Microemulsion as a Function - American Chemical Society

Jul 8, 2008 - Hirosaki UniVersity, Bunkyo-cho 3, Hirosaki, Aomori 036-8561, Japan, and Faculty of Engineering, Tokyo. UniVersity of Science, Kagurazak...
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J. Phys. Chem. B 2008, 112, 8943–8949

8943

Fourier Transform Infrared Spectroscopic Study of Water-in-Supercritical CO2 Microemulsion as a Function of Water Content Yoshihiro Takebayashi,*,† Yasuaki Mashimo,‡ Daisuke Koike,‡ Satoshi Yoda,† Takeshi Furuya,† Masanobu Sagisaka,§ Katsuto Otake,⊥ Hideki Sakai,‡ and Masahiko Abe‡ Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan, Graduate School of Science and Technology, Tokyo UniVersity of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan, Faculty of Science and Technology, Hirosaki UniVersity, Bunkyo-cho 3, Hirosaki, Aomori 036-8561, Japan, and Faculty of Engineering, Tokyo UniVersity of Science, Kagurazaka 1-3, Shinjyuku-ku, Tokyo 162-8601, Japan ReceiVed: March 25, 2008; ReVised Manuscript ReceiVed: May 21, 2008

Fourier transform infrared (FT-IR) spectrum of water-in-supercritical CO2 microemulsion was measured at 60 °C and 30.0 MPa over a wide range of water/CO2 ratio from 0.0 to 1.2 wt % to study the distribution of water into CO2, interfacial area around surfactant headgroup, and core water pool. The microemulsion was stabilized by sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate [8FS(EO)2] equimolarly mixed with sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate [FC6HC4] or with poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether [TMN-6]. The signal area of the O-H stretching band of water suggested that the number of water molecules in the microemulsion increases linearly with the water/CO2 ratio, except for a slow initial increase below 0.4 wt % due to a part of water dissolved in CO2. The amount of water in CO2 was evaluated by decomposing the bending band of water into two components, one at lower frequency ascribed to water in CO2 and the other at higher frequency to water in the microemulsion. The decomposition confirmed that CO2 is saturated with water at the water content of 0.4 wt %. It was also revealed, from the symmetric SdO stretching frequency of the surfactant, that the sulfonate headgroup is completely hydrated at the water/CO2 ratio of 0.4-0.5 wt %. The results demonstrated that water is introduced preferentially into CO2 and the interfacial area at small water content, and then is loaded into the micelle core after the saturation of CO2 with water and the full hydration of the surfactant headgroup. 1. Introduction Water-in-supercritical CO2 microemulsion (W/scCO2 microemulsion) is a reverse micelle encapsulating a nanometer-size water droplet and dispersed in supercritical CO2 (TC ) 31 °C, PC ) 7.4 MPa, and FC ) 0.468 g cm-3).1–3 W/scCO2 microemulsion has attracted increasing attention as a novel and functional medium for nanoparticle synthesis, catalytic and enzymatic reactions, metal extraction, and cleaning, because the water droplet can solubilize polar and large molecules in scCO2, and the solvent CO2 can be easily separated by depressurization after use.2,4–7 To control the new processes, it is essential to understand the microscopic structure of W/scCO2 microemulsion. From the microscopic viewpoint, water molecules in W/scCO2 microemulsion solution are categorized into three: (i) interfacial water bound by the surfactant headgroup, (ii) core water free from the surfactant, and (iii) water dissolved in the background CO2 solvent.5 Because the three types of water play different roles from each other in the applications, it is important to clarify how much water is partitioned into each of the three regions at various water loadings, although it has been rarely explored. For example, the amount of water dissolved in CO2 is necessary for the calculation of “effective” or “corrected” * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Graduate School of Science and Technology, Tokyo University of Science. § Faculty of Science and Technology, Hirosaki University. ⊥ Faculty of Engineering, Tokyo University of Science.

water/surfactant molar ratio W0C, which characterizes the solubilization capacity of W/scCO2 microemulsion:8

WC0 )

