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Investigation on the solubilization of organic dyes and micro-polarity in AOT water-in-CO2 microemulsions with fluorinated co-surfactant by using UV-Vis spectroscopy
J. of Supercritical Fluids 32 (2004) 97–103
Investigation on the solubilization of organic dyes and micro-polarity in AOT water-in-CO2 microemulsions with fluorinated co-surfactant by using UV-Vis spectroscopy Juncheng Liu a,b , Yutaka Ikushima a,b,∗ , Zameer Shervani a,b a
Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan b CREST, Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi 332-0012, Japan Received in revised form 10 February 2004; accepted 10 February 2004
Abstract It was found that organic dyes such as thymol blue, dimidium bromide and methyl orange which are not soluble in pure supercritical CO2 , could be conveniently solubilized in AOT water-in-CO2 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-1-pentanol as co-surfactant. The solubilities of these organic dyes in the microemulsions were measured successfully by using a UV-Vis spectroscopy method newly established in our laboratory; besides that, for a given temperature, a critical micelle pressure (CMP) at which formation of AOT water-in-CO2 reverse micelles starts, was determined in term of the effect of pressure on the absorption intensity of the organic dyes in the microemulsions. Furthermore, the micro-polarity environment of the AOT water-in-CO2 reverse microemulsions was investigated systematically according to the shift of solvatochromic probes such as methyl orange and dimidium bromide with varying water content by using UV-Vis spectroscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical CO2 ; Microemulsions; Solubilization; Micro-polarity; UV-Vis spectroscopy
1. Introduction Conventional dyeing and subsequent washing processes are generally performed in water-based dye-baths. However, waste aqueous effluent containing colored compounds and concentrated electrolytes cause serious environmental problems. Furthermore, both washing step and drying step are essential in the conventional dyeing processes, leading a large amount of energy to be consumed. In order to overcome this problem, new concepts of avoiding the use of water are being evaluated. Recently, the supercritical fluid dyeing process (FDP) has drawn significant attention. Supercritical carbon dioxide (SC CO2 ) is one of the most commonly used supercritical fluids (SCFs) as is naturally abundant, non-flammable, essentially non-toxic, and the least expensive solvent after water. Moreover, CO2 attains the supercritical state at ambient temperature (Tc = 31 ◦ C) and a relatively moderate pres∗ Corresponding author. Tel.: +81-22-237-5211; fax: +81-22-237-5224. E-mail address: [email protected] (Y. Ikushima).
0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2004.02.006
sure (Pc = 73.8 bar). Therefore, SC CO2 dyeing process has been observed as a novel and environmentally friendly technique that would expected to be an attractive alternative to pollution generating and energy-consuming conventional wet-dyeing process. However, SC CO2 is a poor solvent of the hydrophilies because of its very low value of dielectric constant, ε, polarizability per volume, ␣/, and not strongly engaging in van der Waals interactions. So unlike SC CO2 -soluble dyes, water-soluble organic dyes such as thymol blue (TB), dimidium bromide (DB) and methyl orange (MO) do not dissolve in SC CO2 . In the past decade, several approaches have been explored to enhance the solubility of polar substance in SC CO2 [1–3]. For example, the addition of polar entrainer such as alcohols or acetone can increase the polarity, and therefore the solvent power of SC CO2 . However, water-soluble dyes are still insufficiently soluble in this SC CO2 entrainer medium in the dyeing process [4]. Our strategy is to dissolve the water-soluble dyes in thermodynamically stable and optically transparent water-in-CO2 (W/C) reverse microemulsions which can provide nano-sized polar micro-water domains as sites for solubilization of the hydrophilic dye molecules.
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Fig. 1. The structure of Organic dyes and AOT used in this study.
Preliminary works on microemulsions in SC CO2 have indicated that common surfactants used in hydrocarbon solvents are generally not suitable in SC CO2 [5–7]. Certain fluorocarbon [8–10] and silicon-containing surfactants [11,12] are known to be CO2 soluble, and several fluoro-surfactants such as an ammonium carboxylate perfluoropolyether, CF3 O(CF2 CF(CF3 )O)3 CF2 COO− NH4 + (PFPE), fluorinated analogues of AOT and phosphate fluoro-surfactants enable significant water uptake within a continuous CO2 phase, through the formation of W/C microemulsions [13–17]. However, all of these surfactants explored often require specialized synthesis and therefore remain expensive, which make it urgent to explore some water-in-CO2 microemulsions stabilized by low cost commercial surfactant. Certain surfactants, such as commonly used surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT, see Fig. 1) has been shown to form reverse micelles in conventional organic solvent [18] and supercritical alkane [19–21]. However, AOT was completely insoluble in SC CO2 , [7] restricting its wide application in the environmentally benign CO2 . Recently, Hutton et al. [22] proposed that AOT could be solubilized in SC CO2 with ethanol (15–17 mol%) or
1-pentanol (10 mol%) as co-solvent and form AOT reverse microemulsions in SC CO2 . However, a large quantity of co-solvent was required in the system. Subsequently, Wai and co-workers [23] reported that AOT could form reverse microemulsions with perfluoropolyether-phosphate ether (PFPE-PO4 ) as co-surfactant, and this system has been used as nano-reactor for preparing various nanoparticles [24,25]. However, the PFPE-PO4 is also an expensive synthetic chemical and difficult to obtain commercially. In summary, either expensive surfactant or a large amount of co-surfactant was absolutely necessary for the W/C microemulsions so far reported, hindering the industrial application of these microemulsions. In order to overcome this obstacle, 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-pentanol) was first attempted to be used as co-surfactant for promoting the formation of AOT reverse microemulsions in SC CO2 for the reason that F-pentanol is inexpensive and contains ‘CO2 -philic’ fluorinated alkane chain exhibiting very low (much less than zero) dipolarity/polarizability parameters [8]. Thus the addition of only a small quantity of F-pentanol (1.2 mol%) can successfully promote the formation of the AOT W/C reverse microemulsions.
