Thorough understanding of interfaces requires an assessment of both the surface and bulk properties through the use of multiple techniques. In this thesis, infrared absorption, Raman scattering and sum frequency generation were used as vibrational probes of different features of interfacial systems including the ability to measure surface and bulk effects. Two-dimension correlation analysis was used to study the relationship between the spectral response of the different techniques. Attenuated total reflection absorption, bulk Raman scattering and sum frequency generation were used to study the adsorption of ethanol--water mixture on fused silica. With the use of two-dimension correlation analysis, interesting results were observed concerning the behavior of the surface in respect to the bulk. Surface concentration of ethanol were concluded to be higher than in the bulk indicative of competitive adsorption. Furthermore, at low concentration ethanol was shown to adsorb to the surface in dimers, to then form a bilayer of strongly oriented ethanol molecules at higher concentration. At highest concentration, this bilayer is disturbed, leaving only one layer at the surface of oriented ethanol molecules. The same spectroscopic techniques were applied to pressure sensitive adhesives of different composition while drying on a sapphire surface. The presence or absence of acrylic acid in the material was shown to alter the reorientation at the surface while drying. In the case where no acrylic acid is present, the orientation of the polymer at the surface was driven by the packing of the molecules at the surface. When acrylic acid was present in the pressure sensitive adhesive, reorientation occurred much faster and was caused by strong hydrogen bonding with the surface of the sapphire. An increase in acrylic acid composition, increased the rate of reorientation. An experimental set up was constructed to specifically study interfaces with a nonuniform distribution within the plane of the surface. This allows for concomitant measurement of polarized total internal reflection Raman scattering and sum frequency generation spectroscopy along with bright field imaging and cross polarized imaging. This set up was used to study the L-histidine crystal in situ adsorbed on fused silica. The polarized experiments along with calculations allowed for a more in-depth analysis of the crystal orientation effect on the birefringence, the Raman and the sum frequency generation. / Graduate

L'interface (0001) alpha-quartz hydroxylé/eau liquide est modélisée par dynamique moléculaire dans le formalisme de la théorie de la fonctionnelle de la densité (DFT-MD). Poursuivant une étude déjà publiée sur la structure et la réactivité acido-basique de cette interface, nous élargissons l'analyse aux effets d'ions simples et de pH sur la structure et les propriétés de l'interface avec une attention particulière aux phénomènes d'adsorption ainsi qu'aux modifications de l'eau interfaciale et des sites de surface. Nous caractérisons en particulier les changements dans la réactivité de surface dus à la présence d'ions (cations alcalins et halogénures) par calcul direct de pKa et détaillons la structure de la double couche électrique dans le cas de paires d'ions.Nous cherchons de plus à calculer les spectres SFG (Sum Frequency Generation) vibrationnels à ces interfaces, ce qui serait une première pour une interface solide/liquide traitée au niveau DFT-MD. L'approche théorique de cette spectroscopie vibrationnelle non linéaire du second ordre permettrait de proposer une interprétation claire des bandes d'élongation O-H, alors que les études expérimentales continuent à diverger sur ce sujet. / We use Density-Functional-Theory-based molecular dynamics simulations to investigate the hydroxylated (0001) alpha-quartz/liquid water interface. As a follow up of an already published study on the structural and acid/base properties of this interface, we now focus on simple ion and pH effects on these properties and characterize adsorption behaviors and interfacial changes on both solid and liquid sides. In particular, we directly calculate surface pKa's in presence of ions (alkaline cations and/or halide anions) and provide microscopic details on the structure of the electric double layer when ion pairs are concerned.Besides, we try to apply DFT-MD simulations to the computation of vibrational Sum-Frequency Generation spectra at a solid/liquid interface. Indeed, calculations would be necessary to provide a clear interpretation of the vibrational bands in the OH stretching region since experimental band assignment is still a matter of debate.

