Hybrid CO2-philic surfactants with low fluorine content

Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook

CO2 foam injection has been shown to be effective under reservoir conditions for enhanced oil recovery. However, its application requires a certain stability and surfactant absorbability on rock surface, and it is also associated with borehole corrosion in the presence of water. Adding surfactants to CO2 can enhance the interaction between CO2 and crude oil and control the CO2 mobility, thereby improving the performance of CO2 flooding. This paper presents a review of the research of CO2-soluble surfactants and their applications. Molecular dynamics simulation is introduced as a tool for analyzing the behavior of the surfactants in supercritical CO2 (scCO2). The applications of CO2-soluble surfactants, including CO2 thickening, reducing miscibility pressure, and generating supercritical CO2 foam, are discussed in detail. Moreover, some opportunities for the research and development of CO2-soluble surfactants are proposed.

Fabrication and application of composite adsorbents made by one-pot electrochemical exfoliation of graphite in surfactant ionic liquid/nanocellulose mixtures

Previously, surfactant-assisted exfoliated graphene oxide (sEGO) formed with the triple-chain surfactant TC14 (sodium 1,4-bis(neopentyloxy)-3-(neopentylcarbonyl)-1,4-dioxobutane-2-sulfonate) was applied in wastewater treatment. The extent of dye-removal and the adsorption capacity of the sEGO formed with this triple-chain surfactant outperformed those of two other systems, namely, the di-chain version of TC14 (AOT14; sodium 1,2-bis-(2,2-dimethyl-propoxycarbonyl)-ethanesulfonate) and the single-chain surfactant sodium n-dodecylsulfate. In the present study, to further optimise the surfactant chemical structure, the sodium ion of TC14 was substituted with 1-butyl-3-methyl-imidazolium (BMIM) generating surfactant ionic liquids (SAILs; 1-butyl-3-imidazolium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate), hereafter denoted as BMIM-TC14. This SAIL, together with nanofibrillated kenaf cellulose (NFC), was used to electrochemically exfoliate graphite, yielding BMIM-TC14 sEGO/NFC composites. These highly hydrophobic polymer composites were then used for the removal of methylene blue (MB) from aqueous solution. ¹H NMR spectroscopy was used to elucidate the structure of the synthesised SAILs. The morphologies of the resulting nanocomposites were investigated using Raman spectroscopy, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. Analysis using small-angle neutron scattering was performed to examine the aggregation behaviour of sEGO and custom-made SAILs. Zeta potential, surface tension, and dynamic light-scattering measurements were used to study the aqueous properties and colloidal stability of the suspension. Amongst the surfactants tested, BMIM-TC14 sEGO/NFC exhibited the highest MB adsorption ability, achieving 99% dye removal under optimum conditions. These results highlight the importance of modifying the hydrophilic moieties of amphiphilic compounds to improve the performance of sEGO/NFC composites as effective adsorbents for wastewater treatment.

Design of Surfactant Tails for Effective Surface Tension Reduction and Micellization in Water and/or Supercritical CO2

The interfacial properties and water-in-CO₂ (W/CO₂) microemulsion (μE) formation with double- and novel triple-tail surfactants bearing trimethylsilyl (TMS) groups in the tails are investigated. Comparisons of these properties are made with those for analogous hydrocarbon (HC) and fluorocarbon (FC) tail surfactants. Surface tension measurements allowed for critical micelle concentrations (CMC) and surface tensions at the CMC (γ_CMC) to be determined, resulting in the following trend in surface activity FC > TMS > HC. Addition of a third surfactant tail gave rise to increased surface activity, and very low γ_CMC values were recorded for the double/triple-tail TMS and HC surfactants. Comparing effective tail group densities (ρ_layer) of the respective surfactants allowed for an understanding of how γ_CMC is affected by both the number of surfactant tails and the chemistry of the tails. These results highlight the important role of tail group chemical structure on ρ_layer for double-tail surfactants. For triple-tail surfactants, however, the degree to which ρ_layer is affected by tail group architecture is harder to discern due to formation of highly dense layers. Stable W/CO₂ μEs were formed by both the double- and the triple-tail TMS surfactants. High-pressure small-angle neutron scattering (HP-SANS) has been used to characterize the nanostructures of W/CO₂ μEs formed by the double- and triple-tail surfactants, and at constant pressure and temperature, the aqueous cores of the microemulsions were found to swell with increasing water-to-surfactant ratio (W₀). A maximum W₀ value of 25 was recorded for the triple-tail TMS surfactant, which is very rare for nonfluorinated surfactants. These data therefore highlight important parameters required to design fluorine-free environmentally responsible surfactants for stabilizing W/CO₂ μEs.

