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Effect of Fluorocarbon and Hydrocarbon Chain Lengths In Hybrid Surfactants for Supercritical CO2 Masanobu Sagisaka, Shinji Ono, Craig James, Atsushi Yoshizawa, Azmi Mohamed, Frederic Guittard, Sarah E. Rogers, Richard K. Heenan, Ci Yan, and Julian Eastoe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01737 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 21, 2015

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Effect of Fluorocarbon and Hydrocarbon Chain Lengths

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In Hybrid Surfactants for Supercritical CO2

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Masanobu Sagisaka1*, Shinji Ono1, Craig James1, Atsushi Yoshizawa1, Azmi Mohamed 2,3, Frédéric Guittard4,

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Sarah E. Rogers5, Richard K. Heenan5, Ci Yan6, and Julian Eastoe6

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Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University,

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3 Bunkyo-cho, Hirosaki, Aomori 036-8561, JAPAN 2

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Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris,

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35900 Tanjong Malim, Perak, Malaysia 3

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Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan Sultan

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Idris, 35900 Tanjong Malim, Perak, Malaysia 4

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Univ. Nice Sophia-Antipolis, CNRS, Equipe Surfaces et Interfaces, Parc Valrose, 06100 Nice- France

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ISIS-CCLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K.

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.

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*To whom all correspondence should be addressed

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Masanobu SAGISAKA

E-mail: [email protected]

Phone and Fax: +81-172-39-3579

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Abstract

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Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains have

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recently been shown to solubilize water and form elongated reversed micelles in supercritical CO2. To

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clarify the most effective fluorocarbon (FC) and hydrocarbon (HC) chain lengths, the aggregation

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behavior and interfacial properties of hybrid surfactants FCm-HCn (FC length m / HC length n = 4/2,

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4/4, 6/2, 6/4, 6/5, 6/6 and 6/8) were examined in W/CO2 mixtures as functions of pressure, temperature,

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and water-to-surfactant molar ratio (W0). The solubilizing power of hybrid surfactants for W/CO2

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microemulsions was strongly affected by, not only the FC length, but also that of the HC. Although the

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surfactants having short FC and/or HC tails (namely, m/n = 4/2, 4/4, and 6/2) did not dissolve in

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supercritical CO2 (even at ~17 mM, ≤ 400 bar, temperature ≤ 75 ºC, and W0 = 0-40), the other hybrid

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surfactants were able to yield transparent single-phase W/CO2 mixtures identified as microemulsions.

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The solubilizing power of FC6-HCm surfactants reached a maximum (W0 ~80 at 45 ºC and 350 bar)

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with a hydrocarbon length, m, of 4. The W0 value of 80 is the highest for a HC-FC hybrid surfactant,

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matching the highest value reported for a FC surfactant which contained more FC groups. High-pressure

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SANS measurements from FCm-HCn/D2O/CO2 microemulsions were consistent with growth of the

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microemulsion droplets with increasing W0. In addition, not only spherical reversed micelles but also

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non-spherical assemblies (rod-like or ellipsoidal) were found for the systems with FC6-HCn (n = 4-6).

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At fixed surfactant concentration and W0 (17mM and W0 = 20). The longest reversed micelles were

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obtained for FC6-HC6 where a mean aspect ratio of 6.3 was calculated for the aqueous cores.

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Keywords: Supercritical CO2, Microemulsion, Hybrid surfactant, Solubilizing power, Small-Angle

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Neutron Scattering

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1. Introduction

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Supercritical CO2 (scCO2) has received much attention for use in industrial applications due to

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attractive properties such as low cost, inflammability, environmentally benignity, natural abundance,

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high mass transfer, and pressure/temperature-tunable solvency (or CO2 density) 1. ScCO2 is currently

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used as a green solvent for organic synthesis, dry cleaning, polymerization, extraction, nanomaterial

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processing amongst others. Unfortunately, supercritical CO2 can dissolve only nonpolar and small

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molecular mass materials, with large polar materials always separating in neat scCO22. Improving the

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poor solubility of polar materials is important for developing the potential applications of scCO2. One of

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the most promising approaches to increase the solubility of polar substances is to form reversed micelles

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with high-polarity aqueous cores in the continuous scCO2 phase, that is, water-in-scCO2 microemulsions

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(W/CO2 µEs).2 Since such organized fluids have the attractive characteristics of scCO2, as well as the

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solvation properties of bulk water, they have potential as volatile organic compound (VOC)-free and

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energy-efficient solvents for nano-material synthesis, enzymatic reactions, dry-cleaning, dyeing, and

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preparation of inorganic/organic hybrid materials2.

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To be a viable green and economical technology, the amount of surfactant used for W/CO2 µEs

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should be as small as possible, and this needs to be balanced against the need for large interfacial areas

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in W/CO2 µEs and appropriate levels of dispersed water needed to enhance process efficiencies. One

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approach to meet these requirements is to explore or develop highly efficient solubilizers for W/CO2

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µEs, and studies aiming to do this started in the 1990s.3

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The development of CO2-philic hydrocarbon surfactants for scCO2 has been recognized as an

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important task for economic and environmental reasons.3-6 However, most commercial and known

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hydrocarbon surfactants are insoluble and inactive in scCO2 systems3. Note that the commercial

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analogue Aerosol-OT (sodium bis-(2-ethyl-1-hexyl) sulfosuccinate, AOT) is insoluble in scCO2 and

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hence ineffective at stabilizing W/CO2 µEs.3 In this regard, it became apparent that conventional

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surfactant-design theory cannot be applied to W/CO2 systems, and that CO2-philicity is not directly

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comparable to oleo-philicity. Therefore, advancing molecular-design theory for CO2-philic surfactants ACS Paragon Plus Environment

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has required new directions and paradigms in the field of surfactants. In current studies for CO2-soluble

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compounds, highly branched hydrocarbons4,5, especially methyl-branches, ester and ether groups have

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been reported to increase solubility in scCO2. Unfortunately, an efficient and cost effective hydrocarbon

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stabilizer for W/CO2 µEs, like the AOT used commonly for W/O µEs6, has not yet been found.

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Many earlier studies reported that several fluorinated surfactants, including perfluoropolyether

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(PFPF) surfactants and fluorinated AOT analogues, could dissolve in CO2 and exhibit a high interfacial

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activity at the W/CO2 interface, suggesting the feasibility of forming W/CO2 µEs.7-10 The water-

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solubilizing power in CO2 was often discussed in terms of the water-to-surfactant molar ratio W0

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(=[water]/[surfactant]). Hereafter, the maximal W0 achievable in a single-phase W/CO2 microemulsion,

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namely W0max, is used to evaluate the solubilizing power. In the cases of PFPE surfactants the reported

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W0max reached ~20.7-10

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It is not understood why such fluorocarbon surfactants are so good for solubilization of water in

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scCO2. Recent molecular simulation studies11,12 have elucidated that when compared with HC chains,

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FC groups have (1), stronger interactions with CO2 via quadrupolar and dispersion interactions, and (2),

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weaker FC-FC chain-chain interactions which are due to a weak repulsion, electrostatic in origin. These

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properties conspire together to give FC surfactant reversed micelles with better solvation by CO2, and

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this causes lower surfactant interfacial packing densities, and weaker attractive inter-micellar

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interactions compared with hydrocarbon surfactant analogues.

