Probing the Structure of Liquids with 129Xe NMR Spectroscopy: n-Alkanes, Cycloalkanes, and Branched Alkanes

The liquid organization of linear, branched, and cyclic alkanes was studied using atomic 129Xe as a NMR probe. 129Xe chemical shifts have been experimentally determined for xenon dissolved in a total of 21 alkanes. In order to allow the comparison of the different solvents at similar thermodynamic conditions, the measurements were performed over a wide range of temperatures, from the melting point of the solvent up to 350 K. The results were rationalized in terms of the density, nature, and organization of the chemical groups within xenon’s coordination sphere. Additionally, molecular dynamics simulations were performed using established atomistic force fields to interpret and clarify the conclusions suggested by the experimental results. The analysis is able to interpret previous results in the literature for ethane and propane at very different experimental conditions

Diffusion coefficients of perfluorinated n-alcohols in water and heavy water: experiment and computer simulation

Fluorinated surfactants find nowadays many industrial applications due to their enhanced ability to lower surface tension in aqueous solutions [1]. As a result of their extensive use, emissions of fluorinated surfactants became frequent and, because of their persistent character, have been increasingly found in the environment [2]. Both the development of theoretical models to study the environmental fate of those pollutants and the design of unit operations (e.g. adsorption) used for their removal require the knowledge of some key properties such as the diffusion coefficients in water. n-alcohols with perfluorinated carbon chains can be regarded as the most simple fluorinated surfactants, being suitable to be used as model substances that can make easier the molecular interpretation and the theoretical treatment of fluorinated surfactants in a systematic way. On the other hand, the smallest perfluorinated n-alcohols find applications in many fields, such as the pharmaceutical industry, polymer production and refrigerant technology as components of working fluids. We have recently reported intra-diffusion coefficients of 2,2,2-trifluoroethanol in water for dilute solutions as a function of composition and temperature, obtained both experimentally (NMR spin-echo) and by computer simulation (molecular dynamics) [3]. The results obtained by molecular dynamics closely reproduce the experimental ones, which has encouraged us to attempt predicting the dynamic properties of aqueous solutions of the higher fluorinated alcohols and other fluorinated surfactants. In this work, the intra-diffusion coefficients of 2,2,3,3,3-pentafluoropropan-1-ol, 2,2,3,3,4,4,4-heptafluorobutan-1-ol and 2,2,3,3,4,4,5,5,5-nonafluoropentan-1-ol in water and heavy water were measured experimentally by NMR spin-echo technique and compared with results obtained from computer simulation (molecular dynamics). The comparison that can be done between experimental and simulation results is used to test the theoretical models for this chemical family of substances and enriches the molecular interpretation of the results, which can be useful to anticipate trends for more complex fluorinated surfactants. [1] Buck, R. C.; Franklin, J.; Berger, U.; Conder, J. M.; Cousins, I. T.; de Voogt, P.; Jensen, A. A.; Kannan, K.; Mabury, S. A.; van Leeuwen, S., Integr. Environ. Assess. Manage 2011, 7, 513−541 [2] D’Hollander, W.; de Voogt, P.; De Coen, W.; Bervoets, L., Rev. Environ. Contam. Toxicol. 2010, 208, 179–215 [3] Pereira, L. A. M.; Martins, L. F. G.; Ascenso, J. R.; Morgado, P.; Prates Ramalho, J. P.; Filipe, E. J. M., submitted to publicatio

Excess Thermodynamics of Mixtures Involving Xenon and Light Linear Alkanes by Computer Simulation

