Detection and identification of a methanol-water complex by factor analysis of infrared spectra

Fourier transform infrared absorption spectra, between 1850 and 2700 cm(-)(1), were used to study the intermolecular interactions between methanol and water. Digitized spectra obtained from methanol-water mixtures, with methanol mole fraction varying from 0 to 1.00, were subjected to factor analysis. Principal factor analysis indicated that three chemical factors were responsible for the data. Window factor analysis, a model-free method, was used to extract the concentration profiles of three species, which were identified as water, methanol, and a methanol-water complex. The profiles were used to deduce the stoichiometry of the complex, which was found to consist of one molecule of methanol and two molecules of water. The dissociation constant of the complex was determined to be 306 ± 33 M(2). These results differ from those reported by other investigators.     

Participation of Cluster Species in the Solvation Mechanism of a Weak Polar Solute in a Methanol/Water Mixture over a 0.2-0.7 Water Fraction Range: High-Performance Liquid Chromatography Study

It is known that a methanol/water mixture has a quaternary organization consisting of free water W, free methanol M, and two types of methanol/water clusters. A 1:1 methanol/water cluster has been observed at a high methanol concentration, and a 5:1 cluster has been observed at a low methanol concentration. In the water fraction range used in this paper, 0.2-0.7, the number of MW(5) clusters was always inferior to the number of MW clusters. When a weak polar solute is introduced into such a mixture, it is preferentially solvated by free methanol and the MW water clusters. Using this mixture as the mobile phase, a novel mathematical theory is presented to describe, in this water fraction range, the variations of the retention factor k' of alkyl benzoate esters and benzodiazepines in reversed-phase liquid chromatography. Excellent predictions of k' versus free methanol and methanol/water fractions were obtained. For the first time, using this model, enthalpy, entropy, and the Gibbs free energy of the two solute solvation processes were evaluated. Enthalpy-entropy compensation revealed that the main parameters determining retention in the range of the water fraction (0.2-0.7) increased as follows: free methanol ? solute solvation > methanol/water cluster ? solute solvation > RP18 stationary phase ? solute interaction. These results agree well with values obtained when ACN was used instead of methanol.     

Rank deficiency in spectrophotometric study of acid dissociation equilibria in mixed solvent media

A methanol/water mixture, although considered a binary system, does not conform to solute distribution theory. Moreover, similar shortcomings are encountered in studies concerning acid-base reactions in mixtures of these two components. We demonstrate that in mixtures of methanol/water and an acid-base dye as indicator, three components exist for each of the protonated and deprotonated forms. In contrast, analysis of spectrophotometric data obtained from pH-metric titration of the dye in mixtures of methanol/water, shows no more than one component for each form of the dye. This can only be interpreted as a rank deficiency in the acquired data. We used matrix augmentation strategy for eliminating rank deficiency and constructing the corresponding full rank data matrices. In addition, concentration and spectral profiles of the components were obtained using multivariate curve resolution-alternating least squares.     

Equilibrium modeling of mixtures of methanol and water

An understanding of the species that form in mixtures of alcohol and water is important for their use in liquid chromatography applications. In reverse-phase liquid chromatography the retention of solutes on a chromatography column is influenced by the composition of the mobile phase, and in the case of alcohol and water mobile phases, the amount of free alcohol and water present. Previous and similar modeling studies of methanol (MeOH) and water mixtures by near-infrared (NIR) spectroscopy have found up to four species present including free MeOH and water and MeOH and water complexes formed by hydrogen bonding associations. In this work an equilibrium model has been applied to NIR measurements of MeOH and water mixtures. A high-performance liquid chromatography (HPLC) pump was coupled to an NIR flow cell to produce a gradual change in mixture composition. This resulted in a greater mixture resolution than has been achieved previously by manual mixture preparation. It was determined that five species contributed to the data. An equilibria model consisting of MeOH, H2O, MeOH(H2O) (log K(MeOH)H2O = 0.10+/-0.03), MeOH(H2O)4 (log K(MeOH)4H2O = -2.14+/-0.08), and MeOH(H2O)9 (log K(MeOH)9H2O = -8.6+/-0.1) was successfully fitted to the data. The model supports the results of previous work and highlights the progressive formation of MeOH and water complexes that occur with changing mixture composition. The model also supports that mixtures of MeOH and water are not simple binary mixtures and that this is responsible for observed deviations from expected elution behavior.     