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Recently, with the aim of optimizing the surfactant structure of fluorinated AOT analogues for µEs,

double-FC-tail

anionic

surfactants

with

various

FC

lengths13

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W/CO2

and

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sulfoglutarate/sulfosuccinate headgroups were examined to probe the effect of not only FC length, but

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also the addition of methylene spacers between double tails. This examination found that the

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solubilizing powers of the glutarates nFG(EO)2 were higher than those of the succinate analogues

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nFS(EO)2, and the most efficient surfactant was found to be 4FG(EO)2 at 75°C (W0max = 80) despite

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having the shortest FC (perfluorobutyl) tails.13 Considering that fluorocarbons are CO2-philic groups

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and longer FC surfactants generally have higher solubilising power,3,7-12 the fact that this can be ACS Paragon Plus Environment

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obtained by the shortest FC 4FG(EO)2 is at first sight surprising. In further studies, the minimum fluorine content necessary to render a surfactant CO2-philic has been identified by using double-pentyltail surfactants with different fluorination levels14. In these surfactants, at least two fluorinated carbons (CF3CF2-) were required to stabilize W/CO2 µEs.14 Through the studies mentioned above, the understanding behind the effect of FC-tail length on the solubilizing power for symmetrical double FCtail surfactants (i.e. fluorinated AOT-analogues) has been becoming clearer year-by-year. As with the fully fluorinated surfactant series, two-tail hybrid surfactants, having separate HC and FC chains in the same molecule, have also been evaluated for stabilization of W/CO2 microemulsions. The first successful example of a CO2-philic hybrid surfactant was sodium 1pentadecafluoroheptyl-1-octanesulfate (F7H7, (C7H15)(C7F15)CHOSO3Na), which solubilized water up to W0max = 35.15 Further studies16 of hybrid surfactants related to F7H7 but with different FC and HC chain lengths, observed the formation of W/CO2 µEs for most of the analogues, but smaller attainable W0max values than for F7H7. On the other hand, these hybrid surfactant systems were often reported to form elongated reversed micelles16, whereas those formed by non-hybrid surfactants are likely to be spherical. The formation of elongated aggregates with a high aspect ratio (rod length/diameter) can thicken dense CO2 and will be a viable way to achieve higher efficiencies in enhanced oil recovery17. Unfortunately, these unique characteristics have never been examined in detail in terms of the hybrid tail structure (e.g. effect of FC and HC length and the balance of both). How does a hybrid surfactant stabilize the W/CO2 microemulsion to form elongated reversed micelles? What is the role of the non-CO2-philic HC-tail? These questions have never been clarified yet could be very interesting and important topics for chemists in this field. To advance CO2-philic surfactant design theory, this study has focused on the effect of the hybrid tail structure (e.g. FC-HC balance) on solubilizing power, aggregate size and shape in scCO2. This provides important information on the structure-property correlations of hybrid surfactants such as hydrophilic/CO2-philic balance (HCB)18, critical packing parameter (CPP)19, interactions between tail-tail and tail-head groups20.


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Recently, a series of the hybrid surfactant, sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2sulfonates, FCm-HCn (FC length m = 6, HC length n = 2, 4, 6 and 8) were examined for formation of W/CO2 systems.21 However, insufficient data were available to clarify the effect of the FC and HC lengths of the hybrid surfactant on phase stability, and also on the nanostructures of reversed micelles. To reveal the separate roles of FC and HC tails, this study examined in detail the phase behavior, solubilizing power, and aggregate nanostructure for seven FCm-HCn compounds (m/n = 4/2, 4/4, 6/2, 6/4, 6/5, 6/6, and 6/8) in scCO2 by using high-pressure UV-vis light absorbance and small-angle neutron scattering (SANS). The significance of this study to the field of surfactant science is that optimized, super-efficient, low fluorine containing surfactants are now available for stabilization of W/CO2 µEs and formation of CO2-philic elongated reversed micelles.


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2. Experimental Section 2.1. Materials The family of surfactants were based on the hybrid surfactant sodium 1-oxo-1-[4(perfluoroalkyl)phenyl]alkane-2-sulfonates, FCm-HCn (FC length m = 4 or 6, HC length n = 2, 4, 5, 6 and 8) and the non-hybrid analogue surfactant sodium 1-oxo-1-[4-(hexyl)phenyl]-2-hexanesulfonates, HC6-HC6. The surfactants FCm-HCn with m/n = 4/2, 4/4, 6/2, 6/6, and 6/8 and HC6-HC6 were provided by Prof. Yoshino and Dr. Kondo at Tokyo University of Science. The other surfactants FC6HC4 and FC6-HC5 were synthesized and purified as described in supporting information. For the synthesis, iodo-benzene, hexanoyl chloride, heptanoyl chloride were purchased from Tokyo Chemical Industries, and used without further purification. Reagent grade 1,4-dioxane, 1,2-dichloroethane, copper powder, sodium hydroxide were commercially obtained from Wako Pure Chemical Industries and employed as received. Tridecafluorohexyl iodide and sulphur trioxide were purchased from Synquest laboratories and Nacalai tesque, respectively. Structures of FCm-HCn, HC6-HC6, and the other control hybrid surfactants FmHn are shown in Table 1 along with the interfacial properties of aqueous solutions obtained by standard measurements22,23. Ultrapure water with a resistivity of 18.2 MΩ cm was generated from a Millipore Milli-Q Plus system. CO2 was of 99.99% purity (Ekika Carbon Dioxide Co., Ltd.). The structures of the steric models and the length of one surfactant molecule in the absence of other molecules were calculated by MM2 (Molecular Mechanics program 2) calculations (Chem 3D; CambridgeSoft Corp., Cambridge, MA).

2.2. Surface tension measurements Surface tensions of aqueous surfactant solutions were measured using a Wilhelmy tensiometer (CBVP-Z, Kyowa Interface Science) equipped with a platinum plate. The measurements were performed at 35 ± 0.1 °C until constant values of the surface tension of the aqueous surfactant solutions were obtained; the experimental error was less than 0.1 mN/m. The critical micelle concentration ACS Paragon Plus Environment

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(CMC) was obtained from the point of intersection of the curves in the graph of surface tension versus logarithm of the surfactant concentration.

2.3. Phase behavior and UV-visible absorption spectral measurements A high-pressure vessel with an optical window and a moveable piston inside the vessel was used to observe phase behaviour of surfactant/water/scCO2 mixtures with varying pressure and temperature. A detailed description of the experimental apparatus and procedures for the measurements can be found elsewhere.13,14 In order to examine formation of aqueous cores in W/CO2 µEs, UV-visible absorption spectroscopy measurements were performed using methyl orange (MO) as a trace marker dye, on a double-beam spectrophotometer (Hitachi High-Technologies, Co., U-2810), with a quartz window pressure cell (volume: 1.5 cm3) that was connected to the experimental apparatus. The cell was made of stainless steel (SUS316) and had three quartz windows with a thickness of 8 mm. Each window had an inner diameter of 10 mm and was positioned so as to provide a perpendicular 10-mm optical path. Each window was attached to the stainless steel body of the cell using PTFE kel-F packing. The windows were fastened tightly to the steel body, thereby compressing the packing between the stainless steel parts and the quartz window and providing excellent sealing (tested up to 400 bar). The temperature of the cell was controlled by circulating water with a thermostat bath. Spectroscopic measurements were performed and the resulting absorption spectra of the cell windows were compared with those of a standard quartz cell for an aqueous MO solution at ambient pressure; it was observed that both the spectra were in good agreement with each other. The measurements of the water/surfactant/scCO2 systems were performed at temperatures of 35 – 75 °C and pressures of lower than 400 bar. The densities of CO2 were calculated using the SpanWagner equation of state (EOS)34. Pre-determined amounts of surfactant and CO2 (20.0g), where the ACS Paragon Plus Environment

