Characterization of waxes used in pictorial artworks according to their relative amount of fatty acids and hydrocarbons by gas chromatography

A study attempted to characterize natural waxes used in pictorial works of art was carried out by means of gas chromatography. The analytical treatment requires prior hydrolysis of the waxes to release the fatty acids (FA) (myristic (myr), palmitic (pal), oleic (ole), stearic (ste), araquidic (ara), behenic (beh), lignoceric (lig), cerotic (cer)) from the main esters of the waxes. The formation of volatile derivatives of the fatty acids was carried out by derivatization with ethyl choroformate (ECF). This derivatization reagent was chosen due to the speed, safety and quantitativity of the reaction. The analyzed hydrocarbons were n-eicosane, n-heneicosane, n-docosane, n-tricosane, n-tetracosane, n-pentacosane, n-hexacosane, n-heptacosane, n-octacosane, n-nonacosane, n-tricontane n-hentriacontane, n-dotriacontane, n-tritriacontane, n-tetratriacontane, n-pentatriacontane, main constituents of the waxes. No derivatization is needed to analyze the hydrocarbons. Ethyl ester derivatives and hydrocarbons are adequately separated by gas chromatography, identified by flame ionization detection and confirmed by mass spectrometry. To characterize natural waxes, peak area ratios of each fatty acids with respect to the palmitic acid and peak area ratios of each hydrocarbons with respect to n-heptacosane were calculated. The proposed method provides a good characterization of different waxes most frequently used in artworks, such as beeswax, carnauba wax and ceresin, and has been successfully applied to real samples. This is the first report on the application of ECF to the analysis of fatty acids in wax.     

On the protonation of a macrobicyclic cage: an inert tribenzylamine fragment and three robust aminophosphonium units

The protonation of a macrobicyclic cage with four intracyclic nitrogen atoms (one included into a tribenzylamine fragment and the other three into iminophosphorane units) led to tris(aminophosphonium) cations. In the salts, the three anions are situated outside the internal cavity although intercalated within the grooves of its macrobicyclic skeleton. The three aminophosphonium units are remarkably resistant to hydrolysis.     

Comparison of oleyl and elaidyl isomer surfactant-counterion systems in drag reduction, rheological properties and nanostructure

Compared with quaternary ammonium cationic surfactants with saturated alkyl chains, quaternary ammonium cationic surfactants with one double-bond in their alkyl chains, when mixed with appropriate counterions (in certain molar concentration ratios, ξ), can reach much lower effective drag-reduction temperatures, while maintaining the upper drag-reduction temperature limit of the corresponding saturated drag reducing surfactant solutions. No previous study has compared the effects of cis- vs. trans-unsaturated alkyl hydrocarbon tail configurations (oleyl vs. elaidyl) trimethyl ammonium chloride cationic surfactants at different counterion/surfactant concentration ratios on micellar nanostructures, (1)H NMR spectra and on rheological and drag-reduction behavior of their solutions. Since neither pure oleyl (cis-) nor elaidyl (trans-) trimethyl ammonium chloride surfactants are commercially available, they were synthesized and their 5mM solutions with NaSal counterion at concentrations of 5mM, 7.5mM and 12.5mM were studied.     

New method for determination of ochratoxin A in beer using zinc acetate and solid-phase extraction silica cartridges

A new method for the determination of ochratoxin A (OTA) in beer has been developed. The new method has been compared with a reference method currently accepted as AOAC official first action. The limits of detection and quantification of the proposed method were 0.0008 and 0.0025 ng/ml, respectively, while they were 0.0025 and 0.0075 ng/ml, respectively, in the AOAC method used as reference. The recovery levels in the 0.025-0.40 ng OTA/ml spiking range for the proposed and the reference methods were 80.6-87.6% and 78.2-83.8%, respectively. The relative standard deviations of recoveries were 2.6-7.5% for the proposed method and 0.7-6.1% for the reference method. Passing and Bablok regression analysis of recovery data obtained by the proposed method versus data obtained by the reference method on an OTA-spiked beer sample showed good correlation (r2 = 0.9993), while the slope and intercept were 1.049 and -0.0013, respectively. The advantage of the proposed method is the low cost of the materials used in sample preparation because expensive immunoaffinity columns are not needed to clean-up samples while it maintains or even increases the good performance of the reference method. The proposed method was applied to 69 beer samples from different geographic origins (national and imported) but purchased in the Spanish market. They were found to be contaminated with OTA in the range from 0.008 to 0.498 ng/ml (average: 0.070 ng/ml). Five samples surpassed the limit recommended by the European Union (0.2 ng OTA/g).     

