Orientational Analysis of Interfacial Molecular Groups of a 2‑Methoxyethyl Methacrylate Monomer Using Femtosecond Sum Frequency Generation Spectroscopy

This study employs femtosecond sum frequency generation (FS-SFG) spectroscopy to identify the functional groups at the air–liquid interface of the 2-methoxyethyl methacrylate (MEMA) monomer. Based on the collected spectra, the methoxy group (−OCH₃), the methylene (−CH₂) group from the ethyl side chain of MEMA, the α-methyl group (α-CH₃), and the alkene-methylene (CH₂) groups are present at the interface. Tilt angle was determined by calculating the intensity ratio values from the fitting results, and then the ratios are compared to the simulated SFG curves. The results derived from the conventional polarization combination (SSP, PPP, and PSS) approach show average tilt angles of 60° for −OCH₃ symmetric stretch vibration dipoles. Using the polarization mapping method, the tilt angle for the symmetric stretch of the −OCH₃ group was 48° with an intensity ratio (SSP/PPP) of 17. The orientation distribution of the functional groups was also obtained from the amplitude ratios using the two methods. In conclusion, the MEMA monomer is partially ordered at the air–liquid interface as a result of influences from its different functional groups. These results suggest that the α-CH₃ and −OCH₃ symmetric stretches are tilted further away from the surface normal, thus deviating from the presumed model of well-ordered interfacial molecules