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Sum frequency generation (SFG) * Equal contributors. spectroscopy was used to deduce the orientation of the terminal methyl (CH 3 ) group of self-assembled monolayers (SAMs) at the air–solid and air–liquid interfaces at surface concentrations as low as 1% protonated molecules in the presence of 99% deuterated molecules. The SFG spectra of octadecanethiol (ODT) and deuterated octadecanethiol (d 37 ODT) SAMs on gold were used for analysis at the air–solid interface. However, the eicosanoic acid (EA) and deuterated EA (d 39 EA) SAMs on the water were analyzed at the air–liquid interface. The tilt angle of the terminal CH 3 group was estimated to be ∼39 ° for a SAM of 1% ODT : 99% d 37 ODT, whereas the tilt angle of the terminal CH 3 group of the 1% EA : 99% d 39 EA monolayer was estimated to be ∼32 °. The reliability of the orientational analysis at low concentrations was validated by testing the sensitivity of the SFG spectroscopy. A signal-to-noise (S/N) ratio of ∼60 and ∼45 was obtained for the CH 3 symmetric stretch (SS) of 1% ODT : 99% d 37 ODT and 1% EA : 99% d 39 EA, respectively. The estimated increase in S/N ratio values, as a measure of the sensitivity of the SFG spectroscopy, verified the capacity to acquire the SFG spectra at low concentrations of interfacial molecules under ambient conditions. Overall, the orientational analysis of CH 3 SS vibrational mode was feasible at low concentrations of protonated molecules due to increased S/N ratio. In support, the improved S/N ratio on varying incident power density of the visible beam was also experimentally demonstrated. 

The geometry, arrangement, and orientation of a quaternary ammonium surfactant flanked by two methyl groups, a benzyl head, and an octyl tail, were assessed at the air-water and air-deuterium oxide (D2O) interfaces using sum frequency generation vibrational spectroscopy (SFG). Remarkably, symmetric and asymmetric N-CH3 stretches (at ~2979 and ~3045 cm-1, respectively, in the SSP polarization combination) were visible in water but were negligible in deuterium oxide. We concluded that D2O addition triggers the average reorientation of the dimethyl amino units parallel to the interface, and possibly changes the overall conformation of the surfactant. A reduced number of gauche defects in the surfactant octyl chain is also observed in D2O. Tilt angles for the octyl chain (1.0 – 10.8º) are consistent with an ordered monolayer at the air-liquid interface.