NW - NWinCO2 NS

(1)

where NW and NS are the numbers of water and surfactant molecules in the solution, respectively. We should note, however, that NWinCO2, the number of water molecules dissolved in CO2, could not be obtained without the assumptions that the background CO2 is saturated with water and that the solubility of water in CO2 is negligibly affected by the presence of the microemulsion. Fourier transform infrared (FT-IR) spectroscopy is a powerful tool to distinguish the local environments of water molecules in microemulsion, because it is highly sensitive to water-water and surfactant-water interactions.9–16 Since 1994, FT-IR spectroscopy has been often applied to W/scCO2 microemulsion, where the O-H or O-D stretching band of water as well as the bending band were measured to demonstrate the existence of water pool in the microemulsion.1,4,5,17,18 Most of the researches, however, have been limited to a qualitative level. We expect that quantitative analysis of the FT-IR spectrum of W/scCO2 microemulsion as a function of water content will provide fruitful information complementarily to other spectroscopic methods, e.g., small-angle neutron scattering (SANS) measurements of the droplet size,19–23 NMR studies of the dynamics of water and surfactant molecules,24–27 and UV-visible ones of the polarity and acidity in the water droplet.1,4,5,22,28–30

10.1021/jp802578y CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

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Figure 2. Schematic diagram of experimental apparatus. Figure 1. Chemical structures of the surfactants: (a) 8FS(EO)2, (b) FC6HC4, and (c) TMN-6.

Surfactants specially designed and synthesized are needed for W/scCO2 microemulsion, because most of the conventional surfactants used for water-in-oil (W/O) microemulsion, such as Aerosol-OT (AOT), are inactive in scCO2.2,3 Development of new surfactant suited for W/scCO2 microemulsion has been extensively performed by many research groups. We have recently shown that a fluorinated AOT-analogue surfactant, 8FS(EO)2,canefficientlystabilizetheW/scCO2 microemulsion.18,31,32 Although 8FS(EO)2 tends to form a liquid crystal precipitation at large water content, we found that mixing of 8FS(EO)2 with other surfactants, such as FC6HC4 and TMN-6, can prevent the liquid crystal formation,33 probably due to disordering of 8FS(EO)2 by the guest molecule.34 Here we measure the FTIR spectrum of W/scCO2 microemulsion stabilized by the surfactant mixtures. In the present work, we investigate the FT-IR spectrum of the W/scCO2 microemulsion over a wide range of water/CO2 ratio from 0.0 to 1.2 wt %. Not only the O-H stretching and bending bands of water but also the SdO stretching band of the sulfonate headgroup of the surfactant are carefully examined as a function of water content to clarify the distributions of water into CO2, interfacial area, and core water pool. The partition of water was interpreted in terms of the solubility of water in CO2 and the hydration number of the sulfonate headgroup. From the SdO stretching frequency, we also demonstrate the homogeneous mixing of the surfactants, 8FS(EO)2 and TMN-6, at the micelle surface. 2. Experimental Methods 2.1. Reagents. Chemical structures of the surfactants used in this study are summarized in Figure 1. 8FS(EO)2, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate, is a fluorinated dichain surfactant with a sulfonate headgroup, having molecular weight of 1112 g mol-1.35 FC6HC4, sodium 1oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate, is a fluorocarbon-hydrocarbon hybrid dichain surfactant with a sulfonate headgroup, having molecular weight of 596 g mol-1.36 The two surfactants, 8FS(EO)2 and FC6HC4, were synthesized in our laboratory and were purified to >99%, as previously described.35,36 Tergitol TMN-6, poly(ethylene glycol) 2,6,8trimethyl-4-nonyl ether, is a nonionic surfactant with a highly branched hydrocarbon chain and a poly(ethylene glycol) headgroup. TMN-6 (Sanyo Trading) was distilled under vacuum at 100 °C and was dried over anhydrous MgSO4. The amount of residual water after the drying was measured by 1H NMR spectroscopy. Detail of the 1H NMR measurements is described in the Supporting Information. The molar ratio of the residual water to TMN-6 was determined to be 0.25 and was confirmed

