Hybrid CO2-philic surfactants with low fluorine content

The relationships between molecular architecture, aggregation, and interfacial activity of a new class of CO(2)-philic hybrid surfactants are investigated. The new hybrid surfactant CF2/AOT4 [sodium (4H,4H,5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate] was synthesized, having one hydrocarbon chain and one separate fluorocarbon chain. This hybrid H-F chain structure strikes a fine balance of properties, on one hand minimizing the fluorine content, while on the other maintaining a sufficient level of CO(2)-philicity. The surfactant has been investigated by a range of techniques including high-pressure phase behavior, UV-visible spectroscopy, small-angle neutron scattering (SANS), and air-water (a/w) surface tension measurements. The results advance the understanding of structure-function relationships for generating CO(2)-philic surfactants and are therefore beneficial for expanding applications of CO(2) to realize its potential using the most economic and efficient surfactants.     

Surfactant-mixing effects on the interfacial tension and the microemulsion formation in water/supercritical CO2 system

The effects of surfactant mixing on interfacial tension and on microemulsion formation were examined for systems of air/water and water/supercritical CO2 (scCO2) interfaces and for water/scCO2 microemulsions. A fluorinated surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), was mixed with the three hydrocarbon surfactants, Pluronic L31, Tergitol TMN-6, and decyltrimethylammonium chloride (DeTAC), at equimolar ratio. For all the cases, the interfacial tension was significantly lowered by the mixing. The positive synergistic effect suggests that the mixed surfactants tend to pack more closely on the interface than the pure constituents. It was found, however, that the microemulsion formation in scCO2 was never facilitated by the mixing, except for the case of TMN-6. This is probably due to the segregation of the surfactants into hydrocarbon-rich and fluorocarbon-rich phases on the microemulsion surface.     

Water/supercritical CO2 microemulsions with mixed surfactant systems

Phase behavior was investigated for water/supercritical CO 2 (W/scCO2) microemulsions stabilized with sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO) 2) mixed with various guest surfactants. Only for the mixtures with fluorocarbon-hydrocarbon hybrid anionic surfactants (FC6-HC n), the maximum water-to-surfactant molar ratio (W0(c)) was larger than that estimated from linear interpolation of the W0(c) values for pure 8FS(EO) 2 and pure guest surfactant. Fourier transform infrared (FT-IR) measurement for the microemulsion revealed that the mixing of 8FS(EO) 2 with FC6-HC n can prevent a phase transition from the microemulsion to the liquid crystal even in the presence of excess water. It was also found from the measurement of water/scCO 2 interfacial tension that the area occupied per surfactant molecule was markedly increased by the mixing with FC6-HC n. The loose molecular packing, probably due to a microsegregation of 8FS(EO) 2 and FC6-HC n, is consistent with the enhanced stability of the microemulsion upon surfactant mixing.     

Effective and efficient surfactant for CO2 having only short fluorocarbon chains

A previous study (Langmuir2011, 27, 5772) found the fluorinated double-tail sulfogulutarate 8FG(EO)(2) to act as a superefficient solubilizer for water in supercritical CO(2) (W/CO(2)) microemulsions. To explore more economic CO(2)-philic surfactants with high solubilizing power as well as rapid solubilization rates, the effects of fluorocarbon chain length and linking group were examined with sodium 1,5-bis(1H,1H,2H,2H-perfluoroalkyloxy)-1,5-dioxopentane-2-sulfonates (nFG(EO)(2), fluorocarbon chain length n = 4, 6, 8) and sodium 1,4-bis(1H,1H,2H,2H-perfluoroalkyloxy)-1,4-dioxobutane-2-sulfonate (nFS(EO)(2), n = 4, 8). Visual observation and UV-vis spectral measurements with methyl orange as a reporter dye indicated a maximum water-to-surfactant molar ratio (W(0)) in the microemulsions, which was 60-80 for nFG(EO)(2) and 40-50 for nFG(EO)(2). Although it is normally expected that high solubilizing power requires long fluorocarbon surfactant chains, the shortest fluorocarbon 4FG(EO)(2) interestingly achieved the highest W(0) (80) transparent single-phase W/CO(2) microemulsion. In addition, a very rapid solubilization of loaded water into CO(2) was observed for 4FG(EO)(2) even at a high W(0) of ~80.     

