The interface between two fluids provides a unique environment for the manipulation and study of species that are practically insoluble in the bulk phases. The density of molecules, particles and fibres in monolayers can be accurately controlled using a Langmuir trough whose moveable barriers change the area of the interface while maintaining a constant total volume of the bulk phases.
The structure and morphology of the monolayer determine its properties, and can be experimentally observed via a number of techniques.
The simplest and most easily accessible property is the two dimensional ‘pressure’ resulting from repulsion between the surface species on compression. This surface pressure, defined as the change in surface tension, can be conveniently measured using the Wilhelmy plate technique. The surface pressure versus area isotherm provides unique information about the interactions and phase behaviour of the species present in the monolayer.
In cases where microdomains form, the morphology can be observed with Brewster Angle Microscopy (BAM). In this technique, variations in refractive index between associated areas allow visualisation of patches of material only a few nm thick. Determining orientation and chemical structure on floating monolayers can now be performed using infrared reflection absorption spectroscopy (IRRAS), which is a powerful and highly surface specific tool that is now available for water-based systems. While the techniques described above can access the equilibrium behaviour of the monolayer, the stability of many systems, such as emulsions and foams is critically dependent on the non-equilibrium flow that occurs at the interface. This flow is measurable using the highly sensitive interfacial shear rheometer (ISR).
Using the monolayer films prepared in the Langmuir trough to coat solid surfaces opens the door to a highly flexible surface modification procedure, where custom-designed molecules, particles and fibres can be transferred to a surface for further use. This method is referred to as Langmuir-Blodgett (LB) deposition, and is used in a multitude of applications in building biosensor surfaces and in making films of functional molecules for ‘bottom-up’ construction of nanodevices.
KSV Instruments is based in Helsinki, Finland. www.ksvltd.com
