Water-in-Carbon Dioxide Microemulsions with Methylated Branched Hydrocarbon Surfactants

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.

Microscopic properties and stabilization mechanism of a supercritical carbon dioxide microemulsion with extremely high water content

HYPOTHESIS

Developing the supercritical carbon dioxide microemulsion with a broad water content (W₀) window can provide more possibility for designing highly efficient chemical processes, which is challenging due to the lack of comprehension about its formation mechanism. Molecular dynamics simulation method is expected to reveal the microscopic stabilization mechanism of high-W₀ microemulsions.

SIMULATIONS

All-atom molecular dynamics simulations of the ternary systems with varied W₀ stabilized by 4FG(EO)₂ surfactant were designed according to phase behavior experiments. A systematic investigation was performed concerning the self-assembling, equilibrium morphology and detailed microstructure of the microemulsion droplet. An in-depth comparative study about the distribution of both H₂O and CO₂, the interfacial behaviors of 4FG(EO)₂, as well as the microscopic interactions was conducted.

FINDINGS

For the first time, direct evidence was provided for the formation of water-in-carbon dioxide microemulsion with extremely high W₀ (80) under the effect of 4FG(EO)₂. Furthermore, a unique interfacial phenomenon, i. e. CO₂ accumulating at the interface, was revealed to be responsible for the formation and enhanced stability of the nanosized droplet with high W₀. This should set a new guiding star for synthesizing and selecting effective interfacial modifiers to create high-W₀ microemulsions.

Dispersibility of Poly(vinyl acetate) Modified Silica Nanoparticles in Carbon Dioxide with Several Cosolvents

The dispersibility and stabilization of silica nanoparticles with surface-capped poly(vinyl acetate) (PVAc) chains are examined in carbon dioxide with four different cosolvents. Three surface coverages of silica-PVAc were formed by using different weight ratios of the silica and PVAc. The dispersibilities of three silica-PVAc nanoparticles in CO₂ with the four cosolvents were tested in a rotatable high-pressure variable-volume view cell. The effects of surface coverage, cosolvent type, pressure, and particle concentration on dispersion were investigated. Results show that, in the experimental pressure range (5.5 to 20 MPa), the pressure has no significant effect on the dispersion of nanoparticles, and the cosolvent is the key factor in dispersing silica-PVAc particles in CO₂. 1-Butanol is an adequate cosolvent to disperse silica-PVAc in CO₂ with any coverage of PVAc on the surface of the particles when the concentration of particles is smaller than 0.31 wt %. Ethanol can only improve the dispersibility of particles with a high surface coverage of PVAc when the concentration of particles is smaller than 0.14 wt %. 1-Hexanol and ethyl acetate cannot disperse the particles in CO₂ with any coverage of PVAc. Molecular dynamics simulations were carried out to study the nanoparticle-CO₂-cosolvent dispersions. Results suggest that 1-butanol has a good solubility in the CO₂ condensed phase and can effectively absorb onto the nanoparticle surface, which help to prevent the formation of nanoparticle aggregation. The precipitation of nanoparticles in the nanoparticle/1-hexanol/CO₂ and nanoparticle/ethyl acetate/CO₂ systems is attributed to the relatively low solubility of CO₂ in 1-hexanol and ethyl acetate. The precipitation of nanoparticles in the nanoparticle/ethanol/CO₂ system is the result of less hindrance of ethanol molecules to the aggregation of nanoparticles.

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.

