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Aqueous mineral oxide surfaces are ubiquitous in nature, where they play an important role in soil erosion, delta formation etc. Understanding the interfacial solvent environment at mineral oxide surfaces is important as many reactions, e.g., mineral dissolution, heterogeneous catalysis, and electrochemical water splitting occur at interfaces.Vibrational sum frequency generation (vSFG), a second-order nonlinear spectroscopic technique, inherently surface specific under the electric dipole approximation, serves as an excellent tool to study aqueous interfaces. vSFG is forbidden in centrosymmetric environments under the electric dipole approximation, making vSFG inherently specific to non-centrosymmetric environments such as surfaces, where the centrosymmetry is broken. vSFG is capable of measuring interfacial structure and dynamics without contributions from the bulk. Though vSFG has been extensively used to study aqueous interfaces yet there remain fundamental questions that need to be addressed. Is the interface capable of perturbing the environment of a centrosymmetric molecule to render it vSFG active? What higher order multipole terms contribute to vSFG? What are the vibrational energy relaxation pathways and mechanisms at oxide/water interfaces? In this dissertation, we have employed Stark active IR probe molecules (SCN-, N3-), that are sensitive to the local environment and whose frequency shifts depend on the localized electrostatic potential, to understand the interfacial solvent environment and measure the electrostatic potential associated with the charged sites at the aqueous Al2O3(0001) surface. The vibrational lifetime of IR probe molecules sheds information on solvent polarity, H-bonding network, and applied external electric fields. Hence, measuring the vibrational dynamics, whose timescales are comparable to the vibrational lifetime of the IR probe molecules, is a useful tool to understand vibrational energy relaxation (VER) pathways and mechanisms, specific solute-solvent interactions, and localized solvent environment. Though IR probe molecules have been employed to study bulk solvents, the literature for interfaces/surfaces is limited to reverse micelles, air/water interfaces and metal electrode surfaces. The VER rates of IR probe molecules (charged solutes) in bulk solvent and confined solvent environments are significantly different, which reflects the different local properties. The aim of this dissertation is to understand the localized solvent environment as well as the VER pathways and mechanisms of the IR probe molecule (SCN-) at the aqueous mineral oxide interfaces using IR pump-vSFG probe spectroscopy. Bulk H2O and D2O are similar in terms of H-bonding capability, static dielectric constant, and dipole moment. The FTIR spectra of the CN stretch of SCN- in bulk H2O and D2O share a similar central frequency, yet the measured vibrational lifetimes of SCN- reveal accelerated vibrational energy relaxation in bulk H2O vs. bulk D2O, indicating fundamental differences between the two solvent environments. This reflects distinct vibrational energy relaxation pathways. Probing the vibrational lifetime of the CN stretch of SCN- at the alumina(0001)/H2O and alumina(0001)/D2O interfaces enabled us to understand the effect of the interfacial solvent density of states on the solute-solvent vibrational coupling at interfaces. We observed three times faster vibrational energy relaxation (VER) for interfacial D2O (T1 ~7 ps) compared to bulk D2O (T1 ~22 ps). The lifetime of the CN stretch at the α-Al2O3(0001)/H2O interface (T1 ~3 ps) is, however, similar to the dynamics in bulk H2O (T1 ~ 2.7 ps) where effective coupling with the solvent combination band (water bending + librational modes) provides an efficient pathway for intermolecular vibrational energy transfer. Ab-initio simulations show that there is an increase in the vibrational density of states (VDOS) at the interface in the low-frequency region of the O-D stretch, resulting in greater overlap between SCN- and D2O vibrational modes compared to the bulk D2O. The VDOS is not the only factor determining VER. At the interface, there are likely enhanced solute-solvent interactions due to increased transition dipole – transition dipole coupling, as a result of reduced dielectric constant and more net oriented molecules. The two factors (a) availability of accessible energy-accepting states of the solvent and (b) increased solute-solvent coupling, cause acceleration in the vibrational relaxation at the alumina/D2O interface. This work provides insight into the vibrational relaxation pathways and coupling between solute and solvent vibrational modes, which is essential for understanding fundamental condensed phase phenomena in the bulk as well as at interfaces. Our research suggests that VER dynamics cannot be generalized for all interfaces as there are significant differences between how charged solutes behave within confined reverse micelles, at the air/water interface, and