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Solubility data of hydrophilic organic dyes in SC CO2 microemulsion system are of interest for the optimization of SC CO2 microemulsion dyeing technique. In this study, a simple and accurate method was established to measure the solubilities of the organic dyes such as TB, DB, and MO in AOT W/C reverse microemulsions with F-pentanol as co-surfacant by using UV-Vis spectroscopy. In addition, to our knowledge, critical micelle pressure (CMP) as a new concept has not been defined in the fields of combination of colloids and interface science and supercritical fluids technology. The CMP for the AOT W/C reverse microemulsions was defined and measured by plotting absorption intensity of the organic dyes in the W/C microemulsions against system pressures, because the absorption intensity of the organic dyes in the W/C microemulsions increase abruptly at near CMP. Furthermore, solvatochromic probes such as MO and DB were used to explore the polarity of micro-aqueous domains in the AOT W/C reverse microemulsions, which would be benefit to explain the formation of the AOT W/C reverse microemulsions and the solubilization of the organic dyes in the W/C microemulsions.
2. Experimental section 2.1. Materials CO2 (99.999% purity) supplied by the Nippon Sanso Co., Ltd, was used. Sodium bis (2-ethylhexyl) sulfosuccinate (AOT, minimum 99%, Sigma Ultra, MW 444.56) purchased from Sigma Chemical Co. Ltd. (USA) was vacuum dried at 60 ◦ C for 24 h and stored in a vacuum-desicator prior to use. 2,2,3,3,4,4,5,5-octafluoro-1-pentanol, dehydrated methanol, MO and TB were obtained from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan), double distilled and de-ionized water prepared by ultra-filtration, reverse osmosis, deionization and distillation was purchased from Wako Pure Chem. Ind. Ltd. (Tokyo, Japan). DB was purchased from Sigma Chemical Co. Ltd. (USA). 2.2. UV-Vis experiments The solubilities of organic dyes such as TB, DB and MO in AOT W/C reverse microemulsions with F-pentanol as co-surfactant were determined by using UV-Vis spectrometry, which was similar to that reported previously [26]. The high-pressure UV cell consists of a stainless-steel block with two sapphire windows. It has a volume of 2.2 cm3 and withstands maximum pressure of 45 MPa. The mixtures in the cell were stirred by a Teflon-coated bar driven by an outside magnet. Pressure was controlled by a back-pressure regulator (880-81, JASCO Co.) which was accurate to 0.01 MPa in the pressure range of 0–50 MPa. All the measurements were carried out at 38.0 ◦ C, which was controlled within ±0.2 ◦ C by a temperature controller attached to the high-pressure cell. A schematic diagram of the high-pressure cell and ap-
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paratus has been given in a previous publication [27]. For a typical example, a known amount of organic dye solution in methanol was injected into the high-pressure UV cell using a syringe. A gentle stream of N2 was passed through the cell to remove residual methanol. 0.016 M AOT and 0.24 M F-pentanol were charged into the cell. Then the desired amount of double distilled and de-ionized water was injected into the cell using a syringe. After thermal equilibrium had been reached, CO2 was compressed into the cell slowly by an intelligent HPLC pump (PU-980, JASCO Co.) until the desired pressure was reached. The solution was stirred for 30 min and equilibrated for 30 min. Then the UV-Vis spectrophotometer (JASCO V-570) was used to record the absorption spectra of the organic dyes in the AOT W/C reverse microemulsion system.
3. Results and discussions 3.1. The AOT W/C reverse microemulsions The AOT W/C reverse microemulsion system using for solubilizing polar organic dyes was created by mixing 0.016 M AOT/0.24 M F-pentanol/water (W0 )/CO2 system at 38.0 ◦ C and 34.50 MPa. The W0 value has been corrected for the water that dissolves in the SC CO2 continuous phase. F-pentanol acting as co-surfactant in the system would be expected to insert itself between AOT tails and reduce the electrostatic repulsion between the ionic head groups of AOT, thereby apparently overcoming enthalpic destabilization resulting from the mixing of fluorocarbon and hydrocarbon chains [28]. Therefore F-pentanol can effectively stabilize the surfactant interface thereby promoting the formation of the AOT W/C reverse microemulsions. 3.2. The solubilities determination by using UV-Vis spectroscopy A new method for determining the solubilities of different organic dyes in AOT W/C microemulsions with F-pentanol as co-surfactant was established by using UV-Vis spectroscopy. TB (see Fig. 1) has been used as organic dye to be extracted by fluoroether-functional amphiphiles/SC CO2 mixture system [29]. DB (see Fig. 1) has also been used as UV-Vis dye to investigate the supercritical CO2 microemulsions stabilized by synthetic hydrocarbon surfactants [30]. DB does not dissolve in pure SC CO2 , however, if reverse microemulsions were present, the positively charged chromophore should be incorporated owing to favorable interactions with surfactant anionic head groups, thereby making it be dispersed in SC CO2 continuous phase [30]. Besides that, MO (see Fig. 1) is also a commonly used organic dye for investigating the aggregation of the surfactants in SC CO2 [13,31,32]. So, TB, DB, and MO were selected in this study. Fig. 2 shows the UV-Vis spectra for the TB in the AOT W/C reverse microemulsion system. As shown in Fig. 2,
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Fig. 2. Absorption spectra of the different concentration of the TB in AOT W/C reverse microemulsions with F-pentanol as co-surfactant at 38.0 ◦ C and 34.50 MPa.
