Search

The orientation of the cation and the anion of 1-butyl-3-methylimidazolium dicyanamide at the platinum-liquid interface are reported, using sum frequency generation (SFG) spectroscopy. Results suggest that the imidazolium ring in the cation tends to lie flat at the surface of the electrode at negative potentials but is repelled at positive ones, while the anion is strongly adsorbed at the surface of the platinum at more positive potentials with its [C.sub.2] axis aligned with the surface normal.

Sum frequency generation spectroscopy is used to study the influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]. The results suggest that water has no drastic effect on the surface orientation of [BMIM][BF4] cation for concentrations from 0.0002[X.sub.IL] to 1[X.sub.IL].

In situ sum frequency generation vibrational spectroscopy was performed at varied potentials and polarization combinations on polycrystalline copper, platinum and gold samples in 0.5M HCl[O.sub.4] with 50mM 5-methylbenzotriazole 5-methylBTAH added. For copper surfaces orientation of the molecule on the surface was not affected by potential within the potential window studied and sum frequency generation spectra of 5-methyBTAH on platinum show a change in orientation over the potential range studied.

Sum frequency generation vibrational spectroscopy was used to characterize the interfacial properties of two ionic liquids as well as 1-methylimidazole and 1-butylimidazole on a quartz surface and Raman spectroscopy was used to identify vibrations and conclude whether they were symmetric or asymmetric. It was concluded that larger the anion the more the imidazolium ring was tilted towards normal.

Sum frequency generation spectroscopy (SFG) is used for the surface characterization at the gas-liquid interface of the 1-butyl-3-methylimidazolium cation combined with Br(super -), I(super -), PF(sub 6)(super -), BF(sub 4)(super -), (CF(sub 3)SO(sub 2))(sub 2)N(super -), SCN(super -), CH(sub 3)SO(sub 3)(super -), SH(sub 3)SO(sub 4)(super -), and [CN(sub 2)]N(super -) (DCN) anions. The analysis of the polarization-dependent SFG spectra has provided a reliable characterization of the cation at the liquid-gas interface.

The sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has a significant mixing with the molecules deposited from the backfilling solution.

The study investigates the surface of aqueous acetic acid solution using the vibrational sum frequency spectroscopy. It concluded that 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 study of aqueous acetic acid solutions is presented by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. It concluded that even such a small amounts as 0.3% acetic acid disrupt the water surface significantly.

Sum frequency generation vibrational spectroscopy (SFG) and cyclic voltammetry (CV) measurements are conducted on the ionic liquid/platinum electrode system. The SFG spectra indicate that the cation changes orientation as the electrode potential is changed within the double-layer region.

A report on an investigation on the aggregation of 1,2-dilauroyl-sn-glycero-3-phosphatidyluridine (DLPU), a surfactant molecule that merges the self-assembling properties of lecithins with the molecular recognition characteristics of nucleic bases in a phosphate-buffered aqueous solution at physiological pH and as a functin of lipid concentration is presented.

For developing a molecular-level mechanism of the heterogeneous interactions at the aqueous interface the important factor is in determining the changes in the interface with changes in the bulk composition. To probe these changes development of the nonlinear spectroscopy, sum frequency generation (SFG) provides a technique.

Issues concerning the use of sum frequency generation to examine the vapor-liquid interface of aqueous inorganic salt and acid solutions are presented.

Results obtained using sum frequency generation show that carbon monoxide adsorbed on the platinum surface occupies both 3-fold and top-site positions on Pt(111) and only top site on polycrystalline platinum electrodes.

A spectroscopic study was conducted to investigate the vibrational structure of water molecules at the surface of aqueous ionic solutions using sum frequency generation. Results of the study show that the orientation of surface water molecules is influenced by the composition of the bulk solution. Dissolved, non surface-active ions re-orient water at the interface. Experimental methodology and findings are presented.

A study was conducted to gain insight into the self-assembly properties of a cholesteryl-tetraethylenglycol single stranded 18-mer oligonucleotide ([ON.sub.1]TEG-Chol) in aqueous solution and on its spontaneous insertion in fluid phospholipid membranes. The lipophilic oligonucleotide was found to be a suitable building block that could effectively serve as a hydrophobic anchor in the fluid bilayer to assemble supramolecular constructs based on the DNA digital code.

Sum frequency generation vibrational spectroscopy is used for studying the gas-liquid interface of room-temperature ammonium and pyrrolidinium ionic liquids. The chain length dependence studies have shown that the contribution of the N[C[H.sub.3] vibration is masked by the chain for lengths of four carbons and more.

