Molecular simulation of swelling and interlayer structure for organoclay in supercritical CO(2)

In this work, Monte Carlo simulations have been carried out to investigate the swelling stability and interlayer structures of alkylammonium-modified montmorillonite both in vacuum and in supercritical CO(2) (scCO(2)) fluid. In the vacuum (dry) condition, the stable spacing for this kind of organoclay was determined based on the energy minimum. In the stable spacing, the corresponding interlayer structure of dry organoclay is the monolayer arrangement with the intercalated surfactant chains lying parallel to the silicate surface. In scCO(2) fluid medium, the normal pressures within the organoclay gallery and the swelling free energy have been obtained from Gibbs ensemble Monte Carlo simulation. The mechanically and thermodynamically stable spacings of the organoclay have been determined. As compared with the case in vacuum, the simulation shows that the swelling of the organoclay is thermodynamically favorable in the environment of scCO(2) fluid. The interlayer structure and conformation have been used to analyze the mechanism of swelling. The headgroups of surfactant cations are distributed close to the clay surfaces. The presence of CO(2) molecules within the clay gallery can cause a specific steric arrangement of the long-chain alkylammonium cations.     

Olefin metathesis in supercritical carbon dioxide

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.     

Solvation structure and dynamics for passivated Au nanoparticle in supercritical CO2: a molecular dynamic simulation

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO(2)). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO(2) solvent shows a depletion region due to the preclusion of SAM. The presence of scCO(2) solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO(2) fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO(2)-based nanoparticle dispersion behavior.     

Some Organic Reactions in Supercritical Carbon Dioxide

Organic reactions in supercritical carbon dioxide (scCO2) have facilitated great progress in recent years . ScCO2, as an environmentally friendly reaction medium, may be a substitute for 1 volatile and toxic organic solvents and show some special advantages. Firstly, CO2 is inexpensive, nonflammable, nontoxic and chemical inert under many conditions. Secondly, scCO2 possesses hybrid properties of both liquid and gas, to the advantage of some reactions involving gaseous reagents. Control o…     

Interaction of cationic liposomes with cell membrane models

The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO(2) (scCO(2)). The potential of mean force (PMF) between two graphene nanosheets in scCO(2) was simulated, and the effect of scCO(2) density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO(2) fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO(2) molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO(2) fluid density, there are more confined CO(2) molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO(2) solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.     

Peracetylated sugar derivatives show high solubility in liquid and supercritical carbon dioxide

[structure: see text] Acetylated sugars derivatives exhibit high solubility in liquid and supercritical carbon dioxide (scCO(2)). Peracetylated sorbitol and beta-D-galactose are soluble under mild conditions in scCO(2), high pressures are required to dissolve peracetylated beta-cyclodextrin, and peracetoxyalkyl chains impart CO(2)-solubility to amides.     

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.     

Nucleophilic displacements in supercritical carbon dioxide under phase-transfer catalysis conditions. 2. Effect of pressure and kinetics

The nucleophilic displacement on n-octylmesylate (n-C(8)H(17)OSO(2)CH(3), 1) with four different anions (I(-), Br(-), N(3)(-), and SCN(-)) is investigated under liquid-supercritical phase-transfer catalysis (LSc-PTC) conditions, i.e. in a biphase system of supercritical carbon dioxide (scCO(2)) and water, in the presence of both silica supported and conventional onium salts. The CO(2) pressure greatly affects the concentration of 1 in the sc-phase and plays a major role on its conversion. For example, at 50 degrees C and with a supported PT-catalyst, the conversion of 1 into n-octyl iodide drops by a factor of 5 as the CO(2) pressure is increased from 80 to 150 bar, while in the same pressure range, the solubility of n-octylmesylate in scCO(2) shows a 6-fold increase, indicating that the reagent is desorbed from the catalyst. Under LSc-PTC conditions, pseudo-first-order kinetic rate constants, evaluated for the investigated reactions, show that the performance of scCO(2) as a PTC solvent and the relative nucleophilicity order of the anions (N(3)(-) > I(-) > or = Br(-) > SCN(-)) are comparable to those of toluene and n-heptane. The behavior of conventional phosphonium salts in the scCO(2)/H(2)O biphase system suggests that the reaction may take place either within small droplets of PT-catalyst containing water or in a separate third liquid phase of the PT-catalyst itself.     

