New Class of Amphiphiles Designed for Use in Water-in-Supercritical CO2 Microemulsions

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• 作者：Masanobu Sagisaka ; Shunsuke Ogiwara ; Shinji Ono ; Craig James ; Atsushi Yoshizawa ; Azmi Mohamed ; Sarah E. Rogers ; Richard K. Heenan ; Ci Yan ; Jocelyn Alice Peach ; Julian Eastoe
• 刊名：Langmuir
• 出版年：2016
• 出版时间：November 29, 2016
• 年：2016
• 卷：32
• 期：47
• 页码：12413-12422
• 全文大小：442K
• ISSN：1520-5827

文摘

Water-in-supercritical CO₂ microemulsions formed using the hybrid F–H surfactant sodium 1-oxo-1-[4-(perfluorohexyl)phenyl]hexane-2-sulfonate, FC6-HC4, have recently been shown to have the highest water-solubilizing power ever reported. FC6-HC4 demonstrated the ability to outperform not only other surfactants but also other FCm-HCn analogues containing different fluorocarbon and hydrocarbon chain lengths (Sagisaka, M. et al. Langmuir 2015, 31, 7479–7487). With the aim of clarifying the key structural features of this surfactant, this study examined the phase behavior and water/supercritical CO₂ aggregate formation of 1-oxo-1-[4-(perfluorohexyl)phenyl]hexane (Nohead FC6-HC4), which is an FC6-HC4 analogue but now, interestingly, without the sulfonate headgroup. Surprisingly, Nohead FC6-HC4, which would not normally be identified as a classic surfactant, yielded transparent single-phase W/CO₂ microemulsions with polar cores able to solubilize a water-soluble dye, even at pressures and temperatures so low as to approach the critical point of CO₂ (e.g., ∼100 bar at 35 °C). High-pressure small-angle scattering (SANS) measurements revealed the transparent phases to consist of ellipsoidal nanodroplets of water. The morphology of these droplets was shown to be dependent on the pressure, Nohead FC6-HC4 concentration, and water-to-surfactant molar ratio. Despite having almost the same structure as Nohead FC6-HC4, analogues containing both shorter and longer hydrocarbons were unable to form W/CO₂ microemulsion droplets. This shows the importance of the role of the hydrocarbon chain in the stabilization of W/CO₂ microemulsions. A detailed examination of the mechanism of Nohead FC6-HC4 adsorption onto the water surface suggests that the hexanoyl group protrudes into the aqueous core, allowing for association between the carbonyl group and water.