Potential-dependent adsorption and transfer of poly(diallyldialkylammonium) ions at the nitrobenzene|water interface

Electrochemically driven adsorption and partition of a series of poly(diallyldialkylammonium) ions (PDADAA⁺: alkyl = methyl, ethyl, propyl, and butyl) at the nitrobenzene (NB)|water (W) interface have been studied using voltammetry and electrocapillary measurements. When the phase-boundary potential, δπ, that is, the inner potential of the W phase referred to that of the NB phase, is negative, poly(diallyldimethylammonium) (PDADMA⁺) shows little surface activity. The scanning of δπ in the positive direction induces, first, the adsorption of PDADMA⁺ at the interface and, then, the desorption of adsorbed PDADMA⁺ ions into the NB phase, followed by the diffusion-limited transfer of PDADMA⁺ from W to NB. The elongation of the dialkyl chains gives the stronger surface activity of PDADAA⁺ even when δπ < 0. The PDADAA⁺ polyions studied are only slightly more hydrophilic than the corresponding monomers. However, the polycationic character of PDADAA⁺ renders the adsorption, desorption, and ion transfer strongly dependent on δπ and gives rise to unusual, M-shaped electrocapillary curves. The interplay of adsorption-desorption and ion transfer of PDADAA⁺ ions induces the electrochemical instability of the interface and the emulsion formation on the NB side of the interface.