Excess molar enthalpies and excess molar volumes as a function of composition for liquid mixtures of xenon + ethane (at 161.40 K), xenon + propane (at 161.40 K) and xenon + n-butane (at 182.34 K) have been obtained by Monte Carlo computer simulations and compared with available experimental data. Simulation conditions were chosen to closely match those of the corresponding experimental results. The TraPPE-UA force field was selected among other force fields to model all the alkanes studied, whereas the one-center Lennard−Jones potential from Bohn et al. was used for xenon. The calculated and for all systems are negative, increasing in magnitude as the alkane chain length increases. The results for these systems were compared with experimental data and with other theoretical calculations using the SAFT approach. An excellent agreement between simulation and experimental results was found for xenon + ethane system, whereas for the remaining two systems, some deviations that become progressively more significant as the alkane chain length increases were observed

On the Behavior of Solutions of Xenon in Liquid n-Alkanes: Solubility of Xenon in n-Pentane and n-Hexane

The solubility of xenon in liquid n-pentane and n-hexane has been studied experimentally, theoretically, and by computer simulation. Measurements of the solubility are reported for xenon + n-pentane as a function of temperature from 254 to 305 K. The uncertainty in the experimental data is less than 0.15%. The thermodynamic functions of solvation such as the standard Gibbs energy, enthalpy, and entropy of solvation have been calculated from Henry’s law coefficients for xenon + n-pentane solutions and also for xenon + n-hexane, which were reported in previous work. The results provide a further example of the similarity between the xenon + n-alkane interaction and the n-alkane + n-alkane interactions. Using the SAFT-VR approach we were able to quantitatively predict the experimental solubility for xenon in n-pentane and semiquantitatively that of xenon in n-hexane using simple Lorentz−Berthelot combining rules to describe the unlikely interaction. Henry’s constants at infinite dilution for xenon + n-pentane and xenon + n-hexane were also calculated by Monte Carlo simulation using a united atom force field to describe the n-alkane and the Widom test particle insertion method

On the Behaviour of Solutions of Xenon in Liquid Cycloalkanes: Solubility of Xenon in Cyclopentane

The solubility of xenon in liquid cyclopentane has been studied experimentally and theoretically. Measurements of the solubility of xenon in liquid cyclopentane are reported as a function of temperature from 254.60Kto 313.66 K. The imprecision of the experimental data is less than 0.3%. The thermodynamic functions of solvation of xenon in cyclopentane, such as the standard Gibbs energy, enthalpy, entropy and heat capacity of solvation, have been calculated from the temperature dependence of Henry’s law coefficients. The results provide further information about the differences between the xenon + cycloalkanes and the xenon + n-alkane interactions. In particular, interaction enthalpies between xenon and CH2 groups in nalkanes and cycloalkanes have been estimated and compared. Using a version of the soft-SAFT approach developed to model cyclic molecules, we were able to reproduce the experimental solubility for xenon in cyclopentane using simple Lorentz-Berthelot rules to describe the unlike interaction

Diffusion Coefficients of Fluorinated Surfactants in Water:

Intradiffusion coefficients of 2,2,2-trifluoroethanol in water have been measured by the pulsed field gradient (PFG)-NMR spin−echo technique as a function of temperature and composition on the dilute alcohol region. The measurements extend the range of compositions already studied in the literature and, for the first time, include the study of the temperature dependence. At the same time, intradiffusion coefficients of 2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoropropan-1-ol, and 2,2,3,3,4,4,4-heptafluorobutan-1-ol in water were obtained by computer simulation (molecular dynamics) as a function of composition and temperature. The intradiffusion coefficients of 2,2,2-trifluoroethanol in water obtained by simulation agree with the experimental results, while those of 2,2,3,3,3- pentafluoropropan-1-ol and 2,2,3,3,4,4,4-heptafluorobutan-1-ol are the first estimation of this property for those systems. The molecular dynamics simulations were also used to calculate the intradiffusion coefficients of perfluorooctanesulfonic acid and perfluorooctanoic acid in water at infinite dilution as a function of temperature, which are very difficult to obtain experimentally because of the very low solubility of these substances. From the dependence of the intradiffusion coefficients on temperature, diffusion activation energies were estimated for all the solutes in water

