The Curious Case of 1-Ethylpyridinium Triflate: Ionic Liquid Exhibiting the Mpemba Effect

Abstract

Here, we report the results of qualitative and quantitative investigations of the first-order phase transition in the ionic liquid 1-ethylpyridinium triflate exhibiting a high variability of temperature ranges, within which the freezing and melting occur. By two methods, the direct fast quenching/annealing and the slow temperature-controlled differential scanning calorimeter, it is revealed that despite the almost constant absolute enthalpies of phase transition, the freezing occurs faster with the larger temperature contrast (cooling rate) between the initially hotter sample and the colder surrounding. This feature is a clear exhibition of the Mpemba effect. The regularity in the change of the melting point is analyzed as well.

Mpemba effect in crystallization of polybutene-1

“…The Mpemba effect was first systematically observed in freezing water and has recently been reported in various systems, including magnetic alloys, polymers, Clathrate hydrate, ionic liquids, spin glasses, and driven granular gases. [1][2][3][4][5][6][7][8][9][10][11] The Mpemba effect is considered to be a counterintuitive relaxation phenomenon, where an initially hot system cools down faster than an identical system initiated at a warm temperature when both are quenched by a cold bath that is at the same temperature, and the cold bath is large enough relative to either sample so that the temperature of the cold bath does not change. Based on the principle of entropy maximisation in systems that remain at or near thermal equilibrium, the entropy of the initially hot system should be greater than that of the warm one before quenching, and it is impossible to observe the Mpemba effect in a system that passes through all intermediate temperatures during slow cooling.…”