Crystallisation kinetics of some archetypal ionic liquids: isothermal and non-isothermal determination of the Avrami exponent

Abstract

The properties of ionic liquids give rise to applications in diverse technology areas including mechanical engineering, mining, aerospace and defence. The arbitrary physical property that defines an ionic liquid is a melting point below 100 °C, and as such, an understanding of crystallisation phenomena is extremely important. This is the first report dealing with the mechanism of crystallisation in ionic liquids. Assuming crystallisation of the ionic liquids is a thermal or mass diffusion-controlled process, the values of the isothermal Avrami exponent obtained from three different ionic liquids with three different anions and cations all indicate that growth occurs with a decreasing nucleation rate (n=1.8-2.2). For one of the ionic liquids it was possible to avoid crystallisation by fast cooling and then observe a devitrification upon heating through the glass transition. The isothermal Avrami exponent of devitrification suggested growth with an increasing nucleating rate (n=4.1), compared to a decreasing nucleation rate when crystallisation occurs on cooling from the melt (n=2.0). Two non-isothermal methods were employed to determine the Avrami exponent of devitrification. Both non-isothermal Avrami exponents were in agreement with the isothermal case (n=4.0-4.15). The applicability of JMAK theory suggests that the nucleation event in the ionic liquids selected is a random stochastic process in the volume of the material. Agreement between the isothermal and non-isothermal techniques for determining the Avrami exponent of devitrification suggests that the pre-exponential factor and the activation energy are independent of thermal history. The heating rate dependence of the glass transition enabled the calculation of the fragility index, which suggests that the ionic liquid is a "strong" glass former. This suggests that the temperature dependence of the rate constant could be close to Arrhenius, as assumed by JMAK theory. More generally, therefore, it can be concluded that there is nothing unusual about the crystallisation mechanism of the ionic liquids studied here.

Measurement and correlation of electrical conductivity of ionic liquid [EMIM][DCA] in propylene carbonate and γ-butyrolactone

“…Great attention has been given to ILs due to their outstanding physicochemical properties including low melting temperature, good solvation, negligible vapor pressure, high electrical conductivity, wide electrochemical window and high thermal stability [2,3]. ILs have been successfully applied to many processes, such as organic synthesis and catalysis [4,5], separation processes [6][7][8][9], gas and liquid chromatography [10,11], gas capture [12] and electrochemistry [3,13,14]. Because of their excellent electrochemical characteristics, ILs have been used as electrolytes in fuel cells, double layer capacitors, solar cells and lithium batteries [15][16][17][18][19].…”

Crystallization Kinetics of Concurrent Liquid–Metastable and Metastable–Stable Transitions, and Ostwald’s Step Rule

“…Dating back to 1940s, the Kolmogorov, Mehl, Johnson, and Avrami (KMJA) phenomenological model 14−16 or the Avrami Model began to be developed for analyzing the transition kinetics of two phases. This model was broadly used in various systems, such as metallic alloys, 17 a quasicrystal, 18 and an ionic liquid, 19 and continuous efforts have been made to improve the Avrami Model to better explain relevant phase transitions involved in some specified system. 20 For instance, an extension of the KMJA model was used by Wette et al 21 to describe the competition between the wall crystallization and bulk crystallization for a colloid system.…”

Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids

“…For some archetypal ILs it has been shown [6] that is reasonable to assume C (T ) ≈ constant, as proved from the fairly linearity of a plot like that reported in Fig. 8.…”

“…(2) can be verified by checking the linearity of a plot of ln − ln (1 − χ) vs. ln (t). Supposing a diffusion controlled growth, the slope of the line, the Avrami exponent, should have the following values: n = 1.5, in the limit of zero nucleation rate; n < 2.5 for a decreasing nucleation rate; n = 2.5 in the case of constant nucleation rate; n > 2.5 for an increasing nucleation rate [6]. Figure 7 shows the plot of ln − ln (1 − χ) vs. ln (t) obtained from an isothermal measurement carried out with the DMA at 200 K (reached on heating, after cooling the sample at 4 K/min down to 150 K).…”

“…Since a low melting point is central in ILs, an understanding of the crystallization phenomena is extremely important. A study of the factors that influence the crystallization kinetics has been carried out in some archetypal ILs [6].…”