Simultaneous Thickness and Refractive Index Determination of Monolayers Deposited on an Aqueous Subphase by Null Ellipsometry

Abstract

For the first time, the thickness and refractive index of monolayers at the air/water interface have simultaneously been determined by null ellipsometry. Separation of refractive index from film thickness has been achieved by highly precise measurements of the two ellipsometric angles Ψ and Δ. In the solid state, film thicknesses of arachidic acid and valine gramicidin A obtained by ellipsometry are comparable with those obtained by the X-ray techniques. For arachidic acid in the condensed state, our results suggest that only the thickness of the hydrophobic moiety is measured. When highly hydrated, the thickness of the polar headgroup is not detected. This is presumably due to its refractive index being the same as that of the bulk water; hence, the calculated film thickness corresponds to the thickness of the hydrophobic part only. As molecular area is reduced, the polar headgroup gradually loses hydration water molecules causing its refractive index profile to become different from that of the bulk water. Our results suggest that the measurable thickness of the film-forming molecules increases as the degree of dehydration of the headgroup increases.

Three-Dimensional Analysis of the Collapse of a Fatty Acid at Various Compression Rates using In Situ Imaging Ellipsometry

“…With this procedure we obtained the dielectric function of AA from 380 to 1690 nm. The calculated AA monolayer thickness was 28.0 ± 1.5 Å, in good agreement with previous reports [20,25,29]. In previous work [25] data were obtained only at 632.8 nm, where a refractive index n = 1.4461 and extinction coefficient k = 0 were found.…”

“…The calculated AA monolayer thickness was 28.0 ± 1.5 Å, in good agreement with previous reports [20,25,29]. In previous work [25] data were obtained only at 632.8 nm, where a refractive index n = 1.4461 and extinction coefficient k = 0 were found. We emphasize that we observe not only a similar refractive index at 632.8 nm but also the entire dielectric function of AA over a much wider spectral range.…”

“…The agreement is excellent. The AA layer thicknesses found by fitting are not integer multiples of the monolayer thickness, indicating that the molecules in the multilayers are not oriented parallel to the surface normal [20,25,29,34]. The fitting parameters yielding the Gaussian peak positions in Figs.…”

“…Thus IE is particularly suitable for probing the third dimension of the inhomogeneous layer [30,31]. For layers appreciably thinner than the wavelength of the probing light, Δ is sensitive to change in layer thickness, while the Ψ value is hardly affected [25,30]. With n = 1.44 for AA and n = 1.33 for water at 632.8 nm, the connection between Δ and thickness is shown in Fig.…”

Electron Transfer Reactivity Patterns at Chemically Modified Electrodes: Fundamentals and Application to the Optimization of Redox Recycling Amplification Systems

“…Although more complex systems (e.g., a thiolate monolayer on gold) require more terms and a corresponding increase in mathematical complexity, the concepts described here apply in a general sense. In any case, changes in and A can, in principle, be used to measure the refractive index and thickness of a substrate-supported film simultaneously [188]. However, in practice, difficulty arises for thin films such as monolayers, where the thickness to be measured is much less than the wavelength of light employed.…”