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Recent experimental and theoretical results have suggested that organic acids such as pyruvic acid, can be photolyzed in the ground electronic state by the excitation of the OH stretch vibrational overtone. These overtones absorb in the near-infrared and visible regions of the spectrum where the solar photons are plentiful and could provide a reaction pathway for the organic acids and alcohols that are abundant in the earth's atmosphere. In this paper the overtone initiated photochemistry of aqueous pyruvic acid is investigated by monitoring the evolution of carbon dioxide. In these experiments CO(2) is being produced by excitation in the near-infrared, between 850 nm and ∼1150 nm (11,765-8696 cm(-1)), where the second OH vibrational overtone (Δν = 3) of pyruvic acid is expected to absorb. These findings show not only that the overtone initiated photochemical decarboxylation reaction occurs but also that in the aqueous phase it occurs at a lower energy than was predicted for the overtone initiated reaction of pyruvic acid in the gas phase (13,380 cm(-1)). A quantum yield of (3.5 ± 1.0) × 10(-4) is estimated, suggesting that although this process does occur, it does so with a very low efficiency.

Nitrosamines are a class of carcinogenic, mutagenic, and teratogenic compounds generally produced from the nitrosation of amine. This paper investigates the mechanism for the formation of nitrosodimethylamine (NDMA) from the nitrosation of dimethylamine (DMA) by four common nitrosating agents (NO(2)(-), ONOO(-), N(2)O(3), and ONCl) in the absence and presence of CO(2) using the DFT method. New insights are provided into the mechanism, emphasizing that the interactions of CO(2) with amine and nitrosating agents are both potentially important in influencing the role of CO(2) (catalyst or inhibitor). The role of CO(2) as catalyst or inhibitor mainly depends on the nitrosating agents involved. That is, CO(2) shows the catalytic effect when the weak nitrosating agent NO(2)(-) or ONOO(-) is involved, whereas it is an inhibitor in the nitrosation induced by the strong nitrosating agent N(2)O(3) or ONCl. To conclude, CO(2) serves as a "double-edged sword" in the nitrosation of amine. The findings will be helpful to expand our understanding of the pathophysiological and environmental significance of CO(2) and to develop efficient methods to prevent the formation of carcinogenic nitrosamines.

The development of standard extreme ultraviolet (EUV) lithography photoresists with a CO2 compatible salt (CCS) and supercritical carbon dioxide (scCO2) solution has several advantages over typical trimethylammonium hydroxide development, including reduced image collapse and line width roughness in the resulting microchip features. The mechanism and characteristics of the CCS/scCO2 development process are currently being examined. In this paper, the sorption behavior of CO2 and the CCS onto a bare SiO2 surface and into the photoresist was studied using a quartz crystal microbalance (QCM). From the adsorption studies of CO2 and CCS/CO2 onto a bare SiO2 surface, it was found that the CCS begins to adsorb at 8.0 MPa at a temperature of 35 degrees C and at 9.4 MPa at a temperature of 50 degrees C. The adsorption of the CCS was favored and driven by entropy changes. The absorption of CO2 into the glassy photoresist resin was also measured with QCM and found comparable to CO2 absorption in glassy polystyrene for 35 and 50 degrees C up to a pressure where the photoresist is believed to dewet from the substrate. The diffusion behavior during CO2 absorption was found to be comparable to that of small fluorescent molecule diffusion in a CO2 swollen polystyrene.

Flue gas is greatly responsible for acid rain formation and global warming. New generation ionic liquids (ILs) have potential in controlling the flue gas emissions, as they acquire high absorptivity for the component gases SO(2), CO(2), etc. The association of the IL-gas interactions to the absorptivity of gas molecules in ILs is, however, poorly understood. In this paper, we present a molecular level description of the interactions of ILs with SO(2), CO(2), and N(2) and show its implications to the differential gas solubility. Our results indicate that the IL anion-gas interactions play a key role in deciding the gas solubility in ILs, particularly for polar gases such as SO(2). On the other hand, regular solution assumption applies to N(2) solubility. In accordance with the previous theoretical and experimental findings, our results also imply that the IL anions dominate the interactions with gas molecules while the cations play a secondary role and the underlying fluid structures of the ILs remain unperturbed by the addition of gas molecules.

