Rheology of asphaltene-toluene/water interfaces

The stability of water-in-crude oil emulsions is frequently attributed to a rigid asphaltene film at the water/oil interface. The rheological properties of these films and their relationship to emulsion stability are ill defined. In this study, the interfacial tension, elastic modulus, and viscous modulus were measured using a drop shape analyzer for model oils consisting of asphaltenes dissolved in toluene for concentrations varying from 0.002 to 20 kg/m(3). The effects of oscillation frequency, asphaltene concentration, and interface aging time were examined. The films exhibited viscoelastic behavior. The total modulus increased as the interface aged at all asphaltene concentrations. An attempt was made to model the rheology for the full range of asphaltene concentration. The instantaneous elasticity was modeled with a surface equation of state (SEOS), and the elastic and viscous moduli, with the Lucassen-van den Tempel (LVDT) model. It was found that only the early-time data could be modeled using the SEOS-LVDT approach; that is, the instantaneous, elastic, and viscous moduli of interfaces aged for at most 10 minutes. At longer interface aging times, the SEOS-LVDT approach was invalid, likely because of irreversible adsorption of asphaltenes on the interface and the formation of a network structure.     

Mammalian Alkaloids: O-methylation of (S)- and (R)-dideoxynorlaudanosoline-1-carboxylic acid by catechol O-methyltransferase and identification of a yellow pigment obtained at physiological pH

O-Methylation of optically active 3′,4′-dideoxynorlaudanosoline-l-carboxylic acids 1 with O-methyltransferase in vitro afforded almost exclusively the 7-O-methylated acids 3 . A similar result was obtained with the yellow quinonemethide 4A obtained from 1 at neutral or slightly alkaline pH by oxidative decarboxylation and affording the 3,4-dihydroisoquinoline 15 on methylation with catechol O-methyltransferase (COMT). The structure of quinonemethide 4A was determined on the basis of spectral data, by its conversion into isoquinolines of established structure, and by synthesis. Quinonemethide 4A was found to be a weak inhibitor of monoamine oxidase A (MAO A) but not a substrate. Nonenzymatic oxidative decarboxylation of dopamine-derived tetrahydroisoquinoline-l-carboxylic acids to quinonemethides may be a major factor in biochemical experimentation and should be considered in the interpretation of data.     

Ab initio study of the mechanism of singlet-dioxygen addition to hydroxyaromatic compounds: negative evidence for the involvement of peroxa and endoperoxide intermediates

In this article we report our study of two possible mechanisms of photooxidation of hydroxyaromatic compounds, involving the intermediacy of zwitterionic peroxa intermediates or 1,4-endoperoxides. To study the pathway of the first of them, as yet unexplored by theoretical methods, a simpler system composed of 1,3-butadiene-1-ol and singlet ((1)Delta(g)) dioxygen was considered first, for which calculations were carried out at the CASSCF/MCQDPT2 ab initio level, mostly with the 6-31G* basis set. The cumulative activation barrier to this reaction was found to be 20 kcal/mol and corresponded to a proton transfer (from the hydroxy oxygen atom to the attached oxygen molecule) in the cyclic zwitterionic peroxacyclopenta-3-ene-2-ol intermediate. This intermediate and the proton-transfer transition state were found to have a closed-shell character, which enabled us to estimate the corresponding activation barrier for the phenol-dioxygen system by carrying out optimization at the RHF level and single-point calculations at the MP2, CASSCF, and MCQDPT2 levels of theory. The energy barrier to the reaction was estimated to at least about 40 kcal/mol, rendering this mechanism for the phenol-oxygen system unlikely for nonpolar solvents. Similarly, calculations of the barrier to proton transfer from the 1,4-endoperoxide of phenol to its hydroperoxide were found to exceed 60 kcal/mol, eliminating such a mechanism too, which leaves only the earlier postulated mechanisms involving an initial charge or hydrogen-atom transfer to dioxygen as probable.     

Separation of some chiral flavanones by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC) was applied for enantioseparation of selected flavanones, including naringin, hesperidin, neohesperidin, naringenin, hesperetin, pinostrobin, isosakuranetin, eriodictyol, and homoeriodictyol. gamma-Cyclodextrin (gamma-CD) and sodium cholate (SCh) were used as chiral modifiers inducing enantioselectivity to the background electrolyte. From among many investigated selectors only these two appeared to possess the best enantioselective properties in respect to studied flavanones. The mechanisms of their action are a little different; SCh used above critical micelle point concentration forms chiral micelles itself while gamma-CD is deprived of this property and requires addition of surfactants as, e.g., sodium dodecyl sulfate. It was found that SCh enables separation of flavanone glycosides diastereomers while separation of enantiomers of flavanone aglycones may be achieved with gamma-CD. Consideration of structural relation led to the suggestion that interaction of sugar moiety of glycosides with SCh micelles give rise to chiral recognition. MEKC appeared to be a suitable and efficient analytical tool to follow enantiomeric composition of flavanones.     

Adenine ribbon with Watson-crick and Hoogsteen motifs as the "double-sided adhesive tape" in the supramolecular structure of adenine and metal carboxylate

The cocrystals of adenine and metal (II) quinoline-2-carboxylates (M = Mn2+, Fe2+, Co2+) have been obtained by self-assembly. The complexes are composed of adenine ribbons with the AA22 pairing pattern involving both Watson-Crick and Hoogsteen faces in hydrogen bonding and the neutral molecules of carboxylate positioned in inorganic layers. The very compact supramolecular structure is made by the extensive system of hydrogen bonds and face-to-face pi-pi interactions.     

