Molecular insight into the microstructure and microscopic dynamics of pyridinium ionic liquids with different alkyl chains based on temperature response

Abstract

The temperature dependence of the microstructure and microscopic dynamics of three N-alkylpyridinium tetrafluoroborate ionic liquids, namely, [BPy][BF₄], [HPy][BF₄], and [OPy][BF₄], is investigated via molecular dynamics simulation. Under a temperature change, the microstructure changes of [OPy][BF₄] with a longer alkyl chain show a slight difference from that of [BPy][BF₄] and [HPy][BF₄] in terms of the radial and spatial distribution functions. A temperature increase is found to be advantageous to the aggregation of the polar regions as well as the nonpolar regions in the pyridinium ionic liquids. However, the properties of the microscopic dynamics of the three ionic liquids regularly change under conditions with temperature change. The results show a decreased probability of ions to reverse the direction of their translation motion because the negative parts of the velocity autocorrelation functions increase with the temperature. The temperature dependence of rotational dynamics is more obvious than that of translation. A rise in temperature causes rotational autocorrelation functions to decay quickly, and it considerably reduces relaxation time. With an increase in alkyl chain length, the rotational correlation improves and the relaxation time decreases. The time correlation functions show that the association dynamics of nonpolar regions is faster than that of polar regions over time. The association time is short for long-tailed ionic liquids.