Branched and cyclic alkyl groups in imidazolium-based ionic liquids: Molecular organization and physico-chemical properties

• Novel [C4C5Im][Tf2N] ionic liquids with a variation in structure of a C5 alkyl group were synthesised for the first time and characterized as to their thermophysical and transport properties.
• Branching of the alkyl chain has no influence on the ionic liquid density.
• Isopentyl and cyclopentyl groups increase the ionic liquid viscosity.
• Suprisingly the cyclopenty-substituted ionic liquid shows the highest conductivity in spite of its high viscosity. This however is in agreement with a higher degree of its ion dissociation found from the experimental self-diffusion coefficients.

In the present work, three imidazolium bis{(trifluoromethyl)sulfonyl}imide ionic liquids with a linear alkyl chain (1-pentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [C4C5Im][Tf2N]), with branching (1-isopentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide [C4iC5Im][Tf2N]), or with a cyclic group (1-cyclopentyl-3-butylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [C4cC5Im][Tf2N]) were synthesized for the first time. Such variation in structure could be of interest to possible applications of the present ionic liquids as task-specific ionic liquids. To understand the influence of the structure on their macroscopic properties, their density and two transport properties (viscosity and electrical conductivity), were measured as function of temperature. The introduction of branching has no effect on the density values whereas these show an increase when a cycle is substituted to the cation. Both branching of the alkyl chain and the presence of the cyclic group increase the viscosity. Surprisingly, the electrical conductivity of [C4cC5Im][Tf2N] is the highest despite its high viscosity. From the measurement of ion diffusion coefficients by Nuclear Magnetic Resonance (NMR) spectroscopy, the ionicity could be evaluated. The cycloalkane-substituted [C4cC5Im][Tf2N] was found to be more dissociated compared to the other two ionic liquids which explains the behaviour observed for the conductivity. This could mean that the cycloalkane-substituted ionic liquid might find its use in electrochemistry. The experimental viscosity, conductivity, and diffusion coefficient data were described by equations of the Vogel–Tamman–Fulcher type using a regression along a gnostic influence function. In this way, a robust global optimization algorithm could be used to obtain reliable model parameters, but also to critically evaluate the experimental data.

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