at solid/water interfaces. In this dissertation, the basic question of the origin of non-centrosymmetry is also addressed by studying the steady state vSFG response from the azido stretch of N3-, a centrosymmetric molecule, at the α-Al2O3 (0001)/H2O interface. We observed the azide asymmetric stretch peak at the aqueous alumina interface demonstrating that the interface sufficiently perturbs the centrosymmetric environment of the azide ion to make it vSFG active, thereby re-emphasizing the surface-specificity of the vSFG technique. DFT calculations revealed that the application of an external electric field (in the range 0.1 - 0.5 V/Å, similar to the ones typically observed at interfaces), 1-3 the centrosymmetry of the azide ion is broken, introducing Raman activity to the previously IR only active mode (asymmetry azide stretch) thereby making the mode vSFG active. Unlike metal surfaces, where the electrostatic potential is homogeneously distributed over the surface, mineral oxide surfaces have localized and spatially heterogeneous charged sites depending on the bulk pH solution, due to protonation/deprotonation of terminal hydroxyl groups. We employed the asymmetric stretching frequency of N3, an IR probe molecule, that is sensitive to the local solvent environment and applied electric potential to determine the localized interfacial electrostatic potential. Having demonstrated that the interface perturbs the centrosymmetry of N3-, shifts in the central frequency of its asymmetric stretch mode can be used to report on the interfacial localized surface potential of the Al2O3 surfaces. Our previous work using Stark active SCN- to probe the localized charged sites of the alumina (0001)/H2O interface led to the foundation of vSFG spectroscopy as a probe of the local electrostatic potential. Using the N3- Stark tuning rate, the localized electrostatic potential at the negatively charged Al-O- sites was measured to be -170 mV, similar to the one measured by SCN- (-154 mV). In this dissertation, we expand the library of nitrile groups that can be used to measure the interfacial electrostatic potential by using N3-, another Stark active IR molecule, while probing the origin of non-centrosymmetry in this centrosymmetric molecule at mineral oxide/water interfaces. / Chemistry

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Over the past two decades, vibrational sum-frequency generation (VSFG) has been applied as a versatile technique for probing the structure and dynamics of molecules at surfaces and interfaces. The excellent surface specificity of the SFG allows for probing different kinds of liquid interfaces with no or negligible contribution from adjacent and much deeper bulk phase. VSFG spectroscopy has provided evidence that the structure of the water at interfaces is different from the bulk. With the ultrafast pulses, VSFG can also be used as a probe of ultrafast vibrational dynamics at interfaces. However, apart from a few pioneering studies, the extension of VSFG into time domain has not been explored extensively. Here VSFG is used as a probe of ultrafast vibrational dynamics of water at silica interfaces. Silica is an excellent model system for the solid phase where one can systematically vary the surface charge via bulk pH adjustment. The extension of the surface electric field, the interfacial thickness and surface accumulation of ions at a charged silica surface were studied using IR pump-VSFG probe spectroscopy. A vibrational lifetime (T1) of about 250 fs, similar to bulk H2O, was observed for the O-H stretch of H2O/silica interface when the silica surface is negatively charged. At the neutral surface, where the thickness of interfacial water is smaller than at the charged surface, the vibrational lifetime of O-H stretch becomes more than two times longer (T1~ 600 fs) due to the decreased number of neighboring water molecules, probed by SFG. The fast T1 at negatively charged surface begins to slow down by screening of the penetration of surface electric field via adding salt which suggests the primary reason for similar vibrational dynamics of water at charged interface with bulk water is the penetration of electric field. By decoupling of OH of HDO in D2O, a frequency dependent vibrational lifetime is observed with faster T1 at the red compared to the blue side of the hydrogen bond spectral region. This correlates with the redshift of the SFG spectra with increasing charged surface and is consistent with a theoretical model that relates the vibrational lifetime to the strength of the hydrogen bond network. / Chemistry