to have a negligible effect on our FT-IR measurement. Average number of the ethylene oxide groups (nj ) 7.8) and the corresponding number-averaged molecular weight (Mn ) 529 g mol-1) were also determined from the 1H NMR spectrum. Water was purified to the specific resistance of 18 MΩ cm by a Millipore Milli-Q system. CO2 (Showa Tansan, 99.99%) was used without purification. 2.2. Apparatus. Schematic diagram of our high-pressure FTIR apparatus is illustrated in Figure 2. W/scCO2 microemulsion was prepared in a variable-volume view cell and was circulated into an optical cell for FT-IR spectroscopic measurement. Water was injected into the circulation line with a 6-port valve. The circulation/injection system enabled us to measure the FT-IR spectrum of W/scCO2 microemulsion over a wide range of water content in a single experiment. The FT-IR optical cell had a pair of ZnSe windows (8 mm in diameter and 10 mm in thickness). The optical path length l was calculated to be 0.10 mm from the number of interference fringes, N, observed between the wavenumbers ν˜ 1 and ν˜ 2 by the following equation:37

l)

N 2n(ν ˜2 - ˜ ν1)

(2)

where n is the refractive index of medium in the cell. The optical cell was made of SUS-316 stainless steel and can be used up to 30 MPa and 80 °C. Internal volume of the optical cell was ca. 0.9 cm3. The temperature of the optical cell was controlled by flowing thermostated water. The optical cell was placed in an FT-IR spectrometer (JASCO, FT/IR-620) equipped with a Globar infrared source and an MCT detector cooled by liquid nitrogen. The variable-volume view cell (Tama Seiki) has a Pyrex glass window (24 mm in diameter and 20 mm in thickness) for visual observation of the phase behavior by a CCD camera (Olympus, CS230B). The view cell was made of SUS-316 and can be used up to 49 MPa and 100 °C. A piston was installed inside the view cell to vary the pressure without changing the composition of water + surfactant + CO2 mixture. Internal volume of the view cell was ca. 40 cm3 in the presence of the piston. The view cell was heated in an air bath (Yamato, DN410H) thermostated within (1 °C. Temperature of the view cell was monitored by a platinum resistance thermometer (Nimblox). The pressure was measured by a strain gauge (Kyowa Electronics Instruments, PG-500KU), and was controlled within (0.1 MPa. The two cells were connected with each other by a circulation line (SUS-316, 1/16 in. in outer diameter and 0.5 mm in inner diameter). The circulation line involved a circulation pump (Nihon Seimitsu Kagaku, NP-S-321) and a 6-port valve (Valco Instrument, C6W) equipped with a 25 µL sample loop. An inline filter (GL Sciences, 2 µm pore) was inserted in the

FT-IR Study of Water-in-Supercritical CO2 Microemulsion

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Figure 3. Comparison of (a) the FT-IR spectrum of pure CO2 and (b) that of CO2 dissolving 0.2 wt % water at 60 °C and 30.0 MPa.

circulation line to prevent undissolved surfactant from flowing into the optical cell. A part of the circulation line outside of the air bath was thermostated by a tape heater. 2.3. Procedure. An equimolar mixture of the surfactants, 8FS(EO)2 + FC6HC4 (2.27 × 10-4 mol each) or 8FS(EO)2 + TMN-6 (2.27 × 10-4 mol each), was loaded in the view cell. After the temperature was raised to 60 °C, CO2 was pumped into the front room of the view cell as well as the circulation line and the FT-IR optical cell up to 13.0 MPa with an HPLC pump (Shodex, DS-4). Because the total internal volume was 50 cm3 and the density of CO2 at 60 °C and 13.0 MPa was calculated to be 0.50 g cm-3 from the equation of state,38 the mass of CO2 loaded was 25 g (0.568 mol). The CO2 was pressurized by the piston up to the experimental pressure of 30.0 MPa, where the density of CO2 was 0.83 g cm-3. Water was injected into the view cell with the 6-port valve and the circulation pump. The amount of water added (25 mg or 1.39 × 10-3 mol) corresponds to 0.1 wt % in terms of the water/CO2 weight ratio and to 3.06 in terms of water/surfactant molar ratio. The water + surfactant + CO2 mixture was stirred by a magnetic stirrer until phase equilibrium was attained (typically 30 min). After stopping the stirrer and the circulation pump, FT-IR spectrum was measured with nominal resolution of 4 or 0.5 cm-1, and the signal was accumulated 32 times. The water/CO2 weight ratio was varied from 0 to 1.2 wt % at intervals of 0.1 wt %. Because the surfactant/CO2 molar ratio was fixed at 0.08 mol %, the water/surfactant molar ratio ranged from 0 to 37. Solubility of water in CO2 at the experimental condition (60 °C and 30.0 MPa) was determined to be 0.35 ( 0.01 wt % by the flow method. Detail of the solubility measurement is described in the Supporting Information. 3. Results and Discussion 3.1. Spectra of Pure CO2 and Water Dissolved in CO2. Before discussing the FT-IR spectrum of W/scCO2 microemulsion, we examine the spectrum of pure CO2 as a background and that of water dissolved in CO2. Figure 3a shows the spectrum of pure CO2 at 60 °C and 30.0 MPa. The CO2 spectrum had two strong bands; One around 2350 cm-1 is assigned to antisymmetric stretching vibration (ν3) of CO2. The other around 3650 cm-1 arises from the combination of the antisymmetric vibration with symmetric stretching mode (ν1 + ν3) and that with bending overtone (2ν02 + ν3).39 Upon addition of water into CO2, as shown in Figure 3b, a new sharp signal appeared at 1610 cm-1 with a full width at half-maximum (fwhm) of 25 cm-1. The new peak is ascribed to the bending mode (ν2) of H2O.1,5,17 No signal was observed, however, for the O-H stretching modes (ν1, ν3) of water in

