Proton and molecular permeation through the basal plane of monolayer graphene oxide.

Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It is also often presumed that individual GO monolayers contain a large density of nanoscale pinholes that lead to considerable gas leakage across the GO basal plane. Here we show that relatively large, micrometer-scale areas of monolayer GO are impermeable to gases, including helium, while exhibiting proton conductivity through the basal plane which is nearly two orders of magnitude higher than that of graphene. These findings provide insights into the key properties of GO and demonstrate that chemical functionalization of 2D crystals can be utilized to enhance their proton transparency without compromising gas impermeability.

New experimental density data and soft-SAFT models of alkylimidazolium ([C(n)C1im]⁺) chloride (Cl⁻), methylsulfate ([MeSO4]⁻), and dimethylphosphate ([Me2PO4]⁻) based ionic liquids.

Ionic liquids have been shown to have application in several areas of importance in the context of sustainable industrial activity. One application of particular interest is the ability of certain ionic liquids to dissolve biomass. This clearly marks them as useful materials with application within biorefineries. In this contribution, we present new coarse-grained soft-SAFT models and experimental density data of chloride (Cl(-)), methylsulfate ([MeSO4](-)), and dimethylphosphate ([Me2PO4](-)) based ionic liquids which are relevant for biomass deconstruction processes. Model parameters were obtained by fitting to pure component temperature density data, and the models were subsequently tested by assessing their ability to accurately calculate viscosity and interfacial surface tension. We also developed models of mixtures of the ionic liquids with water and short-chain linear alcohols. We decomposed the contributions to the excess Gibbs energy of mixing to chemical and structural contributions, and used this to provide some insight into the driving forces for solubility of molecular species in these ionic liquids.

Free-volume theory coupled with soft-SAFT for viscosity calculations: comparison with molecular simulation and experimental data.

The evaluation of phase equilibria and solubility properties through theoretical approaches is a well-known field, where a significant amount of models are able to describe them with a good degree of accuracy. However, the simultaneous calculation of transport properties together with thermodynamic phase properties still remains a challenge, due to the difficulties in describing the behavior of properties like the viscosity of fluids with the same approach. In this work, the free-volume theory (FVT) has been coupled with the soft-SAFT equation for the first time to extend the capabilities of the equation to the calculation of transport properties. The theory has been first tested using simulation data of the viscosity of the Lennard-Jones (LJ) fluid and LJ chains over a wide range of temperature and pressure. Good agreement has been found at all chain lengths, except for some deviations at near-zero density values. Several trends of the viscosity parameters with the length of the chain are identified, allowing the prediction of other chain fluids. Finally, the new equation has been applied to the n-alkanes family, where viscosity is a key property, and results are compared with experimental data. The three viscosity parameters were fitted to viscosity data of the pure fluid at several isotherms or isobars, whereas the density and pressure (or temperature) were taken from the soft-SAFT output. Again, the effect of these parameters on the viscosity has been investigated and compared with results obtained for the LJ chains and with previous work of other authors. The new equation performs very well in all cases, with a global average absolute deviation of 2.12% and shows predictive capabilities for heavier compounds. This empowers soft-SAFT with new capabilities, allowing the equation to calculate phase, interfacial, and transport properties with the same model and degree of accuracy.

Transport properties of mixtures by the soft-SAFT + free-volume theory: application to mixtures of n-alkanes and hydrofluorocarbons.

In a previous paper (Llovell et al. J. Phys. Chem. B, submitted for publication), the free-volume theory (FVT) was coupled with the soft-SAFT equation of state for the first time to extend the capabilities of the equation to the calculation of transport properties. The equation was tested with molecular simulations and applied to the family of n-alkanes. The capability of the soft-SAFT + FVT treatment is extended here to other chemical families and mixtures. The compositional rules of Wilke (Wilke, C. R. J. Chem. Phys. 1950, 18, 517-519) are used for the diluted term of the viscosity, while the dense term is evaluated using very simple mixing rules to calculate the viscosity parameters. The theory is then used to predict the vapor-liquid equilibrium and the viscosity of mixtures of nonassociating and associating compounds. The approach is applied to determine the viscosity of a selected group of hydrofluorocarbons, in a similar manner as previously done for n-alkanes. The soft-SAFT molecular parameters are taken from a previous work, fitted to vapor-liquid equilibria experimental data. The application of FVT requires three additional parameters related to the viscosity of the pure fluid. Using a transferable approach, the α parameter is taken from the equivalent n-alkane, while the remaining two parameters B and Lv are fitted to viscosity data of the pure fluid at several isobars. The effect of these parameters is then investigated and compared to those obtained for n-alkanes, in order to better understand their effect on the calculations. Once the pure fluids are well characterized, the vapor-liquid equilibrium and the viscosity of nonassociating and associating mixtures, including n-alkane + n-alkane, hydrofluorocarbon + hydrofluorocarbon, and n-alkane + hydrofluorocarbon mixtures, are calculated. One or two binary parameters are used to account for deviations in the vapor-liquid equilibrium diagram for nonideal mixtures; these parameters are used in a transferable manner to predict the viscosity of the mixtures. Very good agreement with available experimental data is found in all cases, with an average absolute deviation ranging between 1.0% and 5.5%, even when the system presents azeotropy, reinforcing the robustness of the approach.