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molar ratio of surfactant to CO2 was fixed at 8 × 10-4, were loaded into a variable-volume high-pressure optical cell. Then, water or an aqueous MO solution (3 mmol L-1), was added into the optical cell through a six-port valve until the clear Winsor-IV W/CO2 µE (i.e. single-phase W/CO2 µE) solution became a turbid macroemulsion or a precipitated hydrated surfactant. Surfactant molar concentration was in the range 10-20 mmol L-1, for example 16.7 mmol L-1 at 45 ºC and 350 bar, as the inner volume of the cell was varied by changing experimental pressure and temperature. During spectroscopic measurements, the scCO2 mixture was stirred and circulated between the optical vessel and the quartz window cell until a constant absorbance was attained. The circulation was then discontinued; the valves between the vessel and the quartz window cell were closed, and the measurement was performed. The physical properties of the continuous phase of scCO2 were assumed to be equivalent to those of pure CO2.

2.4 High-Pressure Small-Angle Neutron Scattering (HP-SANS) measurements and data analysis Due to the range of neutron wavelengths available, time-of-flight SANS is suitable for studying the shapes and sizes of colloidal systems. High-pressure SANS (HP-SANS) is a particularly important technique for determining surfactant aggregation structure in supercritical CO2. The HP-SANS measurements of the D2O/surfactant/scCO2 systems were performed at 45 °C and 350 bar. The LOQ time-of-flight instrument, at the Rutherford Appleton Laboratory at ISIS UK, was used in conjunction with a stirred high-pressure cell (Thar). The path length was 10 mm, the neutron beam diameter was 10 mm. The measurements gave the absolute scattering cross section I(Q) (cm-1) as a function of momentum transfer Q (Å-1), which is defined as Q = (4π/λ)sinθ, where θ is the scattering angle. The accessible Q range was 0.007-0.22 Å-1, arising from an incident neutron wavelength λof 2.2-10 Å. The data were normalized for transmission, empty cell, solvent background, and pressure induced changes in cell volume as before.13


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Pre-determined amounts of D2O and surfactant, where the molar ratio of surfactant to CO2 was fixed at 8 × 10-4 (= 10-20 mM at the appropriate experimental condition), were loaded into the Thar cell. Then, CO2 (11.3g), was introduced into the cell by using a high pressure pump, and the surfactant/D2O/CO2 mixture was pressurized up to 350 bar at 45 ºC by decreasing the inner volume of the Thar cell. With vigorous stirring, visual observation was carried out to identify the mixture as being a transparent single-phase W/CO2 µE or other turbid phase. Finally, the HP-SANS experiment was performed for not only the single-phase W/CO2µE, but also the turbid phase below a cloud point phase transition pressure Ptrans. Because these are dilute solutions/dispersions, the physical properties of the continuous phase of scCO2 were assumed to be equivalent to those of pure CO2. Neutrons are scattered by short-range interactions with sample nuclei, with the “scattering power” of different components being defined by a scattering-length density (SLD), ρ (cm-2). For CO2, ρCO2 ∼ 2.50 × mass density × 1010 cm-2 25; at the experimental pressure of 350 bar and temperature of 45 ºC, the CO2 density is 0.917 g cm-3 so that ρCO2∼ 2.29 × 1010 cm-2. The scattering length density of surfactant (ρsurf) and D2O (ρD2O) were obtained using; = ∑ /

(1)

bi is the nuclear scattering lengths as given in the literature26 and Vm is the molecular volume, which can be obtained from the mass density. Mass densities of FC6-HCn (n = 4, 5, 6, and 8) were assumed to be 1.3 g/cm3 as 1.7 g/cm3 for a typical fluorinated compound27 and 1.0 g/cm3 for a hydrocarbon surfactant. The calculated scattering length densities for the FC6-HCn (n = 4, 5, 6, and 8) were 2.26 × 1010 cm-2 (n = 4), 2.20 × 1010 cm-2 (n = 5), 2.11 × 1010 cm-2 (n = 6) and 2.03× 1010 cm-2 (n = 8), respectively. As the other surfactants FC4-HCn (n = 2 and 4) and FC6-HC2 and HC6-CH6 did not show any SANS under these conditions, their scattering length densities have been omitted. The scattering length density of D2O at 45 ºC was calculated to be ∼ 6.32 × 1010 cm-2. SANS scattering from the D2O/CO2 microemulsions with FC6-HCm shells was assumed to be from only the so-called water core


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contrast, because of very similar scattering densities of surfactant and CO2. Samples in pure CO2 (11.3 g) were run at the constant molar ratio of surfactant to CO2 of 8.0 × 10-4. For model fitting data analysis the W/CO2 µE droplets were treated as cylindrical and ellipsoidal particles with a Schultz distribution in core radius and length.28 The polydispersities in rod radius and length were fixed at 0.3 as found in spherical D2O/CO2 microemulsions with the double FC-tail surfactants (polydispersity = 0.2-0.4).29 Full accounts of the scattering laws are given elsewhere16,23,29-32. Data have been fit to models as described above using the SasView small-angle scattering analysis software package (http://www.sasview.org/).32 The fitted parameters are the core radii perpendicular to the rotation axis (Rf-ell,a) and along the rotation axis (Rf-ell,b) for ellipsoidal particle, or the core radius Rf-cyl and the length Lf-cyl for cylinder particle; these were initially obtained by preliminary Guinier analyses33 (Lg-cyl, Rg-cyl, and Rg-sph).


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3. Results and Discussion 3.1 Interfacial Properties in Water and microemulsion stabilizing ability in scCO2 Table 1 shows basic surfactant properties Krafft temperature, critical micelle concentration (CMC), surface tension at the CMC (γCMC), and the minimal effective area per molecule (Amin). These values except for FC6-HC5 and HC6-HC6 have been reported previously, and are from the literature22. Values for FC6-HC5 and HC6-HC6 were obtained through surface tension (γ) measurements and visual observation of the aqueous surfactant solutions. The behaviour of γ as a function of concentration for both surfactants is given in supporting information (Figure SI2). The parameter γCMC is especially important in predicting the solubilizing power of surfactant in scCO213,14,21; this is because (1) the water/air and water/CO2 interfacial properties of the surfactant are correlated, and (2) microemulsions generally form at an interfacial tension below 1 mN/m.34,35 Interfacial properties of hybrid surfactants, FmHn (m/n =7/4, 8/4, 7/7, 8/8)

16,23,17

, are listed in

Table 1 and were employed as a control hybrid surfactant. In comparison with the FmHn series (γCMC = 21.6-25.4 mN m-1 and Amin = 61-68 Å2), FC4-HCn had similar γCMC (22.5-24.0 mN m-1) but larger Amin values (81-89 Å2). Increasing the FC length of FC4-HCn to form (FC6-HCn), produced lower γCMC values (16.2-21.5 mN m-1) in spite of the larger Amin values (72-105 Å2), indicating less dense intermolecular packing at the surface. The lower γCMC values of FCm-HCn with shorter FC tails were presumably attained due to the structural differences, that is, the presence of a benzene ring and/or the head group (sulfonate/sulphate). These low γCMC values imply increased abilities of FCm-HCn to lower W/CO2 interfacial tension and stabilize W/CO2 microemulsions.13,14,21 On the other hand, the hydrocarbon surfactant HC6-HC6 (which is the analogue of FC6-HC6) exhibited poor interfacial properties, namely significantly higher γCMC. This emphasizes the importance of having a FC tail in hybrid surfactants for generating appropriate interfacial properties for air/water and CO2/water interfaces.