Figure 4. O-H stretching band of water encapsulated in the W/scCO2 microemulsion as a function of water content for the (a) 8FS(EO)2 + FC6HC4 and (b) 8FS(EO)2 + TMN-6 surfactant mixtures.

CO2. This is because the O-H stretching band of water was completely hidden by the strong combination band of CO2 (3700-3600 cm-1).5,40 In the case of the W/O microemulsion, the symmetric O-H stretching band of monomeric water in the oil phase (e.g., carbon tetrachloride) is known to appear at ca. 3630 cm-1, whereas it drastically shifts to lower frequencies (3600-3000 cm-1) upon hydrogen bond formation in the microemulsion.15 The absence of O-H stretching signal in the hydrogen-bonded water region indicates that the water molecules dissolved in CO2 are isolated from each other.41 3.2. Phase Behavior of Water + Surfactant + CO2 Mixture. Phase behavior of the water + surfactant + CO2 mixture in the view cell was monitored with increasing water/ CO2 ratio from 0.0 to 1.2 wt % at intervals of 0.1 wt %. The measurement was made for the two equimolar surfactant mixtures, 8FS(EO)2 + FC6HC4 and 8FS(EO)2 + TMN-6. For 8FS(EO)2 + FC6HC4, a part of the surfactant remained undissolved in the absence of water, and thus we could not measure the FT-IR spectrum at the dry micelle state. Upon addition of 0.1 wt % water, an optically clear solution was obtained without any precipitation, suggesting a formation of W/scCO2 microemulsion. The solution was transparent and homogeneous up to the water/CO2 ratio of 1.1 wt %, but a small amount of water precipitated at the water content of 1.2 wt %. For 8FS(EO)2 + TMN-6, a clear and homogeneous solution was obtained even in the absence of water. This enabled us to measure the surfactant spectrum without water. The solution was transparent and homogeneous up to 1.0 wt %, and a precipitation of water was observed at 1.1 wt %. 3.3. O-H Stretching Band of Water. FT-IR spectrum of the W/scCO2 microemulsion in the O-H stretching region is shown in Figure 4. The O-H stretching band of water was partly overlapped with surfactant signals. The surfactant signals were already subtracted in the figure as follows: For 8FS(EO)2 + FC6HC4, C-H stretching bands of the methyl and methylene groups appeared at 3000-2800 cm-1 10,14,16 but were easily separated from the water signal (>3000 cm-1) when the water content was small enough. For 8FS(EO)2 + TMN-6, the O-H stretching vibration of TMN-6 was also observed at 3600-3400 cm-1. The surfactant signals were subtracted by calculating difference spectrum from the dry micelle state, assuming that the surfactant spectrum is independent of the water content.

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Figure 5. Signal area of the O-H stretching band of water encapsulated in the W/scCO2 microemulsion as a function of water content for the (a) 8FS(EO)2 + FC6HC4 and (b) 8FS(EO)2 + TMN-6 surfactant mixtures.

