Vibrational sum frequency spectroscopy (VSFS) is a nonlinear optical process. The sum frequency signal is proportional to the square of second order nonlinear susceptibility, which is proportional to the average of polarizabilities of molecules, which is related to molecular orientation. Since the polarizabilities of molecules in bulk phase will be canceled out, a sum frequency signal can only be generated from interfaces where the inversion symmetry is broken. Because of its interfacial specificity, VSFS has been applied to study many interfacial phenomena. In this dissertation we investigated various biological interfaces with VSFS. Fibrinogen adsorption was studied at the protein/solid interface in combination with atomic force microscopy (AFM), immunoassay, and VSFS. Astonishing changes in the interfacial water orientation accompanied by the pH changes provided fibrinogen’s adsorption mechanism up to the amino acid level. Enzymatic fragmentation of fibrinogen revealed that the adsorption property of fibrinogen was mainly from the alpha C fragments of the protein. Mimicking of the fibrinogen binding site with polypeptides was successfully performed and showed very similar properties of fibrinogen adsorption. Protein stability is sensitive to the salts in solutions. The ability of ions to stabilize protein was ordered by Hofmeister in 1888 and the order is SO4 2- ≅ HPO4 2- > F- > Cl- > Br- > NO3 - > I- (≅ ClO4 -) > SCN-. Even though the phenomenon was observed in various biological systems, the origin of those ionic effects is still not well understood. We studied ion effects on alkyl chain ordering and interfacial water structure for octadecylamine, dimethyldidodecylammonium bromide, and dilauroylphosphotidyl choline monolayers. Because of its ability to probe a hydrophobic moiety and interfacial water at the same time, VSFS provided further information to understand the Hofmeister series. We found that the Hofmeister effect is a combinatorial effect of screening effects, ion binding, and dispersion forces.

xv, 108 p. : ill. (chiefly col.) / Aqueous aerosol surfaces are an important platform for chemical reactions through which gases are transported in the atmosphere. The chemical complexity of aqueous aerosols is well-established, but many questions remain about the molecular nature of their surfaces, particularly with respect to the uptake of gases. The pollutant sulfur dioxide, SO₂, has been implicated in environmental phenomena such as acid rain, climate change, and cloud formation. SO₂ is fundamentally interesting because it forms spectroscopically identifiable complexes with water at aqueous surfaces. This dissertation aims to understand how temperature and aqueous composition impact the formation of surface complexes between water and SO₂. Vibrational sum frequency spectroscopy (VSFS), a surface specific technique, is used to probe the vibrational modes of water and small organic molecules, investigating changes to the overall orientation, bonding environment, and structure of interfaces when aqueous surfaces are exposed to SO₂. SO₂ adsorption to water at tropospherically relevant temperatures (0--23 °C) is examined first. The results show enhanced SO₂ surface affinity at colder temperatures, with most of the topmost water molecules showing evidence of binding to SO₂ at 0 °C compared to a much lower fraction at room temperature. Surface adsorption results in significant changes in water orientation at the surface but is reversible at the temperatures examined. The surface and vibrational specificity of these studies can be used to distinguish between the effects of surface adsorption compared to bulk accommodation. This distinction is utilized to demonstrate that SO₂ complexation is independent of solution acidity, confirming that bulk absorption is unnecessary for surface adsorption to occur. Finally, the impact of the organic species succinic acid and formaldehyde on the formation of surface SO₂ complexes is examined. These experiments indicate that SO₂ surface complexation occurs primarily with water but that surface active organic species may interact with gases under certain circumstances, namely when the organic species are more chemically reactive towards the gas. These studies have important implications for atmospheric chemistry and the uptake of gases, particularly in the complex aqueous environments expected in the troposphere. / Committee in charge: Dr. Paul C. Engelking, Chair; Dr. Geraldine L. Richmond, Advisor Dr. Jeffrey A. Cina, Member; Dr. Thomas R. Dyke, Member; Dr. Alan D. Johnston, Outside Member