The molecular structure and charge on solid surfaces in aqueous environments is of fundamental importance to various scientific research and applications, yet remain not sufficiently understood. The research herein uses sum frequency generation spectroscopy to reveal the molecular structure of the mineral and polymer surfaces, and also probes the water molecules near the charged aqueous interfaces to get information about the surface charge. The application of visible-infrared sum-frequency generation spectroscopy to polymer thin-films requires a careful interpretation of the results, as the electric field magnitude and phase at each interface must be determined in a manner that takes thin film interference effects into account. A straightforward method that has a concise analytic solution in the case of a single thin film that exhibits interference effects was proposed. This method enabled selective probing of transparent thin-films using sum frequency generation spectroscopy, hence eliminated the ambiguity of the contribution of signal from two interfaces. The method was then extended to multiple polarization schemes, enabling easier and more comprehensive study of the molecular orientation on thin-films. Nonlinear vibrational spectroscopy has also been used to study the temperature-dependent surface structure of polydimethylsiloxane when exposed to water and a perfluorinated hydrophobic liquid. Quantitative analysis of the methyl plane orientation was performed using a combination of vibrational peak ratios and peak amplitudes that enable proposed structures to be identified. For both environments, the tilt and twist of the methyl plane was found to increase with temperature in a reversible manner. This has been attributed to be a consequence of the backbone reorganization due to temperature-dependent density changes. At charged aqueous interfaces, the structure of water adjacent to solid interface is sensitive to the surface potential. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. Nonlinear vibrational spectroscopy was used to monitor the water O-H stretching band over a temperature range of 10-75°C to account for the increase in surface potential from deprotonation. It has been demonstrated that the behavior at the silica surface is a balance between increasing surface charge, and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, the change in enthalpy and entropy for deprotonation at each site were reported. The surface charge density of untreated polydimethylsiloxane surface in water with various ionic strengths was also determined. It was found that the surface charge could be explained with an ion adsorption model. A relationship between the surface potential and measured nonlinear optics response that is valid at high potentials and low ionic strength was proposed. Finally, a universal method was demonstrated to derive the surface potential with nonlinear optics by modulating the coherence length. / Graduate / 2022-09-07

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Many phenomena of interest to science and engineering occur at interfaces; however, access to, or discrimination of, interfacial interactions has been challenging, especially at buried interfaces. Vibrationally resonant sum-frequency generation (VR-SFG) spectroscopy is a powerful tool for investigating the molecular structure of free or buried interfaces, but spectral analysis has relied on many assumptions. To claim accurate new insights, practitioners must be able to make unique determinations from the data without experimental artifacts affecting the final results. For example, two independent and overlapping studies for the polystyrene/air interface were carried out, but reported different surface structures. Initially, this difference was attributed to the use of different substrates, but we have shown that the surface structure is independent of substrate by experimental suppression of the interfering nonresonant signal. These results show difficulties in SFG analysis that have led to faulty determinations of structural changes. Similar problems have been observed in systems assumed to have negligible nonresonant interference, demonstrating the need for proper experimental design instead of relying solely on post-experimental analysis of the data. We have investigated the inherent limitations imposed on the technique from the nature of the signal generation and nonresonant interference, and have developed methods to overcome such difficulties, depending on what is desired from the data. By nature of nonlinear spectroscopy, the desired frequency response is affected by overlapping interactions in the time domain, and these time domain interactions can be exploited to overcome challenges in analysis. By delaying the upconverting pulse, the nonresonant signal can be removed to enable accurate qualitative comparison or even quantify change; however this removal results in incomplete upconversion, or apodization, of the resonant signal, causing distortion in the observed resonant response. If absolute parameters are desired, additional work is necessary to correct the distortion of the resonant response. Correction can be accomplished by further exploiting time domain effects by collecting spectra at various delay times of the upconverting pulse, and this additional data also aids in interpretation of congested spectra. Many practical applications, however, only require a means to quantify change, and measurements of change are unaffected by the effects of apodization. These techniques have been used to more accurately analyze polystyrene and octadecylsilane surfaces.

