Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? Structure and dynamics of CO2 dissolved in [Bmim+] [PF6(-)

When supercritical CO2 is dissolved in an ionic liquid, its partial molar volume is much smaller than that observed in most other solvents. In this article we explore in atomistic detail and explain in an intuitive way the peculiar volumetric behavior experimentally observed when supercritical CO2 is dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim+] [PF6(-)]). We also provide physical insight into the structure and dynamics occurring across the boundary of the CO2 ionic liquid interface. We find that the liquid structure of [Bmim+] [PF6(-)] in the presence of CO2 is nearly identical to that in the neat ionic liquid (IL) even at fairly large mole fractions of CO2. Our simulations indicate, in agreement with experiments, that partial miscibilities of one fluid into the other are very unsymmetrical, CO2 being highly soluble in the ionic liquid phase while the ionic liquid is highly insoluble in the CO2 phase. We interpret our results in terms of the size and shape of spontaneously forming cavities in the ionic liquid phase, and we propose that CO2 occupies extremely well-defined locations in the IL. Even though our accurate prediction of cavity sizes in the neat IL indicates that these cavities are small compared with the van der Waals radius of a single carbon or oxygen atom, CO2 appears to occupy a space that was for the most part a priori "empty".     

Dynamic adsorption properties of n-alkyl glucopyranosides determine their ability to inhibit cytolysis mediated by acoustic cavitation

Suspensions of human leukemia (HL-60) cells readily undergo cytolysis when exposed to ultrasound above the acoustic cavitation threshold. However, n-alkyl glucopyranosides (hexyl, heptyl, and octyl) completely inhibit ultrasound-induced (1057 kHz) cytolysis (Sostaric, et al. Free Radical Biol. Med. 2005, 39, 1539-1548). The efficacy of protection from ultrasound-induced cytolysis was determined by the n-alkyl chain length of the glucopyranosides, indicating that protection efficacy depended on adsorption of n-alkyl glucopyranosides to the gas/solution interface of cavitation bubbles and/or the lipid membrane of cells. The current study tests the hypothesis that "sonoprotection" (i.e., protection of cells from ultrasound-induced cytolysis) in vitro depends on the adsorption of glucopyranosides at the gas/solution interface of cavitation bubbles. To test this hypothesis, the effect of ultrasound frequency (from 42 kHz to 1 MHz) on the ability of a homologous series of n-alkyl glucopyranosides to protect cells from ultrasound-induced cytolysis was investigated. It is expected that ultrasound frequency will affect sonoprotection ability since the nature of the cavitation bubble field will change. This will affect the relative importance of the possible mechanisms for ultrasound-induced cytolysis. Additionally, ultrasound frequency will affect the lifetime and rate of change of the surface area of cavitation bubbles, hence the dynamically controlled adsorption of glucopyranosides to their surface. The data support the hypothesis that sonoprotection efficiency depends on the ability of glucopyranosides to adsorb at the gas/solution interface of cavitation bubbles.     

Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a computational investigation

Efficient technologies/processes for CO(2) capture are greatly desired, and ionic liquids are recognized as promising materials for this purpose. However, the mechanisms for selectively capturing CO(2) by ionic liquids are unclear. In this study, the interactions between CO(2) and 1-n-amino-alkyl-3-methyl-imidazolium tetrafluoroborate, an amino imidazolium ionic liquid (AIIL), in its CO(2) capturing process, are elucidated with both quantum chemistry and molecular dynamics approaches on the molecular level. The effects of the straight aminoalkyl chain length in imidazolium-based cations on CO(2) capture are explored, and thereby the factors governing CO(2) capture for this ionic liquid family, e.g., ionic liquid structure, charge distribution, intermolecular interactions, thermodynamic properties and absorption kinetics, are analyzed. Molecular dynamics simulations are used to study the diffusion of the involved compounds and liquid structures of the CO(2)-AIIL systems. The results show that the amino-alkyl chain length plays an important role in governing the absorption properties of AIILs, including the free energies of absorption, equilibrium constants, desorption temperature, absorption rate constants, diffusion coefficients, and organization of CO(2) around cations and anions. This study provides useful information about rational design of ionic liquids for efficient CO(2) capture.     