Very specific protein-salt interactions are involved in a multitude of biological phenomena such as protein folding/stability, enzymatic activity, and signal transduction events. In this work, we used two very simple, protein-mimic model biopolymers to obtain a better understanding of specific cation effects operating in aqueous protein environments. The two biopolymers used were Elastin-like Polypeptides (ELPs) and poly(N-isopropylacrylamide) (PNIPAM). ELPs are an especially an ideal model system as these polypeptides can be easily genetically engineered to observe the effect of specific amino acid residues and peptide chain length on these salt interactions. Both of these biopolymers are also highly thermoresponsive as their aqueous solutions undergo a hydrophobic collapse/aggregation induced phase transition process above a lower critical solution temperature (LCST). Thermodynamic measurements of these biopolymers were carried out under various salt solution conditions. Additionally, both of these biopolymers are suitable for making surface specific spectroscopic measurements. Vibrational sum frequeny spectroscopy (VSFS), a non-linear interface sensitive spectroscopic technique, was employed here to investigate biologically relevant cation interactions which occur at peptide/protein surfaces. First, the LCST response of a non-polar ELP and a neutral biopolymer, PNIPAM, was investigated in the presence of 12 different alkali, alkaline-earth metal and transition metal chloride salts. Even though the salt interactions for uncharged proteins are dominated by anions, subtle specific cation effects were also observed. The results followed a direct Hofmeister series for cations. Most alkali cations are excluded from the polar amide regions of proteins. More polarizable cations, however, can solvate the hydrophobic moieties and somewhat counter the salting-out effect of the chloride anion. More charged and hydrated ions like lithium and divalent cations showed a weak interaction to the amide moiety through their hydration shell. The role of acidic amino acid residues in inducing cation specificities was investigated using an aspartate-rich ELP system. Both thermodynamic and spectroscopic data conclusively proved that the negative charge on protein surfaces is the main driving force for cation partitioning and specificity under physiological relevant concentration regimes. Apparent binding constants of carboxylate moieties with cations were determined. This is the first quantitative and thoroughly systematic study of such biologically relevant cation-carboxylate interactions prevalent in enzyme active sites and protein surfaces.

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

<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

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.

The primary goal of this thesis was to commission an instrument for vibrational sum frequency spectroscopy (VSFS), and exploit it for the study of solid/gas interfaces; of ultimate interest is characterisation of substrate surfaces in humid environments. Such effort is motivated by interest in understanding the potential for atmospheric corrosion in dry storage facilities of spent nuclear fuels or other nuclear-related wastes. VSFS is a non-linear, interface specific, vibrational spectroscopy, in which two photons of different energies (infrared (IR) and visible (VIS)) impinge upon a surface at the same point at the same time, leading to the generation of a third (sum-frequency generation (SFG)) photon. Features in VSFS spectra can be assigned to vibrational modes of interfacial species, and so enable details of interfacial structure and chemistry to be elucidated. An instrument for such measurements has been developed using laser facilitates located in the Photon Science Institute (PSI) of The University of Manchester. More specifically, an ultra-fast (femtosecond) laser has been employed as a light source, enabling acquisition of spectra (~250 cm-1 in width at a resolution of ~11 cm-1) without the need for scanning the energy of either IR or VIS beams, i.e. so called broad-band VSFS. To test performance, data have been acquired from self-assembled monolayers of alkanethiols (octadecanethiol) on gold substrates, which demonstrate the utility of the instrument. Subsequent to commissioning, the VSFS instrument was initially exploited to study the interaction of two organic molecules, acetonitrile and acetic acid, with a single crystal TiO2(110) substrate; measurements were performed with the sample exposed to the vapour of each organic species under ambient conditions. Surface adsorption was identified through the appearance of the CH3 symmetric stretch. Furthermore, spectra as a function of light (IR/VIS/SFG) polarization combinations have been recorded to explore adsorbate angular geometry. Finally, VSFS measurements have been undertaken from a number of substrates (GaAs, Au, Zn, Fe, Cr, stainless steel), as a function of relative humidity; D2O was employed to overcome the issue of loss of IR beam intensity due to interaction with atmospheric H2O. Signal quality varies significantly with substrate, with the most insight being gained for the interaction of D2O with polycrystalline Zn. Clear vibrational resonances due to both hydroxyls (OD) and molecular water (D2O) are observed, which vary with relative humidity, indicating that there are significant changes in interface structure with relative humidity.

