Sum frequency generation study of the room-temperature ionic liquids/quartz interface

The purpose of this investigation is to study the ionic liquid/quartz interface with sum frequency generation vibrational spectroscopy (SFG). SFG spectroscopy was chosen for this study because of its unique ability to yield vibrational spectra of molecules at an interface. Different polarization combinations are used, which probe different susceptibilities, giving SFG the ability to determine molecular orientation at the interface. The ionic liquids used were 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF(4)], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF(6)]. To determine the influence of the molecular structure and charge on orientation at the interface, neutral, 1-methylimidazole, and 1-butylimidazole were also studied. Raman spectra and depolarization ratios were obtained for neat samples of 1-methylimidazole, 1-butylimidazole, and 1-butyl-3-methylimidazolium tetrafluoroborate recorded from 2700 to 3300 cm(-1). SFG spectra of the 1-methylimidazole/quartz interface showed both methyl and aromatic C-H vibrations. Orientation calculations determined that the ring of the molecule is tilted 45-68 degrees from normal, with the methyl group oriented 32-35 degrees from normal. The SFG spectra of 1-butylimidazole contain several resonances from the alkyl chain with only one weak resonance from the aromatic ring. Orientation calculations suggest that the ring is lying in the plane of the surface with the methyl group pointing 43-47 degrees from normal. The orientation of the [BMIM][PF(6)] ionic liquid was sensitive to trace amounts of water and had to be evacuated to <3 x 10(-5) Torr for the water to be removed. SFG spectra of both ionic liquids were similar, displaying resonances from the alkyl chain as well as the aromatic ring. Orientation analysis suggests the aromatic ring was tilted 45-90 degrees from normal for [BMIM][BF(4)], while the ring for [BMIM][PF(6)] was tilted 38-58 degrees from normal. This suggests the orientation of the molecule is influenced by the size of the anion.     

Orientational motions of vibrational chromophores in molecules at the air/water interface with time-resolved sum frequency generation

The first time-resolved experiments in which interfacial molecules are pumped to excited electronic states and probed by vibrational sum frequency generation (SFG) are reported. This method was used to measure the out-of-plane rotation dynamics, i.e. time dependent changes in the polar angle, of a vibrational chromophore of an interfacial molecule. The chromophore is the carbonyl group, the rotation observed is that of the -C=O bond axis, with respect to the interfacial normal, and the interfacial molecule is coumarin 314 (C314) at the air/water interface. The orientational relaxation time was found to be 220+/-20 ps, which is much faster than the orientational relaxation time of the permanent dipole moment axis of C314 at the same interface, as obtained from pump-second harmonic probe experiments. Possible effects on the rotation of the -C=O bond axis due to the carbonyl group hydrogen bonding with interfacial water are discussed. From the measured equilibrium orientation of the permanent dipole moment axis and the carbonyl axis, and knowledge of their relative orientation in the molecule, the absolute orientation of C314 at the air/water interface is obtained.     

Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

The constructive or destructive spectral interference between the molecular groups oriented up and down at the interface in the sum-frequency generation （SFG） spectra provides a direct measurement of the absolute orientation of these molecular groups. This simple approach can be employed to interrogate absolute molecular orientations other than using the complex absolute phase measurement in the SFG studies. We used the -CN group in the p-cyanophenol （PCP） molecule as the internal phase standard, and we measured the phases of the SFG fields of the -CN groups in the 3,5-dimethyl-4-hydroxy-benzonitrile （35DMHBN） and 2,6-dimethyl-4-hydroxy-benzonitrile （26DMHBN） at the air/water interface by measuring the SFG spectra of the aqueous surfaces of the mixtures of the PCP, 35DMHBN, and 26DMHBN solutions. The results showed that the 35DMHBN had its -CN group pointing into the aqueous phase; while the 26DMHBN, similar to the PCP, had its -CN group pointing away from the aqueous phase. The tilt angles of the -CN group for both the 35DMHBN and 26DMHBN molecules at the air/water interface were around 25°-45° from the interface normal. These results provided insights on the understanding of the detailed balance of the competing factors, such as solvation of the polar head groups, hydrogen bonding and hydrophobic effects, etc., on influencing the absolute molecular orientation at the air/water interface.     

