Faculty

Publications

Saivenkataraman Jayaraman and Edward J. Maginn. Computing the Melting Point and Thermodynamic Stability of the Orthorhombic and Monoclinic Polymorphs of the Ionic Liquid 1-n-Butyl-3-methylimidazolium Chloride. Journal of Chemical Physics, 127:214504, 2007. view abstract // link The melting point, enthalpy of fusion, and thermodynamic stability of two crystal polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride are calculated using a thermodynamic integration-based atomistic simulation method. The computed melting point of the orthorhombic phase ranges from 365 to 369 K, depending on the classical force field used. This compares reasonably well with the experimental values, which range from 337 to 339 K. The computed enthalpy of fusion ranges from 19 to 29 kJ/mol, compared to the experimental values of 18.5−21.5 kJ/mol. Only one of the two force fields evaluated in this work yielded a stable monoclinic phase, despite the fact that both give accurate liquid state densities. The computed melting point of the monoclinic polymorph was found to be 373 K, which is somewhat higher than the experimental range of 318–340 K. The computed enthalpy of fusion was 23 kJ/mol, which is also higher than the experimental value of 9.3−14.5 kJ/mol. The simulations predict that the monoclinic form is more stable than the orthorhombic form at low temperature, in agreement with one set of experiments but in conflict with another. The difference in free energy between the two polymorphs is very small, due to the fact that a single trans-gauche conformational difference in an alkyl sidechain distinguishes the two structures. As a result, it is very difficult to construct simple classical force fields that are accurate enough to definitively predict which polymorph is most stable. A liquid phase analysis of the probability distribution of the dihedral angles in the alkyl chain indicates that less than half of the dihedral angles are in the gauche-trans configuration that is adopted in the orthorhombic crystal. The low melting point and glass forming tendency of this ionic liquid is likely due to the energy barrier for conversion of the remaining dihedral angles into the gauche-trans state. The simulation procedure used to perform the melting point calculations is an extension of the so-called pseudosupercritical path sampling procedure. This study demonstrates that the method can be effectively applied to quite complex systems such as ionic liquids and that the appropriate choice of tethering potentials for a key step in the thermodynamic path can enable first order phase transitions to be avoided.

Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 So Soluble in Imidazolium-Based Ionic Liquids?. Journal of the American Chemical Society, 126:5300-5308, 2004. view abstract // link Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ionic liquids. CO2 absorption isotherms at 10, 25, and 50 C are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO2, while there is little difference in CO2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO2 about hexafluorophosphate anions, but only small differences in CO2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.

Edward J. Maginn. Atomistic Simulation of the Thermodynamic and Transport Properties of Ionic Liquids. Accounts of Chemical Research, 40:1200-1207, 2007. view abstract // link Atomistic simulations have emerged in recent years as an important compliment to experiment for understanding how the properties of ionic liquids are controlled by their underlying chemical structure. The ability to obtain reliable thermodynamic and transport properties from a simulation depends both on the quality of the force field and on the use of a proper simulation method. Properties such as densities and heat capacities may be obtained readily using standard techniques. With more effort and advanced simulation methods, solid–liquid and vapor–liquid phase equilibria may also be determined. Transport properties can also be computed, but the notoriously slow dynamics of many ionic liquid systems means that great care must be taken to ensure that the simulations are accurate.

Manish S. Kelkar, Jake L. Rafferty, Edward J. Maginn and J. Ilja Siepmann. Prediction of Viscosities and Vapor-Liquid Equilibria for Five Polyhydric Alcohols by Molecular Simulation. Fluid Phase Equilibria, 260:218-231, 2007.

