Surface Spectroscopy Reveals Ordering and Alignment of Liquid Carbon Dioxide at Sapphire Interfaces

The interaction between liquid carbon dioxide and material surfaces plays an important role for the function and durability of carbon capture and utilization (CCU) systems. Experimental data on this interaction are scarce due to the difficulty in tracking interfacial layers of CO₂ in contact with solid surfaces. In this work, we use surface-sensitive sum-frequency generation (SFG) spectroscopy combined with molecular dynamics simulations and DFT calculations to measure the geometry and orientation of the first molecular layer of liquid CO₂ in contact with a sapphire surface. The results show that the CO₂ molecules preferentially adopt a tilted orientation with an angle of 14 ± 8° with respect to the surface normal. Furthermore, the electronic structure of CO₂ is perturbed by the interface leading to symmetry breaking: one C═O bond is elongated along the molecular axis while the other is shortened. These results pave the way for using SFG spectroscopy to measure the interactions of liquid CO₂ with functional surfaces, catalysts, and CCU relevant materials.