Glass Transition Behavior of the Quaternary Ammonium-Type Ionic Liquid: N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium Bromide-H2O Mixtures

Abstract

By a simple differential thermal analysis (DTA) system, the concentration dependence of the glass transition temperatures (𝑇𝑔s) for the quaternary ammonium-type ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bromide [DEME][Br] and H2O mixtures, after quick precooling was measured as a function of water concentration 𝑥 (mol% H2O). We compared the results with the previous results of [DEME][I]-H2O and [DEME][BF4]-H2O mixtures in which a double-glass transition behavior was observed. Remarkably, the [DEME][Br]-H2O mixtures basically show one-𝑇𝑔 behavior and the 𝑇𝑔 decreases monotonically with increasing H2O content up to around 𝑥=91.5. But it suddenly jumps to higher 𝑇𝑔 value at a specific 𝑥=∼92. At this very limited point, two 𝑇𝑔s (𝑇𝑔1,𝑇𝑔2) which we might consider as a transition state from the structure belonging to the 𝑇𝑔1 group to another one due to the 𝑇𝑔2 group were observed. These results clearly reflect the difference in the anionic effects among Br−, I−, and BF4−. The end of the glass-formation region of [DEME][Br]-H2O mixtures is around 𝑥=98.9 and moves to more water-rich region as compared to those of [DEME][BF4]-H2O (𝑥=96.0) and [DEME][I]-H2O (𝑥=95.0) mixtures.