Effect of high carbon dioxide atmosphere packaging and soluble gas stabilization pre-treatment on the shelf-life and quality of chicken drumsticks.

The effects of an aerobic modified atmosphere packaging (MAP) (70% CO2, 15% O2 and 15% N2) with and without a CO2 3-h soluble gas stabilization (SGS) pre-treatment of chicken drumsticks were determined for various package and product quality characteristics. The CO2 dissolved into drumsticks was determined. The equilibrium between CO2 dissolved in drumsticks and CO2 in head space was reached within 48h after packaging, showing highest values of CO2 in SGS pre-treated samples. This greater availability of CO2 resulted in lower counts of TAB and Pseudomonas in SGS than in MAP drumsticks. Package collapse was significantly reduced in SGS samples. The average of CO2 dissolved in the MAP treatment was 567mg CO2kg(-1) of chicken and, 361mg CO2kg(-1) of chicken during the MAP treatment, in SGS pre-treated samples. This difference could be the quantity of CO2 dissolved during SGS pre-treatment. These results highlight the advantages of using SGS versus traditional MAP for chicken products preservation.

Surface tension of binary mixtures of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids: experimental measurements and soft-SAFT modeling.

Ionic liquids have attracted a large amount of interest in the past few years. One approach to better understand their peculiar nature and characteristics is through the analysis of their surface properties. Some research has provided novel information on the organization of pure ionic liquids at the vapor-liquid interface; yet, a systematic study on the surface properties of mixtures of ionic liquids and their organization at the surface has not previously been carried out in the literature. This work reports, for the first time, a comprehensive analysis of the surface organization of mixtures of ionic liquids constituted by 1-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [C(n)mim][NTf(2)]. The surface tension of mixtures composed of [C(4)mim][NTf(2)] + [C(n)mim][NTf(2)] (n = 1, 2, 5, 6, 8, and 10) was experimentally determined, at 298.2 K and atmospheric pressure, in the whole composition range. From the experimental data, the surface tension deviations and the relative Gibbs adsorption isotherms were estimated showing how the surface composition of an ionic liquid mixture differs from that of the liquid bulk and that the surface is enriched by the ionic liquid with the longest alkyl chain length. Finally, the soft-SAFT equation of state coupled with the density gradient theory (DGT) was used, for the first time, to successfully reproduce the surface tension experimental data of binary mixtures of ionic liquids using a molecular-based approach. In addition, the DGT was used to compute the density profiles of the two components across the interface, confirming the experimental results for the components distribution at the bulk and at the vapor-liquid interface.

Modeling the [NTf2] pyridinium ionic liquids family and their mixtures with the soft statistical associating fluid theory equation of state.