Fig. 3. Absorption spectra of the different concentration of the DB in AOT W/C reverse microemulsions with F-pentanol as co-surfactant at 38.0 ◦ C and 34.50 MPa.
the wavelength range from 350 to 550 nm, the absorption signals of TB in 0.24 M F-pentanol/SC CO2 (spectrum g) and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system (spectrum f) are very weak even though the pressure was up to 34.50 MPa, mainly resulting from the extremely low solubility of TB in the microemulsion systems. However, as expected, significant TB absorption signals assigned at 427.0 nm can be observed in 0.016 M AOT/0.24 M F-pentanol/water (W0 = 6.0)/SC CO2 mixture system, indicating the formation of AOT W/C reverse microemulsions with polar micro-aqueous domains which provide the solubilization sites for the hydrophilic TB. The solubility of TB was calculated according to the maximum absorption value of TB in the AOT W/C reverse microemulsion system. As can be seen in Fig. 2, when 9.09 × 10−5 M TB was added to the microemulsion system, the maximum absorption value of TB (see spectrum a) was only a little higher than that of the system added to 7.27 × 10−5 M TB (see spectrum b), which suggests that 7.27×10−5 M TB was completely solubilized in the microemulsion system, and the solubility of TB in the AOT W/C reverse microemulsions was a little higher than 7.27 × 10−5 M. Then the maximum absorbance values of (spectrumb–e) against the different TB concentrations were plotted. As shown in Fig. 4, an excellent linear relation curve appeared (R = 0.9988). According to the linear relation curve and the maximum absorption values of (spectrum a), the solubility and molar absorption coefficient of TB in the AOT W/C reverse microemulsions at 38.0 ◦ C and 34.50 MPa can be easily obtained. In addition, an interesting phenomenon was found in the study. As we
can see from Fig. 2, in the wavelength range of 500–650 nm, absorption peaks appeared in 0.24 M F-pentanol/SC CO2 (spectrum g) and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system (spectrum f) respectively; while at this wavelength range there is no absorption signal in 0.016 M AOT/0.24 M F-pentanol/water ( W0 = 6.0)/SC CO2 mixture system (spectrum a–e). The exact reason for this phenomenon is not well understood. As shown in Fig. 3, DB has certain solubility in 0.24 M F-pentanol/SC CO2 and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system, which is reflected by the spectrum f and g in Fig. 3. In addition, the absorption spectra of DB are similar to TB in the AOT W/C reverse microemulsions i.e., the absorption values of DB are linear well with the concentration of the DB, solubilized in the microemulsion system, which can be known clearly from Fig. 4. By using this new UV-Vis spectroscopy method, the accurate solubility and molar absorption coefficients data of TB, DB and MO in the AOT W/C reverse microemulsions were calculated and listed in Table 1. The establishment of this new method for determining the solubilities of the organic dyes in the supercritical CO2 microemulsions has a potential importance in the optimization of this particular SC CO2 microemulsion dyeing technique and separation of hydrophilic organic dyes from waste water. 3.3. CMP for AOT/F-pentanol/water/CO2 system Fig. 5 shows the effect of system pressure on the absorption intensity of the three different dyes in 0.016 M
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Fig. 4. The dependence of the absorbance intensity on the concentration of the organic dyes solubilized in the AOT W/C reverse microemulsions with W0 = 6.0 at 38.0 ◦ C and 34.5 MPa. Table 1 Solubility data of different organic dyes in AOT W/C reverse microemulsions (W0 = 6.0) with F-pentanol determined by UV-Vis spectrometer at 38.0 ◦ C and 34.50 MPa Organic dyes
max (nm)
Absorbance (A)
Molar absorption coefficient (M−1 cm−1 )
Solubility (M)
TB DB MO
427.0 515.0 408.0
0.889 0.341 0.649
2.39 × 104 8.89 × 103 2.25 × 104
7.45 × 10−5 7.67 × 10−5 5.77 × 10−5
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tainly related to the formation of reverse micelles. Because, in contrast to SC CO2 -soluble dyes, the polar hydrophilic organic dyes such as TB, DB, and MO in the AOT W/C reverse microemulsion system would not solubilize in the bulk CO2 phase but solubilize in the polar micro-aqueous domains within the AOT reverse micelles, therefore, a new concept of CMP at a given temperature was brought forward. It is defined as a pressure at which reverse micelles start to form in the SC CO2 continuous phase at a given temperature. Thus, 18.06 MPa was concluded to be the CMP of AOT W/C reverse microemulsions with F-pentanol as co-surfactant. As also seen in Fig. 5, when the system pressure was further increased after achieving CMP, the absorbance of the dyes increased steadily first and then a plateau appeared. Because, higher the pressure, the more loading water within AOT reverse micelles, leading to the enhancement of the solubility of the polar organic dyes in the microemulsion system, until the dyes solubilized completely within the polar micro-aqueous domain. All the experiments were carried out at 38.0 ◦ C. 3.4. Micro-polarity studies
