Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles

Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (γ-Fe₂O₃) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel-Fulcher law gives a large value of T₀ for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent (zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanoparticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C.     

Communication: Thermal rectification in liquids by manipulating the solid-liquid interface

Thermal rectification, the origin of which lies in modifying the thermal resistance in a nonlinear manner, could significantly improve the thermal management of a wide range of nano-devices (both electronic and thermoelectric), thereby improving their efficiencies. Since rectification requires a material to be inhomogeneous, it has been typically associated with solids. However, the structure of solids is relatively difficult to manipulate, which makes the tuning of thermal rectification devices challenging. Since liquids are more amenable to tuning, this could open up new applications for thermal rectification. We use molecular dynamics simulations to demonstrate thermal rectification using liquid water. This is accomplished by creating an inhomogeneous water phase, either by changing the morphology of the surface in contact with the liquid or by imposing an arbitrary external force, which in practice could be through an electric or magnetic field. Our system consists of a bulk fluid that is confined in a reservoir that is bounded by two walls, one hot and the other cold. The interfacial (Kapitza) thermal resistance at the solid-fluid interface and the density gradient of the bulk fluid both influence the magnitude of the thermal rectification. However, we find that the role of the interfacial resistance is more prominent than the application of an external force on the bulk fluid.     

Simulation of multiphase systems utilizing independent force fields to control intraphase and interphase behavior

Fixed‐charge empirical force fields have been developed and widely used over the past three decades for all‐atom molecular simulations. Most simulation programs providing these methods enable only one set of force field parameters to be used for the entire system. Whereas this is generally suitable for single‐phase systems, the molecular environment at the interface between two phases may be sufficiently different from the individual phases to require a different set of parameters to be used to accurately represent the system. Recently published simulations of peptide adsorption to material surfaces using the CHARMM force field have clearly demonstrated this issue by revealing that calculated values of adsorption free energy substantially differ from experimental results. Whereas nonbonded parameters could be adjusted to correct this problem, this cannot be done without also altering the conformational behavior of the peptide in solution, for which CHARMM has been carefully tuned. We have developed a dual‐force‐field approach (Dual‐FF) to address this problem and implemented it in the CHARMM simulation package. This Dual‐FF method provides the capability to use two separate sets of nonbonded force field parameters within the same simulation: one set to represent intraphase interactions and a separate set to represent interphase interactions. Using this approach, we show that interfacial parameters can be adjusted to correct errors in peptide adsorption free energy without altering peptide conformational behavior in solution. This program thus provides the capability to enable both intraphase and interphase molecular behavior to be accurately and efficiently modeled in the same simulation. © 2012 Wiley Periodicals, Inc.     

Dramatic specific-ion effect in supramolecular hydrogels

We report on a pronounced specific-ion effect on the intermolecular and chiral organization, supramolecular structure formation, and resulting materials properties for a series of low molecular weight peptide-based hydrogelators, observed in the presence of simple inorganic salts. This effect was demonstrated using aromatic short peptide amphiphiles, based on fluorenylmethoxycarbonyl (Fmoc). Gel-phase materials were formed due to molecular self-assembly, driven by a combination of hydrogen bonding and π-stacking interactions. Pronounced morphological changes were observed by atomic force microscopy (AFM) for Fmoc-YL peptide, ranging from dense fibrous networks to spherical aggregates, depending on the type of anions present. The gels formed had variable mechanical properties, with G'?values between 0.8?kPa and 2.4?kPa as determined by rheometry. Spectroscopic analysis provided insights into the differential mode of self-assembly, which was found to be dictated by the hydrophobic interactions of the fluorenyl component, with comparable H-bonding patterns observed in each case. The efficiency of the anions in promoting the hydrophobic interactions and thereby self-assembly was found to be consistent with the Hofmeister anion sequence. Similar effects were observed with other hydrophobic peptides, Fmoc-VL and Fmoc-LL. The effect was found to be less pronounced for a less hydrophobic peptide, Fmoc-AA. To get more insights into the molecular mechanism, the effect of anions on sol-gel equilibrium was investigated, which indicates the observed changes result from the specific-ion effects on gels structure, rather than on the sol-gel equilibrium. Thus, we demonstrate that, by simply changing the ionic environment, structurally diverse materials can be accessed providing an important design consideration in nanofabrication via molecular self-assembly.     

