ARTICLE pubs.acs.org/JPCB
Near-Infrared Spectroscopic Study of a Water-in-Supercritical CO2 Microemulsion as a Function of the Water Content Yoshihiro Takebayashi,*,† Masanobu Sagisaka,‡ Kiwamu Sue,† Satoshi Yoda,† Yukiya Hakuta,† and Takeshi Furuya† †
Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan ‡ Faculty of Science and Technology, Hirosaki University, Bunkyo-cho 3, Hirosaki, Aomori 036-8561, Japan ABSTRACT: A water-in-supercritical CO2 microemulsion is a reverse micelle encapsulating a nanometer-size water droplet dispersed in supercritical CO2. In the microemulsion solution, water exists not only in the reverse micelle but also in the solvent CO2. For quantitative analysis of the water distribution, nearinfrared spectra of water þ CO2 and water þ surfactant þ CO2 mixtures were measured over a wide range of water/CO2 ratios from 0.1 to 1.0 wt % at 60 °C and 30.0 MPa. The stretching combination band of water was decomposed into two components, a sharp one peaked at 7194 cm1 assigned to monomeric water dissolved in CO2 and a broad one around 7000 cm1 corresponding to aggregated water in the microemulsion. Integrated molar absorptivities of these types of water were negligibly different from each other, despite the different hydrogen-bonding environments. The spectral decomposition revealed that water is distributed mainly into CO2 at water contents smaller than 0.5 wt % and then is introduced into the microemulsion after saturation of water in CO2 and full hydration of the surfactant headgroup.
1. INTRODUCTION A water-in-supercritical CO2 microemulsion (W/scCO2 microemulsion) is a reverse micelle thermodynamically stable dispersed in supercritical CO2 (TC = 31 °C, PC = 7.4 MPa, and FC = 0.468 g cm3).13 A nanometer-size water droplet is encapsulated in the micelle and stabilized by the surfactant. The W/scCO2 microemulsion has received increasing attention as a functional medium for nanoparticle synthesis, catalytic and enzymatic reactions, metal extraction, and cleaning.48 This is because the water droplet can solubilize polar and large molecules in scCO2 and because the solvent CO2 can be separated simply by depressurization after use. For a better control of these novel processes, it is essential to understand the microscopic structure of the W/scCO2 microemulsion. The water-to-surfactant molar ratio in the microemulsion, i.e., the number of water molecules encapsulated per surfactant molecule, is an important parameter to control the size of the microemulsion and characterize the solubilization capacity of the surfactant.9 For the W/scCO2 microemulsion, however, it should be noted that water molecules exist not only in the reverse micelle but also in the solvent CO2.13,10 The water molecules dissolved in CO2 cannot form droplets and are isolated from each other without hydrogen bond formation. Thus, for the determination of the “effective” or “corrected” water-to-surfactant molar ratio in the W/scCO2 microemulsion, it is necessary to clarify how many water molecules are distributed in the microemulsion and in the solvent CO2 at various water loadings. r 2011 American Chemical Society
Fourier transform infrared (FT-IR) spectroscopy was applied in our previous work for the study of the water distribution in a W/scCO2 microemulsion.11 The stretching and bending bands of water and the SdO stretching band of the surfactant were systematically investigated as functions of the water content. In the FT-IR measurements, however, we encountered several problems. First, the molar absorptivities of water and CO2 in the mid-IR region are so large that a high-pressure cell with an extremely short optical path length (ca. 0.1 mm) is needed. In addition, the strong absorption of CO2 often disturbs the water spectrum; e.g., the stretching band of monomeric water in CO2 was completely hidden by an overlapping combination band of CO2. Furthermore, the molar absorptivity of water changes markedly upon hydrogen bond formation. The molar absorptivity of dimeric water is reported to be 6 times larger than that of monomeric water in the stretching fundamental band.12 The large change in the molar absorptivity complicates the quantitative analysis of the water distribution under various hydrogenbonding conditions. Near-infrared (NIR) spectroscopy is a convenient tool to solve the problems. Since NIR absorption arises from combination and overtone vibrations, the intensity is several orders of magnitude smaller than that of the corresponding fundamental vibrations.13 This allows us to adjust the optical path length to Received: February 21, 2011 Revised: March 30, 2011 Published: April 19, 2011 6111
dx.doi.org/10.1021/jp201722f | J. Phys. Chem. B 2011, 115, 6111–6118
The Journal of Physical Chemistry B
Figure 1. Chemical structures of the applied surfactants: (a) 8FS(EO)2 and (b) FC6HC4.
