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AOT reverse microemulsions in scCO2 — a further investigation
Colloids and Surfaces A: Physicochemical and Engineering Aspects 189 (2001) 177– 181 www.elsevier.com/locate/colsurfa
AOT reverse microemulsions in scCO2 — a further investigation Brenda H. Hutton a, Jilska M. Perera a, Franz Grieser b, Geoffrey W. Stevens a,* a
Department of Chemical Engineering, The Uni6ersity of Melbourne, Victoria 3010, Australia b School of Chemistry, The Uni6ersity of Melbourne, Victoria 3010, Australia Received 18 August 2000; accepted 20 December 2000
Abstract The solubility of riboflavin (RF), potassium ferricyanide (K3Fe(CN)6) and polar components from yeast extract (YX) in a sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) modified supercritical carbon dioxide (scCO2) system was investigated. The results from RF solubility studies indicate that the microemulsion phase formed by AOT contains a polar core with an effective static dielectric constant of 20 9 5. Although this core is not indicative of bulk water, the overall improvement in polarity from pure scCO2 is approximately a factor of 10. K3Fe(CN)6, a highly ionic compound, could not be solubilised into the modified supercritical fluid, indicating that the core was not sufficiently polar to dissolve highly ionic compounds. Finally, successful extraction of polar compounds from YX demonstrated the potential industrial applications of AOT-modified scCO2. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Riboflavin; Inverse microemulsion; Sodium bis-(2-ethylhexyl) sulfosuccinate (AOT)
1. Introduction Supercritical extraction using carbon dioxide (scCO2) as the solvent has become a popular alternative to solvent extraction processes, particularly in food-related areas which necessitate the use of a non-toxic solvent. The industrial usage of scCO2 is increasing, and some of these applications include the extraction of hops for beer production, the decaffeination of tea and coffee, * Corresponding author. Tel.: + 61-3-83446621; fax: +613-83444153. E-mail address: [email protected] (G.W. Stevens).
extraction of flavours such as paprika, fragrances such as lavender, and fish oils [1–7]. Unfortunately, due to the low polarity of scCO2, applications of this extraction technique are limited to substances of similar polarity. Therefore, it has not been possible to extract hydrophilic components, such as the water-soluble vitamins, from a matrix into the supercritical fluid phase. One solution to this problem has been the introduction of fluorinated surfactant systems into scCO2 to form reverse microemulsions [8–10], thereby providing water-soluble sites into which to extract polar substances. Unfortunately these
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surfactants are not suitable for food-related applications. In an earlier study, Hutton et al. [11] solubilised a common food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) into scCO2 using small amounts of either ethanol or n-pentanol as co-solvent. Although this surfactant had been previously assumed to be completely insoluble in scCO2 [12], Hutton et al. also found that it was possible to solubilise two polar substances, methyl orange and riboflavin, but not the highly ionic compound, 8-hydroxy-1,3,6pyrene-trisulphonic acid trisodium salt (HPTS) in this modified supercritical fluid inverse microemulsion [11]. The present study was undertaken in order to gain further information on the nature of the polar sites formed with reverse microemulsions in scCO2, in the presence of co-solvents. In addition, the water-soluble components from yeast extract are shown to be extracted using AOT-modified scCO2 and this demonstrates the potential industrial applications of this type of inverse microemulsion.
2. Experimental procedures
2.1. Materials Carbon dioxide, of supercritical fluid grade, was obtained from BOC gases. Analytical reagent grade organic solvents, n-pentane (99%, BDH), n-decane (\ 99%, Aldrich), 1,4-dioxane (99.5%, AJAX), ethanol (99.7– 100 vol.%, BDH) and npentanol (99%, Riedel– deHaen) were used without further purification. The probes potassium ferricyanide, K3Fe(CN)6 (99%) and Riboflavin (98%) were obtained from Aldrich and used without further purification. Yeast extract (Code L21) was obtained from Oxoid (England). Sodium bis(2-ethylhexyl) sulfosuccinate (99%) was obtained from Sigma and used without further purification. Water used in the experiments was purified by a Millipore Milli-RO 4 purification system, and had an average conductivity of 13.87 mS cm − 1 at 25oC.