A broad and strong band was observed in the frequency range 3600-2800 cm-1, in marked contrast to the monomeric water spectrum shown in Figure 3. The broad O-H signal demonstrates the formation of W/scCO2 microemulsion.5,17 Intensity of the O-H signal monotonically increased with increasing water content, suggesting an increase in the number of water molecules encapsulated in the microemulsion. The spectrum was invariant, however, at further water loading above 1.2 wt % for 8FS(EO)2 + FC6HC4 and above 1.1 wt % for 8FS(EO)2 + TMN-6. This is consistent with the precipitation of water observed in the view cell. The consistency between the FT-IR spectrum and the phase behavior confirms that the structure of the W/scCO2 microemulsion was negligibly influenced during the circulation between the FT-IR optical cell and the view cell, e.g., by a capillary effect in the circulation line and a surface effect of the ZeSe windows. Signal area of the O-H stretching band was calculated by integrating the absorbance in the frequency range 3600-2800 cm-1. The O-H signal area was plotted in Figure 5 as a function of water/CO2 ratio. The signal area increased almost linearly withthewatercontent,asoftenreportedforW/Omicroemulsion,9,12,16 except for a slow increase below 0.4 wt %. The slow initial increase can be explained mainly by the presence of monomeric water dissolved in CO2, which does not contribute to the signal area in the hydrogen-bonded water region. The amount of water dissolved in CO2 is analyzed later in more detail from the bending band of water. Variation in the spectral shape with the water content was investigated by normalizing the O-H spectrum with respect to the signal area: i.e., the absorbance of the O-H spectrum shown in Figure 4 was divided by the signal area plotted in Figure 5. The normalized spectrum is shown in Figure 6. With increasing water content, the normalized absorbance decreased at frequencies higher than ca. 3400 cm-1, whereas it increased at lower frequencies. In the case of W/O microemulsion, the highfrequency component is ascribed to interfacial water molecules adjacent to the surfactant headgroup, and the low-frequency one is assigned to core water molecules strongly hydrogen bonded with each other.9,10,12,15,16 The relative increase in the number of core water molecules to that of interfacial ones suggests an increase in the size of each microemulsion droplet. This is in qualitative agreement with the SANS result that the droplet

Takebayashi et al.

Figure 6. O-H stretching spectrum of water normalized with respect to the signal area as a function of water content for the (a) 8FS(EO)2 + FC6HC4 and (b) 8FS(EO)2 + TMN-6 surfactant mixtures.

Figure 7. Bending band of water as a function of water content (a) for the 8FS(EO)2 + FC6HC4 surfactant mixture including the surfactant signals and (b) for the 8FS(EO)2 + TMN-6 surfactant mixture after separation of the surfactant signals.

radius of W/CO2 microemulsion is an increasing function of the water/surfactant molar ratio.3,20–22 It is expected that FT-IR and SANS measurements at the same experimental condition (surfactant, composition, temperature, and pressure) will further clarify the correlation between the two sets of data at a quantitative level. 3.4. Bending Band of Water. The spectrum in the water bending region is shown in Figure 7. It should be noted that the bending band of water was strongly overlapped with CdO stretching band of the surfactant at 1800-1650 cm-1, as shown in Figure 7a.5 For 8FS(EO)2 + TMN-6, however, the surfactant signals can be easily removed by subtracting the spectrum at the dry micelle state. Difference spectrum after the subtraction is shown in Figure 7b, together with the surfactant spectrum. The bending band consisted of at least two components. At the water content of 0.1 wt %, a sharp peak was observed at 1610 cm-1. Because the peak frequency is equal to that of monomeric water spectrum shown in Figure 3b, the peak was ascribed to water dissolved in CO2. With increasing water content, a broad shoulder appeared on the high-frequency side.

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Figure 8. Decomposition of the bending band of water into two components at the water contents of (a) 0.3, (b) 0.6, and (c) 1.0 wt % for the 8FS(EO)2 + TMN-6 surfactant mixture.

Figure 9. (a) Peak frequency, (b) bandwidth, and (c) signal area of the high-frequency (b) and low-frequency (O) components of the bending band of water as functions of water content for the 8FS(EO)2 + TMN-6 surfactant mixture.

