Computation of surface tensions using expanded ensemble simulations

A method for the direct simulation of the surface tension is examined. The technique is based on the thermodynamic route to the interfacial tension and makes use of the expanded ensemble simulation method for the calculation of the free energy difference between two inhomogeneous systems with the same number of particles, temperature, and volume, but different interfacial area. The method is completely general and suitable for systems with either continuous or discontinuous interactions. The adequacy of the expanded ensemble method is assessed by computing the interfacial tension of the planar vapor-liquid interface of Lennard-Jones, Lennard-Jones dimers, Gay-Berne, and square-well model fluids; in the latter, the interactions are discontinuous and the present method does not exhibit the asymmetry of other related methods, such as the test area. The expanded ensemble simulation results are compared with simulation data obtained from other techniques (mechanical and test area) with overall good agreement.     

Convergence of the Iterated Aluthge Transform Sequence for Diagonalizable Matrices II: λ-Aluthge Transform

Let λ ∈ (0, 1) and let T be a r × r complex matrix with polar decomposition T = U|T|. Then the λ-Aluthge transform is defined by

Structural and textural evolution during folding of layers of layered double hydroxides

Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.     

Deepened chiral cavitands

Deep cavitands bearing eight asymmetric centers on their upper rims are prepared from octamino resorcinarenes. The resorcinarenes are acylated with Fmoc d- and l-alanine or Fmoc glycine acid chlorides. The asymmetric centers create a chiral magnetic environment as shown by binding achiral ?-caprolactam. The chiral steric environment shows modest enantioselectivity (55% de) for chiral guests such as pinane diols bound inside the cavitand.     

Computer simulation of charged hard spherocylinders

In this work we present a computer simulation study of charged hard spherocylinders of aspect ratio L/sigma=5, using NVT and NPT Monte Carlo methods. Coulombic interactions are handled using the Wolf method [D. Wolf, P. Keblinski, S. R. Phillpot, and J. Eggebrecht, J. Chem. Phys. 110, 8254 (1999)]. Thermodynamic and structural properties are in excellent agreement with the results obtained with the standard Ewald summation method. A partial prediction of the corresponding phase diagram is obtained by studying two isotherms of this system. The stability of the liquid crystalline phases is examined and compared with the phase diagrams of neutral hard spherocylinders and dipolar hard spherocylinders.     

Study of the interaction in clusters formed by phenol and CH3X (X=CN,F,Cl) molecules

The characteristics of the interaction between phenol and acetonitrile, methyl fluoride and methyl chloride were studied. The most stable structures for clusters containing one or two CH3X molecules and one phenol moiety were located by means of ab initio and density functional theory calculations. Phenol-acetonitrile dimer presents two almost equally stable structures; one of them is a typical linearly hydrogen bonded minimum, whereas in the other one, a C-H...pi contact is established accompanied by a distorted O-H...N hydrogen bond. Although the latter minimum presents the larger interaction energy, deformation effects favor the formation of the linear hydrogen bonded one. In complexes with methyl fluoride and methyl chloride, this arrangement is the most stable structure and no linear hydrogen bonded structures were located. Our best estimates for the interaction energies amount to -27.8, -21.6, and -19.7 kJ/mol for clusters of phenol with acetonitrile, methyl fluoride, and methyl chloride, respectively. The main contribution to the stabilization of these clusters is of electrostatic nature, although in structures where a C-H...pi contact is present, the dispersion contribution is also significant. In clusters formed by phenol and two CH3X units, the most stable arrangement corresponds to a head to tail disposal with O-H...X, C-H...X, and C-H...pi contacts forming a cycle. Only for this type of arrangement, three body effects are non-negligible even though they constitute a minor effect. The results also indicate that interactions with methyl fluoride and methyl chloride are of similar intensity, although weaker than with acetonitrile. Significant frequency shifts are predicted for the O-H stretching, which increase when increasing the number of CH3X molecules.     

The in, out asymmetric pseudo-triple helical form of a D3h diaza-macropentacycle

A sterically encumbered [N(2)S(6)] macropentacycle (5) related to diazamacrobicycles and cryptands has been synthesized in 53% yield by the [1+1] condensation reaction between functionalized macrocyclic and macrotricyclic precursors. A macrononacycle (18) resulting from the corresponding [2+2] condensation was isolated in 7% yield from the reaction mixture. Both compounds showed broad features in their room-temperature (1)H NMR spectra, but their maximal average symmetry (D(3h) and D(2h), respectively) was achieved at high temperature (380 K). At low temperature (200 K, CD(2)Cl(2) solution), the macropentacycle is "frozen" to a single asymmetric (C1) conformation on the (1)H NMR time scale, which has also the molecular structure observed in the solid state by X-ray crystallography: pseudo-triple helical ( not equalC(3)) shape, io (in, out) form resulting from the endo/exo configuration at the nitrogen bridgehead atoms, and similar orientations of the tosyl substituents. The solution dynamics of the molecule can be described by coupled bridgehead nitrogen inversion, triple helix symmetrization, and reversal of triple helix handedness, with DeltaGc = 54.2 kJ mol(-1) in CD(2)Cl(2) at 300 K. Adoption of the io form by macropentacycle 5 in the crystal and in solution at low-temperature most probably results from the steric crowding and strain introduced by the [15]ane-N(2)S(2) macrocyclic bridging subunits.     

Structure and oxide anion conductivity in Ln2(TO4)O (Ln=La, Nd; T=Ge, Si)

Oxy-silicate and oxy-germanate, Ln₂(TO₄)O (Ln=La and Nd, T=Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd₂(SiO₄)O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd₂(SiO₄)O, La₂(GeO₄)O and Nd₂(GeO₄)O, respectively. The overall conductivities at 1173 K range from 1.2×10⁻⁴ S cm⁻¹ for Nd₂(SiO₄)O to 1.3×10⁻⁶ S cm⁻¹ for La₂(GeO₄)O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.     

In situ preparation of polypropylene/1-butene alloys using a MgCl2-supported Ziegler-Natta catalyst

Sequential polymerizations were carried out using a high-activity MgCl₂/Ziegler-Natta catalyst to evaluate whether the in situ preparation of polypropylene/1-butene alloys was possible inside the reaction vessel and analyze the effects of 1-butene on the final material properties. Propylene/1-butene alloy resins were synthesized in a sequential two-stage process. In the first stage, liquid pool propylene polymerizations were carried out in batch. In the second stage, 1-butene was polymerized inside the polypropylene matrix in gas-phase in semibatch mode. According to the obtained results, it is possible to incorporate 1-butene upon the polypropylene matrix inside the reactor at very low pressures, without affecting the properties of the continuous polypropylene matrix significantly.