Dynamical and rheological properties of fluorinated surfactant films adsorbed at the pressurized CO2-H2O interface

The dynamics of adsorption, interfacial tension, and rheological properties of two phosphocholine-derived partially fluorinated surfactants FnHmPC, designed to compensate for the weak CO(2)-surfactant tail interactions, were determined at the pressurized CO(2)-H(2)O interface. The two surfactants differ only by the length of the hydrocarbon spacer (5 CH(2) in F8H5PC and 11 CH(2) in F8H11PC) located between the terminal perfluoroalkyl chain and the polar head. The length of this spacer was found to have a critical impact on the adsorption kinetics and elasticity of the interfacial surfactant film. F8H5PC is soluble in both water and CO(2) phases and presents several distinct successive interfacial behaviors when bulk water concentration (C(W)) increases and displays a nonclassical isotherm shape. The isotherms of F8H5PC are similar for the three CO(2) pressures investigated and comprise four regimes. In the first regime, at low C(W), the interfacial tension is controlled by the organization that occurs between H(2)O and CO(2). The second regime corresponds to the adsorption of the surfactant as a monolayer until the CO(2) phase is saturated with F8H5PC, resulting in a first inflection point. In this regime, F8H5PC molecules reach maximal compaction and display the highest apparent interfacial elasticity. In the third regime, a second inflection is observed that corresponds to the critical micelle concentration of the surfactant in water. At the highest concentrations (fourth regime), the interfacial films are purely viscous and highly flexible, suggesting the capacity for this surfactant to produce water-in-CO(2) microemulsion. In this regime, surfactant adsorption is very fast and equilibrium is reached in less than 100 s. The behavior of F8H11PC is drastically different: it forms micelles only in the water phase, resulting in a classical Gibbs interface. This surfactant decreases the interfacial tension down to 1 mN/m and forms a strongly elastic interface. As this surfactant forms a very cohesive interface, it should be suitable for formulating stable water-in-CO(2) emulsions. The finding that the length of the hydrocarbon spacer in partially fluorinated surfactants can drastically influence film properties at the CO(2)-H(2)O interface should help control the formation of microemulsions versus emulsions and help elaborate a rationale for the design of surfactants specifically adapted to pressurized CO(2).     

Tailoring porous silica films through supercritical carbon dioxide processing of fluorinated surfactant templates

The tailoring of porous silica thin films synthesized using perfluoroalkylpyridinium chloride surfactants as templating agents is achieved as a function of carbon dioxide processing conditions and surfactant tail length and branching. Well-ordered films with 2D hexagonal close-packed pore structure are obtained from sol-gel synthesis using the following cationic fluorinated surfactants as templates: 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl)pyridinium chloride (HFOPC), 1-(3,3,4,4,5,5,6,6,7,8,8,8-dodecafluoro-7-trifluoromethyl -octyl)pyridinium chloride (HFDoMePC), and 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-decyl)pyridinium chloride (HFDePC). Processing the sol-gel film with CO2 (69-172 bar, 25 and 45 degrees C) immediately after coating results in significant increases in pore diameter relative to the unprocessed thin films (increasing from 20% to 80% depending on surfactant template and processing conditions). Pore expansion increases with CO2 processing pressure, surfactant tail length, and surfactant branching. The varying degree of CO2 induced expansion is attributed to the solvation of the "CO2-philic" fluorinated tail and is interpreted from interfacial behavior of HFOPC, HFDoMePC, and HFDePC at the CO2-water interface.     