The broad shoulder at higher frequency was assigned to water solubilized in the W/scCO2 microemulsion,1,5,17 because the bending band of water is known to show a blue shift upon hydrogen bond formation.42 Here let us decompose the bending band into two component. Each component was fit to a Gauss-Lorentz hybrid function with a peak height of A, a peak frequency of ν˜ 0, and a fwhm of 2δ as follows:

reflecting a variety of local environment in the microemulsion. The band area increased almost linearly with the water content, except for a slow initial increase below ca. 0.4 wt %. The area-water content profile is similar to that for the O-H stretching band of water shown in Figure 5, confirming that the high-frequency component arises from water molecules solubilized in the microemulsion. 3.5. Symmetric SdO Stretching Band of Surfactant. For the W/O microemulsion stabilized by AOT, the symmetric SdO stretching band of the sulfonate headgroup has been often studied to gain microscopic insight into the structure of interfacial water, because the SdO stretching frequency is sensitive to surfactant-water interaction at the interfacial area.9,11,13 For W/scCO2 microemulsion, however, there has been no research on the SdO band, except for an FT-IR study of W/AOT/scC2H6 by Ikushima, et al.43 Here we examine the SdO stretching frequency of the sulfonate headgroup, which is involved in 8FS(EO)2 and FC6HC4, as a function of water content. The symmetric SdO stretching band of the sulfonate group in the W/scCO2 microemulsion is shown in Figure 10, where the result for pure 8FS(EO)2 is also described for comparison. With increasing water content, the SdO stretching band shifted to lower frequency and narrowed for all the surfactants, as observed for the W/O microemulsion.9,11,13 The red shift is known to occur upon hydration of the surfactant sulfonate group by interfacial water molecules. The peak frequency of the SdO stretching band is plotted in Figure 11 as a function of water content. The peak frequency decreased with increasing water content up to 0.4-0.5 wt %, where the frequency shift amounted to ca. 5 cm-1. At larger water content, however, no further frequency shift was observed. This means that the hydration of the sulfonate headgroup completed at the water/CO2 ratio of 0.4-0.5 wt % (or at the

{ [

f(ν ˜) ) A · φG · exp -

(ν ˜-˜ ν0)2 δ2

]

· ln 2

+ (1 - φG) ·

δ2 (ν ˜-˜ ν0)2 + δ2

}

(3)

where φG is the fraction of the Gaussian function. Least-squares fit of the bending spectrum was carried out for 8FS(EO)2 + TMN-6. The results at the water contents of 0.3, 0.6, and 1.0 wt % are shown in Figure 8. The peak frequency, fwhm, and signal area of each component are plotted in Figure 9 as functions of water content. The low-frequency component was centered at ca. 1610 cm-1 with a fwhm of 25-30 cm-1. The peak frequency and the bandwidth were weakly dependent on the water content. This is consistent with the assignment to monomeric water molecules isolated from each other. The signal area increased with increasing water content up to ca. 0.4 wt % but was almost invariant at the larger water/CO2 ratio. This indicates that CO2 was saturated with water at the water/CO2 ratio of 0.4 wt %. For the high-frequency component, in contrast, the peak frequency drastically increased from 1620 to 1643 cm-1 with increasing water content, probably due to hydrogen bond formation between water molecules in the microemulsion. The bandwidth also showed a marked increase from 55 to 80 cm-1,

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Takebayashi et al. water in CO2 is hardly influenced by the presence of the microemulsion. Finally, we should explain why the SdO stretching frequency for the 8FS(EO)2 + TMN-6 mixture was much lower than those for pure 8FS(EO)2 and the 8FS(EO)2 + FC6HC4 mixture at every water content. The low-frequency shift is probably due to hydrogen bond formation between 8FS(EO)2 and TMN-6. O-H proton of TMN-6 can form a hydrogen bond with a sulfonate oxygen of neighboring 8FS(EO)2, whereas no such surfactant-surfactant hydrogen bond can be formed for pure 8FS(EO)2 or the 8FS(EO)2 + FC6HC4 mixture. The surfactantsurfactant hydrogen bond, in addition to the surfactant-water one, resulted in the decrease in the SdO stretching frequency. The frequency shift due to the surfactant-surfactant hydrogen bond (ca. 3 cm-1) was comparable to that upon the hydration of sulfonate group (5 cm-1), suggesting that the two surfactants, 8FS(EO)2 and TMN-6, are homogeneously mixed with each other at the W/CO2 interface without any microscopic phase separation.34 4. Conclusion

Figure 10. Symmetric SdO stretching band of the surfactant as a function of water content for (a) 8FS(EO)2, (b) 8FS(EO)2 + FC6HC4, and (c) 8FS(EO)2 + TMN-6.

