Differing adsorption behavior of environmentally important cyanophenol isomers at the air-water interface

Vibrational sum-frequency spectroscopy and surface tensiometry have been used to study the adsorption of m- and p-cyanophenol at the air-water interface. Spectra of the cyano (CN) group under different polarization schemes are utilized to determine its hydrogen bonding environment and orientation. For both isomers, it is found that the cyano group is hydrogen bonded at the interface but that the CN orientation is independent of surface density. The average CN tilt angle (theta(0)), however, is found to differ between the isomers, such that the CN group points down toward the aqueous phase for m-cyanophenol (theta(0) = 96-106 degrees ) but points up toward the vapor phase for the p-cyanophenol (theta(0) = 65-80 degrees ). In addition, this average tilt angle is distributed over a narrow range, sigma(0) < 10 degrees for the meta isomer and sigma(0) < 16 degrees for the para isomer.     

A vibrational sum frequency spectroscopy study of the liquid-gas interface of acetic acid-water mixtures: 1. Surface speciation

Aqueous acetic acid solutions have been studied by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. The concentration range 0-100% acetic acid has been studied in the CH/OH and the C-O/C=O regions, and in order to clarify peak assignments, experiments with deuterated acetic acid and water have also been performed. Throughout the whole concentration range, the acetic acid is proven to be protonated. It is explicitly shown that the structure of a water surface becomes disrupted even at small additions of acetic acid. Furthermore, the spectral evolution upon increasing the concentration of acetic acid is explained in terms of the different complexes of acetic acid molecules, such as the hydrated monomer, linear dimer, and cyclic dimer. In the C=O region, the hydrated monomer is concluded to give rise to the sum frequency (SF) signal, and in the CH region, the cyclic dimer contributes to the signal as well. The combination of results from the CH/OH and the C-O/C=O regions allows a thorough characterization of the behavior of the acetic acid molecules at the interface to be obtained.     

Influence of temperature on the composition of mixed monolayer formed at the methyl alcohol/acetic acid aqueous solutio–air interface

The influence of temperature on the composition of mixed monolayer formed at the methyl alcohol/acetic acid aqueous solution was examined by surface tension measurements. Surface tension of various two-component solutions was obtained at 10, 20 and 28 °C temperatures in the 0–0.5 M range of bulk concentration of alcohol and acid, respectively. Three independent methods, i.e., the Gibbs adsorption equation (GAE), regular solution approximation (RSA) and Butler adsorption isotherm (BAI) were applied to calculate surface composition of the methyl alcohol/ acetic acid mixed monolayer. It was shown that in the temperature range of 10–28 °C the surface molar fraction of the solutes remained constant for the fixed bulk concentration of alcohol and acid. Additionally, based on the RSA and BAI methods, we showed there were no interactions between solute molecules in the mixed monolayer in the studied range of concentrations. Received: 18 December 1997 Accepted: 8 May 1998     

Sum frequency generation spectroscopy of dicyanamide based room-temperature ionic liquids. Orientation of the cation and the anion at the gas-liquid interface

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 [BMIM]+ and hexyl-tributyl ammonium [N6444]+ together with dicyanamide [DCA]- as the anion. The tilt angle of the C3 axis of the methyl group from the alkyl chain in the cations was found to vary from 52 degrees to 80 degrees as a function of the distribution width sigma (which ranges from 0 degrees to 30 degrees with respect to the surface normal) for [BMIM][DCA] and similarly for [N6444][DCA]. The orientation of the C2 axis in the dicyanamide anion as a function of the twist angle phi, varied between 46 degrees and 90 degrees for [BMIM][DCA] and from 53 degrees to 90 degrees for [N6444][DCA]. 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.     