The surface tension of polymers in a supercritical fluid is one of the most important physicochemical parameters in many engineering processes, such as microcellular foaming where the surface tension between a polymer melt and a fluid is a principal factor in determining cell nucleation and growth. This paper presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendent drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and good qualitative agreement with experiment is obtained. The physical mechanisms for three main experimental trends are explained by using SCFT, and none of the explanations quantitatively depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the slope of surface tension change with temperature is dependent on pressure.

In this paper, we have calculated using the ab initio method the IR vibrational spectra of complexes of CO2 formed with water (sp3 O-donating atom). Binding energies and structures of the CO2-H2O and water-(CO2)2 complexes have been determined at the second-order level of the Moller-Plesset perturbation theory (MP2) using Dunning's basis sets. The results are presented and critically discussed in terms of the nature of the water-CO2 interactions, electron donor acceptor (EDA) and weak O...H-O interactions. For water-(CO2)2 trimer, it is also shown that the contribution to the interaction energy of the irreducible three-bodies remains relatively negligible. We have analyzed the frequency shifts and the IR and Raman intensity variations under the complex formation. We have particularly emphasized the splitting of the 2 bending mode of CO2 and stretching modes of water, which have been revealed as the most pertinent probes to assess the nature of the forces involved in the different complexes. Finally, because water can play the role of Lewis base and acid as well, we found that weak O...H-O interactions can cooperate with EDA interactions in trimer, leading to very specific spectral signatures that are further discussed.

In this paper we explore the effects of water dissociation and CO2contamination on the electrical conductivity of salt-free concentrated suspensions in static electric fields. The conductivity model here presented is based on a new description of the equilibrium double layer for particles in “realistic” salt-free suspensions recently developed by the authors to account for the latter effects (Ruiz-Reina, E.; Carrique, F. J. Phys. Chem. B2008, 112, 11960). It was shown that in most of the cases the neglecting of those effects would lead to a very poor description of common salt-free suspensions, especially, but not only, if the suspensions have been in contact with air. As shown in this paper, the presence of only water dissociation ions suffices to provoke very important changes in the standard salt-free predictions. A realistic aqueous salt-free suspension consists of an aqueous suspension without any electrolyte added during the preparation but including the following ionic species: (i) the “added counterions” stemming from the particle charging process that counterbalance their surface charge (with just this ionic species, the suspension can be considered as an ideal or pure salt-free one), (ii) the H+and OH−ions from water dissociation, and (iii) the ions produced by the atmospheric CO2contamination. The model is based on the classical Poisson−Boltzmann theory, the appropriate local chemical reactions, the standard electrokinetic equations, and the cell model approximation to account for electro-hydrodynamic particle−particle interactions. Thus, we have studied the electrical conductivity of such realistic salt-free suspensions for different particle volume fraction ϕ and surface charge density σ, and compared it with results of pure salt-free conductivity predictions. The numerical results have shown that water dissociation ions and/or CO2contamination has an extreme influence on the suspension conductivity values at low−moderate particle volume fractions. In these situations the role of the added counterions is screened by the other ionic species. Even if the suspensions have not been exposed to the atmosphere, the quantitative changes in conductivity at low volume fractions associated with the presence of water dissociation ions over the added counterions are enormous. It is concluded that it is necessary to take into account the water dissociation influence for ϕ lower than approximately 10−2−10−3, whereas the atmospheric CO2contamination is not negligible if ϕ < 10−1−10−2, depending on the particle charge. The present work sets the basis for further theoretical models concerning, particularly, the dynamic electrophoresis and dielectric response of such systems. [ABSTRACT FROM AUTHOR]

