Oscillatory Motion of a Camphor Disk on a Water Phase with an Ionic Liquid Sensitive to Transition Metal Ions

We investigated oscillatory motion of a camphor disk floating on water containing 5 mM hexylethylenediaminium trifluoroacetate (HHexen-TFA) as an ionic liquid (IL). The frequency of the oscillatory motion increased with increasing concentrations of the transition metal ions Cu²⁺ and Ni²⁺ but was insensitive to Na⁺, Ca²⁺, and Mg²⁺, the typical metal ions in the water phase. The surface tension of the water phase containing 5 mM HHexen-TFA also increased with increasing concentrations of Cu²⁺ and Ni²⁺ but was insensitive to Na⁺, Ca²⁺, and Mg²⁺. Based on density functional theory, metal-ion species-dependent frequency response is discussed with regard to surface tension as the force of self-propulsion and complex formation between HHexen-TFA and metal ions. These results suggest that complex formation between the transition metal ions (Cu²⁺, Ni²⁺) and the ethylenediamine group in the IL increases the surface tension around the camphor disk, resulting in an increase in the frequency of oscillatory motion with increasing concentrations of Cu²⁺ or Ni²⁺. The present study suggests that the nature of self-propulsion can be created by complexation, which changes the force of self-propulsion.