Brownian Dynamics, Molecular Dynamics, and Monte Carlo modeling of colloidal systems

This paper serves as an introductory review of Brownian Dynamics (BD), Molecular Dynamics (MD), and Monte Carlo (MC) modeling techniques. These three simulation methods have proven to be exceptional investigative solutions for probing discrete molecular, ionic, and colloidal motions at their basic microscopic levels. The review offers a general study of the classical theories and algorithms that are foundational to Brownian Dynamics, Molecular Dynamics, and Monte Carlo simulations. Important topics of interest include fundamental theories that govern Brownian motion, the Langevin equation, the Verlet algorithm, and the Metropolis method. Brownian Dynamics demonstrates advantages over Molecular Dynamics as pertaining to the issue of time-scale separation. Monte Carlo methods exhibit strengths in terms of ease of implementation. Hybrid techniques that combine these methods and draw from these efficacies are also presented. With their rigorous microscopic approach, Brownian Dynamics, Molecular Dynamics, and Monte Carlo methods prove to be especially viable modeling methods for problems with challenging complexities such as high-level particle concentration and multiple particle interactions. These methods hold promising potential for effective modeling of transport in colloidal systems.     

Critical study of and improvements in chromatographic methods for the analysis of type B trichothecenes

Various analytical methods used in the analysis of type B trichothecenes (deoxynivalenol, nivalenol, 3- and 15-acetyldeoxynivalenol) in cereals were compared and optimised in this work. These methods use either GC-electron-capture detection (ECD) of trimethylsilyl, trifluoroacetyl and heptafluorobutyryl derivatives or HPLC with UV or photodiode array detection of analytes. A new HPLC procedure using fluorescence detection prior derivatisation with coumarin-3-carbonyl chloride has been also tested. Five extraction solvents and two solid-phase extraction cartridges (silica, Florisil) plus a especial clean-up column (MycoSep 225) were compared in order to obtain the best recovery of the mycotoxins with minimal presence of coextractives in the chromatograms. The chosen extraction solvent was a mixture of acetonitrile-water (84:16, v/v). The MycoSep 225 column was chosen as the best alternative for clean-up of grain samples. For GC-ECD analysis, derivatisation of analytes with heptafluorobutyric anhydride prior the final determination was chosen as the most suitable procedure. HPLC-photodiode array (at 221 nm) analysis was more suitable for determination of type B trichothecenes than HPLC of the fluorescent coumarin-3-carbonyl derivatives. Recoveries obtained in spiked corn, rice and wheat are reported. The utility of the proposed methodology was assayed in cereal cultures of various Fusarium strains.     

Atmospheric degradation of 2-butanol, 2-methyl-2-butanol, and 2,3-dimethyl-2-butanol: OH kinetics and UV absorption cross sections

The absolute rate coefficients for the reactions of hydroxyl radical (OH) with 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), and 2,3-dimethyl-2-butanol (k(3)) were measured as a function of temperature (263-354 K) and pressure (41-193 Torr of He, Ar, and N(2)) by the pulsed laser photolysis/laser-induced fluorescence technique. This work represents the first absolute determination of k(1)(-)k(3) and their temperature dependence. No pressure dependence of the rate coefficients was observed in the range studied. Thus, k(i)(298 K) values (x10(-12) cm(3) molecule(-1) s(-1) with an uncertainty of +/-2sigma) were averaged over the pressure range studied yielding 8.77 +/- 1.46, 3.64 +/- 0.60, and 9.01 +/- 1.00 for 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), and 2,3-dimethyl-2-butanol (k(3)), respectively. k(1) and k(3) exhibit a slightly negative temperature dependence over the temperature range studied. In contrast, the rate coefficient for the reaction of OH with 2-methyl-2-butanol (k(2)) did not show any temperature dependence. Some deviation of the conventional Arrhenius behavior was clearly observed for k(3). In this case, the best fit to our data was found to be described by the three-parameter expression k(T) = A + B exp(-C/T). The UV absorption cross sections of 2-butanol, 2-methyl-2-butanol, and 2,3-dimethyl-2-butanol have also been measured at room temperature between 208 and 230 nm. The values reported constitute the first determination of the UV cross sections of those alcohols. Our results are compared with previous studies, when possible, and are discussed in terms of the H-abstraction by OH radicals. The atmospheric implications of these reactions and the photochemistry of these alcohols are also discussed.     

