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

Towards a sanitation safety plan in the city of braga

A gestão integrada das infra-estruturas urbanas de água e saneamento constitui um desafio das cidades ambientalmente sustentáveis. Alguns avanços no controlo de qualidade dos sistemas de abastecimento de água para consumo humano, baseado em avaliação e gestão de riscos para a saúde pública, têm sido conseguidos através da implementação de planos de segurança da água. Com base nesta metodologia propõem-se, neste trabalho, contributos para a implementação de planos de segurança de saneamento, cobrindo-se, desta forma, todo o ciclo urbano da água com princípios de gestão de riscos. Com esta abordagem, estrutura-se um plano de segurança de saneamento onde são identificados eventos perigosos, avaliados e priorizados riscos para a saúde pública e para o ambiente, assim como se estabelecem medidas de controlo ao longo de todo o sistema de saneamento, percorrendo todas as etapas que o constituem: recolha, tratamento, lançamento no ambiente e reutilização segura de águas residuais. Estes conceitos foram aplicados ao sistema de saneamento da área central da cidade de Braga, para o qual se criou expressamente uma ferramenta informática de suporte, focada nos módulos principais da estrutura de um plano de segurança de saneamento.The integrated management of urban water and sanitation infrastructure is a challenge for environmentally sustainable cities. Positive developments in quality control in drinking water supply systems, based on risk assessment and risk management for public health, have been achieved through the implementation of water safety plans. Based on this methodology, the present work gives contributions for sanitation safety plans implementation. In this way, the urban water cycle is covered with risk management principles. With this approach, a sanitation safety plan is structured in which hazardous events to public health and environment are identified. Also risk assessment, risk prioritization and control measures are established throughout the sanitation system, covering all its steps: collection, treatment, disposal in receiving waters and safe reuse of treated wastewater. These concepts have been applied to the sanitation system of the central area of the city of Braga, for which a support software tool was expressly created, focusing in the core structural modules of a sanitation safety plan

Alkane coiling in perfluoroalkane solutions: a new primitive solvophobic effect

In this work, we demonstrate that n-alkanes coil when mixed with perfluoroalkanes, changing their conformational equilibria to more globular states, with a higher number of gauche conformations. The new coiling effect is here observed in fluids governed exclusively by dispersion interactions, contrary to other examples in which hydrogen bonding and polarity play important roles. FTIR spectra of liquid mixtures of n-hexane and perfluorohexane unambiguously reveal that the population of n-hexane molecules in all-trans conformation reduces from 32% in the pure n-alkane to practically zero. The spectra of peffluorohexane remain unchanged, suggesting nanosegregatiori of the hydrogenated and fluorinated chains. Molecular dynamics simulatiOns support this analysis. The new solvophobic effect is prone to have a major impact on the structure, organization, and therefore thermodynamic properties and phase equilibria of, fluids involving mixed hydrogenated and fluorinated chains.Fundacao para a Ciencia e Tecnologia [UID/NAN/50024/2013, UID/QUI/0100/2013, SFRH/BPD/81748/2011

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

Diffusion coefficients of chlorophenols in water by computer simulation

Cholophenols have found extensive industrial applications as wood preservatives or intermediated compounds for pesticide synthesis, and are also by-products of wood pulp bleaching processes [1]. As a result, they are nowadays common natural water contaminants, being considered priority substances by recent European Directives for Environmental Protection, due to their toxicity and persistence. The establishment of theoretical models to study environmental fate of chlorophenols is an important scientific topic. On the other hand, some unit operations are used to remove them from wastewater, such as adsorption [2] and reverse osmosis [3]. In both approaches, the knowledge of some key properties is needed, such as diffusion coefficients in water. Despite of their obvious importance, the diffusion coefficients of chlorophenols in water are scarce in literature and in many cases have to be estimated. Computer simulation can provide a way to systematically estimate this parameter for chlorophenol chemical family

Viscosity of Liquid Perfluoroalkanes and Perfluoroalkylalkane Surfactants

As part of a systematic study of the thermophysical properties of two important classes of fluorinated organic compounds (perfluoroalkanes and perfluoroalkylalkanes), viscosity measurements of four n-perfluoroalkanes and five perfluoroalkylalkanes have been carried out at atmospheric pressure and over a wide range of temperatures (278–353 K). From the experimental results the contribution to the viscosity from the CF2 and CF3 groups as a function of temperature have been estimated. Similarly, the contributions for CH2 and CH3 groups in n-alkanes have been determined using literature data. For perfluoroalkylalkanes, the viscosity results were interpreted in terms of the contributions of the constituent CF2, CF3, CH2, and CH3 groups, the deviations from ideality on mixing hydrogenated and fluorinated chains, and the contribution due to the formation of the CF2–CH2 bond. A standard empirical group contribution method (Sastri–Rao method) has also been used to estimate the viscosities of the perfluoroalkylalkanes. Finally, to obtain molecular level insight into the behavior of these molecules, all-atom molecular dynamics simulations have been performed and used to calculate the densities and viscosities of the perfluoroalkylalkanes studied. Although both quantities are underestimated compared to the experimental data, with the viscosities showing the largest deviations, the trends observed in the experimental viscosities are captured

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

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

Systems involving hydrogenated and fluorinated chains: Volumetric properties of Perfluoroalkanes and Perfluoroalkylalkane Surfactants

As part of a combined experimental and theoretical study of the thermodynamic properties of perfluoroalkylalkanes (PFAAs), the liquid density of perfluorobutylpentane (F4H5), perfluorobutylhexane (F4H6), and perfluorobutyloctane (F4H8) was measured as a function of temperature from 278.15 to 353.15 K and from atmospheric pressure to 70 MPa. The liquid densities of n-perfluoropentane, n-perfluorohexane, n-perfluorooctane, and n-perfluorononane were also measured at room pressure over the same temperature range. The PVT behavior of the PFAAs was also studied using the SAFT-VR equation of state. The PFAA molecules were modeled as heterosegmented diblock chains, using different parameters for the alkyl and perfluoroalkyl segments, that were developed in earlier work. Through this simple approach, we are able to predict the thermodynamic behavior of the perfluoroalkylalkanes, without fitting to any experimental data for the systems being studied. Molecular dynamics simulations have also been performed and used to calculate the densities of the perfluoroalkylalkanes studied

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