A numerical study of the effect of insoluble surfactants on the stability of a viscous drop translating in a Hele-Shaw cell

A circular drop is a linearly stable solution for the buoyancy-driven motion of drops in a Hele-Shaw cell [Gupta et al. J. Colloid Interface Sci.218(1), 338 (1999)]. In the absence of surface-active agents, an initially prolate drop always goes to a steady circular shape while initially oblate drops exhibit complex dynamics [Gupta et al. J. Colloid Interface Sci.222, 107 (2000)]. In this study, the effect of insoluble surfactant impurities on the critical conditions for drop breakup is explored by using the Langmuir adsorption framework in conjunction with a physically based expression for the depth-averaged tangential stress exerted on a two-phase interface in a Hele-Shaw cell. It is shown that the presence of surfactants can have both a stabilizing and a destabilizing effect on the shape of the drop, depending on the Bond number, the magnitude of the initial perturbation, and the strength of surface convection. Similar to the clean drop dynamics, two marginally stable branches are found. Increasing the surface Peclet number results in the stabilization of the main branch while the secondary branch shifts to higher Bond numbers. The mode of breakup is also found to be strongly influenced by the strength of surface convection.     

Theoretical study of the substituent effect of hydroxy group on tandem Cope rearrangement and [2 + 2] cycloaddition in cis-1,2-diethynylcyclopropane and its mono-hetero analogues

The tandem Cope rearrangement and [2?+?2] cycloaddition of cis-1,2-diethynyl-1,2-dihydroxycyclopropane and its mono-hetero analogues have been investigated at the B3LYP/6-31+G* level. The presence of the hydroxy group lowers the activation enthalpies for the Cope rearrangement, whereas activation enthalpies for the [2?+?2] cycloaddition are raised as compared to those for their non-hydroxy derivatives. The NBO analysis indicates that in the transition structure involved in the Cope rearrangement, lone pairs of the oxygen atoms of the hydroxy groups are transferred into the σ* C–C bond undergoing migration, as a result of which it is weakened. On the other hand, the lone pairs of the oxygen atoms interact with the π* C=C orbitals of the bis-allenic systems in the intermediate thereby stabilizing it and, thus, suppressing its driving ability for the [2?+?2] cycloaddition. In the products so formed, 6π electrons are delocalized conferring stability on them, which is further augmented by extended conjugation with the hydroxy groups. Due to high stability of these products, activation barrier for the change of enol into ketone is very high.

     

Perceiving unstressed vowels in foreign-accented English

This paper investigated how foreign-accented stress cues affect on-line speech comprehension in British speakers of English. While unstressed English vowels are usually reduced to /?/, Dutch speakers of English only slightly centralize them. Speakers of both languages differentiate stress by suprasegmentals (duration and intensity). In a cross-modal priming experiment, English listeners heard sentences ending in monosyllabic prime fragments--produced by either an English or a Dutch speaker of English--and performed lexical decisions on visual targets. Primes were either stress-matching ("ab" excised from absurd), stress-mismatching ("ab" from absence), or unrelated ("pro" from profound) with respect to the target (e.g., ABSURD). Results showed a priming effect for stress-matching primes only when produced by the English speaker, suggesting that vowel quality is a more important cue to word stress than suprasegmental information. Furthermore, for visual targets with word-initial secondary stress that do not require vowel reduction (e.g., CAMPAIGN), resembling the Dutch way of realizing stress, there was a priming effect for both speakers. Hence, our data suggest that Dutch-accented English is not harder to understand in general, but it is in instances where the language-specific implementation of lexical stress differs across languages.     

A Review of the Synthesis and Utility of Some Lipopathic Permanganate Oxidants

Permanganate, the versatile oxidizing agent, has limited solubility in non-polar media, which limits its use for oxidation of organic substrate in the non-aqueous media. But the combination of PTCs with permanganate makes it mild, selective and lipopathic. These lipopathic oxidants are useful for many important organic transformations. In this communication, we have presented the synthesis and utility of some selected lipopathic permanganate oxidants, which oxidize many functional groups, including alcohol, aldehyde, ether, alkane, alkene, alkyne, and oxime and have use in many synthetic organic procedures with chemo-, regio- and stereoselectivity.     