Mineral oxide/aqueous interfaces play an important role in the transport of water through aquafers and streams, erosion, the formation of beaches and river deltas, nuclear waste storage, the sequestration and filtration of small ions, and are widely used in industrial scale catalysis. Unlike metal or semiconductor electrodes, the surface charge resulting from the protonation or deprotonation of insulating mineral oxides is highly localized and heterogeneous in nature. While the unique acid/base chemistry associated with different mineral oxide surfaces leads to their wide variety of applications, the extent to which surface groups found on mineral oxides partake in acid/base chemistry is still controversial due to the difficulty associated with experimentally probing them. Surface specific spectroscopic techniques, such as vibrational Sum Frequency Generation (vSFG), provide an opportunity to investigate how the surface architecture and corresponding chemical nature of various mineral oxide surfaces orient the interfacial solvent at a variety of solvent compositions and surface charges. Although vSFG has been used as a tool to measure the orientation and composition of interfacial O-H species originating from the surface and solvent for many mineral oxide/aqueous interfaces since the late 1990’s, controversy still exists in the assignment of vSFG spectra in the O-H stretching region of SiO2, Al2O3, CaF2, and TiO2/aqueous interfaces. The first section of this dissertation focuses on how the nonlinear optics and computational community’s understanding of the structure associated with mineral oxide/aqueous interfaces has evolved and where it stands now. Of particular interest is how the addition of electrolyte and variation of bulk pH allow modulation of the depth of the interfacial region and surface charge. Electrolyte solutions can vary the length of the interface by screening interfacial charges through non-specific adsorption at the interface, or generating surface charge if accumulation is facilitated by specific adsorption. The specific interaction of small ions with mineral oxide surfaces is relevant in geochemistry and filtration technology, and can also aid in prediction of contaminant mobility in ground water systems. Chapters two and three discuss the theory and application of vSFG, and the experimental setup used to capture vSFG spectra in this work, respectively. The fourth chapter investigates how monovalent or divalent cations accumulate at alpha-Al2O3(0001)/H2O interfaces and reorganize the interfacial solvent structure. The reactivity of these interfaces is strongly impacted by the presence of ions. Thus, it is critical to understand how ions alter the interfacial environment. This is achieved by measuring the changes in the structure and vibrational dynamics of interfacial water induced by the presence of ions in close vicinity to the mineral surface. The alpha-Al2O3(0001) surface represents a flexible platform to study the effect of ions on interfacial aqueous environments at positive, neutral and negative surface charge. Using vibrational sum frequency generation (vSFG) in the frequency and time domain, we investigate how monovalent and divalent cations affect the hydrogen bonding environment of the first few layers of interfacial water next to an alpha-Al2O3(0001) surface. Our results indicate that monovalent cations, such as Li+, Na+, K+, and Cs+, appear to have lower adsorption affinities for the interface compared to Ca2+, Sr2+, and Ba2+. This leads to an interfacial region that is structured in a cation valence dependent manner. Time resolved vSFG measurements reveal that the O-H vibrational lifetime (T1) of interfacial species at pH 10 conditions in the presence of NaCl and BaCl2 remains similar, but restructuring of the surface seen in steady state vSFG is manifested in the degree to which strongly hydrogen bonded species recover to their original populations post excitation. By tracking the accumulation of ions at the interface via the vSFG response, we can characterize the unique surface arrangements of interfacial water molecules induced by a range of monovalent and divalent cations at the alpha-Al2O3(0001)/water interface. In the fifth chapter the Stark active C ≡ N stretch of potassium thiocyanate is used as a molecular probe of interfacial electrostatic potential at the alpha-Al2O3(0001)/H2O interface. We confirm the presence of the thiocyanate ion in the interfacial region via reorganization of surface waters in the O-H stretching region. Changes in electrostatic potential are then tracked via Stark shifts of the vibrational frequency of the C ≡ N stretch. Our vSFG measurements show that we can simultaneously measure the SFG response of SCN- ions experiencing charged and neutral surface sites and assign a local potential of + 308 mV and -154 mV to positively and negatively charged aluminol groups, respectively. Thiocyanate anions at charged surface sites adopt similar relative orientations independent of surface charge, but adopt an opposite orientation at neutral surface sites. MD-DFT simulations of SCN- near the neutral alpha-Al2O3(0001)//H2O interface show that the vSFG response in the C ≡ N stretch region originates from a SCN-H-O-Al complex, suggesting the surface site specificity of these experiments. By tracking how this molecular probe responds to local surface charges we offer insight into the local electrostatic potential at neutral and charged surface aluminol groups. Chapter six investigates the