Fourier transform infrared spectroscopic study of water-in-supercritical CO2 microemulsion as a function of water content

Fourier transform infrared (FT-IR) spectrum of water-in-supercritical CO(2) microemulsion was measured at 60 degrees C and 30.0 MPa over a wide range of water/CO(2) ratio from 0.0 to 1.2 wt % to study the distribution of water into CO(2), interfacial area around surfactant headgroup, and core water pool. The microemulsion was stabilized by sodium bis(1H,1H,2H, 2H-heptadecafluorodecyl)-2-sulfosuccinate [8FS(EO)(2)] equimolarly mixed with sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate [FC6HC4] or with poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether [TMN-6]. The signal area of the O-H stretching band of water suggested that the number of water molecules in the microemulsion increases linearly with the water/CO(2) ratio, except for a slow initial increase below 0.4 wt % due to a part of water dissolved in CO(2). The amount of water in CO(2) was evaluated by decomposing the bending band of water into two components, one at lower frequency ascribed to water in CO(2) and the other at higher frequency to water in the microemulsion. The decomposition confirmed that CO(2) is saturated with water at the water content of 0.4 wt %. It was also revealed, from the symmetric SO stretching frequency of the surfactant, that the sulfonate headgroup is completely hydrated at the water/CO(2) ratio of 0.4-0.5 wt %. The results demonstrated that water is introduced preferentially into CO(2) and the interfacial area at small water content, and then is loaded into the micelle core after the saturation of CO(2) with water and the full hydration of the surfactant headgroup.     

Supercritical carbon dioxide processing of fluorinated surfactant templated mesoporous silica thin films

The effect of processing mesoporous silica thin films with supercritical CO2 immediately after casting is investigated, with a goal of using the penetration of CO2 molecules in the tails of fluorinated surfactant templates to tailor the final pore size. Well-ordered films with two-dimensional hexagonal close-packed pore structure are synthesized using a cationic fluorinated surfactant, 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl)pyridinium chloride, as a templating agent. Hexagonal mesopore structures are obtained for both unprocessed films and after processing the cast films in CO2 at constant pressure (69-172 bar) and temperature (25-45 degrees C) for 72 h, followed by traditional heat treatment steps. X-ray diffraction and transmission electron microscopy analysis reveal significant increases in pore size for all CO2-treated thin films (final pore diameter up to 4.22 +/- 0.14 nm) relative to the unprocessed sample (final pore diameter of 2.21 +/- 0.20 nm) before surfactant extraction. Similar pore sizes are obtained with liquid and supercritical fluid treatments over the range of conditions tested. These results demonstrate that combining the tunable solvent strength of compressed and supercritical CO2 with the "CO2-philic" nature of fluorinated tails allows one to use CO2 processing to control the pore size in ordered mesoporous silica films.     

Morphological control of calcium carbonate crystallized in reverse micelle system with anionic surfactants SDS and AOT

The influence of a surfactant over water on the polymorphism and crystal size of calcium carbonate produced by reaction crystallization in microemulsion systems was investigated in a mixing tank reactor. The crystallization was induced by the reaction between two aqueous micelle solutions (Na2CO3-CaCl2) stabilized by anionic surfactants, SDS (sodium dodecyl sulfate) or AOT (sodium bis(2-ethylhexyl) sulfosuccinate). With increasing surfactant ratio to water, the water-in-oil microemulsion was stably developed and the morphology of the calcium carbonate crystallized in the micelles sharply transformed from calcite to vaterite. The influence of SDS on the polymorphism and crystal size of calcium carbonate was much clearer than that of AOT. In addition, with AOT, certain step changes in the morphology and crystal size occurred around a surfactant ratio to water (R=[H2O]/[surfactant]) of 15 due to a two-phase separation of the microemulsion.     