The aggregation behavior of different 1-R,2-R-sn-glycero-3-phosphatidyl-nucleosides, where R = 8 (Di[C.sub.8]PN) or R = DLPN and N is either adenosine or uridine, a complementary pair in RNA) is compared. The nucleic functionalization has provided an additional parameter to control self-assembly through specific interactions among the polar heads and have altered the aggregation thresholds.

Sum frequency generation vibrational spectroscopy is used for studying the orientation of the cation and the anion of room-temperature ionic liquids. The results have shown that presence of both ionic species at the gas-liquid interface and have described the behavior of a simple inorganic anion at the surface.

The surface orientation of ionic liquids, 1,3-dimethylimidazolium methyl sulfate, [MMIM][MS], 1-ethyl-3-methylimidazolium methyl sulfate [EMIM][MS], and 1-butyl-3-methylimidazolium methylsulfate, [BMIM][MS] at the gas-liquid interface was determined. The observations exhibited that both ions are present at the first layer of the gas-liquid interface and the alkyl chains are extended towards the gas state and far away from the liquid state.

The sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to show that octadecanethiol (ODT) forms ordered monolayers at either electrochemically reduced or oxidized Zn surfaces. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt and acid media, but the protection efficiency is reduced with immersion due to the presence of defects within the monolayers.

Aggregates formed from freshly prepared and annealed samples of dilauroyl-phosphatidyl-adenosine, dilauroyl-phosphatidyl-uridine and their 1:1 mixture were investigated by dynamic light scattering and cryo-TEM. It was found that the adenosine derivative and mixture formed from two complementary adenosine and uridine nucleolipids evolve during aging into helicoidal suprastructures that are formed at the expense of the wormlike micelles.

An n-alkenethiol, octadecanethiol (ODT), monolayer was successfully prepared onto an oxide-free mild steel (MS) surface under cathodic polarization in a 0.1 M LiCl/CH(sub 3)OH solution containing 1mM ODT. Cyclic voltammetry CV) and electrochemical impedence (EIS) and sum frequency generation (SFG) spectroscopy were applied to study and characterize the adsorption of ODT at a MS surface.

The arrangement of ions at the platinum electrode in the room-temperature ionic liquid 1-butyl 3-methylimidazolium tetrafluoroborate is determined using sum frequency generation vibrational spectroscopy (SFG), electrochemical impedance spectroscopy (EIS), and the vibrational Stark effect. The result indicates that the ions of a neat ionic liquid are organized in a Helmholtz layer at the electrified metal electrode interface.

A study is conducted to investigate the structural properties of dilauroyl-phosphatidy-uridine wormlike micelles using the combined use of static light scattering and small angle neutron scattering techniques. The result provides an accurate estimation of micellar flexibility in terms of persistence length.

Sum frequency generation vibrational spectroscopy is used to determine the orientation of the cations in a room-temperature ionic liquid at the air-liquid interface. The ionic liquid studied was 1-butyl-3-methylimidazolium bis-triflouromethylsulfonimade, [BMIM](super +)[imide](super -). At water pressures below 10(super -4) Torr, the imidazolium ring was oriented parallel to the surface plane.

Sum frequency generation (SFG) vibrational spectroscopy in combination with cyclic voltammetry (CV) is used to investigate the oxidation of carbon monoxide (CO) in acetonitrile (CH3CN)/water (H2O) solutions at the platinum (Pt) electrode surface. Spectroscopy and voltammetry data in these essentially nonaqueous electrolytes indicate an oxidation intermediate of CO to CO2 that is nearly parallel to the Pt electrode surface.

Sum frequency generation (SFG) vibrational spectroscopy is used to investigate the orientation of acetonitrile (CH3CN) and Pt (111) electrode. The SFG results indicate that acetronitrile orients in response to the electrode potential with the C-C bond, C3 symmetry axis, perpendicular to the surface.

Adsorption of dimethyldodecylamine oxide (DDAO) and its mixtures with Triton X-100 (TX-100) at the hydrophilic silica/water interface has been studied using total internal reflection (TIR) Raman spectroscopy and target factor analysis (TFA). The use of a linear vibrational spectroscopic technique helps obtain information on molecular behavior, adsorbed amount, and conformational order of surfactant molecules at the interface. The results obtained from polarized Raman measurements of pure DDAO show insignificant changes in the orientation and conformational order of surface molecules as a function of DDAO bulk concentrations. The adsorption isotherm of pure DDAO shows a change in the structure of the adsorbed layer at concentrations close to the critical micelle concentration (cmc). TFA reveals that, for a low concentration of DDAO (0.30 mM in this study), adsorption of both DDAO and TX-100 in the mixed surfactants was enhanced at low TX-100 concentrations. The synergistic effect is dominant at low concentrations of TX-100, with enhanced adsorption of both surfactants. Although competitive adsorption is effective at high concentrations of TX-100, the presence of a small amount of DDAO at the interface still enhances TX-100 adsorption. When DDAO concentrations are increased to 1.00 mM, TX-100 replaces DDAO molecules on the surface when TX-100 concentration is increased. [ABSTRACT FROM AUTHOR]