vibrational dynamics of potassium thiocyanate at the alumina/water interface. Here, we leverage the sensitivity of the C ≡ N stretch vibrational lifetime of potassium thiocyanate to measure the local electrostatic potential at the alpha-Al2O3(0001)/H2O interface. To accomplish this, KSCN was investigated using free induction decay vSFG (FID-vSFG) and time resolved pump probe (TR-vSFG) measurements, which measure the total dephasing time and vibrational lifetime of the excited C ≡ N stretch, respectively. Our FID-vSFG spectra suggest that at all surface charges the total dephasing time of SCN- is on the order of ~300-600 fs. TR-vSFG characterizations of potassium thiocyanate report the vibrational lifetime of the excited C ≡ N stretch between ~0.5-2 ps. TR-vSFG measurements show two distinct vibrational relaxation rates, which are assigned the CN stretch and the HOH bend plus libration combination band of interfacial water. The variation in the T1 lifetime of the CN stretch with bulk pH show that changes in the SCN- net orientation measured using steady-state vSFG can be correlated to the vibrational dynamics in the interfacial region. The energy transfer to the bend plus libration combination band of water is also sensitive to the surface charge, as the lifetime of this species becomes shorter as the bulk pH is increased. Lastly, in chapter seven this thesis is summarized, and future directions of the experiments presented here are discussed. / Chemistry

The distinct structure and dynamics of interfacial water are due to a break in the extended hydrogen bonding network present in bulk water. At solid-aqueous interfaces, the presence of surface charge, which induces a static electric field, and the electrolytes, which are present in most naturally relevant systems, can additionally perturb the hydrogen bonding environment due to polarization. The interplay between the surface-charge-induced electric field and the ions in changing the structure of interfacial water has important consequences in the chemistry of processes ranging from protein-water interactions to mineral-water reactivity in oil recovery. Accessing information about the first few layers of water at buried interfaces is challenging. Vibrational sum-frequency generation (vSFG) spectroscopy is a powerful technique to study exclusively the interfacial region and is used here to investigate the role of interfacial solvent structure on surface reactivity. It is known that the rate of quartz dissolution increases on addition of salt at neat water pH. The reason for this enhancement was hypothesized to be a consequence of perturbations in interfacial water structure. The vSFG spectra, which is a measure of ordering in the interfacial water structure, shows an enhanced effect of salt (NaCl) at neat pH 6~8. The trend in the effect of salt on vSFG spectra versus the bulk pH is remarkably consistent with the enhancement of rate of quartz dissolution, providing the first experimental correlation between interfacial water structure and silica dissolution. If salt alters the structure of interfacial water, it must affect the vibrational energy transfer pathways of water, which is extremely fast in bulk water (~130 fs). Thus far, the role of ions on the vibrational dynamics of water at charged surfaces has been limited to the screening effects and reduction in the depth of the region that contributes to vSFG. Here, we measure the ultrafast vibrational relaxation of the O-H stretch of water at silica at different bulk pH, using time-resolved (TR-vSFG). The fast vibrational dynamics of water (~200 fs) observed at charged silica surfaces (pH 6 and pH 12), slows down (~600 fs) on addition of NaCl only at pH 6 and not at pH 12. On the other hand at pH 2 (neutral surface), the vibrational relaxation shows an acceleration at high ionic strengths (0.5 M NaCl). The TR-vSFG results suggest that there is a surface-charge dependence on the sensitivity of the interfacial dynamics to ions and that reduction in the probe depth of vSFG alone cannot explain the changes in the vibrational lifetime of interfacial O-H. This is further supported by the cation specific effects observed in the TR-vSFG of the silica/water interface. While the vibrational relaxation of O-H stretch slows on addition of all salts (LiCl, NaCl, RbCl, and CsCl), the degree of slowing down is sensitive to the cation identity. The vibrational lifetime of O-H stretch in the presence of different cations follows the order: Li+ < Na+ < Rb+, consistent with previous Hofmeister effect reported in vSFG spectroscopy as well as AFM measurements at silica/water interface. To provide molecular insight on the effect of surface charge density and ionic strength on the changes in interfacial water structure, Molecular Dynamics (MD) simulations were performed on water at different types of surfaces. It was shown that the properties of water near the interface, e.g., a net orientation and the depth to which this persists, depend on the degree of specific adsorption of the counter ions. Our vSFG results, along with the insights from MD simulations, highlight the importance of considering the role of ions on the solvent structure within the electric double layer region, beyond the screening effects predicted by classical electrochemical models. / Chemistry

