J. of Supercritical Fluids 32 (2004) 97–103 Investigation on the solubilization of organic dyes and micro-polarity in AOT water-in-CO 2 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 CO 2 , could be conveniently solubilized in AOT water-in-CO 2 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-CO 2 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-CO 2 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 CO 2 ; Microemulsions; Solubilization; Micro-polarity; UV-Vis spectroscopy 1. Introduction Conventional dyeing and subsequent washing processes are generally performed in water-based dye-baths. How- ever, 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 sig- nificant attention. Supercritical carbon dioxide (SC CO 2 ) is one of the most commonly used supercritical fluids (SCFs) as is naturally abundant, non-flammable, essen- tially non-toxic, and the least expensive solvent after water. Moreover, CO 2 attains the supercritical state at ambient temperature (T c = 31 ◦ C) and a relatively moderate pres- ∗ Corresponding author. Tel.: +81-22-237-5211; fax: +81-22-237-5224. E-mail address: y-ikushima@aist.go.jp (Y. Ikushima). sure (P c = 73.8 bar). Therefore, SC CO 2 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 CO 2 is a poor solvent of the hydrophilies because of its very low value of di- electric constant, ε, polarizability per volume, /, and not strongly engaging in van der Waals interactions. So unlike SC CO 2 -soluble dyes, water-soluble organic dyes such as thymol blue (TB), dimidium bromide (DB) and methyl or- ange (MO) do not dissolve in SC CO 2 . In the past decade, several approaches have been explored to enhance the solu- bility of polar substance in SC CO 2 [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 CO 2 . However, water-soluble dyes are still insufficiently soluble in this SC CO 2 entrainer medium in the dyeing process [4]. Our strategy is to dissolve the water-soluble dyes in thermodynamically stable and optically transparent water-in-CO 2 (W/C) reverse microemulsions which can provide nano-sized polar micro-water domains as sites for solubilization of the hydrophilic dye molecules. 0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2004.02.006