Liquid structure of acetic acid-water and trifluoroacetic acid-water mixtures studied by large-angle X-ray scattering and NMR

The structures of acetic acid (AA), trifluoroacetic acid (TFA), and their aqueous mixtures over the entire range of acid mole fraction xA have been investigated by using large-angle X-ray scattering (LAXS) and NMR techniques. The results from the LAXS experiments have shown that acetic acid molecules mainly form a chain structure via hydrogen bonding in the pure liquid. In acetic acid-water mixtures hydrogen bonds of acetic acid-water and water-water gradually increase with decreasing xA, while the chain structure of acetic acid molecules is moderately ruptured. Hydrogen bonds among water molecules are remarkably formed in acetic acid-water mixtures at xA相似文献   

From simple amphiphilic to surfactant behavior: analysis of (1)H NMR chemical shift variations

Analysis of the (1)H NMR chemical shift variations for the methyl protons of sodium decanoate and decanoic acid in D(2)O solutions using reduced variables is consonant with a narrow distribution of sizes about the mean aggregation number for decanoate ion micelles, in contrast with decanoic acid polydisperse aggregates which increase their size with concentration, until phase separation is reached. At defined temperatures between 10 and 50 degrees C, the chemical shift coefficients for the methyl group protons exhibit a negative temperature slope (shielding) for decanoate ion micelles and a positive temperature slope (deshielding) for decanoic acid aggregates. These results suggest that an increase of temperature improves the mobility of the decanoate ion chains in the micelles, thus inducing the methyl groups of the decanoate ion micelles to spend more time near the micelle-water interfaces. In turn, the size of polydisperse decanoic acid aggregates increases with temperature.     

Effect of ring substitution position on the structural conformation of mercaptobenzoic acid self-assembled monolayers on Au(111)

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, photoemission spectroscopy (PES), and contact angle measurements have been used to examine the structure and bonding of self-assembled monolayers (SAMs) prepared on Au(111) from the positional isomers of mercaptobenzoic acid (MBA). The isomer of MBA and solvent chosen in SAM preparation has considerable bearing upon film morphology. Carbon K-edge NEXAFS measurements indicate that the monomers of 2-, 3-, and 4-MBA have well-defined orientations within their respective SAMs. Monomers of 3- and 4-MBA assume an upright orientation on the Au substrates in monolayers prepared using an acetic acid in ethanol solvent. The aryl ring and carboxyl group of these molecules are tilted from the surface normal by a colatitudal angle of approximately 30 degrees . Preparation of 4-MBA SAMs using pure ethanol solvent, a more traditional means of synthesis, had no appreciable effect upon the monomer orientation. Nonetheless, S(2p) PES measurements illustrate that it results in extensive bilayer formation via carboxyl group hydrogen-bonding between 4-MBA monomers. In 2-MBA monolayers prepared using acetic acid/ethanol solvent, the monomers adopt a more prostrate orientation on the Au substrates, in which the aryl ring and carboxyl group of the molecules are tilted approximately 50 degrees from the surface normal. This configuration is consistent with an interaction between both the mercaptan sulfur and carboxyl group of 2-MBA with the underlying substrate. S(2p) and C(1s) PES experiments provide supporting evidence for a bidentate interaction between 2-MBA and Au(111).     

Alpha-gamma hybrid peptides that contain the conformationally constrained gabapentin residue: characterization of mimetics of chain reversals