2.2. Methods Details of the supercritical extraction unit have been reported elsewhere [11]. Briefly, however, it can be described as a recirculating system with an on-line UV–Vis spectrophotometric facility that directly measures the uptake of the relevant material into the supercritical carbon dioxide phase. Experiments were performed by initially preparing an AOT solution in n-pentanol, such that the final concentrations once mixed with scCO2 were 0.03 M AOT and 10 mol% alcohol. These preparations contained 1.7 mol% water initially, and this solution was then added to the scCO2. A small quantity of probe material was injected (as a dispersed solid) into the modified supercritical phase. Subsequently, water was injected in small quantities (generally 100–200 ml amounts) into the system during the experiment, and UV–Vis spectra were obtained. Operating conditions were generally 250 bar, 40oC, and 20 Hz circulating pump speed [13]. It is assumed that when water is present in the modified scCO2 that an inverse microemulsion system is produced [14,15]. Experiments under ambient conditions using alkanes as the continuous phase were performed prior to the supercritical experiments to give a first approximation of the likely behaviour of the probe under investigation in the modified scCO2. The same quantities of surfactant and co-solvent were used as described above, and the supercritical fluid was replaced with n-decane or n-pentane. Water was added to these solutions and spectra obtained after each addition.
3. Results and discussions
3.1. Ribofla6in The UV –Vis spectrum of RF in water shown in the insert of Fig. 1 indicates umax at 37392 and 44492 nm. In the supercritical phase these peaks are shifted to 36092 and 4409 2 nm, respectively. Experiments with the following combinations of materials all resulted in effectively no solubility of the RF: RF/scCO2, RF/scCO2/water,
B.H. Hutton et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 177–181
RF/pentanol/scCO2 and RF/pentanol/scCO2/H2O. This demonstrates that the presence of AOT is essential for the solubility of the RF. Similarly if water is excluded from the solvent the solubility of RF is very small. Fig. 1 shows that an increase in W0, i.e. the water content of the reverse microemulsion phase, allows an increase in the uptake of RF. This data indicates further solubilisation of precipitated RF as the water content of the reverse microemulsion is increased. Based on the solubility limit of RF in water at 40°C [16], it is expected that a significant quantity of the injected RF would be in the solid form. The next step was to estimate the dielectric environment of the water pools in the reverse microemulsion. The peak at 3609 2 nm is solvatochromic and was therefore used to estimate the dielectric constant of the supercritical phase [17–19]. The UV– Vis spectrum of RF in various 1,4-dioxane/water mixtures (of known static dielectric constant [20]) was monitored. Assuming that 1,4-dioxane/water mixtures are an adequate reference for the dielectric environment of the water pools in the supercritical phase, an effective dielectric constant can be assigned to each umax.
Fig. 1. Absorbance (umax = 440 nm) vs. W0 for 200 ml (of 0.031 M in water) RF in 0.03 M AOT/10 mol% pentanol/88.3 mol% scCO2/1.7 mol% water (initial conditions) at 40oC, 250 bar and 20 Hz pump speed [13]. The insert shows the UV – Vis spectrum of RF in water.
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Based on this calibration scale, it was estimated that RF in AOT-modified scCO2 was situated in an environment having a dielectric constant of 2095. The estimated dielectric constant via the 1,4-dioxane/water system shows relatively large changes in effective dielectric constant with small changes in wavelength over this range, and therefore it is likely that the actual polarity of the aggregates is just sufficiently high enough to allow solubility of RF into the core. Regardless of this uncertainty, these results have shown that the aggregate core in the supercritical fluid is not indicative of bulk-like water. Nevertheless, the overall polarity of the supercritical fluid system has been enhanced by a factor of approximately 10 (scCO2 has an effective dielectric constant between 1.0 and 1.6, depending on temperature and pressure [21]).