AOT/0.24 M F-pentanol/water (W0 = 6.0)/CO2 system. As can be seen in Fig. 5, the absorbance of the three dyes increased abruptly as pressure was increased to 18.06 MPa (density of 0.837 g/cm3 ) approximately. The results in this study are quite different to those obtained from SC CO2 -soluble dyes in pure SC CO2 [33,34]. As reported by Hori and Tabata [35] the solubility of dispersed dyes in SC CO2 are greatly dependent on temperature and pressure. At constant temperature, the solubility of these dyes increases linearly with increasing system pressure. The abrupt increase in the absorbance value of the three different dyes at approximately 18.06 MPa, observed in this study, are cer-
The shift in the maximum absorption wavelength (λmax ) of a solvatochromic probes can sensitively reflect the local environment about the probe. MO is a useful probe molecule to investigate the micro-polar environment of W/C microemulsions [13,17,22,31]. The λmax of MO shifts to larger wavelength (red shift) as the solvent polarity increases. No detectable MO absorption signals were observed in 0.24 M F-pentanol/SC CO2 and 0.24 M F-pentanol/water (saturated water in bulk CO2 )/SC CO2 system, while as shown in Fig. 6, the remarkable absorption peaks of MO could be observed in 0.016 M AOT/0.24 M F-pentanol/water/SC CO2 system, and with an increase in W0 from 0 to 6.0, λmax increased from 402 to 408 nm. It is clear that MO molecules resided in an environment of increasing polarity due to the increasing water content. As shown in Fig. 7, the maximum absorbance of probe molecule DB
Fig. 5. Effect of pressure on the absorbance of organic dyes (5.45 × 10−5 M) in the AOT W/C reverse microemulsions with W0 = 6.0 at 38.0 ◦ C and 34.50 MPa.
Fig. 6. Absorbance spectra of MO in 0.016 M AOT/0.24 M F-pentanol/ water/SC CO2 mixture system with various W0 values at 34.5 MPa and 38.0 ◦ C.
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croemulsion system against system pressures. Furthermore, solvatochromic probes were used to explore the polarity of micro-aqueous domains in the AOT W/C reverse microemulsions, which well demonstrated the formation of the AOT W/C reverse microemulsions and the solubilization of the organic dyes in the microemulsions. The introduction of commercial available surfactant AOT into environmentally benign SC CO2 , thereby forming water-in-CO2 reverse microemulsions with the help of a small quantity of commercially inexpensive F-pentanol will have an industrially promising application potential in dyeing process and separation for water soluble organic dyes from waste water. Fig. 7. Absorbance spectra of DB in 0.016 M AOT/0.24 M F-pentanol/ water/SC CO2 mixture system with various W0 values at 34.5 MPa and 38.0 ◦ C.
in 0.016 M AOT/0.24 M F-pentanol/water/SC CO2 system increased with increasing W0 and λmax of DB shifted from 515 to 511 nm as W0 increased from 0 to 6.0. The λmax of MO and DB in aqueous solution were measured to be 482 and 464 nm, respectively. Compared the UV spectra of DB with those of MO in the same AOT W/C reverse microemulsion system and the λmax of DB in aqueous solution, obviously, DB is a blue shift molecular spectroscopic probe. Thus, the spectra of DB in the microemulsion system are identical with those of MO in the polar change of the micro-aqueous domains within AOT reverse micelles with varying water content. The MO and DB spectra in the AOT W/C reverse microemulsions and λmax in the aqueous solution proved the existence of polar micro-aqueous domains within AOT W/C reverse microemulsions with F-pentanol, even though the polarity of the aggregated water is lower than that of bulk water, implying the different properties of the aggregate water from the highly polar water pool. Therefore, we conclude that MO would be expected to host in the polar outer shell of the reverse micelle rather than residing in the highly polar water pool. All the molecular spectroscopic probe studies were carried out at 38.0 ◦ C.
4. Conclusion In the study, it was found that organic dyes such as TB, DB, and MO which are not soluble in pure SC CO2 could conveniently be solubilized in AOT water-in-CO2 reverse microemulsions with F-pentanol as co-surfactant. The solubilities of these organic dyes in the AOT W/C reverse microemulsions were measured successfully by using a UV-Vis spectroscopy method newly established in our laboratory. In addition, CMP as a new concept in the fields of combination of colloids and interface chemistry and supercritical fluids technology was defined and obtained easily by plotting absorption intensity of organic dyes in the mi-
Acknowledgements The authors are grateful to the Japanese Science Promotion Society (JSPS) for financial support.
