Predicting the Phase Equilibria of Mixtures of Hydrogen Fluoride with Water, Difluoromethane (HFC-32), and 1,1,1,2-Tetrafluoroethane (HFC-134a) Using a Simplified SAFT Approach

Assessment of hydrogen bond strengths and cooperativity in self- and cross-associating cyclic (HF)m(H2O)n (m + n = 2 to 8) clusters

In this work, we investigated the strengths of various self- and cross-associating hydrogen bonds (HBs) in mixed hydrogen fluoride–water cyclic (HF)m(H2O)n (m + n = 2 to 8) clusters, employing a molecular tailoring approach (MTA)-based method.

The phase and interfacial properties of azeotropic refrigerants: the prediction of aneotropes from molecular theory

Predicting the azeotropic (bulk) and aneotropic (interface) behavior of refrigerant mixtures using polar PC-SAFT coupled with DGT.

Molecular dynamics of a model dimerizing fluid

A model dimer forming fluid has been investigated by continuous molecular dynamics simulations. This study emphasizes the volume fraction and temperature dependence of the dynamic properties of the system, including the self and collective diffusion coefficients and the forward and reverse rate constants. The self and collective diffusion coefficients are found to be well described by a monomer fraction controlled interpolation formula. The forward rate constant (dimer formation) is found to be weakly temperature dependent and strongly volume fraction dependent. The opposite holds for the reverse rate constant. The dimer and monomer decay rates are not found to affect the intermediate scattering functions at the conditions studied.

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.

Association patterns in (HF)(m)(H2O)(n) (m + n = 2-8) clusters

In an attempt to understand the phase behavior of aqueous hydrogen fluoride, the clustering in the mixture is investigated at the molecular level. The study is performed at the mPW1B95/6-31+G(d,p) level of theory. Several previous studies attempted to describe the dissociation of HF in water, but in this investigation, the focus is only on the association patterns that are present in this binary mixture. A total of 214 optimized geometries of (HF)n(H2O)m clusters, with m + n as high as 8, were investigated. For each cluster combination, several different conformations are investigated, and the preferred conformations are presented. Using multiple linear regressions, the average strengths of the four possible H-bonding interactions are obtained. The strongest H-bond interaction is reported to be the H2O...H-F interaction. The most probable distributions of mixed clusters as a function of composition are also deduced. It is found that the larger (HF)n(H2O)m clusters are favored both energetically and entropically compared to the ones that are of size m + n < or = 3. Also, the clusters with equimolar contributions of HF and H2O are found to have the strongest interactions.

Isothermal compressibility maxima of hydrogen fluoride in the supercritical and superheated vapor regions

The highly nonideal behavior of hydrogen fluoride (HF) vapor has been considered to be the origin of its numerous vapor phase anomalies. In this work, we report one such potential vapor phase anomaly for HF. For a nonassociating substance like propane, the response functions go through a maximum only once in the supercritical region. However, for HF, when an association model is used to predict the isothermal compressibility (KT), it exhibits a maximum in the supercritical region more than once, and this peak extends well in to the superheated vapor region upon decompression. This theoretical prediction is also supported by two other models recently developed for HF. Note that experimental values of KT for HF have not been reported in the literature so far. Preliminary investigations on this KT maximum for HF have suggested no reentrant spinodal, singularity-free scenario, or any additional first-order phase transition, unlike water, and, also, no lambda (or higher-order phase) transitions, unlike liquid helium. However, this KT peak is similar to the experimentally supported heat capacity (CP) peak of HF which extends into the supercritical and superheated vapor regions. Similar to the CP peak, which is understood based on vapor-phase clustering in HF, we relate KT to the derivatives of enthalpy and entropy of the system. Also, we analyze some of the P-v-T experimental data that are available to provide an overview of the KT behavior in the region of interest, and compare them with the model results. Finally, to explore the effect of including a distribution pattern for the oligomers, we report the results on a model that only includes association. Using this approach, we report KT results with and without a Poisson-type oligomer distribution and show that the KT appears once this distribution scheme is specified.

Examination of the Excess Thermodynamic Properties of n-Alkane Binary Mixtures: A Molecular Approach

A modification of the statistical associating fluid theory, the so-called Soft-SAFT equation of state, is proposed to predict the excess thermodynamic properties of binary mixtures of n-alkanes. n-Alkane molecules are modeled as fully flexible Lennard-Jones chains. This molecular model accounts for the most important microscopic features of real chainlike molecules: attractive and repulsive interactions between different chemical groups and the connectivity of the segments that form the molecules. In this work we consider an additional microscopic effect that can profoundly affect certain thermodynamic properties, namely, the conformational changes when two different n-alkane molecules are mixed. We propose, following the work of Vega and co-workers [J. Chem. Phys. 1999, 111, 3192], a simple model to account for the conformational changes in molecules. The resulting free energy is combined with the SAFT free energy to describe the excess thermodynamic properties of binary mixtures of n-alkanes. Predictions from the theory are compared with experimental data taken from the literature. The agreement between the experiments and the theoretical predictions is excellent in all cases. This work shows that although minor microscopic effects, such as the conformational changes in the molecules that form the mixtures, have only a very small effect on the usual thermodynamic properties, such as pressure, chemical potential, phase equilibria, and excess volumes, they can contribute significantly to other thermodynamic properties. In fact, one of the main conclusions of this work is that it is essential that conformational effects be taken into account in molecular-based theories if an accurate description of certain excess properties (excess enthalpy for instance) is desired.

Spectral Intensity and Lineshape Measurements in the First Overtone Band of HF Using Tunable Diode Lasers

High-resolution absorption lineshapes for the P(3) and P(6) transitions of the first overtone (v = 2-0) band of HF at 296 K have been measured using a pair of distributed feedback diode lasers operating near 1.31 and 1.34 µm, respectively. Spectral line intensities and self-broadening parameters were determined by fitting the measured spectra with Voigt, Galatry, and Rautian lineshape models. Voigt profiles fit the low-pressure (<10 Torr) spectra of the P(3) transition reasonably well due to the relatively strong collisional broadening effect. Lineshape measurements of the P(6) transition (for pressures ranging from 5 to 60 Torr) show significant variation from the Voigt lineshape model due to velocity-changing collisions that effectively reduce the Doppler component of the spectral line. Lineshape models that include motional (Dicke) narrowing effects, Galatry (soft collision) and Rautian (hard collision) profiles yield significant improvements in the spectral lineshape fits compared with Voigt profiles. The collisional broadening coefficient (gamma) of the P(6) transition obtained from a Voigt fit is approximately 4% lower than those found with either Galatry or Rautian profile fits. The measured intensities and self-broadening coefficients are compared with values in the HITRAN database and previous measurements. Copyright 1999 Academic Press.