Adsorption of 1- and 2-butylimidazoles at the copper/air and steel/air interfaces studied by sum frequency generation vibrational spectroscopy

The structure of thin films of 1- and 2-butylimidazoles adsorbed on copper and steel surfaces under air was examined using sum frequency generation (SFG) vibrational spectroscopy in the ppp and ssp polarizations. Additionally, the SFG spectra of both isomers were recorded at 55 °C at the liquid imidazole/air interface for reference. Complementary bulk infrared, reflection-absorption infrared spectroscopy (RAIRS), and Raman spectra of both imidazoles were recorded for assignment purposes. The SFG spectra in the C-H stretching region at the liquid/air interface are dominated by resonances from the methyl end group of the butyl side chain of the imidazoles, indicating that they are aligned parallel or closely parallel to the surface normal. These are also the most prominent features in the SFG spectra on copper and steel. In addition, both the ppp and ssp spectra on copper show resonances from the C-H stretching modes of the imidazole ring for both isomers. The ring C-H resonances are completely absent from the spectra on steel and at the liquid/air interface. The relative intensities of the SFG spectra can be interpreted as showing that, on copper, under air, both butylimidazoles are adsorbed with their butyl side chains perpendicular to the interface and with the ring significantly inclined away from the surface plane and toward the surface normal. The SFG spectra of both imidazoles on steel indicate an orientation where the imidazole rings are parallel or nearly parallel to the surface. The weak C-H resonances from the ring at the liquid/air interface suggest that the tilt angle of the ring from the surface normal at this interface is significantly greater than it is on copper.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.     

Evidence for epitaxial arrangement and high conformational order of an organic monolayer on Si(111) by sum frequency generation spectroscopy

The structure of an octadecyl monolayer formed on a hydrogen-terminated Si(111) surface in neat octadecene was studied by infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra in the CH vibration region were dominated by peaks corresponding to those of the methyl group, confirming that the monolayer is essentially in the all-trans conformation. The shapes of the spectra were strongly dependent on the azimuthal angle, and the strength of the asymmetric vibration mode obtained from the theoretical fitting shows threefold symmetry with respect to the azimuthal angle, suggesting the epitaxial arrangement of the monolayer with the Si(111) substrate. The orientation angle of the methyl group estimated from SFG anisotropy was in good agreement with the theoretical prediction.     

Molecular restructuring at poly(n-butyl methacrylate) and poly(methyl methacrylate) surfaces due to compression by a sapphire prism studied by infrared-visible sum frequency generation vibrational spectroscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, performed in visible wavelength total internal reflection (TIR) geometry, was used to determine the molecular structures of poly(n-butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) surfaces in air and in contact with a smooth sapphire surface with and without the application of pressure. C-H vibrational resonances were probed optically to nondestructively examine the buried polymer/sapphire interfaces and obtain information about the molecular orientation in situ. These findings are contrasted with those of the same polymers cast from a toluene solution directly on the sapphire prism surface and annealed. Compared to polymer surface conformation in air, the SFG spectra of the deformed (compressed) PBMA at the sapphire interface illustrate that the ester butyl side chain restructures and tilts away from the surface normal. However, the molecular conformation in the similarly deformed PMMA at the sapphire interface is identical to that obtained in air, which is dominated by the upright-oriented ester methyl side chains. For PBMA and PMMA spin cast on sapphire and annealed, the surface structure of the undeformed PBMA at the sapphire interface is identical to that of the deformed PBMA at the sapphire interface, while the PMMA conformation is different and shows alpha-methyl group ordering. Since the glass transition temperature of PBMA is below room temperature, the rubbery state of PBMA demonstrates a melt-like behavior, evidenced by the fact that PBMA is in conformation chemical equilibrium at the sapphire surface even under compression. Due to the high glass transition temperature of PMMA, compression freezes PMMA in a metastable state, revealed by the restructured molecular conformation when annealed against the sapphire surface. The results of this study demonstrate that structural changes at buried polymer surfaces due to the application of contact pressure can be detected in situ by TIR-SFG vibrational spectroscopy.     