3.2. Potassium ferricyanide K3Fe(CN)6 is a highly ionic compound requiring complete hydration for solubility in a water environment. For this reason, it was chosen in order to determine whether or not the conclusions reached with the RF results were consistent with the expected behaviour of an ionic solute in the modified scCO2. Also, Fe(CN)36 − with its negative charge can be expected to be repelled by the anionic head groups of AOT, and forced away from the interfacial region and into the core of the inverse micelle, the more polar region of the emulsion. An experiment at ambient conditions with npentane as the solvent and pentanol as co-solvent, was performed to determine the suitability of this compound for solubilisation into an AOT reverse microemulsion. It was possible to solubilise K3Fe(CN)6 in the reverse microemulsion (W0 = 10.5) and the resulting UV–Vis spectrum was similar to that obtained in pure water. The equivalent experiment was performed under supercritical conditions, and although the initial W0 was 10.5, no K3Fe(CN)6 whatsoever was detected in the modified supercritical fluid. These results are consistent with our earlier study [11], where it was found that HPTS, another ionic compound, could not be solubilised in the modified scCO2. Together, these results suggest that although the water pools were sufficiently polar to
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solubilise moderately polar components such as RF and MO, they were insufficiently polar to promote solubilisation and hydration of two highly ionic compounds. Researchers [22] using a fluorinated surfactant, ammonium carboxylate perfluoropolyether (PFPE), in scCO2 were able to solubilise the ionic compound potassium permanganate. This result indicates that bulk-like water exists in the reverse microemulsions formed with the fluorinated surfactant, and therefore the type of inverse microemulsion formed in the present study is quite different from that formed using PFPE.
3.3. Yeast extract Yeast is one of the major food sources of RF, and was therefore chosen to demonstrate the potential applications of using AOT-modified scCO2 to extract water-soluble components from this raw material. Current techniques for the extraction of RF from natural sources involves the use of some relatively hazardous organic solvents, such as, pyridine– acetic acid [23]. Yeast is autolysed yeast, i.e. the cell wall has been removed leaving only the contents of the yeast cells. Yeast extract typically contains approximately two-thirds amino acid (as organic peptide material), with the remainder a combination of moisture, ash, salt and nitrogen/phosphate material. A water-soluble vitamin analysis supplied by Oxoid indicated 60– 90 ppm RF in the sample, as well as the other vitamins, thiamin B1, pantothenic acid, pyridoxin B6, niacin and cyanocobalamin B12 [24]. Fig. 2 shows the absorption spectra obtained for 0.04 wt.% YX dissolved in water and also in AOT-modified scCO2. The spectrum from the supercritical experiment has had the AOT peak subtracted from the data to show only the peaks resulting from the water-soluble portion of YX. As seen from this plot, the spectra are quite similar in both systems. The major peak for YX in water occurred at 25892 nm, while the umax in the supercritical phase was between 260–263 nm. These similarities indicate that the
Fig. 2. UV – Vis spectra of polar compounds extracted from YX (2800 ml of 11.5 wt.% in water) into 0.03 M AOT/10 mol% pentanol/88.3 mol% scCO2/1.7 mol% water (initial conditions). Conditions of operation were 40oC, 250 bar and 20 Hz pump speed [13]. Also shown is a spectrum of 0.04 wt.% YX in water at ambient conditions. The AOT peak has been subtracted in the supercritical extraction profile.
water-soluble portion of YX was successfully extracted into scCO2 with the microemulsion phase present, indicating the potential of this technique for a wider range of applications. It should be noted that no extraction was observed when pure scCO2 only was used. At this stage of the process development, selective extraction of a particular vitamin from the others is not possible. However, this could be achieved by considering such factors as pH, pKa, the isoelectric point, or the introduction of a complexing agent.
4. Conclusions Solutes that cannot be solubilised in pure scCO2 have been successfully solubilised in inverse microemulsions of AOT-modified scCO2. Data obtained for the solubilisation of the vitamin, RF, indicated that the water pools in the supercritical phase contain a polar environment with an effective dielectric constant of 209 5.
B.H. Hutton et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 177–181
Ionic compounds requiring a highly polar environment could not be solubilised in the modified supercritical fluid, even though at ambient conditions using an organic solvent to replace scCO2 (and the same emulsion phase) these extractions were quite successful. These results lend support to the conclusion that the polarity of water in aggregates in scCO2 is much lower than bulk water. Finally, supercritical extraction of polar compounds from yeast extract was possible using AOT-modified scCO2. This extraction is not possible in pure scCO2. These results demonstrate the potential of using this alternative extraction technique on a wider range of applications — namely extraction of non-CO2 soluble compounds.