High-Precision and High-Resolution Measurements of Thermal Diffusivity and Infrared Emissivity of Water–Methanol Mixtures Using a Pyroelectric Thermal Wave Resonator Cavity: Frequency-Scan Approach

The thermal diffusivity and effective infrared emissivity of water–methanol mixtureswere measured at atmospheric pressure and ambient temperature using a pyroelectric thermal-wave resonator cavity. The applied frequency-scan method allows keeping the cavity length fixed, which eliminates instrumental errors and substantially improves the precision and accuracy of the measurements. A theoretical model describing conduction and radiation heat transfer in the cavity was developed. The model predictions and the frequency-scan experimental data were compared, showing excellent agreement. The measurements were performed for methanol volume fractions of 0, 0.5, 1, 2, 5, 10, 20, 40, 75, and 100%. The fitted thermal diffusivity and effective emissivity vs. concentration results of the mixtures were compared to literature theoretical and experimental data. The maximum resolution of 0.5% by volume of methanol in water by means of the thermal-wave cavity method is the highest reported to date using thermophysical techniques. Semi-empirical expressions for the mixture thermal diffusivity and infrared emissivity as functions of methanol concentration have been introduced. The expression for infrared emissivity is consistent with the physical principle of detailed balance (Kirchhoff’s law). The expression for thermal diffusivity was found to describe the data satisfactorily over the entire methanol volume-fraction range.     

Ion-pair chromatographic separation of water-soluble gold monolayer-protected clusters

We demonstrate the efficacy of ion-pair chromatography for separations of samples of charged, polydisperse, water-soluble gold nanoparticles protected by monolayers of N-acetyl-l-cysteine and of tiopronin ligands. These nanoparticle mixtures have 1-2-nm-diameter Au core sizes as estimated from UV-visible spectra of the separated components. This size range encompasses the transition from bulk metal to molecular properties. The nanoparticle mixtures were resolved, the smallest nanoparticles eluting first, on an octadecylsilyl (C18) column using isocratic elution with a methanol/water mobile phase containing tetrabutylammonium fluoride (Bu4N+F-) and phosphate buffer. The column retention increases with Bu4N+F- concentration, lowered pH, and decreasing methanol volume fraction. The retention mechanism is dominated by ion-pairing in either the mobile phase or at the stationary/mobile-phase interface. Size exclusion effects, used in many previous nanoparticle separations, are insignificant.     

Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number

Critical heat flux (CHF) in nucleate pool boiling of binary mixtures was newly measured with a horizontal platinum wire, 0.5 mm in diameter, and heated by DC, over the full range of concentrations. Seven mixtures were selected with the intent to cover various types of mixtures: methanol/water, ethanol/water, methanol/ethanol, ethanol/n-butanol, methanol/benzene, benzene/n-heptane and water/ethylene glycol, each in the saturated state at atmospheric pressure. Total 311 raw CHF data were obtained at 75 concentrations including pure components.Aqueous mixtures of methanol and ethanol revealed significant increase of CHF compared to either CHF linearly interpolated between pure components or CHF predicted from a single component correlation with use of the mixture properties. Three organic mixtures showed more or less the same level as an interpolated CHF, while the remaining two mixtures of methanol/benzene and water/ethylene glycol gave the reduced CHF by 20% and 50% at most, respectively. Marangoni number was introduced as a controlling variable to explain the observed increased, invariable, or reduced CHF, and an empirical correlation was developed.     