The adsorption of the cationic surfactantbenzyldimethylhexadecylammonium(BDMHA+) chloride was studied at an octadecyltrichlorosilane(OTS)-monolayer-modified silica–water interface by Raman spectroscopyin total internal reflection (TIR) geometry. The present study demonstratesthe capabilities of this spectroscopic technique to evaluate thermodynamicand kinetic BDMHA+Cl–adsorption propertiesat the hydrophobic silica surface. The surface coverage of BDMHA+decreased by 50% at the hydrophobic OTS-silica surface relativeto the surface coverage on bare silica; the dominating driving mechanismsfor surfactant adsorption were identified as hydrophobic effects andhead group charge screening by the electrolyte counterions. Additionof magnesium metal salt (MgCl2) to the aqueous solution(∼ neutral pH) lowered the surface coverage and moderatelyincreased the Langmuir adsorption constants relative to those of thepure surfactant. These trends were previously observed at the hydrophilic,negatively charged silica surface but with a smaller change in theGibbs free energy of adsorption at the hydrophobic silica surface.The hydrophobic OTS-silica surface properties resulted in shortertimes for the surfactant to reach steady-state adsorption conditionscompared to the slow adsorption kinetics previously seen with thesurfactant at the hydrophilic surface. Adsorption isotherms, basedon Raman signal intensities from spectral analysis, were developedaccording to the Langmuir adsorption model for the pure surfactantat the OTS-silica–water interface; the modified Langmuir modelwas applied to the surfactant adsorption in the presence of 5, 10,50, and 100 mM magnesium chloride. Spectral analysis of the Ramanscattering intensities and geometric considerations suggests a hemimicelle-typesurface aggregate as the most likely surfactant structure at the OTS-silicasurface. The different kinetics observed at the hydrophilic versusthe hydrophobic silica surface further indicate that the surface chargeand potential influence the surfactant diffusion and kinetic ratesof adsorption at the silica–water interface. [ABSTRACT FROM AUTHOR]

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium(BDMHA+) chloride has been studied at the hydrophilic silica–waterinterface by Raman spectroscopy in total internal reflection geometry(TIR Raman). This Raman spectroscopic technique takes advantage ofan evanescent electric field that is generated at the silica–waterinterface in TIR mode with specific probing depth. The present studydemonstrates the capabilities of the TIR Raman sampling configurationto provide structural information and simultaneously serve as an experimentalplatform for studying thermodynamic and kinetic properties of BDMHA+Cl–at the silica–water interfaceat neutral pH and compare its adsorption behavior with the modifiedadsorption properties in the presence of four different concentrationsof a divalent metal salt. Spectral analysis of the Raman scatteringintensities as a function of time and concentration provided the inputdata for evaluating adsorption properties of the surfactant in theabsence and presence of the metal salt additive. Addition of the magnesiummetal salt lowered the cmc, altered the surface excess of the surfactant,and increased the Langmuir adsorption constants, as well as the magnitudeof the free energy of adsorption, and adsorption kinetics, proportionalto the concentrations of the metal salt. Adsorption isotherms basedon a modified Langmuir adsorption model were established for fivesystems: the pure surfactant in aqueous solution, and the surfactantin the presence of 5, 10, 50, and 100 mM of magnesium chloride. Themetal salt did not enhance surfactant adsorption at very low surfactantconcentrations below 5 μM, where adsorption occurs by electrostaticattraction; the divalent metal salt, however, favorably influencedthe adsorption behavior in the aggregate formation region by reducingthe electrostatic repulsion between the polar surfactant head groups,and enhancing the hydrophobic effect between the hydrophobic surfactantalkyl chains and the polar water molecules. [ABSTRACT FROM AUTHOR]