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Investigation on the solubilization of organic dyes and micro-polarity in AOT water-in-CO2 microemulsions with fluorinated co-surfactant by using UV-Vis spectroscopy Juncheng Liu a,b , Yutaka Ikushima a,b,∗ , Zameer Shervani a,b a
Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan b CREST, Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi 332-0012, Japan Received in revised form 10 February 2004; accepted 10 February 2004
Abstract It was found that organic dyes such as thymol blue, dimidium bromide and methyl orange which are not soluble in pure supercritical CO2 , could be conveniently solubilized in AOT water-in-CO2 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-1-pentanol as co-surfactant. The solubilities of these organic dyes in the microemulsions were measured successfully by using a UV-Vis spectroscopy method newly established in our laboratory; besides that, for a given temperature, a critical micelle pressure (CMP) at which formation of AOT water-in-CO2 reverse micelles starts, was determined in term of the effect of pressure on the absorption intensity of the organic dyes in the microemulsions. Furthermore, the micro-polarity environment of the AOT water-in-CO2 reverse microemulsions was investigated systematically according to the shift of solvatochromic probes such as methyl orange and dimidium bromide with varying water content by using UV-Vis spectroscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical CO2 ; Microemulsions; Solubilization; Micro-polarity; UV-Vis spectroscopy
1. Introduction Conventional dyeing and subsequent washing processes are generally performed in water-based dye-baths. However, waste aqueous effluent containing colored compounds and concentrated electrolytes cause serious environmental problems. Furthermore, both washing step and drying step are essential in the conventional dyeing processes, leading a large amount of energy to be consumed. In order to overcome this problem, new concepts of avoiding the use of water are being evaluated. Recently, the supercritical fluid dyeing process (FDP) has drawn significant attention. Supercritical carbon dioxide (SC CO2 ) is one of the most commonly used supercritical fluids (SCFs) as is naturally abundant, non-flammable, essentially non-toxic, and the least expensive solvent after water. Moreover, CO2 attains the supercritical state at ambient temperature (Tc = 31 ◦ C) and a relatively moderate pres∗ Corresponding author. Tel.: +81-22-237-5211; fax: +81-22-237-5224. E-mail address: [email protected] (Y. Ikushima).
0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2004.02.006
sure (Pc = 73.8 bar). Therefore, SC CO2 dyeing process has been observed as a novel and environmentally friendly technique that would expected to be an attractive alternative to pollution generating and energy-consuming conventional wet-dyeing process. However, SC CO2 is a poor solvent of the hydrophilies because of its very low value of dielectric constant, ε, polarizability per volume, ␣/, and not strongly engaging in van der Waals interactions. So unlike SC CO2 -soluble dyes, water-soluble organic dyes such as thymol blue (TB), dimidium bromide (DB) and methyl orange (MO) do not dissolve in SC CO2 . In the past decade, several approaches have been explored to enhance the solubility of polar substance in SC CO2 [1–3]. For example, the addition of polar entrainer such as alcohols or acetone can increase the polarity, and therefore the solvent power of SC CO2 . However, water-soluble dyes are still insufficiently soluble in this SC CO2 entrainer medium in the dyeing process [4]. Our strategy is to dissolve the water-soluble dyes in thermodynamically stable and optically transparent water-in-CO2 (W/C) reverse microemulsions which can provide nano-sized polar micro-water domains as sites for solubilization of the hydrophilic dye molecules.
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Fig. 1. The structure of Organic dyes and AOT used in this study.
Preliminary works on microemulsions in SC CO2 have indicated that common surfactants used in hydrocarbon solvents are generally not suitable in SC CO2 [5–7]. Certain fluorocarbon [8–10] and silicon-containing surfactants [11,12] are known to be CO2 soluble, and several fluoro-surfactants such as an ammonium carboxylate perfluoropolyether, CF3 O(CF2 CF(CF3 )O)3 CF2 COO− NH4 + (PFPE), fluorinated analogues of AOT and phosphate fluoro-surfactants enable significant water uptake within a continuous CO2 phase, through the formation of W/C microemulsions [13–17]. However, all of these surfactants explored often require specialized synthesis and therefore remain expensive, which make it urgent to explore some water-in-CO2 microemulsions stabilized by low cost commercial surfactant. Certain surfactants, such as commonly used surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT, see Fig. 1) has been shown to form reverse micelles in conventional organic solvent [18] and supercritical alkane [19–21]. However, AOT was completely insoluble in SC CO2 , [7] restricting its wide application in the environmentally benign CO2 . Recently, Hutton et al. [22] proposed that AOT could be solubilized in SC CO2 with ethanol (15–17 mol%) or
1-pentanol (10 mol%) as co-solvent and form AOT reverse microemulsions in SC CO2 . However, a large quantity of co-solvent was required in the system. Subsequently, Wai and co-workers [23] reported that AOT could form reverse microemulsions with perfluoropolyether-phosphate ether (PFPE-PO4 ) as co-surfactant, and this system has been used as nano-reactor for preparing various nanoparticles [24,25]. However, the PFPE-PO4 is also an expensive synthetic chemical and difficult to obtain commercially. In summary, either expensive surfactant or a large amount of co-surfactant was absolutely necessary for the W/C microemulsions so far reported, hindering the industrial application of these microemulsions. In order to overcome this obstacle, 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-pentanol) was first attempted to be used as co-surfactant for promoting the formation of AOT reverse microemulsions in SC CO2 for the reason that F-pentanol is inexpensive and contains ‘CO2 -philic’ fluorinated alkane chain exhibiting very low (much less than zero) dipolarity/polarizability parameters [8]. Thus the addition of only a small quantity of F-pentanol (1.2 mol%) can successfully promote the formation of the AOT W/C reverse microemulsions.
