Solvation of halogens in fluorous phases. Experimental and simulation data for F2, Cl2, and Br2 in several fluorinated liquids

The solubility of halogen gases--fluorine, chlorine and bromine--has been determined experimentally in several fluorinated solvents between 283 and 323 K at atmospheric pressure. The solubility of chlorine was studied in perfluorooctane, perfluorohexane, perfluorohexylethane, perfluoromethylcyclohexane, perfluoro-1,3-dimethylcyclohexane, perfluoro-2-butyltetrahydrofuran, and perfluoroperhydrophenanthrene and was found to be on the order of 10(-2) in mole fraction. The solubility of fluorine in the studied fluorinated solvents at 298 K is 1 order of magnitude lower than the solubility of chlorine. The solubility of bromine was studied as a function of temperature in perfluorooctane, and it was found to be higher than that of chlorine but of the same order of magnitude. The experimental studies were complemented by molecular simulation calculations. The molecular force fields used for the halogen gases and for the fluorinated solvents were taken, when possible, from the literature. An intermolecular potential model had to be developed for perfluoro-2-butyltetrahydrofuran, with a functional form of the Lennard-Jones plus point charges type. The solubility of the three gases was calculated by molecular simulation using Widom test-particle insertion. Dissimilar interaction parameters of 0.89 and 0.75 in the Lennard-Jones well depths between the solute and the solvent had to be introduced to reach agreement with the experimental results for chlorine and fluorine solubilities, respectively. The structure of the solutions was studied by analysis of solute-solvent radial distribution functions. It was found that the preferential solvation sites for the halogen gases are the terminal CF3 groups of the different fluorinated solvents.     

Interactions of fluorinated gases with ionic liquids: solubility of CF4, C2F6, and C3F8 in trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide

The interactions between ionic liquids and totally fluorinated alkanes are investigated by associating gas solubility measurements with molecular simulation calculations. Experimental values for the solubility of perfluoromethane, perfluoroethane, and perfluoropropane in one ionic liquidtrihexyltetradecylphophonium bis(trifluoromethylsulfonyl)amide [P 6,6,6,14][Ntf 2]are reported between 303 and 343 K and close to atmospheric pressure. All mole fraction solubilities decrease with increasing temperature. From the variation of Henry's law constants with temperature, the thermodynamic functions of solvation were calculated. The precision of the experimental data, considered as the average absolute deviation of the Henry's law constants from appropriate smoothing equations, is always better than +/-3%. By the analysis of the differences between the solute-solvent radial distribution functions of perfluoromethane and perfluoropropane obtained by molecular simulation, it was possible to explain why solubility increases with the size of the perfluoroalkane. The trend of solubility is explained on the basis of the location of the solute with respect to the solvent ions as well as on the differences in the solute-solvent energies of interaction.     

Anion effects on gas solubility in ionic liquids

This work presents the results of solubility measurements for a series of gases in 1-n-butyl-3-methyl imidazolium tetrafluoroborate and 1-n-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) imide. The gases considered include benzene, carbon dioxide, nitrous oxide, ethylene, ethane, oxygen, and carbon monoxide. Carbon dioxide and oxygen solubilities are also reported in methyl-tributylammonium bis(trifluoromethylsulfonyl) imide, butyl-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide, and tri-isobutyl-methyl phosphonium p-toluenesulfonate. We report the associated Henry's constants and enthalpies and entropies of absorption. In general, benzene, followed by carbon dioxide and nitrous oxide, have the highest solubilities and strongest interactions with the ionic liquids, followed by ethylene and ethane. Oxygen had very low solubilities and weak interactions. Carbon monoxide had a solubility below the detection limit of our apparatus. Ionic liquids with the bis(trifluoromethylsulfonyl) imide anion had the largest affinity for CO(2), regardless of whether the cation was imidazolium, pyrrolidinium, or tetraalkylammonium. These results suggest that the nature of the anion has the most significant influence on the gas solubilities.     