The crystal structures of four dipeptides that contain the stereochemically constrained gamma-amino acid residue gabapentin (1-(aminomethyl)cyclohexaneacetic acid Gpn) are described. The molecular conformation of Piv-Pro-Gpn-OH (1), reveals a beta-turn mimetic conformation, stabilized by a ten atom C[bond]H...O hydrogen bond between the Piv CO group and the pro S hydrogen of the Gpn CH(2)[bond]CO group. The peptides Boc-Gly-Gpn-OH (2), Boc-Aib-Gpn-OH (3), and Boc-Aib-Gpn-OMe (4) form compact, folded structures, in which a distinct reversal of polypeptide chain direction is observed. In all cases, the Gpn residue adopts a gauche,gauche (g,g) conformation about the C(gamma)[bond]C(beta) (theta(1)) and C(beta)[bond]C(alpha) (theta(2)) bonds. Two distinct Gpn conformational families are observed. In peptides 1 and 3, the average backbone torsion angle values for the Gpn residue are phi=98 degrees, theta(1)=-62 degrees, theta(2)=-73 degrees, and psi=79 degrees, while in peptide 2 and 4 the average values are phi=-103 degrees, theta(1)=-46 degrees, theta(2)=-49 degrees, and psi=-92 degrees. In the case of 1 and 3, an intramolecular nine-membered O[bond]H...O hydrogen bond is formed between the C[double bond]O of the preceding residue and the terminal carboxylic acid OH group. All four alpha-gamma dipeptide sequences yield compact folded backbone conformations; this suggests that the Gpn residue may be employed successfully in the design of novel folded structures.     

Structure and phase transition in self-assembled films of an anti-ferroelectric liquid crystal studied by two-dimensional correlation FTIR spectroscopy

The FTIR spectra were measured for a self-assembled film of the anti-ferroelectric liquid crystal (AFLC) molecules over a temperature range of 40-150 degrees C. The frequency and intensity variations of the infrared spectra were discussed in terms of changes in molecular orientation, conformation and intra- and/or inter-molecular interaction during phase transitions. The two-dimensional (2d) correlation analysis revealed that two hydrocarbon chains have different orientation behaviors, the shorter hydrocarbon chain is parallel to molecular long axis and the longer one is perpendicular to it. The splitting of the C=O stretching band indicated that the s-cis and the s-trans conformers of the ester group coexist in the self-assembly film. During the phase transitions, in mesogen part the C=O group adjacent to the chiral part is first response with temperature changing, following the C=O group between two phenyl rings, and eventually the phenyl rings; and the respondence with temperature of methyl is slower than that of methylene in alkyl chains.     

The adsorption of cetyltrimethylammonium bromide and propanol mixtures with regard to wettability of polytetrafluoroethylene II. Adsorption at polytetrafluoroethylene-aqueous solution interface and wettability

Measurements of contact angles (theta) of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and propanol mixtures at constant CTAB concentration equal to 1x10(-5), 1x10(-4), 6x10(-4) and 1x10(-3) M on polytetrafluoroethylene (PTFE) were carried out. The obtained results indicate that the wettability of PTFE by aqueous solutions of these mixtures depends on their composition and concentration. They also indicate that, contrary to Zisman, there is no linear relationship between cos theta and the surface tension (gamma(LV)), but a linear relationship exists between the adhesional (gamma(LV)cos theta) and surface tension of aqueous solutions of CTAB and propanol mixtures. Curve gamma(LV)cos theta vs gamma(LV) has a slope equal -1 suggesting that adsorption of CTAB and propanol mixtures and the orientation of their molecules at aqueous solution-air and PTFE-aqueous solution interfaces is the same. Extrapolating this curve to the value of gamma(LV)cos theta corresponding to theta=0, the value of the critical tension of PTFE wetting equal 23.4 mN/m was determined. This value was higher than that obtained from contact angles of n-alkanes on PTFE surface (20.24 mN/m). The difference between the critical surface tension values of wetting probably resulted from the fact that at cos theta=1 the PTFE-aqueous solution of CTAB and propanol mixture interface tension was not equal to zero. This tension was determined on the basis of the measured contact angles and Young equation. It appeared that the values of PTFE-aqueous solution of the CTAB and propanol mixtures interface tension can be satisfactorily determined by modified Szyszkowski equation only for solutions in which probably CTAB and propanol molecules are present in monomeric form. However, it appeared that using the equation of Miller et al., in which the possibility of aggregation of propanol molecules in the interface layer is taken into account, it is possible to describe the PTFE-solution interfacial tension for all systems studied in the same way as by the Young equation. On the basis of linear dependence between the adhesional and surface tension it was established that the work of adhesion of aqueous solution of CTAB and propanol mixtures does not depend on its composition and concentration, and the average value of this work was equal to 46.85 mJ/m(2), which was similar to that obtained for adhesion of aqueous solutions of two cationic surfactants mixtures to PTFE surface.     

