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Molecular dynamics study of the effect of flexibility over the melting point of ionic liquids

Grant number: 19/04785-9
Support Opportunities:Scholarships abroad - Research Internship - Post-doctor
Effective date (Start): August 01, 2019
Effective date (End): January 31, 2020
Field of knowledge:Physical Sciences and Mathematics - Chemistry - Physical-Chemistry
Principal Investigator:Mauro Carlos Costa Ribeiro
Grantee:Kalil Bernardino
Supervisor: Edward J Maginn
Host Institution: Instituto de Química (IQ). Universidade de São Paulo (USP). São Paulo , SP, Brazil
Research place: University of Notre Dame, United States  
Associated to the scholarship:17/12063-8 - Non-equilibrium molecular dynamics of ionic liquids, BP.PD

Abstract

One of the most fundamental properties of the ionic liquids (ILs) is the melting point.Although several relations can be made between the chemical structure of the ions and the melting point, their accurate prediction and explanation for ILs is far from trivial. Computer simulations are valuable tools in order to give molecular level insights over several physical properties of the ILs, however, the observation of phase transitions in molecular dynamics is a challenging task since the time scale accessible, between nanoseconds to a few microseconds, is too short to enable the melting of a crystal upon heating in the correct melting point, being the latter largely overestimated. The observation of spontaneous freezing is even harder, since the nucleation step is too slow and the liquid will probably be trapped in a vitreous state bellow the melting point instead of forming the desired crystal. In order to avoid this problem, thermodynamics based methods relies in the calculation of the free energy difference between the two phases without the need to observe a realistic process of melting or freezing and the melting point can be obtained by calculating the temperature where the free energy difference goes to zero. Between those methods, the pseudosupercritical path method (PSCP), as proposed by the Professor Edward Maginn, was showed to be a reliable way to determine the melting point in computer simulations of ILs with reasonable agreement with experimental data. In this project, two different imidazolium based ILs ([C2MIM][PF6] and [C4MIM][PF6]) will be studied with and without the use of external potential torestrain the conformations of the alkyl chains in order to determine quantitatively the effect of the flexibility of alkyl chain over the melting point and also over the thermodynamic functions differences in the phase transition. Since in the liquid phase a mixture of conformations is expected, when the restraint potential is applied to force a given conformation, a reductionover the entropy is expected and the melting point will increase. However, no study was done so far to determine how large should be this increase, which would be a direct effect of the flexibility of the alkyl chains over the melting. Also, different from the entropy, is not obvious a priori how the enthalpy of melting will respond to the loss of flexibility. This study can also helpin the understanding of the formation of different polymorphs when a IL freeze, being the difference between the polymorphs often related to the conformation of an alkyl chain. The PSCP method learned during the time abroad can also be applied in future investigations of other liquids of interest in our research group in Brazil.

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Scientific publications (6)
(References retrieved automatically from Web of Science and SciELO through information on FAPESP grants and their corresponding numbers as mentioned in the publications by the authors)
BERNARDINO, KALIL; RIBEIRO, MAURO C. C.. Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids - a molecular dynamics study. Physical Chemistry Chemical Physics, v. 24, n. 11, p. 14-pg., . (16/21070-5, 17/12063-8, 19/04785-9)
BERNARDINO, KALIL; RIBEIRO, MAURO C. C.. Relating the structure and dynamics of ionic liquids under shear by means of reverse non-equilibrium molecular dynamics simulations. Physical Chemistry Chemical Physics, v. 23, n. 25, p. 13984-13995, . (19/04785-9, 17/12063-8, 16/21070-5)
BERNARDINO, KALIL; RIBEIRO, MAURO C. C.. Confined ionic liquids films under shear: The importance of the chemical nature of the solid surface. Journal of Chemical Physics, v. 158, n. 9, p. 14-pg., . (17/12063-8, 19/04785-9, 16/21070-5)
BERNARDINO, KALIL; ZHANG, YONG; RIBEIRO, MAURO C. C.; MAGINN, EDWARD J.. Effect of alkyl-group flexibility on the melting point of imidazolium-based ionic liquids. Journal of Chemical Physics, v. 153, n. 4, . (16/21070-5, 17/12063-8, 19/04785-9)
BERNARDINO, KALIL; LIMA, THAMIRES A.; RIBEIRO, MAURO C. C.. Low-Temperature Phase Transitions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Dicyanamide. Journal of Physical Chemistry B, v. 123, n. 44, p. 9418-9427, . (16/21070-5, 17/12063-8, 14/15049-8, 19/04785-9)
BERNARDINO, KALIL; GOLOVIZNINA, KATERYNA; GOMES, MARGARIDA COSTA; PADUA, AGILIO A. H.; RIBEIRO, MAURO C. C.. Ion pair free energy surface as a probe of ionic liquid structure. Journal of Chemical Physics, v. 152, n. 1, . (16/21070-5, 17/12063-8, 19/04785-9)

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