Conventional and unconventional infrared spectrometry and their quantitative interpretation

In condensed matter, optical properties can be described by a dielectric function (DF), and the structures observed in spectra are then related to the poles and zeros of the DF. As an example, model functions are calculated by a fit to measured spectroscopic data for polystyrene and silica. The first material shows weak, narrow bands and the latter strong, broad bands and a negative real part of the DF.Based on these model DFs, spectra are simulated which are expected to be obtained by “conventional” methods such as transmittance or reflectance measurements, or by “unconventional” methods such as reflectance at oblique incidence, diffuse reflectance, photoacoustic spectroscopy and attenuated total reflectance. A variety of simulated, typical spectra are plotted as a small “atlas”. Conditions are discussed that allow a straightforward procedure for interpreting the spectra quantitatively, i.e., the evaluation of the resonance frequency and the concentration of the oscillators under consideration.It is shown that for systems characterized by weak, narrow oscillator lines, mostly an intuitive interpretation is possible, looking only at the position and strength of “lines” in the spectra. Materials showing strong polar vibrations, however, require more sophisticated procedures for interpreting the spectra.     

Quantitative analysis of functional groups in HDPE powder by DRIFT spectroscopy

FT-IR spectra of ethylene homopolymers and ethylene/1-hexene copolymers polymerized under different conditions were studied by transmission and diffuse reflection (DRIFT) spectroscopy. The absorbance spectra of film samples were compared with the DRIFT spectra of powders ground from the films. For determining the concentration of the methyl and unsaturated (vinyl, vinylidene and trans-vinylene) groups of polyethylene powders the DRIFT spectra were calibrated by comparing the IR intensities of the corresponding bands measured by the two methods. The results proved that the effect of differences in scattering of the polymer powder originating from the irregularity of the top surface, as well as the size and shape of the particles can be eliminated by the use of a proper internal standard. Linear correlation was established between the logarithms of the normalized intensities measured in absorbance and Kubelka-Munk units. In the case of polyethylene the selection of the internal reference band affects significantly the accuracy of the calibration due to the difference in the refractive indices of the crystalline and amorphous phases.     

Phase behavior of the orange essential oil/sodium bis(2-ethylhexyl)sulfosuccinate/water system

The ternary phase diagram for the orange essential oil (OEO)/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water system was constructed at 25 °C. It indicates a large single phase region, comprising an isotropic water-in-oil (W/O) microemulsion (ME) phase (L₂), a liquid crystal (LC) (lamellar or hexagonal) and a large unstable emulsion phase that separates in two phases of normal and reverse micelles (L₁ and L₂). In this communication the properties of the ME are investigated by viscosity, electric conductivity and small angle X-ray scattering (SAXS) indicating that the isotropic ME phase exhibits different behaviors depending on composition. At low water content low viscous “dry” surfactant structures are formed, whereas at higher water content higher viscous water droplets are formed. The experimental data allow the determination of the transition from “dry” to the water droplet structures within the L₂ phase. SAXS analyses have also been performed for selected LC samples.     

Organic anions : VIII. A simple electrostatic model for benzyl- and related alkyl-lithiums

Often the structures of 1:1 organolithium “contact ion pairs”, in which the organic portion consists of a delocalised π system, have been interpreted in terms of FMO theory or in terms of Möbius/Hückel aromaticity. We have now shown that some of these structures including those of benzyl-, allyl-, indenyl- and trityl-lithium can be explained on the basis of a simple electrostatic model.     

Measurement of surface orientation in uniaxial poly(ethylene terephthalate) films using polarised specular reflectance Fourier transform infrared microscopy

Polarised Fourier transform infrared (FT-IR) external-reflection spectroscopy has been used to measure molecular orientation at the surface of uniaxially drawn poly(ethylene terephthalate) (PET) films as a function of draw ratio. The spectra, which were obtained using an FT-IR microscope operating in the reflectance mode, were of high signal-to-noise (S/N) ratio, and contained only the specular component of the reflected radiation. The dichroic ratio of the 1019 cm⁻¹ ring stretching band was used to measure the PET orientation function, P₂₀₀. Prior to quantifying band intensities, the reflectance spectra were either transformed using the Kramers-Kronig algorithm, or simply differentiated; it was found that the dichroic ratios obtained using either pretreatment were similar. It was shown that the P₂₀₀ values obtained using the FT-IR method compared well with those obtained using surface refractive index measurements, provided that the intensity of the 1019 cm⁻¹ band was normalised relative to a non-dichroic band in the spectrum prior to computing dichroic ratios. The use of a reflecting microscope to perform the analysis of surface orientation not only simplifies sample alignment compared with “macro” reflectance accessories, but also allows awkward-shaped or small samples to be examined with relative ease. Unfortunately, a significant current limitation is that samples must be sufficiently “optically thick” to prevent double-pass (“transflectance”) radiation reaching the detector and yielding non-specular features in the reflectance spectrum. For polyaromatics, this probably means thicknesses in excess of 50 μm to obtain good spectra in the fingerprint region. Polarised attenuated total reflectance microscopy may present an alternate approach for examining thinner films.     