Phase behavior in model homopolymer/CO2 and surfactant/CO2 systems: discontinuous molecular dynamics simulations

Discontinuous molecular dynamics simulations are performed on surfactant (HmTn)/solvent systems modeled as a mixture of single-sphere solvent molecules and freely jointed surfactant chains composed of m slightly solvent-philic head spheres (H) and n solvent-philic tail spheres (T), all of the same size. We use a square-well potential to account for the head-head, head-solvent, tail-tail, and tail-solvent interactions and a hard-sphere potential for the head-tail and solvent-solvent interactions. We first simulate homopolymer/supercritical CO2 (scCO2) systems to establish the appropriate interaction parameters for a surfactant/scCO2 system. Next, we simulate surfactant/scCO2 systems and explore the effect of the surfactant volume fraction, packing fraction, and temperature on the phase behavior. The transition from the two-phase region to the one-phase region is located by monitoring the contrast structure factor of the equilibrated surfactant/scCO2 system, and the micelle to unimer transition is located by monitoring the aggregate size distribution of the equilibrated surfactant/scCO2 system. We find a two-phase region, a micelle phase, and a unimer phase with increasing packing fraction at fixed temperature or with increasing temperature at fixed packing fraction. The phase diagram for the surfactant/scCO2 system in the surfactant volume fraction-packing fraction plane and the density dependence of the critical micelle concentration are in qualitative agreement with experimental observations. The phase behavior of a surfactant/scCO2 system can be directly related to the solubilities of the corresponding homopolymers that serve as the head and tail blocks for the surfactant. The influence of surfactant structure (head and tail lengths) on the phase transitions is explored.     

Electrochemical synthesis of a polypyrrole thin film with supercritical carbon dioxide as a solvent

A conductive polypyrrole (PPy) film was successfully synthesized in a homogeneous supercritical carbon dioxide (scCO2)/acetonitrile (AN) system. The occurrence of a homogeneous supercritical state was confirmed by observations of the phase behavior of the system through a high-pressure cell with a viewing window. The concentration of a supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), significantly changed the phase behavior of the scCO2/AN system. The polymerization rate of the film in that system decreased with further addition of CO2. This result suggested that the low viscosity of scCO2 did not play an important role in improving the growth rate of the PPy film. The low polymerization rate might have been due to the electron-transfer resistance arising from the low dielectric constant of scCO2/AN mixture. The roughness of the film prepared in the homogeneous scCO2/AN system was 1/10 that synthesized in AN itself as a solvent. The slow growth of film and the high diffusion rate of the monomer seemed to account for the smooth flat film formation.     

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.     

Deposition of small organic molecules by the displacement of two immiscible supercritical phases

A new coating process is described (deposition from two immiscible supercritical phases, or DISP) in which a solution of supercritical carbon dioxide (scCO2) with a desired solute is displaced by supercritical helium (scHe). After depressurization, the solute is deposited on substrates initially submerged in the coating solvent. Micron-sized particles and thin films of sucrose octaacetate (SOA) were formed on silicon wafer substrate coupons from DISP at relatively low temperatures and pressures (< or = 6500 psi and < or = 60 degrees C). The particle size, film thickness, and morphology of SOA were characterized as a function of coating conditions-solution concentrations, withdrawal velocities, and pressures. Particles in the range of 1-14 microm in diameter were deposited at low solute concentrations (< or = 0.2 wt % at 4500 psi), whereas films in the range of 0.1-0.5 microm in thickness were deposited at higher solute concentrations (> or = 1.5 wt % at 4500 psi). Particle sizes decreased with increasing displacement velocity and increasing pressure. Estimates of characteristic times for diffusion and nucleation indicate that DISP is a diffusion-limited process. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology, including defect formations and film roughness. Highly uniform films with low root-mean-square (RMS) roughness (approximately 10 angstroms) were obtained at a low displacement velocity of 0.0035 cm/s, while ring-like defect structures were observed in films deposited at a higher displacement velocity of 0.035 cm/s. The film thickness and morphology of the films deposited from DISP were compared with films from normal dip coating with typical organic solvents (acetone and toluene). Films deposited from scCO2 by DISP were much thicker, more uniform, and exhibited much fewer drying defects and lower RMS roughness compared with films from the organic solvents.     

Selective hydrogenation of phenol in supercritical carbon dioxide

Liquid phase hydrogenation of phenol over Pt/C catalysts was investigated under conventional conditions and supercritical carbon dioxide (scCO2). The equivalent ration of hydrogen to phenol shows a significant effect on the product selectivity. Hydrogenation of phenol in different solvents was also studied, the experimental results show that polarity of solvents influences the yield of cyclohexanone remarkably, scCO2 has the highest one. Catalytic hydrogenation of phenol in scCO2 or sub-scCO2 was emphatically researched. The result is that near the critical point of CO2 phenol has higher reaction activity than that of normal organic solvents, cyclohexanone has 47% in yield and 87% in selectivity.     

Ab initio molecular-dynamics study of supercritical carbon dioxide

Car-Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO(2)) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO(2) molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO(2) have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO(2). The results have also been compared to classical molecular-dynamics calculations employing an empirical potential.     

