Quantitative determination of element distributions at the gas/liquid interface by grazing-incidence X-ray fluorescence

FERNANDEZ BORDÍN, S.P.; OLIVEIRA, R.G.; GALVAN V.; DUPUY F. G.; PUSTERLA J.

2018-11-06/2020-04-22

Química

Prom.Gral.del Conoc.-Cs.Exactas y Naturales

In this scientific case we propose to mount a setup for detecting fluorescence excited at X-ray grazing incidence geometry on a Langmuir trough suited for monolayers of amphiphilic molecules at the air/water interface. The setup for grazing incidence X-ray scattering on a Langmuir trough has already been installed at XRD-2 where the technical possibilities expand over two main techniques: 1) grazing incidence X-ray diffraction (GIXD) to probe lateral  correlations, and 2) grazing incidence X-ray off-specular scattering (GIXOS) to probe the perpendicular structure of the monolayer.This setup will be translated to SIRIUS at the SAGUI beam-line where the performance will increase significantly (Antonio Gasperini, personal communication). Moreover, the time consuming and sometimes difficult experiments will be performed more quickly and easier due to the higher photon flux (with a flux 10 4 higher at LNLS), allowing getting GIXOS and GIXD good signals on time scales in the order of a second. This is in contrast to the current time scale of two hours. Then, performing a third additional measurements on the same monolayer can be affordable and desirable. In this scientific case, we propose to couple the existing grazing incidence setup of the horizontal Langmuir trough with a fluorescence detector on top of it to perform grazing incidence X-ray fluorescence (GIXRF). The penetration length of the resulting evanescent wavecan be controlled by slights modifications of the incident grazing angle, by using the very same system build to deflect the beam below the critical angle for total reflectionin grazing incidence setup. This allows the differential excitation of atoms present along the vertical profile of the monolayer itself, with the resulting fluorescence. Moreover, the counter-ion layers beneath the monolayer can also be excited. In this way, not onlyorganic surfactants, but also (in)organic ions can be studied in combination withLangmuir monolayers and even without it. This will help during the electron density modeling of the monolayer itself and/or to account for the distribution of ionic atmospheres associated to interfaces. Of course, this setup will be useful in other biomembrane model systems like solid supported layers. In such cases, even more optically refined experiments can be performed recurring to the modulation of the X-ray standing waves produced by reflection from solids. Nevertheless, since the Langmuirtrough allows for in-situ manipulations (surface density, surface tension, composition, etc) the flexibility of the biological experiment is normally higher at the air/water interface, for this reason is important to keep the horizontal sample geometry alreadydeveloped at XRD-2.