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Solubility data of hydrophilic organic dyes in SC CO2 microemulsion system are of interest for the optimization of SC CO2 microemulsion dyeing technique. In this study, a simple and accurate method was established to measure the solubilities of the organic dyes such as TB, DB, and MO in AOT W/C reverse microemulsions with F-pentanol as co-surfacant by using UV-Vis spectroscopy. In addition, to our knowledge, critical micelle pressure (CMP) as a new concept has not been defined in the fields of combination of colloids and interface science and supercritical fluids technology. The CMP for the AOT W/C reverse microemulsions was defined and measured by plotting absorption intensity of the organic dyes in the W/C microemulsions against system pressures, because the absorption intensity of the organic dyes in the W/C microemulsions increase abruptly at near CMP. Furthermore, solvatochromic probes such as MO and DB were used to explore the polarity of micro-aqueous domains in the AOT W/C reverse microemulsions, which would be benefit to explain the formation of the AOT W/C reverse microemulsions and the solubilization of the organic dyes in the W/C microemulsions.
2. Experimental section 2.1. Materials CO2 (99.999% purity) supplied by the Nippon Sanso Co., Ltd, was used. Sodium bis (2-ethylhexyl) sulfosuccinate (AOT, minimum 99%, Sigma Ultra, MW 444.56) purchased from Sigma Chemical Co. Ltd. (USA) was vacuum dried at 60 ◦ C for 24 h and stored in a vacuum-desicator prior to use. 2,2,3,3,4,4,5,5-octafluoro-1-pentanol, dehydrated methanol, MO and TB were obtained from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan), double distilled and de-ionized water prepared by ultra-filtration, reverse osmosis, deionization and distillation was purchased from Wako Pure Chem. Ind. Ltd. (Tokyo, Japan). DB was purchased from Sigma Chemical Co. Ltd. (USA). 2.2. UV-Vis experiments The solubilities of organic dyes such as TB, DB and MO in AOT W/C reverse microemulsions with F-pentanol as co-surfactant were determined by using UV-Vis spectrometry, which was similar to that reported previously [26]. The high-pressure UV cell consists of a stainless-steel block with two sapphire windows. It has a volume of 2.2 cm3 and withstands maximum pressure of 45 MPa. The mixtures in the cell were stirred by a Teflon-coated bar driven by an outside magnet. Pressure was controlled by a back-pressure regulator (880-81, JASCO Co.) which was accurate to 0.01 MPa in the pressure range of 0–50 MPa. All the measurements were carried out at 38.0 ◦ C, which was controlled within ±0.2 ◦ C by a temperature controller attached to the high-pressure cell. A schematic diagram of the high-pressure cell and ap-
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paratus has been given in a previous publication [27]. For a typical example, a known amount of organic dye solution in methanol was injected into the high-pressure UV cell using a syringe. A gentle stream of N2 was passed through the cell to remove residual methanol. 0.016 M AOT and 0.24 M F-pentanol were charged into the cell. Then the desired amount of double distilled and de-ionized water was injected into the cell using a syringe. After thermal equilibrium had been reached, CO2 was compressed into the cell slowly by an intelligent HPLC pump (PU-980, JASCO Co.) until the desired pressure was reached. The solution was stirred for 30 min and equilibrated for 30 min. Then the UV-Vis spectrophotometer (JASCO V-570) was used to record the absorption spectra of the organic dyes in the AOT W/C reverse microemulsion system.
3. Results and discussions 3.1. The AOT W/C reverse microemulsions The AOT W/C reverse microemulsion system using for solubilizing polar organic dyes was created by mixing 0.016 M AOT/0.24 M F-pentanol/water (W0 )/CO2 system at 38.0 ◦ C and 34.50 MPa. The W0 value has been corrected for the water that dissolves in the SC CO2 continuous phase. F-pentanol acting as co-surfactant in the system would be expected to insert itself between AOT tails and reduce the electrostatic repulsion between the ionic head groups of AOT, thereby apparently overcoming enthalpic destabilization resulting from the mixing of fluorocarbon and hydrocarbon chains [28]. Therefore F-pentanol can effectively stabilize the surfactant interface thereby promoting the formation of the AOT W/C reverse microemulsions. 3.2. The solubilities determination by using UV-Vis spectroscopy A new method for determining the solubilities of different organic dyes in AOT W/C microemulsions with F-pentanol as co-surfactant was established by using UV-Vis spectroscopy. TB (see Fig. 1) has been used as organic dye to be extracted by fluoroether-functional amphiphiles/SC CO2 mixture system [29]. DB (see Fig. 1) has also been used as UV-Vis dye to investigate the supercritical CO2 microemulsions stabilized by synthetic hydrocarbon surfactants [30]. DB does not dissolve in pure SC CO2 , however, if reverse microemulsions were present, the positively charged chromophore should be incorporated owing to favorable interactions with surfactant anionic head groups, thereby making it be dispersed in SC CO2 continuous phase [30]. Besides that, MO (see Fig. 1) is also a commonly used organic dye for investigating the aggregation of the surfactants in SC CO2 [13,31,32]. So, TB, DB, and MO were selected in this study. Fig. 2 shows the UV-Vis spectra for the TB in the AOT W/C reverse microemulsion system. As shown in Fig. 2,
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Fig. 2. Absorption spectra of the different concentration of the TB in AOT W/C reverse microemulsions with F-pentanol as co-surfactant at 38.0 ◦ C and 34.50 MPa.
