Vibrational sum frequency spectroscopy har använts för att studera interaktionerna mellan monovalenta joner och laddade fettsyramonolager. Subfasen bestod av utvalda alkalihydroxidsaltlösningar med pH12, med huvudfokus på cesiumjonen. Molekylär information erhölls genom att rikta in sig på vibrationerna hos karboxylsyragruppen, alkylkedjor, och vattenmolekyler i kontakt med monolagret, samt i det diffusa dubbellagret. Ytans laddning uppskattades genom att observera sträckningsvibrationerna hos hydratiserade karboxylat och protonerade karboxylsyror. Ytans potential beräknades utifrån signalresponsen hos vattenmolekylerna i det diffusa dubbellagret. Beteendet hos ytans laddning och potential överensstämmer väl med Gouy-Chapman-teorin för stora områden per molekyl, men avviker avsevärt vid kompression av monolagret. Vid små områden per molekyl, eller hög laddningsdensitet, stämmer resultaten bättre överens med en storleksmodifierad version av Poisson-Boltzmann-teorin. Resultaten från experiment med en cesiumhydroxidsubfas jämfördes med motsvarande experiment med natrium och litium, vilket beskriver cesiumjonen i relation till de andra alkalijonerna vad gäller jonspecifika effekter. Resultaten som har samlats in här ger insikt i hur effektiv jonstorlek kan användas för att modellera och förutsäga jonspecifika effekter med karboxylsyragruppen i ett Langmuir-monolager. / Vibrational sum frequency spectroscopy has been used to study the interactions of monovalent ions with highly charged fatty acid monolayers. The subphase consisted of selected alkali hydroxide salt solutions of pH 12, primarily focusing on the cesium ion. Molecular information was obtained by targeting the vibrational modes of the carboxylic acid headgroups, alkyl chains, and water molecules in the immediate surface and diffuse double layers. The surface charge was estimated by monitoring the stretching modes of the hydrated carboxylate and protonated carboxylic acid. The surface potential was estimated from the signal response of the water molecules in the diffuse double layer. The behaviour of the surface charge and surface potential agrees well with Gouy-Chapman theory for large areas per molecule, but deviates significantly upon monolayer compression. At small areas per molecule or high surface charge density, the results better align with a size-modified version of the Poisson-Boltzmann theory. The results obtained with a cesium hydroxide subphase were compared to equivalent experiments with sodium and lithium, which puts the cesium ion into context with the other alkali ions in terms ion specific effects. The results collected here provide insight into how effective ion size can be used in modelling and predicting ion specific effects with the carboxylic acid moiety in a Langmuir monolayer.

<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 aim of this work is to study the interfacial properties of amphiphilic compounds at the liquid–air interface in an attempt to develop a comprehensive understanding of their orientation as well as the influence of their interaction with the solvent on the interfacial layer properties. Using Vibrational Sum Frequency Spectroscopy (VSFS) as the main tool, the molecular structure of the amphiphilic layer and the amphiphile–solvent relation can be illuminated in great detail – it is arguably the most sensitive surface spectroscopy currently available. Due to its second order nature, the VSFS technique is capable of distinguishing molecules at the interface even in the presence of a vast excess of similar molecules in the bulk.Ionic liquids (Ils) form a class of solvent which are increasingly receiving attention as ``green solvents´´. Some of these, such as ethyl ammonium nitrate (EAN), a protic IL, have the capacity to hydrogen bond extensively which is one of the important features they share with water. Since the interaction with solvent is an important consideration for self assembly and it is known that surfactant self assembly in the EAN bulk is analogous to in water, it was considered of interest to probe self assembly at EAN–air interface. To this end the interfacial structure of the pure EAN interface was probed, as was the conformation and ordering of nonionic surfactants. These studies reveal that EAN is highly ordered at the interface, exposing the ethyl moiety to the gas phase. Additionally, polarization studies have enabled the average orientation of the ethyl group to be determined. Adsorption of nonionic surfactants at the interface appears to significantly displace the EAN from the interface. The headgroup of the surfactant, a linear ethylene oxide group, appears to be highly disordered.The disorder of the linear ethylene oxide groups has led to difficulties in their surface spectroscopic fingerprinting in this and other works. In an attempt to study the interfacial behaviour of ethylene oxide and the temperature dependence of its hydration, closed loop structures of PEO attached to hydrophobic groups were also probed. This essentially locks their conformation. Such molecules are known as crown ethers and display interesting interfacial behaviour and also the ability to bind cations. The presence of even small amounts of adsorbed crown ethers at the water interface is shown to considerably perturb the water structure. The NO, CN, COC and CH vibrational modes of these compounds at the air-water interface as well as OH vibrational modes of the surface water hydrating this compound have been targeted in order to obtain molecular information about arrangement and conformation. The CH2 vibrational modes of crown ethers have been identified and found to be split due to their interaction with ether oxygen. The spectra provide evidence for the existence of a protonated crown complex moiety at the surface leading to the appearance of strongly ordered water species. The orientation of Nitrobenzo crown (NB15C5) was monitored as a function of solution concentration, by targeting the ratio of peak intensities of the CN and NO2 vibrational modes. The water of hydration has also been probed as a function of crown concentration, salt concentration, and temperature. The latter study strongly suggests that the surface can be treated as a charged interface, and that the associated ordered water decreases with increasing ionic strength of the bulkFinally, insoluble monolayers of fatty acids spread on a water surface have also been studied in an effort to further understand the relationship between molecular architecture and film structure. Fatty acid (Arachidic Acid – AA and Eicosenoic Acid – EA) monolayers are compared to investigate the effect on the monolayer structure of introducing unsaturation into the alkyl chain. For AA, at very large area per molecule, floating domains of crystalline nature exist rather than any classical gaseous phase. The measured conformational disorder in EA decreases continuously with monolayer compression and no crystalline domains are observed at low density. Addition of NaCl to the subphase does not affect the monolayer order for either of the compounds; instead, a dramatic increase in the signal of the water hydrating the headgroups is observed. The effect of introducing further unsaturations (up to three) was also studied in order to probe the resulting interfacial structure. Remarkably the double bonds appear to adopt the same orientation, irrespective of how many they are in the chain. By monitoring the vinyl CH stretch it was possible to study the film stability towards oxidative degradation and it was found that all three unsaturated species studied showed rapid degradation. The rate of degradation could be controlled by adjusting the film pressure. However, the monolayers could be stabilised by performing the experiments in an inert nitrogen atmosphere. / QC20100629

