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Oak Ridge, TN, United States

Kalyuzhnyi Y.V.,Institute for Condensed Matter Physics | Cummings P.T.,Oak Ridge National Laboratory | Cummings P.T.,Center for Nanophase Material science
Journal of Chemical Physics

We propose a second-order thermodynamic perturbation theory for a hard-sphere patchy colloidal model with two doubly bondable patches of type A and B. AB bonding results in the formation of a three-dimensional network of the particles and AA and BB bonding promotes chain formation. The theory is applied to study the phase behaviour of the model at different values of the potential model parameters. Competition between network and chain formation gives rise to a re-entrant phase behaviour with upper and lower critical points. The model with an additional van der Waals type of interaction may have a re-entrant phase diagram with three critical points and two separate regions of the liquid-gas phase coexistence. We analyze our results in terms of the fractions of the particles in different bonding states and conclude that re-entrant phase coexistence can be seen as a coexistence between a gas phase rich in chain ends and a liquid phase rich in branch points. © 2013 AIP Publishing LLC. Source

Goswami M.,Center for Nanophase Material science | Borreguero J.M.,Oak Ridge National Laboratory | Pincus P.A.,University of California at Santa Barbara | Sumpter B.G.,Center for Nanophase Material science

Self-assembly and dynamics of a polyelectrolyte (PE)-surfactant complex (PES) are investigated using molecular dynamics simulations. The complexation is systematically studied for five different PE backbone charge densities. At a fixed surfactant concentration the PES complexation exhibits pearl necklace to agglomerated double spherical structures with a PE chain decorating the surfactant micelles. The counterions do not condense on the complex but are released in the medium with a random distribution. The relaxation dynamics for three different length scales, the polymer chain, segmental, and monomer, show distinct features of the charge and neutral species; the counterions are fastest followed by the PE chain and surfactants. The surfactant heads and tails have the slowest relaxation due to their restricted movement inside the agglomerated structure. At the shortest length scale, all the charge and neutral species show similar relaxation dynamics confirming Rouse behavior at monomer length scales. Overall, the present study highlights the structure-property relationship for polymer-surfactant complexation. These results help improve the understanding of PES complexes and should aid in the design of better materials for future applications. © 2015 American Chemical Society. Source

Chi M.,Center for Nanophase Material science | Veith G.M.,Oak Ridge National Laboratory | Gallego N.C.,Oak Ridge National Laboratory | Dai S.,University of Tennessee at Knoxville
Journal of the American Chemical Society

Here we describe a lab-in-a-shell strategy for the preparation of multifunctional core-shell nanospheres consisting of a core of metal clusters and an outer microporous silica shell. Various metal clusters (e.g., Pd and Pt) were encapsulated and confined in the void space mediated by the entrapped polymer dots inside hollow silica nanospheres acting first as complexing agent for metal ions and additionally as encapsulator for clusters, limiting growth and suppressing the sintering. The Pd clusters encapsulated in hybrid core-shell structures exhibit exceptional size-selective catalysis in allylic oxidations of substrates with the same reactive site but different molecular size (cyclohexene ∼0.5 nm, cholesteryl acetate ∼1.91 nm). The solvent-free aerobic oxidation of diverse hydrocarbons and alcohols was further carried out to illustrate the benefits of such an architecture in catalysis. High activity, outstanding thermal stability and good recyclability were observed over the core-shell nanocatalyst. © 2014 American Chemical Society. Source

Bauer J.C.,Oak Ridge National Laboratory | Mullins D.R.,Oak Ridge National Laboratory | Oyola Y.,Oak Ridge National Laboratory | Overbury S.H.,Oak Ridge National Laboratory | And 4 more authors.
Catalysis Letters

Supported AuCu and AuCuPd catalysts were synthesized through the diffusion of Pd and Cu into Au nanoparticle seeds. When supported on SiO2, the AuCuPd nanoparticles were found to be the most active for the oxidation of CO after being exposed to reductive pretreatment conditions as opposed to oxidative pretreatment conditions. In contrast, AuCu/SiO2 was found to be more active for CO oxidation after the alloy phase was segregated into a Au-CuO x heterostructure. In situ XRD and EXAFS were used to monitor the structural changes of AuCu and AuCuPd catalysts as they were subjected to different pretreatment conditions. Graphical Abstract: [Figure not available: see fulltext.] © 2013 Springer Science+Business Media New York. Source

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