Fig. 3. Absorption spectra of the different concentration of the DB in AOT W/C reverse microemulsions with F-pentanol as co-surfactant at 38.0 ◦ C and 34.50 MPa.
the wavelength range from 350 to 550 nm, the absorption signals of TB in 0.24 M F-pentanol/SC CO2 (spectrum g) and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system (spectrum f) are very weak even though the pressure was up to 34.50 MPa, mainly resulting from the extremely low solubility of TB in the microemulsion systems. However, as expected, significant TB absorption signals assigned at 427.0 nm can be observed in 0.016 M AOT/0.24 M F-pentanol/water (W0 = 6.0)/SC CO2 mixture system, indicating the formation of AOT W/C reverse microemulsions with polar micro-aqueous domains which provide the solubilization sites for the hydrophilic TB. The solubility of TB was calculated according to the maximum absorption value of TB in the AOT W/C reverse microemulsion system. As can be seen in Fig. 2, when 9.09 × 10−5 M TB was added to the microemulsion system, the maximum absorption value of TB (see spectrum a) was only a little higher than that of the system added to 7.27 × 10−5 M TB (see spectrum b), which suggests that 7.27×10−5 M TB was completely solubilized in the microemulsion system, and the solubility of TB in the AOT W/C reverse microemulsions was a little higher than 7.27 × 10−5 M. Then the maximum absorbance values of (spectrumb–e) against the different TB concentrations were plotted. As shown in Fig. 4, an excellent linear relation curve appeared (R = 0.9988). According to the linear relation curve and the maximum absorption values of (spectrum a), the solubility and molar absorption coefficient of TB in the AOT W/C reverse microemulsions at 38.0 ◦ C and 34.50 MPa can be easily obtained. In addition, an interesting phenomenon was found in the study. As we
can see from Fig. 2, in the wavelength range of 500–650 nm, absorption peaks appeared in 0.24 M F-pentanol/SC CO2 (spectrum g) and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system (spectrum f) respectively; while at this wavelength range there is no absorption signal in 0.016 M AOT/0.24 M F-pentanol/water ( W0 = 6.0)/SC CO2 mixture system (spectrum a–e). The exact reason for this phenomenon is not well understood. As shown in Fig. 3, DB has certain solubility in 0.24 M F-pentanol/SC CO2 and 0.24 M F-pentanol/water (saturated water in bulk CO2 phase)/SC CO2 system, which is reflected by the spectrum f and g in Fig. 3. In addition, the absorption spectra of DB are similar to TB in the AOT W/C reverse microemulsions i.e., the absorption values of DB are linear well with the concentration of the DB, solubilized in the microemulsion system, which can be known clearly from Fig. 4. By using this new UV-Vis spectroscopy method, the accurate solubility and molar absorption coefficients data of TB, DB and MO in the AOT W/C reverse microemulsions were calculated and listed in Table 1. The establishment of this new method for determining the solubilities of the organic dyes in the supercritical CO2 microemulsions has a potential importance in the optimization of this particular SC CO2 microemulsion dyeing technique and separation of hydrophilic organic dyes from waste water. 3.3. CMP for AOT/F-pentanol/water/CO2 system Fig. 5 shows the effect of system pressure on the absorption intensity of the three different dyes in 0.016 M
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Fig. 4. The dependence of the absorbance intensity on the concentration of the organic dyes solubilized in the AOT W/C reverse microemulsions with W0 = 6.0 at 38.0 ◦ C and 34.5 MPa. Table 1 Solubility data of different organic dyes in AOT W/C reverse microemulsions (W0 = 6.0) with F-pentanol determined by UV-Vis spectrometer at 38.0 ◦ C and 34.50 MPa Organic dyes
max (nm)
Absorbance (A)
Molar absorption coefficient (M−1 cm−1 )
Solubility (M)
TB DB MO
427.0 515.0 408.0
0.889 0.341 0.649
2.39 × 104 8.89 × 103 2.25 × 104
7.45 × 10−5 7.67 × 10−5 5.77 × 10−5
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tainly related to the formation of reverse micelles. Because, in contrast to SC CO2 -soluble dyes, the polar hydrophilic organic dyes such as TB, DB, and MO in the AOT W/C reverse microemulsion system would not solubilize in the bulk CO2 phase but solubilize in the polar micro-aqueous domains within the AOT reverse micelles, therefore, a new concept of CMP at a given temperature was brought forward. It is defined as a pressure at which reverse micelles start to form in the SC CO2 continuous phase at a given temperature. Thus, 18.06 MPa was concluded to be the CMP of AOT W/C reverse microemulsions with F-pentanol as co-surfactant. As also seen in Fig. 5, when the system pressure was further increased after achieving CMP, the absorbance of the dyes increased steadily first and then a plateau appeared. Because, higher the pressure, the more loading water within AOT reverse micelles, leading to the enhancement of the solubility of the polar organic dyes in the microemulsion system, until the dyes solubilized completely within the polar micro-aqueous domain. All the experiments were carried out at 38.0 ◦ C. 3.4. Micro-polarity studies