Insights into the structure and dynamics of a room-temperature ionic liquid: ab initio molecular dynamics simulation studies of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and the [bmim][PF6]-CO2 mixture

Ab initio molecular dynamics (AIMD) studies have been carried out on liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and its mixture with CO2 using the Car-Parrinello molecular dynamics (CPMD) method. Results from AIMD and empirical potential molecular dynamics (MD) have been compared and were found to differ in some respects. With a strong resemblance to the crystal, the AIMD simulated neat liquid exhibits many cation-anion hydrogen bonds, a feature that is almost absent in the MD results. The anions were observed to be strongly polarized in the condensed phase. The addition of CO2 increased the probability of this hydrogen bond formation. CO2 molecules in the vicinity of the ions of [bmim][PF6] exhibit larger deviations from linearity in their instantaneous configurations. The polar environment of the liquid induces a dipole moment in CO2, lifting the degeneracy of its bending mode. The calculated splitting in the vibrational mode compares well with infrared spectroscopic data. The solvation of CO2 in [bmim][PF6] is primarily facilitated by the anion, as seen from the radial and spatial distribution functions. CO2 molecules were found to be aligned tangential to the PF6 spheres with their most probable location being the octahedral voids of the anion. The structural data obtained from AIMD simulations can serve as a benchmark to refine interaction potentials for this important room-temperature ionic liquid.     

Ultrasound-induced polymerization of methyl methacrylate in liquid carbon dioxide: a clean and safe route to produce polymers with controlled molecular weight

Ultrasound-induced cavitation is known to enhance chemical reactions as well as mass transfer at ambient pressures. Ultrasound is rarely studied at higher pressures, since a high static pressure hampers the growth of cavities. Recently, we have shown that pressurized carbon dioxide can be used as a medium for ultrasound-induced reactions, because the static pressure is counteracted by the higher vapor pressure, which enables cavitation. With the use of a dynamic bubble model, the possibility of cavitation and the resulting hot-spot formation upon bubble collapse have been predicted. These simulations show that the implosions of cavities in high-pressure fluids generate temperatures at which radicals can be formed. To validate this, radical formation and polymerization experiments have been performed in CO₂-expanded methyl methacrylate. The radical formation rate is approximately 1.5*10¹⁴ s⁻¹ in this system. Moreover, cavitation-induced polymerizations result in high-molecular weight polymers. This work emphasizes the application potential of sonochemistry for polymerization processes, as cavitation in CO₂-expanded monomers has shown to be a clean and safe route to produce polymers with a controlled molecular weight.     

Phase field modeling of CH4 hydrate conversion into CO2 hydrate in the presence of liquid CO2

We present phase field simulations to estimate the conversion rate of CH(4) hydrate to CO(2) hydrate in the presence of liquid CO(2) under conditions typical for underwater gas hydrate reservoirs. In the computations, all model parameters are evaluated from physical properties taken from experiment or molecular dynamics simulations. It has been found that hydrate conversion is a diffusion controlled process, as after a short transient, the displacement of the conversion front scales with t(1/2). Assuming a diffusion coefficient of D(s) = 1.1 x 10(-11) m(2) s(-1) in the hydrate phase, the predicted time dependent conversion rate is in reasonable agreement with results from magnetic resonance imaging experiments. This value of the diffusion coefficient is higher than expected for the bulk hydrate phase, probably due to liquid inclusions remaining in the porous sample used in the experiment.     

Molecular dynamics simulations of ordering of poly(dimethylsiloxane) under uniaxial stress

Molecular dynamics simulations of bulk melts of poly(dimethylsiloxane) (PDMS) are utilized to study chain conformation and ordering under constant stress uniaxial extension at room temperature. We find that large extensions induce chain ordering in the direction of applied stress. During the extension, we also find that voids are created via a cavitation mechanism. At the end of our simulations, by visual inspection, we distinguish cavity, fibril, and amorphous regions that coexist together. The surrounding material about the formed cavities is fibril-like, while the remaining material remains amorphous. We also estimate the surface energy of the cavity. The cavity size continually increases in the dimension of applied stress but saturates in the lateral dimensions, most likely due to the finite size of the system. Despite chain orientation and ordering in the direction of applied stress, crystallization is absent in the time and stress range of our simulation. This study represents a baseline for the future study of mechanical properties of PDMS melts enriched with fillers under stress.     