Synthesis of complex shape gold nanoparticles in water-methanol mixtures

Gold nanoparticles with shapes which varied from spheres to multipods and polygons were prepared with a seedless approach in water/methanol mixtures in the presence of polyvinyl alcohol using sodium ascorbate as the reducing agent. The shape of Au nanoparticles is critically affected by the water/methanol ratio, as well as by the ratio of hydrogen tetrachloroaurate (HAuCl4) to sodium ascorbate and the concentration of HAuCl4 in the reaction mixture. A decreased ratio of water to methanol below 30/70 leads to the formation of multi-branched nanoparticles with the size in the range of 30-70 nm at relatively low HAuCl4 concentration, whereas polygons are formed when HAuCl4 concentration increases. The polyvinyl alcohol stabilized multi-branched Au nanoparticle colloids were stable at room temperature for a period of at least six-month.     

Characterization of methanol-water and acetonitrile-water association using multivariate curve resolution methods

The solvatochromic comparison method has been used to probe the interactions of solutes with binary solvent mixtures of methanol-water and acetonitrile-water. The solute spectra recorded in these mixtures are composed of the additive spectral contributions of the different solvated species of the solute, i.e., the water-solvated species, the cosolvent-solvated species, and the species solvated by water-solvent complexes. Multivariate curve resolution-alternating least squares has been used to model the solvation of the solutes as a function of the composition of the binary solvent mixture. Spectra and concentration profiles of the dye surrounded by the different solvation environments have been isolated. For the first time, solute spectra solvated exclusively by methanol-water and acetonitrile-water complexes have been obtained, and the solvatochromic parameters of dipolarity/polarizability and hydrogen-bonding acidity have been estimated for these complex species.     

Studies of Ergotamine Tartrate-Caffeine Interactions by Differential Scanning Calorimetry

Differential scanning calorimetry was used to evaluate the interactions between ergotamine tartrate and caffeine. Physical mixtures of ergotamine tartrate and caffeine, as well as mixtures formed by evaporation of ergotamine tartrate and caffeine from a methanol solution, were evaluated. The preparation and characterization of eleven different molar ratios of ergotamine tartrate-caffeine complexes is described. The drug was found to interact with caffeine in the solid state with the formation of complexes. The mixture obtained from the methanol solution showed complexes with ergotamine tartrate-caffeine molar ratios of 2:3 and 2:15, with possible complexes having 1:10, 1:15 and 1:20 molar ratios. A phase diagram was constructed. Physical mixtures of the same molar ratios showed possible complex formation at molar ratios of 1:1, 1:1 and 1:3.     

藻朊酸盐膜对水／有机物混合液的PV分离

研究了藻朊酸、藻朊酸钠及二价金属离子交联的藻朊酸钠对水／乙醇体系的渗透汽化性能，同时，还研究了藻朊酸盐膜对水／甲醇、水／异丙醇、水／丙三醇、水／丙酮体系的分离性能，表明，藻朊酸盐膜上述几种体系均具有良好的分离性能。     

Near-infrared spectroscopy for cocrystal screening. A comparative study with Raman spectroscopy

Near-infrared (NIR) spectroscopy is a well-established technique for solid-state analysis, providing fast, noninvasive measurements. The use of NIR spectroscopy for polymorph screening and the associated advantages have recently been demonstrated. The objective of this work was to evaluate the analytical potential of NIR spectroscopy for cocrystal screening using Raman spectroscopy as a comparative method. Indomethacin was used as the parent molecule, while saccharin and l-aspartic acid were chosen as guest molecules. Molar ratios of 1:1 for each system were subjected to two types of preparative methods. In the case of saccharin, liquid-assisted cogrinding as well as cocrystallization from solution resulted in a stable 1:1 cocrystalline phase termed IND-SAC cocrystal. For l-aspartic acid, the solution-based method resulted in a polymorphic transition of indomethacin into the metastable alpha form retained in a physical mixture with the guest molecule, while liquid-assisted cogrinding did not induce any changes in the crystal lattice. The good chemical peak selectivity of Raman spectroscopy allowed a straightforward interpretation of sample data by analyzing peak positions and comparing to those of pure references. In addition, Raman spectroscopy provided additional information on the crystal structure of the IND-SAC cocrystal. The broad spectral line shapes of NIR spectra make visual interpretation of the spectra difficult, and consequently, multivariate modeling by principal component analysis (PCA) was applied. Successful use of NIR/PCA was possible only through the inclusion of a set of reference mixtures of parent and guest molecules representing possible solid-state outcomes from the cocrystal screening. The practical hurdle related to the need for reference mixtures seems to restrict the applicability of NIR spectroscopy in cocrystal screening.     

