Volume 35, Issue 3, September 2006
Index of content:
35(2006); http://dx.doi.org/10.1063/1.1901689View Description Hide Description
The thermochemical behavior of more than 3000 organic compounds known to form liquid crystals is reported along with references to the original literature. A group additivity approach used to estimate total phase changeentropies of organic molecules applied to 627 of these liquid crystals is found to significantly overestimate their total phase changeentropies. Comparison of experimental and estimated values also show significant scatter relative to databasecompounds. The origins of these discrepancies are discussed in terms of a model used to explain liquid crystal formation.
Viscosity, Second -Virial Coefficient, and Diffusion of Pure and Mixed Small Alkanes , , , , , , , and Calculated by Means of an Isotropic Temperature-Dependent Potential. I. Pure Alkanes35(2006); http://dx.doi.org/10.1063/1.2201308View Description Hide Description
Reference tables of second -virial coefficients , viscosity, and self-diffusion are given for all neat alkanes , , for temperatures starting at for , for , and for , , , , , and . Restricting ourselves to low densities the thermophysical properties are calculated by means of an isotropic Lennard-Jones temperature dependent potential (LJTDP). In this model the potential well depth and the separation at minimum energy are explicitly temperature dependent, whereas the repulsive term is independent of . The LJTDP has been used before in order to construct reference tables of thermophysical properties of neat gases [Zarkova and Hohm, J. Phys. Chem. Ref. Data31, 183 (2002)] and binary mixtures [Zarkova, Hohm, and Damyanova, J. Phys. Chem. Ref. Data32, 1591 (2003)]. However, those studies were restricted to atoms and globularly shaped nondipolar molecules. Here the approach is extended to elongated, not necessarily spherically symmetric, and in part slightly dipolar molecules. As in previous works the potential parameters , , and are determined by minimizing the root-mean-square deviation between calculated and experimentally obtained thermophysical properties, , , and the second acoustic virial coefficient normalized to their experimental error. In extension of our previous efforts we present a thorough statistical analysis of the experimental input data which gives us the possibility to select primary data which could be used to build up a database.
The Fragment Constant Method for Predicting Octanol–Air Partition Coefficients of Persistent Organic Pollutants at Different Temperatures35(2006); http://dx.doi.org/10.1063/1.2203356View Description Hide Description
The octanol–air partition coefficient is a key physicochemical parameter for describing the partition of organic pollutants between air and environmental organic phases. Experimental determination of is costly and time consuming, and sometimes restricted by lack of sufficiently pure chemicals. There is a need to develop a simple but accurate method to estimate . In the present study, a fragment constant model based on five fragment constants and one structural correction factor, was developed for predicting at temperatures ranging from 10 to . The model was validated as successful by statistical analysis and external experimental data. Compared to other quantitative structure–property relationship methods, the present model has the advantage that it is much easier to implement. As aromatic compounds that contain C, H, O, Cl, and Br atoms, were included in the training set used to develop the model, the current fragment model applies to a wide range of chlorinated and brominated aromatic pollutants, such as chlorobenzenes, polychlorinated naphthalenes, polychlorinated biphenyls, polychlorinated dibenzo- -dioxins and dibenzofurans, polycyclic aromatic hydrocarbons, and polybrominated diphenyl ethers, all of which are typical persistent organic pollutants. Further study is necessary to expand the utility of the method to all halogenated aliphatic and aromatic compounds.
35(2006); http://dx.doi.org/10.1063/1.2201867View Description Hide Description
(methyltrichlorosilane) (MTS) is one of the most important precursors for manufacturing both an oxidation resistant SiC coating and a functional SiC film by chemical vapor deposition(CVD). In order to analyze the decomposition products of MTS with a thermodynamic calculation, correct thermodynamic data must be obtained from the authoritative data sources. G3(MP2) has been applied to evaluate the thermodynamic data of MTS(gas). The calculated value of the Gibbs energy of formation, , compares with a value, from the 4th edition of the NIST-JANAF Thermochemical Tables. Further analyses have been conducted: (1) by using G3, G3//B3LYP, and G3(MP2)//B3LYP theories; (2) by using variable scale factors for G3(MP2) theory; and (3) by investigating the accuracy of both experimental and calculated thermodynamic data. The calculated values can provide values for MTS above . The final fitted equation for MTS(gas) is: where is absolute temperature.
35(2006); http://dx.doi.org/10.1063/1.2203354View Description Hide Description
The recommended liquid–liquid equilibrium (LLE) data for 19 binary 1-alkanol–water systems have been obtained after critical evaluation of all data (527 data sets) reported in the open literature up to the end of 2004. An equation for prediction of the 1-alkanol solubility was developed. The predicted 1-alkanol solubility was used for calculation of water solubility in the second liquid phase. The LLE calculations were done with the equation of state appended with a chemical term proposed by Góral. The recommended data were presented in the form of individual pages containing tables and all the references.
35(2006); http://dx.doi.org/10.1063/1.2213629View Description Hide Description
A multiparameter viscosityequation for propane, valid in wide temperature and pressure ranges, was developed through an optimization technique for the functional form. The obtained results are very satisfactory, showing an average absolute deviation of 0.28% for the currently available 1024 primary data points. This is a significant improvement with respect to the reference equation available in the literature. As usual, both the development and the evaluation of the viscosityequation requires a highly accurate equation of state in order to convert the independent variables used for the experimental data, in most applications, , into the independent variables of the viscosityequation,. The heuristic technique used to develop the equation allows to select consistent data sets and thus it is a powerful tool for screening the available experimental data. The present limit for the accuracy achievable in the representation of the viscosity surface of a pure fluid is set by the uncertainty level of the experimental data rather than by the effectiveness of the proposed modeling method.
Prediction of Enthalpy of Formation in the Solid State (at ) using Second-Order Group Contributions. Part 1. Carbon-Hydrogen and Carbon-Hydrogen-Oxygen Compounds35(2006); http://dx.doi.org/10.1063/1.2203111View Description Hide Description
A predictive method, based on Benson’s group additivity technique, is developed for calculating the enthalpy of formation in the solid phase, at , of carbon-hydrogen compounds and carbon-hydrogen-oxygen compounds. A complete database compiles 398 experimental enthalpies of formation. The whole group contribution values, ring strain corrections, and nonnearest neighbor interactions evaluated are listed. Finally a comparison with Cohen’s method indicates that this new estimation method leads to higher precision and reliability.