Project Details
Projekt
The molecular scale of switchable wetting
Applicant
Professorin Dr. Ellen Backus
Subject Area
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term
from 2019 to 2024
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 422852727
Intermolecular interfacial interactions co-determine macroscopic wetting properties, yet insights into wetting at the molecular level have been lacking. Here, we propose to use photoswitchable surfaces based on spiropyran/merocyanine isomerization to instantaneously switch the surface’s wetting properties, and follow the molecular response of water in real-time. In the closed spiropyran form, the molecule is nonpolar, while it is zwitterionic in the open merocyanine form. Under UV light irradiation the merocyanine form is obtained; visible light switches the molecule back to the closed form. It is well-known that surfaces functionalized with the spiropyran/merocyanine pair behave hydrophobic for the spiropyran case, but hydrophilic for the merocyanine form. The ability to switch the hydrophobicity of these surfaces using ultrashort laser pulses – shorter than the timescales on which molecules reorient –provides a unique way to follow the response of water to a step change in the hydrophobic surface properties. In this manner, we aim to obtain molecular-level information about the (de)wetting dynamics at switchable substrates and to correlate molecular-level details on the water organization with macroscopic wetting properties. The structure of water and the organic coating at the interface will be investigated before, during, and after photoswitching using sum frequency generation (SFG) spectroscopy. In SFG, an infrared laser pulse and a visible laser pulse are overlapped at the interface. If the infrared laser pulse is in resonance with a molecular vibration, the signal is strongly enhanced. Due to its selection rules, SFG probes specifically the interfacial layers and does not see the bulk water. The vibrational frequency provides information about the strength of the hydrogen bond network, while the intensity of the signal is a measure for the amount of water alignment. Furthermore, we can obtain information about the ordering of the polymer from its CH vibrations. By combining the SFG probe method with an optical pulse to initiate the transition between hydrophobic and hydrophilic surface, we can obtain dynamical information on sub-picosecond timescales, to probe the molecular timescales on which the water molecules adapt to the new surface structure. Typical questions we aim to address are: - What are differences in the hydrogen bond network of water and the water orientation for the hydrophobic and hydrophilic structure? - How fast does the photoswitch switch and how fast does the rest of the polymer change ordering and orientation? - How fast does the water adapt to the new situation?- What is the dynamics of the contact line spreading? This study will provide unprecedented insights into wetting phenomena at a molecular level, expected to open avenues not only for a better fundamental understanding, but also for designing superior active surfaces.
DFG Programme
Priority Programmes
International Connection
Austria
Co-Investigator
Professor Dr. Mischa Bonn
