Modified Sauerbrey equation: a facile method to quantitatively probe the conformation of isolated molecules at solid-liquid interfaces.

Despite the increasingly popular application of the quartz crystal microbalance (QCM) technique in monitoring phenomena taking place at solid-liquid interfaces, ranging from changes in mass to changes in conformation, a simple, direct relationship between QCM signal and surface mass remains elusive. In this paper, we report that the proportional relationship between the QCM signal and the surface mass arises from the linear relationship between the viscosity of the layer adsorbed at the solid-liquid interface and the surface coverage, as well as a small viscosity shift. The proportionality coefficient depends on the intrinsic viscosity of adsorbates, solvent density, and quartz crystal thickness. The intrinsic viscosity is dominated by the conformation of the entire molecular chain and the adsorption blob for end-grafted and physisorbed molecules, respectively. Using this modified Sauerbrey equation, the phenomena relating to the conformation of discrete chains at the solid-liquid interfaces can be semi-quantitatively described.

Viscoelastic response and profile of adsorbed molecules probed by quartz crystal microbalance.

The acoustic responses of molecular films with continuous viscoelastic profiles adsorbed on solid-liquid interfaces probed by quartz crystal microbalance (QCM) are analyzed using a continuum mechanics model. The numerically calculated results show that the shift of resonant frequency and the change of dissipation factor of a QCM are determined mostly by the change of viscoelastic profile of the layers in solution adjacent to the quartz-solution interface due to the adsorbed molecular film. For films with the same amount of absorbed mass, the changes in resonant frequency and the dissipation factor vary approximately linearly with the width of the film-solution interface in the profiles. Other viscoelastic properties of the adsorbed films are also affected by the profiles.

Viscoelastic signature of physisorbed macromolecules at the solid-liquid interface.

Viscoelastic behavior of a solution boundary layer at a solid-liquid interface could differ from that of bulk solution due to molecular adsorption at the interface. Such a property can be used as a characteristic signature to indicate the molecular adsorption at the interface. In this work, we systematically measured the viscoelastic properties of polyethylene glycol (PEG) solution boundary layers in contact with a gold surface using a quartz crystal resonator technique. The results show that viscosity and shear modulus of the PEG boundary layer increase with the PEG concentration in the solution; the increasing rate depends on the molecular weight. For relatively small PEG molecules, the viscoelastic property of the PEG solution boundary layer is almost indistinguishable from that of the bulk solution of the same concentration, indicating no adsorption at the interface. For larger PEG polymers (with molecular weights above a few thousands grams per mole), the viscoelastic property of the solution boundary layer differs distinctively from that of the corresponding bulk solution. The difference can be attributed to physisorption of PEG molecules on the Au surface, which alters the viscoelastic behaviors of the boundary layer. The results suggest that adsorption behaviors of macromolecules at a solid-liquid interface might be inferred from the changes of the viscoelastic properties of a solution boundary layer.