Retention of ionizable compounds in reversed-phase liquid chromatography. Effect of the ionic strength of the mobile phase and the nature of the salts used on the overloading behavior

The retention mechanism of the protonated cation in propranolol chloride on C18-Xterra was investigated using mobile phases of various compositions. Accurate adsorption data were measured by frontal analysis, with a mixture of methanol and water (25% methanol), with no salt, as the mobile phase. The experimental isotherm has at least two inflection points, at concentrations of about 0.2 and 6.0 g/L, respectively. This precludes the modeling of these data with a simple convex-upward isotherm (e.g., Langmuir). The adsorption energy distribution or relationship between the number of sites on the adsorbent surface and the energy of adsorption on these sites was calculated by assuming Moreau isotherm behavior (S-shaped isotherm). This model has never been applied to describe the surface heterogeneity of any RPLC adsorbent. The calculation converged toward a bimodal energy distribution. Accordingly, the bi-Moreau model is the simplest theoretical model accounting for the adsorption data of propranolol from a mobile phase without salt. The complex-overloaded band profiles of propranolol measured in the presence of increasing concentrations of a supporting salt (KCl) in the mobile phase demonstrate that the same isotherm model applies also under these conditions, as was merely assumed in a previous work. The elution band profiles of propranolol calculated with the bi-Moreau isotherm model for solutions of salts of different natures (CaCl2, CsCl, Na2SO4) in the same mobile phase agree very well with the experimental band profiles.     

Spectroscopic characterization of the oligomeric surface structures on polyamide materials formed during accelerated aging

Crystalline surface species were observed at the surface of polyamide 12 materials upon accelerated aging. After detection of the depositions with scanning electron microscopy (SEM), the crystalline surface precipitations were analyzed with Fourier transform infrared (FT-IR) and Raman imaging microscopy. These surface species were supposed to be cyclic oligomers (dimer and trimer) of the PA12 monomer laurolactam, which are usually present in polyamide materials and tend to migrate to the surface when the material is subjected to accelerated aging. The evidence for the chemical identity of the crystalline surface structures to be mainly the cyclic dimer and trimer of laurolactam was given by melting-point identification and mass spectroscopic analysis of the methanol eluate of the surface. The Raman and FT-IR spectra of the mixture were extracted from the recorded images.     

Stability-indicating spectrophotometric and densitometric methods for determination of aceclofenac

Three methods were developed for the determination of aceclofenac in the presence of its degradation product, diclofenac. In the first method, third-derivative spectrophotometry (D₃) is used. The D₃ absorbance is measured at 283 nm where its hydrolytic degradation product diclofenac does not interfere. The suggested method shows a linear relationship in the range of 4-24 µg mL^-1 with mean percentage accuracy of 100.05±0.88. This method determines the intact drug in the presence of up to 70% degradation product with mean percentage recovery of 100.42±0.94. The second method depends on ratio-spectra first-derivative (RSD₁) spectrophotometry at 252 nm for aceclofenac and at 248 nm for determination of degradation product over concentration ranges of 4-32 µg mL^-1 for both aceclofenac and diclofenac with mean percentage accuracy of 99.81±0.84 and 100.19±0.72 for pure drugs and 100.17±0.94 and 99.73±0.74 for laboratory-prepared mixtures, respectively. The third method depends on the quantitative evaluation of thin-layer chromatography of aceclofenac using chloroform:methanol: ammonia (48:11.5:0.5 v/v/v) as a mobile phase. Chromatograms were scanned at 274 and 283 nm for aceclofenac and diclofenac, respectively. The method determined aceclofenac and diclofenac in concentration ranges of 2-10 and 1-9 µg spot^-1 with mean percentage accuracy of 100.20±1.03 and 100.14±0.98 for pure drugs and 99.77±0.74 and 100.07±0.78 for laboratory-prepared mixtures, respectively. This method retains its accuracy in the presence of up to 80% degradation product for the studied drug.