Surfactants having polyfluoroalkyl chains. II. Syntheses of anionic surfactants having two polyfluoroalkyl chains including a trifluoromethyl group at each tail and their flocculation-redispersion ability for dispersed magnetite particles in water

Anionic surfactants having two polyfluoroalkyl chains per molecule, i.e. the sodium salt of bis(1H, 1H, 2H,2H-heptadeca-fluorodecyl)-2-sulfosuccinate, CF₃(CF₂)₇(CH₂)₂OCOCH₂CH(SO₃Na)COO(CH₂)₂(CF₂)₇CF₃, the sodium salt of bis(1H, 1H, 2H, 2H-tridecafluoro-octyl)-2-sulfosuccinate, CF₃(CF₂)₅(CH₂)₂OCOCH₂CH(SO₃Na)COO(CH₂)₂(CF₂)₅CF₃, and the sodium salt of bis(1H, 1H, 2H, 2H-nonafluorohexyl)-2-sulfosuccinate, CF₃(CF₂)₃(CH₂)₂OCOCH₂CH(SO₃Na)COO(CH₂)₂(CF₂)₃CF₃, have been prepared from maleic anhydride, the corresponding alcohols possessing a polyfluoroalkyl chain and sodium hydrogen sulfite. The flocculation and redispersion abilities of these surfactants for dispersed magnetic particles in water have been examined to investigate the effect of the chain length. It was found that this ability was enhanced by an increase in the chain length. The contact angles for water for pelleted surface-modified magnetite have been measured. In order to compare this ability and the contact angles, data for other fluorinated surfactant have been obtained. The Kraff point, the surface tension and the pNa of the aqueous surfactant solutions have also been measured.     

Concentrated CO(2)-in-Water Emulsions with Nonionic Polymeric Surfactants

Concentrated CO(2)-in-water (C/W) emulsions are reported for amphiphiles containing alkylene oxide-, siloxane-, and fluorocarbon-based tails as a function of temperature and salinity. Poly(ethylene oxide)-b-poly(butylene oxide) (EO(15)-b-BO(12)) can emulsify up to 70% CO(2) with droplet sizes from 2 to 4 &mgr;m in diameter, as determined by video-enhanced microscopy. This emulsion is stable over 48 h against both flocculation and coalescence. In contrast, it is extremely difficult to form concentrated water-in-CO(2) (W/C) emulsions with surfactants containing alkylene oxide moieties due to limited solvation of such tails by CO(2). In several cases, C/W emulsions are formed even when the surfactant prefers CO(2). This violation of Bancroft's rule may be attributed in part to the low viscosity of the compressed CO(2), which governs several mass and momentum transport mechanisms relevant to emulsion formation and stabilization. For the first time, W/C microemulsions are observed in a system with a nonionic amphiphile, namely F(CF(2)CF(2))(3-8)CH(2)CH(2)O(CH(2)CH(2)O)(10-15)H. For the same system, the emulsion morphology changes from C/W to W/C as the temperature increases. The electrical conductivity of C/W emulsions is predicted successfully as a function of the dispersed phase volume fraction of CO(2) with Maxwell's theory for inhomogeneous systems. Copyright 2001 Academic Press.     

Optimum tail length of fluorinated double-tail anionic surfactant for water/supercritical CO2 microemulsion formation

The effect of surfactant tail structure on the stability of a water/supercritical CO2 microemulsion (W/scCO2 muE) was examined for various fluorinated double-tail anionic surfactants of different fluorocarbon chain lengths, F(CF2)n (n = 4, 6, 8, and 10), and oxyethylene spacer lengths, (CH2CH2O)(m/2) (m = 2 and 4). The phase behavior of the water/surfactant/CO2 systems was studied over a wide range of CO2 densities from 0.70 to 0.85 g/cm(3) (temperatures from 35 to 75 degrees C and pressures up to 500 bar) and corrected water-to-surfactant molar ratios (W0c). All of the surfactants yielded a W/scCO2 muE phase, that is, a transparent homogeneous phase with a water content larger than that permitted by the solubility of water in pure CO2. With increasing W0c, a phase transition occurred from the muE phase to a macroemulsion or a lamella-like liquid crystal phase. The maximum W0c value was obtained at a tail length of 12-14 A, indicating the presence of an optimum surfactant tail length for W/scCO2 muE formation.     

