3.1 SFG-VS Results at room temperature.
We collected the SFG-VS spectra of the BUT-EG binary mixtures with different BUT bulk mole fractions (x but) which are included in Figure 2, while the SFG-VS spectra for the PPP, SPS polarizations are displayed in Figure S3 in supporting information (SI). The peak assignments of the pure components are shown in part II of SI. It is clearly shown that two broad peaks in EG spectrum are well-separated with the two shark peaks in BUT’s spectrum. Atx but = 0.1, the\(r^{+}\)mode of BUT has emerged evidently compared to the pure EG spectrum. At this concentration, both CH2-ss mode in EG molecules and \(r^{+}\) mode in BUT molecules are combined into a broad peak around 2885 cm-1. It can be further observed that the intensity of\(r^{+}\) mode of BUT molecules is larger than the intensity in EG’s CH2-ss mode at x but = 0.1. At the same time the broad peak around 2948 cm-1 is replaced by a shark peak. From x but = 0.10 to 0.30, the amplitude of \(r^{+}\)mode of BUT molecules increased and achieved at a maximum atx but = 0.30. From x but = 0.30 to 1.00, the spectra are quite same as the spectrum of pure BUT which means the change in amplitude and position of peaks could be ignored. The frequency shifts of the C-H vibration modes in the mixture are usually attribute to the changes of the solvation environments or the intermolecular interaction such as hydrogen-bonding. It is believed that the intensity of the SFG-VS spectra is related to the molar interfacial concentration and the molecular orientation of the interfacial groups. The obvious frequency shifts from x but = 0.10 to 0.30 might be due to the change of the molar interfacial concentration of the methyl group and the methylene group and the competitive vibration mode between them. Further investigations are needed to examine the validity of such a change.
Figure 2 SFG-VS spectra of SSP polarization of BUT-EG mixtures at the vapor/liquid interfaces.
The terminal CH3 group of BUT molecules in SFG studies is commonly regarded as C3vsymmetry.43-47 In this model, the methyl group is assumed to rotate about the C3 axis freely, yielding degenerate \(r_{\text{ip}}^{-}\) and \(r_{\text{op}}^{-}\)states.48 According to the features revealed from the spectra in Figure S1(a), the observed splitting in the\(r^{-}\)mode of BUT molecules could be negligible. That means other parts of this molecule make little effect on the C-H high frequency stretching vibration of methyl group. So the methyl group could be treated as C3v symmetry in this work. A slow-motion limit model means the molecules’ motion within the observation time could be ignored. According to Wei’s work49, the rapid-motion limit would be a better approximation when the SPS mode amplitudes is very small, which does not accord with this work. The 2960cm-1 peak’s amplitude in SPS spectra is comparable to the other two polarization modes. It is reasonable to use the slow-motion limit treatment to analyze the orientation of the in the rest of this work.