Figure 2. Schematic diagram of the experimental apparatus.
several centimeters. It is also reported that the hydrogen-bonding effect on the molar absorptivity of water is much milder in the NIR region than in the mid-IR.14,15 These advantages can simplify the measurement and analysis of the NIR spectrum of the microemulsion.16,17 In fact, Eastoe et al. measured the NIR spectrum of a W/scCO2 microemulsion as a function of temperature and showed that a broad signal assigned to encapsulated water can be distinguished from a sharp one due to monomeric water in CO2.3,18 In the present study, we measure the NIR spectra of water þ CO2 and water þ surfactant þ CO2 mixtures as functions of the water content. The spectrum of the W/scCO2 microemulsion solution is decomposed into two components to perform a quantitative analysis of the water distribution in the microemulsion and in the solvent CO2. The variation in the water distribution with increasing water content is interpreted in terms of the solubility of water in CO2 and the presence of interfacial water around the surfactant headgroup. We also compare the molar absorptivities of these forms of water to discuss the hydrogen-bonding effect on the absorptivity.
2. EXPERIMENTAL METHODS 2.1. Reagents. The chemical structures of the surfactants used in this study are shown in Figure 1. 8FS(EO)2, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate, is a fluorinated dichain surfactant with a sulfonate headgroup.19 FC6HC4, sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate, is a fluorocarbonhydrocarbon hybrid dichain surfactant also with a sulfonate headgroup.20 These surfactants were synthesized and purified to 99%, as described elsewhere.19,20 The two surfactants are used as an equimolar mixture, because the mixing can prevent a transition from a microemulsion to liquid crystals at large water contents.21,22 Water was purified to the specific resistance of 18 MΩ cm by a Millipore Milli-Q system. CO2 (Showa Tansan, 99.99%) was used after drying over
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Figure 3. NIR spectrum of CO2 at 60 °C and 30.0 MPa.
activated molecular sieves 4A (Wako, 0.50 to 1.18 mm). Chloroform (Wako, 99%), n-heptane (Wako, 99%), and sodium bis (2-ethylhexyl)sulfosuccinate (AOT; Aldrich, 98%) were used as purchased. 2.2. Apparatus. A schematic diagram of our high-pressure NIR apparatus is shown in Figure 2. The W/scCO2 microemulsion was prepared in a high-pressure optical cell (Taiatsu Techno). The cell was made of SUS-316 stainless steel and can be used up to 200 °C and 30 MPa. The optical cell had a pair of sapphire windows (25 mm in diameter and 12 mm in thickness), whose optical path length l was 22.5 mm. The cell was settled in a UV/vis/NIR spectrometer (Jasco, V-570) equipped with a tungsten halogen lamp and a PbS detector. The optical cell was connected to a dual-stem three-way valve (SSI, 02-0127) via an SUS-316 stainless steel tube (0.5 mm in inner diameter and 1/16 in. in outer diameter). The total internal volume of the optical cell, the connecting tube, and the three-way valve was 5.9 cm3. The cell was heated by four cartridge heaters. The temperature in the cell was measured by a sheathed thermocouple (Chino) and controlled to (0.2 °C by a temperature controller (Shimaden, SR53). The three-way valve and the connecting tube were thermostated by a tape heater at the same temperature as that of the optical cell. 2.3. Procedure. An equimolar mixture of the surfactants, 8FS(EO)2 and FC6HC4 (4.4 105 mol each), and a given amount of water were loaded into the optical cell. After the cell was heated to 60 °C, CO2 was injected into the cell up to a pressure