| Presented here is the study of a unique type of dendritic macromolecule known as a hydraamphiphile. The basic structure consists of a poly(L-lysine) (PLL) dendrimer, constructed on a poly(ethylene glycol) (PEG) support. As the name suggests, these molecules were designed to exploit their amphiphilic nature. Accordingly, one of the target applications is as a biomedical surfactant. The PEG core provides the hydrophilic portion of the structure, and the amine terminal ends of the PLL dendrimer can be functionalized with a wide variety of hydrophobic moieties (typically t-butoxycarbonyl or perfluorocarbon groups). Both linear-dendritic and dendritic-linear-dendritic (barbell) structures have been prepared. Synthetic flexibility is also accomplished by varying the length of the PEG chain and the size of the dendrimer. The perfluorocarbon-terminated hydraamphiphiles will stabilize perfluorocarbon-in-water emulsions with droplet diameters of 100--200 nm, but these emulsions exhibit limited lifetimes due to a creaming phenomenon. Based on current observations, several improvements have been proposed to remedy this problem. To further probe the surfactant properties of the hydraamphiphiles, Langmuir films have been spread at the air-water interface, and pressure-area isotherms were recorded. The fluorinated hydraamphiphiles correlate well their expected areas per molecule, but the Boc-terminated species are approximately half, or less, of their expected values. This discrepancy led to the use of direct monolayer imaging (fluorescence microscopy, Brewster angle microscopy, and atomic force microscopy) to determine the cause. Collapse structure were observed at well below what was initially though to be the collapse pressure, and it was concluded that the film instability is due to a high surface viscosity (probably caused by the PEG chains). Finally, our experience with direct fluorescence imaging of Langmuir monolayers led to an improvement over the traditional practice of doping the bulk surfactant with a small amount of amphiphilic dye. A dye has been covalently attached to the hydraamphiphile to prevent the possibility of phase separation. The work presented in this thesis has contributed significantly to our understanding of hydraamphiphiles and their amphiphilic behavior. |
