Chronocoulometry and the thermodynamic analysis of charge density data were employed to describe the energetics of sodium dodecyl sulfate (SDS) adsorption at the Au(111) electrode surface. Thermodynamic data such as the Gibbs excess, Gibbs energy of adsorption, and the film pressure of adsorbed SDS were determined for a broad range of electrode potentials, charge densities, and bulk SDS concentrations. The present results, combined with our previous scanning probe microscopy (SPM) studies, show that adsorption of SDS at the Au( 111) electrode surface has a two-state character. At small or moderate absolute charge densities, the adsorbed SDS molecules aggregate into hemicylindrical stripelike micelles. This state is well-ordered. The unit cell of its two-dimensional lattice consists of two vectors that are 44 and 5.0 Å long and are oriented at an angle of 70°. The Gibbs excess data indicate that five SDS molecules are accommodated into the unit cell. At large positive charge densities, the hemimicellar aggregates melt to form a condensed film. The surface concentration of SDS doubles upon transition from the hemimicellar to the condensed state. We have performed neutron reflectivity experiments to determine the thickness of the hemimicellar and condensed films. The neutron reflectivity data indicate that the thickness of the condensed film is equal to 20.5 Å and is only 30% larger than the thickness of the hemimicellar state. The electrochemical and neutron reflectivity data indicate that the properties of the condensed state are best explained by a model of an interdigitated film in which half of the sulfate groups are turned toward the metal and half toward the solution.

Electrochemistry and polarization modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS) was employed to investigate fusion of small unilamellar vesicles of 1,2dioyl-sn-glycero-3-phosphatidyl choline (DOPC) onto the Au(111) electrode. Electrochemical studies demonstrated that the DOPC vesicles fuse and spread onto the gold electrode surface at small charge densities -8 microC cm(-2)

No abstract

Polarization modulation Fourier transform infrared reflection-absorption spectroscopy (PM FTIRRAS) has been combined with electrochemistry to monitor the potential-induced transformations of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15-4Py), a model amphiphilic compound, at a Au(111) electrode surface. The optical constants for a solution of randomly oriented C15-4Py molecules have been determined and used to calculate the integrated band intensities for a monolayer and bilayer of randomly oriented molecules. The orientation of the hydrophobic and hydrophilic parts of the molecule with respect to the surface normal was determined from the ratio of the intensity of experimental to calculated bands. The CH stretch region of the spectra was used to determine the tilt angle between hydrocarbon chains and the surface normal. The tilt angle varied with potential. The minimum value of the tilt angle was 16°f or the monolayer and 27°for the bilayer. In the monolayer, the headgroup of the C15-4Py molecule lies almost flat on the Au(111) surface. At negative potentials, the angle between the plane of the pyridine ring and the surface normal is 68°. As applied potential becomes more positive, the pyridine group stands up gradually and the tilt angle decreases to 63°. In the bilayer, the C2 axis of the pyridine group in the leaflet turned to the electrolyte solution is tilted at ∼70°with respect to the normal and this tilt angle changes little with potential. In the leaflet turned to the electrode, the angle between the C2 axis of the headgroup and the surface normal changes from 64°to 54°by moving from negative to positive potentials. The PM IRRAS data also show that in the bilayer, the plane of the pyridine group rotates with respect to the C 2 axis when the electrode potential changes. In the monolayer, the tilt angle of the C2 axis changes but the plane of the pyridine moiety does not rotate with potential.

No abstract