Synthesis of PDMS-b-POEGMA Diblock Copolymers and Their Application for the Thermoresponsive Stabilization of Water-Supercritical Carbon Dioxide Emulsions

In this study, PDMS₁₃-b-POEGMA_x diblock copolymers consisting of a CO₂-philic poly(dimethylsiloxane) (PDMS) block connected to a thermosensitive hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block were synthesized by reversible addition-fragmentation chain-transfer (RAFT) radical polymerization. Their ability to decrease the water-supercritical CO₂ (scCO₂) interfacial tension (γ) and to stabilize water-scCO₂ emulsions was investigated using an original homemade device developed in the laboratory. This device is able to control the pressure from 1 to 250 bar and the temperature from 40 to 80 °C. It was implemented with 2 visualization windows, a drop tensiometer and a remote optical head for dynamic light scattering (DLS) measurements. These experiments revealed that PDMS-b-POEGMA decreased γ down to 1-2 mN/m and was the most efficient at high pressure (250 bar) and low temperature (40 °C) where PDMS and POEGMA blocks exhibited the highest affinity for their respective phase. The diblock copolymers were shown to stabilize water-scCO₂ emulsions. Moreover, the thermosensitive behavior of the POEGMA block in water (with a lower critical solubility temperature around 65 °C) resulted in the formation of temperature-responsive emulsions that could reversibly switch at 100 bar from stable at 40 °C to unstable at 80 °C. These results were rationalized based on the solubility of each individual block of the copolymers in water and scCO₂ as a function of temperature and pressure.

Water-in-CO2 Microemulsions Stabilized by an Efficient Catanionic Surfactant

To facilitate potential applications of water-in-supercritical CO₂ microemulsions (W/CO₂ μEs) efficient and environmentally responsible surfactants are required with low levels of fluorination. As well as being able to stabilize water-CO₂ interfaces, these surfactants must also be economical, prevent bioaccumulation and strong adhesion, deactivation of enzymes, and be tolerant to high salt environments. Recently, an ion paired catanionic surfactant with environmentally acceptable fluorinated C₆ tails was found to be very effective at stabilizing W/CO₂ μEs with high water-to-surfactant molar ratios (W₀) up to ∼50 (Sagisaka, M.; et al. Langmuir 2019, 35, 3445-3454). As the cationic and anionic constituent surfactants alone did not stabilize W/CO₂ μEs, this was the first demonstration of surfactant synergistic effects in W/CO₂ microemulsions. The aim of this new study is to understand the origin of these intriguing effects by detailed investigations of nanostructure in W/CO₂ microemulsions using high-pressure small-angle neutron scattering (HP-SANS). These HP-SANS experiments have been used to determine the headgroup interfacial area and volume, aggregation number, and effective packing parameter (EPP). These SANS data suggest the effectiveness of this surfactant originates from increased EPP and decreased hydrophilic/CO₂-philic balance, related to a reduced effective headgroup ionicity. This surfactant bears separate C₆F₁₃ tails and oppositely charged headgroups, and was found to have a EPP value similar to that of a double C₄F₉-tail anionic surfactant (4FG(EO)₂), which was previously reported to be one of most efficient stabilizers for W/CO₂ μEs (maximum W₀ = 60-80). Catanionic surfactants based on this new design will be key for generating superefficient W/CO₂ μEs with high stability and water solubilization.

Water-in-CO2 Microemulsions Stabilized by Fluorinated Cation-Anion Surfactant Pairs

High-water-content water-in-supercritical CO₂ (W/CO₂) microemulsions are considered to be green, universal solvents, having both polar and nonpolar domains. Unfortunately, these systems generally require environmentally unacceptable stabilizers like long and/or multifluorocarbon-tail surfactants. Here, a series of catanionic surfactants having more environmentally friendly fluorinated C₄-C₆ tails have been studied in terms of interfacial properties, aggregation behavior, and solubilizing power in water and/or CO₂. Surface tensions and critical micelle concentrations of these catanionic surfactants are, respectively, lowered by ∼9 mN/m and 100 times than those of the constituent single fluorocarbon-tail surfactants. Disklike micelles in water were observed above the respective critical micelle concentrations, implying the catanionic surfactants have a high critical packing parameter, which should be suitable for the formation of reverse micelles. Based on visual observation of phase behavior and Fourier transform infrared spectroscopic and small-angle neutron scattering studies, one of the three catanionic surfactants tested was found to form transparent single-phase W/CO₂ microemulsions with a water-to-surfactant molar ratio of up to ∼50. This is the first successful demonstration of the formation of W/CO₂ microemulsions by synergistic ion-pairing of anionic and cationic single-tail surfactants. This indicates that catanionic surfactants offer a promising approach to generate high-water-content W/CO₂ microemulsions.