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

In this thesis the structure and hydration of small organic and amphipilic compounds adsorbed at the air-liquid interface, have been studied using the nonlinear optical technique Vibrational Sum Frequency Spectroscopy (VSFS). The second order nature of the sum frequency process makes this technique particularly surface sensitive and very suitable for interfacial studies, as molecules at the surface can be distinguished even in the presence of a vast excess of the same molecules in the bulk. Particular emphasis was given to the surface water structure and how it is affected by the presence of small model compounds such as acetic acid and formic acid, and also non-ionic surfactants with sugar based and ethylene oxide based polar headgroups. Understanding the structure of water at these interfaces is of considerable fundamental importance, and here VSFS provided unique information. Upon addition of tiny amounts of these surface active compounds, the ordered surface structure of water was found to be significantly perturbed, as revealed by the changes observed in the characteristic spectroscopic signature of the dangling OH bond of water molecules, which vibrate free in air and are present in the top monolayer. Dramatic differences between the different compounds were also observed in the bonded OH region, providing a valuable insight into the hydration of polar groups at interfaces. Additionally, by employing different polarization combinations of the laser beams involved in the sum frequency process, information about the different water species present at the surface and their average orientation were extracted. In particular an unusual state of water was found with a preferred orientation in a non-donor configuration in close proximity to the hydrophobic region formed by the hydrocarbon tails of the surfactant molecules. The conformation and orientation of the different adsorbates were also characterized, targeting their specific vibrational frequencies. Noteworthy is the orientation of the fluorocarbon chain of ammonium perfluorononanoate (APFN), which in contrast to the hydrocarbon chains of the other surfactant molecules studied, remained constant over a wide range of surface densities. This behaviour was also observed for the anionic headgroup of sodium dodecyl sulphate (SDS). Other interesting findings were the formation of a cyclic dimer bilayer at the surface of concentrated aqueous solutions of acetic acid and the water structuring effect induced by poly(ethylene-oxide) headgroups, in spite of being themselves disordered at the air-liquid interface.

<p>In this thesis the structure and hydration of small organic and amphipilic compounds adsorbed at the air-liquid interface, have been studied using the nonlinear optical technique Vibrational Sum Frequency Spectroscopy (VSFS). The second order nature of the sum frequency process makes this technique particularly surface sensitive and very suitable for interfacial studies, as molecules at the surface can be distinguished even in the presence of a vast excess of the same molecules in the bulk. Particular emphasis was given to the surface water structure and how it is affected by the presence of small model compounds such as acetic acid and formic acid, and also non-ionic surfactants with sugar based and ethylene oxide based polar headgroups. Understanding the structure of water at these interfaces is of considerable fundamental importance, and here VSFS provided unique information. Upon addition of tiny amounts of these surface active compounds, the ordered surface structure of water was found to be significantly perturbed, as revealed by the changes observed in the characteristic spectroscopic signature of the dangling OH bond of water molecules, which vibrate free in air and are present in the top monolayer. Dramatic differences between the different compounds were also observed in the bonded OH region, providing a valuable insight into the hydration of polar groups at interfaces. Additionally, by employing different polarization combinations of the laser beams involved in the sum frequency process, information about the different water species present at the surface and their average orientation were extracted. In particular an unusual state of water was found with a preferred orientation in a non-donor configuration in close proximity to the hydrophobic region formed by the hydrocarbon tails of the surfactant molecules.</p><p>The conformation and orientation of the different adsorbates were also characterized, targeting their specific vibrational frequencies. Noteworthy is the orientation of the fluorocarbon chain of ammonium perfluorononanoate (APFN), which in contrast to the hydrocarbon chains of the other surfactant molecules studied, remained constant over a wide range of surface densities. This behaviour was also observed for the anionic headgroup of sodium dodecyl sulphate (SDS). Other interesting findings were the formation of a cyclic dimer bilayer at the surface of concentrated aqueous solutions of acetic acid and the water structuring effect induced by poly(ethylene-oxide) headgroups, in spite of being themselves disordered at the air-liquid interface.</p>