Figure 11. Peak frequency of the symmetric SdO stretching band of the surfactant as a function of water content for 8FS(EO)2 (0), 8FS(EO)2 + FC6HC4 (O), and 8FS(EO)2 + TMN-6 (b).

water/surfactant molar ratio of 12-15). After the complete hydration of the surfactant headgroup at 0.4-0.5 wt %, where CO2 is also saturated with water, water molecules are loaded selectively into the micelle core, resulting in the swelling of the microemulsion. The number of water molecules required for the full hydration per sulfonate headgroup in the W/scCO2 microemulsion (12-15) was much larger than that reported for W/O microemulsion (3-5).9,11,13 The excess water molecules are attributed to water dissolved in CO2, because in the case of the W/O microemulsion water is negligibly soluble in the oil phase (e.g., toluene, heptane, and carbon tetrachloride). In fact, the amount of excess water (ca. 10) is almost equal to the solubility of water in CO2 at the experimental condition (0.35 wt %, which corresponds to 10.7 in the water/surfactant molar ratio). This supports that CO2 is already saturated with water at the complete hydration of the surfactant headgroup and suggests that the solubility of

FT-IR measurement was carried out for W/scCO2 microemulsion stabilized by the surfactant mixtures, 8FS(EO)2 + FC6HC4 and 8FS(EO)2 + TMN-6. Variations in the O-H stretching band of water, the bending band of water, and the symmetric SdO stretching band of the surfactant with increasing water/CO2 ratio from 0.0 to 1.2 wt % were studied to clarify how water molecules were distributed into the three regions: (i) monomeric water dissolved in CO2, (ii) interfacial water bound by the surfactant headgroup, and (iii) core water free from the surfactant. Water was introduced preferentially into CO2 and the interfacial area, until CO2 was saturated with water at 0.4 wt % and the surfactant headgroup was fully hydrated at 0.4-0.5 wt %. The number of water molecules required for the full hydration per surfactant headgroup (12-15) was much larger than that for W/O microemulsion (3-5), the excess water being due to the solubility of water in CO2 (0.35 wt % or 10.7 in terms of water/surfactant molar ratio). Thereafter, water was loaded into the micelle core, resulting in an increase in the size of each microemulsion droplet. At the maximum water content (1.2 wt % or 37 in terms of water/surfactant molar ratio), the number of water molecules in the micelle core per surfactant molecule reached 22-25 and was shown to be much larger than those in CO2 (ca. 11) and the interfacial area (3-5). The quantitative understanding of the distributions of water into CO2 and microemulsion will facilitate the development in processes using W/scCO2 microemulsion. Supporting Information Available: (i) Determinations of the average molecular weight of TMN-6 and the amount of residual water after drying by 1H NMR spectroscopy and (ii) measurement of the solubility of water in supercritical CO2 by flow method. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (2) Eastoe, J.; Gold, S. Phys. Chem. Chem. Phys. 2005, 7, 1352. (3) Eastoe, J.; Gold, S.; Steytler, D. C. Langmuir 2006, 22, 9832. (4) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (5) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399.