Syntheses, structures, and magnetic properties of mixed-valent diruthenium(II,III) diphosphonates with discrete and one-dimensional structures

Four mixed-valent ruthenium diphosphonates, namely, Na(4)[Ru(2)(hedp)(2)X]x16H(2)O [X = Cl (1), Br (2)], K(3)[Ru(2)(hedp)(2)(H(2)O)(2)]x6H(2)O (3), and Na(7)[Ru(2)(hedp)(2)Fe(CN)(6)]x24H(2)O (4), where hedp represents 1-hydroxyethylidenediphosphonate [CH(3)C(OH)(PO(3))(2)](4-), were synthesized and structurally characterized. Compounds 1, 2, and 4 show linear chain structures in which the mixed-valent [Ru(2)(hedp)(2)](3-) dimers are linked by X(-) or [Fe(CN)(6)](4-) bridges. Compound 3 contains discrete species of [Ru(2)(hedp)(2)(H(2)O)(2)](3-) where the axial positions of [Ru(2)(hedp)(2)](3-) paddlewheel are terminated by water molecules. Magnetic studies show that significant antiferromagnetic exchanges are mediated between the [Ru(2)(hedp)(2)](3-) (S = 3/2) units through halide bridges in compounds 1 and 2.     

Physical and electrochemical characterizations of microwave-assisted polyol preparation of carbon-supported PtRu nanoparticles

PtRu nanoparticles supported on Vulcan XC-72 carbon and carbon nanotubes were prepared by a microwave-assisted polyol process. The catalysts were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which were uniformly dispersed on carbon, were 2-6 nm in diameter. All PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic structure, excepting that the 2theta values were shifted to slightly higher values. XPS analysis revealed that the catalysts contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV), and Ru(IV). The electro-oxidation of methanol was studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. It was found that both PtRu/C catalysts had high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a direct methanol fuel cell single stack test cell using the Vulcan-carbon-supported PtRu alloy as the anode catalyst showed high power density.     

Highly specific spectrophotometric method for palladium(II) determination with 3-(5'-tetrazolylazo)-2,6-Diaminotoluene in the presence of chlorides. Kinetic and equilibrium study of reactions

3-(5'-tetrazolylazo)-2,6-Diaminotoluene (TEADAT, H(3)L(2+)) forms stable 1:1 and 1:2 (metal:ligand) pink-red complexes (lambda(max) 506 and 536 nm) with palladium(II). The apparent molar absorptivity of 1:2 complex is 5.2 x 10(4) 1.mol(-1). cm(-1) at 536 nm. Equilibrium constants beta*(nl) for reactions PdCl(2-)(4) + nH(3)L(2+) right harpoon over left harpoonright harpoon over left harpoon PdCl(4-n) (H(2)L)(2n-2)(n) + n Cl(-) + n H(+) were determined: logbeta*(1) = 4.09 +/- 0.05, logbeta*(2) = 8.40 +/- 0.02, corresponding stability conditional constants of PdCl(3)(H(2)L) and PdCl(2)(H(2)L)(2+)(2) were log beta(1) = 19.03, log beta(2) = 26.74. The formation of complexes was rather slow but could be speeded up considerably by the catalytic effect of trace amounts of thiocyanate. Constant absorbance values were thus reached in 2-5 min. A rapid, sensitive and highly specific method for the determination of palladium(II) at pH 1.42 in 0.25M NACl has been worked out with a detection limit of 0.54 mug. Interference of precious and common metal ions have been studied and the method has been applied for the determination of palladium in Pd asbestos, oakay alloys and various catalysts and for the determination of palladium in precious metals.     