Cavitation and crystallization in a metastable Lennard-Jones liquid at negative pressures and low temperatures

Molecular dynamics simulations have been used to investigate the kinetics of spontaneous cavitation and crystallization in a Lennard-Jones liquid at negative pressures in the temperature range where these processes compete with each other. The nucleation rate has been calculated in NVE and NpT ensembles by the method of mean lifetime and the transition interface sampling method with parallel path swapping. The data obtained have been used to determine in the framework of classical nucleation theory the value of the ratio of the solid-liquid and the liquid-void interfacial free energy for critical crystals and cavities and the values of their volumes at points where the cavitation rate of the liquid is equal to the rate of its crystallization.     

Force field of the TMGL ionic liquid and the solubility of SO2 and CO2 in the TMGL from molecular dynamics simulation

An all-atom force field is developed using a combination of density functional theory calculations and OPLS force field parameter values for the 1,1,3,3-tetramethylguanidium lactate (TMG) lactic acid (LAC) ionic liquid (TMGL). The computed density of the TMGL is in good agreement with available experimental values. The internal energy components, cohesive energy density, and the self-diffusion constants are also discussed. Molecular dynamics simulations are then conducted to investigate the solubility of the SO2 and CO2 gases in the TMGL. The simulation results show strong organization of SO2 about the TMG cation and the LAC anion, especially the LAC anion, but relatively weak organization of CO2 about the cation and the anion of the TMGL, which explained well the selectivity of the TMGL toward the SO2 and CO2.     

Molecular dynamics study of ethanolamine as a pure liquid and in aqueous solution

Molecular dynamics simulations have been carried out for ethanolamine as a pure liquid and in aqueous solution at 298 and 333 K. The ethanolamine force field has been parametrized to reproduce intramolecular energies from quantum mechanical calculations and experimentally determined properties of the liquid. The results are presented for conformer distributions, density, enthalpy of vaporization, self-diffusion constant, dielectric constant, and radial distribution functions. The results strongly suggest that the main (O-C-C-N) dihedral tends to stay in its gauche conformers in solution and that the ethanolamine molecules populate conformers with a significant degree of intramolecular hydrogen bonding. This result is also supported by results from a continuum solvation model. Simulation of a 10 mol % aqueous ethanolamine system suggests that ethanolamine is preferentially solvated to by water molecules. The results suggest that ethanolamine dimer formation in aqueous solution is very limited. Simulations were also carried out for CO2 in an aqueous ethanolamine system. The results suggest that CO2 has a comparable level of attraction to ethanolamine and water. The degree of interaction between CO2 and the amine and alcohol functionalities in ethanolamine also appear to be of comparable strength.     

Structural Identification of Spectroscopic Substates in Neuroglobin

The structural origins of infrared absorptions of photodissociated CO in murine neuroglobin (Ngb) are determined by combining Fourier transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulations. Such an approach allows to identify and characterize both the different conformations of the Ngb active site and the transient ligand docking sites. To capture the influence of the protein environment on the spectroscopy and dynamics, experiments and simulations are carried out for the wild type protein and its F28L and F28W mutants. It is found that a voluminous side chain at position 28 divides site B into two subsites, B’ and B”. At low temperatures, CO in wt Ngb only migrates to site B’ from where it can rebind, and B” is not populated. The spectra of CO in site B’ for wt Ngb from simulations and experiments are very similar in spectral shift and shape. They both show doublets, red‐shifted with respect to gas‐phase CO and split by≈8 cm^?1. The FTIR spectra of the F28L mutant show additional bands which are also found in the simulations and can be attributed to CO located in substate B”. The different bands are mainly related to different orientations of the His64 side chain with respect to the CO ligand. Large red‐shifts arise from strong interactions between the Histidine? NH and the CO oxygen. After dissociation from the heme iron, the CO ligand visits multiple docking sites. The locations of the primary docking site B and a secondary site C, which corresponds to the Mb Xe4 cavity, could be identified unambiguously. Finally, by comparing experiment and simulations it is also possible to identify protonation of its ε position (His_ε64 NgbCO) as the preferred heme‐bound conformation in the wild type protein with a signal at 1935 cm^?1.     