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To examine the solubility and microemulsion stabilization by surfactants in scCO2, the pressures at which a transparent single phases cloud, Ptrans, were measured for water/surfactant/CO2 mixtures at various temperatures (35 – 75 ºC) and W0 values. Figure 1 shows changes in Ptrans as a function of temperature for each surfactant at W0 = 0 and ~10. At values higher than Ptrans, FC6-HCm (m = 4, 5, 6, 8) at [surfactant]/[CO2] = 8 × 10-4 were soluble in scCO2 with and without water. However, FC4-HCm (n = 2, 4) and HC6-HC6 always remained as a precipitate even at the highest pressure and temperature 400 bar and 75 ºC. FC6-HC2 was also insoluble without added water (W0 = 0), but it became soluble for W0 ≤ 4. Ptrans values at W0 = 0 were comparable for the three FC6-HCn (n = 4, 5, and 6) surfactants but those for FC6-HC8 were higher than the others by 80-110 bar. At W0 = 10, Ptrans values of FCm-HCn elevated with HC length, n, in most cases, and were higher by >50 bar than those of the double FC-tail surfactant 4FG(EO)2 reported earlier13. These significant increases in Ptrans values with HC-tail length and number could be from increased attractive intermolecular interactions4,14,21, promoting aggregation between reversed micelles. This effect of HC-tail causing high Ptrans was found to be emphasized clearly upon addition of water. As Ptrans became higher at lower temperature for FC6-HCn, the effect is likely to be accelerated due to the decreased temperature.21 Mid-range HC-tail lengths (n = 4, 5, 6) of FC6-HCn were observed to yield transparent phases with W0 values up to at least 35, and the Ptrans data obtained are shown in Figures SI3-SI6 (see supporting information). However, at W0 higher than the solubilizing power of each surfactant as examined in Sec 3.2, the mixtures did not turn into a single phase even at the highest pressure available, and the symbols for Ptrans at each W0 are not displayed.

3.2 Solubilizing power of W/CO2 microemulsions To determine the solubilizing power, W0max (i.e. maximal W0 solubilized by surfactant) 3 mmol L-1 aqueous methyl orange (MO) solution was loaded as a dispersed phase into the transparent single-phase CO2 solutions with FC6-HCn (n = 4, 5, 6, and 8), and then UV-vis adsorption spectra were ACS Paragon Plus Environment

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measured with increasing W0. Alone, MO does not dissolve in pure CO2 but it does dissolve in water, and is generally incorporated within the water-rich pockets of a single-phase W/CO2 µE, dyeing the systems red.13,14 All transparent single–phases composed of surfactants and CO2 were initially colorless, but turned reddish after loading the MO solution. The color became deeper with an addition of the MO solution, reflecting the reversed micelles encapsulating the loaded MO solution. With addition of aqueous MO solution to surfactant/CO2 mixtures, clear absorption spectra were obtained at different W0 (see Figure SI7 in supporting information). A large and broad absorption peak of MO solubilized in the microemulsions was found at 360~520 nm. As the absorbance maximum λmax shift to longer wavelengths8,13,14 when MO molecules are solubilized in the more polar environment, λmax can be employed as a probe of microenvironment polarity. The λmax values for each hybrid surfactant were 410-430 nm and were close to those observed for double FC-tail surfactants (nFG(EO)2 and nFS(EO)2)13, and therefore the reversed micellar polar microenvironments are expected to be similar. This implies that the surfactant tail structure did not affect aqueous core polarity, although the nature of the hydrophilic group, temperature and W0 were found to affect polarity in previous studies using the double FC-tail surfactants8,13,14. From the spectra, changes in the absorbance at λmax were plotted as a function of W0, as shown in Fig. 2. Linear functions were obtained until certain W0 values for each surfactant. The linear relationship and the appearance of the transparent single-phases demonstrates that the W/CO2 µEs disperse increasing amounts of MO with increasing W0. The point at which the linear relationship was broken was identified as the solubilizing power W0max. At higher than W0max the absorbance gradually decreased with increasing W0, consistent with phase separation (e.g. Winsor-II W/CO2 µE or liquid crystal-like surfactant precipitates) 13,14,29 occurring. The Winsor-II W/CO2 µE (i.e. a W/CO2 µE having separated excess water) and W/CO2-type liquid crystal were indeed found over W0max for the double FC-tail surfactants by using high-pressure SANS.29 The solubilizing powers W0max have been extracted from Figure 2 and are clearly presented in Figure 3. As mentioned above, the hydrocarbon FC6-HC6 analogue HC6-HC6 and FC4-HCn (n=2 and ACS Paragon Plus Environment

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4) always gave two-phases of W/CO2 mixtures, and were suggested to have no solubilizing power, namely W0max = 0. In comparison, for FC6-HCn, even for n-numbers of (n= 4, 6, 8), W0max decreased from 80 to 20 with increasing HC-tail length at 45 ºC, but it became almost constant (W0max = 50-60) with increasing temperature up to 75 ºC. Linear HC surfactant tails have been known to induce the attractive interactions between reversed micelles and destabilize W/CO2 microemulsions (i.e. a poor W0max and/or a high Ptrans)4,14,21. However, the comparison in W0max values of FC6-HCn (n= 4, 6, 8) suggested the undesirable interactions of HC-tails are less prominent at higher temperatures due to increased thermal motion. On the other hand, the shortest HC-tail hybrid surfactant FC6-HC2 exhibited poor solubilizing power W0max = 4. This probably resulted from the short chain ethyl group, which is an ineffective tail and hence gives a large critical packing parameter19 and a low hydrophilic/lipophilic balance18. In the case of FC6-HC5, the W0max values at both temperatures were lower by 20-45 than those of FC6-HC4 and FC6-HC6. Due to small structural differences of just one methylene unit, the hydrophilic/lipophilic (or hydrophilic/CO2-philic) balance of FC6-HC5 should be similar to those of n = 4 and 6. These data therefore display an odd-even effect36 of surfactant HC-tails on W0max, offering a possible reason why the pentyl tail decreased W0max compared with the butyl and hexyl tails. From this stand point, even n-numbered HC-tails are suggested to be more suitable as hybrid surfactants for W/CO2 microemulsions. Most importantly, FC6-HC4 was found to have the highest solubilizing power of W0max = 80, a performance that is almost equal to the double FC-tail surfactant 4FG(EO)2.13 Here, to evaluate the effectiveness of fluorocarbon on W0max enhancement, W0max/ NF (where NF is number of fluorines in molecule) values were compared for each surfactant. Those were 6.2 for FC6-HC4 and 4.4 for 4FG(EO)2 13, and the effectiveness per F-atom for FC6-HC4 was 1.5 times larger than that of 4FG(EO)2. This implies that if a hybrid tail structure can be optimized it could boost the efficiency compared with a symmetrical tail structure of typical fluorocarbon surfactants used previously.