Oxygenated hydrocarbon ionic surfactants exhibit CO2 solubility

Several oxygenated hydrocarbons, including acetylated sugars, poly(propylene glycol), and oligo(vinyl acetate), have been used to generate CO2-soluble ionic surfactants. Surfactants with vinyl acetate tails yielded the most promising results, exhibiting levels of CO2 solubility comparable to those associated with fluorinated ionic surfactants. For example, a sodium sulfate with single, oligomeric vinyl acetate (VAc) tails consisting of 10 VAc repeat units was 7 wt % soluble in CO2 at 25 degrees C and 48 MPa. Upon introduction of water to these systems, only surfactants with the oligomeric vinyl acetate tails exhibited spectroscopic evidence of a polar environment that was capable of solubilizing the methyl orange into the CO2-rich phase. For example, a single-phase solution of CO2, 0.15 wt % sodium bis(vinyl acetate)8 sulfosuccinate, and water, at water loading (W) values ranging from 10 to 40 at 25 degrees C and 34.5 MPa, exhibited a methyl orange peak at 423 nm. This result indicated that the core of a reverse micelle provided a microenvironment with a polarity similar to that of methanol. Quantum chemical calculations indicate that the acetylated sugars may be too hydrophilic to readily form reverse micelles, whereas the VAc-based surfactants appear to have the correct balance of hydrophilic and hydrophobic forces necessary to form reverse micelles.     

UV-Vis Investigation on the Hydrocarbon Length Effect on the α-Tocopherol Solubility in an AOT/n-Alkane/Water System

The influence of n-alkane hydrocarbon chain length on both binding and distribution constants of α-tocopherol and Aerosol-OT [sodium bis(2-ethylhexyl) sulfosuccinate] reversed micelles were studied with the UV-vis method using n-heptane and n-decane as solvents. The amount of water in the system was determined by R defined as the ratio of water to surfactant molalities ( $R=[\hbox{H$_{2}$O}]/[\hbox{AOT}]$ ). No significant water influence on distribution and binding constants was observed. This finding is consistent with earlier results, which indicated that the location of α-tocopherol molecules in the AOT reversed micelles is the palisade layer. The results obtained indicate that the longer hydrocarbon chain length makes surroundings of micelles more ordered and causes some limitations in α-tocopherol access to its palisade layer. This implies a weaker connection to the micellar structure and an increase of α-tocopherol freedom of motion in the space of hydrocarbon tails around polar core.     

NMR and SANS studies of aggregation and microemulsion formation by phosphorus fluorosurfactants in liquid and supercritical carbon dioxide

(1)H NMR relaxation and diffusion studies were performed on water-in-CO(2) (W/C) microemulsion systems formed with phosphorus fluorosurfactants of bis[2-(F-hexyl)ethyl] phosphate salts (DiF(8)), having different counterions (Na(+), NH(4)(+), N(CH(3))(4)(+)) by means of high-pressure in situ NMR. Water has a low solubility in CO(2) and is mainly solubilized by the microemulsion droplets formed with surfactants added to CO(2) and water mixtures. There is rapid exchange of water between the bulk CO(2) and the microemulsion droplets; however, NMR relaxation measurements show that the entrapped water has restricted motion, and there is little "free" water in the core. Counterions entrapped by the droplets are mostly associated with the surfactant headgroups: diffusion measurements show that counterions and the surfactant molecules move together with a diffusion coefficient that is associated with the droplet. The outer shell of the microemulsion droplets consists of the surfactant tails with some associated CO(2). For W/C microemulsions formed with the phosphate-based surfactant having the ammonia counterion (A-DiF(8)), the (1)H NMR signal for NH(4)(+) shows a much larger diffusion coefficient than that of the surfactant tails. This apparent paradox is explained on the basis of proton exchange between water and the ammonium ion. The observed dependence of the relaxation time (T(2)) on W(0) (mole ratio of water to surfactant in the droplets) for water and NH(4)(+) can also be explained by this exchange model. The average hydrodynamic radius of A-DiF(8) microemulsion droplets estimated from NMR diffusion measurements (25 degrees C, 206 bar, W(0) = 5) was R(h) = 2.0 nm. Assuming the theoretical ratio of R(g)/R(h) = 0.775 for a solid sphere, where R(g) is the radius of gyration, the equivalent hydrodynamic radius from SANS is R(h) = 1.87 nm. The radii measured by the two techniques are in reasonable agreement, as the two techniques are weighted to measure somewhat different parts of the micelle structure.     

