Estimation of surface desorption times in hydrophobically coated nanochannels and their effect on shear-driven and pressure-driven chromatography

The present paper provides a detailed analysis of the analyte-wall adsorption effects in nanochannels, including a random walk study of the analyte-wall collision frequency, and uses these insights to estimate wall desorption times from chromatographic experiments in nanochannels. Using coumarin dye analytes and using a methanol/water mixture buffered at pH 3 in 120-nm deep channels, the surface desorption times on naked fused-silica glass were found to be maximally of the order of 60 to 150 μs, while they were found to be on the order of 100 to 500 μs on a hydrophobically coated wall. These nonzero adsorption and desorption times lead to an additional band broadening when conducting chromatographic separations. Shear-driven flows, requiring a noncoated moving wall and a stationary coated wall, intrinsically turn out to be more prone to this effect than pressure-driven or electro-driven flows for example. The present study also shows that, interestingly, the number of analyte-wall collisions increases with the inverse of the channel depth and not with its second power, as would be expected from the Einstein–Smoluchowski relationship for molecular diffusion.     

Effects of counterion size on the attraction between similarly charged surfaces

Interaction between similarly charged surfaces can be attractive at high electrostatic coupling constants Ξ = l(B)Z(2)/μ(GC), where l(B) is the Bjerrum length, μ(GC) the Gouy-Chapman length, and Z the valency of counterions. While this effect has been studied previously in detail, as a function of surface charge density and valency of the pointlike counterions, much less is known about the effect of counterion size. We apply the Wang-Landau sampling Monte Carlo (MC) simulation method to compute the free energy F as a function of the scaled distance between the plates D?=D/μ(GC) for a range of Ξ and scaled counterion radii R?=R/μ(GC). We find that for large Ξ and small ion radius, there is a global equilibrium distance D?=D?(eq)=2(1+R?), correctly giving the expected value at the point counterion limit. With increasing R? the global minimum in F(D?) changes to a metastable state and finally this minimum vanishes when R? reaches a critical value, which depends on Ξ. We present a state diagram indicating approximate boundaries between these three regimes. The Wang-Landau MC method, as it is applied here, offers a possibility to study a wide spectrum of extended problems, which cannot be treated by the use of contact value theorem.     

Solving inverse eigenvalue problems by a projected newton method

The inverse eigenvalue problem for symmetric matrices (IEP) is formulated as a system of two matrix equations. For solving the system a variation of Newton's method is used which has been proposed by Fusco and Zecca [Calcolo XXIII (1986), pp. 285–303] for the simultaneous computation of eigenvalues and eigenvectors of a given symmetric matrix. an iteration step of this method consists of a Newton step followed by an orthonormalization with the consequence that each iterate satisfies one of the given equations. The method converges locally y quadratically to regular solutions. The algorithm and some numerical examples are presented. In addition, it is shown that the so called Method III proposed by Friedland, Nocedal, and Overton [SIAM J. Numer. Anal.,24 (1987), pp. 634–667] for solving IEP may be constructed similarly to the method presented here     

A Hypergeometric Inequality

A sequence of coefficients that appeared in the evaluation of a rational integral has been shown to be unimodal. The original proof is based on a inequality for hypergeometric functions. A generalization is presented.     

Nitrogen-Rich Oxetanes Based on the Combination of Azides and Tetrazoles

Literature known energetic oxetane derivatives have a nitrogen content of up to 49.98 %. Through the introduction of azide and tetrazole functionalities attached to an oxetane ring, energetic oxetanes with higher nitrogen contents than previously reported in the literature were obtained. The newly synthesized oxetane derivatives were extensively characterized via ¹H NMR, ¹³C{¹H} NMR, ¹⁴N NMR, ¹⁵N NMR, ¹H-¹⁵N HMBC, FT-IR spectroscopy and/or DTA. Their crystal structures were elucidated using X-ray diffraction, their sensitivities towards impact, friction and electrostatic discharge were determined and their energetic properties were calculated using the EXPLO5 code.     

Strong influence of intramolecular Si⋯O proximity on reactivity: Systematic molecular structure,solvolysis, and mechanistic study of cyclic N-trimethylsilyl carboxamide derivatives

A comparative alcoholysis study of N-silylated derivatives of simple heterocyclic carboxamides (lactams, imides, ureas) is presented. The second-order rate constant values span a range as wide as three orders of magnitude. On the basis of DFT calculations, a good correlation between reactivity and the Si?O distance was found within each family of compounds. The viability of two different reaction pathways was evaluated using a detailed computational mechanistic study of the methanolysis of cyclic urea homologues. Peculiarities in the single-crystal X-ray diffraction structures of the trimethylsilyl and trimethylsiloxy phthalimides are also discussed.     

Latoxanthin, a minor carotenoid isolated from the fruits of yellow paprika (Capsicum annuum var. lycopersiciforme flavum)

Latoxanthin was isolated as a minor carotenoid from the ripe fruits of yellow tomato shaped paprika (Capsicum annuum var. lycopersiciforme flavum) and identified as (all-E,3S,5R,6R,3′S,5′R,6′S)-5′,6′-epoxy-5,6,5′,6′-tetrahydro-β,β-carotene-3,5,6,3′-tetrol based on spectral data.     

Effect of numerical integration in the DGFEM for nonlinear convection‐diffusion problems

This paper is concerned with the effect of numerical integration applied to the discontinuous Galerkin finite element discretization of nonlinear convection‐diffusion problems in 2D. In the space semidiscretization the volume and line integrals are evaluated by numerical quadratures. Our goal is to estimate the error caused by the numerical integration and to show what numerical quadratures guarantee that the accuracy of the method with exact integration is preserved. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007