Studying soft matter interactions at liquid interfaces

At the interface between a liquid and a gas, many interesting chemical reactions feature soft matter. Foams, protein structures, and atmospheric aerosols all have complex interactions at the liquid surface, and many of these depend heavily on the location, density, and orientation of soft matter molecules. However, determining these properties is difficult, as experimental data does not always agree with computational models.

Yu et al. developed an experimental method using sum-frequency generation spectroscopy to accurately measure the three-dimensional structure of molecules at a liquid interface. They demonstrated this method experimentally and hope others can adapt the method for their own work.

“We developed the methodology to understand the three-dimensional structure of molecules at water interfaces by using a surface-specific vibrational spectroscopic technique,” said author Yuki Nagata. “We examined the methodology with a small molecule, formic acid, at the water-air interface.”

The team combined phase- and polarization-resolved sum-frequency generation spectroscopy to accurately measure molecular orientation at an air-water interface. They tested their approach by determining the orientation of formic acid molecules at this interface and comparing the results to those obtained through molecular dynamics simulations. The results they obtained were in agreement.

With their method proven, the researchers believe it can be used to study many different kinds of molecular interactions at surface boundaries, in particular protein structures and interactions.

“Our novel methodology to determine the three-dimensional structural conformation will be useful to unveil the catalytic reaction pathway on a heterogeneous catalysis and to monitor the protein conformational change at biological interfaces,” said Nagata.

Source: “Accurate molecular orientation at interfaces determined by multimode polarization-dependent heterodyne-detected sum-frequency generation spectroscopy via multidimensional orientational distribution function,” by Chun-Chieh Yu, Sho Imoto, Takakazu Seki, Kuo-Yang Chiang, Shumei Sun, Mischa Bonn, and Yuki Nagata, Journal of Chemical Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0081209 .