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.