The possibility of symmetry breaking of the fluid (argon) density distribution across a long closed slit with identical walls composed of solid carbon dioxide was noted in previous papers by the authors. The main conclusion was that there is a range of average densities in which symmetry breaking occurs and that outside that range the fluid density profile is symmetrical. A critical temperature Tsbwas also identified below which symmetry breaking can occur. In this paper, symmetry breaking is examined for walls made of other materials and it is shown that it occurs only when the energy parameter fwof the fluid−wall interaction in the Lennard-Jones potential satisfies the inequalities fw1≤ fw≤ fw2, where fw1and fw2are temperature-dependent critical values of fw. The value of fw1increases and that of fw2decreases with increasing temperature. The comparison of the theory with Monte Carlo simulations confirms the existence of symmetry breaking across the slit. The possibility of symmetry breaking along the slit is also noted. [ABSTRACT FROM AUTHOR]

In this paper, a general electrokinetic theory for concentrated suspensions in salt-free media is derived. Our model predicts the electrical conductivity and the electrophoretic mobility of spherical particles in salt-free suspensions for arbitrary conditions regarding particle charge, volume fraction, counterion properties, and overlapping of double layers of adjacent particles. For brevity, hydrolysis effects and parasitic effects from dissolved carbon dioxide, which are present to some extent in more “realistic” salt-free suspensions, will not be addressed in this paper. These issues will be analyzed in a forthcoming extension. However, previous models are revised, and different sets of boundary conditions, frequently found in the literature, are extensively analyzed. Our results confirm the so-called counterion condensation effect and clearly display its influence on electrokinetic properties such as electrical conductivity and electrophoretic mobility for different theoretical conditions. We show that the electrophoretic mobility increases as particle charge increases for a given particle volume fraction until the charge region where counterion condensation takes place is attained, for the above-mentioned sets of boundary conditions. However, it decreases as particle volume fraction increases for a given particle charge. Instead, the electrical conductivity always increases with either particle charge for fixed particle volume fraction or volume fraction for fixed particle charge, whatever the set of boundary conditions previously referred. In addition, the influence of the electric permittivity of the particles on their electrokinetic properties in salt-free media is examined for those frames of boundary conditions. [ABSTRACT FROM AUTHOR]

The adjustable solvent properties, vanishingly low surface tensions, and environmentally green characteristics of supercritical carbon dioxide present certain advantages in nanoparticles synthesis and processing. Unfortunately, most current techniques employed to synthesize and disperse nanoparticles in carbon dioxide use environmentally persistent fluorinated compounds as metal precursors and/or stabilizing ligands. This paper illustrates a one-step process for synthesis and stabilization of silver nanoparticles in carbon dioxide using only fluorine-free compounds. Isostearic acid coated silver nanoaparticles were formed and stably dispersed through arrested precipitation. Silver bis(3,5,5-trimethyl-1-hexyl)sulfosuccinate (Ag-AOT-TMH) was reduced in the presence of isostearic acid as a capping ligand in carbon dioxide solvent to form silver nanoparticles. The addition of cyclohexane as cosolvent or an increase in carbon dioxide solvent density enhances the dispersibility of the particles due to an increase in solvent strength. The dispersibility of the isostearic acid capped silver nanoparticles diminished with time until a stable dispersion was achieved due to the precipitation of a fraction of particle sizes too large to be stabilized by the solvent medium, thereby leaving a smaller size fraction of nanoparticles stably dispersed in the CO2mixtures. This paper presents the one-step synthesis and stabilization of metallic nanoparticles in neat carbon dioxide without the aid of any fluorinated compounds. [ABSTRACT FROM AUTHOR]