Comparison of random-walk density functional theory to simulation for bead-spring homopolymer melts

Density profiles for a homopolymer melt near a surface are calculated using a random-walk polymeric density functional theory, and compared to results from molecular dynamics simulations. All interactions are of a Lennard-Jones form, for both monomer-monomer interactions and surface-monomer interactions, rather than the hard core interactions which have been most investigated in the literature. For repulsive systems, the theory somewhat overpredicts the density oscillations near a surface. Nevertheless, near quantitative agreement with simulation can be obtained with an empirical scaling of the direct correlation function. Use of the random phase approximation to treat attractive interactions between polymer chains gives reasonable agreement with simulation of dense liquids near neutral and attractive surfaces.     

The wettability of fluoropolymer surfaces: influence of surface dipoles

The wettabilities of fluorinated polymers were evaluated using a series of contacting probe liquids ranging in nature from nonpolar aprotic to polar aprotic to polar protic. Fully fluorinated polymers were wet less than partially fluorinated polymers, highlighting the weak dispersive interactions of fluorocarbons. For partially fluorinated polymers, the interactions between the distributed dipoles along the polymer backbone and the dipoles of the contacting liquids were evaluated using both polar and nonpolar probe liquids. The results demonstrate that the surface dipoles of the fluoropolymers generated by substituting fluorine atoms with hydrogen or chlorine atoms can strongly interact with polar contacting liquids. The wettabilities of the partially fluorinated polymers were enhanced by increasing the density of dipoles across the surfaces and by introducing differentially distributed dipoles.     

Why Is CO2 so soluble in imidazolium-based ionic liquids?

Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ionic liquids. CO2 absorption isotherms at 10, 25, and 50 degrees C are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO2, while there is little difference in CO2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO2 about hexafluorophosphate anions, but only small differences in CO2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.     

A computer simulation study of the ordered phases of some mesogenic fullerene derivatives

We present a Monte Carlo simulation study of the phase behaviour and molecular organization of a system of fullerene-based mesogens, represented by a three-site model composed of a fullerene sphere and two mesogenic moieties rigidly attached to it. It is shown that a combination of suitably modified Lennard-Jones and Gay–Berne attractive–repulsive potentials allows a satisfactory qualitative modeling of the interactions between the fullerene derivatives under investigation. Indeed, simulation results show that, despite the crude representation of the molecular structure, our model generates nematic and smectic phases, thus accounting qualitatively for the basic experimental observations on the class of compounds considered.     

Solubility measurements for carbon dioxide and nitrous oxide in liquid oxygen at temperatures down to 90 K

The solubility of nitrous oxide and carbon dioxide in liquid oxygen at temperatures between 90 and 110 K has been measured using a static-analytic method. The NRTL model has been used to calculate the liquid-phase activity. The overall experimental data-set is compared both with the results of the model and with experimental data available in literature.     

Modeling the solubility behavior of CO(2), H(2), and Xe in [C(n)-mim][Tf(2)N] ionic liquids

We present here a new model for the imidazolium-based ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide anion [Tf(2)N](-) in the context of the soft-SAFT EoS. The model is used to predict the solubility of several compounds in these ILs, and results are compared to available experimental data. Since in the soft-SAFT EoS an associating site is used to represent a short-range and highly directional attractive force among molecules, we have used this feature to mimic the main interactions between the anion and the cation for the alkylimidazolium-[Tf(2)N] ILs. The members of the alkylimidazolium-[Tf(2)N] family are modeled as Lennard-Jones chains with three associating sites in each molecule (one "A" site and two "B" sites). An "A" site represents the nitrogen atom interactions with the cation, and a "B" site represents the delocalized charge due the oxygen molecules on the anion. Each type of associating site is identically defined, but only AB interactions between different IL molecules are allowed. Model parameters for the ionic liquids were estimated with experimental density data from different authors, following a similar approach taken in our previous work [Andreu and Vega, J. Phys. Chem. C 2007, 111, 16028]. The new set of parameters was used to study the solubility behavior of hydrogen, carbon dioxide, and xenon in these ILs over a wide range of temperature and pressure. It has been observed that no binary parameters are needed to correlate the solubility of hydrogen in [C(6)-mim][Tf(2)N] at different temperatures, and predictions up to 100 MPa are presented here. The model is able to correlate with very good agreement the experimental data for the systems [C(n)-mim][Tf(2)N] + CO(2) with only one temperature-independent mixture parameter, while two temperature-independent mixture parameters are needed to correlate the experimental solubility data for the systems IL + Xe, attaining an excellent agreement in a wide range of temperatures. The work presented here reinforces previous results, proving that a reasonable simple model for the IL within the framework of soft-SAFT is able to describe the physical absorption of different gases in ILs with good accuracy, in spite of the most complex nature of the anion, without the need of further parameters or terms. In addition, since these parameters do not depend on the particular conditions at which they were fitted, soft-SAFT is used then to analyze the solubility dependence of these gases in ILs, according to the anion nature and the alkyl chain length of the imidazolium cation by the use of the models developed within this approach.     