<p>In the research presented in this licentiate thesis the surface specific technique Vibrational Sum Frequency Spectroscopy, VSFS, combined with the Langmuir trough has been utilized to investigate Langmuir monolayers and Langmuir-Blodgett (LB) deposited mono- and bilayers of phospholipids. Their molecular structure, stability, and hydration were probed to gain additional understanding of important properties aiming at facilitating the use of such layers as model systems for biological membranes.</p><p>VSFS was applied to <em>in situ</em> studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with identical C-18 chains having various degrees of unsaturation. The time-dependent change of the monolayer area at constant surface pressure as well as the sum frequency intensity of the vinyl-CH stretch at the C=C double bonds were measured to monitor the degradation. It was shown that a rapid degradation of the monolayers of unsaturated phospholipids occurred when exposed to the laboratory air compared to the fully saturated lipid, and that the degradation could be inhibited by purging the ambient air with nitrogen. The degradation was attributed to oxidation mediated by reactive species in the air.</p><p>The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The spectroscopic data indicated a well ordered monolayer at all surface pressures with a more intense signal at higher pressures attributed to the subsequent increase of the number density and more ordered lipid molecules due to the tighter packing. Water molecules hydrating the headgroups or being in contact with the hydrophobic parts were observed and distinguished by their vibrational frequencies, and found to have different average orientations.</p><p>Additionally, monolayers of 18:0 PC, its fully deuterated analogue, and 1,2-distearoyl-phosphoserine (18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF₂ substrates and VSFS was used to investigate the structure and order of the films as well as the hydrating water. The CH-region, water region, and lower wavenumber region containing phosphate, ester, carboxylic acid, and amine signals were probed to obtain a complete picture of the molecule. The data indicates that all deposited monolayers formed a well ordered and stable film and the average orientation of the aliphatic chains was determined using the antisymmetric methyl stretch.</p> / <p>I forskningen som presenteras i denna licentiatavhandling har den ytspecifika vibrationssumfrekvensspektroskopin, VSFS, använts tillsammans med Langmuirtråget för att studera Langmuir-monolager och Langmuir-Blod-gett (LB) deponerade monolager och bilager av fosfolipider. För att utvidga förståelsen av egenskaper som är viktiga för att underlätta användandet av dem som modellsystem för biologiska membran undersöktes såväl deras molekylära struktur som stabilitet och hydratisering.</p><p>VSFS användes för att genomföra <em>in situ</em>-studier av nedbrytningen av Langmuir-monolager av 1,2-diacyl-fosfokoliner med identiska 18 kolatomer långa sidokedjor med varierande antal omättade kol-kol-bindningar. För att övervaka nedbrytningen mättes såväl den tidsberoende förändringen av monolagernas area vid konstant yttryck som sumfrekevensintensiteten från dubbelbindningarnas CH-vibration. När monolagerna bestående av omättade fosfolipider utsattes för laboratorieluften bröts de ner hastigt jämfört med det helt mättade monolagret. Denna nedbrytning som sannolikt orsakades av reaktiva ämnen i luften kunde inhiberas fullständigt genom att ersätta den omgivande luften med kvävgas.</p><p>Den molekylära strukturen och ordningen hos Langmuir-monolager av 1,2-distearoyl-fosfokolin (18:0 PC) och deras hydratiseringsvatten undersöktes vid olika yttryck med VSFS. Den spektroskopiska datan visar att monolagerna är välordnade vid alla yttryck samt att sumfrekvenssignalens styrka ökar med ökande yttryck på grund av såväl det större antalet molekyler per ytenhet som den högre ordningen då molekylerna packas tätare. Vattenmolekyler som hydratiserar huvudgrupperna eller är i kontakt med hydrofoba delar och har olika medelorientering observerades och kunde identifieras genom sina vibrationsfrekvenser.</p><p>Vidare deponerades monolager av 18:0 PC, dess fullt deuterade analog och 1,2-distearoyl-fofsfoserin (18:0 PS) på substrat av CaF₂ och VSFS användes för att undersöka filmernas struktur och ordning såväl som hydratiseringsvattnet. CH- och vattenregionerna samt lågvågtalsområdet som innehåller fosfat-, ester-, karboxylsyra- och aminsignaler undersöktes för att få en fullständig bild av den molekylära strukturen. Data visar att alla deponerade monolager bildade en välordnad och stabil film och kolvätekedjornas medelorientering bestämdes med hjälp av signalen från den antisymmetriska metylvibrationen.</p> / QC 20100924