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.

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.

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

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains have recently been shown to solubilize water and form elongated reversed micelles in supercritical CO2. To clarify the most effective FC and HC chain lengths, the aggregation behavior and interfacial properties of hybrid surfactants FCm-HCn (FC length m/HC length n = 4/2, 4/4, 6/2, 6/4, 6/5, 6/6, and 6/8) were examined in W/CO2 mixtures as functions of pressure, temperature, and water-to-surfactant molar ratio (W0). The solubilizing power of hybrid surfactants for W/CO2 microemulsions was strongly affected by not only the FC length but also by that of the HC. Although the surfactants having short FC and/or HC tails (namely, m/n = 4/2, 4/4, and 6/2) did not dissolve in supercritical CO2 (even at ∼17 mM, ≤400 bar, temperature ≤ 75 °C, and W0 = 0-40), the other hybrid surfactants were able to yield transparent single-phase W/CO2 mixtures identified as microemulsions. The solubilizing power of FC6-HCm surfactants reached a maximum (W0 ∼ 80 at 45 °C and 350 bar) with a hydrocarbon length, m, of 4. The W0 value of 80 is the highest for a HC-FC hybrid surfactant, matching the highest value reported for a FC surfactant which contained more FC groups. High-pressure small-angle neutron scattering measurements from FCm-HCn/D2O/CO2 microemulsions were consistent with growth of the microemulsion droplets with increasing W0. In addition, not only spherical reversed micelles but also nonspherical assemblies (rodlike or ellipsoidal) were found for the systems with FC6-HCn (n = 4-6). At fixed surfactant concentration and W0 (17 mM and W0 = 20), the longest reversed micelles were obtained for FC6-HC6 where a mean aspect ratio of 6.3 was calculated for the aqueous cores.

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.

High pressure CO2-controlled reactors: enzymatic chiral resolution in emulsions

Chiral separation of ibuprofen catalyzed by enzyme conducted in CO₂-based micelle makes the reaction more effective and greener.

A liquid CO2-compatible hydrocarbon surfactant: experiment and modelling

Surfactants soluble in liquid CO2 are rare and knowledge on interfacial and self-assembly behaviour is fragmented. We found that polyoxyethylene (5) isooctylphenyl ether is interfacially active at the water-liquid CO2 interface. Water-liquid CO2 interfacial tension was measured at various surfactant concentrations at 50 bar and 283 K using the pendant drop method, and a CMC like cusp was observed at a surfactant concentration of ~50 mM in the bulk liquid CO2. This system was modelled applying the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We use a free-volume approach, wherein the chemical potential of the vacancies was linked to the pressure and the molecules were described using a freely-jointed chain model on a united atom level. The model indicates that typically the water-vapour interface is wet by CO2. Interestingly, a window of partial wetting was identified at the water-vapour interface as a function of the chemical potential of the surfactant. The second-order nature of both wetting transitions is attributed to the close proximity to the critical point of the CO2-vapour system. Furthermore, the SF-SCF theory was used to study the self-assembly of the surfactant in bulk CO2 or water, focusing on the three-phase coexistence, that is at P/Psat = 1. Above ~40 mM in the CO2-rich phase, the theory indicates stable water swollen reverse micelles with an aggregation number of ~100. The analysis further shows the stability of compressible CO2-swollen surfactant bilayers in the bulk water phase at elevated surfactant concentrations. Finally it was found that the critical reverse micellar concentration (in liquid CO2) increases and the aggregation number decreases with increasing pressure.