How trifluoroacetone interacts with water

The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetone-water have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V(3) = 3.29 kJ·mol(-1) and ∠(a,i) = 67.5°, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a O-H···O hydrogen bond and a C-H···O intermolecular contact, lying in the effective plane of symmetry of the complex.     

Molecular orientation of asphaltenes and PAH model compounds in Langmuir-Blodgett films using sum frequency generation spectroscopy

Asphaltenes are an important class of compounds in crude oil whose surface activity is important for establishing reservoir rock wettability which impacts reservoir drainage. While many phenomenological interfacial studies with crude oils and asphaltenes have been reported, there is very little known about the molecular level interactions between asphaltenes and mineral surfaces. In this study, we analyze Langmuir-Blodgett films of asphaltenes and related model compounds with sum frequency generation (SFG) vibrational spectroscopy. In SFG, the polarization of the input (vis, IR) and output (SFG) beams can be varied, which allows the orientation of different functional groups at the interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen functionality have PAHs oriented transverse to the interface. Computational quantum chemistry is used to support corresponding band assignments, enabling robust determination of functional group orientations.     

Structure of poly(methyl methacrylate) chains adsorbed on sapphire probed using infrared-visible sum frequency generation spectroscopy

We have studied the orientation of the train segments of a poly(methyl methacrylate) (PMMA) adsorbed layer at the CCl4-sapphire interface using surface-sensitive IR-visible sum frequency generation (SFG) spectroscopy. The SFG spectra of PMMA chains adsorbed on sapphire indicate ordered ester methyl groups. In comparison, we did not observe any significant contributions from the backbone methylene and alpha methyl groups, suggesting that these groups are disordered. No change in the structure of the adsorbed layer is observed upon cooling the solvent below the theta temperature; this is consistent with the picture of flat adsorbed chains on the surface. Interestingly, the orientation of the ester methyl groups of a spin-coated PMMA film at the PMMA-sapphire interface is similar to that of the same groups in the chains adsorbed from solution.     

Surface restructuring behavior of various types of poly(dimethylsiloxane) in water detected by SFG

Surface structures of several different poly(dimethylsiloxane) (PDMS) materials, tetraethoxysilane-cured hydroxy-terminated PDMS (TEOS-PDMS), platinum-cured vinyl-terminated PDMS (Pt-PDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer in air and water have been investigated by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra collected from all PDMS surfaces in both air and water are dominated by methyl group stretches, indicating that all the surfaces are mainly covered by methyl groups. Other than surface-dominating methyl groups, some -Si-CH2-CH2- moieties on the Pt-PDMS surface have also been detected in air, which are present at cross-linking points. Information about the average orientation angle and angle distribution of the methyl groups on the PDMS surface has been evaluated. Surface restructuring of the methyl groups has been observed for all PDMS surfaces in water. Upon contacting water, the methyl groups on all PDMS surfaces tilt more toward the surface. The detailed restructuring behaviors of several PDMS surfaces in water and the effects of molecular weight on restructuring behaviors have been investigated. For comparison, in addition to air and water, surface structures of PDMS materials mentioned above in a nonpolar solvent, FC-75, have also been studied. By comparing the different response of phenyl groups to water on both PDPS-co-PDMS and PS-co-PDMS surfaces, we have demonstrated how the restructuring behaviors of surface phenyl groups are affected by the structural flexibility of the molecular chains where they are attached.     