Mixtures of hydrogenated and fluorinated lactobionamide surfactants with cationic surfactants: study of hydrogenated and fluorinated chains miscibility through potentiometric techniques

The work reported herein deals with the aqueous behavior of hydrocarbon and/or fluorocarbon ionic and nonionic surfactants mixtures. These mixtures were studied using potentiometric techniques in NaBr (0.1 mol L-1) aqueous solution as well as in pure water. Mixed micelles were formed from a cationic surfactant (dodecyl or tetradecyltrimethylammonium bromide respectively called DTABr or TTABr) and neutral lactobionamide surfactants bearing a hydrogenated dodecyl chain (H12Lac) or a fluorinated chain (CF3-(CF2)5-(CH2)2- or CF3-(CF2)7-(CH2)2-). We showed that concentrations of ionic and nonionic surfactants in the monomeric form as well as the composition of the mixed micelles can be specified thanks to a potentiometric technique. The complete characterization does not request any model of micellization a priori. The activities of the micellar phase constituents, as well as the free enthalpies of mixing, were calculated. The subsequent interpretation only relies on the experimental characterization. Comparison of the behaviors of the various systems with a model derived from the regular solution theory reveals the predominant part of electrostatic interactions in the micellization phenomenon. It also appears that the energy of interaction between hydrogenated and fluorinated chains is unfavorable to mixing and is of much lower magnitude than the electric charges interactions.     

Electron density matching as a guide to surfactant design

The effectiveness at reducing interfacial tension between water and different organic solvents was studied, with 14 structurally different dichain sulfosuccinate surfactants. Variations in chemical structure ranged from linear/branched alkyl tail groups, to phenyl-tipped tail units, to partially and fully fluorinated tails. The solvents n-heptane, toluene, and perfluoroheptane were used as example oil phases. Interfacial activity was measured in terms of a reduced interfacial tension scale, R(IFT), based on the value in the presence of surfactants compared to that for the pure solvent-water interface. Overall surfactant chain structure was determined to be the key factor affecting R(IFT). Furthermore, a strong correlation was observed between R(IFT) and the electron density rho(e) of the different surfactants: with any given oil, the most effective surfactants have rho(e) values closest to that for the solvent. For example, phenyl-tipped surfactants were shown to be comparatively more effective at the interface with an aromatic solvent (toluene) than with an aliphatic n-alkane (heptane). Furthermore, fluorination of the tail groups decreased effectiveness at the hydrocarbon/water interface, which was substantially increased at the fluorocarbon/water interface: this too followed the electron density-matching pattern. The importance of chain-tip chemical structure was also noted, with regard to the introduction of phenyl, CF3-, and H-CF2- terminal moieties. For branched alkyl-tailed surfactants, it was found that effectiveness could be linked to an empirical "branching factor". The significance of the electron density matching of organic solvent and surfactant for the prediction of interfacial activities is highlighted, and this concept may prove useful for the future design of new high-efficiency surfactants.     