Coarse-grained potential models for structural prediction of carbon dioxide (CO2) in confined environments

In this paper, we propose coarse-grained single-site (CGSS), wall-CO(2), and CO(2)-CO(2) interaction potential models to study the structure of carbon dioxide under confinement. The CGSS potentials are used in an empirical potential based quasi-continuum theory, EQT, to compute the center-of-mass density and potential profiles of CO(2) confined inside different size graphite slit pores. Results obtained from EQT are compared with those obtained from all-atom molecular dynamics (AA-MD) simulations, and are found to be in good agreement with each other. Though these CGSS interaction potentials are primarily developed and parameterized for EQT, they are also used to perform coarse-grained molecular dynamics (CG-MD) simulations. The results obtained from CG-MD simulations are also found to be in reasonable agreement with AA-MD simulation results.     

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.     

Enlightened sonochemistry

Sonochemistry and photochemistry are initiated by high-energy transient species, which may be prone to mutual interaction. Electronic excitation of solutes by energy transfer from high energy species generated in collapsing bubbles is already supported by experimental evidence. The rates of photochemical reactions can be affected by ultrasound-induced mixing of liquids caused by microstreaming near pulsating cavitation bubbles and shockwaves due to bubble collapse. This may not only improve light absorption but also modify the pathway of reaction by increasing the contact between reagents. Finally, one may speculate about a potentially new chemistry of photoexcited solutes under the extreme conditions inside cavitation microreactors. This work reviews research on the excitation of solutes by sonoluminescence, the combined effects of ultrasound and light on liquid systems and the effect of ultrasound on photocatalytic reactions.     

Hydrogen bond dynamics in heavy water studied with quantum dynamical simulations

The structure and dynamics of the hydrogen-bond network in heavy water (D(2)O) is studied as a function of the temperature using quantum dynamical simulations. Our approach combines an ab initio-based representation of the water interactions with an explicit quantum treatment of the molecular motion. A direct connection between the calculated linear and nonlinear vibrational spectra and the underlying molecular dynamics is made, which provides new insights into the rearrangement of the hydrogen-bond network in heavy water. A comparison with previous calculations on liquid H(2)O suggests that tunneling does not effectively contribute to the dynamics of the water hydrogen-bond network above the melting point. However, the effects of nuclear quantization are not negligible at all temperatures and become increasingly important near the melting point, which is in agreement with recent experimental analysis of the structural properties of liquid water as well as of the proton momentum distribution in supercooled water.     

Free-radical generation from collapsing microbubbles in the absence of a dynamic stimulus

Free radicals are generated by the collapse of ultrasound-induced cavitation bubbles when they are forcefully compressed by dynamic stimuli. Radical generation occurs as a result of the extremely high temperatures induced by adiabatic compression during the violent collapse process. It is generally believed that extreme conditions are required for this type of radical generation. However, we have demonstrated free-radical generation from the collapse of microbubbles (diameter = <50 microm) in the absence of a harsh dynamic stimulus. In contrast to ultrasound-induced cavitation bubbles, which collapse violently after microseconds, the microbubbles collapsed softly under water after several minutes. Electron spin-resonance spectroscopy confirmed free-radical generation by the collapsing microbubbles. The increase of the surface charges (zeta potentials) of the microbubbles, which were measured during their collapse, supported the hypothesis that the significant increase in ion concentration around the shrinking gas-water interface provided the mechanism for radical generation. This technique of radical generation from collapsing microbubbles could be employed in numerous engineering applications, including wastewater treatment.     

Molecular simulation study of cavity-generated instabilities in the superheated Lennard-Jones liquid

