) Phase Diagram Determination and Relative Dielectric Constant Measurements of the Butyronitrile-Chloroethane System by Robin Beth Michnick B. S. School of Engineering at Columbia University (1987) New York, New York Submitted to the Department of Materials Science and Engineering in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY at the Massachusetts Institute of Technology Cambridge, Massachusetts January, 1995 © Massachusetts Institute of Technology, 1995 All rights reserved. Signature of Author LD - . i. a - Dept. f Mat. Sci. and Eng., January 1995 !, /, Certified by I Dofiald R. Soway, Prof/of Materials Chemistry, Thesis Supervisor / Accepted by Prof. C. V. Thompson, Chairman, De*t. Grad. Comm. ,CU.T-SNSTUE; OF - c OLOG JUL 2 01995 Se~v, | .r- PHASE DIAGRAM DETERMINATION AND RELATIVE DIELECTRIC CONSTANT MEASUREMENTS OF THE BUTYRONITRILE- CHLOROETHANE SYSTEM by Robin Beth Michnick Submitted to the Department of Materials Science and Engineering on January 13, 1995 in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Metallurgy. A systematic study of the physical chemistry of butyronitrile-chloroethane solutions was conducted with the intention of assessing their utility as electrolytic solvents for subambient electrochemistry. Both the solid-liquid phase diagram and the relative dielectric constants as a function of temperature and solution composition were measured. The solid-liquid phase diagram was determined by differential thermal analysis. Apparatus was designed with a low temperature limit near the normal boiling point of liquid nitrogen. This is approximately 20° colder than commercially available equipment. The subambient boiling point of the chloroethane required the development of new protocols to ensure compositional control of the solutions. The butyronitrile-chloroethane phase diagram is a simple eutectic, the eutectic point being -185°Cand 48 mole percent butyronitrile. The thermodynamics of the liquid phase were tested against a number of solution models and were found to be best represented by the Associated Solution Model. The electrical conductivity and relative dielectric constant of butyronitrile, chloroethane, and their solutions were determined by electrochemical impedance spectroscopy (EIS) over the temperature range spanning -35°Cto -105°C. The electrical conductivity exhibited Arrhenius behavior due to reduced ion mobility at lower temperatures. The relative dielectric constant varied linearly with inverse temperature, a behavior consistent with the greater alignment of the electric dipoles at reduced levels of thermal energy. The temperature dependence of the relative dielectric constant was fit to the Kirkwood model for dielectric media in order to determine the correlation factor of the molecules. On the basis of the quasi-chemical solution model and the compositional variation of the correlation factor, the following was inferred to be the molecular configuration of the liquid: an associated solution composed of uncomplexed butyronitrile and chloroethane, dimolecular self-associates of butyronitrile, and dimolecular complexes composed of one butyronitrile and one chloroethane. Thesis Supervisor: Donald R. Sadoway Professor of Materials Chemistry 2 A bstract .............................................................................................................................2 L ist of Figures .................................................................................................................. 5 List of Tables .................................................................................................................... 9 Sym bols ........................................................................................................................... 10 Acknowledgments ........................................................................................................ 12 1. Introduction ....................................................... 13 2. Phase Diagram ....................................................... 20 2.1 Theory ....................................................... 20 2.2 Experimental Methods ........................................ ............... 21 2.2.1 Thermal Methods ....................................... ................ 23 2.2.1.1 Thermometry ....................................................... 23 2.2.1.2 Differential Analysis ....................................... ................ 24 2.2.2 Experimental Design ........................................ ............... 26 2.2.2.1 Sample Holder ........................................ ............... 26 2.2.2.2 Thermocouples ........................................ ............... 28 2.3 Procedure ....................................................... 28 2.3.1 Experimental ....................................... ................ 28 2.3.2 Data Analysis ....................................... ................ 31 2.3.3 Temperature Error of Thermocouples ....................................... 32 2.3.4 Error of Phase Diagram Data ....................................................... 34 2.4 Discussion ....................................................... 35 2.4.1 Qualitative Analysis ....................................... ................ 35 2.4.2 Quantitative Analysis ....................................... ................ 38 2.4.3 Model of Phase Diagram ........................................ ............... 44 2.4.3.1 Excess Enthalpy Models ........................................ ............... 45 2.4.3.1.1 One Term Redlich-Kister Model .......................................... 45 2.4.3.1.2Hildebrand-Scatchard Regular Solution Model ............. 56 2.4.3.1.3Weimer-Prausnitz Polar Regular Solution Model .............59 2.4.3.1.4 Two Term Redlich-Kister Model ......................................... 64 2.4.3.1.5Z-Fraction Model ....................................................... 67 2.4.3.2 Excess Entropy Models ........................................ ............... 73 2.4.3.2.1 Flory Model ........................................ ............... 73 2.4.3.2.2 Associated Solution Model ................................................... 78 3. Dielectric Constant ....................................................... 87 3.1 Theory ....................................................... 88 3.1.1 Pure Polar Solvents ........................................ ............... 88 3 3.1.2 Mixture of Polar Solvents .................................................................. 93 3.2 Experimental Methods ........................................ .............. 94 3.2.1 Electrochemical Impedance Spectroscopy ..................................... 95 3.2.2 Experimental Design ........................................ .............. 99 3.2.2.1 Electrode Selection ........................................ .............. 99 3.2.2.2 Electrochemical Impedance Cell .......................................... 107 3.3 Procedure ...................................................... 110 3.3.1 Experimental ..................................................... 110 3.3.2 Data Analysis ................. ..................................... 114 3.3.3 Error Analysis ..................................................... 115 3.4 Discussion ...................................................... 124 3.4.1 Qualitative Analysis ...................................................... 124 3.4.1.1 Cell Constant Calibration and Confirmation ......................124 3.4.1.2 Relative Dielectric Constant Measurements ....................... 125 3.4.1.3 Conductivity Measurements ................................................. 129 3.4.2 Quantitative Analysis ...................................................... 130 3.4.2.1 Liquid Structure ..................................................... 133 3.4.2.1.1 Pure Butyronitrile ........................................ ............. 136 3.4.2.1.2 Pure Chloroethane ........................................ .............. 138 3.4.2.1.3 Solutions of Butyronitrile-Chloroethane ........................ 141 3.4.2.2 Summary of Liquid Structure ...................................................... 148 4. Conclusion ..................................................... 152 4.1 Contributions of Dissertation ........................................ ............. 152 4.2 Determination of Molecular Arrangements in the Liquid ..................154 4.3 Solvent Selection ...................................................... 156 4.4 Recommendations for a More Detailed Determination of the Liquid Structure and Thermodyanmics .................................................157 5. Appendices ...................................................... 159 A. Derivation of Van't Hoff Equation ......................... .......... .......... 159 B. Relative Dielectric Constant and Electrical Conductivity Data ........... 160 C. Purification of Solvents ...................................................... 161 D. Rationalized MKS and Gaussian CGS Unit Conversion Table ........... 162 E. Estimated Vapor Pressure ........................................ .............. 163 6. References ...................................................... 164 4 List of Figures Figure 1.1 Oxygen Pump ............................................ 14 Figure 1.2 Electrochemically Modulated Ba2YCu307_xat 500°C............. 15 Figure 2.2.1 Cooling Curve Sample ........................................ .... 23 Figure 2.2.2 DTA Schematic ...........................................