Infrared-visible sum frequency generation spectroscopic study of molecular orientation at polystyrene/comb-polymer interfaces

Surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) in total internal reflection geometry has been used to study the structure of poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC) or poly(octadecyl acrylate) (PA-18) in contact with a deuterated or hydrogenated polystyrene (dPS or hPS) layer. SFG spectra from the PVNODC (or PA-18)/hPS interface show methyl and methylene peaks corresponding to PVNODC (or PA-18) and phenyl peaks corresponding to the PS. Analysis suggests that the methyl groups are tilted at angles less than 30 degrees with respect to the surface normal. The presence of a strong methylene peak suggests the PVNODC alkyl side chains contain more gauche defects at the PS/PVNODC interface in comparison to PVNODC (or PA-18)/air interfaces. On heating, the SFG intensity from the PS/PA-18 interface drops sharply near the bulk melting temperature (T(m)) of PA-18. Interestingly, a similar drop in SFG signal is also observed for the PS phenyl groups. This demonstrates the ability of the phenyl group at the PS/PA-18 interface to rearrange itself upon the solid-to-liquid transition of the PA-18 alkyl side chain, at a temperature well below the bulk PS glass transition temperature. For PS/PVNODC interfaces, the drop in SFG intensity is gradual and in agreement with the three broad transitions of PVNODC observed in the bulk.     

Molecular surface structure of comb-like polyoxyethylene with alkyl sulfonyl side-chains studied by sum-frequency generation vibrational spectroscopy

The orientation of alkyl sulfonyl side-chains on a series of polyoxyethylenes, CH₃–nSE (n?=?6, 8 or 10), and the effect of annealing and rubbing on the molecular orientation of the alkyl side-chains at the substrate surface, were investigated using sum-frequency generation (SFG) vibrational spectroscopy. Based on the SFG spectra and their quantitative interpretation, we deduced that the alkyl chains of CH₃–10SE are almost vertically oriented at the surface and that the terminal methyl groups of the alkyl chains are tilted from the surface normal as much as θ ≈ 40?±?5°, with a broad distribution of tilt angles. We also found that rubbing treatment induced the anisotropic orientation of the alkyl side-chains perpendicular to the rubbing direction, but their orientation was unchanged by annealing.     

Measuring polymer surface ordering differences in air and water by sum frequency generation vibrational spectroscopy

Molecular structures of poly(n-butyl methacrylate) (PBMA) at the PBMA/air and PBMA/water interfaces have been studied by sum frequency generation (SFG) vibrational spectroscopy. PBMA surfaces in both air and water are dominated by the methyl groups of the ester side chains. The average orientation and orientation distribution of these methyl groups at the PBMA/air and PBMA/water interfaces are different, indicating that surface restructuring occurs when the PBMA sample contacts water. Analysis shows that the orientation distribution of side chain methyl groups on the PBMA surface is narrower in water than that in air, indicating that the PBMA surface can be more ordered in water. To our knowledge, this is the first time that quantitative comparisons between molecular surface structures of polymers in air and in water have been made. Two assumptions on the orientation distribution function, including a Gaussian distribution and a formula based on the maximum entropy approach, are used in the analysis. It has been found that the orientation angle distribution function deduced by the Gaussian distribution and the maximum entropy distribution are quite similar, showing that the Gaussian distribution is a good approximation for the angle distribution. The effect of experimental error on the deduced orientational distribution is also discussed.     

Spatially resolved vibrational energy transfer in molecular monolayers

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.     

Cd2+-induced interfacial structural changes of Langmuir-Blodgett films of stearic acid on solid substrates: a sum frequency generation study

The molecular structures and their stabilities at the outmost-layer of the Langmuir-Blodgett (LB) films of stearic acid on solid substrates have been investigated by a highly surface-sensitive spectroscopic technique, sum frequency generation (SFG), in air and in aqueous solution, using the combination of both normal and deuterated stearic acid. Peaks observed in the SFG spectra are mainly attributed to the terminal methyl group at the outmost layer of the LB films. The SFG spectra in air are virtually identical and are independent of the odd-even property and thickness (1-12) of the LB films, indicating that the even-numbered LB film changes its surface structure after passing through the interface between the water subphase and air, especially when the Cd2+ cation was included in the water subphase. Furthermore, we have demonstrated for the first time using in situ SFG measurement that the interfacial molecular structure at the LB bilayer of stearic acid on the hydrophilic substrates significantly change with immersion in the water subphase containing Cd2+ cation while such a structural change has not been observed in the water subphase without Cd2+. These results clearly indicate that a reorganization process takes place on the surface of the stearic acid bilayer induced by the Cd2+ cation. The electrostatic interaction between the carboxylate headgroup of stearic acid via the Cd2+ cation seems to play an important role in the surface reorganization process both in air and in solution.     