The refined structure of ribonuclease-A at 1.45 Å resolution

Features of the refined X-ray crystal structure of bovine pancreatic ribonuclease-A at 1.45 Å resolution are described. The positions of the protein atoms have been determined within the range 0.004–0.05 Å, and of solvent atoms, assumed to be oxygens, within the range 0.08–0.13 Å. The present model contains 127 solvent molecules, taken to be water, and a sulfate anion located in the active site. Mean square atomic displacement parameters,U _iso, refined for each atom, give an indication of the mobility of different parts of the structure. Main-chainU _iso values tend to be less than side-chain values, having an average value of 0.15 Ų compared to 0.25 Ų. Both main-chain and side-chain averageU _iso values tend to increase with distance from the center of gravity of the molecule. Side-chain averageU _iso values also tend to increase with the number of atoms in the side-chain, with different distributions for ring and chain type residues. Side-chain conformations have been analyzed and found on the whole to follow commonly observed distributions. A notable exception to this is the active-site residue His-119 which occupies two distinct sites. Apart from two small clusters of eight and seven atoms respectively, the solvent molecules are distributed in quite small numbers on the protein surface. The solvent clusters occur in the active-site region and, together with the sulfate anion, appear to stabilize residues in this region. Sixty-three solvent atoms have only one identified hydrogen bond contact. Of the rest, 36 form two, 22 form three, and 6 form four hydrogen bonds. There is a marked tendency for the mean square displacement parameter,U _iso, for the solvent atoms to be lower for atoms with many hydrogen bond contacts than for those with fewer contacts.     

Rheological behavior of silver nanowire conductive inks during screen printing

The rheological behavior of silver nanowire (AgNW) suspensions adapted for screen printing inks was investigated. Aqueous silver nanowire inks consisting of AgNW (length of 30 μm, and diameter of 40 and 90 nm), dispersant and binder were formulated. The effect of AgNW content on the rheological behavior of the ink and the build-up of ink structure after screen printing were examined as they depend on applied shear and temperature. Rheological measurements under conditions that mimic the screen printing process were done to assess viscoelastic properties induced by flow alignment of the wires and the subsequent recovery of the low shear structure. The Stretched Exponential model (SEmo) was used to model the recovery process after screen printing to obtain the characteristic time of the recovery or build-up process. The characteristic time was determined at several temperatures to obtain the activation energy of recovery. The domination of Brownian motion or non-Brownian motion behavior can be characterized by a Peclet number, which is the ratio of shear rate to the rotational diffusion coefficient. The Peclet number and the dimensionless concentration of wires were used to assess the recovery mechanism. The steady viscosity at low and high shear rates was also treated by an activation energy analysis.     

Unveiling the synthesis of spirocyclic,tricyclic, and bicyclic triazolooxazines from intramolecular [3 + 2] azide-alkyne cycloadditions with a molecular electron density theory perspective

Structural Chemistry - The intramolecular [3+2] cycloaddition (32CA) reactions of azido alkynes leading to spirocyclic, tricyclic, and bicyclic triazolooxazines has been studied within the...     

Development and validation of a reversed-phase HPLC method for monitoring of synthetic reactions during the manufacture of a key intermediate of an anti-hypertensive drug

A reversed-phase high-performance liquid-chromatographic method for monitoring of reactions involved in process development of a key intermediate of antihypertensive drugs, e.g, doxazosin mesylate, prazosin, alfuzosin, terazosin, etc., has been developed and validated. The HPLC profiles of impurities of 4-amino-2-chloro-6,7-dimethoxyquinazoline were used as fingerprints to follow the synthetic procedures in the manufacturing unit. The separation was accomplished on an Inertsil ODS-3 column with isocratic elution using acetonitrile-ammonium acetate (10 mM; pH 4.0; 50:50 v/v) as mobile phase and a photodiode array detector set at 240 nm at ambient temperature. The method was validated with respect to accuracy, precision, linearity, and limits of detection and quantification. The method could detect the impurities at a level of 0.01 to 0.20 microg/mL and it was found to be suitable not only for monitoring of reactions but also for quality assurance of 4-amino-2-chloro-6,7-dimethoxyquinazoline.     

Unravelling the regio- and stereoselective synthesis of bicyclic N,O-nucleoside analogues within the molecular electron density theory perspective

Structural Chemistry - The [3?+?2] cycloaddition (32CA) reactions of 1-pyrroline-1-oxide with N-vinyl nucleobases leading to bicyclic N,O nucleoside analogues have been studied within...