Ion-Selective Marangoni Instability Coupled with the Nonlinear Adsorption/Desorption Rate

An oil/water interface containing bis(2-ethylhexyl)phosphate and Ca2+ or Fe3+ exhibits spontaneous Marangoni instability associated with the fluctuation in interfacial tension. This instability rarely appears for oil/water systems with Mg2+, Sr2+, Ba2+, Cu2+, or Co2+. 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 Ca2+-containing systems and probably for Fe3+-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.