The atomistic simulation of the gas permeability of poly(organophosphazenes). Part 1. Poly(dibutoxyphosphazenes)

Self-diffusion and solubility coefficients of six gas molecules (He, Ne, O₂, N₂, CH₄, and CO₂) in two poly(dibutoxyphosphazenes)—poly[bis(n-butoxy)phosphazene] (PnBuP) and poly[bis(sec-butoxy)phosphazene] (PsBuP)—have been investigated by means of molecular simulation using the COMPASS molecular mechanics force field. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) using up to 3 ns simulation time. Solubility coefficients were obtained by means of a Grand Canonical Monte Carlo (GCMC) method. Results of both simulations were in generally good agreement with experimental data with the exception of the simulation results for gas solubility in PsBuP where differences from the data may be attributed to microcrystallinity of the experimental sample. In the case of diffusivity, diffusion coefficients correlated well with the square of the effective diameter of the diffusing gas. Similarly a good correlation was found between the solubility coefficients obtained by GCMC simulation of sorption isotherms and the Lennard-Jones potential well depth parameter, ϵ/k.The transition-state model of Gusev and Suter was used to determine free volume and free volume distribution for PnBuP, PsBuP, and poly[bis(iso-butoxy)phosphazene] (PiBuP). The diffusion coefficient for a given gas in each polyphosphazene was found to correlate exponentially with its accessible free volume fraction. A model for the distribution of accessible free volume, derived from the Cohen–Turnbull theory for the self diffusion of a liquid of hard spheres, was found to provide excellent fit with the simulation results.     

In silico prediction of drug solubility: 2. Free energy of solvation in pure melts

The solubility of drugs in water is investigated in a series of papers and in the current work. The free energy of solvation, DeltaG*(vl), of a drug molecule in its pure drug melt at 673.15 K (400 degrees C) has been obtained for 46 drug molecules using the free energy perturbation method. The simulations were performed in two steps where first the Coulomb and then the Lennard-Jones interactions were scaled down from full to no interaction. The results have been interpreted using a theory assuming that DeltaG*(vl) = DeltaG(cav) + E(LJ) + E(C)/2 where the free energy of cavity formation, DeltaG(cav), in these pure drug systems was obtained using hard body theories, and E(LJ) and E(C) are the Lennard-Jones and Coulomb interaction energies, respectively, of one molecule with the other ones. Since the main parameter in hard body theories is the volume fraction, an equation of state approach was used to estimate the molecular volume. Promising results were obtained using a theory for hard oblates, in which the oblate axial ratio was calculated from the molecular surface area and volume obtained from simulations. The Coulomb term, E(C)/2, is half of the Coulomb energy in accord with linear response, which showed good agreement with our simulation results. In comparison with our previous results on free energy of hydration, the Coulomb interactions in pure drug systems are weaker, and the van der Waals interactions play a more important role.     