The gas−liquid interface of halide-free 1,3-dialkylimidazolium alkyl sulfates [RMIM][R−OSO3] with R chain length from C1−C4and C8has been studied systematically using the surface-specific sum frequency generation (SFG) vibrational spectroscopy and surface tension measurements. From the SFG spectra, vibrational modes from the methyl group of both cation and anion are observed for all ionic liquid samples considered in the present study. These results suggest the presence of both ions at the gas−liquid interface, which is further supported by surface tension measurements. Surface tension data show a decreasing trend as the alkyl chain in the imidazolium cation is varied from methyl to butyl chain, with a specific anion. A similar trend is observed when the alkyl chain of the anion is modified and the cation is fixed. [ABSTRACT FROM AUTHOR]

DNA nanoconstructs are obtained in solution by using six unique 42-mer DNA oligonucleotides, whose sequences have been designed to form a pseudohexagonal structure. The required flexibility is provided by the insertion of two non-base-paired thymines in the middle of each sequence that work as flexible hinges and constitute the corners of the nanostructure when formed. We show that hexagonally shaped nanostructures of about 7 nm diameter and their corresponding linear open constructs are formed by self-assembly of the specifically designed linear oligonucleotides. The structural and dynamical characterization of the nanostructure is obtained in situ for the first time by using dynamic light scattering (DLS), a noninvasive method that provides a fast dynamic and structural analysis and allows the characterization of the different synthetic DNA nanoconstructs in solution. A validation of the LS results is obtained through Monte Carlo (MC) simulations and atomic force microscopy (AFM). In particular, a mesoscale molecular model for DNA, developed by Knotts et al., is exploited to perform MC simulations and to obtain information about the conformations as well as the conformational flexibilities of these nanostructures, while AFM provides a very detailed particle analysis that yields an estimation of the particle size and size distribution. The structural features obtained by MC and AFM are in good agreement with DLS, showing that DLS is a fast and reliable tool for characterization of DNA nanostructures in solution. [ABSTRACT FROM AUTHOR]

Phosphatidyl-nucleosides are a class of functional amphiphiles, where a nucleic acid monomer is conjugated to a lipid skeleton. These derivatives self-organize in aqueous solution as assemblies of various size, shape, and interfacial curvature. This paper presents a comparison of the aggregation behavior of different 1-R,2-R-sn-glycero-3-phosphatidyl-nucleosides, where R 8 (DiC8PN) or R 12 (DLPN) and N is either adenosine (a purine) or uridine (a pyrimidine), a complementary pair in RNA. Surface tension, small angle neutron scattering, cryo-TEM, and circular dichroism are used to highlight and distinguish the impact of the hydrophobic assembler and of the base substitution on the solution phase behavior. Our main conclusion is that the nucleic functionalization provides an additional parameter to control self-assembly through specific interactions among the polar heads. Further nonideal effects are induced by mixing nucleolipids with complementary base substitution. We show that these contributions alter the aggregation thresholds and modulate properties of the aggregates on the mesoscale. [ABSTRACT FROM AUTHOR]

The orientation of the cation and the anion of room-temperature ionic liquids using sum frequency generation vibrational spectroscopy is reported. The ionic liquids are based on butyl-methyl imidazolium BMIMand hexyl-tributyl ammonium N6444together with dicyanamide DCA-as the anion. The tilt angle of the C3axis of the methyl group from the alkyl chain in the cations was found to vary from 52° to 80° as a function of the distribution width (which ranges from 0° to 30° with respect to the surface normal) for BMIMDCA and similarly for N6444DCA. The orientation of the C2axis in the dicyanamide anion as a function of the twist angle , varied between 46° and 90° for BMIMDCA and from 53° to 90° for N6444DCA. These results suggest the presence of both ionic species at the gas−liquid interface and help describe the behavior of a simple inorganic anion at the surface. [ABSTRACT FROM AUTHOR]

The orientation of the ions at the surface of ionic liquids has been determined by the surface-sensitive technique sum-frequency generation vibrational spectroscopy. The results indicate that both ions are present at the first layer of the gas−liquid interface. Furthermore, the alkyl chains are found to be extended toward the gas phase and away from the liquid phase. The proposed models for the preferred orientation of the ions are in good agreement in comparison with the results obtained from the recent MD simulation studies. The salts considered here are 1,3-dimethylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium methylsulfate, and 1-butyl-3-methylimidazolium methylsulfate. [ABSTRACT FROM AUTHOR]

The orientation of the cation and the anion of 1-butyl-3-methylimidazolium dicyanamide at the platinum−liquid interface, using sum frequency generation (SFG) spectroscopy is reported. Sum frequency spectra at two different polarizations and different potentials are recorded, and analysis of polarization-dependent spectra is performed to estimate the orientation of the dicyanamide anion and the alkyl and ring moieties in the cation as a function of the potential applied to the platinum electrode. In addition, cyclic voltammetry and electrochemical impedance spectroscopy are conducted. A model of the double-layer structure at the electrified interface is presented from the analysis of capacitance and SFG data. [ABSTRACT FROM AUTHOR]

