Nanostructures in Water-in-CO2 Microemulsions Stabilized by Double-chain Fluorocarbon Solubilizers

Masanobu Sagisaka, Shuho Iwama, Shinji Ono, Atsushi Yoshizawa, Azmi Mohamed, Stephen Cummings, Ci Yan, Craig James, Sarah E Rogers, Richard K Heenan, Julian Eastoe

Research output: Contribution to journalArticle (Academic Journal)peer-review

20 Citations (Scopus)
256 Downloads (Pure)

Abstract

High-pressure small-angle neutron scattering (HP-SANS) studies were conducted to investigate nanostructures and interfacial properties of water-in-supercritical CO2 (W/CO2) microemulsions with double-fluorocarbon-tail anionic surfactants, having different fluorocarbon chain lengths and linking groups (glutarate or succinate). At constant pressure and temperature, the microemulsion aqueous cores were found to swell with an increase in water-to-surfactant ratio, W0, until their solubilizing capacities were reached. Surfactants with fluorocarbon chain lengths of n = 4, 6, and 8 formed spherical reversed micelles in supercritical CO2 even at W0 over the solubilizing powers as determined by phase behavior studies, suggesting formation of Winsor-IV W/CO2 microemulsions and then Winsor-II W/CO2 microemulsions. On the other hand, a short C2 chain fluorocarbon surfactant analogue displayed a transition from Winsor-IV microemulsions to lamellar liquid crystals at W0 = 25. Critical packing parameters and aggregation numbers were calculated by using area per headgroup, shell thickness, the core/shell radii determined from SANS data analysis: these parameters were used to help understand differences in aggregation behavior and solubilizing power in CO2. Increasing the microemulsion water loading led the critical packing parameter to decrease to ∼1.3 and the aggregation number to increase to >90. Although these parameters were comparable between glutarate and succinate surfactants with the same fluorocarbon chain, decreasing the fluorocarbon chain length n reduced the critical packing parameter. At the same time, reducing chain length to 2 reduced negative interfacial curvature, favoring planar structures, as demonstrated by generation of lamellar liquid crystal phases.
Original languageEnglish
Pages (from-to)7618-7628
Number of pages11
JournalLangmuir
Volume29
Issue number25
Early online date23 May 2013
DOIs
Publication statusPublished - 25 Jun 2013

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