AOT/0.24 M F-pentanol/water (W0 = 6.0)/CO2 system. As can be seen in Fig. 5, the absorbance of the three dyes increased abruptly as pressure was increased to 18.06 MPa (density of 0.837 g/cm3 ) approximately. The results in this study are quite different to those obtained from SC CO2 -soluble dyes in pure SC CO2 [33,34]. As reported by Hori and Tabata [35] the solubility of dispersed dyes in SC CO2 are greatly dependent on temperature and pressure. At constant temperature, the solubility of these dyes increases linearly with increasing system pressure. The abrupt increase in the absorbance value of the three different dyes at approximately 18.06 MPa, observed in this study, are cer-
The shift in the maximum absorption wavelength (λmax ) of a solvatochromic probes can sensitively reflect the local environment about the probe. MO is a useful probe molecule to investigate the micro-polar environment of W/C microemulsions [13,17,22,31]. The λmax of MO shifts to larger wavelength (red shift) as the solvent polarity increases. No detectable MO absorption signals were observed in 0.24 M F-pentanol/SC CO2 and 0.24 M F-pentanol/water (saturated water in bulk CO2 )/SC CO2 system, while as shown in Fig. 6, the remarkable absorption peaks of MO could be observed in 0.016 M AOT/0.24 M F-pentanol/water/SC CO2 system, and with an increase in W0 from 0 to 6.0, λmax increased from 402 to 408 nm. It is clear that MO molecules resided in an environment of increasing polarity due to the increasing water content. As shown in Fig. 7, the maximum absorbance of probe molecule DB
Fig. 5. Effect of pressure on the absorbance of organic dyes (5.45 × 10−5 M) in the AOT W/C reverse microemulsions with W0 = 6.0 at 38.0 ◦ C and 34.50 MPa.
Fig. 6. Absorbance spectra of MO in 0.016 M AOT/0.24 M F-pentanol/ water/SC CO2 mixture system with various W0 values at 34.5 MPa and 38.0 ◦ C.
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croemulsion system against system pressures. Furthermore, solvatochromic probes were used to explore the polarity of micro-aqueous domains in the AOT W/C reverse microemulsions, which well demonstrated the formation of the AOT W/C reverse microemulsions and the solubilization of the organic dyes in the microemulsions. The introduction of commercial available surfactant AOT into environmentally benign SC CO2 , thereby forming water-in-CO2 reverse microemulsions with the help of a small quantity of commercially inexpensive F-pentanol will have an industrially promising application potential in dyeing process and separation for water soluble organic dyes from waste water. Fig. 7. Absorbance spectra of DB in 0.016 M AOT/0.24 M F-pentanol/ water/SC CO2 mixture system with various W0 values at 34.5 MPa and 38.0 ◦ C.
in 0.016 M AOT/0.24 M F-pentanol/water/SC CO2 system increased with increasing W0 and λmax of DB shifted from 515 to 511 nm as W0 increased from 0 to 6.0. The λmax of MO and DB in aqueous solution were measured to be 482 and 464 nm, respectively. Compared the UV spectra of DB with those of MO in the same AOT W/C reverse microemulsion system and the λmax of DB in aqueous solution, obviously, DB is a blue shift molecular spectroscopic probe. Thus, the spectra of DB in the microemulsion system are identical with those of MO in the polar change of the micro-aqueous domains within AOT reverse micelles with varying water content. The MO and DB spectra in the AOT W/C reverse microemulsions and λmax in the aqueous solution proved the existence of polar micro-aqueous domains within AOT W/C reverse microemulsions with F-pentanol, even though the polarity of the aggregated water is lower than that of bulk water, implying the different properties of the aggregate water from the highly polar water pool. Therefore, we conclude that MO would be expected to host in the polar outer shell of the reverse micelle rather than residing in the highly polar water pool. All the molecular spectroscopic probe studies were carried out at 38.0 ◦ C.
4. Conclusion In the study, it was found that organic dyes such as TB, DB, and MO which are not soluble in pure SC CO2 could conveniently be solubilized in AOT water-in-CO2 reverse microemulsions with F-pentanol as co-surfactant. The solubilities of these organic dyes in the AOT W/C reverse microemulsions were measured successfully by using a UV-Vis spectroscopy method newly established in our laboratory. In addition, CMP as a new concept in the fields of combination of colloids and interface chemistry and supercritical fluids technology was defined and obtained easily by plotting absorption intensity of organic dyes in the mi-
Acknowledgements The authors are grateful to the Japanese Science Promotion Society (JSPS) for financial support.
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