Aggregates formed from freshly prepared and annealed samples of dilauroyl-phosphatidyl-adenosine, dilauroyl-phosphatidyl-uridine, and their 1:1 mixture have been investigated by dynamic light scattering, cryo-transmission electron microscopy (cryo-TEM) observations, and circular dichroism. The two surfactants differ only for the nucleoside at the phospholipid polar headgroup and self-assemble in solution to form supramolecular structures that behave dissimilarly. The uridine derivative forms long wormlike aggregates that are invariant with the aging of the solution, while the wormlike aggregate of the adenosine derivative undergoes, as the sample ages, a subsequent self-assembling process forming giant helicoidal aggregates that coexist with the smaller wormlike aggregates. Dynamic light scattering and cryo-TEM show that the large helicoidal structures are formed at the expense of the small wormlike micelles. The 1:1 mixture behaves as the adenosine derivative and evolves to form giant superstructures for all the lipid concentrations investigated. Circular dichroism measurements suggest that the formation of the supramolecular helicoidal structure might not be driven by a purely chiral effect, but rather stacking and hydrogen bonding, present at the phospholipid headgroups of the self-assembled nucleosides, contribute to the final supramolecular structure. [ABSTRACT FROM AUTHOR]

Sum frequency generation spectroscopy (SFG) was used to study the influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. The orientation of the cation at the gas−liquid interface was analyzed as a function of ionic liquid concentration in water for concentrations from 0 to 1 mole fraction of the ionic liquid. The cation was found to be oriented with the imidazolium ring nearly parallel to the surface plane with a tilt angle ≥ 70° when the ionic liquid was dry. Furthermore, no noticeable change in the orientation was observed when high concentrations of water were mixed with the ionic liquid. The cation butyl chain is projecting into the gas phase with a CH3tilt angle of 54 ± 2° when the ionic liquid is dry and 46 ± 4° when mixed with water. Water is oriented at the surface only for concentration ≤ 0.02 mole fraction of the ionic liquid. At higher ionic liquid concentrations (mole fractions ≥ 0.05) the gas−liquid interface resembles that of the pure ionic liquid. [ABSTRACT FROM AUTHOR]

In situ sum frequency generation vibrational spectroscopy, at varied potentials and polarization combinations, was performed on polycrystalline copper, polycrystalline platinum, and polycrystalline gold samples in 0.5 M HClO4with 50 mM 5-methylbenzotriazole (5-methylBTAH) added. These studies were performed to determine the orientation of 5-methylBTAH on the surface at different potentials. For copper surfaces, orientation of the molecule on the surface is not affected by potential within the potential window studied (−500 to −100 mV vs saturated calomel electrode (SCE)). Sum frequency generation spectra of 5-methylBTAH on platinum show a change in orientation over the potential range studied (−250 to 750 mV vs SCE). The orientation of the methyl group tilts more toward the plane of the interface as the potential is scanned in the positive direction. This orientation change is correlated to hydrogen coadsorption on the platinum surface at low potentials. 5-Methylbenzotriazole lies in the surface plane or does not orient on gold at lower potentials but the orientation is tilted toward normal at more positive potentials over the potential range studied (−500 to 900 mV vs SCE). To compliment these results, cyclic voltammetry and electrochemical impedance spectroscopy measurements were performed. Cyclic voltammograms of copper show that addition of 5-methylBTAH protects the surface from copper dissolution, increasing the electrochemical window by 450 mV. Cyclic voltammetry of 5-methylBTAH on platinum showed a partial blockage of adsorbed hydrogen and also prevented the adsorption of oxygenated species at 450−600 mV. Cyclic voltammetry on gold shows that 5-methylBTAH blocks oxide formation for 400 mV thus increasing the electrochemical window. Electrochemical impedance spectroscopy has been performed to determine the potential of zero charge of 5-methylBTAH on copper. [ABSTRACT FROM AUTHOR]

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][BF4], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]. 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° from normal, with the methyl group oriented 32−35° 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° from normal. The orientation of the [BMIM][PF6] ionic liquid was sensitive to trace amounts of water and had to be evacuated to <3 × 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° from normal for [BMIM][BF4], while the ring for [BMIM][PF6] was tilted 38−58° from normal. This suggests the orientation of the molecule is influenced by the size of the anion. [ABSTRACT FROM AUTHOR]