Investigation of the interactions between ginsenosides and amino acids by mass spectrometry and theoretical chemistry

In order to evaluate the essence of the interactions of ginsenosides and proteins which are composed by α-amino acids, electrospray ionization mass spectrometry was employed to study the noncovalent interactions between ginsenosides (Rb₂, Rb₃, Re, Rg₁ and Rh₁) and 18 kinds of α-amino acids (Asp, Glu, Asn, Phe, Gln, Thr, Ser, Met, Trp, Val, Gly, Ile, Ala, Leu, Pro, His, Lys and Arg). The 1:1 and 2:1 noncovalent complexes of ginsenosides and amino acids were observed in the mass spectra. The dissociation constants for the noncovalent complexes were directly calculated based on peak intensities of ginsenosides and the noncovalent complexes in the mass spectra. Based on the dissociation constants, it can be concluded that the acidic and the basic amino acids, Asp, Glu, Lys and Arg, bound to ginsenosides more strongly than other amino acids. The experimental results were verified by theoretical calculations of parameters of noncovalent interaction between ginsenoside Re and Arg which served as a representative example. Two kinds of binding forms, “head–tail” (“H–T”) and “head–head” (“H–H”), were proposed to explain the interaction between ginsenosides and amino acids. And the interaction in “H–T” form was stronger than that in “H–H” form.     

Photoresist Modification of Sol–Gel Solutions for Texturing of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> and Bi<Subscript>3</Subscript>TiNbO<Subscript>9</Subscript> Thin Films

Bismuth titanate (Bi₄Ti₃O₁₂) and bismuth titanium niobate (Bi₃TiNbO₉) c-axis textured thin films were fabricated using the sol–gel processing technique. Chemical modification of precursor solutions was performed using a proprietary photosensitive chemical (photoresist). Increases in crystallinity and texture of resulting films were seen over films that were made from unmodified solutions. Analysis by X-ray diffraction (XRD) revealed c-axis orientation factors for Bi₄Ti₃O₁₂ near 88% and Bi₃TiNbO₉ near 63% in “modified” films. Atomic force microscopy images of films show microstructural improvement between “modified” and “unmodified” films. Images generally show smaller randomly oriented grains in “unmodified” films, and larger oriented platelet structures related to growth due to crystal habit in “modified” films. Light scattering experiments show the addition of photoresist to the precursor solution initiates accelerated particle growth. AFM imaging of soft-baked films also suggests an enhancement of texture.     

Quantitative assessment of organic compounds adsorbed on silica gel by FTIR and UV–VIS spectroscopies: the contribution of diffuse reflectance spectroscopy

Different spectroscopic methods are compared to quantitatively determine organic compounds adsorbed or grafted on silica. The studied example is a derivative 1 of a well known photosensitizer, benzophenone. Transmission FTIR, diffuse reflectance FTIR (DRIFT) and UV (DRUV) spectroscopy are used to measure the concentration of adsorbed 1 on silica. The diffuse reflectance spectra are treated according to the Kubelka–Munk theory. It is shown that for such a compound absorbing in the UV range, DRUV spectra actually display a linear correlation between the remission function F(R) and the concentration of adsorbed 1. As this method neither implies any sample preparation nor any problems arising from the supporting silica, it proves to be fast, efficient and with a low detection threshold. For the IR spectra, it is necessary to numerically substrate the spectrum of silica which absorbs the light in this range. Provided that some care is paid to this numerical treatment which takes the sample heterogeneity into account, transmission and DRIFT spectra also display a linear correlation between the absorbance or the F(R) function and the concentration of adsorbed 1. However, sample preparation is much easier for DRIFT spectra as no pellets are made. Moreover, the DRIFT spectra of ground samples appear easier to process than the transmission spectra because of a better baseline and resolution. Although less resolved under 1500 cm⁻¹, the DRIFT spectra of non-ground samples can also be processed and give satisfactorily and rapid results without any possible perturbation of the structural integrity of the sample.     

