ACS PRF | ACS
All e-Annual Reports
42286-G6
A Comparative Study of Solvation Dynamics in Different Classes of Room-Temperature Ionic Liquids
Progress for the period between September 1, 2006 and August 31, 2007 has been significant. The primary objective of the project, a detailed comparison of the solvation dynamics of different types of ionic liquids, has been achieved and has offered interesting insights on the relationship between the chemical structure and solvation behavior of a room-temperature ionic liquid. In comparing the ionic motions responsible for the photophysical response of the liquid to the excitation of a chromophore, we have confirmed that the rovibrational response of large, flexible ions is much larger than that of small, rigid species. More importantly, it has been shown that the sub-picosecond dynamics of ionic liquids are dominated by the translational response of ions, and that collective motion dominates behavior even on sub-picosecond timescales. This result stands in marked contrast to the dominance of inertial rovibrational motion known in molecular liquids, and offers considerable insight on the relationship between the chemical structure of an ionic liquid and its role in determining the kinetics of reaction.
An unintended consequence of this insight has been the development of a new line of research into the electrostatics of solute-solvent interactions. The dominance of ionic translational response suggested that simple, primitive models of molten salts would be adequate to the description of the basic features of solvation in a molecular ionic liquid. A modified version of Debye-Hückel theory was constructed to obtain an analytical theory describing the distribution of charge about a molecular dipole in an ionic solution. This theory, confirmed in simulation, offers a key insight: The electrostatic energy of interaction between a molecular solute and an ionic solvent depends strongly on the solvent charge density.
This prediction was confirmed by comparison of experimental energies for solute-solvent electrostatic interactions (estimated from Kamlet-Taft theory) with the experimentally-known molar volumes of ionic liquids. This connection represents the first structure-property relationship capable of interpreting the relative polarities of two ionic liquid solvents. This result has been parlayed into a predictive method using a recently-developed semi-empirical scheme for the estimation of ionic liquid molar volumes [C. Ye and J. M. Shreeve, J. Phys. Chem. A, 111 4755 (2007)]. The scheme, which can be implemented with a pocket calculator in a matter of minutes, makes it possible to estimate the relative polarity of two ionic liquid solvents. This is a practical guide to researchers seeking to design ionic liquids of a particular character, and represents a milestone for the field.
