Elution of organic solutes from different polarity sorbents using subcritical water

The intermolecular interactions between organic solutes and sorbent matrices under subcritical water conditions have been investigated at a pressure of 50 bar and temperatures ranging from 50 to 250°C. Both polar and nonpolar organics (chlorophenols, amines, n-alkanes, and polycyclic aromatic hydrocarbons) and five different sorbent matrices (glass beads, alumina, Florisil, silica-bonded C₁₈, and polymeric XAD-4 resins) were used. From the same matrix, the polar solutes always eluted at lower temperatures, while the moderately polar and nonpolar solutes only eluted at higher temperatures. Similar to matrix effects previously observed using supercritical carbon dioxide, the sorbent type greatly influenced the elution efficiency under subcritical water conditions. Lower temperatures are sufficient to elute a particular solute from glass beads, alumina, and Florisil, but higher temperatures (less polar water) are needed to elute the same solute from silica-bonded C₁₈. The highest temperatures were required to elute aromatic organics from XAD-4. These matrix effects demonstrate that, while low temperature water can break inert or dipole interactions between solutes and glass beads, alumina, and Florisil, higher temperature water is required to interrupt the van der Waals attractions between solutes and silica-bonded C₁₈, and even higher temperatures needed to overcome the π-electron interactions between aromatic solutes and XAD-4.     

Semi-micro solvent extraction as a rapid and efficient preconcentration technique for the determination of n-alkanes in oil-contaiminated water and soil samples by capillary gas chromatography

Summary In this study, a rapid and efficient semi-micro extraction procedure is presented for the extraction of some higher n-alkanes from water and soil samples. In the case of water samples n-hexane was used as the organic phase in a phase volume ratio (volume of aqueous phase/volume of organic phase) higher than 285, while in the case of soil samples, extraction with n-hexane was carried out in the presence of an excess of 2 M NaCl solution. The extraction rate from soil samples is very high and is better than Soxhlet extraction, comparable with supercritical fluid extraction. High preconcentration factor in water samples allows the limits of detection to be in the ng.mL⁻¹ level with the use of gas chromatographic analysis. Flame ionization detector was used for monitoring the analytes. The obtained recoveries of all studied compounds from both water and soil samples are higher than 90%. This method was successfully used to determine some n-alkanes in municipal wastewater and contaminated soil.     

Evaluating perturbation contributions in SAFT models by comparing to molecular simulation of n-alkanes

SAFT models are generally written as a perturbation series of the Helmholtz energy with reciprocal temperature as the argument. The perturbation coefficients are then functions of density and molecular size. The variation of the perturbation coefficients with molecular size is given primarily by Wertheim's theory [6], [7], [8] and [9], but there may be additional variations as in the PC-SAFT model. In the present work, we compare the characterization of perturbation coefficients inferred from PC-SAFT to those derived from molecular simulations.The molecular simulations are based on Discontinuous Molecular Dynamics (DMD) and second order Thermodynamic Perturbation Theory (TPT). DMD simulation is applied to the repulsive part of the potential model with molecular details like fused hard spheres for the interaction sites and 110° bond angles. The thermodynamic effects of disperse attractions are treated by rigorous application of TPT. The present work re-examines the related work of Elliott and Gray [35] in the low density and critical regions, focusing on n-alkanes with carbon numbers ranging from 3 to 80.We find that SAFT theory overestimates the repulsive contribution (A₀) and underestimates the first order contribution (A₁) of Helmholtz energy relative to simulation. Nevertheless, the correlations are qualitatively reasonable. Significant inconsistencies arise when considering the second order contribution (A₂). For example, the PC-SAFT characterization of A₂ becomes larger than A₁ in the low density, long chain limit, raising concerns about the convergence of the series. Furthermore, fluctuations are underestimated in the critical region and overestimated in the liquid region. In each case, we can suggest improved characterizations. Altogether, these results suggest ways to modify the SAFT formalism to achieve greater consistency between atomistic and coarse-grained models.     

