Photo-Induced Dynamics at Liquid Interfaces

Understanding and controlling structure and function of liquid interfaces has been a constant challenge in biology, environmental science, nanoscience and nanotechnology. From the ion distribution of inorganic ionic compounds at the liquid-air interface, which is crucial for understanding fundamental surface and atmospheric chemistry, to photo-active molecules for r controlled drug release applications. X-ray diffraction and reflectivity provide invaluable probes for studying the atomic scale structure at liquid–vapour interfaces. I will introduce the basics of X-ray scattering and reflectivity [1, 2] and show how the method can be employed in combination with optical excitation to investigate surface dynamics. 

Following the reaction of biological membranes to external stimuli reveals fundamental insights into cellular function. Self-assembled lipid monolayers act as model membranes containing photoswitchable azobenzene glycolipids [3] for investigating structural response during isomerization by combining Langmuir isotherms with X-ray scattering. Controlled in situ trans/cis photoswitching of the azobenzene N = N double bond alters the DPPC monolayer structure, causing reproducible changes in surface pressure and layer thickness, indicating monolayer reorientation[4]. Studying the morphology of azobenzene containing lyotropic liquid crystals with in situ SAXS identified their shapes and mesophases. Reversible light-induced mesophase transitions from a lamellar to a cubic mesophase for combinations with the azobenzene amphiphiles in vesicules were revealed. [5,6]

Studying ion enrichment and depletion of inorganic ionic compounds at liquid interfaces contributes to understanding and explaining chemical reactions at liquid surfaces. Moreover, ions can be excited by UV laser pulses, photoreactions occur on time scales of few fs to s. The laser pump - X-ray probe setup at the LISA endstation at P08, PETRA III [7] allows to study ions at liquid-air interfaces to understand the role they play in atmospheric, biological, and chemical processes, including reactions of ozone with sea-salt aerosols in the troposphere. We used complementary X-ray reflectivity and total reflection X-ray fluorescence measurements to investigate the ion distribution at the liquid-air interface of aqueous salt solutions to consider accumulation and depletion behaviour at the liquid interface.

References

[1] B. M. Murphy et al., Journal of synchrotron radiation21, 45-56 (2024).

[2] O. Seeck et al., Journal of synchrotron radiation19, 30-38 (2012).

[3] F. Reise et al., Chemistry – A European Journal 24, 17497-17505 (2018).

[4] J. E. Warias et al., Scientific reports, 13(1):11480, (2023)

[5] SC Hövelmann et al.,IUCrJ 11 (4), 486-493 (2024)

[6] SC Hövelmann et al., Applied Crystallography 58 (4) (2025)

[7] J. E. Warias et al., Journal of synchrotron radiation31, 779-790 (2024).