3.2 Thermal effect on SFG-VS spectra.
Temperature change usually leads to a bright variation on the formal vibrations, such as intensity change or vibrational shift in spectra.23,24,35,50 As reported by Souna et al, the methyl-stretching region of the spectrum of acetonitrile/silica interface for all polarization would lead to a decline in magnitude and shift to the blue as the temperature was raised from 22 ℃ to 60 ℃, suggesting the second sublayer at the acetonitrile/silica interface would be substantially influenced by the temperature araisment.51 Similar phenomenon could also be observed at the vapor/water interface. Strazdaite and coworkers proposed that a temperature rise from 20 ℃ to 60 ℃ caused a 40% decrease in the hydrogen-bonded OH stretch signal at the D2O/air interface, and the decline in the same stretch region was rather more distinct at the D2O/hexane interface.22 These researches indicate that the ordering of the molecules at the interface influenced by the temperature at various degree.
Here we measured the C-H stretching region of the BUT-EG mixtures at six different mole fractions at 64 ℃. The SSP vibrational mode are shown in Figure 3 at 22 and 64 ℃, while their PPP and SPS polarization spectra are shown in the SI (Figure S3-S7). The obvious decline could be observed in all spectra when the temperature rose to 64 ℃. The SSP polarizations spectra of pure in Figure 3(a), the decline of the intensity of the CH2-ss at 2880cm-1larger than the Fermi stretch’s vibration. Clear similarities could be also seen in the SSP spectra of xbut = 0.10 which in Figure 3(b). But the peculiarity of this case could be remarkably observed. From Figure 2, we know that the CH2-ss peak of EG and CH3-ss peak of BUT are both clear atxbut = 0.10 under a room temperature. At 64 ℃, both peaks mentioned above were notably weaker, but the intensity of CH3-ss peak decreased in a “faster” mode. It could be noticed that CH2-ss of EG was weaker than the CH3-ss of BUT at room temperature, and both peaks become equal in intensity at 64 ℃.
At an ambient temperature, the spectra from xbut = 0.30 to 1.00 are almost the same in Figure 2. But after the temperature turn to 64℃, the changes in intensity for different mole fractions are different. The pure BUT spectra in Figure 3(f) showed that the rise of temperature from 22 ℃ to 64 ℃ lead to an approximate 30% decrease in intensity of CH3-ss. Through the PPP and SPS spectra in Figure S4-S8 (SI), we can also notice that the CH3-as vibrational mode also has an obvious decline in intensity. The decrease in magnitude could also be observed in Figure 3(c)-3(e), but the extent of the decline becomes much weaker around the azeotropic mole fraction. It is also clear that we did not notice any distinct vibrational shifts of these peaks during the spectra measurements of the temperature rise.
Figure 3. SFG-VS spectra of mixture at the vapor/liquid interface at 22 ℃, 64 ℃ under SSP polarization conditions at different bulk mole fractions of BUT.
There are various possible causes of a decline in the intensity. In Moberg et al’s work, a temperature rise from 238K to 368 K lead to a decline in the negative hydrogen bonded region of water between ~3200cm-1 and ~3600cm-1, which attributed to a separating contributions associated with water molecules donating zero, one or two hydrogen bonds.23 But for acetonitrile at the silica/liquid interface, a decline in intensity caused by temperature rise from 298K to 333K was attributed to the more disordered first sublayer of acetonitrile molecules at the silica/liquid interface.24 In summary, the cause of a decline in intensity is always related to the organization change of the corresponding molecules at the interface. We will further investigate this variation by molecular dynamics simulation in the following section.