemical Applications of Infrared Spectroscopy; Academic Press: New York, 1963.

7416 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 Jain et al.

2 4

2 2

2 0

I 8

16

t 2 ' 4 + d

I 2 - yl

z W

;; I O

0 8

0 6

0 4

0 2

0 0 I I I I I I 4 a 12 I C 2 0 2 4 z e 32

wo - Figure 12. Variation of intensity ratio (11736/117,9) as a function of W, for the water/AOT/isooctane system.

be the same for all types of conformations, particularly when the molecule is situated at the interface between strongly polar and strongly apolar media.

A plot of the intensity ratio (I , ) of the peaks at 1736 and 1719 cm-' for the system water/AOT/isooctane at different Wo values is shown in the Figure 12. It is observed that the peak intensity ratio increases up to Wo = 10, above which it decrease up to Wo = 18-20, beyond which the intensity ratio ( I , ) remains more or

less constant. This particular behavior of the intensity ratio can be explained in the following way. In the dry micelles and those containing a very low amount of water, the ratio is very small, indicating that the intensity of the low-frequency band that is responsible for gauche conformation is very large. The micro- environment of the ester group vicinal to the sulfonic groups seems to be predominantly apolar. The surfactant molecule in this region exists largely in the monomeric state so that the polar as well as the apolar part of the molecule experiences only an apolar mi- croenvironment. With the gradual increase of water in the system, the equilibrium between the monomers and aggregates shifts more toward the aggregate side with the formation of a distinct po- lar/apolar interface. The ester group, under these circumstances, gradually experiences a polar microenvironment, and as a result, the intensity of the low-frequency band diminishes, giving rise to an increase of intensity ratio as shown in Figure 12. At about Wo = 10, the peak intensity ratio becomes maximum, beyond which it falls once again. In the region 10 < Wo < 20, we assume that due to rotational isomerism the carbonyl ester group shifts from the polar side of the interface to the apolar side so that the intensity of the low-frequency band increases once again.

Conclusions The FT-IR investigations of water/AOT/isooctane micro-

emulsion systems show that three different types of water coexist in the aqueous core of the droplets. These are bound water, free water, and trapped water. It has been concluded from the results that bound water is composed of primary hydration and secondary hydration layers. The structure of secondary hydration layer can be changed by changing the composition of the aqueous core. In the microemulsion droplets the surfactant molecules can be hy- drated to a maximum extent of 12. The studies have also revealed the phenomena of rotational isomerism of AOT molecule, which have been established earlier with different techniques.

Acknowledgment. We thank Dr. Dinesh, Head, University Science Instrumentation Centre of Delhi University, for providing the necessary instrumental facility and for his interest in the work. T.K.J. thanks the University Grants Commission, New Delhi, for providing a research fellowship.

Registry No. AOT, 577-1 1-7; isooctane, 540-84-1.