High Pressure as a Key Factor to Identify the Conductivity Mechanism in Protic Ionic Liquids

Z. Wojnarowska, Y. Wang, J. Pionteck, K. Grzybowska, A. P. Sokolov, and M. Paluch

Phys. Rev. Lett. 111, 225703 – Published 26 November 2013

Abstract

In this Letter we report the relation between ionic conductivity and structural relaxation in supercooled protic ionic liquids (PILs) under high pressure. The results of high-pressure dielectric and volumetric measurements, combined with rheological and temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties under isothermal and isobaric conditions for three PILs with different charge transport mechanisms (Grotthuss vs vehicle). Our findings indicate a breakdown of the fractional Stokes-Einstein relation and Walden rule when the ionic transport is controlled by fast proton hopping. Consequently, we demonstrate that the studied PILs exhibit significantly higher conductivity than one would expect taking into account that they are in fact a mixture of ionic and neutral species. Thus, the examined herein samples represent a new class of “superionic” materials desired for many advanced applications.

Received 19 May 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.225703

Authors & Affiliations

Z. Wojnarowska¹, Y. Wang², J. Pionteck³, K. Grzybowska¹, A. P. Sokolov^2,4, and M. Paluch¹

¹Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
²Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, D-01069 Dresden, Germany
⁴Department of Chemistry, University of Tennessee Knoxville, Knoxville, Tennessee 37996, USA

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Vol. 111, Iss. 22 — 27 November 2013

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Figure 1
(a) Dielectric loss modulus spectra due to conductivity relaxation at various temperatures in the liquid and glassy states for carvedilol HCl; (b) and (c) present the temperature dependence of viscosity, conductivity relaxation time, and structural dynamics of carvedilol salts. Open circles: conductivity relaxation time ( $τ_{σ}$ ). Open stars: ( $τ_{α}$ ) measured by TMDSC. Solid squares: viscosity ( $η$ ). The solid lines are Vogel-Fulcher-Tammann fits to the experimental data. $τ_{σ}$ shows weaker temperature dependence below $T_{g}$ . Insets: relation of $τ_{σ}$ to $η$ . The $τ_{σ}$ in the glassy states of C-HCl (crossed circles) were estimated by horizontal shift of the $σ$ peak from the region above $T_{g}$ to the temperatures below $T_{g}$ .Reuse & Permissions
Figure 2
(a) Isothermal dielectric measurements of conductivity relaxation time of carvedilol dihydrogen phosphate. The inset presents the temperature dependence of specific volume $V_{s p}$ in the pressure range of 10–200 MPa for C phosphate. The data above $T_{g}$ are fitted by the Tait equation [27, 28] (solid lines). (b) Open circles and triangles denote isothermal dielectric measurements of the conductivity relaxation time of C phosphate and C-HCl, respectively. Open squares denote the pressure dependence of structural relaxation time of C base. The values of $Δ V$ were calculated at $P = 0.1 MPa$ for each isotherm. The inset presents the pressure dependence of the $T_{g}$ temperature for C compounds. Solid lines are fits of the Andersson equation [29] to the experimental data. $T_{g} (P_{g})$ values for C salts were taken from the isoviscosity line while the data for the C base was calculated as $P_{g} = P$ ( $τ_{α} = 1000 s$ ).Reuse & Permissions
Figure 3
Test of the fractional Debye-Stokes-Einstein relation (fDSE) at ambient and elevated pressures for carvedilol salts (with Grotthuss conducting mechanism) [panels (a) and (b)] and verapamil HCl (vehicle mechanism) [panel (c)]. Closed triangles, diamonds, and circles indicate the data collected at ambient pressure while open symbols are the data recorded under high pressure. Solid lines are determined using the pressure dependence of viscosity generated by the Avramov entropic model.Reuse & Permissions
Figure 4
Walden plot under high pressure conditions for C salts. Closed symbols: isobaric measurements. Open symbols: isothermal measurements. The ideal line is constructed from the data of KCl aqueous solution. The insets present the molar conductivity and viscosity as a function of $1 / T$ for C phosphate (solid circles) and C-HCl (solid triangles), respectively.Reuse & Permissions

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