Diffusion coefficients of 2,2,2-trifluoroethanol/water mixtures

Aqueous mixtures of 2,2,2-trifluoroethanol have received significant attention in the last years because their applications, such as working fluid in Rankine cycle thermal engines or as solvent in studies of protein stability (protein folding). From the fundamental point of view, fluoroalcohols have an amphyphile character, due to the simultaneous presence of a hydrophobic (and alkane-phobic) fluorinated surface and a hydroxyl group. Mixtures of 2,2,2-trifluoroethanol with hydrogenated alcohols were studied recently in our research group (experimental determination of pVT surfaces as well as thermal, volumetric and structural properties by computer simulation) [1]. In the case of 2,2,2-trifluoroethanol/water this study suggested a preference of cross hydrogen bond. Hydrogen bonds play a determinant role in the structure of aqueous mixtures of alcohols, and should influence the dynamic properties of binary systems of fluorinated alcohols with water. The goal of this work is to study a dynamic property (diffusion coefficient) of binary mixtures 2,2,2-trifluoroethanol/water, comparing the results with equilibrium and structural properties already known for this and related systems

Liquid Mixtures Involving Hydrogenated and Fluorinated Chains: (p, ρ, T, x) Surface of (Ethanol + 2,2,2-Trifluoroethanol), Experimental and Simulation

The effect of mixing hydrogenated and fluorinated molecules that simultaneously interact through strong hydrogen bonding was investigated: (ethanol + 2,2,2-trifluoroethanol) binary mixtures were studied both experimentally and by computer simulation. This mixture displays a very complex behavior when compared with mixtures of hydrogenated alcohols and mixtures of alkanes and perfluoroalkanes. The excess volumes are large and positive (unlike those of mixtures of hydrogenated alchools), while the excess enthalpies are large and negative (contrasting with those of mixtures of alkanes and perfluoroalkanes). In this work, the liquid density of the mixtures was measured as a function of composition, at several temperatures from 278.15 to 353.15 K and from atmospheric pressure up to 70 MPa. The corresponding excess molar volumes, compressibilities, and expansivities were calculated over the whole (p, ρ, T, x) surface. In order to obtain molecular level insight, the behavior of the mixture was also studied by molecular dynamics simulation, using the OPLS-AA force field. The combined analysis of the experimental and simulation results indicates that the peculiar phase behavior of this system stems from a balance between the weak dispersion forces between the hydrogenated and fluorinated groups and a preferential hydrogen bond between ethanol and 2,2,2-trifluoroethanol. Additionally, it was observed that a 25% reduction of the F−H dispersive interaction in the simulations brings agreement between the experimental and simulated excess enthalpy but produces no effect in the excess volumes. This reveals that the main reason causing the volume increase in these systems is not entirely related to the weak dispersive interactions, as it is usually assumed, and should thus be connected to the repulsive part of the intermolecular potential

Supramolecular hydrogel based on a sodium deep eutectic solvent

A supramolecular hydrogel based on a metal-containing deep eutectic solvent (DES) is presented here for the first time. The phase diagram of the DES-based hydrogel was drawn and its rheological properties were determined

Prediction of diffusion coefficients of chlorophenols in water by computer simulation

Intra-diffusion coefficients of seven chlorophenols (2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 2,4,6-dichlorophenol and pentachlorophenol) in water were determined by computer simulation (molecular dynamics) for dilute solutions at three different temperatures and the corresponding mutual diffusion coefficients estimated. The mutual diffusion coefficients of 2-chlorophenol in water agree with the available experimental results from the literature for all the temperatures studied. From the dependence of the diffusion coefficients on temperature, diffusion activation energies were estimated for all the solutes inwater. Analyzing the radial distribution functions and spatial distribution functions of water around chlorophenols sites enable a discussion about intermolecular interactions (dominated by hydrogen bonding) between solute and solvent and its importance on the relative magnitude of diffusion coefficients. Finally the mutual diffusion coefficients obtained by simulation were correlated by the well-known Wilke–Chang equation