title: 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.
authors.affiliation: CIQ, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, P-, 4169-007, Porto, Portugal. asmcrodrigues@fc.up.pt.
authors.forename: Ana S M C
authors.initials: AS
authors.lastname: Rodrigues
authors.affiliation: CIQ, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, P-, 4169-007, Porto, Portugal. lbsantos@fc.up.pt.
authors.forename: Luís M N B F
authors.initials: LM
authors.lastname: Santos
keywords: glasses; ionic liquids; melting transition; nanostructures; phase behavior