Oscillatory Motion\nof a Camphor Disk on a Water Phase\nwith an Ionic Liquid Sensitive to Transition Metal Ions

We investigated oscillatory motion of a camphor disk\nfloating on\nwater containing 5 mM hexylethylenediaminium trifluoroacetate (HHexen-TFA)\nas an ionic liquid (IL). The frequency of the oscillatory motion increased\nwith increasing concentrations of the transition metal ions Cu<sup>2+</sup> and Ni<sup>2+</sup> but was insensitive to Na<sup>+</sup>, Ca<sup>2+</sup>, and Mg<sup>2+</sup>, the typical metal ions in\nthe water phase. The surface tension of the water phase containing\n5 mM HHexen-TFA also increased with increasing concentrations of Cu<sup>2+</sup> and Ni<sup>2+</sup> but was insensitive to Na<sup>+</sup>, Ca<sup>2+</sup>, and Mg<sup>2+</sup>. Based on density functional\ntheory, metal-ion species-dependent frequency response is discussed\nwith regard to surface tension as the force of self-propulsion and\ncomplex formation between HHexen-TFA and metal ions. These results\nsuggest that complex formation between the transition metal ions (Cu<sup>2+</sup>, Ni<sup>2+</sup>) and the ethylenediamine group in the IL\nincreases the surface tension around the camphor disk, resulting in\nan increase in the frequency of oscillatory motion with increasing\nconcentrations of Cu<sup>2+</sup> or Ni<sup>2+</sup>. The present\nstudy suggests that the nature of self-propulsion can be created by\ncomplexation, which changes the force of self-propulsion.