In CO2 geological storage, the interfacial tension (IFT) between supercritical CO2 and brine is critical for the storage capacitance design to prevent CO2 leakage. IFT relies not only on the interfacial molecule properties but also on the environmental conditions at different storage sites. In this paper, supercritical CO2-NaCl solution systems are modeled at 343-373 K and 6-35 MPa under the salinity of 1.89 mol/L using molecular dynamics simulations. After computing and comparing the molecular density profile across the interface, the atomic radial distribution function, the molecular orientation distribution, the molecular Gibbs surface excess (derived from the molecular density profile), and the CO2-hydrate number density under the above environmental conditions, we confirm that only the molecular Gibbs surface excess of CO2 molecules and the CO2-hydrate number density correlate strongly with the temperature- and pressure-dependent IFTs. We also compute the populations of two distinct CO2-hydrate structures (T-type and H-type) and attribute the observed dependence of IFTs to the dominance of the more stable, surfactant-like T-type CO2-hydrates at the interface. On the basis of these new molecular mechanisms behind IFT variations, this study could guide the rational design of suitable injecting environmental pressure and temperature conditions. We believe that the above two molecular-level metrics (Gibbs surface excess and hydrate number density) are of great fundamental importance for understanding the supercritical CO2-water interface and engineering applications in geological CO2 storage. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (scCO2) on its own can be a relatively poor solvent. However, the addition at relatively modest concentration of "entrainers", simple solvent molecules such as ethanol or acetone, can provide a significant boost in solubility, thereby enabling its industrial use. However, how entrainers work is still under debate; without an unambiguous explanation, it is hard to optimize entrainers for any specific solute. This paper demonstrates that a fundamental, assumption-free statistical thermodynamic theory, the Kirkwood-Buff (KB) theory, can provide an unambiguous explanation of the entrainer effect through an analysis of published experimental data. The KB theory shows that a strong solute-entrainer interaction accounts for the solubility enhancement, while CO2 density increase and/or CO2-entrainer interactions, which have been assumed widely in the literature, do not account for solubilization. This conclusion, despite the limited completeness of available data, is demonstrably robust; this can be shown by an order-of-magnitude analysis based upon the theory, and can be demonstrated directly through a public-domain "app", which has been developed to implement the theory. [ABSTRACT FROM AUTHOR]

In this paper, we have reported theCO2solubility indifferent pure alkyl carbonate solvents (EC, DMC, EMC, DEC) and theirbinary mixtures as EC/DMC, EC/EMC, and EC/DEC and for electrolytes[solvent + lithium salt] LiX (X = LiPF6, LiTFSI, or LiFAP)as a function of the temperature and salt concentration. To understandthe parameters that influence the structure of the solvents and theirability to dissolve CO2, through the addition of a salt,we first analyzed the viscosities of EC/DMC + LiX mixtures by meansof a modified Jones–Dole equation. The results were discussedconsidering the order or disorder introduced by the salt into thesolvent organization and ion solvation sphere by calculating the effectivesolute ion radius, rs. On the basis ofthese results, the analysis of the CO2solubility variationswith the salt addition was then evaluated and discussed by determiningspecific ion parameters Hiby using theSetchenov coefficients in solution. This study showed that the CO2solubility has been affected by the shape, charge density,and size of the ions, which influence the structuring of the solventsthrough the addition of a salt and the type of solvation of the ions. [ABSTRACT FROM AUTHOR]

Supportedionic liquid membranes (SILMs) are promising tools for the separationof carbon dioxide from other gases. In this paper, new imidazolium,pyrrolidinium, piperidinium, and morpholinium ionic liquids with atriethylene glycol side chain and tosylate anions, as well as theirsymmetrical dicationic analogues, have been synthesized and incorporatedinto SILMs. The selectivities for CO2/N2andCO2/CH4separations have been measured. Theselectivities exhibited by the dicationic ionic liquids are up totwo times higher than the values of the corresponding monocationicionic liquids. Quantum chemical calculations have been used to investigatethe difference in the interaction of carbon dioxide with monocationicand dicationic ionic liquids. The reason for the increased gas separationselectivity of the dicationic ionic liquids is two-fold: (1) a decreasein permeance of nitrogen and methane through the ionic liquid layer,presumably due to their less favorable interactions with the gases,while the permeance of carbon dioxide is reduced much less; (2) anincrease in the number of interaction sites for the interactions withthe quadrupolar carbon dioxide molecules in the dicationic ionic liquids,compared to the monocationic analogues. [ABSTRACT FROM AUTHOR]

