Hydrophobic and low-density amino acid ionic liquids

A series of tetraalkylphosphonium amino acid salts was synthesized to determine the effect of ion structure, in particular the length of alkyl chains, on physico-chemical properties such as density, viscosity, glass transition temperature, and decomposition temperature. The resulting alkylphosphonium amino acid ionic liquids (AAILs) all had low density, from 0.886 to 0.945 g cm− 3 at 25 °C. The density was tunable by varying the alkyl-chain length, regardless of the symmetry of the phosphonium cations. When the alkyl chains on the phosphonium cation were hexyl or longer, the AAILs were phase-separated upon mixing with water because of their hydrophobicity, and floated on top of the water phase. The water content in the ILs depended on the amino acid side chains and on the alkyl-chain length on the cations.