In this work, the soft statistical associating fluid theory (soft-SAFT) equation of state (EoS) has been used to provide an accurate thermodynamic characterization of the pyridinium-based family of ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide anion [NTf(2)](-). On the basis of recent molecular simulation studies for this family, a simple molecular model was proposed within the soft-SAFT EoS framework. The chain length value was transferred from the equivalent imidazolium-based ILs family, while the dispersive energy and the molecular parameters describing the cation-anion interactions were set to constant values for all of the compounds. With these assumptions, an appropriate set of molecular parameters was found for each compound fitting to experimental temperature-density data at atmospheric pressure. Correlations for the nonconstant parameters (describing the volume of the IL) with the molecular weight were established, allowing the prediction of the parameters for other pyridiniums not included in the fitting. Then, the suitability of the proposed model and its optimized parameters were tested by predicting high-pressure densities and second-order thermodynamic derivative properties such as isothermal compressibilities of selected [NTf(2)] pyridinium ILs, in a large range of thermodynamic conditions. The surface tension was also provided using the density gradient theory coupled to the soft-SAFT equation. Finally, the soft-SAFT EoS was applied to describe the phase behavior of several binary mixtures of [NTf(2)] pyridinium ILs with carbon dioxide, sulfur dioxide, and water. In all cases, a temperature-independent binary parameter was enough to reach quantitative agreement with the experimental data. The description of the solubility of CO(2) in these ILs also allowed identification of a relation between the binary parameter and the molecular weight of the ionic liquid, allowing the prediction of the CO(2) + C(12)py[NTf(2)] mixture. The good agreement with the experimental data shows the excellent ability of the soft-SAFT EoS to describe the thermophysical properties of ILs as well as their phase behavior. Results prove that this equation of state can be a valuable tool to assist the design of ILs (in what concerns cation and anion selection) in order to obtain ILs with the desired properties and, consequently, enhancing their potential industrial applications.

Modeling the absorption of weak electrolytes and acid gases with ionic liquids using the soft-SAFT approach.

In this work, the solubility of three common pollutants, SO(2), NH(3), and H(2)S, in ionic liquids (ILs) is studied using the soft-SAFT equation of state with relatively simple models. Three types of imidazolium ionic liquids with different anions are described in a transferable manner using the recently published molecular models (Andreu, J. S.; Vega, L. F. J. Phys. Chem. C 2007, 111, 16028; Llovell et al. J. Phys. Chem. B 2011, 115, 4387), whereas new models for SO(2), NH(3), and H(2)S are proposed here. Alkyl-imidazolium ionic liquids with the [PF(6)](-) and [BF(4)](-) anions are considered to be Lennard-Jones chainlike molecules with one associating site in each molecule describing the specific cation-anion interactions. Conversely, the cation and anion forming the imidazolium [Tf(2)N](-) ionic liquids are modeled as a single molecule with three associating sites, taking into account the delocalization of the anion electric charge due to the presence of oxygen groups surrounding the nitrogen of the anion. NH(3) is described with four associating sites: three sites of type H mimicking the hydrogen atoms and one site of type e representing the lone pair of electrons. H(2)S is modeled with three associating sites: two for the sites of type H for the hydrogen atoms and one site of type e for the electronegativity of the sulfur. SO(2) is modeled with two sites, representing the dipole moment of the molecule as an associative interaction. Soft-SAFT calculations with the three models for the pollutants provide very good agreement with the available phase equilibria, enthalpy of vaporization, and heat capacity experimental data. Then, binary mixtures of these compounds with imidazolium-based ionic liquids were calculated in an industrially relevant temperature range. Unlike association interactions between the ionic liquids and the pollutant gases have been explicitly accounted for using an advanced association scheme. A single temperature independent energy binary parameter is sufficient to describe every family of mixtures in good agreement with the available data in the literature. In addition, a vapor-liquid-liquid equilibrium (VLLE) region, never measured experimentally, has been identified for mixtures of hydrogen sulfide + imidazolium ionic liquids with the [PF(6)](-) anion at high H(2)S concentrations. This work illustrates that relatively simple models are able to capture the phase absorption diagram of different gases in ionic liquids, provided accurate models are available for the pure components as well as an accurate equation of state to model the behavior of complex systems.

Influence of the concentration of CO2 and SO2 on the absorption of CO2 by a lithium orthosilicate-based absorbent.

A novel, high temperature solid absorbent based on lithium orthosilicate (Li(4)SiO(4)) has shown promise for postcombustion CO(2) capture. Previous studies utilizing a clean, synthetic flue gas have shown that the absorbent has a high CO(2) capacity, >25 wt %, along with high absorption rates, lower heat of absorption and lower regeneration temperature than other solids such as calcium oxide. The current effort was aimed at evaluating the Li(4)SiO(4) based absorbent in the presence of contaminants found in typical flue gas, specifically SO(2), by cyclic exposure to gas mixtures containing CO(2), H(2)O (up to 25 vol. %), and SO(2) (up to 0.95 vol. %). In the absence of SO(2), a stable CO(2) capacity of ∼ 25 wt % over 25 cycles at 550 °C was achieved. The presence of SO(2), even at concentrations as low as 0.002 vol. %, resulted in an irreversible reaction with the absorbent and a decrease in CO(2) capacity. Analysis of SO(2)-exposed samples revealed that the absorbent reacted chemically and irreversibly with SO(2) at 550 °C forming Li(2)SO(4). Thus, industrial application would require desulfurization of flue gas prior to contacting the absorbent. Reactivity with SO(2) is not unique to the lithium orthosilicate material, so similar steps would be required for other absorbents that chemically react with SO(2).