Acknowledgements This work was funded by an Australian Food Industry Science Centre (AFISC) postgraduate scholarship, and also the Advanced Mineral Products Special Research Centre (AMPC) at The University of Melbourne. Special thanks to Dr Neville Pamment for helpful discussions with the yeast work.
References [1] C. O’Toole, P. Richmond, J. Reynolds, Chem. Eng. June (1986) 73. [2] D.H. Logsdail, Process Eng. September (1983) 32. [3] R.N. Skinner, Chem. Aust. 50 (1983) 89. [4] H. Brogle, Chem. Ind. June (1982) 385.
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[5] P. Hubert, O. Vitzthum, Angew. Chem. Int. Ed. Engl. 17 (1978) 710. [6] R.P. de Filippi, Chem. Ind. June (1982) 390. [7] J.P. Calame, R. Steiner, Chem. Ind. June (1982) 399. [8] R.M. Lemert, K.P. Johnston, Ind. Eng. Chem. Res. 30 (1991) 1222. [9] G.S. Gurdial, S.J. MacNaughton, D.L. Tomasko, N.R. Foster, Ind. Eng. Chem. Res. 32 (1993) 1488. [10] S.S. Ting, S.J. MacNaughton, D.L. Tomasko, N.R. Foster, Ind. Eng. Chem. Res. 32 (1993) 1471. [11] B.H. Hutton, J.M. Perera, F. Grieser, G.W. Stevens, Coll. Surf. A Physicochem. Eng. Aspects 146 (1999) 227. [12] K. Harrison, J. Goveas, K.P. Johnston, Langmuir 10 (1994) 3536. [13] 20 Hz 103 ml water min − 1 at 20oC, 1 atm. To convert to CO2 flow rate, multiply by zwater (20oC, 1 atm)/zCO2 (T, P specified). [14] G.J. McFann, K.P. Johnston, Supercritical microemulsions, in: P. Kumar, K.L. Mittal (Eds.), Handbook of Microemulsion Science and Technology, Marcel-Dekker, New York, 1999, p. 9 (Chapter 9). [15] J. Eastoe, B.M.H Cazelles, D. Steytler, J.D. Holmes, A.R. Pitt, T.J. Wear, R.K. Heenan, Langmuir 13 (1997) 6980. [16] G.F.M. Ball, Water-Soluble Vitamin Assays in Human Nutrition, Chapman & Hall, London, 1994. [17] C.J. Drummond, F. Grieser, T.W. Healy, Faraday Discuss. Chem. Soc. 81 (1986) 95. [18] M.S. Fernandez, P. Fromherz, J. Phys. Chem. 81 (1977) 1755. [19] J.M. Perera, G.W. Stevens, F. Grieser, Coll. Surf. A Physicochem. Eng. Aspects 95 (1995) 185. [20] O. Madelung, Static dielectric constant of pure liquids and binary liquid mixtures, in: Numerical Data and Functional Relationships in Science and Technology, vol. 6, Springer, Berlin, 1991, p. 459. [21] A. Wesch, N. Dahmen, K. Ebert, Berichte der BunsenGesellschaft, Phys. Chem. Chem. Phys. 100 (1996) 1368. [22] M.J. Clarke, K.L. Harrison, K.P. Johnston, S.M. Howdle, JACS 119 (1997) 6399. [23] L.J. Machlin, Handbook of Vitamins, Marcel-Dekker, New York, 1991. [24] Oxoid, England; personal communication.
