Μελέτη της διεπιφανειακής συμπεριφοράς μικυλλίων στο σύστημα Hg-ηλεκτρολυτικό διάλυμα

Abstract

In the present study, the adsorption of micelles on an electrode surfaceis studing theoretically, by means of Statistical Thermodynamics, andexperimentally, by means of differential capacitance measurements.A simple model is developed to describe the single adsorption of amonomer neutral organic compound on an ideally polarized electrode surface.The equilibrium properties are determined within the framework of latticestatistics using the mean field approximation under the assumption of a meanelectrical field acting at each site. Neglecting short-range interactions, themodel for single adsorption predicts langmuirian adsorption isotherms, whichare congruent only with respect to potential, and a quadratic dependence of theGibbs energy of adsorption on the potential.The model is extended to include aggregation phenomena leading to theformation of surface micelles and phase transformations of the adsorbed layer.The model predicts that the formation of oligomers is not depicted in thecapacitance plots, which exhibit the same features as those for adsorption ofmonomers under all circumstances. The equilibrium between monomers andoligomers extends throughout the polarization range where these two states ofthe adsorbate coexist. As the number of monomer units in the micellesincreases, monomers and micelles tend to separate, occupying differentpolarization regions. In addition, the equilibrium between monomers and largeaggregates leads in general to complicated capacitance plots characterized bythe appearance of very sharp capacitance peaks. The same featurescharacterize the formation of mixed two-dimensional micellar films on electrodesurfaces. Phase transformations of adsorbed monolayers on electrodesurfaces, which may lead to the formation of either two immiscible concentratedsurface solutions of adsorbate in solvent and vice versa or pure adsorbateprecipitate, are examined within the framework of this model. The separation o the adsorbed layer to immiscible surface solutions is related to the short-rangeparticle-particle interactions, whereas the surface precipitation process isconsidered as an aggregation process where the aggregation number tends toinfinity. It is shown that in general there is an acceptable agreement betweentheory and experiment.The formation and properties of mixed micellar films on the Hg electrodein the presence of two surfactants in the electrolyte solution is studied bymeans of differential capacity measurements. The experimental study isperformed with a system consisting of a potensiostat, a lock-in amplifier and astandard electrochemical cell with a HDME electrode. The system wascontrolled through a PC with an appropriate software, which was developed forthis study.The experimental data are analyzed on the basis of the present andprevious theoretical treatments. It is found that the system sodiumdodecylsulphate - cetyltrimethylammonium bromide exhibits two differentbehaviours: At bulk solutions rich in sodium dodecylsulphate a film of mixedmicelles covers the electrode surface from positive polarizations up to - I.6V(see). In contrast, at bulk solutions rich in cetyltrimethylammonium bromide theelectrode surface is covered with a bilayer of micelles composed exclusively ofcetyltrimethylammonium cations, which negative of - 1.2 V (see) collapses to acondensed layer due to the penetration of dodecylsulphate anions. When theelectrolyte solution contains sodium dodecylsulphate and tween 80, then underfavourable conditions the electrode is covered with micelles of puredodecylsulphate anions from positive potentials up to the first capacitance peakat negative potentials. Negative of this peak and up to the second peak at farnegative potentials the electrode is covered with mixed micelles rich in tween80. Finally, in the system of tween 80 -cetyltrimethylammonium bromide theelectrode is covered with a film of mixed micelles, possibly of variablecomposition, throughout the range of potentials we studied.