Nanostructures in water-in-CO2 microemulsions stabilized by double-chain fluorocarbon solubilizers

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

Equilibrium gold nanoclusters quenched with biodegradable polymers

Although sub-100 nm nanoclusters of metal nanoparticles are of interest in many fields including biomedical imaging, sensors, and catalysis, it has been challenging to control their morphologies and chemical properties. Herein, a new concept is presented to assemble equilibrium Au nanoclusters of controlled size by tuning the colloidal interactions with a polymeric stabilizer, PLA(1k)-b-PEG(10k)-b-PLA(1k). The nanoclusters form upon mixing a dispersion of ~5 nm Au nanospheres with a polymer solution followed by partial solvent evaporation. A weakly adsorbed polymer quenches the equilibrium nanocluster size and provides steric stabilization. Nanocluster size is tuned from ~20 to ~40 nm by experimentally varying the final Au nanoparticle concentration and the polymer/Au ratio, along with the charge on the initial Au nanoparticle surface. Upon biodegradation of the quencher, the nanoclusters reversibly and fully dissociate to individual ~5 nm primary particles. Equilibrium cluster size is predicted semiquantitatively with a free energy model that balances short-ranged depletion and van der Waals attractions with longer-ranged electrostatic repulsion, as a function of the Au and polymer concentrations. The close spacings of the Au nanoparticles in the clusters produce strong NIR extinction over a broad range of wavelengths from 650 to 900 nm, which is of practical interest in biomedical imaging.

Growth of textured thin Au coatings on iron oxide nanoparticles with near infrared absorbance

A homologous series of Au coated iron oxide nanoparticles with hydrodynamic diameters smaller than 60 nm was synthesized with very low Au-to-iron mass ratios, as low as 0.15. The hydrodynamic diameter was determined by dynamic light scattering and the composition by atomic absorption spectroscopy and energy dispersive x-ray spectroscopy. Unusually low Au precursor supersaturation levels were utilized to nucleate and grow Au coatings on iron oxide relative to the formation of pure Au nanoparticles. This approach produced unusually thin coatings by lowering autocatalytic growth of Au on Au, as shown by transmission electron microscopy. Nearly all of the nanoparticles were attracted by a magnet, indicating a minimal number of pure Au particles. The coatings were sufficiently thin to shift the surface plasmon resonance to the near infrared with large extinction coefficients, despite the small particle hydrodynamic diameters observed from dynamic light scattering to be less than 60 nm.

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.

Colloidal interactions in liquid CO2--a dry-cleaning perspective

Liquid CO(2) is a viable alternative for the toxic and environmentally harmful solvents traditionally used in dry-cleaning industry. Although liquid CO(2) dry-cleaning is being applied already at a commercial scale, it is still a relatively young technique which poses many challenges. The focus of this review is on the causes of the existing problems and directions to solve them. After presenting an overview of the state-of-the-art, we analyze the detergency challenges from the fundamentals of colloid and interface science. The properties of liquid CO(2) such as dielectric constant, density, Hamaker constant, refractive index, viscosity and surface tension are presented and in the subsequent chapters their effects on CO(2) dry-cleaning operation are delineated. We show, based on theory, that the van der Waals forces between a model soil (silica) and model fabric (cellulose) through liquid CO(2) are much stronger compared to those across water or the traditional dry-cleaning solvent PERC (perchloroethylene). Prevention of soil particle redeposition in liquid CO(2) by electrostatic stabilization is challenging and the possibility of using electrolytes having large anionic parts is discussed. Furthermore, the role of different additives used in dry-cleaning, such as water, alcohol and surfactants, is reviewed. Water is not only used as an aid to remove polar soils, but also enhances adhesion between fabric and soil by forming capillary bridges. Its role as a minor component in liquid CO(2) is complex as it depends on many factors, such as the chemical nature of fabrics and soil, and also on the state of water itself, whether present as molecular solution in liquid CO(2) or phase separated droplets. The phenomena of wicking and wetting in liquid CO(2) systems are predicted from the Washburn-Lucas equation for fabrics of various surface energies and pore sizes. It is shown that nearly complete wetting is desirable for good detergency. The effect of mechanical action and fluid dynamic conditions on dry-cleaning is analyzed theoretically. From this it follows that in liquid CO(2) an order of magnitude higher Reynold's number is required to exceed the binding forces between fabric and soil as opposed to PERC or water, mainly due to the strong van der Waals forces and the low viscosity of CO(2) at dry-cleaning operational conditions.

