Determination of the refractive index and thickness of phospholipid Langmuir monolayers from reflectivity

VARGAS FONDACARO JUAN M; RAFAEL GUSTAVO OLIVEIRA

Congreso; XLIX Reunión Anual de la Sociedad Argentina de Biofísica; 2021

Using the reflectivity and refractive index obtained on lipid monolayers (DMPC & DPPC)and a mathematical model (Drude?s Approximation), the thicknesses of themonomolecular films were calculated. The thicknesses were then compared withreference thickness values measured directly by Grazing Incidence X-Ray Off SpecularScattering (GIXOS) at the beamline XRD-2, Laboratorio Nacional de Luz Sincrotron (LNLS,Brazil) also in Langmuir monolayers. The reflectivity was obtained in a Brewster?s AngleMicroscopy (BAM) and the refractive index was obtained in the same setup via ContrastMatching by varying the refractive index subphase. The results showed an acceptableapproximation of thickness for one of the lipids (DMPC) with an overestimation of around5.5% and an error margin 1 order of magnitude smaller than the reference technique(GIXOS). If GIXOS are used with a correction proposed in our group, then 0.17 nm shouldbe added to the raw measurement, the agreement is also acceptable. Nevertheless, theother lipid (DPPC) showed a big overestimation and the same error margin as before. Ifcorrection of 0.17 applies, then the thickness (2.14 nm) is closer to the 2.6 nm by BAM.This side-by-side comparison allows us to better asses the limitations of each techniquewhen measuring monolayer thickness and provides insight into plausible sources of error,such as unaccounted anisotropy in the liquid condensed phase of DPPC, due to axial tilt oflipid molecules, this deviations from the ideal Fresnel Interface on which the model isbased and thus help to improve future experiments.