Optical potential theory of diatom-diatom scattering. II. Indistinguishable molecules

The theory of vibro-rotational energy exchange in “indistinguishable” diatom-diatom scattering is formulated in terms of effective potentials. Due the necessity of considering both “symmetric” and “antisymmetric” molecular two-particle states, the formalism has a characteristic (two-by-two matrix structure. A mathematical generalization of Schwinger's theory of sources allows a compact derivation of exact and approximate expressions for t “optical” and “transition” potentials of elastic and inelastic processes, respectively. Finally, considerations based on a partial wave analysi of the working equations, suggest that the present theory should be more readily amenable to numerical implementations than the close coupling approach     

Esca studies of polyurethanes: blood platelet activation in relation to surface composition

Segmented polyurethanes (SPU) were synthesized with polyethylene oxide (PEO), polypropylene oxide, or polytetramethylene oxide as the “soft segment,” from the respective polyether diols, of which molecular weight varied from 600 to 2000. The “hard segment” was created from ethylene diamine and tolylene diisocyanate or 4,4′-diphenylmethane diisocyanate. Platelet activation was assessed using columns packed with beads coated with each of the SPU by solutions from which the solvent was subsequently evaporated. Citrated whole human blood was passed through the columns and the platelet count in aliquots leaving the columns was compared with the platelet count in blood that had not contacted the column surface. By this method the fraction of platelets retained in the column averaged for several donors, ρ, was determined. In parallel experiments, SPU surfaces formed under identical conditions by evaporation of solvent were examined by X-ray electron spectroscopy for carbon, oxygen, and nitrogen content of the surface. The carbon C_1s spectra proved to be particularly useful, when analyzed for the components with peaks respectively at 286 eV (carbon not bonded to an ether oxygen) and at 288 eV (carbon bonded to an ether oxygen). The platelet retention index ρ was found to increase nearly linearly with the ratio of the 286-eV intensity to the 288-eV intensity, and extrapolated to nearly zero for zero value of the intensity ratio, which would correspond to amorphous PEO, suggesting that if a surface were only amorphous PEO it would be remarkably inactive toward platelets. In contrast, nitrogen spectra show no systematic relationship with ρ.     

Polymer multilayer film formation studied by in situ ellipsometry and electrochemistry

Polyelectrolyte multilayer films adsorbed on gold surfaces were studied by combined ellipsometric and electrochemical methods. Multilayers were composed of “synthetic” (poly(4-styrenesulfonic acid) ammonium salt (PSS) and poly(allylamine hydrochloride) (PAH) (PSS/PAH)) and “semi-natural” (carboxymethyl cellulose (CMC) and chitosan (CHI) (CMC/CHI)) polyelectrolytes. It was found that only PSS/PAH Layer-by-Layer (LbL) assembled structures result in dense surface confined films that limit permeability of small molecules, such as ferri-/ferrocyanide. The PSS/PAH assemblies can be envisaged as films with pinholes, through which small molecules diffuse. During the LbL deposition process of these films a number of pinholes quickly decay. A representative pinhole diameter was found to be approximately 20 μm, which determines the diffusion of small molecules through LbL films, and yet remains constant when the film consists of a few LbL assembled polyelectrolyte bilayers. CMC/CHI LbL assemblies at gold electrode surfaces give very low density films, which do not limit the diffusion of ferri-/ferrocyanide between the surface of the electrode and the solution.     

The role of a reference sample in the study of the titanium-containing silicas by ultraviolet-visible diffuse reflectance spectroscopy

On the example of the silica gel with grown titanium oxide structures, in which titanium has two coordination enviromments (anatase, rutile) we showed the effect of the choice of a reference (MgO, SiO₂) on the shape of the ultraviolet-visible diffuse reflectance spectra and the result of their mathematical processing (the Kubelka-Munk function, first derivative).     

Effect of nanobubbles on friction forces between hydrophobic surfaces in water

Micro- and nanoscale combined hierarchical polymer structures were fabricated by UV-assisted capillary force lithography. The method is based on the sequential application of engraved polymer molds with a UV-curable resin of polyurethane acrylate (PUA) followed by surface treatment with a trichloro(1H,1H,2H,2H-perfluorooctyl) silane in vapor phase. Two distinct wetting states were observed on these dual-roughness structures. One is “Cassie–Wenzel state” where a water droplet forms heterogeneous contact with microstructures and homogeneous contact with nanostructures. The other is “Cassie–Cassie state” where a droplet makes heterogeneous contact both with micro- and nanostructures. A simple thermodynamic model was developed to explain static contact angle, hysteresis, and wetting transition on dual-roughness structures.     