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3.3 Effect of hybrid-tail structure of FC6-HCn on formation of W/CO2 microemulsions To examine the effect of the hybrid-tail structure on the shape and size of aggregates in FCmHCn /D2O/CO2 mixtures, SANS I(Q) profiles were measured at W0 = 20, 45 ºC and 350 bar. SANS data along with the fitted I(Q) functions are shown in Figure 4. SANS profiles are useful in determining the shape of nano- and colloidal particles. Under these conditions, HC6-HC6, FC4-HC2 and FC4-HC4 remained as a precipitate, and therefore no clear SANS profiles were obtained. On the other hand, FC6HCn with n = 4-6 exhibited transparent single-phases giving clear SANS profiles. In the case of FC6HC8, although a turbid phase appeared as W0 reached the solubilizing power (W0max = 20), a SANS curve from D2O droplets was observed. In the low Q region (typically in the case of droplet microemulsions < 0.01 Å-1), the scattering may scale as I(Q) ~ Q-D, where D is a characteristic “fractal dimension” for the colloids; hence, the gradient of a log-log plot will be –D. In the case of non-interacting spheres, D should be zero in this low Q region, whereas D = 1 for cylinders and 2 for disks.29,33 The FC6-HCn SANS profiles show D = ~0 for n = 4 and 8 and D = ~1 for n = 5 and 6, suggesting the presence of globular, and then rod-like nanodomains, respectively. One method to approximate radii and cylinder length from SANS data for the globular and rod-like microemulsions is via Guinier plots29,33 (Ln [I(Q)] vs Q2 for spheres and Ln [I(Q) Q] vs Q2 for cylinders) as shown in supporting information (Figure SI7). In the all plots of Ln [I(Q)] vs Q2, linearity was obtained, and the gradients allowed estimation of radii of gyration, Rg (the slope = –Rg2/3). This Rg may also be related to a principal sphere radius Rg-sph as Rg = (3/5)0.5 Rg-sph and cylinder length Lg-cyl as Rg = Lg-cyl /(12)0.5.29,33 In other cases the Ln [I(Q) Q] and Q2 plots also exhibited linearity, and gradients were used to estimate cylinder radii, Rgui-cyl (the slope = Rgui-cyl2 / 4). The values Rg-sph were calculated for the spherical microemulsions of FC6-HC4 and FC6-HC8, and Lg-cyl and Rg-cyl were calculated for FC6-HC5 and FC6-HC6: these values are listed in Table 2 along with Rg values. The values of Rg-sph, Rg-cyl, and Lg-cyl were employed as the starting points for model fit analyses using the full polydisperse Schultz ellipsoid and cylinder models. The parameter outputs are the average values of radii for the ellipsoidal D2O cores (Rf-ell,a and Rf-ell,b) and radius and length of the cylindrical ACS Paragon Plus Environment

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one (Rf-cyl and Lf-cyl). The polydispersity width was set at 0.3, which was assumed to be typical value for W/CO2 microemulsion systems (e.g. 0.17-0.40 for double FC-tail sulfonate surfactants)29. These fitted parameters are also listed in Table 2. For FC6-HCn reversed micelles, a very unique change in the D2O core morphology was observed as a function of HC-tail length. Cylindrical core morphology was obtained for middle HC-tail lengths of n = 5 and 6, close to the FC-tail length (m = 6), with transitions into spherical nanodroplets occurring as the HC-tail became either shorter (n = 4) or longer (n = 8). The aspect ratios of the D2O cores were calculated as 1.39 for n = 4, 5.45 for n = 5, 6.32 for n = 6, and 1.09 for n = 8. These results suggest an optimal FC/HC balance to stabilize cylindrical reversed micelles in scCO2 where m/n = 6/6 (i.e. same FC and HC lengths). Why does this specific FC-HC balance drive the elongation of reversed micelles? From the standpoint that in the earlier reports most cylindrical reversed micelles in CO2 have been obtained using hybrid surfactants, and only rarely with other types of surfactants. A key to resolving the question may be in characteristics special to hybrid surfactant, for example, FC- HC micro-segregation of self-assembled hybrid surfactant molecules. To investigate aggregation properties of FC6-HCn (n = 4, 5, 6) in scCO2, the aggregation number of surfactant per reversed micelle (Nagg) and occupied area per surfactant molecule at the W/CO2 microemulsion surface (AC/W) were calculated by following equations. Nagg = Csurf/Cmicelle

(2)

Cmicelle = (VD2O CD2O+Vhead Csurf)/(Vcore) = Csurf (VD2O W0 + vhead NA)/(vcore NA) AC/W = score/Nagg

(3)

(4)

where NA is Avogadro’s number, Csurf and CD2O, Cmicelle are molar concentrations of surfactant, D2O and reversed micelle, VD2O, Vhead, and Vcore are molar volumes of D2O, surfactant headgroup and D2O core, vcore and vhead are volumes per D2O core (Vcore = vcore NA) and headgroup (e.g. vhead = ~215 Å3 for sulfonate group, Vhead = vhead NA)29, respectively. For AC/W calculation, score is surface area per D2O core, and calculated from the shape parameters (Rf-ell,a, Rf-ell,b, Rf-cyl, and Lf-cyl) as well as the calculation of vcore for Cmicelle. ACS Paragon Plus Environment

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According to critical packing parameter (CPP) theory19, CPP can be also obtained by CPP = vtail/(AC/W ltail)

(5)

where vtail and ltail are hydrophobic tail volume and length, respectively. According to this approach reversed micelles would be formed for surfactants with CPP > 1 (reversed cones form if the double-tail orients upward) to ~1 (cylindrical). If the hydrophobic part is assumed to be a truncated core, the volume should be29 vtail = ltail{AC/W + Atail + (AC/W Atail)0.5}/3

(6)

where Atail is area per hydrophobic tail terminus, respectively. Then eq.(5) can be simply expressed as CPP = {smicelle + score + (smicelle score)0.5}/(3score)

(7)

where smicelle is surface area per reversed micelle. In this study, the values of smicelle were calculated from the shape parameters (Rf-ell,a, Rf-ell,b, Rf-cyl, and Lf-cyl) and surfactant tail length ltail assumed to be 13.4 Å (the length between the terminal F-atom and the C-atom bearing the sulfonate group). The calculated aggregation properties Nagg, AC/W, and CPP are listed in Table 2. Another way to estimate AC/W is by using the Porod approximation to analyze SANS data29. However, unfortunately, for these weakly scattering systems, sufficiently accurate I(Q) Q4}Q→∞ values could not be obtained due to background noise. A previous study reported that double FC-tail surfactants with sulfonate headgroups have AC/W = 117-129 Å2, which were slightly larger than AA/W (AA/W = 94-118 Å2)13,29. The larger AC/W values were suggested to occur due to CO2-solvation into the surfactant tails as reported in the earlier papers13,29, and this trend of AC/W being larger than AA/W was also observed in this study of hybrid surfactants. One large difference in AC/W for hybrid surfactants and double-tail surfactants is that AC/W values of FC6-HCn gradually increased from 101 to 149 Å2 with increasing n from 4 to 6, in contrast with those of doubletail surfactants which were independent of tail length29. The differing effects of tail length on AC/W between hybrid and double FC-tail surfactants suggests intramolecular FC-HC repulsion to be more active in scCO2, causing hybridized tails to be bulkier with a longer HC-tail. If so, intermolecular FCHC micro-segregation would be also accelerated in scCO2 compared with in atmospheric conditions. ACS Paragon Plus Environment

