Force microscopy of self-assembled amphiphilic films

Scanning force microscopy is a powerful technique to study surfaces in vacuum, air or liquids because of its nanometer spatial resolution and piconewton force resolution. The work presented here uses colloidal (steric and double layer) forces to investigate the shape of surfactant micelles on an isotropic hydrophobic surface (Chapter 3). Frictional forces are used in this work as a useful contrast mechanism in the study of monolayer water films (Chapter 4), self-assembled monolayers (Chapter 6), and to detect the presence and concentration of a 2D gas phase of molecules on a surface (Chapter 5). The invention of "soft contact" mode Atomic Force Microscopy imaging allows the direct imaging of surfactant micelles at the solid-liquid interface. One of the results in this work was the determination of the structure of surface micelles on an isotropic hydrophobic surface. This model system is useful for comparison to crystalline surfaces to determine the relative importance of simple hard-wall confinement versus the influence of the underlying substrate lattice. Lateral (friction) forces have also proven to be a useful contrast mechanism in imaging surfaces. The results presented here demonstrate the use of lateral force (friction) microscopy as a highly sensitive method to detect a 2D gas of amphiphiles on a mica surface, making this technique useful in studying the early stages of monolayer formation. This sensitivity also makes friction a useful contrast mechanism when imaging patterned self-assembled monolayers and ultra-thin water films. A simple and inexpensive method of creating micrometer to sub-micrometer structures was reported by Kumar and Whitesides in 1993. This technique, called microcontact printing, uses an elastomeric replica of a master to pattern a surface, typically with self-assembled, covalently bonded molecules. Microcontact printing of non-covalently bonding molecules has not been as extensively studied, with the exceptions of lipid bilayers of phosphatidylcholine and proteins. Chapters 5 and 6 will show results of stamping several types of alkyl derivatives on a model surface (mica). These non-covalently bonding molecules show much greater diffusion and spreading during stamping, which reveal details of the mechanisms of microcontact printing that are obscured with the strongly interacting, covalently bonding systems