Antibody nanoparticle dispersions formed with mixtures of crowding molecules retain activity and in vivo bioavailability

Monoclonal antibodies continue to command a large market for treatment of a variety of diseases. In many cases, the doses required for therapeutic efficacy are large, limiting options for antibody delivery and administration. We report a novel formulation strategy based on dispersions of antibody nanoclusters that allows for subcutaneous injection of highly concentrated antibody (≈ 190 mg/mL). A solution of monoclonal antibody 1B7 was rapidly frozen and lyophilized using a novel spiral-wound in-situ freezing technology to generate amorphous particles. Upon gentle stirring, a translucent dispersion of approximately 430 nm protein clusters with low apparent viscosity (≈ 24 cp) formed rapidly in buffer containing the pharmaceutically acceptable crowding agents such as trehalose, polyethylene glycol, and n-methyl-2-pyrrolidone. Upon in vitro dilution of the dispersion, the nanoclusters rapidly reverted to monomeric protein with full activity, as monitored by dynamic light scattering and antigen binding. When administered to mice as an intravenous solution, subcutaneous solution, or subcutaneous dispersion at similar (4.6-7.3 mg/kg) or ultra-high dosages (51.6 mg/kg), the distribution and elimination kinetics were within error and the protein retained full activity. Overall, this method of generating high-concentration, low-viscosity dispersions of antibody nanoclusters could lead to improved administration and patient compliance, providing new opportunities for the biotechnology industry.

Stabilization of superparamagnetic iron oxide nanoclusters in concentrated brine with cross-linked polymer shells

Iron oxide nanoparticles, in the form of sub-100-nm clusters, were synthesized in the presence of poly(acrylic acid) (PAA) or poly(styrene sulfonate-alt-maleic acid) (PSS-alt-MA) to provide electrosteric stabilization. The superparamagnetic nanoclusters were characterized using a superconducting quantum interference device (SQUID), transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), and zeta potential measurements. To anchor the polymer shell on the nanoparticle surface, the polymer was cross-linked for a range of cross-linking densities. For nanoclusters with only 12% (w/w) PSS-alt-MA, electrosteric stabilization was sufficient even in 8 wt % NaCl. For PAA, the cross-linked polymer shell was essentially permanent and did not desorb even upon dilution of the nanoparticles for iron oxide concentrations down to 0.014 wt %. Without cross-linking, over half of the polymer desorbed from the particle surfaces. This general approach of the adsorption of polymer stabilizers onto nanoparticles followed by cross-linking may be utilized for a wide variety of cross-linkable polymers without the need to form covalent bonds between the nanoparticles and polymer stabilizer. Thus, this cross-linking approach is an efficient and inexpensive method of stabilizing nanoparticles for large-scale applications, including the electromagnetic imaging of subsurface reservoirs, even at high salinity.

Amphiphiles for supercritical CO2

A semi-fluorinated hybrid amphiphile, pentadecafluoro-5-dodecyl (F7H4) sulfate, has been shown to form reversed micelles in dense CO(2); the aggregates evolve to form water-in-CO(2) (w/c) microemulsion droplets on addition of water. Aggregation structures in these w/c phases have been characterised by small-angle neutron scattering (SANS), showing the presence of cylindrical droplets, which change into dispersed lamellar phases at even higher water loadings. Other systems are also introduced, being high internal phase emulsions (HIPEs) with brine, and liquid and supercritical CO(2), stabilized by certain commercially available nonylphenol ethoxylates (Dow Tergitol NP-, and Huntsman Surfonic N- amphiphiles). These dispersions have been characterised by SANS for the first time. Quantitative analyses of the HIPEs SANS profiles show that they behave similarly to hydrocarbon-water emulsion analogues, with regard to total interfacial areas and the effects of amphiphile concentration on the underlying structures. Finally, the advantages and disadvantages of both approaches for controlling the physico-chemical properties of liquid/supercritical CO(2) in potential applications are compared and contrasted. These results highlight the importance of using specially designed CO(2)-philic amphiphiles for generating self-assembly structures in dense CO(2).

