3.3 The orientation of interfacial BUT and EG molecules at different temperature.
To confirm a comparable discussion between the SFG-VS measurement and Molecular dynamics simulation,\(\chi_{\text{ppp}}^{(2)}/\chi_{\text{sps}}^{(2)}\) for the CH3-as mode at the liquid/air interfaces of BUT-EG mixtures at 22℃ calculated by SFG-VS analysis and MD simulation are plotted in Figure S2 (SI). The result show that the value of\(\chi_{\text{ppp}}^{(2)}/\chi_{\text{sps}}^{(2)}\) is unchanged with a changing concentration at 22℃. On the other hand, the orientation of the methyl group could not illustrate the average tilt angle and orientational distributions of interfacial molecules are unchangeable with x but. A more detailed analysis of the orientation of the interfacial molecules at both 22℃ and 64℃ are presented in this section by MD simulation.
The simulated density profiles and ρ< cosθμ> (index could be 1,2,3,4 corresponding to the Figure 1) profiles for pure BUT and pure EG are ploted in Figure 4. Herein, ρBUT or ρEG represents the molecular density or the amounts of the corresponding molecules along the Z axis of the simulation box, while the < cos θμ>refers to the mean value of the cosine of the tile angleθμ which shown in Figure 1. The generation of SFG-VS is forbidden in the isotropic bulk phase but allowed at surface where there is a lack of inversion symmetry. Such ρ< cosθμ> profiles could help us to compare the MD results with the SFG-VS results for the molecules with broken centrosymmetry will contribute to the SFG-VS signal.34,52,53
Figure 4. Simulated ρ<cosθμ > profiles in 22 ℃ and 64 ℃ of (a) the C3-C4, C2=O2 bond in pure 1,2-butanediol box and (b) the C5-C6 bond and the angular bisector of CH2 group of C6 in pure ethylene glycol box . The simulated density profile for the corresponding components in different temperature are also plotted.
The ρ and ρ< cosθμ>profiles for x but = 0.10 to 0.59 are plotted in Figure S9 (SI). The enrichment of BUT molecules can be observed in the transition regions for these mixtures. And the enrichment at low mole fractions is more distinct. According to the SFG-VS theory, the microscopic susceptibility is related to the density and the anisotropic orientation of interfacial molecules. On the other way, the value of ρ< cosθμ> could represent the contribution to the microscopic susceptibility from the corresponding groups. At x but = 0.10, although the mole fraction of BUT molecules is rather small, the ρ< cosθμ> of BUT molecules at interface is comparable with EG’s. The phenomenon indicated that BUT is stronger surfactant and would quickly occupy the topmost surface region even at a rather low BUT mole fraction. This explains why the \(r^{+}\) mode and \(r_{\text{FR}}^{+}\) mode of BUT emerged evidently at x but = 0.10 in Figure 2.
The thickness change of the interfacial region is an important impact on the signal of SFG-VS. A variation in surface thickness could lead to a change of the density of interfacial molecules and affect the intensity of the signal of SFG-VS of corresponding groups pairwise. In the Nagata et al’s work, the surface roughness would vary with temperature at the water-air interface, and lead to the variation of the SFG signal.35
In the current work, it can be seen that the interfacial region of EG molecules gets thicker with x but increasing. The thickness of EG’s interfacial region is about 0.7nm atx but =0.0, 0.8 nm at x but= 0.10 and 0.9 nm at x but = 0.30. Furthermore the temperature change from 22℃ to 64℃ would also cause 0.1nm increase of both BUT’s and EG’s interfacial region. Such a variation of surface roughness would cause a decline in the density of the molecules at the interfacial region which could be observed in both ρ profile in Figure 4 and Figure S9. This decline in interfacial intensity finally leads to a decline in the intensity of the SFG-VS signal pairwise.
For each molecule in the simulation box has a fixed z coordinates with an orientational angle θμ at the final frame of the equilibrium state. Herein, the temperature effect on the surface molecular structure is also evidenced by the joint number density plotted in Figure 5. We calculated θ1 in BUT molecules and θ4 in EG molecules at the equilibrium state ofx but=0.457, for θ1 could also represent the orientation of the methyl group in BUT molecules and θ4 could be on behalf of the orientation of the methylene group. The n (θμ, Z) was measured by a basic unit of 0.1nm in Z coordinates, and 1° in θμ .
In Figure 5(a), the predominant orientation of methyl group in BUT molecules centered on 45° and range from 20° to 80°, inferring that the methyl group in BUT molecules prefer an upward direction to the vapor side. Compared to the EG molecules at 22℃ in Figure 5(b), the θ4 in EG molecules is centered at about 90°. Actually, an orientation at around 90° may lead to a relatively small signal in SFG-VS experiment.1 And this behavior is consistent with the phenomenon we observed at the SFG-VS measurements, of which the spectra collected at x but=0.457 are lack of characteristics of EG’s spectrum in Figure 2. When the temperature rises to 64℃, shown in Figure 5(b) and 5(d), the outer boundary of the anisotropic part turned wider. It is also obvious that most of the red region turns to the green which indicates the orientation of the methyl group gets disordered. This characteristic can also be observed in n(θ4, Z) in EG plotted in Figure 5(d).
Figure 5. Joint number density, nμ,Z) calculated from MD simulations of θ1 in BUT molecules at (a) 22℃ and (b) 64℃, θ4 in EG molecules at (c) 22℃ and (d) 64℃ of x but = 0.457.
We measured < cosθμ> of interfacial region in different temperature in order to illustrate the temperature effect on the orientation of these two kinds of molecules at the liquid/air surface, which is shown in Figure 6. The < cosθ1> , which is the average cosine of the tilt angle of the C3-C4 bond is around 0.30 at 22 ℃ and turned to about 0.20 at 64℃. At the same time, the < cosθ2> corresponding to the tilt angle of the C2-O2 bond is around 0.0. This transition indicates that the BUT molecules would like to lean on the vapor/liquid interface at 22℃. The temperature rise would magnify the angle between the carbon chain and the Z axis towards the vapor side. According to the SFG theory, the tilt angle that rotates towards 90 degrees will weaken its SFG contribution. This could illustrate the tilt angle change would also contribute to the decline of the SFG signal from 22℃ to 64℃. In contrast to the BUT molecules, the tilt angle between the carbon chain of the EG molecules and the interface is rather small at 22℃, and a temperature rise would not significantly affect the orientation of EG molecules.