Highly branched triple-chain surfactant-mediated electrochemical exfoliation of graphite to obtain graphene oxide: colloidal behaviour and application in water treatment

The generation of surfactant-assisted exfoliated graphene oxide (sEGO) by electrochemical exfoliation is influenced by the presence of surfactants, and in particular the hydrophobic tail molecular-architecture. Increasing surfactant chain branching may improve the affinity for the graphite surfaces to provide enhanced intersheet separation and stabilisation of exfoliated sheets. The resulting sEGO composites can be readily used to remove of a model pollutant, the dye, methylene blue (MB), from aqueous solutions by providing abundant sites for dye adsorption. This article explores relationships between surfactant structure and the performance of sEGO for MB adsorption. Double-branched and highly branched triple-chain graphene-compatible surfactants were successfully synthesised and characterised by ¹H NMR spectroscopy. These surfactants were used to produce sEGO via electrochemical exfoliation of graphite, and the sEGOs generated were further utilised in batch adsorption studies of MB from aqueous solutions. The properties of these synthesised surfactants were compared with those of a common single-chain standard surfactant, sodium dodecyl-sulfate (SDS). The structural morphology of sEGO was assessed using Raman spectroscopy and field emission scanning electron microscopy (FESEM). To reveal the links between the hydrophobic chain structure and the sEGO adsorption capacity, UV-visible spectroscopy, zeta potential, and air-water (a/w) surface tension measurements were conducted. The aggregation behaviour of the surfactants was studied using small-angle neutron scattering (SANS). The highly branched triple-chain surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentylcarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) displayed enhanced exfoliating efficiency compared to those of the single-and double-chain surfactants, leading to ∼83% MB removal. The findings suggest that highly branched triple-chain surfactants are able to offer more adsorption sites, by expanding the sEGO interlayer gap for MB adsorption, compared to standard single-chain surfactants.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO₂) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO₂, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO₂-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO₂, avoiding direct contacts between CO₂ and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO₂ are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO₂. The average reverse micelle structure equilibrated in supercritical CO₂ is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO₂. We note that the diverse pathways of surfactant self-aggregation in gaseous CO₂ could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO₂.

Tuning Micellar Structures in Supercritical CO2 Using Surfactant and Amphiphile Mixtures

For equivalent micellar volume fraction (ϕ), systems containing anisotropic micelles are generally more viscous than those comprising spherical micelles. Many surfactants used in water-in-CO₂ (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO₂-philic surfactants with hydrotropes and cosurfactants may generate elongated micelles in w/c systems at high-pressures (e.g., 100-400 bar). A range of novel w/c microemulsions, stabilized by new custom-synthesized CO₂-phillic, partially fluorinated surfactants, were formulated with hydrotropes and cosurfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure, and hydrotrope tail length were all investigated. Dispersed water domains were probed using high pressure small-angle neutron scattering (HP-SANS), which provided evidence for elongated reversed micelles in supercritical CO₂. These new micelles have significantly lower fluorination levels than previously reported (6-29 wt % cf. 14-52 wt %), and furthermore, they support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio; from this value a relative viscosity value can be estimated through combination with the micellar volume fraction (ϕ). Combining these new results with those for all other reported systems, it has been possible to "map" predicted viscosity increases in CO₂ arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.

Aqueous Self-Assembly of Y-Shaped Amphiphilic Block Copolymers into Giant Vesicles

The preparation and aqueous self-assembly of newly Y-shaped amphiphilic block polyurethane (PUG) copolymers are reported here. These amphiphilic copolymers, designed to have two hydrophilic poly(ethylene oxide) (PEO) tails and one hydrophobic alkyl tail via a two-step coupling reaction, can self-assemble into giant unilamellar vesicles (GUVs) (diameter ≥ 1000 nm) with a direct dissolution method in aqueous solution, depending on their Y-shaped structures and initial concentrations. More interesting, the copolymers can self-assemble into various distinct nano-/microstructures, such as spherical micelles, small vesicles, and GUVs, with the increase of their concentrations. The traditional preparation methods of GUVs generally need conventional amphiphilic molecules and additional complicated conditions, such as alternating electrical field, buffer solution, or organic solvent. Therefore, the self-assembly of Y-shaped PUGs with a direct dissolution method in aqueous solution demonstrated in this study supplies a new clue to fabricate GUVs based on the geometric design of amphiphilic polymers.