Selective targeting of antibody conjugated multifunctional nanoclusters (nanoroses) to epidermal growth factor receptors in cancer cells

The ability of smaller than 100 nm antibody (Ab) nanoparticle conjugates to target and modulate the biology of specific cell types may enable major advancements in cellular imaging and therapy in cancer. A key challenge is to load a high degree of targeting, imaging, and therapeutic functionality into small, yet stable particles. A versatile method called thin autocatalytic growth on substrate (TAGs) has been developed in our previous study to form ultrathin and asymmetric gold coatings on iron oxide nanocluster cores producing exceptional near-infrared (NIR) absorbance. AlexaFluor 488 labeled Abs were used to correlate the number of Abs conjugated to iron oxide/gold nanoclusters (nanoroses) with the hydrodynamic size. A transition from submonolayer to multilayer aggregates of Abs on the nanorose surface was observed for 54 Abs and an overall particle diameter of ∼60-65 nm. The hydrodynamic diameter indicated coverage of a monolayer of 54 Abs, in agreement with the prediction of a geometric model, by assuming a circular footprint of 16.9 nm diameter per Ab molecule. The targeting efficacy of nanoclusters conjugated with monoclonal Abs specific for epidermal growth factor receptor (EGFR) was evaluated in A431 cancer cells using dark field microscopy and atomic absorbance spectrometry (AAS) analysis. Intense NIR scattering was achieved from both high uptake of nanoclusters in cells and high intrinsic NIR absorbance of individual nanoclusters. Dual mode imaging with dark field reflectance microscopy and fluorescence microscopy indicates the Abs remained attached to the Au surfaces upon the uptake by the cancer cells. The ability to load intense multifunctionality, specifically strong NIR absorbance, conjugation of an Ab monolayer in addition to a strong r2 MRI contrast that was previously demonstrated in a total particle size of only 63 nm, is an important step forward in development of theranostic agents for combined molecular specific imaging and therapy.

Phase equilibrium for surfactant Ls-54 in liquid CO(2) with water and solubility estimation using the Peng-Robinson equation of state

It is known that the commercial surfactant Dehypon® Ls-54 is soluble in supercritical CO(2) and that it enables formation of water-in-CO(2) microemulsions. In this work we observed phase equilibrium for the Ls-54/CO(2) and Ls-54/water/CO(2) systems in the liquid CO(2) region, from 278.15 - 298.15 K. In addition, the Peng-Robinson equation of state (PREOS) was used to model the phase behavior of Ls-54/CO(2) binary system as well as to estimate water solubilities in CO(2). Ls-54 in CO(2) can have solubilities as high as 0.086 M at 278.15 K and 15.2 MPa. The stability of the microemulsion decreases with increasing concentration of water, and lower temperatures favor increased solubility of water into the one-phase microemulsion. The PREOS model showed satisfactory agreement with the experimental data for both Ls-54/CO(2) and water/CO(2) systems.

CO2: a wild solvent, tamed

This article reviews approaches for modification of solvent properties of supercritical carbon dioxide (scCO(2)), with particular reference to self-assembly of oligomeric and polymeric solute additives. Of special interest are viscosity modifiers for scCO(2) based on molecular self-assembly. Background on polymers and surfactants with CO(2)-compatible functionalities is covered, leading on to the attempts made so far to increase the scCO(2) viscosity, which are described in detail. The significance of this field, and the implications a breakthrough could bring environmentally and economically will be addressed.