Mixtures of octanol and an ionic liquid: Structure and transport

Ionic liquids (ILs) with long alkyl substituents are amphiphilic, which leads to a bicontinuous liquid structure. The strongly interacting anionic and cationic head groups form a long range charge network, with the hydrocarbon tails forming a nonpolar domain. Such nonpolar domains have been shown to dissolve a variety of neutral organic solvents. In mixtures of ILs with solvents the neutral organic molecules residing in the nonpolar domains experience different environments and friction from the charged cations and anions. Thus, the neutral molecules diffuse much faster than predicted by hydrodynamic scaling using the average viscosity of the mixture. In this work, we report studies on the structure and transport properties of mixtures of 1-octanol with the IL trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (P_6,6,6,14 ⁺/NTf₂ ^-). The majority of the atom fraction in the P_6,6,6,14 ⁺ cation comprises four hydrocarbon substituents. The unique amphiphilic nature of ILs with the P_6,6,6,14 ⁺ cation makes 1-octanol fully miscible with the IL at ambient temperatures. X-ray scattering experiments show that the IL structure persists in the mixtures for 1-octanol mole fractions as large as x_oct = 0.90. The self-diffusion coefficients of the three molecular species in the mixtures were measured by NMR experiments. The self-diffusion of the P_6,6,6,14 ⁺ cation is well described by the Stokes-Einstein equation, while the diffusivity of the NTf₂ ^- anion is slightly lower than the hydrodynamic prediction. The measured diffusivities of octanol in these mixtures are 1.3-4 times higher than the hydrodynamic predictions.