G. Szymanski

Electric Field-Driven Transformations of a Supported Model Biological Membrane—An Electrochemical and Neutron Reflectivity Study

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the supports surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the supports surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the supports surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

STM studies of fusion of cholesterol suspensions and mixed 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol vesicles onto a Au(111) electrode surface.

Electrochemical scanning tunneling microscopy (EC-STM) has been applied to study the structure of the film formed by fusion of cholesterol suspensions and mixed dimyristoylphosphatidylcholine (DMPC)/cholesterol vesicles on a Au(111) electrode surface. It has been demonstrated that cholesterol molecules assemble at the gold support into several structures templated by the crystallography of the metal surface and involving flat or edge-on adsorbed molecules. Studies of the film formed by fusion of mixed DMPC/cholesterol vesicles revealed that ordered domains of either pure DMPC or pure cholesterol were formed. These results indicate that, at the metal surface, the molecules released by the rupture of a vesicle initially self-assemble into a well-ordered monolayer. The self-assembly is controlled by the hydrocarbon skeleton-metal surface interaction. In the case of mixed DMPC/cholesterol vesicles, the molecule-metal interactions induce segregation of the two components into single component domains. However, the molecule-metal interaction induced monolayer is a transient phenomenon. When more molecules accumulate at the surface, the molecule-molecule interactions dominate the assembly, and the monolayer is transformed into a bilayer.

Neutron reflectivity studies of field driven transformations in a monolayer of 4-pentadecyl pyridine at Au electrode surfaces

Neutron reflectometry (NR) has been employed to study the structure and composition of thin films formed by 4-pentadecylpyridine (C15-4Py) at a gold electrode surface. The thickness and the water content of films of C15-4Py have been measured as a function of the potential applied to the electrode. At positive potentials, where condensed film is formed, the monolayer contains defects that are filled with water. At very negative potentials, the film is desorbed from the electrode surface. NR has demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface as a thicker and water rich film. # 2003 Elsevier B.V. All rights reserved.

Spectroscopic and Electrochemical Studies of Coordination of Organic Molecules to Gold Single Crystal Surfaces

This lecture gives a review of our efforts to describe adsorption of organic molecules at gold electrodes. First, we will discuss adsorption of pyridine and benzonitrile at the Au(111) electrode surface. We will combine chronocoulometric, and in situ infrared spectroscopy to describe; (i) energetics of molecular adsorption at the gold electrode surface, (ii) character of the interaction of the adsorbed molecule with the metal substrate and (iii) influence of the electric field on the orientation of the adsorbed molecule.

Ionic Adsorption and Co-Adsorption at Single Crystal Electrodes

J.LIPKOWSKI, X.CAl, A.CHEN, z.sm, G.SZYMANSKI This lecture gives a review of thermodynamic, spectroscopic, STM and AFM imaging and X-ray diffraction studies of co-ordination of ions to the Au(lt1) electrode surface. In the first part, adsorption of simple ions such as sulfate, chloride, bromide and iodide will be discussed. Thermodynamic experiments, used to determine surface concentrations of adsorbed anions, Gibbs energies of adsorption and the polarity of the chemisorption bond formed between the anion and the metal surface, will be described. At high coverages, anions adsorbed at a single crystal surface form ordered 2D adlayers. We will show that structure of these adlayers can be studied by STM and surface X-ray diffraction techniques. In the second part of the lecture, we will describe mixed adlayers formed by coadsorption of anions and metal adatoms. We will demonstrate how to combine electrochemical experiments with in situ polarization dependent Cu K-edge X-ray absorption spectroscopy to determine the composition and the structure of mixed films formed by deposition of Cu on Au(l11) in the presence of cr, sot or Br-. The anion influences the amount of deposited metal atoms, the ordering of the overlayer and the amount of charge transfer to the deposited copper. Present work provides quantitative details of the structure for the cases of co-adsorption of Cu + sot on Au(111) electrodes. Copper deposited in the presence of sulfate ions are packed in registry with respect to the Au(l11) substrate. The coadsorbed copper and sulfate form a highly corrugated overlayer. The copper adatoms assume a