Planar Lattice Graphs with Gallai’s Property

In 1966 T. Gallai asked whether connected graphs with empty intersection of their longest paths do or do not exist. After examples of such graphs were found, the question was extended to graphs of higher connectivity, and to cycles instead of paths. Examples being again found, for connectivity up to 3, the question has been asked whether there exist large families of graphs without Gallai’s property. The family of grid graphs, a special kind of graphs embedded in the planar square lattice ${\mathcal {L}}$ , has been shown by B. Menke to contain no graph enjoying Gallai’s property. In this paper we find several examples of graphs embedded in ${\mathcal {L}}$ and enjoying that property with respect to both paths and cycles.     

Lie ideals,Morita context and Generalized (α, β)-Derivations

A classical problem in ring theory is to study conditions under which a ring is forced to become commutative. Stimulated from Jacobson's famous result, several techniques are developed to achieve this goal. In the present note, we use a pair of rings, which are the ingredients of a Morita context, and obtain that if one of the ring is prime with the generalized (α, β)-derivations that satisfy certain conditions on the trace ideal of the ring, which by default is a Lie ideal, and the other ring is reduced, then the trace ideal of the reduced ring is contained in the center of the ring. As an outcome, in case of a semi-projective Morita context, the reduced ring becomes commutative.     

Blood flow of Jeffrey fluid in a catherized tapered artery with the suspension of nanoparticles

Current letter deals with the mathematical models of Jeffrey fluid via nanoparticles in the tapered stenosed atherosclerotic arteries. The convection effects of heat transfer with catheter are also taken into account. The nonlinear coupled equations of nanofluid model are simplified under mild stenosis. The solutions for concentration and temperature are found by using homotopy perturbation method, whereas for velocity profile the exact solution is calculated. Moreover, the expressions for flow impedance and pressure rise are computed and discussed through graphs for different physical quantities of interest. The streamlines have also been presented to discuss the trapping bolus discipline.     

Synthesis and characterization of palladium(II) complexes of thioureas. X-ray structures of [Pd( N , N ′-dimethylthiourea)4]Cl2?·?2H2O and [Pd(tetramethylthiourea)4]Cl2

Palladium(II) complexes of thiones having the general formula [Pd(L)₄]Cl₂, where L = thiourea (Tu), methylthiourea (Metu), N,N′-dimethylthiourea (Dmtu), and tetramethylthiourea (Tmtu) were prepared by reacting K₂[PdCl₄] with the corresponding thiones. The complexes have been characterized by elemental analysis, IR and NMR spectroscopy, and two of these, [Pd(Dmtu)₄]Cl₂ · 2H₂O (1) and [Pd(Tmtu)₄]Cl₂ (2), by X-ray crystallography. An upfield shift in the >C=S resonance of thiones in ¹³C NMR and downfield shift in N–H resonance in ¹H NMR are consistent in showing sulfur coordination with palladium(II). The crystal structures of the complexes show a square-planar coordination environment around the Pd(II) ions with the average cis and trans S–Pd–S bond angles of 89.64° and 173.48°, respectively. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. An erratum to this article can be found at     