Modeling complex associating mixtures with [Cn-mim][Tf2N] ionic liquids: predictions from the soft-SAFT equation.

In a previous work (Andreu and Vega, J. Phys. Chem.B2008, 111, 16028), we presented a simple model for the imidazolium-based ionic liquids (ILs) with the bis(trifluorosulfonyl)imide anion [Tf(2)N](-) in the context of the soft-SAFT equation of state. The model was successfully used to predict the solubility of several gases in these ILs. However, the small amount of experimental data made the predictions less accurate when going into more complex mixtures and one or two fitted binary parameters were needed in some cases. In this work, we have reparameterized our previous model and evaluated its reliability to predict the behavior of these ionic liquids in binary mixtures with other associating compounds. Model parameters for the ionic liquids were estimated using new experimental density data at atmospheric pressure in an extended range of temperatures, from 273 until 473 K, consistent within the range of temperatures previously measured by other authors. The new set of molecular parameters has been tested to predict the density of several members of the family at higher pressures up to 60 MPa with the same degree of accuracy than at atmospheric values. In addition to density-temperature data, interfacial tensions and the isothermal compressibility of some compounds were predicted in reasonable good agreement with experimental data. The molecular parameters of the pure compounds were used then, in a predictive manner, to describe the behavior of binary mixtures with other imidazolium ionic liquids, changing either the cation or the anion. Predictions for some mixtures with methanol, ethanol, and water were compared with experimental data, providing an excellent description of the systems, with no fitting to mixture data in almost all the cases. The excellent results obtained in this work reinforce the need to have accurate data, showing that molecular based models can be used to assess the validity of these data. In addition, this work also shows that a simple model in which the physics of the system is kept is good enough to describe the complex behavior of associating mixtures of ionic liquids, without the need of additional parameters that may obscure the real physics of the system.

Vapor-liquid and critical behavior of binary systems of hydrogen chloride and n-alkanes: experimental data and soft-SAFT modeling.

The purpose of this work is to check the capability of the crossover soft-SAFT equation of state to predict the phase behavior of hydrogen chloride/n-alkane mixtures based on experimental data. The hydrogen chloride parameters were optimized using the experimental information, while the parameters for the n-alkanes were obtained from published correlations to the molecular weight of the compounds. We have found that a unique binary parameter with a constant value for the whole family provides an excellent description of the behavior of hydrogen chloride + propane and hydrogen chloride + dodecane mixtures in a broad range of temperatures and pressures, as well as the critical line of the mixture. The model confirms that HCl + propane exhibits type-I critical behavior, while HCl + dodecane shows a type-II critical behavior. Taking advantage of the transferability of the parameters, the critical transition from type-II to type-III has been investigated with the equation in a predictive manner. Although results are very sensitive to the binary parameter value, there are indications to assert that type-III is achieved close to the HCl + heneicosane binary system.

Water liquid-vapor equilibria predicted by refined ab initio derived potentials.

Coexistence properties for water near the critical point using several ab initio models were calculated using grand canonical Monte Carlo simulations with multiple histogram reweighting techniques. These models, that have proved to yield a good reproduction of the water properties at ambient conditions, perform rather well, improving the performance of a previous ab initio model. It is also shown that bulk geometry and dipole values, predicted by the simulation, can be used and a good approximation obtained with a polarizable rigid water model but not when polarization is excluded.

Neodymium: YAG laser treatment of persistent pupillary membrane.

A 16-year-old girl had a cosmetically unappealing persistent iridopupillary membrane in the left eye. Her best corrected visual acuity was 20/60, but this improved to 20/40 with pupillary dilation. Neodymium: YAG laser membranectomy resulted in 20/20 vision. This is the first reported case of persistent pupillary membrane treated with the Neodymium: YAG laser.