Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.     

Coarse-grained molecular dynamics simulations of the sphere to rod transition in surfactant micelles

Surfactant molecules self-assemble in aqueous solutions to form various micellar structures such as spheres, rods, or lamellae. Although phase transitions in surfactant solutions have been studied experimentally, their molecular mechanisms are still not well understood. In this work, we show that molecular dynamics (MD) simulations using the coarse-grained (CG) MARTINI force field and explicit CG solvent, validated against atomistic MD studies, can accurately represent micellar assemblies of cetyltrimethylammonium chloride (CTAC). The effect of salt on micellar structures is studied for aromatic anionic salts, e.g., sodium salicylate, and simple inorganic salts, e.g., sodium chloride. Above a threshold concentration, sodium salicylate induces a sphere to rod transition in the micelle. CG MD simulations are shown to capture the dynamics of this shape transition and support a mechanism based on the reduction in the micelle-water interfacial tension induced by the adsorption of the amphiphilic salicylate ions. At the threshold salt concentration, the interface is nearly saturated with adsorbed salicylate ions. Predictions of the effect of salt on the micelle structure in different CG solvent models, namely, single-site standard water and three-site polarizable water, show qualitative agreement. This suggests that phase transitions in aqueous micelle solutions could be investigated by using standard CG water models which allow for 3 orders of magnitude reduction in the computational time as compared to that required for atomistic MD simulations.     

Elastic properties of the Ta–V system: bcc Ta0.33V0.67 and C15 TaV2

Resonant ultrasound spectroscopy was used to measure the elastic constants of bcc Ta_0.33V_0.67 over the temperature range 3.5–300?K; the results were compared to earlier measurements on C15 TaV₂. The temperature dependence of the polycrystalline shear modulus is completely different in the two phases; that of the bcc phase decreases with temperature whereas that of the C15 phases increases in an anomalous fashion. This difference is consistent with a model involving doubly-degenerate levels at the X point of the Brillouin zone in the C15 phase with the Fermi level lying near the doubly degenerate level. This model accounted for the unusual behaviour of the C15 phase. Debye temperatures were determined from the ultrasonic measurements: 295?K for the C15 phase and 315?K for the bcc phase.     

Confinement induced conformational changes in n-alkanes sequestered within a narrow carbon nanotube

While alkanes in solution exhibit predominantly extended conformations, nanoscale confinement of these chains within protein binding sites and synthetic receptors can significantly alter the conformer distribution. As a simple model for the effect of confinement on the conformation, we report molecular simulations of n-alkanes absorbed from a bulk solvent into narrow carbon nanotubes. We observe that confinement of butane, hexane, and tetracosane induces a trans to gauche conformational redistribution. Moreover, confined hexane and tetracosane exhibit cooperative interactions between neighboring dihedral angles, which promote a helical gauche conformation for the portions of the chain within the nanotube. Hexane absorbed into the nanotube from water or benzene exhibits essentially the same conformation regardless of the bulk solvent. The PMF between the nanotube and hexane along the central nanotube axis finds that nanotube absorption is favorable from aqueous solution but neutral from benzene. The interaction between hexane and the nanotube in water is dominated by the direct interaction between the alkane and the nanotube and weakly opposed by indirect water-mediated forces. In benzene, however, the direct alkane/nanotube interaction is effectively balanced by the indirect benzene-mediated interaction. Our simulations in water stand in difference to standard interpretations of the hydrophobic effect, which posit that the attraction between non-polar species in water is driven by their mutual insolubility.     

Binary interactions and salt-induced coalescence of spherical micelles of cationic surfactants from molecular dynamics simulations

A direct estimation of salt-mediated potential of mean force (PMF) between spherical micelles of cationic surfactants is obtained for the first time using molecular dynamics (MD) simulations. Coarse-grained (CG) potentials benchmarked in an earlier study [Langmuir, 2011, 27(11), 6628-6638] are used to model a binary system of cetyltrimethylammonium chloride (CTAC) surfactant micelles at varying concentrations of sodium chloride (NaCl) or sodium salicylate (NaSal). The shape and structure of micelles are not subject to external constraints. NaSal is significantly more efficient in screening the intermicelle repulsive interactions shown by the PMF compared to NaCl due to a stronger binding of salicylate counterions to the micelle corona. Upon contact with each other, the micelles coalesce in the presence of NaSal to form a cylindrical structure which is stabilized by the adsorbed salicylate anions. Comparison of the PMF with Derjaguin-Landau-Verwey-Overbeek (DLVO) potentials shows qualitative agreement, while the magnitude of PMF is significantly greater than that of the DLVO potentials. To understand this discrepancy, PMF is evaluated by turning off (a) long-ranged electrostatic interactions and (b) solvent polarizability. The above effects are shown to play an important role in determining the solvent-mediated and ion-correlated interactions between the two micelles, which are not explicitly captured by mean-field double layer theories such as DLVO.     

Dynamic viscoelastic properties of free radical bulk polymerizing systems under near-isothermal and non-isothermal conditions

Dynamic viscoelastic moduli of an example polymerizing system namely the free radical bulk polymerization of methyl methacrylate (MMA), is studied using a Haake® rheometer–reactor assembly and an adapted Haake® HV-DIN cup-and-bob assembly. A series of experiments on the bulk polymerization of MMA under different temperature histories (isothermal, step-increase and step-decrease) and at two different initiator [2, 2′-azoisobutyronitrile (AIBN)] concentrations, have been carried out. The data on the storage modulus, G′, the loss modulus, G″, and the phase shift, δ, are measured during the course of polymerization, well into the gel effect region. A new correlation is developed for these properties. The kinetic model and the correlation for the zero-shear viscosity, presented in our earlier studies, are used in this generalized correlation. The correlation so developed is observed to represent experimental data quite well for a variety of temperature histories. The characteristic relaxation time, τ_i, representing the bulk polymerization of MMA is observed to be larger than the Rouse value, τ_R, by a factor of about seven. The findings are observed to be in agreement with several other studies for entangled non-polymerizing systems.