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See detailFrom Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions
Philipp, Martine UL; Kyriakos, Konstantinos; Silvi, Luca et al

in Journal of Physical Chemistry B (2014), 118

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The ... [more ▼]

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H2O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions. [less ▲]

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See detailNonequilibrium Thermodynamics and Nose-Hoover Dynamics
Esposito, Massimiliano UL; Monnai, Takaaki

in Journal of Physical Chemistry B (2011), 115(18),

We show that systems driven by an external force and described by Nose-Hoover dynamics allow for a consistent nonequilibrium thermodynamics description when the thermostatted variable is initially assumed ... [more ▼]

We show that systems driven by an external force and described by Nose-Hoover dynamics allow for a consistent nonequilibrium thermodynamics description when the thermostatted variable is initially assumed in a state of canonical equilibrium. By treating the “real” variables as the system and the thermostatted variable as the reservoir, we establish the first and second law of thermodynamics. As for Hamiltonian systems, the entropy production can be expressed as a relative entropy measuring the system-reservoir correlations established during the dynamics. [less ▲]

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See detailTrans-Interfacial Polymerization and Matter Transport Processes in Epoxy-Alumina Nanocomposites Visualized By Scanning Brillouin Microscopy
Sanctuary, Roland UL; Philipp, Martine UL; Kieffer, J. et al

in Journal of Physical Chemistry B (2010), 114(25), 8396-8404

The structural developments in the vicinity of the interface between the reactants of an epoxy are investigated using time- and space-resolved scanning Brillouin microscopy. The hypersonic profile across ... [more ▼]

The structural developments in the vicinity of the interface between the reactants of an epoxy are investigated using time- and space-resolved scanning Brillouin microscopy. The hypersonic profile across the phase boundary evolves with strong spatial asymmetry and exhibits erratic behavior within the resin-rich region, which is attributed to a complex interplay between matter transport, dissolution, polymerization, and molecular unravelling process. The presence of alumina nanoparticles in the resin changes the character of these matter transport and reaction processes significantly. On the one hand, the nanoparticles act as transport barriers, hindering the mixing of the reactive components; on the other hand they seem to have a catalytic influence on the epoxy polymerization under certain circumstances. Their transport against gravity is tentatively attributed to gradients in surface tension. [less ▲]

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See detailAtomistic Simulation of Water Percolation and Proton Hopping in Nation Fuel Cell Membrane
Devanathan, Ram; Venkatnathan, Arun; Rousseau, Roger et al

in Journal of Physical Chemistry B (2010), 114(43), 13681-13690

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration ... [more ▼]

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H(2)O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (lambda) between 5 and 6 H(2)O/SO(3)(-) as the percolation threshold of Nation. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the lambda value is increased from 5 to 15. The proton diffusion coefficient in Nation at a lambda value of 15 is about 1.1 x 10(-5) cm(2)/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity. [less ▲]

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See detailMixing Behavior and Interphase Formation in the Diethylene Triamine-Water System Studied by Optical Imaging and Spatially Resolved Brillouin Scattering
Philipp, Martine UL; Collette, Florimond; Veith, Michael et al

in Journal of Physical Chemistry B (2009), 113(38), 12655-12662

The injection of water beneath liquid diethylene triamine in a glass cuvette leads to an unexpected phase evolution behavior of the two liquids. The space and time dependent developments of the molecular ... [more ▼]

The injection of water beneath liquid diethylene triamine in a glass cuvette leads to an unexpected phase evolution behavior of the two liquids. The space and time dependent developments of the molecular structure and the underlying transport associated with mixing of the two liquids are monitored by optical imaging and scanning Brillouin microscopy. Apparently, results obtained by either experimental technique lead to disparate interpretations. Whereas optical imaging suggests the existence of a two phase structure, which disappears within a few hours, acoustic microscopy indicates the evolution of a more gradually evolving and longer-lived three phase structure. According to molecular acoustics, the transport of diethylene triamine into water and vice versa behaves strongly asymmetric in time. An attempt is made to reconcile the observed optical and acoustic manifestations of the mixing process on the basis of molecular complex formation. [less ▲]

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See detailElectrolyte effects on the stability of nematic and lamellar lyotropic liquid crystal phases – colligative and ion-specific aspects
Dawin, Ute; Lagerwall, Jan UL; Giesselmann, Frank

in Journal of Physical Chemistry B (2009), 113(33), 11414-11420

We investigated the electrolyte effects on the stability of nematic and lamellar lyotropic liquid crystalline (LLC) phases formed by the simple anionic surfactant cesium pentadecafluorooctanoate (CsPFO ... [more ▼]