Photoinitiated polymerization of methacrylic monomers in a polystyrene matrix: Kinetic,mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6‐hexanediol dimethacrylate (HDDMA) and 2‐ethylhexyl methacrylate (EHMA)] in a polystyrene (PS) matrix were studied. The aggregation state, vitreous or rubbery, of the monomer/matrix system and the intermolecular strength of attraction in the monomer/matrix and growing macroradical/matrix systems are the principal factors influencing the kinetics and mechanism. For the PS/HDDMA system, where a relatively high intermolecular force of attraction between monomer and matrix and between growing macroradical and matrix occurs, a reaction‐diffusion mechanism takes place at low monomer concentrations (<30–40%) from the beginning of the polymerization. For the PS/EHMA system, which presents low intermolecular attraction between monomer and matrix and between growing macroradical and matrix, the reaction‐diffusion termination is not clear, and a combination of reaction‐diffusion and diffusion‐controlled mechanisms explains better the polymerization for monomer concentrations below 30–40%. For both systems, for which a change from a vitreous state to a rubbery state occurs when the monomer concentration changes from 10 to 20%, the intrinsic reactivity and k_p/k_t^1/2 ratio (where k_p is the propagation kinetic constant and k_t is the termination kinetic constant) increase as a result of a greater mobility of the monomer in the matrix (a greater k_p value). The PS matrix participates in the polymerization process through the formation of benzylic radical, which is bonded to some extent by radical–radical coupling with the growing methacrylic radica, producing grafting on the PS matrix. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2049–2057, 2001     

Role of quaternary ammonium salts as new additives in the enantioselective organocatalytic β-benzylation of enals

We report herein the efficiency of quaternary ammonium salts as co-catalysts in organocatalytic Michael reactions involving iminium activation of α,β-unsaturated aldehydes. The enantioselective formal benzylation of these substrates has been optimized and used to rationalize the role of the ammonium salts in these processes.     

Hydricity-promoted [1,5]-H shifts in acetalic ketenimines and carbodiimides

2-monosubstituted 1,3-dioxolanes and dithiolanes act as hydride-releasing fragments, transferring intramolecularly their acetalic H atom to the central carbon of ketenimine functions. The presumed products of these migrations, o-quinomethanimines, undergo in situ 6pi-electrocyclization. A computational study supports this mechanism and the hydride-shift character of the first step. Carbodiimides were also suitable substrates, although less reactive. [reaction: see text].     

(3h)J((15)N-(31)P) spin-spin coupling constants across N[bond]H....O[bond]P hydrogen bonds

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to evaluate three-bond (15)N-(31)P coupling constants ((3h)J(N[bond]P)) across N[bond]H....O[bond]P hydrogen bonds in model cationic and anionic complexes including NH(4)(+):OPH, NH(4)(+):OPH(3), NH(3):(-)O(2)PH(2), NFH(2):(-)O(2)PH(2), and NF(2)H:(-)O(2)PH(2). Three-bond coupling constants can be appreciable when the phosphorus is P(V), but are negligible with P(III). (3h)J(N[bond]P) values in complexes with cyclic or open structures are less than 1 Hz, a consequence of the nonlinear arrangement of N, H, O, and P atoms. For complexes with these structures, (3h)J(N[bond]P) may not be experimentally measurable. In contrast, complexes in which the N, H, O, and P atoms are collinear or nearly collinear have larger values of (3h)J(N[bond]P), even though the N[bond]P distances are longer than N[bond]P distances in cyclic and open structures. In linear complexes, (3h)J(N[bond]P) is dominated by the Fermi-contact term, which is distance dependent. Therefore, N[bond]P (and hydrogen-bonding N[bond]O) distances in these complexes can be determined from experimentally measured (15)N-(31)P coupling constants.     

Photochemical behavior of p-dimethylaminobenzoylated polystyrene as polymerization photoinitiator: Synthesis of graft copolymers

The photochemical behavior of p-dimethylaminobenzoylated polystyrene (PS-MI) in benzene solution has been investigated, both in the presence and absence of methyl methacrylate (MMA). This behavior has been compared with that of the model compound, 4-isopropyl-4′-N,N-dimethylaminobenzophenone (CU-MI). PS-MI photoreduction takes place only through excimer formation due to the high local chromophore concentration, and therefore, PS-MI disappearance quantum yield is close to the previously calculated limiting value (0.02) and independent of chromophore concentration. Several parameters that characterize the polymerization process have been determined; it has been found that the obtained PMMA is photografted onto PS-MI backbone. This is in agreement with the proposed mechanism for radical generation. No homo-PMMA formation has been detected. © 1993 John Wiley & Sons, Inc.     

Preparation and application of silicalite-1 micromembranes on laser-perforated stainless steel sheets

Silicalite-1 micromembranes of approximately 80 μm in diameter have been prepared by synthesizing the zeolite material inside a network of perforations created by using a Nd–YAG laser on 75 μm thick stainless steel sheets. Several strategies have been introduced here to reduce the thickness of the micromembranes synthesized; among these the most successful involved using a polyamide filling to reduce the volume of the microperforations accessible to the synthesis solution. The density of microperforations could be tailored to prepare arrays of micromembranes of different overall porosity (1.8, 3.7, 6.6 and 7.7%). The micromembranes performed well in the propane/N₂ separation, showing a maximum propane/N₂ separation factor of 46, at a propane permeance of about 5 × 10⁻⁶ mol/(m² s Pa).