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The elongated reversed micelles might result from the intramolecular repulsion and/or the intermolecular micro-segregation promoted with longer HC-tails (n = 5 and 6). Earlier papers29 found Nagg and CPP values at W0 = 20 to be ~30 and ~1.5 for the double perfluorobutyl-tail surfactants in W/CO2 microemulsions, respectively. In comparison, hybrid surfactants FC6-HCn have larger values of Nagg (=78-130) and CPP (> 1.6). The larger Nagg values were considered to be due to stronger intermolecular interactions such as π-π interactions and/or molecular shape37 allowing the easy packing into large non-spherical aggregates. The larger CPP would act as an advantage to stabilize reversed micelle geometrically, and can be caused by larger AC/W, as mentioned above.


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4. Conclusions W/CO2 microemulsions are promising universal green-solvents for applications such as extraction, dyeing, dry cleaning, metal-plating, and organic or nanomaterial synthesis. These microemulsions should ideally be prepared with low levels of surfactant, be inexpensive and environmentally-benign. Therefore, finding a low F-content surfactant which generates a high solubilizing power is key to designing useful CO2-philic surfactants. This study examined the optimal tail structure for stabilizing microemulsions in hybrid FCmHCn surfactants, and elucidated the causes of important findings seen previously [1]-[3]. As the results of phase behavior observations and absorbance measurements previously indicated, [1] optimal HC-tail length in the hybrid surfactant was found to be n = 4 at the shortest FC-tail length able to yield microemulsions (m = 6). The identification of an optimal HC-length is very interesting, since straightchain HCs were commonly considered not to be CO2-philic3-5 and their inclusion in FC-surfactant molecules was limited in order to retain CO2-philicity [2]. However, the maximum solubilizing power of FC6-HC4 was W0max = 80, a value equal to the highest performance yet reported in W/CO2 systems3-5,1317,21,29

. To evaluate the effectiveness of each F-atom in the solubilization, the solubilizing power was

divided by number of F-atoms in the molecule, and calculated as 6.2. This is 1.5 times larger than the most effective FC-surfactant in earlier papers13. The highest effectiveness per F-atom generated by hybrid structures is also an interesting concept for developing design theory of new CO2-philic surfactants. HP-SANS measurements characterized the D2O cores of the FC6-HCn reversed micelles at W0 = 20, and demonstrated [3] shape transitions in core morphology upon increasing HC-tail length (i.e. ellipsoid for n = 4 → cylinder for n = 5 and 6 → sphere for n = 8). The aspect ratio was seen to reach 6.3 as a maximum at HC-tail length n = 6. Earlier papers16,17 demonstrated the formation of reversed cylindrical micelles of hybrid surfactants; however, the role of the HC-tails in micelle elongation was unclear. Finding the optimal HC-tail length for the elongated reversed micelle could be a key to open this black box, and it implies the driving force belongs to intermolecular FC-HC micro-segregation. ACS Paragon Plus Environment

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A high water content W/CO2 microemulsions, and the formation of elongated reversed micelles, could offer a new generation of universal solvents with unique properties. This is especially true if the elongated reversed micelles can increase CO2 viscosity, which would significantly improve the poor efficiency of EOR CO2-flooding17. Further W/CO2 microemulsion studies on FC6-HCn will be focused on how reversed micelle morphology changes with concentration, W0, and/or the other additives (metal ions, cosurfactants or hydrotropes)38-40 and explore the use of elongated reversed micelle generation for increasing CO2 viscosity for efficient EOR technologies.


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5. ASSOCIATED CONTENT Supporting Information. Surfactant synthesis for two hybrid surfactants (FC6-HC4 and FC6-HC5) and characterization by 1H-NMR, FT-IR, and elemental analysis. Surface tension vs log (surfactant concentration) curves for aqueous surfactant (FC6-HC5 and HC6-HC6) solutions at 35 ºC. Change in Ptrans as a function of temperature for W/CO2 mixtures with FC6-HCn (n = 4, 5, 6 and 8) at [surfactant]/[CO2] = 8 × 10-4. UV-vis absorption spectra for FC6-HCn (n = 4, 5, 6 and 8)/CO2 mixtures with 0.1wt% methyl orange aqueous solution at different W0 values. Guinier plots (Ln [I(Q)] vs Q2 and Ln [I(Q) Q] vs Q2) for D2O/FC6-HCn/CO2 mixtures at W0 = 20, 350 bar and 45 ºC. This material is available free of charge via the Internet at “http://pubs.acs.org.”

6. AUTHOR INFORMATION Corresponding Author. *E-mail [email protected]; FAX +81-172-39-3579 (M.S.) Notes. The authors declare no competing financial interest.

7.ACKNOWLEDGEMENT This project was supported by JSPS [KAKENHI, Grant-in-Aid for Scientific Research (B), No. 26289345, Grant-in-Aid for Challenging Exploratory Research, No. 26630383], and Leading Research Organizations (RCUK [through EPSRC EP/I018301/1], ANR [13-G8ME-0003]) under the G8 Research Councils Initiative for Multilateral Research Funding –G8-2012. CJ thanks the Japan Society for the Promotion of Science (JSPS) for an 18-month fellowship (The JSPS Postdoctoral Fellowship for Foreign Researchers) and EPSRC (grants EP/I018301 and EP/I018212/1). We also acknowledge STFC for the allocation of beam time, travel, and consumables grants at ISIS, and Dr. Kondo and Prof. Yoshino at Tokyo University of Science for providing hybrid surfactants FCm-HCn and the hydrocarbon surfactant HC6-HC6.


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Figure captions Figure 1. Changes in Ptrans for surfactant/W/CO2 mixtures with W0 = 0 and 10 as a function of temperature. The molar ratio of the surfactant to CO2 was fixed at 8 × 10-4. Figure 2. Absorbance maximum, λmax, of MO in surfactant/W/CO2 mixtures for different W0 values at 350 bar. The MO concentration in water was 3 mmol L-1. Molar ratio of surfactant-to-CO2 was fixed at 8 × 10-4. Figure 3. Solubilizing power W0max of surfactants in scCO2 at 350 bar and 45 or 75 ºC. Figure 4. SANS profiles for surfactant/D2O/CO2 mixtures with W0 = 20 at 45 ºC and 350 bar (CO2 density = 0.92 g/cm3). Fitted curves were based on a model incorporating a Schultz distribution of polydisperse ellipsoid or cylinder particles. The molar ratio of the surfactant to CO2 was fixed at 8 × 104

.


