We have derived the general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid (quartz) surfaces in a fluid environment. Within the Voight model of viscoelastic element, we calculate the acoustic response of the system to an applied shear stress, i.e. we find the shift of the quartz generator resonance frequency and of the dissipation factor, and show that it strongly depends on the viscous loading of the adsorbed layers and on the shear storage and loss moduli of the overlayers. These results can readily be applied to quartz crystal acoustical measurements of the viscoelasticity of polymers which conserve their shape under the shear deformations and do not flow , and layered structures such as protein films adsorbed from solution onto the surface of self-assembled monolayers.

We have measured the kinetics of adsorption of small (12.5-nm radius) unilamellar vesicles onto SiO2, oxidized gold, and a self-assembled monolayer of methyl-terminated thiols, using a quartz crystal microbalance (QCM). Simultaneous measurements of the shift in resonant frequency and the change in energy dissipation as a function of time provide a simple way of characterizing the adsorption process. The measured parameters correspond, respectively, to adsorbed mass and to the mechanical properties of the adsorbed layer as it is formed. The adsorption kinetics are surface specific; different surfaces cause monolayer, bilayer, and intact vesicle adsorption. The formation of a lipid bilayer on SiO2 is a two-phase process in which adsorption of a layer of intact vesicles precedes the formation of the bilayer. This is, to our knowledge, the first direct evidence of intact vesicles as a precursor to bilayer formation on a planar substrate. On an oxidized gold surface, the vesicles adsorb intact. The intact adsorption of such small vesicles has not previously been demonstrated. Based on these results, we discuss the capacity of QCM measurements to provide information about the kinetics of formation and the properties of adsorbed layers.

An experimental setup has been constructed for simultaneous measurements of the frequency, the absolute Q factor, and the amplitude of oscillation of a quartz crystal microbalance (QCM). The technical solution allows operation in vacuum, air, or liquid. The crystal is driven at its resonant frequency by an oscillator that can be intermittently disconnected causing the crystal oscillation amplitude to decay exponentially. From the recorded decay curve the absolute Q factor (calculated from the decay time constant), the frequency of the freely oscillating crystal, and the amplitude of oscillation are obtained. AI1 measurements are fully automated. One electrode of the QCM in our setup was connected to true ground which makes possible simultaneous electrochemistry. The performance is illustrated by experiments in fluids of varying viscosity (gas and liquid) and by protein adsorption in situ. We found, in addition to the above results, that the amplitude of oscillation is not always directly proportional to the Q factor, as the commonly used theory states. This puts limitations on the customary use of the amplitude of oscillation as a measure of the Q factor. 8 1995 American Institute of Physics.

The adsorption kinetics of small unilamellar egg-yolk phosphatidylcholine vesicles was investigated by the quartz crystal microbalance-dissipation (QCM-D) technique, as a function of surface chemistry (on SiO2, Si3N4, Au, TiO2, and Pt), temperature (273-303 K), vesicle size (25-200 nm), and osmotic pressure. On SiO2 and Si3N4, the vesicles adsorb intact at low coverage, followed by transformation to a bilayer at a critical coverage. On TiO2, oxidized Pt, and oxidized Au, the vesicles adsorb intact at all coverages and all studied temperatures. Variation of vesicle size does not change the qualitative behavior on any of the surfaces, but the quantitative differences provide important information about surface-induced vesicle deformation. In the low-coverage regime (where vesicles adsorb intact on all surfaces), the deformation is much larger on SiO2 than on the surfaces where bilayer formation does not occur. This is attributed to stronger vesicle-surface interaction on SiO2. The bilayer formation is thermally activated with an apparent activation energy of 63-78 kJ/mol. Osmotic pressure promotes bilayer formation, especially when the external salt concentration is higher than the internal one. Depending on preparation conditions, a varying amount of nonruptured vesicles are trapped in the saturated bilayer on SiO2, but the fraction can be efficiently reduced to below the detection level using elevated temperature and/or high osmotic stress.

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