Wei Shi, and Edward J. Maginn. Atomistic Simulation of the Absorption of Water in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]). Journal of Physical Chemistry B, 112:2045-2055, 2008. view abstract // link The solubility of water and carbon dioxide in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) is computed using atomistic Monte Carlo simulations. A newly developed biasing algorithm is used to enable complete isotherms to be computed. In addition, a recently developed pairwise damped electrostatic potential calculation procedure is used to speed the calculations. The computed isotherms, Henry's Law constants, and partial molar enthalpies of absorption are all in quantitative agreement with available experimental data. The simulations predict that the excess molar volume of CO2/ionic liquid mixtures is large and negative. Analysis of ionic liquid conformations shows that the CO2 does not perturb the underlying liquid structure until very high CO2 concentrations are reached. At the highest CO2 concentrations, the alkyl chain on the cation stretches out slightly, and the distance between cation and anion centers of mass increases by about 1 Å. Water/ionic liquid mixtures have excess molar volumes that are also negative but much smaller in magnitude than those for the case of CO2.

Manish S. Kelkar and Edward J. Maginn. Effect of Temperature and Water Content on the Shear Viscosity of the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide As Studied by Atomistic Simulations. Journal of Physical Chemistry B, 111:4867-4876, 2007. view abstract // link Atomistic simulations are conducted to examine the dependence of the viscosity of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on temperature and water content. A nonequilibrium molecular dynamics procedure is utilized along with an established fixed charge force field. It is found that the simulations quantitatively capture the temperature dependence of the viscosity as well as the drop in viscosity that occurs with increasing water content. Using mixture viscosity models, we show that the relative drop in viscosity with water content is actually less than that that would be predicted for an ideal system. This finding is at odds with the popular notion that small amounts of water cause an unusually large drop in the viscosity of ionic liquids. The simulations suggest that, due to preferential association of water with anions and the formation of water clusters, the excess molar volume is negative. This means that dissolved water is actually less effective at lowering the viscosity of these mixtures when compared to a solute obeying ideal mixing behavior. The use of a nonequilibrium simulation technique enables diffusive behavior to be observed on the time scale of the simulations, and standard equilibrium molecular dynamics resulted in sub-diffusive behavior even over 2 ns of simulation time.

Awards

John A. Kaneb Award

Given on January 1, 2001 by University of Notre Dame

Outstanding New Faculty Award

Given on July 12, 1998 by American Association of Engineering Education and Dow Chemical

College of Engineering Teacher of the Year

Given on May 1, 2006 by BP

NSF Faculty Early Career Development (CAREER) Award

Given on January 1, 1997 by National Science Foundation

John A. Kaneb Award

Given on January 1, 2006 by University of Notre Dame

Courses

• CBE 20256 - Chemical Engineering Thermodynamics - This course covers the basic concepts of chemical engineering thermodynamics. It includes a review of the First Law (which was originally covered in CBE 20255), and an introduction to the Second ... more >
• CBE 40443 - Separation Processes - This course demonstrates the application of the principles of phase equilibria, transport processes, and chemical kinetics to the design and characterization of stagewise and continuous separation ... more >
• CBE 40448 - Chemical Process Design - This course represents a capstone in the chemical engineering curriculum. In this course students will have the opportunity to apply the basic concepts learned in previous courses to the design and... more >
• CBE 40485 - Biological Thermodynamics - This course provides a survey of the use and application of classical and statistical thermodynamics to biological systems. It covers how the First and Second Laws can be applied to living syste... more >
• CBE 60553 - Advanced Chemical Engineering Thermodynamics - This course is focused on an advanced treatment of thermodynamic concepts. An introduction to molecular thermodynamics is given, followed by detailed treatments of phase equilibrium, equation-of-st... more >

Images

	Location of water in the tunnel of the proton-exchanged form of a crystalline titanosilicate ion exchanger. The waters form a hydrogen bondng network with both the lattice and each other.	This image shows the computed principal axes for three different pyridinium-based ionic liquid cations, as well as the bistrifylimide anion.
In collaboration with Steve Leone's group in the Departments of Chemistry and Physics at Berkeley, we have used molecular dynamics simulations along with their photoionization experiments to probe the nature of the vapor phase of ionic liquids. This is an image from a simulation of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The experiments and simulations indicate that the volatile species is single ion pairs and not larger clusters or non-neutral species.