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References (1) Beckman, E. J. Supercritical and Near-Critical CO2 in Green Chemical Synthesis and Processing. J. Supercrit. Fluids. 2004, 28, 121-191. (2) Goetheer, E. L. V.; Vortaman, M. A. G.; Keurentjes, J. T. F. Opportunities for Process Intensification Using Reverse Micelles in Liquid and Supercritical Carbon Dioxide. Chem. Eng. Sci. 1999, 54, 1589-1596. (3) Consani, K. A.; Smith, R. D. Observations on the Solubility of Surfactants and Related Molecules in Carbon Dioxide at 50 ºC. J. Supercrit. Fluids 1990, 3, 51-65. (4) Ryoo, W.; Webber, S. E.; Johnston, K. P. Water-in-Carbon Dioxide Microemulsions with Methylated Branched Hydrocarbon Surfactants. Ind. Eng. Chem. Res., 2003, 42, 6348-6358. (5) Lee, H.; Pack, J W.; Wang, W.; Thurecht, K. J.; Howdle, S. M. Synthesis and Phase Behavior of CO2-Soluble Hydrocarbon Copolymer: Poly(Vinyl Acetate-alt-Dibutyl Maleate). Macromolecules 2010, 43, 2276-2282. (6) Zulauf, M.; Eicke, H. F. Inverted micelles and microemulsions in the ternary system water/aerosolOT/isooctane as studied by photon correlation spectroscopy. J. Phys. Chem. 1979, 83, 480–486. (7) Lee, C. T., Jr.; Psathas, P. A.; Johnston, K. P.; deGrazia, J.; Randolph, T. W. Water-in-Carbon Dioxide Emulsions: Formation and Stability. Langmuir 1999, 15, 6781-6791. (8) Johnston, K. P.; Harrison, K. L.; Klarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Water-in-Carbon Dioxide Microemulsions: A New Environment for Hydrophiles Including Proteins. Science 1996, 271, 624-626. (9) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. A Small-Angle Neutron Scattering Study of Water in Carbon Dioxide Microemulsions. Langmuir 1997, 13, 3934-3937.


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(10) Niemeyer, E. D.; Bright, F. V. The pH within PFPE Reverse Micelles Formed in Supercritical CO2. J. Phys. Chem. B 1998, 102, 1474-1478. (11) Dalvi, V. H.; Srinivasan, V.; Rossky, P. J. Understanding the Effectiveness of Fluorocarbon Ligands in Dispersing Nanoparticles in Supercritical Carbon Dioxide. J. Phys. Chem. C 2010, 114, 15553-15561. (12) Dalvi, V. H.; Srinivasan, V.; Rossky, P. J. Understanding the Relative Effectiveness of Alkanethiol Ligands in Dispersing Nanoparticles in Supercritical Carbon Dioxide and Ethane. J. Phys. Chem. C 2010, 114, 15562-15573. (13) Sagisaka, M.; Iwama, S.; Yoshizawa, A.; Mohamed, A.; Cummings S.; Eastoe, J. An Effective and Efficient Surfactant for CO2 Having Only Short Fluorocarbon Chains. Langmuir 2012, 28, 10988-10996. (14) Mohamed, A.; Sagisaka, M.; Guittard, F.; Cummings, S.; Paul, A.; Rogers, S. E.; Heenan, R. K.; Dyer, R.; Eastoe, J. Low Fluorine Content CO2-Philic Surfactants. Langmuir 2011, 27, 10562-10569. (15) Harrison, K.; Goveas, J.; Johnston, K. P.; O'Rear III, E. A. Water-in-Carbon Dioxide Microemulsions with a Fluorocarbon-Hydrocarbon Hybrid Surfactant. Langmuir 1994, 10, 3536-3541. (16) Dupont, A.; Eastoe, J.; Martin, L.; Steytler, D. C.; Heenan, R. K.; Guittard, F.; Taffin de Givenchy, E. Hybrid Fluorocarbon-Hydrocarbon CO2-philic Surfactants. 2. Formation and Properties of Water-inCO2 Microemulsions. 2004, 20, 9960-9967. (17) Peach , J.; Eastoe, J. Supercritical carbon dioxide: a solvent like no other. Beilstein J. Org. Chem. 2014, 10, 1878–1895. (18) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide. Langmuir 1999, 15, 419–428.


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(19) Israelachvili, J. N. Measurements of Hydration Forces Between Macroscopic Surfaces. Chem. Scr. 1985, 25, 7-14. (20) Winsor, P. A. Hydrotropy, Solubilisation and Related Emulsification Processes. Trans. Faraday. Soc. 1948, 54, 376-398. (21) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kitiyanan, B.; Kondo, Y.; Yoshino, N.; Takebayashi, K.; Sakai, H.; Abe, M. Preparation of a W/scCO2 Microemulsion Using Fluorinated Surfactants. Langmuir 2003, 19, 220-225. (22) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Micellar Solution Properties of Fluorocarbon−Hydrocarbon Hybrid Surfactants. Langmuir 1996, 12, 5768–5772. (23) Dupont, A.; Eastoe, J.; Murray, M; Martin, L.; Guittard, F.; Taffin de Givenchy, E.; Heenan, R. K. Hybrid fluorocarbon-hydrocarbon CO2-philic surfactants.1. Synthesis and properties of aqueous solutions. Langmuir 2004, 20, 9953 - 9959. (24) Span, R.; Wahner, W. A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures up to 800 MPa. J. Phys. Chem. Ref. Data 1996, 25, 1509-1596. (25) McClain, J. B.; Londono, D.; Combes, J. R.; Romack, T. J.; Canelas, D. A.; Betts, D. E.; Wignall, G. D.; Samulski, E. T.; DeSimone, J. M. Solution Properties of a CO2-Soluble Fluoropolymer via Small Neutron Scattering. J. Am. Chem. Soc. 1996, 118, 917-918. (26) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26-37. (27) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. A Small-Angle Neutron Scattering Study of Water in Carbon Dioxide Microemulsions. Langmuir 1997, 13, 3934-3937.


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1 2 3 565 4 5 566 6 7 8 567 9 10 11568 12 13569 14 15 16570 17 18 19571 20 21572 22 23 24573 25 26 27574 28 29 30575 31 32576 33 34 35577 36 37 38 39578 40 41 42579 43 44580 45 46 47 581 48 49 50582 51 52 53583 54 55584 56 57 58585 59 60

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(28) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Structure of Three-Component. Microemulsions in the Critical Region Determined by Small Angle Neutron Scattering Data. Phys. Rev. A 1984, 29, 2054-2069. (29) Sagisaka, M.; Iwama, S.; Ono, S.; Yoshizawa, A.; Mohamed, A.; Cummings, S.; Yan, C.; James, C.; Rogers, S. E.; Heenan, R. K.; Eastoe, J. Nanostructures in Water-in-CO2 Microemulsions Stabilized by Double-chain Fluorocarbon Solubilizers. Langmuir 2013, 29, 7618−7628. (30) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Structure of Three-Component. Microemulsions in the Critical Region Determined by Small Angle Neutron Scattering Data. Phys. Rev. A 1984, 29, 2054-2069. (31) Hayter, J. B.; Penfold, J. Determination of Micelle Structure and Charge by Neutron Small Angle Scattering. Colloid Polym. Sci. 1983, 261, 1022-1030. (32) Smith, G. N.; Grillo, I.; Rogers, S. E.; Eastoe, J. Surfactants with colloids: Adsorption or absorption? J. Colloid Interface Sci. 2015, 449, 205–214. (33) Guinuier, A.; Fournet, G. Small-Angle Scattering of X-Rays, Wiley, New York, 1956. (34) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (35) Kumar, P.; Mittal, K. L.; eds., Handbook of Microemulsion Science and Technology, Marcel Deckker, 1999, p. 400. (36) Tao, F.; Bernasek, S. L. Understanding Odd-Even Effects in Organic Self-Assembled Monolayers. Chem. Rev. 2007, 107, 1408-1453. (37) Praefcke, K.; Marquardt, P.; Kohne, B.; Stephan, W.; Levelut, A.-M.; Wachtel, E. Inositol Liquid Crystals, X-Ray Diffraction Studies of Their Hydrogen-Bond Supported Supramolecular Mesophases. Mol. Cryst. Liq. Cryst. 1991, 203, 149-158.