of 30.0 MPa with an HPLC pump (Shodex, DS-4). The pressure was measured by a strain gauge (Kyowa Electronics Instruments, PG-500KU). Under the thermodynamic condition, the density of CO2 is 0.83 g cm3 according to the equation of state,23 and the molar ratio of the surfactants to CO2 is 0.08 mol %. The mixture was stirred by a magnetic stirrer (AS ONE, Octopus S-1) for 15 min. After the stirring was stopped, NIR spectra were measured in the range of 10002500 nm (100004000 cm1) at intervals of 1 nm. The stirring and the spectroscopic measurement were repeated typically three times, until no further spectral change was observed. The water/CO2 weight ratio was varied from 0.1 to 1.0 wt % at intervals of 0.1 wt %. This corresponds to the “uncorrected” water-to-surfactant molar ratios of 3.131. The water þ 8FS(EO)2 þ FC6HC4 þ CO2 mixture is known to form a homogeneous microemulsion phase without any precipitation at each water content examined here.11 The solubility of water in CO2 at 60 °C and 30.0 MPa has been determined to be 0.35 ( 0.01 wt % by a flow method.11 Thus, the maximum water content studied (1.0 wt %) is about 3 times as high as the solubility of water in CO2. 6112
dx.doi.org/10.1021/jp201722f |J. Phys. Chem. B 2011, 115, 6111–6118
The Journal of Physical Chemistry B
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Table 1. Integrated Molar Absorptivities B10 of the 3ν3 and Fermi Tetrad Bands of CO2 at Various Densities integrated molar absorptivity, B10 (m mol1) 3ν3 band (73006700 cm1)
Fermi tetrad band (66005850 cm1)
scCO2 (0.83 g cm3)
4.5
3.1
present paper
scCO2 (0.41.0 g cm3)
4.5 ( 0.3
2.9 ( 0.2
ref 25
gaseous CO2
4.5
3.0
ref 26
Integrated molar absorptivities B of these bands were calculated for comparison with those in the literature. The decadic integrated molar absorptivity B10 is defined by Z Z Z 1 1 I0 ð~ νÞ Að~ν Þ d~ν ¼ log B10 εð~ν Þ d~ν ¼ d~ ν Cl Cl Ið~ νÞ ð1Þ where ε(ν~) is the molar absorptivity at wavenumber ν ~ and A(ν ~) is the decadic absorbance. C is the molar concentration of the ~) absorbing species, and l is the optical path length. I(ν ~) and I0(ν are the transmitted intensities with and without the absorbing species, respectively. The napierian integrated molar absorptivity Be is also commonly used in the literature and related to B10 by Z 1 I0 ð~ν Þ ln Be ð2Þ d~ν ¼ ðln10ÞB10 Cl Ið~ν Þ
Figure 4. Spectra of (a) water in CO2 and (b) water in CHCl3 as functions of the water content.
NIR spectra of water/CHCl3 solutions and water/AOT/ n-heptane microemulsions were measured at 25 °C and atmospheric pressure using a quartz cell (l = 10 mm). The spectra of water/CHCl3 solutions were measured at various water concentrations of 0.60, 0.80, and 1.00 g dm3 and those of water/AOT/ n-heptane microemulsions at water-to-surfactant molar ratios from 2.0 to 20.0 at a fixed AOT concentration of 0.030 mol dm3.
3. RESULTS AND DISCUSSION 3.1. Pure CO2. Before discussing the NIR spectrum of the W/ scCO2 microemulsion, we examine the spectrum of pure CO2 as a background and that of water dissolved in CO2. Figure 3 shows the spectrum of pure CO2 at 60 °C and 30.0 MPa. The spectrum consists of the three bands:24 (i) a single peak centered at 6954 cm1 assigned to the second overtone 3ν3 of the antisymmetric stretching vibration ν3 of the CO2 molecule, (ii) a tetrad around 6250 cm1 arising from the Fermi resonance of 3ν1 þ ν3, 2ν1 þ 2ν20 þ ν3, ν1 þ 4ν20 þ ν3, and 6ν20 þ ν3, where ν1 is the symmetric stretching mode of CO2 and ν2 is the bending mode, and (iii) a triad around 5000 cm1 due to the Fermi resonance of 2ν1 þ ν3, ν1 þ 2ν20 þ ν3, and 4ν20 þ ν3. The absorption of CO2 in the NIR region is weak enough (absorbance
