Synthesis of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: Effects of the Water/Surfactant Molar Ratio and Ammonia Concentration

The effect of ammonia concentration on the region of existence of single-phase water-in-oil microemulsions has been investigated for the system polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/ammonium hydroxide. The presence of ammonia decreases the size of the microemulsion region. A minimum concentration of surfactant (estimated at about 1.1 wt%) is required for solubilization of the aqueous phase; this value is not significantly affected by ammonia concentration. As indicated by fluorescence spectral data, the transition between bound and free water occurs when the water-to-surfactant molar ratio is about 1 and the presence of ammonium hydroxide does not appear to have a significant effect on this. Ultrafine (30-70 nm diameter), monodisperse silica particles produced by hydrolysis of tetraethoxysilane (TEOS) in the microemulsion show a complex dependence of the particle size on the water-to-surfactant molar ratio (R) and on the concentration of ammonium hydroxide. At relatively low ammonia concentration in the aqueous pseudophase (1.6 wt% NH3) the particle size decreases monotonically with increase in R. However, for higher ammonia concentrations (6.3-29.6 wt% NH3) a minimum in particle size occurs as R is increased. These trends are rationalized in terms of (a) the effects of the concentration, structure, and dynamics of the NP-5 reverse micelles on the hydrolysis and condensation reactions of TEOS, and (b) the effects of ammonia concentration on the stability of the microemulsion phase, the hydrolysis/condensation reactions of TEOS, and the depolymerization of siloxane bonds. Copyright 1999 Academic Press.     

Theoretical insights into the regioselectivity of a Pictet‐Spengler reaction: Transition state structures leading to salsolinol and isosalsolinol

The mechanism of the cyclization step of the Pictet‐Spengler reaction between acetaldehyde and dopamine to give salsolinol and isosalsolinol was studied computationally, using density functional theory. The preferential formation in acidic media of salsolinol, the product of para‐cyclization, and the requirement of a neutral pH for the formation of the ortho‐cyclized isosalsolinol are explained in terms of 2 different mechanistic routes with an iminium ion or a phenolate‐iminium zwitterion as starting reactants.     

Adsorption of polycyclic aromatic hydrocarbons onto graphyne: Comparisons with graphene

Density functional theory calculations were implemented to expand the knowledge about graphyne and its interaction with polycyclic aromatic hydrocarbons (PAHs). Due to the porous character of graphyne, the adsorption strength of PAHs onto graphyne surfaces is expected to be lower with respect to graphene (a perfect π‐extended system). However, there are not quantitative evidences for this assumption. This work shows that the adsorption strength of adsorbed PAHs onto γ‐graphyne nanosheets (GY) is weakened in 12 ? 23% with respect to the adsorption onto graphene, with a decrease of 10 ? 20% in the dispersive interactions. The adsorption energies (in eV) of the GY–PAH systems can be straightforward obtained as E _ads/eV≈0.033N _H + 0.031N _C, where N _H and N _C is the number of H and C atoms in the aromatic molecule, respectively. This equation predicts the binding energy of graphene–graphyne bilayers with a value of ～31 meV/atom. Analysis of the electronic properties shows that PAHs behaves as n‐dopants for GY, introducing electrons in GY and also reducing its bandgap in up to ～0.5 eV. Strong acceptor or donor substituted PAHs decrease the bandgap of γ‐graphyne in up to ～0.8 eV, with changes in its valence or conduction band, depending on the chemical nature of the adsorbate. Finally, these data will serve for future studies related to the bandgap engineering of graphyne surfaces by nonaggressive molecular doping, and for the development of graphyne‐based materials with potential applications in the removal of persistent aromatic pollutants.     

Removal of 4‐chlorophenol using graphene,graphene oxide,and a‐doped graphene (A = N,B): A computational study

Density functional theory (including van der Waals correction with the PBE‐D functional) is applied to the study of 4‐chlorophenol (4‐CP) adsorption on graphene oxide (GO), A‐doped graphene (A = N, B), and pristine graphene and test their possible application for 4‐CP removal. Results show that on GO adsorption is improved by the hydrogen bond interactions between the adsorbents and 4‐CP, suggesting that functionalized graphene is a preferable alternative than pristine graphene for 4‐CP removal. In addition, the stability of hydrogen bonds is confirmed by molecular dynamics calculations using the PM6 potential. Without hydrogen bonds, A‐doped graphene models show a comparable performance for 4‐CP removal than pristine graphene. Finally, even in a solvent medium, 4‐CP adsorption is strong. © 2013 Wiley Periodicals, Inc.     

Optical properties of binuclear zinc (II) macrocyclic complexes derived from 4-methyl-2,6-diformylphenol and 1,2-diaminobenzene

In this work, we present the synthesis and optical study of the binuclear zinc(II) macrocyclic complexes, derived from 4-methyl-2,6-diformylphenol and 1,2-diaminobenzene (H₂L). Two zinc macrocyclic complexes with different anions were prepared and characterized: [Zn₂LCl₂]·H₂O (1) and [Zn₂L](NO₃)₂ (2).     

Determination of thermal parameters of one-dimensional nanostructures through a thermal transient method

We present an improved methodology for a thermal transient method enabling simultaneous measurement of thermal conductivity and specific heat of nanoscale structures with one-dimensional heat flow. The temporal response of a sample to finite duration heat pulse inputs for both short (1 ns) and long (5 μs) pulses is analyzed and exploited to deduce the thermal properties. Excellent agreement has been obtained between the recovered physical parameters and computational simulations through choosing an optimized pulse width.