Spectrofluorimetric determination of cisatracurium and mivacurium in spiked human serum and pharmaceuticals

Spectrofluorimetric methods to determine cisatracurium and mivacurium are proposed and applied to the determination of both substances in human serum and to the determination of mivacurium in pharmaceuticals. The fluorimetric methods allow the determination of 5-500 ng ml(-1) of mivacurium in aqueous solutions and 5-500 ng ml(-1) of cisatracurium in water-acetonitrile solutions, both containing acetic acid-sodium acetate buffer (pH 5.5) with lambda(exc)=230 nm and lambda(em)=324 nm.     

Limited flexibility of lactose detected from residual dipolar couplings using molecular dynamics simulations and steric alignment methods

The conformational flexibility of lactose in solution has been investigated by residual dipolar couplings (RDCs). One-bond carbon-proton and proton-proton coupling constants have been measured in two oriented media and interpreted in combination with molecular dynamics simulations (MD). Two different approaches, known as PALES (Zweckstetter et al., J. Am. Chem. Soc. 2000, 122, 3791-3792) and TRAMITE (Azurmendi et al., J. Am. Chem. Soc. 2002, 124, 2426-2427), have been used to determine the alignment tensor from a shape-induced alignment model with the oriented medium. The steric alignment of the structures from several MD trajectories has provided ensemble averaged RDCs that have been compared with the experimental ones. The obtained results reveal the almost exclusive presence of a major low energy region defined as syn-phi/syn-psi (> 97%), for which sampling occurs in a dynamic manner. This result satisfactorily agrees with that determined by standard NOE-based methods.     

Temperature Dependence of the Solubility of Acetaminophen in Propylene Glycol + Ethanol Mixtures

The thermodynamic functions Gibbs energy, enthalpy and entropy of solution, mixing and solvation of acetaminophen in propylene glycol (PG) + ethanol (EtOH) cosolvent mixtures were evaluated from solubility data measured at several temperatures, using the van't Hoff and Gibbs equations. The solubility was greater at 50% m/m of PG at 20.0^C, while it was greater at 80% of PG at 40.0 ^C where m/m refers to mass percent. The solvation of this drug is appreciably greater in the mixtures than in the pure solvents. By means of an enthalpy–entropy compensation analysis, complex behavior was found for the solution. From 0 up to 20% of PG and from 60 up to 100% of PG the solution process is enthalpy driven, whereas from 20 up to 60% of PG it is entropy driven. These facts can be explained in terms of a decrease in the energy required for cavity formation in the solvent for mixtures containing 20–60% of PG.     

Mineral content and botanical origin of Spanish honeys

Eleven elements (Zn, P, B, Mn, Mg, Cu, Ca, Ba, Sr, Na and K) were determined by inductively plasma coupled spectrometry in 40 honey samples from different places of Spain and four different botanical origins: Eucalyptus (Eucalyptus sp.), Heather (Erica sp.), Orange-blossom (Citrus sinensis) and Rosemary (Rosmarinus officinalis). K, Ca and P show the higher levels with average concentrations ranged between 434.1-1935 mg kg⁻¹ for K; 42.59-341.0 mg kg⁻¹ for Ca and 51.17-154.3 mg kg⁻¹ for P. Levels of Cu (0.531-2.117 mg kg⁻¹), Ba (0.106-1.264 mg kg⁻¹) and Sr (0.257-1.462 mg kg⁻¹) are the lowest in all honey samples. Zn (1.332-7.825 mg kg⁻¹), Mn (0.133-9.471 mg kg⁻¹), Mg (13.26-74.38 mg kg⁻¹) and Na (11.69-218.5 mg kg⁻¹) concentrations were found strongly dependent on the kind of botanical origin.Results were submitted to pattern recognition procedures, unsupervised methods such as cluster and principal components analysis and supervised learning methods like linear discriminant analysis in order to evaluate the existence of data patterns and the possibility of differentiation of Spanish honeys from different botanical origins according to their mineral content. Cluster analysis shows four clusters corresponding to the four botanical origins of honey and PCA explained 71% of the variance with the first two PC variables. The best-grouped honeys were those from heather; eucalyptus honeys formed a more dispersed group and finally orange-blossom and rosemary honeys formed a less distinguishable group.