This paper presents experimentaland modeling results on water–CO2interfacial tension(IFT) together with wettability studiesof water on both hydrophilic and hydrophobic surfaces immersed inCO2. CO2–water interfacial tension (IFT)measurements showed that the IFT decreased with increasing pressureand the negative slopes of IFT–pressure isotherms decreasedwith increasing temperature. Water contact angle on a cellulose surface(hydrophilic) immersed in CO2increased with pressure,whereas the water contact angle on a hydrophobic surface such as hexamethyldisilazane (HMDS) coated silicon surface was almost independent ofpressure. These experimental findings were augmented by modeling usingthe self-consistent field theory. The theory applies the lattice discretizationscheme of Scheutjens and Fleer, with a discretization length closeto the size of the molecules. In line with this we have implementeda primitive molecular model, with just small variations in the molarvolume. The theory makes use of the Bragg-Williams approximation andhas binary Flory–Huggins interaction parameters (FH) betweenCO2, water, and free volume. Using this model, we generatedthe complete IFT–pressure isotherms at various temperatures,which coincided well with the trends reported in literature, thatis, the water–CO2interfacial tension decreasedwith increasing pressure for pressures ≤100 bar and becameindependent of pressure >100 bar. The transition point occurredathigher pressures with increasing temperature. At three-phase coexistence(water–CO2–free volume) and at the water–vaporinterface (water–free volume), we always found the CO2phase in between the water-rich and free volume-rich phases. Thismeans that for the conditions studied, the water–vapor interfaceis always wet by CO2and there are no signs of a nearbywetting transition. Calculation of the water contact angle on a solidsurface was based on the computed adsorption isotherms of water froma vapor or from a pressurized CO2-rich phase and analysisof surface pressures at water–vapor or water–CO2coexistence. The results matched reasonably well with theexperimental contact angle data. Besides, we also computed the volumefraction profiles of the CO2, H2O, and the Vphase, from which the preferential adsorption of CO2near the hydrophilic surface was deduced. [ABSTRACT FROM AUTHOR]

In the first paper of this series[C. Avendaño, T. Lafitte,A. Galindo, C. S. Adjiman, G. Jackson, and E. A. Müller, J. Phys. Chem. B2011, 115, 11154] we introduced the SAFT-γ force field for molecularsimulation of fluids. In our approach, a molecular-based equationof state (EoS) is used to obtain coarse-grained (CG) intermolecularpotentials that can then be employed in molecular simulation overa wide range of thermodynamic conditions of the fluid. The macroscopicexperimental data for the vapor–liquid equilibria (saturatedliquid density and vapor pressure) of a given system are representedwith the SAFT-VR Mie EoS and used to estimate effective intermolecularparameters that provide a good description of the thermodynamic propertiesby exploring a wide parameter space for models based on the Mie (generalizedLennard-Jones) potential. This methodology was first used to developa simple single-segment CG Mie model of carbon dioxide (CO2) which allows for a reliable representation of the fluid-phase equilibria(for which the model was parametrized), as well as an accurate predictionof other properties such as the enthalpy of vaporization, interfacialtension, supercritical density, and second-derivative thermodynamicproperties (thermal expansivity, isothermal compressibility, heatcapacity, Joule-Thomson coefficient, and speed of sound). In our currentpaper, the methodology is further applied and extended to developeffective SAFT-γ CG Mie force fields for some important greenhousegases including carbon tetrafluoride (CF4) and sulfur hexafluoride(SF6), modeled as simple spherical molecules, and for longlinear alkanes including n-decane (n-C10H22) and n-eicosane (n-C20H42), modeled as homonuclearchains of spherical Mie segments. We also apply the SAFT-γ methodologyto obtain a CG homonuclear two-segment Mie intermolecular potentialfor the more challenging polar and asymmetric compound 2,3,3,3-tetrafluoro-1-propene(HFO-1234yf), a novel replacement refrigerant with promising properties.The description of the fluid-phase behavior and the prediction ofthe other thermophysical properties obtained by molecular simulationusing our SAFT-γ CG Mie force fields are found to be of comparablequality (and sometimes superior) to that obtained using the more sophisticatedall-atom (AA) and united-atom (UA) models commonly employed in thefield. We should emphasize that though the focus of our current workis on simple homonuclear models, the SAFT-γ methodology is basedon a group contribution methodology which is naturally suited to thedevelopment of more sophisticated heteronuclear models. [ABSTRACT FROM AUTHOR]