AOT reverse microemulsions in scCO2 — a further investigation Brenda H. Hutton a, Jilska M. Perera a, Franz Grieser b, Geoffrey W. Stevens a,* a
Department of Chemical Engineering, The Uni6ersity of Melbourne, Victoria 3010, Australia b School of Chemistry, The Uni6ersity of Melbourne, Victoria 3010, Australia Received 18 August 2000; accepted 20 December 2000
Abstract The solubility of riboflavin (RF), potassium ferricyanide (K3Fe(CN)6) and polar components from yeast extract (YX) in a sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) modified supercritical carbon dioxide (scCO2) system was investigated. The results from RF solubility studies indicate that the microemulsion phase formed by AOT contains a polar core with an effective static dielectric constant of 20 9 5. Although this core is not indicative of bulk water, the overall improvement in polarity from pure scCO2 is approximately a factor of 10. K3Fe(CN)6, a highly ionic compound, could not be solubilised into the modified supercritical fluid, indicating that the core was not sufficiently polar to dissolve highly ionic compounds. Finally, successful extraction of polar compounds from YX demonstrated the potential industrial applications of AOT-modified scCO2. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Riboflavin; Inverse microemulsion; Sodium bis-(2-ethylhexyl) sulfosuccinate (AOT)
1. Introduction Supercritical extraction using carbon dioxide (scCO2) as the solvent has become a popular alternative to solvent extraction processes, particularly in food-related areas which necessitate the use of a non-toxic solvent. The industrial usage of scCO2 is increasing, and some of these applications include the extraction of hops for beer production, the decaffeination of tea and coffee, * Corresponding author. Tel.: + 61-3-83446621; fax: +613-83444153. E-mail address: [email protected] (G.W. Stevens).
extraction of flavours such as paprika, fragrances such as lavender, and fish oils [1–7]. Unfortunately, due to the low polarity of scCO2, applications of this extraction technique are limited to substances of similar polarity. Therefore, it has not been possible to extract hydrophilic components, such as the water-soluble vitamins, from a matrix into the supercritical fluid phase. One solution to this problem has been the introduction of fluorinated surfactant systems into scCO2 to form reverse microemulsions [8–10], thereby providing water-soluble sites into which to extract polar substances. Unfortunately these
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surfactants are not suitable for food-related applications. In an earlier study, Hutton et al. [11] solubilised a common food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) into scCO2 using small amounts of either ethanol or n-pentanol as co-solvent. Although this surfactant had been previously assumed to be completely insoluble in scCO2 [12], Hutton et al. also found that it was possible to solubilise two polar substances, methyl orange and riboflavin, but not the highly ionic compound, 8-hydroxy-1,3,6pyrene-trisulphonic acid trisodium salt (HPTS) in this modified supercritical fluid inverse microemulsion [11]. The present study was undertaken in order to gain further information on the nature of the polar sites formed with reverse microemulsions in scCO2, in the presence of co-solvents. In addition, the water-soluble components from yeast extract are shown to be extracted using AOT-modified scCO2 and this demonstrates the potential industrial applications of this type of inverse microemulsion.
2. Experimental procedures
2.1. Materials Carbon dioxide, of supercritical fluid grade, was obtained from BOC gases. Analytical reagent grade organic solvents, n-pentane (99%, BDH), n-decane (\ 99%, Aldrich), 1,4-dioxane (99.5%, AJAX), ethanol (99.7– 100 vol.%, BDH) and npentanol (99%, Riedel– deHaen) were used without further purification. The probes potassium ferricyanide, K3Fe(CN)6 (99%) and Riboflavin (98%) were obtained from Aldrich and used without further purification. Yeast extract (Code L21) was obtained from Oxoid (England). Sodium bis(2-ethylhexyl) sulfosuccinate (99%) was obtained from Sigma and used without further purification. Water used in the experiments was purified by a Millipore Milli-RO 4 purification system, and had an average conductivity of 13.87 mS cm − 1 at 25oC.
2.2. Methods Details of the supercritical extraction unit have been reported elsewhere [11]. Briefly, however, it can be described as a recirculating system with an on-line UV–Vis spectrophotometric facility that directly measures the uptake of the relevant material into the supercritical carbon dioxide phase. Experiments were performed by initially preparing an AOT solution in n-pentanol, such that the final concentrations once mixed with scCO2 were 0.03 M AOT and 10 mol% alcohol. These preparations contained 1.7 mol% water initially, and this solution was then added to the scCO2. A small quantity of probe material was injected (as a dispersed solid) into the modified supercritical phase. Subsequently, water was injected in small quantities (generally 100–200 ml amounts) into the system during the experiment, and UV–Vis spectra were obtained. Operating conditions were generally 250 bar, 40oC, and 20 Hz circulating pump speed [13]. It is assumed that when water is present in the modified scCO2 that an inverse microemulsion system is produced [14,15]. Experiments under ambient conditions using alkanes as the continuous phase were performed prior to the supercritical experiments to give a first approximation of the likely behaviour of the probe under investigation in the modified scCO2. The same quantities of surfactant and co-solvent were used as described above, and the supercritical fluid was replaced with n-decane or n-pentane. Water was added to these solutions and spectra obtained after each addition.
