On the behaviour of molecules of the quinoline group at the water—Mercury interface: Part II. Electrocapillary study of isoquinoline in solutions containing only an “inert” electrolyte

Abstract

Isoquinoline has been examined in a 0.5 M Na 2SO 4 aqueous solution, by using various electrocapillary methods. In the range of small potentials, and rather surprisingly, isoQ and Q present a very similar behaviour, despite the considerable difference in their dipole orientation with respect to the median carbons. Esin-Markov plots and capacity—potential—concentration maps indicate that there is gradual desorption towards negative potentials. Three distinct states (dilute flat molecules, a mixture of flat and erect molecules, and a monolayer with a variable extent of local clusters) have been characterized and their domain of prevalence demarcated. At more negative potentials, a sudden reorientation is observed, which reflects the lack of miscibility between two superficial states internally stabilized by their own set of lateral interactions. The effects of the surfactant concentration, the electrolyte concentration and the temperature on the position of the transition potential are interpreted on the basis of the relative position of two γ=f( E) curves, whose vertical shifts are controlled by the Gibbs isotherm equation, with two distinct discrete values for the superficial excesses corresponding to the two antagonizing monolayers. The most compact of these monolayers is stable in the region with stretches between the phase transition potential and the electrolyte potential, both very sharply defined. Its structure has been determined on the basis of a number of converging facts, such as the limiting value of τ, the values of the capacity and the position of the pzc, extrapolated from the σ M=f( E) plots.