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. VSFS was applied to in situ 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. 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. 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. / 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. VSFS användes för att genomföra in situ-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. 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. 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. / QC 20100924

Laddningsbeteendet hos amingruppen vid ytgränsskiktet mellan vatten och luft har studerats under ändring av vattenfasens pH och NaCl-koncentration via den ickelinjära laserspektroskopiska tekniken, vibrationssumfrekvensspektroskopi (VSFS). Modellen som användes för ytan bestod av ett Langmuirmonolager av 1-docosanamin, en icke-löslig fettkedjeamin med NH2-gruppen riktad mot vattenfasen. En av de huvudsakliga syftena med projektet var att bestämma det skenbara samt yt-pKa:t för amingruppen, såväl som att testa gränserna för de klassiska Poisson-Boltzmann-formuleringarna av teorin för det elektriska dubbellagret. Molekylär information av laddningsbeteendet hos aminen erhölls från VSFS-spektra genom att följa NH-, OH- och CH-sträckningsvibrationsmoderna. Specifikt så identifierades de spektrala dragen från den neutrala formen (R-NH2) och den laddade formen (R-NH3+) av aminen, vilka direkt korrelerades med monolagrets ytladdning. Intensiteten hos OH-banden från vattenmolekylerna i det diffusa dubbellagret kunde länkas till ytpotentialen och CH-vibrationerna från alkylkedjan av amino-tensiden kopplades slutligen till packningstätheten hos monolagret. Ytterligare experiment utfördes med en deutererad vattenfas (D2O) för att bekräfta sträckningsvibrationsmoderna hos NH3+, vilka aldrig tidigare har rapporterats. Resultaten visar på att fettkedjeaminen får ett avsevärt lägre yt- och skenbart pKa jämfört med bulken (∼ 4 jämfört med 10,5). Detta är i enlighet med vad Gouy-Chapmann-modellen av det elektriska dubbellagret förutser. Dessutom så indikerar datat att det inneboende pKa:t hos aminen också blir lägre vid ytan än för bulken (9,7±0,7 jämfört med 10,5), vilket kan beskrivas som en effekt av begränsningen i frihetsgrader hos den ytbundna aminen. Dock, så hindrades en mer utförlig kvantitativ jämförelse mot teorin av ett sämre val av det experimentella referensvärdet som användes för att jämföra data mellan olika dagar. Dessutom så var anpassningsrutinen för datat begränsad på grund av spektrala vibrationsöverlapp mellan de relativt svaga aminbanden och de mycket starkare vatten- och alkylsträckningssignalerna. Intressant nog, för högre pH-värden, då aminen är helt oladdad, bevisade sumfrekvensspektra att hydroxidjonen föredrar att adsorbera till ytan genom att monolagret fick en negativ nettoladdning. Överlag så förbättrar studierna som presenteras i denna master-projektuppsats vår molekylära förståelse kring hur den biofysiskt betydande amingruppen beter sig vid ytgränsskikt. / The charging behaviour of the amine moiety at the water-air interface upon changes in the aqueous subphase pH and NaCl concentration has been studied using  the non-linear laser spectroscopy technique, Vibrational Sum Frequency Spectroscopy (VSFS).  The model surface consisted of a Langmuir monolayer of 1-docosaneamine, an insoluble fatty amine that exposes its NH2  group to solution. One of the main purposes of the project is to determine the surface, and the apparent pKa of the amine moiety, as well as testing the limits of validity of classical formulations of the electrical double layer theory within the Poisson-Boltzmann formalism. Molecular information of the charging behaviour was obtained from the VSFS spectra by targeting the NH, OH and CH stretching modes. Specifically, spectral features from the neutral amine (i.e. R-NH2) and charged (R-NH3+) groups could be identified and directly correlated to the surface charge of the monolayer. The intensity of the  OH bands from water molecules in the diffuse double layer, were linked to the surface potential, and finally, the CH modes from the surfactant alkyl chain gave information of the packing density in the monolayer. Additional experiments were also carried out in D2O to help confirm the assignment of the NH3+ stretching modes that had not been previously reported. The results show that as predicted from the Gouy-Chapman electric double layer model, the apparent pKa of the fatty amine monolayer is significantly lower than in the bulk  (∼ 4 compared with 10.5) . However, the data show indication that the intrinsic pKa at the surface is also lower than in the bulk (9.7+/- 0.7, compared to 10.5), an effect that is ascribed to the 2D molecular confinement in the monolayer. A more quantitative comparison with the theoretical predictions was nonetheless hampered by a poor selection of the experimental reference for comparing data collected in different days, and the limitations in the fitting routines due spectral overlap of the relatively weak amine bands with the OH and CH stretching modes. Interestingly, at high pH when the fatty amine is fully uncharged, the sum frequency spectra show evidence that OH- ion preferentially adsorbed to the surface, making it effectively net negatively charged. Overall, the studies presented in this master thesis, improve our molecular understanding of the behaviour of the biophysically relevant amine-functionality at interfaces.

