Mechanism of a Four-Phase Liquid Membrane Oscillator Containing Hexadecyltrimethylammonium Bromide

Abstract

The oscillatory behavior of a nitromethane based liquid membrane oscillator was investigated to contribute to the oscillation mechanism at the molecular level. At the beginning the system contains three phases: the aqueous donor phase in which the cationic surfactant, hexadecyltrimethylammonium bromide and ethanol are present and the aqueous acceptor phase made up by sucrose solution separated by the liquid membrane containing a constant amount of picric acid. During experiment a new phase x is created between the liquid membrane and acceptor phase. It was established that the oscillations take place at the membrane/phase x and the phase x/acceptor phase interfaces. Five basic regions can be distinguished in the oscillation pattern. The molecular events provoking the oscillations of electric potential difference between the two aqueous phases involve essentially the diffusion of hexadecyltrimethylammonium bromide and ion pairs formed by the cation of the surfactant and the picrate anion to the vicinity of the membrane/phase x interface, sudden adsorption of these ion pairs at this interface in noncatalytic and autocatalytic steps, desorption of ion pairs from the membrane/phase x interface into phase x, diffusion of ion pairs to the vicinity of phase x/acceptor phase interface, and sudden adsorption at this interface followed by desorption to the aqueous acceptor phase. It is shown by numerical simulations that the proposed mechanism may account for the observed oscillations and for the species distribution throughout the system as found experimentally. This four-phase system behaves like two coupled oscillators.