Video-microscopic observation of ionic liquid/alcohol interface and the corresponding molecular simulation study
Description
This research is aimed at studying the ionic liquid/n-pentanol interface via video-microscopy and molecular dynamic simulations. Understanding the interfacial phenomena and interfacial transport between ionic liquids and other liquids is of interest to the development and application of ionic liquids in a number of areas. One such area is the biphasic hydroformylation of alkenes to obtain alcohol and aldehyde, in which case ionic liquid is the reaction medium where a catalyst resides. The dissolution of an ionic liquid into an alcohol was studied by microscopically observing and measuring the shrinking of a micropipette-produced droplet in real time. Although microscopic investigation of droplet dissolution has been studied before, no attempt had been made to measure the di↵usion coefficient D of the droplet species in the surrounding medium. A key finding of this work is that the Epstein-Plesset mathematical model, which describes the dissolution of a droplet/bubble in another fluid medium, can be used to measure D. Other experimental studies of the ionic liquid/alcohol system include electrical conductivity and UV-visible spectroscopy measurements of solutions of 1-hexyl-3-methylimidazolium tetrafluoroborate in n-pentanol. Those experiments were done in order to understand the molecular state of the particular ionic liquid in n-pentanol, as well as obtaining the dissociation constant K of such weak electrolyte solution. The experimental results provide an entry to the assessment of ionic liquid interaction with n-pentanol at molecular scale. Subsequently, molecular dynamics simulation was implemented for the investigation of such interaction. The computation started with simulation of the bulk phase of 1-butyl-3-methylimidazolium tetrafluoroborate, an affine ionic liquid on which molecular simulations had already been reported. A generalized probability based on Fuoss approximation for the closest ion to a distinguished countercharge ion was developed. In addition to 1-butyl-3- methylimidazolium tetrafluoroborate, the generalization was tested also on tetraethyl ammonium tetrafluoroborate in propylene carbonate from low to high concentrations, and on the corresponding primitive model. Such generalization helps us understand paring of ions in electrolyte solution, especially for elevated concentrations. Two cases of 1-hexyl-3-methylimidazolium tetrafluoroborate ionic liquid/npentanol system were studied, which are (i) liquid-liquid interface; and (ii) solution of the former in the latter. Computation of biphasic interface revealed interaction at the liquid-liquid junction, e.g., the transport of molecules from one phase to another, and lead to evaluation of di↵usion coefficient that has good agreement with experimental measurement. The simulation of dilute electrolyte solution, i.e., an ionic liquid pair in n-pentanol, gives free energy change as a function of ion separation distance. The dissociation constant K was evaluated and found to be closed to experimental value that was obtained from solution conductivity measurement. The investigation of ion dynamics, especially the memory function transformed from velocity autocorrelation function, lead to the finding of dielectric friction in the system. Furthermore, precise evaluation of D gives satisfied agreement with experimental measurement from micropipette technique.