Nanostructure–Thermal Conductivity Relationships in Protic Ionic Liquids

Abstract

The thermal conductivities of nine protic ionic liquids (ILs) have been investigated between 293 and 340 K. Within this range, the thermal conductivities are between 0.18 and 0.30 W · m(-1) · K(-1). These values are higher than those typically associated with oils and aprotic ILs, but lower than those of strongly hydrogen bonding solvents like water. Weak linear decreases in thermal conductivity with temperature are noted, with the exception of ethanolammonium nitrate (EtAN) where the thermal conductivity increases with temperature. The dependence of thermal conductivity on IL type is analyzed with use of the Bahe-Varela pseudolattice theory. This theory treats the bulk IL as an array of ordered domains with intervening domains of uncorrelated structure which enable and provide barriers to heat propagation (respectively) via allowed vibrational modes. For the protic ILs investigated, thermal conductivity depends strongly on the IL cation alkyl chain length. This is because the cation alkyl chain controls the dimensions of the IL bulk nanostructure, which consists of charged (ordered domains) and uncharged regions (disordered domains). As the cation alkyl chain controls the dimensions of the disordered domains, it thus limits the thermal conductivity. To test the generality of this interpretation, the thermal conductivities of propylammonium nitrate (PAN) and PAN-octanol mixtures were examined; water selectively swells the PAN charged domain, while octanol swells the uncharged regions. Up to a certain concentration, adding water increases thermal conduction and octanol decreases it, as expected. However, at high solute concentrations the IL nanostructure is broken. When additional solvent is added above this concentration the rate of change in thermal conductivity is greatly reduced. This is because, in the absence of nanostructure, the added solvent only serves to dilute the salt solution.

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 1. Nanostructure

“…is the volume of the PAN hydrocarbon chain (107 Å 3 ), 51 These values are listed in Table 1. As the polar group of the n-alkanols (the alcohol moeity) is smaller than the polar part of a PAN ion pair (ammonium group plus nitrate ion), and the hydrocarbon group of the nalkanols are all larger than that of PAN, addition of butanol, octanol or dodecanol always increases the apolar vol : polar vol.…”

“…) is the volume of the PAN charged groups (68 Å 3 ), 51 (-OH) is the volume of the hydroxyl group (20 Å 3 ), 51 (-C 3 H 7 ) is the volume of the PAN hydrocarbon chain (107 Å 3 ), 51 and (-C n H 2n+1 ) is the volume of the n-alkanol group hydrocarbon chain as determined from the Tanford equation. 52 These values are listed in Table 1.…”

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 2. Nanotribology and fluid dynamics

“…ILs generally have low vapour pressures, 2,3 high thermal stabilities, 4 wide electrochemical windows, 5 good thermal conductivities, 6,7 and are non-flammable. 4 There is considerable capacity to vary the physicochemical properties of an IL to suit a particular application by changing ion structures.…”

Sub-μL measurements of the thermal conductivity and heat capacity of liquids

“…This shows that this relationship is probably general in the absence of strong intermolecular interactions, like for instance hydrogen bonds in water 51 and protic-ionic liquids. 52…”