Homo‐ and Heterovalent Polynuclear Cerium and Cerium/Manganese Aggregates

Reactions of Ce^III(NO₃)₃?6 H₂O or (NH₄)₂[Ce^IV(NO₃)₆] with Mn‐containing starting materials result in seven novel polynuclear Ce or Ce/Mn complexes with pivalato (^tBuCO ) and, in most cases, auxiliary N,O‐ or N,O,O‐donor ligands. With nuclearities ranging from 6–14, the compounds present aesthetically pleasing structures. Complexes [Ce^IV₆(μ₃‐O)₄(μ₃‐OH)₄(μ‐O₂C^tBu)₁₂] ( 1 ), [Ce^IV₆Mn^III₄(μ₄‐O)₄(μ₃‐O)₄(O₂C^tBu)₁₂(ea)₄(OAc)₄]?4 H₂O?4 MeCN (ea^?=2‐aminoethanolato; 2 ), [Ce^IV₆Mn^III₈(μ₄‐O)₄(μ₃‐O)₈(pye)₄(O₂C^tBu)₁₈]₂[Ce^IV₆(μ₃‐O)₄(μ₃‐OH)₄(O₂C^tBu)₁₀(NO₃)₄] [Ce^III(NO₃)₅(H₂O)]?21 MeCN (pye^?=pyridine‐2‐ethanolato; 3 ), and [Ce^IV₆Ce^III₂Mn^III₂(μ₄‐O)₄(μ₃‐O)₄(^tbdea)₂(O₂C^tBu)₁₂(NO₃)₂(OAc)₂]?4 CH₂Cl₂ (^tbdea^2?=2,2′‐(tert‐butylimino]bis[ethanolato]; 4 ) all contain structures based on an octahedral {Ce^IV₆(μ₃‐O)₈} core, in which many of the O‐atoms are either protonated to give (μ₃‐OH)^? hydroxo ligands or coordinate to further metal centers (Mn^III or Ce^III) to give interstitial (μ₄‐O)^2? oxo bridges. The decanuclear complex [Ce^IV₈Ce^IIIMn^III(μ₄‐O)₃(μ₃‐O)₃(μ₃‐OH)₂(μ‐OH)(bdea)₄(O₂C^tBu)_9.5(NO₃)_3.5(OAc)₂]?1.5 MeCN (bdea^2?=2,2′‐(butylimino]bis[ethanolato]; 5 ) contains a rather compact Ce^IV₇ core with the Ce^III and Mn^III centers well‐separated from each other on the periphery. The aggregate in [Ce^IV₄Mn^IV₂(μ₃‐O)₄(bdea)₂(O₂C^tBu)₁₀(NO₃)₂]?4 MeCN ( 6 ) is based on a quasi‐planar {Mn^IV₂Ce^IV₄(μ₃‐O)₄} core made up of four edge‐sharing {Mn^IVCe^IV₂(μ₃‐O)} or {Ce^IV₃(μ₃‐O)} triangles. The structure of [Ce^IV₃Mn^IV₄Mn^III(μ₄‐O)₂(μ₃‐O)₇(O₂C^tBu)₁₂(NO₃)(furan)]?6 H₂O ( 7 ?6 H₂O) can be considered as {Mn^IV₂Ce^IV₂O₄} and distorted {Mn^IV₂Mn^IIICe^IVO₄} cubane units linked through a central (μ₄‐O) bridge. The Ce₆Mn₈ equals the highest nuclearity yet reported for a heterometallic Ce/Mn aggregate. In contrast to most of the previously reported heterometallic Ce/Mn systems, which contain only Ce^IV and either Mn^IV or Mn^III, some of the aggregates presented here show mixed valency, either Mn^IV/Mn^III (see 7 ) or Ce^IV/Ce^III (see 4 and 5 ). Interestingly, some of the compounds, including the heterovalent Ce^IV/Ce^III 4 , could be obtained from either Ce^III(NO₃)₃?6 H₂O or (NH₄)₂[Ce^IV(NO₃)₆] as starting material.     

Suppression of methane formation during Fisher-Tropsch synthesis using manganese-cobalt oxide supported on H-5A zeolite as a catalyst

In Fischer-Tropsch synthesis reaction, methane formation is one of the side reactions which must be suppressed in order to get better catalytic selectivity for light olefins. In the present study, we have modified cobalt based Fischer-Tropsch catalyst and developed a process to minimize methane production, consequently to produce maximum yield of light olefins. Manganese-cobalt oxide supported on H-5A zeolite catalyst was synthesized using modified H-5A zeolite, to increase its surface acid sites. Increased acidity of zeolite plays a major part in the suppression of methane formation during the Fischer-Tropsch reaction. The modified zeolite results in the electronic modification of catalyst surface by creating new active catalytic sites. The results are compared with other supported catalysts along with unmodified zeolite. Appreciable reduction in methane formation is achieved on modified zeolite supported catalyst in comparison with unsupported catalyst.