Effect of Fluorocarbon and Hydrocarbon Chain Lengths in Hybrid Surfactants for Supercritical CO2

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.

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Rapid characterization of the activities of lignin-modifying enzymes based on nanostructure-initiator mass spectrometry (NIMS)

BACKGROUND

Producing valuable fuels and chemicals from lignin is a key factor for making lignocellulosic biomass economically feasible; however, significant roadblocks exist due to our lack of detailed understanding of how lignin is enzymatically depolymerized and of the range of possible lignin fragments that can be produced. Development of suitable enzymatic assays for characterization of putative lignin active enzymes is an important step towards improving our understanding of the catalytic activities of relevant enzymes. Previously, we have successfully built an assay platform based on glycan substrates containing a charged perfluorinated tag and nanostructure-initiator mass spectrometry to study carbohydrate active enzymes, especially various glycosyl hydrolyses. Here, we extend this approach to develop a reliable and rapid assay to study lignin-modifying enzymes.

RESULTS

Two β-aryl ether bond containing model lignin dimer substrates, designed to be suitable for studying the activities of lignin-modifying enzymes (LMEs) by nanostructure-initiator mass spectrometry (NIMS), were successful synthesized. Small-angle neutron scattering experiments showed that these substrates form micelles in solution. Two LMEs, laccase from the polypore mushroom Trametes versicolor, and manganese peroxidase (MnP) from white rot fungus Nematoloma frowardii, were tested for catalytic activity against the two model substrates. We show that the reaction of laccase and MnP with phenolic substrate yields products that arise from the cleavage of the carbon-carbon single bond between the α-carbon and the adjacent aryl carbon, consistent with the mechanism for producing phenoxy radical as reaction intermediates. Reactions of the nonphenolic substrate with laccase, on the other hand, adopt a different pathway by producing an α-oxidation product; as well as the cleavage of the β-aryl ether bond. No cleavage of the carbon-carbon bond between the α-carbon and the aryl carbon was observed. To facilitate understanding of reaction kinetics, the reaction time course for laccase activity on the phenolic substrate (I) was generated by the simultaneous measurement of all products at different time points of the reaction. Withdrawal of only a small sample aliquot (0.2 μL at each time point) ensured minimum perturbation of the reaction. The time course can help us to understand the enzyme kinetics.

CONCLUSIONS

A new assay procedure has been developed for studying lignin-modifying enzymes by nanostructure-initiator mass spectrometry. Enzyme assays of a laccase and a MnP on phenolic and nonphenolic β-aryl ether substrates revealed different primary reaction pathways due to the availability of the phenoxy radical intermediates. Our assay provides a wealth of information on bond cleavage events not available using conventional colorimetric assays and can easily be carried out in microliter volumes and the quantitative analysis of product formation and kinetics is rapidly achieved by NIMS. This is the first time that NIMS technology was applied to study the activities of lignin-modifying enzymes. Unlike other previous works, our use of amphiphilic guaiacylglycerol β-O-4 substrate (I) enables the formation of micelles. This approach helps avoid the re-polymerization of the resulting monomeric product. As a result, our assay can clearly demonstrate the degradation pathways of phenolic guaiacylglycerol β-O-4 type of molecules with laccase and MnP.

Anisotropic reversed micelles with fluorocarbon-hydrocarbon hybrid surfactants in supercritical CO2

Previous work (M. Sagisaka, et al. Langmuir 31 (2015) 7479-7487), showed the most effective fluorocarbon (FC) and hydrocarbon (HC) chain lengths in the hybrid surfactants FCm-HCn (sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2-sulfonates, where m = FC length and n = HC length) were m and n = 6 and 4 for water solubilization, whereas m 6 and n 6, or m 6 and n 5, were optimal chain lengths for reversed micelle elongation in supercritical CO₂. To clarify why this difference of only a few methylene chain units is so effective at tuning the solubilizing power and reversed micelle morphology, nanostructures of water-in-CO₂ (W/CO₂) microemulsions were investigated by high-pressure small-angle neutron scattering (SANS) measurements at different water-to-surfactant molar ratios (W₀) and surfactant concentrations. By modelling SANS profiles with cylindrical and ellipsoidal form factors, the FC6-HCn/W/CO₂ microemulsions were found to increase in size with increasing W₀ and surfactant concentration. Ellipsoidal cross-sectional radii of the FC6-HC4/W/CO₂ microemulsion droplets increased linearly with W₀, and finally reached ∼39 Å and ∼78 Å at W₀ = 85 (close to the upper limit of solubilizing power). These systems appear to be the largest W/CO₂ microemulsion droplets ever reported. The aqueous domains of FC6-HC6 rod-like reversed micelles increased in size by 3.5 times on increasing surfactant concentration from 35 mM to 50 mM: at 35 mM, FC6-HC5 formed rod-like reversed micelles 5.3 times larger than FC6-HC6. Interestingly, these results suggest that hybrid HC-chains partition into the microemulsion aqueous cores with the sulfonate headgroups, or at the W/CO₂ interfaces, and so play important roles for tuning the W/CO₂ interfacial curvature. The super-efficient W/CO₂-type solubilizer FC6-HC4, and the rod-like reversed micelle forming surfactant FC6-HC5, represent the most successful cases of low fluorine content additives. These surfactants facilitate VOC-free, effective and energy-saving CO₂ solvent systems for applications such as extraction, dyeing, dry cleaning, metal-plating, enhanced oil recovery and organic/inorganic or nanomaterial synthesis.

Shape transition of water-in-CO2 reverse micelles controlled by the surfactant midpiece

Designing CO₂-philic surfactants for generating wormlike reverse micelles (RMs) is an effective approach to enhance the viscosity of supercritical CO₂ (scCO₂), however this remains challenging.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO₂), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C₈Benz). It is found that adding the C₈Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C₈Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C₈Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C₈Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO₂.

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.

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

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

Molecular dynamics study of di-CF4 based reverse micelles in supercritical CO2

Reverse micelles (RMs) in supercritical CO₂ (scCO₂) are promising alternatives for organic solvents, especially when both polar and non-polar components are involved. Fluorinated surfactants, particularly double-chain fluorocarbon surfactants, are able to form well-structured RMs in scCO₂. The inherent self-assembly mechanisms of surfactants in scCO₂ are still subject to discussion. In this study, molecular dynamics simulations are performed to investigate the self-aggregation behavior of di-CF4 based RMs in scCO₂, and stable and spherical RMs are formed. The dynamics process and the self-assembly structure in the RMs reveal a three-step mechanism to form the RMs, that is, small RMs, rod-like RMs and fusion of the rod-like RMs. Hydrogen-bonds between headgroups and water molecules, and salt bridges linking Na⁺ ions, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The results show that di-CF4 molecules have a high surfactant coverage at the RM interface, implying a high CO₂-philicity. This mainly results from bending of the short chain (C-COO-CH₂-(CF2)₃-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful in understanding surfactant self-assembly phenomena and designing new CO₂-philic surfactants.

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.