Vibrational spectroscopic study on the polytypism of n-alkanes and fatty acids

An important characteristic of n-alkanes and fatty acids is the great diversity of solid-state structures, which we can hardly imagine from their simple molecular structures. The variety of crystalline states can be categorized with two concepts: polymorphism and polytypism. Polytypism refers to higher-order structural variation caused solely by the stacking mode of molecular layers. Two basic polytypic structures, single-layer and double-layer, were confirmed in several modifications of n-alkanes and fatty acids having an O⊥ subcell. During the last decade, the two basic polytypes of n-alkanes have been intensively studied mainly by vibrational spectroscopy, with a focus on their formation conditions, thermodynamic stability, and molecular motions. This review summarizes new progress in the context of research on the polytypism of long-chain compounds.     

Dilute gas viscosity of n-alkanes represented by rigid Lennard-Jones chains

ABSTRACT

The shear viscosity in the dilute gas limit has been calculated by means of the classical trajectory method for a gas consisting of chain-like molecules. The molecules were modelled as rigid chains made up of spherical segments that interact through a combination of site–site Lennard-Jones 12-6 potentials. Results are reported for chains consisting of 2, 3, 4, 6, 8, 12 and 16 segments in the reduced temperature range of 0.3–50 for site–site separations of 0.25σ, 0.333σ, 0.40σ, 0.60σ and 0.80σ, where σ is the Lennard-Jones length scaling parameter. The results were used to determine the shear viscosity of n-alkanes in the zero-density limit by representing an n-alkane molecule as a rigid linear chain consisting of n_c ? 1?spherical segments, where n_c?is the number of carbon atoms. We show that for a given n-alkane molecule, the scaling parameters ? and σ are not unique and not transferable from one molecule to another. The commonly used site–site Lennard-Jones 12-6 potential in combination with a rigid-chain molecular representation can only accurately mimic the viscosity if the scaling parameters are fitted. If the scaling parameters are estimated from the scaling parameters of other n-alkanes, the predicted viscosity values have an unacceptably high uncertainty.     

Review: active homogeneous reagents and catalysts in n-alkane activation

The development of selective, efficient, and direct routes for activation and functionalization of naturally abundant n-alkanes could lead to a new paradigm in materials and energy technologies. In this context, the use of homogeneous catalysts to functionalize C–H bonds of unactivated hydrocarbons is of particular interest from a scientific as well as an economic viewpoint. Despite the large body of work on stoichiometric C–H activation reactions produced over the last three decades, relatively few systems have been developed to allow catalytic functionalization of hydrocarbons. This review deals with homogeneous catalytic processes available in the literature for paraffin activation and functionalization. The key intermediates involved in catalytic systems are highlighted, providing important information in the design of new and efficient catalysts. Also, some of the key challenges and approaches to rational development of the next generation of organometallic catalysts will be highlighted.     

An analytical perturbed equation of state for hard chain fluids: Application to n-alkanes and n-perfluoroalkanes

A completely analytical equation of state for pure hard chain fluids, derived on the basis of perturbation theory and reported in our previous work, is applied for the calculation of pVT properties and the prediction of vapour–liquid equilibria of n-alkanes and n-perfluoroalkanes. The molecules are treated as a chain formed from freely joined spheres which interact via an extended site-site square-well potential. The molecular parameters of compounds are obtained from the experimental compressibility factor data above the critical temperature. These parameters are capable of relatively satisfactory prediction of the vapour–liquid equilibrium coexistence curves of compounds. Linear relationships have been found between the potential parameters of fluids and their molecular weight, which make it possible to predict the pVT data and vapour–liquid phase equilibria of heavier compounds.     

Predictive use of a SAFT EOS for phase equilibria of some hydrocarbons and their binary mixtures

A systematic study of several hydrocarbons with a SAFT equation of state (EOS) is presented. First, it is shown that the phase behavior of the whole family of n-alkanes may be represented with the use of only three parameters. The approach is then extended to moderately branched alkanes, alkenes and ring compounds using one additional property, namely normal boiling point. Binary mixtures are also investigated and reasonable results are obtained with no additional binary parameters.