Comparison between Phosphonium Docusate Ionic Liquids and Their Equimolar Mixtures with Alkanes: Temperature-Dependent Viscosity, Glass Transition, and Fragility

In this study, we determined the temperature-dependent viscosities, glass transition temperatures, and fragilities of tetraalkylphosphonium docusate ionic liquids (ILs) and their equimolar mixtures with alkanes to elucidate the effects of the alkyl groups on the phosphonium cation. The target ILs were the docusate salts with tributylheptylphosphonium ([P₄₄₄₇][doc]), tributyltetradecylphosphonium ([P_444,14][doc]), butyltrihexylphosphonium ([P₄₆₆₆][doc]), trihexylheptylphosphonium ([P₆₆₆₇][doc]), and trihexyltetradecylphosphonium cations ([P_666,14][doc]). The comparable IL/alkane mixtures were equimolar mixtures of IL and alkane with the same carbon numbers of the target ILs: [P₄₄₄₇][doc]/hexane to [P₆₆₆₇][doc]; [P₄₄₄₇][doc]/heptane to [P_444,14][doc]; [P_444,14][doc]/hexane to [P_666,14][doc]; [P₄₆₆₆][doc]/decane to [P_666,14][doc]; and [P₆₆₆₇][doc]/heptane to [P_666,14][doc]. The viscosities and glass transition temperatures of the neat ILs were higher than those of their respective IL/alkane mixtures. Based on the analysis of temperature-dependent viscosities, including a viscosity value of 10¹³ mPa·s at the glass transition temperature using the Vogel-Fulcher-Tammann equation, the neat ILs were stronger liquids than the corresponding IL/alkane mixtures. By comparing several combinations of the neat ILs and IL/alkane mixtures, we found that the larger the alkane, the more fragile the mixture.