Hur elektrolytlösningar interagerar med laddade ytor är viktigt för många fenomen. I detta arbete undersöktes samspelet mellan flera joner med en negativt laddad yta som exponerade sulfonat funktionella grupper mot lösning. Särskild vikt lades på deprotoneringsbeteendet vid ytan. Samspelet mellan olika joner och sulfonatgruppen hade inte tidigare beaktats ur ett molekylärt perspektiv. Här användes ett Langmuir-monolager bestående av dokosan-1-sulfonsyra deponerat på olika elektrolytlösningar som modellsystem. För att studera molekylära interaktioner mellan ytan och elektrolyterna användes Vibrational Sum Frequency Spectroscopy (VSFS). Denna inneboende ytkänsliga teknik gör det möjligt att observera små förändringar i vibrationsenergier i sulfonagruppen vid kontakt med olika joner, här hydronium, litium, natrium och cesium. Ett av huvudsyftena med detta arbete var att jämföra de experimentellt bestämda parametrarna med teoretiska modeller av det elektriska dubbelskiktet vid laddade gränssnitt, såsom Gouy-Chapman-teorin och andra mer avancerade Poisson-Boltzmann-baserade modeller, för att utforska deras lämplighet och gränser av giltighet. Docosan-1-sulfonsyra packades snyggt i monolager, med packningstätheten ökande med starkare jonstyrka I underfasen. Två toppar i vibrationsspektra sågs för sulfonatgruppen, den symmetriska och asymmetriska sträckningen vid 1048 cm-1 respektive 1150 cm-1. Dessa band sågs blåskiftas vid bindning av litium- och natriummotjoner. Amplituden för den symmetriska sträckningen kunde kopplas direkt till mängden laddade arter. Den teoretiska Gouy-Chapman-modellen var tillräcklig att användas för relativt låga ytpotentialer (&lt;|150mV|). För högre potentialer var motjonens storlek nödvändig att inkludera i modellen för mer exakta förutsägelser. Sulfonsyrans surhetsgrad uppmättes till att vara pKa=-1.8±0.4 och bindningskonstanterna för olika hårda joner till den sulfonatfunktionella gruppen uppskattades vara pKLi=0 och pKNa=-0.7. Däremot binder den mjuka jonen, cesium, inte till sulfonatgruppen. Implikationen är att sulfonatgruppen bör betraktas som en hård jon I enlighet med Collins lag om matchande vattenaffiniteter. / How electrolyte solutions interact with charged surfaces is essential for many phenomena in physics, chemistry and biology. In this work, the interactions of several ions with a negatively charged surface exposing sulfonate functional groups to solution were investigated. Specific emphasis was made on the deprotonation behaviour at the surface. The interplay between different ions and the sulfonate moiety had not been previously considered from a molecular perspective.  Here, a Langmuir monolayer consisting of docosane-1-sulfonic acid deposited on top of different electrolyte solutions was used as model system. To study the molecular interactions between the surface and the electrolytes, Vibrational Sum Frequency Spectroscopy (VSFS) was used. This intrinsically surface sensitive technique allows observing small changes in vibrational energies of the sulfonate functional group when in contact with different ionic species, here hydronium, lithium, sodium and cesium. One of the main objectives of this work was to compare the experimentally determined parameters with theoretical models of the electric double layer at charged interfaces, such as the Gouy-Chapman theory and other more advanced Poisson-Boltzmann based models, to explore their suitability and limits of validity. Docosane-1-sulfonic acid packed nicely into monolayers, with the packing density increasing with stronger ionic strength in the subphase. Two peaks in the vibrational spectra were seen for the sulfonate functional group, the symmetric and asymmetric stretch at 1048 cm-1 and 1150 cm-1, respectively. These bands were seen to blue-shift upon binding of lithium and sodium counterions. The amplitude of the symmetric stretch could be directly linked to the amount of charged species. The Gouy-Chapman theoretical model was adequate to use for relative low surface potentials (&lt;|150mV|). For higher potentials, the size of the counterion was necessary to include in the model for more accurate predictions. The acidity of the sulfonic acid was measured to be pKa=-1.8±0.4 and the binding constants for different hard ions to the sulfonate functional group were estimated to be pKLi=0 and pKNa=-0.7. In contrast, the soft ion, cesium, does not bind to the sulfonate. The implication is that the sulfonate moiety should be considered a hard ion in accordance with Collins law of matching water affinities.