Applications controlled by interfacial rheology, including
Langmuir monolayer structural studies, including:
The KSV NIMA ISR method is based on interfacial shear rheology. A magnetized probe positioned at the interface is used to create shear deformation on the interface for which response can be measured by optically recording the probe movement.
For more information about the technology, see: Interfacial Shear Rheometry
KSV NIMA ISR enables both dynamic and static measurements to define the viscoelasticity of the interfacial layers. With dynamic measurement, viscoelastic properties are measured as a function of frequency, time, strain, temperature or surface pressure. Static measurement enables creep testing to be performed and indicates whether the system behaves like an ideal Newtonian liquid (dashpot model) or ideal elastic (spring model). These measurements enable the following parameters to be defined:
The KSV NIMA ISR can be equipped with either a KSV NIMA Langmuir Trough (or Liquid-Liquid Trough) for simultaneous control of the film packing density or a Low Volume Measurement cell to work with small interfacial areas and reduced subphase volumes.
Both systems enable surface pressure measurement thanks to the integrated highly sensitive Wilhelmy balance. The Langmuir Trough and the Low Volume Measurement Cell are divided into an upper and lower compartment, which can be used to study film viscoelasticity at the liquid-air or liquid-liquid interface.
All the ISR systems have hardware solutions designed for easy injection of the chemicals to enable real-time chemical interactions studies. For example, the low volume cell has two injection ports at each end of the cell to enable easy injection of materials (e.g. proteins, enzymes) in the subphase and to allow gradual subphase exchange while measuring.
Graph 1 illustrates the evolution of the interfacial viscosity of a protein monolayer (lysozyme) residing between water and decane plotted as a function of time. The surface pressure of the layer is also plotted. The change in surface pressure shows the evolution of the adsorption, interfacial viscosity and the crosslinking of the protein as a viscoelastic "skin" develops at the interface as a function of time. The surface pressure data complements the interfacial rheology data.
In a KSV NIMA ISR Low Volume Measurement Cell, a 20 mg/mL solution of lysozyme was injected in the subphase and interfacial viscolelastic properties were monitored (single frequency mode, 0.1 Hz) at an air-water interface (AW) and at an oil-water interface (OW). Graph 2 gives the storage and loss moduli obtained during both experiments. The lyzozyme injection was made at time 0s. The adsorption to the AW interface had only a slight effect on the viscoelastic properties. There was no network formation, the adsorption ended at a plateau and the viscosity dominated during the whole experiment. In the OW experiment the interfacial elasticity clearly developed faster than the interfacial viscosity and a gel point was reached after approximately 11,600 s (3.2 hours).
Graph 3 demonstrates the capability to observe a phase transition in eicosanol by measuring changes in the viscoelastic behavior as a function of surface pressure. The purple crosses show the viscous modulus (surface loss, G’’) that reaches a maximum value at a surface pressure of 5mN/m while nothing is visible on the surface pressure isotherm. The blue crosses show the elastic modulus (surface storage, G’). Both G’ and G’’reach a constant value when the surface pressure reaches approximately 15 mN/m. The value corresponds to a phase transition in the packing of the eicosanol monolayer from tilted liquid to a non-tilted liquid phase. After the phase transition value is reached the film retains some viscous properties while the elasticity is practically zero.