Interfacial Behavior of Benzoic Acid and Phenylphosphonic Acid on Nanocrystalline TiO2 Surfaces

The interfacial behavior of self‐assembled thin films of benzoic acid (BA) and phenylphosphonic acid (PPOA) anchored on TiO₂ surfaces was studied by using temperature‐dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. On the basis of the disappearance of the OH band from the infrared spectra at room temperature, BA and PPOA appear to adsorb onto TiO₂ surfaces through carboxylate and phosphonate groups, respectively. Above 420 °C, DRIFT spectra indicated that both BA and PPOA desorb from TiO₂ surfaces; however, dissimilar desorption behavior could be inferred for BA and PPOA from their temperature‐dependent spectral changes. The benzene ring modes of PPOA remained above 420 °C, whereas those of BA disappeared. Density functional theory calculations showed that the adsorption of BA and PPOA on TiO₂ surfaces corresponded to bidentate bridging geometry on TiO₂ surfaces, and the adsorption of PPOA is stronger than that of BA. The monodentate structures with energy differences of 4.9 and 9.1 kcal mol^?1 from the most stable bidentate structures of BA and PPOA, respectively, from the DFT calculations appeared to be possible, particularly at the high temperatures above 420 °C, as indicated by the intensified OH bands. The geometry of PPOA was also estimated to be more upright standing than that of BA on TiO₂ surfaces, which may lead a rather straight detachment from the TiO₂ surfaces based on the presence of in‐plane ring modes in the DRIFT spectra at the higher temperature.     

Variations in the chemical composition within pine (Pinus sylvestris) trunks determined by diffuse reflectance infrared spectroscopy and chemometrics

The feasibility of using DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy combined with a multivariate analysis method (a PLS (projection to latent structures), regression) for predicting the distribution of the main organic constituents (cellulose, glucomannan, xylan, lignin, and extractives) within the Scots pine (Pinus sylvestris) stemwood was examined. PLS calibrations were carried out to establish a mathematical correlation between the data sets of conventional (“wet-chemistry-based”) wood analysis and the DRIFT spectra of the corresponding wood samples. Based on this approach, different surface maps on variations in the content of the main organic constituents within the stemwood matrix were shown.     

Prebiotic chemistry: A fuzzy field

If prebiotic chemistry is defined as the study of the chemical steps, which lead to the first organisms, a clear-cut definition of “living organism” is needed. Unfortunately, no unambiguous and universally accepted definition exists for the concept “living”. Under these conditions, fuzzy logic is probably the methodological tool that can best be used to handle questions pertaining to “the origin of life”. A conventional scale must, however, be defined which interestingly enough, depends necessarily on our present-day scientific knowledge.     

Diffuse reflectance spectroscopy—Important developments

Effect of front surface reflectance is incorporated into standard Kubelka-Munk theory of diffuse reflection. A method of obtaining the absorption coefficient independent of scattering coefficient is presented.     

An infrared spectroscopic study of hydrogen bonding in ethyl phenol: A model system for polymer phenolics

A detailed analysis of the bands appearing in the OH stretching region of the infrared spectrum of ethyl phenol solutions is presented. In cyclohexane solutions, the band due to “free” (non-hydrogen-bonded groups) contains overlapping contributions from both monomeric and end-group species. Other assignments are made on the basis of whether the proton and oxygen in a particular OH group are both involved in hydrogen bonds (as “donors” and “acceptors”, respectively), or if only the proton is acting as a donor. The strongest band in the spectra obtained at the highest concentration of ethyl phenol is due to OH groups present in linear chains of hydrogen-bonded OH groups (as recognized in numerous other studies), but a band due to cyclic trimers has also been identified. The assignment of other modes is more uncertain and various possibilities are discussed. In toluene solutions, assignments are more complicated, because bands due to OH–π hydrogen bonds are observed instead of free groups. Finally, the data from cyclohexane solutions was used to calculate equilibrium constants capable of describing the distribution of species present. A new methodology for determining the equilibrium constant describing association in the form of dimers is described.     

Recent developments and applications of NSE in soft matter

Neutron Spin Echo is probably the youngest spectroscopic method in neutron scattering. After a brief introduction I will try to give a short account of recent progress on the instrumentation. Through some recent examples I will illustrate that NSE is a powerful tool in colloid science applied to both “old” and recent problems. With new instrumentation we will be able to tackle new problems and one of those areas in which there has been recently exciting progress is the field of surface science.