Nanostructuration Effect on the Thermal Behavior of Ionic Liquids

Abstract

This work shows how the nanostructuration of ionic liquids (ILs) governs the glass and melting transitions of the bistriflimide imidazolium-based [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series, which highlights the trend shift that occurs at the critical alkyl size (CAS) of n=6. An initial increase in the glass temperature (Tg ) with an increase in the alkyl side chain was observed due to the intensification of the dispersive interactions (van der Waals). Above the CAS, the -CH2 - increment has the same effect in both glass and liquid states, which leads to a plateau in the glass transition after nanostructuration. The melting temperature (Tm ) of the [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series presents a V-shaped profile. For the short-alkyl ILs, the -CH2 - increment affects the electrostatic ion pair interactions, which leads to an increase in the conformational entropy. The -CH2 - increment disturbs the packing ability of the ILs and leads to a higher entropy value (ΔslSm○ ) and consequently a decrease in Tm . Above the CAS, the -CH2 - contribution to the melting temperature becomes more regular, as a consequence of the nanostructuration of the IL into polar and nonpolar domains. The dependence of the alkyl chain on the temperature, enthalpy, and entropy of melting in the ILs above the CAS is very similar to the one observed for the alkane series, which highlights the importance of the nonpolar alkyl domains on the ILs thermal behavior.

Magnitude of Dynamically Correlated Molecules as an Indicator for a Dynamical Crossover in Ionic Liquids

“…In this context, it is worth noting that this statement agrees with the results reported by Koperwas et al and Grzybowski et al Namely, in the mentioned works, the authors observed that reducing the intermolecular distances as a result of compressing the system also causes its homogenization, i.e., a decrease in N α . It should be stressed that we do not observe visible changes in N α D ( T g ) values around n = 6 for [C n C 1 im][TCM] ( n = 2, 4, 6, 8) and [C n C 1 im][NTf 2 ] ( n = 4, 8, 10) when, as was reported in the literature, 24 , 32 , 33 the investigated ILs begin to form nanoscale aggregates (local microseparation leads to the formation of segregated polar and apolar domains in the bulk).…”

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

“…Blokhin et al (2006) and Rodrigues and Santos (2016) have continuously presented high quality thermal properties data of ILs and the latter demonstrated how the nanostructuration of [C n C 1 im][NTf 2 ] and [C n C n im][NTf 2 ] family influences their glass and melting temperatures. The work demonstrates a variation of the Tg behavior at the critical alkyl size, when n = 6 (Figure 6).…”

“…Glass transition temperatures (T g ) for the [C n C 1 im][Ntf 2 ] (black circles) and [C n C n im][Ntf 2 ] (red squares) series as a function of n (number of carbon atoms in one alkyl side chain); [C 1 C 1 im][Ntf 2 ] (gray circle). (Reprinted from (Rodrigues and Santos, 2016) with permission from John Wiley and Sons).…”

The effect of n vs. iso isomerization on the thermophysical properties of aromatic and non-aromatic ionic liquids

“…An uncertainty of 10 J·K -1· mol -1 was estimated for ΔslgsCp,m∘. Literature data: [ b ][50],[ c ][48],[ d ][49] (differential scanning calorimetry), [ e ][36]; [ f ][51] (adiabatic calorimetry); [ g ] ratio between T g / T m . [ h ] The ration between the enthalpy of melting (isotropization) and this transition is in order of 13%, which indicate that this transition could correspond to a solid to crystal liquid phase transition.…”