Nitrosamines are a class of carcinogenic, mutagenic, and teratogenic compounds generally produced from the nitrosation of amine. This paper investigates the mechanism for the formation of nitrosodimethylamine (NDMA) from the nitrosation of dimethylamine (DMA) by four common nitrosating agents (NCl) in the absence and presence of CO2 using the DFT method. New insights are provided into the mechanism, emphasizing that the interactions of CO2 with amine and nitrosating agents are both potentially important in influencing the role of CO2 (catalyst or inhibitor). The role of CO2 as catalyst or inhibitor mainly depends on the nitrosating agents involved. That is, CO2 shows the catalytic effect when the weak nitrosating agent Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. is involved, whereas it is an inhibitor in the nitrosation induced by the strong nitrosating agent N2O3 or ONCl. To conclude, CO2 serves as a "double-edged sword" in the nitrosation of amine. The findings will be helpful to expand our understanding of the pathophysiological and environmental significance of CO2 and to develop efficient methods to prevent the formation of carcinogenic nitrosamines. [ABSTRACT FROM AUTHOR]

In a very recent paper (Ruiz-Reina, E.; Carrique, F.J. Phys. Chem. B2008, 112, 11960.) we studied the effect of water dissociation and CO2contamination on the equilibrium electrical double layer of spherical particles in salt-free concentrated suspensions in aqueous solutions. It was shown that in most cases (dilute to moderately concentrated suspensions), the neglecting of those effects would lead to a very poor description of common salt-free suspensions, especially if the suspensions have been in contact with air. In the present contribution we explore the influence of the latter effects on the dc electrophoresis in realistic salt-free suspensions. This kind of system consists of aqueous suspensions without any electrolyte added during the preparation. The ionic species in solution can solely be (i) the “added counterions” stemming from the particles that counterbalance their surface charge, (ii) the H+and OH−ions from water dissociation, and (iii) the ions produced by the atmospheric CO2contamination. Our model follows the classical Poisson−Boltzmann approach, a spherical cell model and the appropriate local chemical reactions. We have applied it to the study of the electrophoretic mobility of a spherical particle for different particle volume fractions ϕ and surface charge densities. The numerical results have shown the quite large influence that water dissociation ions and/or CO2contamination have on the electrophoretic mobility at low-moderate particle volume fractions. In those situations the role of the added counterions is screened by the other ionic species. These effects yield the mobility to reach plateau values instead of further increasing as volume fraction decreases. It is concluded that it is necessary to take into account the water dissociation influence for ϕ lower than approximately 10−2, whereas the atmospheric contamination, if the suspensions have been exposed to the atmosphere, is not negligible if ϕ < 10−1. The present work sets the basis for further theoretical models concerning particularly the ac electrokinetics and dielectric response of such systems. [ABSTRACT FROM AUTHOR]

The surface tension of polymers in a supercritical fluid is one of the most important physicochemical parameters in many engineering processes, such as microcellular foaming where the surface tension between a polymer melt and a fluid is a principal factor in determining cell nucleation and growth. This paper presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendent drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and good qualitative agreement with experiment is obtained. The physical mechanisms for three main experimental trends are explained by using SCFT, and none of the explanations quantitatively depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the slope of surface tension change with temperature is dependent on pressure. [ABSTRACT FROM AUTHOR]

