Monolayers at the air/water interface have been widely used as biomembrane models under controlled intermolecular organization (lateral molecular area). Surface pressure, surface potential, reflectivity (R) and other magnitudes can be precisely determined on these planar monomolecular films. However, some physical parameters such as the refractive index (nm) still remain elusive. This is important because its precise quantification allows for the determination of the thickness of the film by R, for instance on a Brewster Angle Microscope (BAM)1. The uncertainties of nm determine important errors in the calculation of monolayers thickness that propagates non-linearly.
Here we present an analytical method for the experimental determination of nm in monolayers based on the principle of refractive index matching as used in bulk suspensions of liposomes2. By using a BAM setup and monolayers spread over subphases with different and known refractive index (ns), a minimum in R is search as a function of ns. In these conditions, the nm is equal to the known ns. The results shown correspond to monolayers of isolated bovine myelin and two lipids with nm previously reported, DMPC and cerebrosides. The nm values remain approximately constant, slightly increasing in a range of 0.005 to 0.01 upon compression. The obtained values allows for determination of thickness.