Solubility of dioxygen in seven fluorinated liquids

Experimental values for the solubility of oxygen in seven fluorinated organic liquids with potential for bio-medical applications—perfluorohexylethane, perfluoromethylcyclohexane, perfluorodimethylcyclohexane, perfluorophenanthrene, perfluorooctane, perfluorodecalin, and perfluorooctylbromide—are reported as a function of temperature and at pressures close to atmospheric. An apparatus based on a saturation method at constant pressure was used to measure solubility at temperatures from 288 to 316 K. The precision of the results, expressed as Ostwald coefficients, is estimated as ±1%. The solubility of oxygen is constant with temperature for perfluorooctylbromide, perfluorodimethylcyclohexane, perfluorohexylethane, perfluorodecalin and perfluorophenanthrene and decreases with temperature for perfluorooctane and perfluoromethylcyclohexane. Some of the experimental results are compared with existing literature values at 298 K.     

Free energy calculations for a flexible water model

In this work, we consider the problem of calculating the classical free energies of liquids and solids for molecular models with intramolecular flexibility. We show that thermodynamic integration from the fully-interacting solid of interest to a Debye crystal reference state, with anisotropic harmonic interactions derived from the Hessian of the original crystal, provides a straightforward route to calculating the Gibbs free energy of the solid. To calculate the molecular liquid free energy, it is essential to correctly account for contributions from both intermolecular and intramolecular motion; we employ thermodynamic integration to a Lennard-Jones reference fluid, coupled with direct evaluation of the molecular ro-vibrational partition function. These approaches are used to study the low-pressure classical phase diagram of the flexible q-TIP4P/F water model. We find that, while the experimental ice-I/liquid and ice-III/liquid coexistence lines are described reasonably well by this model, the ice-II phase is predicted to be metastable. In light of this finding, we go on to examine how the coupling between intramolecular flexibility and intermolecular interactions influences the computed phase diagram by comparing our results with those of the underlying rigid-body water model.     

A theoretical study of structure-solubility correlations of carbon dioxide in polymers containing ether and carbonyl groups

This work involves a theoretical study to investigate the effects of the structure on CO(2) sorption in polymers, where poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), poly(vinyl acetate) (PVAc), poly(ethylene carbonate) (PEC) and poly(propylene carbonate) (PPC) were examined. In the theoretical approach, the multi-site semiflexible chain model and the renormalized technique of electrostatic potentials were incorporated into the polymer reference interaction site model (PRISM). To test the theory, molecular dynamic simulations were performed using the TraPPE-UA force field. The theoretically calculated reduced X-ray scattering intensities and intermolecular correlation functions of these five polymers are found to be in qualitative agreement with the corresponding molecular simulation data. The theory was then employed to investigate the distribution functions between CO(2) and different sites of the polymers with consideration of the Lennard-Jones, potential of mean force, and columbic contributions. Based on the detailed structure characteristics of CO(2) in contact with different groups, the CO(2) coordination molecular numbers were obtained and their sorption intensities analyzed. Finally, the sorption isotherms of CO(2) in these five polymers were calculated. The results for PEO, PPO and PVAc are close to the available experimental curves, and the trend of CO(2) solubility is PPC > PEC > PVAc ~ PPO > PEO.     

Improving carbon dioxide solubility in ionic liquids

Previously we showed that CO2 could be used to extract organic molecules from ionic liquids without contamination of the ionic liquid. Consequently a number of other groups demonstrated that ionic liquid/CO2 biphasic systems could be used for homogeneously catalyzed reactions. Large differences in the solubility of various gases in ionic liquids present the possibility of using them for gas separations. More recently we and others have shown that the presence of CO2 increases the solubility of other gases that are poorly soluble in the ionic liquid phase. Therefore, a knowledge and understanding of the phase behavior of these ionic liquid/CO2 systems is important. With the aim of finding ionic liquids that improve CO2 solubility and gaining more information to help us understand how to design CO2-philic ionic liquids, we present the low- and high-pressure measurements of CO2 solubility in a range of ionic liquids possessing structures likely to increase the solubility of CO2. We examined the CO2 solubility in a number of ionic liquids with systematic increases in fluorination. We also studied nonfluorinated ionic liquids that have structural features known to improve CO2 solubility in other compounds such as polymers, for example, carbonyl groups and long alkyl chains with branching or ether linkages. Results show that ionic liquids containing increased fluoroalkyl chains on either the cation or anion do improve CO2 solubility when compared to less fluorinated ionic liquids previously studied. It was also found that it was possible to obtain similar, high levels of CO2 solubility in nonfluorous ionic liquids. In agreement with our previous results, we found that the anion frequently plays a key role in determining CO2 solubility in ionic liquids.     