3. Results and discussions
3.1. Ribofla6in The UV –Vis spectrum of RF in water shown in the insert of Fig. 1 indicates umax at 37392 and 44492 nm. In the supercritical phase these peaks are shifted to 36092 and 4409 2 nm, respectively. Experiments with the following combinations of materials all resulted in effectively no solubility of the RF: RF/scCO2, RF/scCO2/water,
B.H. Hutton et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 177–181
RF/pentanol/scCO2 and RF/pentanol/scCO2/H2O. This demonstrates that the presence of AOT is essential for the solubility of the RF. Similarly if water is excluded from the solvent the solubility of RF is very small. Fig. 1 shows that an increase in W0, i.e. the water content of the reverse microemulsion phase, allows an increase in the uptake of RF. This data indicates further solubilisation of precipitated RF as the water content of the reverse microemulsion is increased. Based on the solubility limit of RF in water at 40°C [16], it is expected that a significant quantity of the injected RF would be in the solid form. The next step was to estimate the dielectric environment of the water pools in the reverse microemulsion. The peak at 3609 2 nm is solvatochromic and was therefore used to estimate the dielectric constant of the supercritical phase [17–19]. The UV– Vis spectrum of RF in various 1,4-dioxane/water mixtures (of known static dielectric constant [20]) was monitored. Assuming that 1,4-dioxane/water mixtures are an adequate reference for the dielectric environment of the water pools in the supercritical phase, an effective dielectric constant can be assigned to each umax.
Fig. 1. Absorbance (umax = 440 nm) vs. W0 for 200 ml (of 0.031 M in water) RF in 0.03 M AOT/10 mol% pentanol/88.3 mol% scCO2/1.7 mol% water (initial conditions) at 40oC, 250 bar and 20 Hz pump speed [13]. The insert shows the UV – Vis spectrum of RF in water.
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Based on this calibration scale, it was estimated that RF in AOT-modified scCO2 was situated in an environment having a dielectric constant of 2095. The estimated dielectric constant via the 1,4-dioxane/water system shows relatively large changes in effective dielectric constant with small changes in wavelength over this range, and therefore it is likely that the actual polarity of the aggregates is just sufficiently high enough to allow solubility of RF into the core. Regardless of this uncertainty, these results have shown that the aggregate core in the supercritical fluid is not indicative of bulk-like water. Nevertheless, the overall polarity of the supercritical fluid system has been enhanced by a factor of approximately 10 (scCO2 has an effective dielectric constant between 1.0 and 1.6, depending on temperature and pressure [21]).
3.2. Potassium ferricyanide K3Fe(CN)6 is a highly ionic compound requiring complete hydration for solubility in a water environment. For this reason, it was chosen in order to determine whether or not the conclusions reached with the RF results were consistent with the expected behaviour of an ionic solute in the modified scCO2. Also, Fe(CN)36 − with its negative charge can be expected to be repelled by the anionic head groups of AOT, and forced away from the interfacial region and into the core of the inverse micelle, the more polar region of the emulsion. An experiment at ambient conditions with npentane as the solvent and pentanol as co-solvent, was performed to determine the suitability of this compound for solubilisation into an AOT reverse microemulsion. It was possible to solubilise K3Fe(CN)6 in the reverse microemulsion (W0 = 10.5) and the resulting UV–Vis spectrum was similar to that obtained in pure water. The equivalent experiment was performed under supercritical conditions, and although the initial W0 was 10.5, no K3Fe(CN)6 whatsoever was detected in the modified supercritical fluid. These results are consistent with our earlier study [11], where it was found that HPTS, another ionic compound, could not be solubilised in the modified scCO2. Together, these results suggest that although the water pools were sufficiently polar to
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solubilise moderately polar components such as RF and MO, they were insufficiently polar to promote solubilisation and hydration of two highly ionic compounds. Researchers [22] using a fluorinated surfactant, ammonium carboxylate perfluoropolyether (PFPE), in scCO2 were able to solubilise the ionic compound potassium permanganate. This result indicates that bulk-like water exists in the reverse microemulsions formed with the fluorinated surfactant, and therefore the type of inverse microemulsion formed in the present study is quite different from that formed using PFPE.
