Jeffrey Errington of the State University of New York at Buffalo is supported by an award from the Theory, Models and Computational Methods program in the Chemistry division to carry out the development of molecular simulation methods to compute interfacial properties of electrolytes. The award is co-funded by the Interfacial Processes and Thermodynamics program in CBET. The PI and his research group develop free energy-based methods to study interfacial properties of a system via connection to rigorous thermodynamic relationships. The group pursues two approaches. The first technique focuses on the evaluation of contact angles and solid-fluid interfacial tensions via determination of the surface density dependence of the so-called surface excess free energy of a system. The free energy approach has a number of advantages over the commonly-used nanodroplet route to the contact angle. The second method focuses on the determination of liquid-vapor surface tensions via area sampling techniques, which deduce the interfacial tension via measurement of the change in system free energy upon variation of the interfacial area. The research involves extending the use of transition matrix Monte Carlo methods to compute free energies to the study of electrolytes.
Numerous technologies (e.g. batteries, fuel cells) contain an electrolytic fluid in contact with one or more solid surfaces. In many cases the performance of such devices is influenced significantly by the properties of the electrolyte-solid interface. It follows that the design of these applications benefits from fundamental knowledge regarding the relationship between microscopic substrate-fluid and fluid-fluid interactions within a system and the macroscopic properties it exhibits. Information of this type provides insight into how the chemistry and/or structure of a substrate can be tuned to obtain a desired behavior.
Both undergraduates and graduate students are involved in this research. The PI has initiated an outreach program to middle school students that focuses on lithium-ion batteries and how they work.
Overview The goal of this project was to develop and implement novel computational methods for studying electrolytes (e.g., room temperature ionic liquids, molten salts, charged colloids) in the vicinity of a solid surface. These fluids are used in a number of emerging technologies, including solar cells, supercapacitors, antimicrobial coatings, paper coatings, and within electronic and sensor applications. We pursued an approach that is based upon the so-called interface potential. The method provides advantages in terms of the quality and quantity of information that can be obtained from molecular simulation. From the potential, scientists and engineers can readily deduce macroscopic interfacial properties of interest, such as the contact angle of a droplet (see Figure 1) and interfacial tensions. The interface potential has traditionally proven difficult to calculate, and therefore this valuable technique experienced limited use until recently. We focused our attention on the development of strategies that increase the capabilities of this approach, with the aim of providing a tool that can be applied to a broad range of complex systems. The interfacial behavior of a system is intimately linked to its bulk phase behavior, and therefore one typically needs to understand well the latter before studying the former. As a result, we invested considerable efforts in developing methods to determine the bulk properties of ionic fluids with complex intramolecular topologies. These methods were applied to study the properties of a simple model for ionic fluids as well as a realistic model for a room temperature ionic liquid. In what follows below, we highlight a couple of results and discuss how our results were disseminated and how students benefitted from this project. Bulk Phase Behavior of Room Temperature Ionic Liquids (RTILs) We studied the liquid-vapor saturation properties of the 1-alkyl-3-methyl imidazolium bis (trifluoromethylsulfonyl) imide [Cnmim][Ntf2] series, where the n subscript denotes the length of the alkyl chain attached to the imidazolium ring. Figure 2 provides our data for the temperature dependence of the vapor pressure. The data span from room temperature to around 1000 K, which is slightly below the expected critical temperature of the fluids. These liquids have been suggested as potential green solvents, in part due to their extremely low vapor pressures. Estimating the vapor pressure of these molecules via experiment or simulation at room temperature has proven difficult. Experimental data have been collected at elevated temperatures of approximately 450 K. Comparison between our simulation data and experimental data in this region are provided within the inset. The agreement between our estimates and experimental data is excellent. These results provide optimism that molecular modeling can be used to quantitatively describe the bulk and interfacial properties of RTILs. Our calculations suggest that the vapor pressure of typical RTILs is approximately sixteen orders of magnitude lower than that of the standard atmosphere. New Methods for Computing Interfacial Properties As noted above, much of this project was directed toward the development of general rigorous methods that can be broadly used by the scientific community. The central construct that we compute is the interfacial potential, which provides the surface excess free energy of a thin fluid film in contact with a surface of interest as a function of the film thickness. Examples of this potential are provided in Figures 3 and 4. In one case, we compute the potential associated with a thin liquid film grown on a substrate in a vapor background. Such a potential is useful for studying fluids at relatively strong surfaces (e.g., water at a mineral surface). The second case is associated with the growth of a vapor film from a surface in a background liquid. This potential is useful for studying fluids at relatively weak surfaces (e.g., water at a superhydrophobic surface, such as a teflon-coated pan or rain jacket). These potentials provide a means to quantitatively determine macroscopic interfacial properties, such as the contact angle. Dissemination of Results The project produced fifteen published peer-reviewed journal articles, with a few additional articles expected as we wrap-up the remaining components of the study. In addition, two Ph.D. dissertations were produced from students associated with this project. Training of Students This project has had a significant impact on the training of scientists and engineers. Two of the students associated with this project completed PhD degrees in Chemical Engineering, a third student is close to finishing a PhD degree, and a fourth is making steady progress towards completing their degree. One of the graduates is now employed as a post-doctoral associate at the Massachusetts Institute of Technology. The second graduate recently began a post-doctoral position at Technische Universität Darmstadt.