Ultrathin film deposition by liquid CO2 free meniscus coating-uniformity and morphology

Ultrathin organic films of sucrose octaacetate (SOA) were deposited on 12.5 cm diameter silicon wafer substrates using high-pressure free meniscus coating (hFMC) with liquid CO2 (l-CO2) as a coating solvent. The dry film thickness across the wafer and the morphology of deposited films were characterized as a function of coating conditions-withdrawal velocity, solution concentration, and evaporation driving force (deltaP). When no evaporation driving force was applied (deltaP = 0), highly uniform films were deposited with thickness in the range of 8-105 angstroms over the entire concentration range (3-11 wt%). Uniform films were also obtained at low concentrations (3-5 wt%) with a low evaporation driving force (deltaP = 0.0138 MPa). However, films deposited at medium to high concentrations (7-11 wt%) were thicker (110-570 angstroms) and less uniform, with larger nonuniformities at higher applied evaporation driving forces. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology including drying defects and film roughness. Films deposited without evaporation had no apparent drying defects and very low root-mean-square (RMS) roughness (1.4-3.8 angstroms). Spinodal-like dewetting morphologies including holes with diameters in the range of 100-300 nm, and surface undulations were observed in films deposited at medium concentration (7 wt%) and low deltaP (0.0138-0.0276 MPa). At higher concentrations and higher evaporative driving forces, spinodal-like dewetting morphologies disappeared but concentric ring defect structures were observed with diameters in the range 20-125 microm. The film thickness and morphology of SOA films deposited from 1-CO2 hFMC were compared to those deposited from toluene and acetone under normal dip coating. Films deposited from l-CO2 hFMC were much thinner, more uniform, and exhibited much fewer drying defects and lower RMS roughness.     

Kinetico-mechanistic studies of C-H bond activation on new Pd complexes containing N,N'-chelating ligands

The hybrid imine/amine palladium(II) coordination complexes [PdX2(kappa2-N(imino),N(amino))](X = Cl, AcO; kappa2-N(imino),N(amino)= 4ClC6H4CHNCH2(CH2)nN(CH3)2, n= 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the C[double bond]N bond of the hybrid ligand and {Pd(kappa(2)-N(imino),N(amino))} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(kappa2-N(imino),N(amino))} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(kappa2-C,N(imino))} palladacycle are obtained, as characterized by 1H NMR spectroscopy. In these compounds the total opening of the N(amino) moiety of the ligand has occurred. The C-H bond activation process has been studied kinetico-mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX2(kappa2-N(imino),N(amino))] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N(imino)-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.     

In situ molecular spectroscopic evidence for CO2 intercalation into montmorillonite in supercritical carbon dioxide

The interaction of anhydrous supercritical CO(2) (scCO(2)) with both kaolinite and ~1W (i.e., close to but less than one layer of hydration) calcium-saturated montmorillonite was investigated under conditions relevant to geologic carbon sequestration (50 °C and 90 bar). The CO(2) molecular environment was probed in situ using a combination of three novel high-pressure techniques: X-ray diffraction, magic angle spinning nuclear magnetic resonance spectroscopy, and attenuated total reflection infrared spectroscopy. We report the first direct evidence that the expansion of montmorillonite under scCO(2) conditions is due to CO(2) migration into the interlayer. Intercalated CO(2) molecules are rotationally constrained and do not appear to react with waters to form bicarbonate or carbonic acid. In contrast, CO(2) does not intercalate into kaolinite. The findings show that predicting the seal integrity of caprock will have complex dependence on clay mineralogy and hydration state.     

Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene ionic liquids and catalysis for hydrogenation

In this work we synthesized Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene (NHC) ionic liquids (ILs) with different alkyl chain lengths. The catalysts were characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), low-angle X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen sorption. The catalysts were used for the hydrogenation of alkenes and allyl alcohol. The results indicated that the catalysts were very active, selective, and stable. The selectivity for the hydrogenation of allyl alcohol to 1-propanol increased with the increase of the alkyl chain length of the ILs. The effect of supercritical CO(2) (scCO(2)) on the hydrogenation of allyl alcohol was also studied, and it was demonstrated that scCO(2) could enhance the selectivity of the reaction considerably. The XPS study showed that the valence of Pd(II) remained unchanged under hydrogenation conditions.     

Complexation of phosphine ligands with peracetylated beta-cyclodextrin in supercritical carbon dioxide: spectroscopic determination of equilibrium constants

The interaction between peracetylated beta-cyclodextrin and several triphenyl phosphine derivatives was studied in supercritical carbon dioxide (scCO2) by UV-visible spectroscopy. The equilibrium constant for a 1:1 complexation reaction was obtained from titration spectra and calculated using two established mathematical models. The values of the equilibrium constants are 1-3 orders of magnitude smaller than those obtained in aqueous solution with analogous phosphines. This is likely due to the absence in scCO2 of the hydrophobic effect, which is replaced by a corresponding, but weaker, CO2-phobic effect. The largest value of Kf was found for complexes of diphenyl(4-adamantylphenyl)phosphine, which is rationalized on the basis of the excellent fit of the phosphine in the cyclodextrin cavity, leading to enhanced host-guest van der Waals interactions. This study can be considered the first step toward the comprehension of the complexation thermodynamics of modified cyclodextrins soluble in scCO2.