The suggested procedures were checked using laboratory-prepared mixtures and were successfully applied for the analysis of their pharmaceutical preparation. The validity of the proposed methods was further assessed by applying a standard addition technique. The obtained results agreed statistically with those obtained by the reported method.     

High-concentration band profiles and system peaks for a ternary solute system

High-concentration HPLC band profiles of single solutes and the individual band profiles of the components of binary and ternary mixtures are reported for benzyl alcohol, 2-phenylethanol, and 2-methylbenzyl alcohol. These solutes were eluted from a C18 column by a binary mobile phase (MeOH:H2O = 1:1, v/v). High-concentration system peaks were obtained using mixtures of benzyl alcohol and 2-phenylethanol at different relative concentrations as the feed and 2-methylbenzyl alcohol as the strong mobile phase additive. Band profiles and system peak profiles were calculated using the equilibrium-dispersive model of chromatography. The adsorption equilibrium in the multicomponent system was characterized by the competitive Langmuir model. Excellent quantitative agreement was found between the experimental and the calculated profiles. This work confirms that extremely unusual system peak profiles can be obtained even when the adsorption behavior is quite simple. Under certain circumstances, the use of a properly chosen additive could markedly increase the separation between bands and hence the production rate, the recovery yield, and/or the purity of the fractions.     

a-C:N膜的制备及结构分析

采用射频溅射法在锗衬底上沉积无定形碳氮膜 (a- C:N) ,用 X射线光电子谱 (XPS)、傅立叶变换红外光谱 (FT- IR)和喇曼光谱 (RS)分析了膜的成分与结构。结果表明 ,氮是以化学键的形式存在于膜中 ,且有三种不同的 C- N键合状态。随着反应气体中氮气分压的增加 ,a- C:N膜中氮浓度增加 ,C≡N键的含量增加。     

Studies of Ergotamine Tartrate-Caffeine Interactions by Differential Scanning Calorimetry

Abstract

Differential scanning calorimetry was used to evaluate the interactions between ergotamine tartrate and caffeine. Physical mixtures of ergotamine tartrate and caffeine, as well as mixtures formed by evaporation of ergotamine tartrate and caffeine from a methanol solution, were evaluated. The preparation and characterization of eleven different molar ratios of ergotamine tartrate-caffeine complexes is described. The drug was found to interact with caffeine in the solid state with the formation of complexes. The mixture obtained from the methanol solution showed complexes with ergotamine tartrate-caffeine molar ratios of 2:3 and 2:15, with possible complexes having 1:10, 1:15 and 1:20 molar ratios. A phase diagram was constructed. Physical mixtures of the same molar ratios showed possible complex formation at molar ratios of 1:1, 1:1 and 1:3.     

Effect of acetonitrile/water mobile-phase composition on adsorption characteristics of reversed-phase liquid chromatography

The influence of volumetric composition of acetonitrile in a mobile phase on adsorption characteristics of reversed-phase liquid chromatography using octadecylsilyl (ODS)-modified silica gel was studied by the pulse response method and the moment analysis. The results were compared with those obtained for the reversed-phase system consisting of ODS-silica gel and methanol/water mixtures. In both systems, surface diffusion was dominant for intraparticle diffusion in ODS-silica gel particles. The contributions of three mass transfer steps in a column to peak broadening were of about the same order of magnitude. The activation energy of surface diffusion, E(s), was found to be larger than the isosteric heat of adsorption, Q(st). Because similar tendencies were observed for these adsorption characteristics, adsorption mechanisms may be analogous in both chromatographic systems. However, absolute values of the adsorption equilibrium constant, K, the decreasing ratio of hydrophobic surface area, ΔA/A, Q(st), and E(s) for acetonitrile/water systems were smaller than the corresponding values for methanol/water systems. Oppositely, greater values of D(s) were obtained for acetonitrile/water systems. It was concluded that the interaction between ODS ligands and adsorbate molecules was weaker compared with that in methanol/water mobile-phase systems when acetonitrile was used as an organic modifier in a mobile phase of reversed-phase liquid chromatography.