The effect of surface coverage on conformation changes of bovine serum albumin molecules at the air-solution interface detected by sum frequency generation vibrational spectroscopy

The air-BSA solution interface has been investigated by various techniques for years. From these studies we know that BSA molecules segregate at the BSA solution-air interface, and the surface coverage increases with the increase of the bulk solution concentration. However, questions still remain as to whether the protein changes conformation, orientation, or a combination of the two upon adsorption. In this paper, by using sum frequency generation (SFG) vibrational spectroscopy we found that the conformation of interfacial BSA molecules changes dramatically at the solution-air interface, compared to that of the native BSA in solution. The hydrophobic methyl groups of BSA molecules at this interface tend to align along the surface normal. The degree of such conformational changes of surface BSA molecules depend on the surface coverage, indicating that the protein-protein interaction plays a very important role in determining the conformation of interfacial protein molecules. At very low surface concentration, the adsorbed BSA molecules unfold substantially. Our results can provide a molecular interpretation of results obtained from other studies such as protein layer thickness and surface tension measurements of protein solution.     

DPPC Langmuir monolayer at the air-water interface: probing the tail and head groups by vibrational sum frequency generation spectroscopy

Dipalmitoylphosphatidylcholine (DPPC) is the predominant lipid component in lung surfactant. In this study, the Langmuir monolayer of deuterated dipalmitoylphosphatidylcholine (DPPC-d62) in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase has been investigated at the air-water interface with broad bandwidth sum frequency generation (BBSFG) spectroscopy combined with a Langmuir film balance. Four moieties of the DPPC molecule are probed by BBSFG: the terminal methyl (CD3) groups of the tails, the methylene (CD2) groups of the tails, the choline methyls (CH3) in the headgroup, and the phosphate in the headgroup. BBSFG spectra of the four DPPC moieties provide information about chain conformation, chain orientation, headgroup orientation, and headgroup hydration. These results provide a comprehensive picture of the DPPC phase behavior at the air-water interface. In the LE phase, the DPPC hydrocarbon chains are conformationally disordered with a significant number of gauche configurations. In the LC phase, the hydrocarbon chains are in an all-trans conformation and are tilted from the surface normal by 25 degrees. In addition, the orientations of the tail terminal methyl groups are found to remain nearly unchanged with the variation of surface area. Qualitative analysis of the BBSFG spectra of the choline methyl groups suggests that these methyl groups are tilted but lie somewhat parallel to the surface plane in both the LE and LC phases. The dehydration of the phosphate headgroup due to the LE-LC phase transition is observed through the frequency blue shift of the phosphate symmetric stretch in the fingerprint region. In addition, implications for lung surfactant function from this work are discussed.     

A vibrational sum frequency spectroscopy study of the liquid-gas interface of acetic acid-water mixtures: 2. Orientation analysis

Vibrational sum frequency spectroscopy has been used to investigate the surface of aqueous acetic acid solutions. By studying the methyl and carbonyl vibrations with different polarization combinations, an orientation analysis of the acetic acid molecules has been performed in the concentration range 0-100%. The surface tension of acetic acid solutions was also measured in order to obtain the surface concentration. The orientation of the interfacial acetic acid molecules was found to remain essentially constant in an upright position with the methyl group directed toward the gas phase in the whole concentration range. The tilt angle (theta(CH)3) of the symmetry axis of the methyl group with respect to the surface normal was found to be lower than 15 degrees when considering a delta distribution of angles or as narrow as 0 +/- 11 degrees when assuming a Gaussian distribution. Further investigations showed that the C=O bond tilt (theta(C)(=)(O)) of the acetic acid hydrated monomer was constant and close to 55 degrees in the concentration range where it was detected. Finally, the orientation information is discussed in terms of different species of acetic acid, where the formation of a surface layer of acetic acid cyclic dimers is proposed at high acid concentrations.