On the mechanism of nitrobenzene liquid membrane oscillators containing hexadecyltrimethylammonium bromide

The oscillatory behaviour of a liquid membrane oscillator was investigated in order to contribute to the oscillation mechanism at the molecular level. The chosen system involved nitrobenzene as liquid membrane containing a constant amount of picric acid. The aqueous donor phase contained the cationic surfactant, hexadecyltrimethylammonium bromide, and ethanol. The aqueous acceptor phase was made up by sucrose solution. It was established that the oscillations take place exclusively at the aqueous acceptor phase/membrane interface. Three stages mechanism of the oscillation (I--induction period, II--first peak formation, III--creation of the first peak) together with appropriate processes were proposed. The molecular events provoking the oscillations of electric potential difference between the two aqueous phases concern: 1) the diffusion of hexadecyltrimethylammonium bromide and ion pairs formed by cation of the surfactant and the picrate anion to the vicinity of the membrane/acceptor phase interface, 2) sudden adsorption of these ion-pairs at this interface in non-catalytic and autocatalytic steps, 3) desorption of ion pairs from the a/m interface to the 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.