The self-assembly structure and the CO2-philicity of a hybrid surfactant in supercritical CO2: effects of hydrocarbon chain length

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains, as effective CO₂-philic surfactants, could improve the solubility of polar substances in supercritical CO₂. Varying the length of the HC of hybrid surfactants is an effective way to improve the CO₂-philicity. In this paper, we have investigated the effects of the HC length on the self-assembly process and the CO₂-philicity of hybrid surfactants (F7Hn, n = 1, 4, 7 and 10) in water/CO₂ mixtures using molecular dynamics simulations. It is found that the self-assembly time of F7Hn exhibits a maximum when the length of the HC is equal to that of the FC (F7H7). In this case, the investigation of H-bonds between the water core and CO₂ phase shows that F7H7 has the strongest CO₂-philicity because it has the best ability to separate water and CO₂. To explain the origin of the differences in separation ability, the analysis of the structures of the reverse micelles shows that there are two competing mechanisms with a shortening HC. Firstly, the volume of F7Hn is reduced, which thus decreases the separation ability. Moreover, this also leads to the curved conformation of the FC. As a result, the separation ability is enhanced. These two mechanisms are balanced in F7H7, which has the best ability to separate water and CO₂. Our simulation results demonstrate that the increased volume and the curved conformation of the hybrid surfactant tail could enhance the CO₂-philicity in F7Hn surfactants. It is expected that this work will provide valuable information for the design of CO₂-philic surfactants.

Shape Modification of Water-in-CO2 Microemulsion Droplets through Mixing of Hydrocarbon and Fluorocarbon Amphiphiles

An oxygen-rich hydrocarbon (HC) amphiphile has been developed as an additive for supercritical CO2 (scCO2). The effects of this custom-designed amphiphile have been studied in water-in-CO2 (w/c) microemulsions stabilized by analogous fluorocarbon (FC) surfactants, nFG(EO)2, which are known to form spherical w/c microemulsion droplets. By applying contrast-variation small-angle neutron scattering (CV-SANS), evidence has been obtained for anisotropic structures in the mixed systems. The shape transition is attributed to the hydrocarbon additive, which modifies the curvature of the mixed surfactant films. This can be considered as a potential method to enhance physicochemical properties of scCO2 through elongation of w/c microemulsion droplets. More importantly, by studying self-assembly in these mixed systems, fundamental understanding can be developed on the packing of HC and FC amphiphiles at water/CO2 interfaces. This provides guidelines for the design of fluorine-free CO2 active surfactants, and therefore, practical industrial scale applications of scCO2 could be achieved.

Exploration of CO2-Philicity of Poly(vinyl acetate-co-alkyl vinyl ether) through Molecular Modeling and Dissolution Behavior Measurement

Hydrocarbon CO2-philes are of great interest for use in expanding CO2 applications as a green solvent. In this work, multiscale molecular modeling and dissolution behavior measurement were both applied to explore CO2-philicity of the poly(vinyl acetate) (PVAc)-based copolymer. Introduction of a favorable comonomer, i.e., vinyl ethyl ether (VEE), could significantly reduce the polymer-polymer interaction on the premise that the polymer-CO2 interaction was not weakened but enhanced. The ab initio calculated interaction of the model molecules with CO2 demonstrated that the ether group in VEE or VBE was the suitable CO2-philic segment. From the molecular dynamics (MD) simulations of polymer/CO2 systems, the interaction energy and Flory-Huggins parameter (χ12) of poly(VAc-alt-VEE)/CO2 supported that poly(VAc-alt-VEE) possessed better CO2-philicity than PVAc. The dissolution behaviors of the synthesized poly(VAc-co-alkyl vinyl ether) copolymers in CO2 showed the best CO2-phile had the VEE content of about 34 mol %. The MD simulations also indicated that the interaction of random poly(VAc-co-VEE) containing about 30 mol % VEE with CO2 was the strongest and the χ12 was the smallest in these polymer/CO2 systems. Not only could the VEE monomer reduce the polymer-polymer interaction, but it could also enhance the polymer-CO2 interaction with an optimized composition. Introducing a suitable comonomer with a certain composition might be a promising strategy to form the synergistic effect of polymer-polymer interaction and polymer-CO2 interaction for screening the hydrocarbon CO2-philes.