Probing the impact of advanced melting and advanced adsorption phenomena on the accuracy of pore size distributions from cryoporometry and adsorption using NMR relaxometry and diffusometry

The penetration of compressed CO(2) in hydrocarbon and fluorocarbon regions of concentrated surfactant mesophases are interpreted from differences in the CO(2)-processed pore expansion of mesoporous silica thin films templated by three surfactants containing varying degrees of hydrocarbon and fluorocarbon functionality. Ordered silica thin films are synthesized for the first time using the 16-carbon (C(16)) partly fluorinated surfactant, 11,11,12,12,13,13,14,14,15,15,16,16,16-tridecafluorocetyl pyridinium bromide (HFCPB), as a templating agent. Silica films templated with surfactants containing a 8-carbon (C(8)) fluorocarbon tail (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl pyridinium chloride (HFOPC)) and a 16-carbon (C(16)) hydrocarbon tail (cetyl pyridinium bromide (CPB)) and HFCPB (C(16)) are processed in compressed CO(2) (69-172 bar, 25 °C and 45 °C) during synthesis. CO(2) processing results in significant pore expansion for films templated with both fluorinated surfactants, while pore expansion is negligible for the hydrocarbon templated material suggesting that preferential CO(2) penetration occurs in the 'CO(2)-philic' fluorocarbon segments of the surfactant template. The effect of substrate surface energy on the final uniformity of the dip-coated films is studied by varying the substrate from unmodified glass to a fluorocarbon-capped substrate. The ability to create dip-coated thin films on low surface energy substrates through favorable interaction of surfactant template tail with the substrate surface functional groups is demonstrated.     

Low fluorine content CO2-philic surfactants

The article addresses an important, and still unresolved question in the field of CO(2) science and technology: what is the minimum fluorine content necessary to obtain a CO(2)-philic surfactant? A previous publication (Langmuir 2002, 18, 3014) suggested there should be an ideal fluorination level: for optimization of possible process applications in CO(2), it is important to establish just how little F is needed to render a surfactant CO(2)-philic. Here, optimum chemical structures for water-in-CO(2) (w/c) microemulsion stabilization are identified through a systematic study of CO(2)-philic surfactant design based on dichain sulfosuccinates. High pressure small-angle neutron scattering (HP-SANS) measurements of reversed micelle formation in CO(2) show a clear relationship between F content and CO(2) compatibility of any given surfactant. Interestingly, high F content surfactants, having lower limiting aqueous surface tensions, γ(cmc), also have better performance in CO(2), as indicated by lower cloud point pressures, P(trans). The results have important implications for the rational design of CO(2)-philic surfactants helping to identify the most economic and efficient compounds for emerging CO(2) based fluid technologies.     

Micellization of fluorinated amphiphiles

New cationic fluorinated surfactants and new types of fluorinated surfactants having fluorocarbon–hydrocarbon hybrids, dimeric and polymeric structure have been synthesized recently. Their synthesis requires many steps and consequently requires much time and high expense. Since the fluorinated surfactants have unusual molecular aggregation properties, ¹⁹F-NMR, novel fluorescence probes and cryo-transmission electron microscope techniques have been applied to study their aggregation behaviour in aqueous systems. Their unique characteristics are summarized as follows: (1) the dissolution process from solid state to dissolved aggregate state requires a very long time for the long chain fluorinated surfactants under thermodynamic equilibrium. The equilibration time can be reduced at higher temperatures; (2) interfacial properties and critical micelle concentration (CMC) are influenced by the nature of the hydrophobic terminal groups (CF₃− or HCF₂−); (3) the fluorocarbon functionality can make it possible even for single-chain amphiphiles to form vesicles or lamellar structures; (4) the hybrid surfactant made of both hydrocarbon and fluorocarbon chains showed a life time of 2.0×10⁻³ s for the exchange rate between the monomeric and the micellar states at the CMC and moreover, these detergents can cosolubilize fluorocarbon–hydrocarbon mixed solubilizates.