Absolute orientation of molecules at interfaces

A method to determine the absolute orientation of molecules at liquid interfaces by sum frequency generation (SFG) is reported. It is based on measurements of the orientations of two nonparallel vibrationally active chromophores in the molecule of interest combined with a rotation matrix formulation to obtain the absolute molecular orientation. We chose m-tolunitrile, a planar molecule adsorbed to the air/water interface, as a proof-of-method experiment. Quantitative analysis of different polarization sum frequency intensities facilitate unique peak assignments of the methyl and nitrile groups of m-tolunitrile. The SFG analysis of the measurement yields a nitrile group tilting at 53 degrees to the surface normal, and the C3 axis of the methyl group is almost upright at 23 degrees with respect to the surface normal. Using a rotation matrix formulation, we found that the angle between the surface plane and the m-tolunitrile molecular plane is 70 degrees.     

Atmospheric oxidation pathways of acetic acid

One of the most abundant carboxylic acids measured in the atmosphere is acetic acid (CH(3)C(O)OH), present in rural, urban, and remote marine environments in the low-ppb range. Acetic acid concentrations are not well reproduced in global 3-D atmospheric models because of the poor inventory of sources and sinks to model its global distribution. To understand the complete oxidation of acetic acid in the atmosphere initiated by OH radicals, ab initio calculations are performed to describe in detail the energetics of the reaction potential energy surface (PES). The proposed reaction mechanism suggests that the CH(3)C(O)OH + OH reaction takes place via three pathways: the addition of OH to the central carbon, the abstraction of a methyl hydrogen, and the abstraction of an acidic hydrogen. The PES is characterized by prereactive H-complexes, transition states, and more interestingly unique radical-mediated isomerization reactions. From the analysis of the energetics, acetic acid atmospheric oxidation will proceed mainly via the abstraction of the acidic hydrogen, consistent with previous experimental and theoretical studies. The major byproducts from each pathway are identified. Glyoxylic acid is suggested to be a major byproduct of the atmospheric oxidation of acetic acid. The atmospheric fate of glyoxylic acid is discussed.     

Characterization of Langmuir-Blodgett films of a ferroelectric liquid crystal

Molecular orientation, structure, and phase transition behaviors in Langmuir-Blodgett (LB) and cast films of a ferroelectric liquid crystal of sec-butyl 6-(4-(nonyloxy)benzoyloxy)-2-naphthoate (FLC-1) are determined by ultraviolet (UV) spectroscopy, X-ray diffraction, and infrared (IR) spectroscopy. It is found that the orientation angle of chromophores theta in LB films is 41 degrees from the surface normal. The tilt angle of the chromophore changes at 56, 70, and 88 degrees C, respectively, which denotes the presence of phase transitions. Two kinds of layered or isomeric crystal structures of the LB films with layer spacings of 3 and 3.5 nm at room temperature have been found while the latter disappears above 45 degrees C, as confirmed by measurement of temperature-dependent IR spectra.     