This paper presents an environmentally friendly, inexpensive, rapid, and efficient process for size-selective fractionation of polydisperse metal nanoparticle dispersions into multiple narrow size populations. The dispersibility of ligand-stabilized silver and gold nanoparticles is controlled by altering the ligand tails−solvent interaction (solvation) by the addition of carbon dioxide (CO2) gas as an antisolvent, thereby tailoring the bulk solvent strength. This is accomplished by adjusting the CO2 pressure over the liquid, resulting in a simple means to tune the nanoparticle precipitation by size. This study also details the influence of various factors on the size-separation process, such as the types of metal, ligand, and solvent, as well as the use of recursive fractionation and the time allowed for settling during each fractionation step. The pressure range required for the precipitation process is the same for both the silver and gold particles capped with dodecanethiol ligands. A change in ligand or solvent length has an effect on the interaction between the solvent and the ligand tails and therefore the pressure range required for precipitation. Stronger interactions between solvent and ligand tails require greater CO2 pressure to precipitate the particles. Temperature is another variable that impacts the dispersibility of the nanoparticles through changes in the density and the mole fraction of CO2 in the gas-expanded liquids. Recursive fractionation for a given system within a particular pressure range (solvent strength) further reduces the polydispersity of the fraction obtained within that pressure range. Specifically, this work utilizes the highly tunable solvent properties of organic/CO2 solvent mixtures to selectively size-separate dispersions of polydisperse nanoparticles (2 to 12 nm) into more monodisperse fractions (±2 nm). In addition to providing efficient separation of the particles, this process also allows all of the solvent and antisolvent to be recovered, thereby rendering it a green solvent process. [ABSTRACT FROM AUTHOR]

In previous work on the density fluctuation theory of transport coefficients of liquids, it was necessary to use empirical self-diffusion coefficients to calculate the transport coefficients (e.g., shear viscosity of carbon dioxide). In this work, the necessity of empirical input of the self-diffusion coefficients in the calculation of shear viscosity is removed, and the theory is thus made a self-contained molecular theory of transport coefficients of liquids, albeit it contains an empirical parameter in the subcritical regime. The required self-diffusion coefficients of liquid carbon dioxide are calculated by using the modified free volume theory for which the generic van der Waals equation of state and Monte Carlo simulations are combined to accurately compute the mean free volume by means of statistical mechanics. They have been computed as a function of density along four different isotherms and isobars. A Lennard-Jones site-site interaction potential was used to model the molecular carbon dioxide interaction. The density and temperature dependence of the theoretical self-diffusion coefficients are shown to be in excellent agreement with experimental data when the minimum critical free volume is identified with the molecular volume. The self-diffusion coefficients thus computed are then used to compute the density and temperature dependence of the shear viscosity of liquid carbon dioxide by employing the density fluctuation theory formula for shear viscosity as reported in an earlier paper (J. Chem. Phys. 2000, 112, 7118). The theoretical shear viscosity is shown to be robust and yields excellent density and temperature dependence for carbon dioxide. The pair correlation function appearing in the theory has been computed by Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

In this paper, we report the results of static and dynamic light scattering investigations of polydimethyl siloxane (PDMS) solutions in both liquid and supercritical carbon dioxide (CO2). This study was performed below the theta point and provides quantitative information on the CO2 solvent quality over a large range of temperature (25-54 °C) and density (0.97-1.01 g/mL). The solvent quality of the CO2 can be adjusted by independently varying temperature or density, as demonstrated by the dependence of the second virial coefficient on these two parameters. The theta temperature was observed to be a strong function of CO2 density and may be a weak function of the PDMS molecular weight. The strength of the excluded volume interactions in the PDMS-CO2 solution was determined to be weaker than predicted, and no universal behavior was observed. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were employed to both estimate the solubility of nonelectrolyte solids, such as acetanilide, acetaminophen, phenacetin, methylparaben, and lidocaine, in supercritical carbon dioxide and understand the underlying molecular-level driving forces. The solubility calculations involve the estimation of the solute's limiting activity coefficient, which may be computed using conventional staged free-energy calculations. For the case of lidocaine, wherein the infinite dilution approximation is not appropriate, we demonstrate how the activity coefficient at finite concentrations may be estimated without additional effort using the dilute solution approximation and how this may be used to further understand the solvation process. Combining with experimental pure-solid properties, namely, the normal melting point and enthalpy of fusion, solubilities were estimated. The results are in good quantitative agreement with available experimental data, suggesting that molecular simulations may be a powerful tool for understanding supercritical processes and the design of carbon dioxide-philic molecular systems. Structural analyses were performed to shed light on the microscopic details of the solvation of different functional groups by carbon dioxide and the observed solubility trends. [ABSTRACT FROM AUTHOR]