TY - JOUR

T1 - Controlling the Melting Temperatures and Ion Dynamics of Carboxy-Functionalized Ionic Liquids

AU - Khudozhitkov, Alexander E

AU - Hunger, Lasse

AU - Al Sheakh, Loai

AU - Stepanov, Alexander G

AU - Kolokolov, Daniil I

AU - Ludwig, Ralf

N1 - This work was supported by has been supported by the Russian Science Foundation (grant no. 24-13-00129) and by DFG Research Grants LU-506/17-1, project no. 470038970 and LU-506/18-1, and project no. 517661181.

PY - 2025/11/6

Y1 - 2025/11/6

N2 - We measured the 2H spectra of carboxy-functionalized ionic liquids [Py-(CH2)n-COOD][NTf2] with alkyl chains n = 1,2,4,5,6,7,9 by means of NMR solid-state spectroscopy. The change from the broad Pake patterns in the glassy state to the narrow lines in the liquid with increasing temperatures indicated the glass-liquid phase transition and allowed the determination of the melting temperatures Tm of these unique ILs characterized by the formation of doubly hydrogen-bonded cationic dimers (c+═c+) present in the gas, as well as in the condensed phases. The melting occurs at 248 K, only 30 K above the glass transition temperature Tg. The Tm/Tg ratio is 1.13 and thus significantly smaller than the reported values for H-bonded molecular liquids such as water and alcohols. The small ratio is usually referred to the dynamical heterogeneities in the liquid material, which is illustrated by the simultaneously observed solid and liquid NMR spectra. Additionally, we measured the NMR relaxation times T1 and T2 that could be modeled by a slow isotropic and a fast anisotropic motion. The maximum between the two T1 minima coincides with the observed melting temperatures. The microscopic ILs viscosity reflected by the cation dynamics reaches optimal values for n > 4. Our results show that the doubly H-bonded carboxy-functionalized ILs provide low melting temperatures and thus might be suitable for electrolytes.

AB - We measured the 2H spectra of carboxy-functionalized ionic liquids [Py-(CH2)n-COOD][NTf2] with alkyl chains n = 1,2,4,5,6,7,9 by means of NMR solid-state spectroscopy. The change from the broad Pake patterns in the glassy state to the narrow lines in the liquid with increasing temperatures indicated the glass-liquid phase transition and allowed the determination of the melting temperatures Tm of these unique ILs characterized by the formation of doubly hydrogen-bonded cationic dimers (c+═c+) present in the gas, as well as in the condensed phases. The melting occurs at 248 K, only 30 K above the glass transition temperature Tg. The Tm/Tg ratio is 1.13 and thus significantly smaller than the reported values for H-bonded molecular liquids such as water and alcohols. The small ratio is usually referred to the dynamical heterogeneities in the liquid material, which is illustrated by the simultaneously observed solid and liquid NMR spectra. Additionally, we measured the NMR relaxation times T1 and T2 that could be modeled by a slow isotropic and a fast anisotropic motion. The maximum between the two T1 minima coincides with the observed melting temperatures. The microscopic ILs viscosity reflected by the cation dynamics reaches optimal values for n > 4. Our results show that the doubly H-bonded carboxy-functionalized ILs provide low melting temperatures and thus might be suitable for electrolytes.

U2 - 10.1021/acs.jpcb.5c05946

DO - 10.1021/acs.jpcb.5c05946

M3 - Article

C2 - 41146488

VL - 129

SP - 11557

EP - 11563

JO - Journal of Physical Chemistry B

JF - Journal of Physical Chemistry B

SN - 1520-6106

IS - 44

ER -