Colloids and Surfaces A: Physicochemical and Engineering Aspects
AOT reverse microemulsions in scCO2 — a further investigation
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, extraction of flavours such as paprika, fragrances such as lavender, and fish oils [1], [2], [3], [4], [5], [6], [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], [9], [10], thereby providing water-soluble sites into which to extract polar substances. Unfortunately these 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-1,3,6-pyrene-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.
Section snippets
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 n-pentanol (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).
Riboflavin
The UV–Vis spectrum of RF in water shown in the insert of Fig. 1 indicates λmax at 373±2 and 444±2 nm. In the supercritical phase these peaks are shifted to 360±2 and 440±2 nm, respectively. Experiments with the following combinations of materials all resulted in effectively no solubility of the RF: RF/scCO2, RF/scCO2/water, 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
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 20±5.
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
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.
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