We investigated the electrolyte effects on the stability of nematic and lamellar lyotropic liquid crystalline (LLC) phases formed by the simple anionic surfactant cesium pentadecafluorooctanoate (CsPFO) in water. To the lyotropic guest phase, at the constant CsPFO-mass fraction of 0.55, the series of electrolytes LiCl, NaCl, KCl, CsCl, CsI, and Cs2SO4, respectively, was added at concentrations ranging from 0.5 to 2.5 mol %. With increasing electrolyte concentration two substantially different effects were observed. At low concentrations all added electrolytes caused an increase of the thermal stability of the LLC phases, favoring the lamellar phase over the nematic phase. This behavior is, at least qualitatively, understood within the packing parameter model. The extent of the stabilization clearly depends on the chemical nature of the added cation. For a given cation, however, the effect is colligative, i.e., independent of the chemical nature of the added anion. At higher salt concentrations a salting-out-like phase separation was induced. This effect is clearly ion-specific as the salting-out concentration varied for each cation following the order of the Hofmeister series for cations. [less ▲]

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See detailAre solvation free energies of homogeneous helical peptides additive?
Staritzbichler, R.; Gu, Wei UL; Helms, V.

in Journal of Physical Chemistry B (2005), 109(40), 19000-19007

We investigated the additivity of the solvation free energy of amino acids in homogeneous helices of different length in water and in chloroform. Solvation free energies were computed by ... [more ▼]

We investigated the additivity of the solvation free energy of amino acids in homogeneous helices of different length in water and in chloroform. Solvation free energies were computed by multiconfiguration thermodynamic integration involving extended molecular dynamics simulations and by applying the generalized-born surface area solvation model to static helix geometries. The investigation focused on homogeneous peptides composed of uncharged amino acids, where the backbone atoms are kept fixed in an ideal helical conformation. We found nonlinearity especially for short peptides, which does not allow a simple treatment of the interaction of amino acids with their surroundings. For homogeneous peptides longer than five residues, the results from both methods are in quite good agreement and solvation energies are to a good extent additive. [less ▲]

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See detailUnequal-sphere packing model for simulation of the uniaxially compressed iodine adlayer on Au(111)
Tkatchenko, Alexandre UL; Batina, Nikola

in JOURNAL OF PHYSICAL CHEMISTRY B (2005), 109(46), 21710-21715

A simple unequal-sphere packing (USP) model, based on pure geometrical principles, was applied to study the centered-rectangular iodine c(p x root 3)R30 degrees adlayer on the Au(111) surface, well-known ... [more ▼]

A simple unequal-sphere packing (USP) model, based on pure geometrical principles, was applied to study the centered-rectangular iodine c(p x root 3)R30 degrees adlayer on the Au(111) surface, well-known from surface X-ray structure (SXS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) experiments. To reproduce the exact patterns observed in experiments, two selective conditions-minimum average adsorbate height and minimum adlayer roughness-were imposed. As a result, a series of adlayer patterns with c(p x root 3)R30 degrees symmetry (2.3 < p < 3), with precise structural details, including atomic registry and identification of the p-bisector as the most likely trajectory for the iodine adatom movement during the so-called uniaxial compression phenomenon, were identified. In addition, using the same model, the difference between the iodine adlayer arranged in hexagonal and centered-rectangular c(p x root 3)R30 degrees patterns, as in the case of Pt(111) and Au(111) surfaces, was investigated. Qualitative and quantitative comparison shows that iodine adatoms in these two arrangements differ significantly in atomic registry, distance from the substrate, and the adlayer corrugation. Our findings could be of special interest in the study of the nature of the iodine adatom bonding to different substrates (i.e., Au vs Pt). [less ▲]

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See detailSolvation free energies and transfer free energies for amino acids from hydrophobic solution to water solution from a very simple residue model
Gu, Wei UL; Rahi, S. J.; Helms, V.

in Journal of Physical Chemistry B (2004), 108(18), 5806-5814

Solvation free energies of neutral amino acids in water and in chloroform were computed from extensive molecular dynamics simulations using multiconfiguration thermodynamic integration. The values ... [more ▼]

Solvation free energies of neutral amino acids in water and in chloroform were computed from extensive molecular dynamics simulations using multiconfiguration thermodynamic integration. The values computed for the AMBER force field are in very good agreement with available experimental data (rms differences of 5.1 kJ mol(-1) for the solvation free energies and 6.4 kJ mol(-1) for the transfer free energies of amino acids between water and chloroform) and with existing calculations. We derived an additive residue-scale solvation model formulated as the sum of a nonpolar term that is proportional to the molecular surface area and an electrostatic term (Kirkwood-Onsager model) for the hydration free energy of a dipole in a solvated cavity. This model can surprisingly well describe the solvation free energies in water and chloroform as well as the transfer free energies of amino acids between the two solvents when suitably adapted cavity radii are used. Root-mean-square differences of the predicted values with respect to the values calculated from thermodynamic integration are 1.8, 5.9, and 7.7 kJ mol(-1), respectively. [less ▲]

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