FT-IR Study of Water-in-Supercritical CO2 Microemulsion (6) Kometani, N.; Toyoda, Y.; Asami, K.; Yonezawa, Y. Chem. Lett. 2000, 682. (7) Eastoe, J.; Hollamby, M. J.; Hudson, L. AdV. Colloid Interface Sci. 2006, 128, 5. (8) Harrison, K.; Goveas, J.; Johnston, K. P.; Orear, E. A. Langmuir 1994, 10, 3536. (9) MacDonald, H.; Bedwell, B.; Gulari, E. Langmuir 1986, 2, 704. (10) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. (11) Christopher, D. J.; Yarwood, J.; Belton, P. S.; Hills, B. P. J. Colloid Interface Sci. 1992, 152, 465. (12) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (13) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1995, 11, 738. (14) Li, Q.; Weng, S. F.; Wu, J. G.; Zhou, N. F. J. Phys. Chem. B 1998, 102, 3168. (15) Profio, P. D.; Germani, R.; Onori, G.; Santucci, A.; Savelli, G.; Bunton, C. A. Langmuir 1998, 14, 768. (16) Brubach, J. B.; Mermet, A.; Filabozzi, A.; Gerschel, A.; Lairez, D.; Krafft, M. P.; Roy, P. J. Phys. Chem. B 2001, 105, 430. (17) Loeker, F.; Marr, P. C.; Howdle, S. M. Colloid Surf. A-Physicochem. Eng. Asp. 2003, 214, 143. (18) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kitiyanan, B.; Kondo, Y.; Yoshino, N.; Takebayashi, K.; Sakai, H.; Abe, M. Langmuir 2003, 19, 220. (19) Eastoe, J.; Bayazit, Z.; Martel, S.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 1423. (20) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934. (21) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980. (22) Lee, C. T.; Psathas, P. A.; Ziegler, K. J.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2000, 104, 11094. (23) Lee, C. T.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2001, 105, 3540. (24) Fremgen, D. E.; Smotkin, E. S.; Gerald, R. E.; Klingler, R. J.; Rathke, J. W. J. Supercrit. Fluids 2001, 19, 287. (25) Nagashima, K.; Lee, C. T.; Xu, B.; Johnston, K. P.; DeSimone, J. M.; Johnson, C. S. J. Phys. Chem. B 2003, 107, 1962. (26) Xu, B.; Lynn, G. W.; Guo, J.; Melnichenko, Y. B.; Wignall, G. D.; McClain, J. B.; DeSimone, J. M.; Johnson, C. S. J. Phys. Chem. B 2005, 109, 10261.

J. Phys. Chem. B, Vol. 112, No. 30, 2008 8949 (27) Thurecht, K. J.; Hill, D. J. T.; Whittaker, A. K. J. Supercrit. Fluids 2006, 38, 111. (28) Holmes, J. D.; Ziegler, K. J.; Audriani, M.; Lee, C. T.; Bhargava, P. A.; Steytler, D. C.; Johnston, K. P. J. Phys. Chem. B 1999, 103, 5703. (29) Liu, J. C.; Ikushima, Y.; Shervani, Z. J. Supercrit. Fluids 2004, 32, 97. (30) Shervani, Z.; Liu, J. C.; Ikushima, Y. Chem. Lett. 2004, 33, 280. (31) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Langmuir 2003, 19, 8161. (32) Sagisaka, M.; Fujii, T.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2004, 20, 2560. (33) Sagisaka, M.; Fujii, T.; Koike, D.; Yoda, S.; Takebayashi, Y.; Furuya, T.; Yoshizawa, A.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2007, 23, 2369. (34) Mixed Surfactant Systems, 2nd ed.; Abe, M., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 2005. (35) Yoshino, N.; Komine, N.; Suzuki, J.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (36) Yoshino, N.; Hamano, K.; Omiya, Y.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (37) Bulgarevich, D. S.; Otake, K.; Sako, T.; Sugeta, T.; Takebayashi, Y.; Kamizawa, C.; Shintani, D.; Negishi, A.; Tsurumi, C. J. Chem. Phys. 2002, 116, 1995. (38) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (39) Buback, M.; Schweer, J.; Tups, H. Z. Naturforsch., A: Phys. Sci. 1986, 41, 505. (40) Substitution of H2O with D2O can prevent the overlap of the stretching band of monomeric water (ca. 3630 cm-1 for H2O and ca. 2650 cm-1 for D2O) with the combination band of CO2 (3700-3600 cm-1), as reported in refs 4, 5, and 18. It should be noted, however, that the substitution causes another interference; O-D stretching band of hydrogen-bonded water (2600-2300 cm-1) is partly hidden by the antisymmetric stretching band of CO2 (2500-2200 cm-1). (41) Oparin, R.; Tassaing, T.; Danten, Y.; Besnard, M. J. Chem. Phys. 2004, 120, 10691. (42) Zilles, B. A.; Person, B. J. Chem. Phys. 1983, 79, 65. (43) Ikushima, Y.; Saito, N.; Arai, M. J. Colloid Interface Sci. 1997, 186, 254.

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