The surface-specific technique of vibrational sum frequency spectroscopy (VSFS) can provide vibrational information about chemical bonds at surfaces and interfaces. Two photons, of visible and infrared frequency, are spatially and temporally overlapped at a surface/interface to produce a photon at the sum frequency (SF) of the two input photons. As well as this process only being allowed in non-centrosymmetric media (i.e. VSFS is surface/interface specific), the SF process is enhanced when the IR beam is resonant with vibrational resonances. Broadband VSFS has been used in this project to study surfaces of two distinct classes of materials, namely graphene and polymers. Firstly, broadband VSFS was used to investigate the heating polymeric thin films using a home-built heated sample cell. The cell was tested using self-assembled monolayers (SAMs) of 1-octadecanethiol (ODT) grown on gold substrates. It was subsequently used to investigate thin films of poly(methyl methacrylate) (PMMA) of four different thicknesses and two different molecular weights that were spin-coated onto gold substrates. It was shown that the monolayers of ODT become disordered upon heating and solidified to incorporate the disorder introduced by the heating process. The PMMA films were also shown to become more disordered as a function of temperature. Secondly, broadband VSFS was used to investigate modified graphene, motivated by the fact that modifications to pristine graphene, be it intentional (i.e. functionalisation) or unintentional (i.e. contamination), cause the properties of graphene to change. This project focused on studying hydrogenated graphene, N-methylbenzamide functionalised graphene and contamination on commercial graphene. A method for calculating the number of hydrogen atoms in a hydrogen island was developed. VSF spectra of CH stretches in N-methylbenzamide functionalised graphene were obtained. Residues on commercially bought graphene were detected using VSFS and RAIRS. These residues were assigned to PMMA that remained on the CVD graphene by the process of transferring the CVD graphene from the copper foil on which it was grown onto the gold substrates.

Reversed-phase liquid chromatography (RPLC) is a widely used technique for analytical separations but routinely requires empirical optimization. Gaining a better understanding of the molecular reasons for retention may mean more efficient separations with fewer trial and error runs to obtain optimized separations. Vibrationally resonant sum frequency generation (VR-SFG) is a surface specific technique that has allowed for in situ examination of model RPLC stationary phases under various solvent and pressure conditions. In order to improve on past work with model RPLC stationary phases two challenges had to be overcome. First, improved vibrational mode assignments of the C18 stationary phase were needed for proper understanding of this model system. Second, the synthesis of back-surface reference mirrors used in these VR-SFG experiments allowed us to better correct the relative intensities of the various spectral peaks present in typical spectra. After examination of model RPLC systems under various conditions, we have found that these model substrates have a significant amount of interference from nonresonant signal. This interference of resonant and nonresonant signals on fused silica surfaces has not been previously examined and further studies of the model RPLC stationary phase must properly deal with the non-negligible nonresonant interference that is present. We have seen changes in the VR-SFG spectra of these model systems under a variety of conditions including elevated pressure, however the changes are mostly due to nonresonant interference. These spectral changes, although apparently not solely from structural changes, need to be investigated further to better understand the molecular basis of retention in model RPLC systems.

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No description available.

Applying linear programming (LP) to spectroscopy techniques, such as IR, Raman and SFG, is a new approach to extract the molecular orientation information at surfaces. In Hung’s previous research, he has shown how applying LP results in the computational gain from O(n!) to O(n). However, this LP approach does not always return the known molecular orientation distribution information when mock spectral information is used to build the instance of the model. The first goal of our study is to figure out the cause for the failed LP instances. After that, we also want to know for different cases with what spectral information, can the correct molecular orientation be expected when using LP. To achieve these goals, a simplified molecular model is designated to study the nature of our LP model. With the information gained, we further apply the LP approach to various test cases in order to verify whether it can be systematically applied to different circumstances. We have achieved the following conclusions: with the help of simplified molecular model, the inability to extract a sufficient data set from the given spectral information to build the LP instances is the reason that the LP solver does not return the target composition. When candidates coming from one same molecule, even combining all three spectral information of IR, Raman and SFG, the data set extracted is still not sufficient in order to obtain the target composition for most cases. When candidates are coming from different molecules, Raman or SFG spectral information alone contains sufficient data set to obtain the target composition when candidates of each molecule expanded in [0◦, 90◦) on θ. When candidates of each molecule expanded in [0◦, 180◦] on θ, excluding 90◦, SFG spectral information needs to combine with IR or Raman in order to obtain the sufficient data set to obtain the target composition. When the slack variable is introduced to each spectral technique, for the case of candidates coming from different molecules, when candidates expanded in [0◦, 90◦) on θ, Raman spectral information carries sufficient data set to obtain the target composition. When candidates expanded in [0◦, 180◦] on θ, excluding 90◦, SFG and Raman spectral information together carries sufficient data set in order to obtain the target composition. / Graduate / chenfei.cp@gmail.com