Pore surface fractal analysis of palladium-alumina ceramic membrane using Frenkel-Halsey-Hill (FHH) model

The alumina ceramic membrane has been modified by the addition of palladium in order to improve the H(2) permeability and selectivity. Palladium-alumina ceramic membrane was prepared via a sol-gel method and subjected to thermal treatment in the temperature range 500-1100 degrees C. Fractal analysis from nitrogen adsorption isotherm is used to study the pore surface roughness of palladium-alumina ceramic membrane with different chemical composition (nitric acid, PVA and palladium) and calcinations process in terms of surface fractal dimension, D. Frenkel-Halsey-Hill (FHH) model was used to determine the D value of palladium-alumina membrane. Following FHH model, the D value of palladium-alumina membrane increased as the calcinations temperature increased from 500 to 700 degrees C but decreased after calcined at 900 and 1100 degrees C. With increasing palladium concentration from 0.5 g Pd/100 ml H(2)O to 2 g Pd/100 ml H(2)O, D value of membrane decreased, indicating to the smoother surface. Addition of higher amount of PVA and palladium reduced the surface fractal of the membrane due to the heterogeneous distribution of pores. However, the D value increased when nitric acid concentration was increased from 1 to 15 M. The effect of calcinations temperature, PVA ratio, palladium and acid concentration on membrane surface area, pore size and pore distribution also studied.     

Solvent-dependent assembly of terphenyl- and quaterphenyldithiol on gold and gallium arsenide

The assembly of terphenyldithiol (TPDT) and quaterphenyldithiol (QPDT) on gold and gallium arsenide from ethanol (EtOH), tetrahydrofuran (THF), and solutions consisting of both solvents has been characterized by near-edge X-ray absorption fine structure spectroscopy. The surface coverage and the average orientation of both TPDT and QPDT on gold are solvent-independent. These molecules readily form monolayers on gold with an ensemble-average backbone tilt of 30 degrees +/- 3 degrees from the substrate normal. In sharp contrast, the assembly of TPDT and QPDT on gallium arsenide is extremely solvent-sensitive. At high ethanol fractions, both molecules form monolayers with an ensemble-average orientation that is indistinguishable from those on gold substrates. At low ethanol fractions and in pure THF, however, these molecules are disordered on gallium arsenide and the surface coverage is poor.     

Changes in the structure of water in aqueous solutions of acetic acid, depending on concentration and temperature according to densitometry, viscosimetry, and IR spectroscopy data

The structural features of aqueous solutions of acetic acid are studied by means of viscosimetry, densimetry, and IR spectroscopy within the temperature range of 283.15–333.15 K and a concentration range of 0–80%. The results from our experiments are used to calculate the parameters of viscous flow activation, the structural temperature, the partial molar volume of acetic acid in the solution, the thermal expansion coefficient, and the energies and lengths of the hydrogen bonds between water molecules. The concentration dependences of these parameters are analyzed. Based on the obtained results, we conclude that acetic acid disrupts the structure of water.     

Hydrogen Bonding of Carboxylic Acids in Aqueous Solutions—UV Spectroscopy, Viscosity, and Molecular Simulation of Acetic Acid

The UV spectra of aqueous acetic acid solutions up to 2M were investigated. At these wavelengths, the carboxylic acids exhibit an absorption peak, attributed to the C=O group, which shifts when hydrogen bonds are formed.. The measured spectra were best fitted to several bands, either of Gaussian or Lorentzian shape, which can be explained as several types of structural units formed by hydrogen bonds established between acetic acid and water molecules and between acetic acid molecules themselves. Molecular dynamics simulation of these mixtures was also performed, confirming the occurrence of several types of hydrogen bonds and showing the presence of dimers at higher concentrations. The viscosity and density of these solutions were also measured at different concentrations and temperatures. These results give a more complete picture of the hydrogen bond network of the system.     