Previous equilibrium-based density-functional theory (DFT) analyses of cavity formation in the pure component superheated Lennard-Jones (LJ) liquid [S. Punnathanam and D. S. Corti, J. Chem. Phys. 119, 10224 (2003); M. J. Uline and D. S. Corti, Phys. Rev. Lett. 99, 076102 (2007)] revealed that a thermodynamic limit of stability appears in which no liquidlike density profile can develop for cavity radii greater than some critical size (being a function of temperature and bulk density). The existence of these stability limits was also verified using isothermal-isobaric Monte Carlo (MC) simulations. To test the possible relevance of these limits of stability to a dynamically evolving system, one that may be important for homogeneous bubble nucleation, we perform isothermal-isobaric molecular dynamics (MD) simulations in which cavities of different sizes are placed within the superheated LJ liquid. When the impermeable boundary utilized to generate a cavity is removed, the MD simulations show that the cavity collapses and the overall density of the system remains liquidlike, i.e., the system is stable, when the initial cavity radius is below some certain value. On the other hand, when the initial radius is large enough, the cavity expands and the overall density of the system rapidly decreases toward vaporlike densities, i.e., the system is unstable. Unlike the DFT predictions, however, the transition between stability and instability is not infinitely sharp. The fraction of initial configurations that generate an instability (or a phase separation) increases from zero to unity as the initial cavity radius increases over a relatively narrow range of values, which spans the predicted stability limit obtained from equilibrium MC simulations. The simulation results presented here provide initial evidence that the equilibrium-based stability limits predicted in the previous DFT and MC simulation studies may play some role, yet to be fully determined, in the homogeneous nucleation and growth of embryos within metastable fluids.     

The dynamics of a bubble ensemble in a cavitation field

A system of equations was obtained to describe the dynamics of bubbles in a cavitation cloud taking into account the interaction of pulsating bubbles involved in translational motion. The kinetics of cavitation bubble concentration changes, changes in the compressibility of the liquid, and phase transitions within a cavitation bubble and in the neighboring volume of the liquid were taken into account. The role played by bubble deformation in a cavitation cloud was considered. The Bernoulli pressure effect was shown to be negligible. The interaction of cavitation bubbles was a substantial factor that strongly influenced the dynamics of bubbles. It was suggested that there was at least one more mechanism that reduced sonoluminescence intensity from the multiple-bubble cavitation field, namely, a fairly high efficiency of sonoluminescence quenching could additionally be related to the arrival of a cumulative liquid stream at the central cavitation bubble region, where the concentration of active species was high. The dynamics of bubbles in the cavitation field is not only related to the expansion and compression of cavitation bubbles in the acoustic field, but also governed to a great extent by their interaction, translational motion, deformation, and the influence of cumulative streams penetrating the bubbles.     

Solvation and solvatochromism in CO2-expanded liquids. 3. The dynamics of nonspecific preferential solvation

Subpicosecond time-resolved fluorescence of trans-4-dimethylamino-4'-cyanostilbene (DCS) is used to measure solvation dynamics in the gas-expanded liquid (GXL) system CH(3)CN + CO(2) at 25 degrees C along the liquid-vapor coexistence curve. These measurements are supplemented by measurements of the steady-state solvatochromism of DCS and of its rotation and isomerization times. Molecular dynamics computer simulations and a semiempirical spectral model that reproduces the observed solvatochromism in this system are used to interpret the experimental results. Simulations indicate that at compositions of x(CO2) > 0.5, the cybotactic region surrounding DCS is enriched in CH(3)CN molecules, and the extent of this enrichment is greater in S(1) than that in S(0). Solvation dynamics are dominated by the CH(3)CN component. These dynamics are biphasic, consisting of a subpicosecond inertial component, followed by a slower picosecond component, related to the redistribution of CH(3)CN molecules between the cybotactic region and the bulk solvent.     

Atomistic simulation of the absorption of carbon dioxide and water in the ionic liquid 1-n-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N

The solubility of water and carbon dioxide in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) is computed using atomistic Monte Carlo simulations. A newly developed biasing algorithm is used to enable complete isotherms to be computed. In addition, a recently developed pairwise damped electrostatic potential calculation procedure is used to speed the calculations. The computed isotherms, Henry's Law constants, and partial molar enthalpies of absorption are all in quantitative agreement with available experimental data. The simulations predict that the excess molar volume of CO2/ionic liquid mixtures is large and negative. Analysis of ionic liquid conformations shows that the CO2 does not perturb the underlying liquid structure until very high CO2 concentrations are reached. At the highest CO2 concentrations, the alkyl chain on the cation stretches out slightly, and the distance between cation and anion centers of mass increases by about 1 angstroms. Water/ionic liquid mixtures have excess molar volumes that are also negative but much smaller in magnitude than those for the case of CO2.