Employing two different quartz crystal microbalance models to study changes in viscoelastic behavior upon transformation of lipid vesicles to a bilayer on a gold surface

By analyzing the viscoelastic properties of two distinct layers, a layer of "soft" vesicles and a "rigid" bilayer, we have created a model system to permit the study of film behavior in the region of nonlinear mass and frequency change (non-Sauerbrey). The structural transformation of lipid vesicles to a bilayer is shown to be accompanied by significant changes in their physical properties. After the adsorption and saturation of intact vesicles on gold surfaces, the adsorbed vesicle layer exhibits a soft, water-rich, viscoelastic state. The AH peptide, a vesicle-destabilizing agent, is then added to trigger the formation of a much thinner (approximately 5 nm), compact, and rigid bilayer. In this study, we used the quartz crystal microbalance with dissipation technique. Large non-Sauerbrey frequency and energy dissipation changes characterize the viscoelastic nature of adsorbed intact vesicle films thicker than approximately 10 nm. Once the transformation is complete, the frequency changes along with zero energy dissipation for sufficiently thin films (t approximately 5 nm) were effectively modeled with the Sauerbrey equation. Furthermore, we checked the validity of the Voigt-Voinova model in which the quartz substrate is treated as a Voigt element, which is beyond the Sauerbrey description. The calculations treating the film as having a constant viscosity agreed well with the Voigt-Voinova model. These results were compared to calculations done using the electromechanical (EM) model, which does not require a series expansion. The Voigt-Voinova results were in excellent agreement with the EM model, providing evidence that the expansion used in their study is quite accurate.