Effect of bromine substitution on the solubility of gases in hydrocarbons and fluorocarbons

The interactions of oxygen and of carbon dioxide with normal and brominated octane are studied by analysing original experimental gas solubility data as a function of temperature in the range between 288 and 313 K. The temperature dependence of the solubility yields the thermodynamic properties of solvation. The influence of bromine substitution was studied by comparing the present data with that for perfluorooctane and 1-bromoperfluorooctane. A molecular interpretation of the effects observed was done using atomistic simulation. In order to simulate 1-bromooctane, parameters of the molecular force field were developped for bromoalkanes within the OPLS-AA framework. In general, simulation provides correct predictions of the solubility and of its temperature dependence, except in cases where values are too close (within the error bars of the simulation). Structural aspects of the solvation of the two gases were analysed in light of the site–site solute–solvant radial distribution functions. The relative importance of electrostatic interactions is assessed by modifying the intermolecular potential models for the gases.     

Mesoscopic structural organization in fluorinated room temperature ionic liquids

The presence of fluorous tails in room-temperature ionic liquids imparts new properties to their already rich spectrum of appealing features. The interest towards this class of compounds that are of ionic nature with melting point less than 25 °C is accordingly growing; in particular, compounds bearing relatively long fluorous tails have begun to be considered. In this invited presentation, we show recent results arising from the systematic study of structural properties of a series of fluorinated room temperature ionic liquids, with growing fluorous chain length. At odd with the current understanding of this class of compounds, we show experimentally that they are characterized by the presence of segregated fluorous domains whose size depends on the fluorous chain length. This experimental finding, based on the synergic use of X-ray and neutron scattering, provides a structural scenario at the mesoscopic spatial scale that is in agreement with the recent state of the art molecular dynamic simulations. We speculate on the potential role of this significant compartmentalization of the bulk liquid phase into different nanoscale domains, as relevant in a series of applications including separation, solubility, catalysis, and so forth.     

A polarizable model for ethylene oxide

A series of interaction models for ethylene oxide are developed for use in molecular simulation of the thermal properties of both the gas and liquid phases. While it is possible to develop nonpolarizable models to accurately generate either the gas or liquid properties separately, it was not possible to do so using a single model for both phases. A polarizable, rigid all-atom model was developed that reproduces the temperature dependence of the second virial coefficient B(T) and the pressure of the liquid at ambient conditions. The model consists of Lennard-Jones and Coulomb interactions between intermolecular atomic sites plus a scalar polarizability located at the midpoint of the line joining the carbon sites. The electrostatic charges and the polarizability are set to match the experimentally determined dipole and quadrupole moments and the molecular polarizability.     

Molecular simulation of the hydration Gibbs energy of barbiturates

In the present work, molecular dynamics calculations of the Gibbs energy of hydration of 10 different substituted barbiturates in SPC/E water were performed using thermodynamic integration. Given that experimental determination of the Gibbs hydration energy for this class of compounds is currently unfeasible, computer simulations appear as the only alternative for the estimation of this important quantity. Several simulation parameters are discussed and optimized based on calculations for barbituric acid. It is concluded that accounting for electrostatic interactions with the Reaction-Field method can be up to two times faster than with Particle-Mesh-Ewald method, without loss of accuracy. Different number of solvent molecules and simulation lengths were also tested. Lennard-Jones and electrostatic contributions were scaled down to zero in an independent way. It is shown that the electrostatic contribution is dominant (representing approximately 90% of the total Gibbs energy of hydration) and that barbiturate intra-molecular interactions cannot be neglected. The importance of the electrostatic contribution is attributed to the formation of hydrogen bonds between the barbiturates and water, which play an important role in the solvation process. The influence of the different substituents and their contribution to the Gibbs energy of hydration was assessed. Finally, the Lennard-Jones contributions and the total hydration Gibbs energy can both be correlated against molecular weight or partition coefficient data for mono- and di-substituted barbiturates.