Unexpected efficiency boosting in CO2-microemulsions: a cyclohexane depletion zone near the fluorinated surfactants evidenced by a systematic SANS contrast variation study

Concentration gradient of cyclohexane in a CO₂/cyclohexane swollen micelle stabilized by fluorinated surfactants revealed by the GIFT analysis of a SANS contrast variation.

Properties and sodium salicylate induced aggregation behavior of a tail-branched cationic surfactant with a hydroxyl-containing hydrophilic head

This paper introduced a cationic amphiphile with remarkable surface activity, whose aggregation behavior could be modulated by salt addition.

Supercritical carbon dioxide: a solvent like no other

Supercritical carbon dioxide (scCO2) could be one aspect of a significant and necessary movement towards green chemistry, being a potential replacement for volatile organic compounds (VOCs). Unfortunately, carbon dioxide has a notoriously poor solubilising power and is famously difficult to handle. This review examines attempts and breakthroughs in enhancing the physicochemical properties of carbon dioxide, focusing primarily on factors that impact solubility of polar and ionic species and attempts to enhance scCO2 viscosity.

Low-surface energy surfactants with branched hydrocarbon architectures

Surface tensiometry and small-angle neutron scattering have been used to characterize a new class of low-surface energy surfactants (LSESs), "hedgehog" surfactants. These surfactants are based on highly branched hydrocarbon (HC) chains as replacements for environmentally hazardous fluorocarbon surfactants and polymers. Tensiometric analyses indicate that a subtle structural modification in the tails and headgroup results in significant effects on limiting surface tensions γcmc at the critical micelle concentration: a higher level of branching and an increased counterion size promote an effective reduction of surface tension to low values for HC surfactants (γcmc ∼ 24 mN m(-1)). These LSESs present a new class of potentially very important materials, which form lamellar aggregates in aqueous solutions independent of dilution.

One F-octyl versus two F-butyl chains in surfactant aggregation behavior

An easy synthetic procedure in two or three steps from perfluoroalkylethyl iodide derivatives led to six novel fluorinated carboxylates monomeric and gemini surfactants with one or two hydrophobic tails, respectively: RF(C2H4)CH(CO2(-))2,2Na(+) and [RF(C2H4)]2C(CO2(-)),Na(+), where RF = C4F9, C6F13, and C8F17. These anionic surfactants exhibited very low surface tension from 15 to 33 mN/m as well as low critical micelle concentration until 1.3 × 10(-4) mol/L. Furthermore, the surface properties of the gemini compound with two short fluoroalkyl chains (RF = C4F9) were found to be almost equal to those of the monomeric surfactant with one long fluoroalkyl chain (RF = C8F17), which could provide an interesting alternative to the bioaccumulative long-chain perfluorinated surfactant.

Aggregation behavior of Triton X-100 with a mixture of two room-temperature ionic liquids: can we identify the mutual penetration of ionic liquids in ionic liquid containing micellar aggregates?

In this manuscript, we have characterized two different micellar aggregates containing all nonvolatile components. We have shown (i) the effect of ethylammonium nitrate (EAN) addition on the properties of micellar solution of Triton X-100 in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)) and (ii) the effect of bmimPF(6) addition on the properties of micellar solution of Triton X-100 in EAN. To investigate the effect, we have used (1)H NMR, pulsed-field gradient spin-echo NMR (PFGSE NMR), and methyl orange (MO) and coumarin 153 (C-153) as absorption and emission probes, respectively. The penetration of added EAN inside the Triton X-100/bmimPF(6) micellar aggregates is indicated by (i) red shift in both the absorption spectra of MO and emission spectra of C-153 and (ii) downfield shift of proton signals of ethylene oxide units in Triton X-100. On the other hand, (1)H NMR and PFGSE NMR indicates the penetration of added bmimPF(6) inside the Triton X-100/EAN micellar aggregates. However, the constancy of both the absorption spectra of MO and emission spectra of C-153 indicates that the microenvironment around the probe molecules remains unaffected. We have also investigated the effect of micelle formation and the effect of penetration of ionic liquids (ILs) in micellar aggregates, on the solvation dynamics of C-153. The solvent relaxation around C-153 gets retarded on going from neat ILs to the micellar solution of Triton X-100 in ILs. In addition to this, we have also observed that with the addition of EAN in Triton X-100/bmimPF(6) micellar aggregates the solvation dynamics becomes faster, whereas with the addition of bmimPF(6) in Triton X-100/EAN micellar aggregates we did not observe any notable change in solvation dynamics. This observation further supports the conclusions drawn from UV-visible and NMR studies.