3.3. Yeast extract Yeast is one of the major food sources of RF, and was therefore chosen to demonstrate the potential applications of using AOT-modified scCO2 to extract water-soluble components from this raw material. Current techniques for the extraction of RF from natural sources involves the use of some relatively hazardous organic solvents, such as, pyridine– acetic acid [23]. Yeast is autolysed yeast, i.e. the cell wall has been removed leaving only the contents of the yeast cells. Yeast extract typically contains approximately two-thirds amino acid (as organic peptide material), with the remainder a combination of moisture, ash, salt and nitrogen/phosphate material. A water-soluble vitamin analysis supplied by Oxoid indicated 60– 90 ppm RF in the sample, as well as the other vitamins, thiamin B1, pantothenic acid, pyridoxin B6, niacin and cyanocobalamin B12 [24]. Fig. 2 shows the absorption spectra obtained for 0.04 wt.% YX dissolved in water and also in AOT-modified scCO2. The spectrum from the supercritical experiment has had the AOT peak subtracted from the data to show only the peaks resulting from the water-soluble portion of YX. As seen from this plot, the spectra are quite similar in both systems. The major peak for YX in water occurred at 25892 nm, while the umax in the supercritical phase was between 260–263 nm. These similarities indicate that the
Fig. 2. UV – Vis spectra of polar compounds extracted from YX (2800 ml of 11.5 wt.% in water) into 0.03 M AOT/10 mol% pentanol/88.3 mol% scCO2/1.7 mol% water (initial conditions). Conditions of operation were 40oC, 250 bar and 20 Hz pump speed [13]. Also shown is a spectrum of 0.04 wt.% YX in water at ambient conditions. The AOT peak has been subtracted in the supercritical extraction profile.
water-soluble portion of YX was successfully extracted into scCO2 with the microemulsion phase present, indicating the potential of this technique for a wider range of applications. It should be noted that no extraction was observed when pure scCO2 only was used. At this stage of the process development, selective extraction of a particular vitamin from the others is not possible. However, this could be achieved by considering such factors as pH, pKa, the isoelectric point, or the introduction of a complexing agent.
4. Conclusions Solutes that cannot be solubilised in pure scCO2 have been successfully solubilised in inverse microemulsions of AOT-modified scCO2. Data obtained for the solubilisation of the vitamin, RF, indicated that the water pools in the supercritical phase contain a polar environment with an effective dielectric constant of 209 5.
B.H. Hutton et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 189 (2001) 177–181
Ionic compounds requiring a highly polar environment could not be solubilised in the modified supercritical fluid, even though at ambient conditions using an organic solvent to replace scCO2 (and the same emulsion phase) these extractions were quite successful. These results lend support to the conclusion that the polarity of water in aggregates in scCO2 is much lower than bulk water. Finally, supercritical extraction of polar compounds from yeast extract was possible using AOT-modified scCO2. This extraction is not possible in pure scCO2. These results demonstrate the potential of using this alternative extraction technique on a wider range of applications — namely extraction of non-CO2 soluble compounds.
Acknowledgements This work was funded by an Australian Food Industry Science Centre (AFISC) postgraduate scholarship, and also the Advanced Mineral Products Special Research Centre (AMPC) at The University of Melbourne. Special thanks to Dr Neville Pamment for helpful discussions with the yeast work.
References [1] C. O’Toole, P. Richmond, J. Reynolds, Chem. Eng. June (1986) 73. [2] D.H. Logsdail, Process Eng. September (1983) 32. [3] R.N. Skinner, Chem. Aust. 50 (1983) 89. [4] H. Brogle, Chem. Ind. June (1982) 385.
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