Vapochromic behaviour of a nickel(II)-quinonoid complex with dimensional changes between 1D and higher.

The nickel(ii)-chloranilato complex {Ni(ca)(VM)2}n (H2ca = chloranilic acid, VM = coordinated vapour molecules, such as water) shows reversible vapochromism upon exposure to various vapours and subsequent drying by heating. In contrast to the Ni(ii)-quinonoid complex, [Ni(HLMe)2] (H2LMe = 4-methylamino-6-methyliminio-3-oxocyclohexa-1,4-dien-1-olate), which was reported to exhibit vapochromic spin-state switching between high and low spin states, the chloranilato complex does not change its spin state even after the removal of coordinated vapour molecules. X-ray absorption fine structure (XAFS) analysis revealed that the six-coordinate geometry of {Ni(ca)(VM)2}n was maintained even after the removal of vapour molecules, in contrast to the [Ni(HLMe)2] complex. The unique vapochromism that follows the dimensional change between 1D and higher is influenced by the relatively weaker ligand field of the chloranilate ligand.

Ion-selective Marangoni instability--chemical sensing of specific cation for macroscopic movement.

Spontaneous motion and tension oscillation of an oil/water interface responding to specific cation Ca(2+) or Fe(3+) were observed when the oil phase containing the anionic surfactant bis(2-ethylhexyl) phosphate came in contact with the cation-containing water. Both the dynamics were the results of Marangoni instability. Complex formation between the anionic surfactant and cation caused the instability. The results showing the level of cation extraction and degree of interfacial tension revealed that the surfactant-cation combination forms an oil-soluble complex with reduced surface activity. Brewster angle microscopy indicated that molecules of the complex tend to aggregate at the interface. This aggregation affected the desorption rate of the complex. We were able to generate ion-selective instability by imposing mechanical and electrochemical perturbations to the interface at equilibrium. The results from these efforts suggested that the aggregation is a type of thermodynamic transition and is required for the onset of instability: Desorption probably occurs as an exfoliation of the aggregated complex, which generates the gradient of interfacial tension. For the standard experiment of biphasic contact, two neighboring interfacial flows compress the local interface between them. We considered that this compression provides mechanical work to the local interface, resulting in desorption of the aggregates and occurrence of instability. Both complex formation and aggregation are possible in the presence of the specific cation. The interface detects the cation via the chemical and thermodynamic processes in order to develop the macroscopic movement, a form of biomimetic motion of the oil/water interface.

Ion-selective Marangoni instability coupled with the nonlinear adsorption/desorption rate.

An oil/water interface containing bis(2-ethylhexyl)phosphate and Ca(2+) or Fe(3+) exhibits spontaneous Marangoni instability associated with the fluctuation in interfacial tension. This instability rarely appears for oil/water systems with Mg(2+), Sr(2+), Ba(2+), Cu(2+), or Co(2+). The same ion selectivity is observed for n-heptane and nitrobenzene despite their significant differences in density, viscosity, and the dielectric constant of oil. We studied this instability under acidic pH conditions to avoid the neutralization reaction effects. The result of the equilibrium interfacial tension and the extraction ratio of cations indicates that a large number of oil-soluble complexes form at the interfaces of Ca(2+)-containing systems and probably for Fe(3+)-containing systems. The results obtained by oscillating drop tensiometry and Brewster angle microscopy indicate that desorption, rather than adsorption, is more significant to the onset of instability and that the resulting complex tends to form aggregates in the interface. This aggregation gives the nonlinear desorption rate of the oil-soluble complex. Then, exfoliation of the aggregating matter occurs, which triggers the Marangoni instability. The induced convection removes the oil-soluble complex accumulated at the interface, creating a renewed interface, which is necessary for the successive occurrence of the Marangoni instability. For the other cations, the oil-soluble compounds are insignificant, and they rarely form aggregates. In such cases, adsorption/desorption proceeds without instability.