Speaker
Description
Liquid-vapor (especially aqueous-vapor) interfaces play a major role in atmospheric science, for example in the interaction of the oceans with the atmosphere, in aqueous aerosols where they govern important chemical reactions, or in the evaporation of lakes and reservoirs. Sparely soluble, amphiphilic organic molecules (e.g. long chain fatty acids) tend to accumulate at the surface of aqueous solutions and can change dramatically the properties of the interface. One of the crucial steps towards a better understanding of the effect of surfactants is to obtain molecular-scale information on the arrangement and behavior of these molecules at the interface.
Here we present an investigation of octanoic acid and perfluorinated pentanoic acid as model surfactants in aqueous solution at various pH and concentrations. We use X-ray photoelectron spectroscopy (XPS), which has been established as an important technique to study the liquid-vapor interface at the molecular level [1], coupled with a liquid microjet. We show how the measurement of photoelectron angular distributions (PADs) in XPS, a so far little explored parameter, can provide information on the relative depth of molecules and functional groups within molecules. In parallel, we use molecular dynamics (MD) calculations to simulate the behavior of protonated and deprotonated octanoic acid at the interface and compare with the experimental results.
Our combined experimental and theoretical investigations demonstrate that the carboxylate (COO-) group of octanoic acid is located deeper into the solution than the carboxylic acid (COOH) group, and that the deprotonated molecule stands more upright than the protonated molecule at all concentrations.[2] Building on this result we demonstrate for the case of perfluorinated pentanoic acid that a spatial resolution in the direction perpendicular to the surface of 1 Å can be achieved in PAD measurements.[3] These results open up new opportunities for a systematic investigation of the interaction of ions and surfactants at the liquid-vapor interface, in particular the effect of ionic charge and surfactant functional group character on their relative propensity for the interface.
References
[1] R. Dupuy, C. Richter, B. Winter, G. Meijer, R. Schlögl, H. Bluhm, J. Chem. Phys. 154, 060901 (2021).
[2] R. Dupuy, J. Filser, C. Richter, R. Seidel, F. Trinter, T. Buttersack, C. Nicolas, J. Bozek, U. Hergenhahn, H. Oberhofer, B. Winter, K. Reuter, H. Bluhm, Phys. Chem. Chem. Phys. 24, 4796 (2022).
[3] R. Dupuy, J. Filser, C. Richter, T. Buttersack, F. Trinter, S. Gholami, R. Seidel, C. Nicolas, J. Bozek, H. Oberhofer, K. Reuter, U. Hergenhahn, B. Winter, H. Bluhm, in preparation.
| Abstract Number (department-wise) | AC 5.1 |
|---|---|
| Department | AC (Schlögl) |
