In Situ Infrared Attenuated Total Reflection Membrane Spectroscopy
Optimum use of infrared attenuated total reflection (IR ATR) spectroscopy can be made if the model membrane is immobilized at the flat surfaces of the reflection element in the form of a planar bilayer or four-layered assembly. The monolayer-coated dry plate is then mounted in a liquid sample cell, which has been hydrodynamically optimized for flow-through experiments. The transfer of the second monolayer makes use of the energetically unfavorable state of the hydrophobic surface of the first monolayer in contact with the aqueous phase in the ATR cell. Most procedures for scaling the water reference spectrum as described in literature fail to take into account that bulk water and hydration water generally exhibit different wavenumbers and different molar adsorption coefficients. As a consequence, unambiguous water compensation requires a very high degree of reproducibility of a measurement and the use of scaling factors of unity throughout.