Pulsed Electric Linear Dichroism of Triphenylmethane Dyes Adsorbed on Montmorillonite K10 in Aqueous Media

Electric linear dichroism (ELD) spectra of two cationic triphenylmethane dyes, crystal violet (CV) and malachite green (MG), bound to sodium montmorillonite K10 (MK-10) were studied at 20 degrees C in aqueous media at two mixing ratios, D/S, of 0.10 and 0.24 in the 700- to 400-nm wavelength region and in the applied electric field strength range between 0 and 3 kV/cm. The specific parallel and perpendicular dichroism (Delta A( ||)/A and Delta A( perpendicular)/A) spectra of dye-adsorbed MK-10 suspension were measured at a fixed field strength with an apparatus equipped with a 512-channel photodiode array detector. By changing the field strength over a wide range, a series of the reduced dichroism values of the bound dyes were measured at a fixed wavelength. By fitting these dichroism values to theoretical orientation functions, the intrinsic reduced dichroism (Delta A/A)(int) spectra at the limiting high fields (ELD spectrum) were determined for CV and MG bound to MK-10. No appreciable difference was observed at the two D/S values. The ELD spectra of these bound dyes are undulatory but never constant, throughout their absorption region; thus, the dye plane does not lie flatly either on the surface or between layers of MK-10 particle. The isotropic absorption spectra, A, of bound CV and MG were each deconvoluted to eight partial absorption bands, which were grouped into three differently polarized transitions, i.e., one out-of-plane and two mutually perpendicular in-plane. The optical transition dipole moment direction of each group was found to make a considerable angle with respect to the symmetry axis of the disklike MK-10 particle, whose plane (or surface) tends to orient toward the applied electric field at the limiting high fields. By simulating the observed ELD spectra of bound CV and MG with those deconvoluted bands, the roll, tilt, and inclination angles of both dyes were evaluated quantitatively with a new analytical method. The average angles (+/-θ(R), +/- θ(T), |θ(N)|) are -(34-47) degrees, 34 degrees, and 51 degrees for bound CV and -44 degrees, 32 degrees, and 53 degrees for bound MG at two D/S values; thus, the triangular plane of each dye is rolled as well as tilted with a large inclination angle. Copyright 2000 Academic Press.     

Study on the conversion of three natural statins from lactone forms to their corresponding hydroxy acid forms and their determination in Pu-Erh tea

Conversions of statins, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, from lactone forms to their corresponding hydroxy acid form in 0.1 N NaOH or 0.05 N KOH (prepared with 25, 50, 75, 90% acetonitrile or methanol in water or 100% water) were evaluated. Results showed that lactone form statins could be transformed almost completely only in alkaline solutions prepared with 25 or 50% acetonitrile. In all methanolic alkaline solutions, lactone form statins could also be converted entirely, nevertheless, they would be further transformed to the methyl ester of the hydroxy acid form and the transformation increased as methanol rises. When lactone and hydroxy acid forms of statins were in methanol, ethyl acetate, 70% acetonitrile in water (with 0.5% acetic acid or no) for 0-48 h at room temperature or in 100 degrees C water for 0-2 h, lactone form statins were converted to their corresponding hydroxy acids, which were raised as time extends and the highest conversions of them were about 35% in 100 degrees C water and 70% acetonitrile, slightly transformed for lactone form statins in 70% acetonitrile (with 0.5% acetic acid) after 8 h, and the other treatments for all statins showed no significant changes. Interferences would be reduced efficiently when statins were extracted from Pu-Erh tea with methanol, ethyl acetate or 100 degrees C water followed by purifying through a C18 solid-phase extraction cartridge. Lovastatin was the only statin found in Pu-Erh tea and the highest content of it was found under ethyl acetate extraction. In ethyl acetate and methanol extracts, lovastatin existed merely as lactone form. The lowest content of lovastatin was found in the 100 degrees C water extract of Pu-Erh tea, however, both of lactone and hydroxy acid forms were found to exist in the extract.     

Influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate: a sum frequency generation analysis

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 > or = 70 degrees 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 CH(3) tilt angle of 54 +/- 2 degrees when the ionic liquid is dry and 46 +/- 4 degrees when mixed with water. Water is oriented at the surface only for concentration < or = 0.02 mole fraction of the ionic liquid. At higher ionic liquid concentrations (mole fractions > or = 0.05) the gas-liquid interface resembles that of the pure ionic liquid.