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(38) Vlachy, N.; Drechsler, M.; Verbavatz, J.-M.; Touraud, D.; Kunz, W. Role of The Surfactant Headgroup on The Counterion Specificity in The Micelle-to-Vesicle Transition Through Salt Addition. J. Colloid Interface Sci. 2008, 319, 542–548. (39) Marques, E. F.; Regev, O.; Khan, A.; Lindman, B. Self-Organization of Double-Chained and Pseudodouble-Chained Surfactants: Counterion and Geometry Effects. Adv. Colloid Interface Sci. 2003, 100-102, 83-104. (40) Hodgdon, T. K.; Kaler, E. W. Hydrotropic Solutions. Curr. Opin. Colloid Interface Sci. 2007, 12, 121–128.


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Page 29 of 35 Figure 1.

400

Pressure / bar

FC6-HC4 FC6-HC8

FC6-HC5 4FG(EO) 2

FC6-HC6

300

200 W0 = 10

100 400

Pressure / bar

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Sagisaka, M. et al.

Langmuir

40

50 FC6-HC4 FC6-HC6

60

70

FC6-HC5 FC6-HC8

300

200 W0 = 0

100

40

50

60

Temperature / ºC


70

Absorbance / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Absorbance / a.u.

Figure 2.

Langmuir

0.25

0.80 0.60 0.40

of 35 Sagisaka, M.Pageet30al.

45 °C 75 °C

0.20

45ºC 75ºC

(B)

0.15

(A)

0.10

0.20 0.00 0 0.60

0.05

20

0.50

60

45 ºC 75 ºC

0.40 0.30

40

0.00 80 100 120 0 1.00

0.60

(C)

40

60

45 ºC 75 ºC

0.80

(D)

0.40

0.20

0.20

0.10 0.00 0

20

20

40

60

80

0.00 100 0

W0


20

40

W0

60

80

Page 31 of 35 Figure 3.

100

80

45 ºC 75 ºC

80 60

60

W0max

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Sagisaka, M. et al.

Langmuir

40

55 35

30 20

20

0

55

50

0 0 0 0

0 0

4 4


Figure 4.

1

FC4-HC2 FC6-HC2 FC6-HC5 FC6-HC8

4

-1

2

I(Q) / cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58


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FC4-HC4 FC6-HC4 FC6-HC6 HC6-HC6

Q -1

0.1 4 2

0.01 4 78

2

3

4 5 6 78

0.01

2

0.1 Q/Å

-1


3

Page 33 of 35 Table 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

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Sagisaka, M. et al.

Table 1 Interfacial properties of hybrid surfactants FCn-HCm and FmHn, and the double hydrocarbon surfactant HC6-HC6 in water at 25 ºC. Main structure

Surfactant

FC4-HC2 FC4-HC4 FC6-HC2 FC6-HC4 O -C-CH-R2 FC6-HC5 R1SO3Na FC6-HC6 FC6-HC8 HC6-HC6 F7H4 R1-CH-R2 F8H4 OSO3Na F7H7 F8H8

R1 / R 2 F(CF2)4- / H(CH2)2F(CF2)4- / H(CH2)4F(CF2)6- / H(CH2)2F(CF2)6- / H(CH2)4F(CF2)6- / H(CH2)5F(CF2)6- / H(CH2)6F(CF2)6- / H(CH2)8H(CH2)6- / H(CH2)6F(CF2)7- / H(CH2)4F(CF2)8- / H(CH2)4F(CF2)7- / H(CH2)7F(CF2)8- / H(CH2)7-

TK a) / ºC

CMC b) CMC c) AA/W d) /(mol L−1) /(mN m−1) / Å2

< 0 8.2  10−3 < 0 3.5  10−3 < 0 8.3  10−4 15 2.3  10−4 10 1.3  10−4 e) 48 5.5  10−5 f) ― > 95 < 0 6.0  10−3 e) 1.9  10−3 7.8  10−4 6.3  10−4 1.6  10−4

a)

24.0 81 22.5 89 21.5 89 20.4 105 18.0 e) 72 e) 16.2 f) 98 f) ― ― e) 36.9 71 e) 23.8 67 25.4 68 60 22.1 21.6 61

Krafft temperature, b) Critical micelle concentration in water. c) Surface tension at CMC. Molecular area at CMC obtained by using the Gibbs adsorption equation. The uncertainties of AA/W are 5 Å2. e) at 35 ºC. f) at 50 ºC. d)


Table 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58


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Table 2 Characteristics of FCm-HCn reversed micelles in CO2 at 45 ºC, 350 bar, W0=20, and [Surfactant]=16.7 mM. Radius or cylinder length / Å m-n

Guinier plot Rg

6-4 32.2

Curve fitting

Lg-cyl Rg-cyl Rg-sph Lf-cyl Rf-cyl ―

―

41.6

―

―

Nagg

AC/W CPP / Å2

78

101 1.63

Rf-ell,a Rf-ell,b 22.3

31.0

6-5 44.5 154.2 17.5

―

159.3 14.6

―

―

130 122 1.63

6-6 44.8 155.2 18.2

―

150.5 11.9

―

―

81

149 1.76

12.9

11.8

―

―

6-8 25.7

―

―

33.2

―

―

―

Rg, Rg-sph, Rg-cyl and Lg-cyl are gyration, sphere, and cylinder radii, and cylinder length obtained from the slopes of Guinier plots, respectively. Rf-ell,a and Rf-ell,b are radii perpendicular to the rotation axis and along the rotation axis of the ellipsoid, respectively. Rf-cyl and Lf-cyl are cylinder radius and the length, respectively. Rf-ell,a, Rf-ell,b, Rf-cyl and Lf-cyl obtained by fitting the experimental data by theoretical curve with ellipsoidal or cylindrical particles. Nagg is aggregation number of surfactant per reversed micelle, calculated by the equation (2). AC/W is area per molecule obtained by the equation (4). CPP is critical packing parameter calculated by the equation (7).


Page 35 of 35 SYNOPSIS TOC

Effect of hydrocarbon length n on solubilizing power W0max of O the hybrid surfactant in scCO2 (45 ºC -C-CH-CnH2n+1 and 350 bar) and aspect ratio of D2O C6F13SO3Na core Rasp at W0 ([Water]/[Surfactant])=20

Optimal n for W0max

Optimal n for Rasp

max

Solubilizing power W00max

7.0 80

6.0 Aqueous core

60 40

5.0 Elongated W/CO2 micelle

scCO2

4.0 3.0

20 0

2

2.0 3

4

5

6

Hydrocarbon length n


7

8

1.0

Aspect ratio of D 2O core Rasp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

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Effect of Fluorocarbon and Hydrocarbon Chain ... - ACS Publications

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