Investigation of AOT reverse microemulsions in supercritical carbon dioxide

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Abstract

Bis(2-ethylhexyl) sodium sulphosuccinate (AOT) was successfully solubilised in supercritical carbon dioxide (scCO2), with ethanol or pentanol as co-solvent. Three molecular spectroscopic probes: methyl orange (MO), 8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt (HPTS), and riboflavin (RF) were used to examine the solubilisation characteristics of the water/scCO2 microemulsions formed with AOT. MO was extracted at various operating conditions, although the wavelength of its solvatochromic absorption maximum was not indicative of bulk water properties. Instead, the spectral results imply that MO may be located at the surfactant/water interface. The highly water-soluble dye HPTS was unable to be extracted into scCO2/AOT/water systems, suggesting that the water in the reverse micelle core was not as polar under supercritical conditions as those at ambient conditions. Finally, RF was extracted into the supercritical phase (40°C, 175 bar) with pentanol co-solvent, with an apparent enhanced uptake compared with the value at 40°C and ambient pressure in bulk water. This appears to be due to the presence of microcrystals dispersed in the supercritical phase.

Introduction

Supercritical extraction using carbon dioxide (scCO2) as bulk solvent has become very popular in recent years, due to its environmentally benign nature. It is particularly popular in food-related applications, where the need for a non-toxic solvent in the extraction of many foodstuffs is highly desirable. While this technology has been adapted industrially in areas such as the extraction of fats, oils, flavours and fragrances [1], [2], [3], no significant progress has been made in other food related areas, such as in the extraction of water-soluble vitamins, water-soluble proteins and food colourings that tend to be more polar in nature. This has been attributed to the non-polar nature of scCO2, which exhibits a dielectric constant even lower than most organic solvents.

Over the past decade or so, researchers have investigated ways of increasing the polarity of scCO2. Some of these techniques include the addition of polar co-solvents, such as methanol or ethanol [4], [5], [6], treating the sample before its extraction [7], and introducing a reverse microemulsion phase into bulk scCO2 to provide microwater pools as sites for solubilisation [8], [9], [10], [11], [12].

Wesch et al. [13] measured the static dielectric constant of carbon dioxide at various pressures and temperatures, and found that it varied between 1.0 at atmospheric conditions to 1.6 at 300 bar, 40°C. They subsequently showed an increase in the dielectric constant when small amounts of ethanol were added—12 vol.% resulted in a dielectric constant of 2.62, at 100 bar, 40°C. Other researchers have also reported the enhancement in extractions of moderately polar components with the addition of a co-solvent [5], [6], [14].

Preliminary work with microemulsions in scCO2 has indicated that common surfactants used in hydrocarbon solvents are generally not suitable in carbon dioxide [8], [9], [12], [15], [16], [17], [18], and specialised surfactants designed to be ‘CO2-philic’ are required. The most successful of these surfactants are those containing fluorinated functional groups, and to a lesser extent, silicone groups. The surfactant bis(2-ethylhexyl)sodium sulphosuccinate (AOT), generally thought to be completely insoluble in scCO2 [8], was shown by Ihara et al. [19] to be soluble with 7.5% ethanol as a co-solvent. Furthermore, Jackson et al. [20] demonstrated that the dielectric constant of a range of solvents needed to be greater than 1.61 for AOT to dissolve in the solvent. These two groups, along with Wesch et al. [13], have demonstrated that AOT can be solublised in scCO2 if a small amount of co-solvent such as ethanol, is present.

These findings have prompted the current study to investigate AOT reverse microemulsions in supercritical carbon dioxide, with ethanol as co-solvent (and later, 1-pentanol). The bulk of this study used the solvatochromic probe methyl orange (MO), since it had been used by other researchers to investigate reverse microemulsions in various systems [21], [22], [23], [24]. An appreciation of the effects of temperature and pressure on the extraction process were also considered. The highly water-soluble probe 8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt (HPTS) was then investigated [25], and was chosen to obtain further information on the properties of the water pool in scCO2. Finally, to demonstrate the possibilities of using this technique from an industrial viewpoint, extraction of water-soluble vitamin B2 or riboflavin (RF) was performed. Fig. 1 shows the molecular formulae of all dyes used in this study, as well as AOT.

Section snippets

Materials used

All high pressure experiments used supercritical fluid grade carbon dioxide from BOC gases. This grade had <50 ppm moisture, typically having a 20 ppm moisture content [26]. AR grade methanol (99.8%), ethanol (99.7–100 vol.%) and n-pentane (99%) were obtained from BDH. n-Decane (99+%) was obtained from Aldrich, and 1-pentanol (99%) was from Riedel-deHaen. 2-Propanol (99.8%) was from Fisons, while n-butanol (99%) was from Tokyo Kasei Kogyo. AOT (99%, FW 444.6), was obtained from Sigma and used

Ambient experiments

The solvatochromic behaviour of MO was examined by dissolving it in various solvents having different static dielectric constants. The absorption maximum shifted to larger wavelengths (red shift) as the solvent’s dielectric strength was increased, this behaviour also being noted by other researchers [21], [24]. Preliminary investigations of MO extraction into reverse microemulsion organic phases were performed, with the results summarised in Table 1.

For the experiments described, MO was

Conclusions

AOT/water reverse microemulsions with ethanol or pentanol co-solvent allowed significant amounts of the solvatochromic probe MO to be extracted into supercritical carbon dioxide. Only small quantities were extracted when this surfactant phase was not present. The position of the MO peak in both ambient and supercritical systems was not indicative of a bulk-like water environment, indicating that the MO is most likely residing in the interfacial region of the surfactant core. The polarity of

Acknowledgements

This work has been 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. Many thanks to Dr Brian Imison (Afisc) for helpful discussions with the trans-β-carotene extraction work.

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