The extraction of Yb(III) and Lu(III) with chloroform from aqueous solutions in the presence of cupferron

The equilibrium of distribution of Yb(III) and Lu(III) between chloroform and the aqueous phase in the presence of cupferron (the ammonium salt of N-nitrosophenylhydroxylamine) were studied as apH function of the aqueous phase and the concentration of N-nitrosophenylhydroxylamine (HL). The stability constants for theLnL _n ^3–n ) complexes (n=1÷3) being formed in the aqueous phase were established, as well as the equilibrium constants of the extraction reaction $$Ln(H_2 O)_m^{3 + } + 3HL_{(O)} \mathop \rightleftharpoons \limits^{K_{ex} } LnL_{3(O)} + 3H^ + + mH_2 O(Ln^{3 + } = Yb,Lu),$$ two-phase stability constants for theLnL ₃ complexes,pH _0.5 and the separation factor Lu(III) from Yb(III).     

Chiral building blocks: enantioselective syntheses of benzyloxymethyl phenyl propionic acids

The synthesis of (2S)-2-benzyloxymethyl-3-(2-fluoro-4-methoxyphenyl)- propionic acid, (2S)-2-benzyloxymethyl-3-(2-fluoro-4-methylphenyl)propionic acid and (2S)-2-benzyl-oxymethyl-3-(2,4-dimethylphenyl)propionic acid has been achieved by TiCl4 mediated alkylation of the corresponding (4R)-4-benzyl-3-[3-(2-fluoro-4-methoxyphenyl-, 2-fluoro-4-methylphenyl-, 2,4- dimethylphenyl-)propionyl]-2-oxazolidinones, followed by hydrolysis of the chiral auxiliary. The stereochemistry of the alkylation reaction was confirmed by an X-ray crystal structure of (4R)-4-benzyl-3-[(2S)-2-benzyloxymethyl-3-(2- fluoro-4-methylphenyl)propionyl]-2-oxazolidinone.     

PHOTO ACOUSTIC SPECTRA OF PHYCOBILIPROTEINS and CHLOROPHYLLIN IN ISOTROPIC and ANISOTROPIC POLYVINYL ALCOHOL FILMS

Abstract— Photoacoustic spectra (PAS) of biliproteins: R-phycoerythrin (PE) with R-phycocyanin (PC) admixture in isotropic and anisotropic polyvinyl-alcohol (PVA) films were measured. The mixtures of biliproteins and chlorophyllin (Chllin) in PVA were also investigated. In order to establish the influence of PVA anisotropy on PAS amplitude, the stretched and unstretched PVA films colored by Congo red were studied. The films with PE and Chllin mixture when illuminated in the region of PE absorption, show higher PAS signals than the films with PE alone. This is explained by the excitation energy transfer from efficiently fluorescent PE to Chllin that has much lower yield of fluorescence. The stretched films always show a higher PAS signal than isotropic films because of the increase in the thermal conductivity of PVA. But normalized PAS of Congo red and Chllin in stretched PVA are similar to those in unstretched films.
Pigment molecules oriented parallel to the direction of film stretching have a lower rate of thermal deactivation compared to that of unoriented molecules as has been found by comparing PAS of differently oriented Congo red molecules using photoselection by polarized light. The same result was obtained for various PE chromophores which are differently oriented in anisotropic films.     

Complex formation studies on Ho(III) and Lu(III) with 1-(2-pyridylazo)-2-naphthol (PAN) in alcohol-water solutions

The formation ofPAN complexes in the systemsLn(III)—PAN—alcohol-water (where:Ln(III)=Ho, Lu and alcohol=ethanol,n-propanol,iso-propanol) was investigated by a spectrophotometric method. Equilibrium constants for the reactionLn ³⁺ + HLLnL ²⁺ + H⁺ (HL=PAN) and stability constants of complexesLnL ²⁺ were calculated.

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Untersuchungen zur Komplexbildung von Ho(III) und Lu(III) mit 1-(2-Pyridylazo)-2-naphthol (PAN) in alkoholisch-;wä\rigen Lösungen

Zusammenfassung Die Bildung der Komplexe vonPAN in den SystemenLn(III)—PAN—Alkohol-Wasser (Ln(III)=Ho, Lu; Alkohol=Ethanol,n-Propanol,iso-Propanol) wurde mit einer spektrophotometrischen Methode untersucht. Die Gleichgewichtskonstanten der ReaktionenLn ³⁺ + HLLnL ²⁺ + H⁺ (HL==PAN) und die Stabilitätskonstanten der KomplexeLnL ²⁺ wurden berechnet.

     

4,4′-Dipyridyl complexes of rare-earth thiocyanates

4,4-Dipyridyl complexes of rare-earth thiocyanates of the formulaLn(4-dipy)₂(NCS)₃·5H₂O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 4-dipy = 4,4-dipyridyl) have been synthesized. The IR spectra of these compounds and other physical properties are discussed. The thermal decomposition of some compounds (in the order Gd ÷ Lu) has been investigated.

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4,4-Dipyridylkomplexe von Seltenerdmetallthiocyanaten

Zusammenfassung Es wurden 4,4-Dipyridylkomplexe des TypesLn(4-dipy)₂(NCS)₃·5H₂O mitLn = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu und Y dargestellt. Die IR-Spektren und andere physikalische Eigenschaften werden diskutiert und die thermische Zersetzung von einigen Verbindungen (in der Reihe Gd ÷ Lu) untersucht.