Frequency Generation Vibrational Spectroscopy Research Articles

Comment on "Bi-layering at ionic liquid surfaces: a sum - frequency generation vibrational spectroscopy - and molecular dynamics simulation-based study" by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565.

This Comment raises several questions concerning the surface structure concluded in the paper referenced in the title. Specifically, that paper ignores previous experiments and simulations which demonstrate for the same ionic liquids depth-decaying, multilayered surface-normal density profiles rather than the claimed molecular mono- or bi-layers. We demonstrate that the claimed structure does not reproduce the measured X-ray reflectivity, which probes directly the surface-normal density profile. The measured reflectivities are found, however, to be well-reproduced by a multilayered density model. These results, and previous experimental and simulation results, cast severe doubt on the validity of the surface structure claimed in the paper referenced in the title.

Polarization-Dependent SFG Spectroscopy of Near Ambient Pressure CO Adsorption on Pt(111) and Pd(111) Revisited

Polarization-dependent sum frequency generation (SFG) vibrational spectroscopy was employed to examine CO overlayers on Pt(111) and Pd(111) single crystal surfaces at room temperature. Utilizing different polarization combinations (SSP and PPP) of the visible and SFG light allows to determine the molecular orientation (tilt angle) of interface molecules but the analysis of the measured I_{text{ppp}}/I_{text{ssp}} is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio left( {R={beta _{aac}}/{beta _{ccc}}={beta _{bbc}}/{beta _{ccc}}} right) is not exactly known and varying R in the range 0.1–0.5 yields tilt angles of 40°–0°, respectively. Based on the known perpendicular adsorption of CO on Pt, an exact R-value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10−4 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of {I_{{text{ppp}}}}/{I_{{text{ssp}}}} on the incidence angles of visible and IR laser beams. Complementing previous low temperature/low pressure data, room temperature CO adsorption on Pd(111) was examined from 10−6 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratio for Pd (48 vs. 27 on Pt) renders the detection of adsorbed CO in SSP spectra difficult. The presence or absence of CO species in SSP spectra can thus not simply be correlated to tilted or perpendicular CO molecules, respectively. Careful modeling, including not only molecular and interface properties, but also the experimental configuration (incidence angles), is certainly required even for seemingly simple adsorbate–substrate systems.

Coherent Vibrational Dynamics and High-resolution Nonlinear Spectroscopy: A Comparison with the Air/DMSO Liquid Interface

We present a comparative study on the C—H stretching vibrations at air/DMSO (dimethyl sulfoxide) interface with both the free-induction decay (FID) coherent vibrational dynamics and the sub-wavenumber high resolution sum-frequency generation vibrational spectroscopy measurements. In principle the frequency-domain and time-domain spectroscopic measurements should generate identical information for a given molecular system. However, when the molecular systems are with several coupled or overlapping vibrational modes, obtaining detailed spectroscopic and coherent dynamics information is not as straightforward and rather difficult from either the time-domain or the frequency domain measurements. For the case of air/DMSO interface that is with moderately complex vibrational spectra, we show that the frequency-domain measurement with sub-wavenumber high-resolution sum- frequency generation vibrational spectroscopy is probably more advantageous than the time- domain measurement in obtaining quantitative understanding of the structure and coherent dynamics of the molecular interface.

Surface Characterization of Poly(DL-Lactide-co<i></i>-Glycolide) (PLGA) Membranes Fabricated by the Various Methods for Biomedical Applications

A systematic study on molecular structure of Poly (DL-lactide-co-glycolide) (PLGA) films by the methods of being chemically immobilized onto self-assembled monolayers (SAMs) and spin coating using Fourier transform infrared spectroscopy (FTIR), sum frequency generation (SFG) vibrational spectroscopy is reported. Especially, a surface sensitive second order nonlinear optical technique, SFG vibrational spectroscopy, was applied to study molecular structure on PLGA films. It is shown that the more chemical groups of PLGA thin films with being chemically immobilized have orientation than that of PLGA films made by spin coating on the basis of SFG vibrational spectroscopy data. The characterization of the other analytical techniques indicates similar results. This work will be an important step for future biomaterials design and understanding biosensors, biocompatibility and biomacromolecule (e.g. proteins and peptides) activity.

A Powerful Nonlinear Optical Technique to Characterize Surfaces and Interfaces Sum Frequency Generation Vibrational Spectroscopy

In the last two decades, a nonlinear optical technique - sum frequency generation (SFG) vibrational spectroscopy was developed into a powerful characterization method to probe the molecular structures of various surfaces and interfaces in situ. In this paper, we gave a general introduction of the SFG theory and two case studies to demonstrate the power of the SFG spectroscopy.

Surface properties and chain structure of fluorinated acrylate copolymers prepared by emulsion polymerization

The fluorinated copolymer poly (MMA–co–FMA), composed of methyl methacrylate (MMA) and 2-perfluorooctylethyl methacrylate (FMA), was prepared by emulsion polymerization using a pre-emulsified monomer addition process. The results showed that the contact angle of water on its solvent-borne film increased dramatically and reached 118o when the FMA content in the copolymer was only 0.65 mol%, approaching that of poly(2-perfluorooctylethyl methacrylate) homopolymer. Unlike the copolymer prepared by solution polymerization, X-ray photoelectron spectroscopy (XPS), and sum frequency generation (SFG) vibrational spectroscopy analysis results indicated that the fluorinated moieties in this polymer were very easily segregated at the surface of the film. The interfacial structure and properties of this polymer in solution were investigated using SFG, surface tension, and dynamic laser light scattering (DLS). It was found that emulsion polymerization produced a chain structure of the fluorinated copolymer similar to that of FMA capped PMMA, thereby providing a possible way to produce fluorinated end-capped polymers using a popular polymerization method.

Detection of Interfacial Structures of Poly(ethylene glycol), Poly(propylene glycol) and Their Copolymers at Liquid/Solid Interfaces Using Sum Frequency Generation Vibrational Spectroscopy

ABSTRACTWe have applied sum frequency generation (SFG) vibrational spectroscopy, a submonolayer surface sensitive analytical technique, to study interfacial structures of poly (ethylene glycol) (PEG), poly (propylene glycol) (PPG), their solutions, their copolymers, and copolymer solutions while contacting different media including air, fused silica, and various polymers.For the first time, conformations of polyether materials at various interfaces have been shown at the molecular level in situ. Depending on the hydrophobicity of the solid contacting media, the liquid polymers PEG and PPG show different conformations at different interfaces, which can be correlated to molecular interactions at these interfaces. The favorable interaction between hydrophobic media and the hydrophobic segments, methylene or methyl groups, of polyethers causes an ordered conformation with these groups standing up at the interface. The unfavorable interaction between hydrophilic media and hydrophobic segments of the polyethers induces interfacial methylene or methyl groups to have a random structure or to lie down at the interfaces, indicated by the weakening or even absence of SFG signals. For comparison, interfaces between aqueous PEG or PPG solutions and air, polystyrene (PS), poly(methyl methacrylate) (PMMA), and fused silica have also been investigated. In addition, we have studied conformations of pluronics at various interfaces, showing that different blocks have different interface activities. The interfaces are always dominated by the PPG segments. The studies on molecular level interfacial structures of PEG, PPG and pluronics will help us to understand and control the interfacial behavior of liquids at interfaces.