3,788 research outputs found
Influence of pH on the Transport of Silver Nanoparticles in Saturated Porous Media: Laboratory Experiments and Modeling
Given the ubiquity of silver nanoparticles (AgNPs), the largest and fastest growing category of nanomaterials, and their potential for toxic effects to both humans and the environment, it is important to understand their environmental fate and transport. The purpose of this study is to gain information on the transport properties of unmodified AgNP suspensions in a glass bead-packed column under saturated flow conditions at different solution pH levels. Commercial AgNPs were characterized using high resolution transmission spectroscopy (HRTEM), dynamic light scattering (DLS) and ultraviolet (UV) visible spectroscopy. Transport data were collected at different pH levels (4, 6.5 and 9) at fixed ionic strength. Capture of AgNPs increased as the pH of the solution increased from 4 to 6.5. Further increase in pH to 9 decreased the attachment of AgNPs to the glass beads. AgNP concentration versus time breakthrough data were simulated using an advection-dispersion model incorporating both irreversible and reversible attachment. In particular, a reversible attachment model is required to simulate breakthrough curve tailing at near neutral pH, when attachment is most significant
Proteins and polymers
Proteins, chain molecules of amino acids, behave in ways which are similar to
each other yet quite distinct from standard compact polymers. We demonstrate
that the Flory theorem, derived for polymer melts, holds for compact protein
native state structures and is not incompatible with the existence of
structured building blocks such as -helices and -strands. We
present a discussion on how the notion of the thickness of a polymer chain,
besides being useful in describing a chain molecule in the continuum limit,
plays a vital role in interpolating between conventional polymer physics and
the phase of matter associated with protein structures.Comment: 7 pages, 6 figure
Universal Formulae for Percolation Thresholds
A power law is postulated for both site and bond percolation thresholds. The
formula writes , where is the space
dimension and the coordination number. All thresholds up to are found to belong to only three universality classes. For first two
classes for site dilution while for bond dilution. The last one
associated to high dimensions is characterized by for both sites and
bonds. Classes are defined by a set of value for . Deviations
from available numerical estimates at are within and
for high dimensional hypercubic expansions at . The
formula is found to be also valid for Ising critical temperatures.Comment: 11 pages, latex, 3 figures not include
Glass transition of an epoxy resin induced by temperature, pressure and chemical conversion: a configurational entropy rationale
A comparative study is reported on the dynamics of a glass-forming epoxy
resin when the glass transition is approached through different paths: cooling,
compression, and polymerization. In particular, the influence of temperature,
pressure and chemical conversion on the dynamics has been investigated by
dielectric spectroscopy. Deep similarities are found in dynamic properties. A
unified reading of our experimental results for the structural relaxation time
is given in the framework of the Adam-Gibbs theory. The quantitative agreement
with the experimental data is remarkable, joined with physical values of the
fitting parameters. In particular, the fitting function of the isothermal
tau(P) data gives a well reasonable prediction for the molar thermal expansion
of the neat system, and the fitting function of the isobaric-isothermal tau(C)
data under step- polymerization conforms to the prediction of diverging tau at
complete conversion of the system.Comment: 16 pages, 8 figures, from the talk given at the 4th International
Discussion Meeting on Relaxations in Complex Systems (IDMRCS), Hersonissos,
Helaklion, Crete (Greece), 17-23 June 200
Theoretical and numerical study of the phase diagram of patchy colloids: ordered and disordered patch arrangements
We report theoretical and numerical evaluations of the phase diagram for a
model of patchy particles. Specifically we study hard-spheres whose surface is
decorated by a small number f of identical sites ("sticky spots'') interacting
via a short-range square-well attraction. We theoretically evaluate, solving
the Wertheim theory, the location of the critical point and the gas-liquid
coexistence line for several values of f and compare them to results of Gibbs
and Grand Canonical Monte Carlo simulations. We study both ordered and
disordered arrangements of the sites on the hard-sphere surface and confirm
that patchiness has a strong effect on the phase diagram: the gas-liquid
coexistence region in the temperature-density plane is significantly reduced as
f decreases. We also theoretically evaluate the locus of specific heat maxima
and the percolation line.Comment: preprint, 32 pages, 6 figures, 3 tables, J. Chem. Phys. in pres
The Administration of Senior Design Projects in a Distance Learning Environment
A method for administering a senior level capstone design course in Electrical Engineering Technology in a distance learning environment is described. Several avenues are explored that help the students successfully conceive, develop, and present their design projects from off- campus locations that are consistent with the requirements placed upon their on-campus peers. Several problems that are unique to offering a senior project course in a distance learning environment are explored and solutions are described
Absorption/Expulsion of Oligomers and Linear Macromolecules in a Polymer Brush
The absorption of free linear chains in a polymer brush was studied with
respect to chain size and compatibility with the brush by means of
Monte Carlo (MC) simulations and Density Functional Theory (DFT) /
Self-Consistent Field Theory (SCFT) at both moderate, , and
high, , grafting densities using a bead-spring model.
Different concentrations of the free chains are
examined. Contrary to the case of when all species are almost
completely ejected by the polymer brush irrespective of their length , for
we find that the degree of absorption (absorbed amount)
undergoes a sharp crossover from weak to strong () absorption,
discriminating between oligomers, , and longer chains. For a
moderately dense brush, , the longer species, ,
populate predominantly the deep inner part of the brush whereas in a dense
brush they penetrate into the "fluffy" tail of the dense
brush only. Gyration radius and end-to-end distance of absorbed
chains thereby scale with length as free polymers in the bulk. Using both
MC and DFT/SCFT methods for brushes of different chain length , we demonstrate the existence of unique {\em critical} value of
compatibility . For the energy of free
chains attains the {\em same} value, irrespective of length whereas the
entropy of free chain displays a pronounced minimum. At all density
profiles of absorbing chains with different intersect at the same distance
from the grafting plane. The penetration/expulsion kinetics of free chains into
the polymer brush after an instantaneous change in their compatibility
displays a rather rich behavior. We find three distinct regimes of penetration
kinetics of free chains regarding the length : I (), II (), and III (), in which the time of absorption grows with
at a different rate. During the initial stages of penetration into the
brush one observes a power-law increase of with power
whereby penetration of the free chains into the
brush gets {\em slower} as their concentration rises
What thermodynamic features characterize good and bad folders? Results from a simplified off-lattice protein model
The thermodynamics of the small SH3 protein domain is studied by means of a
simplified model where each bead-like amino acid interacts with the others
through a contact potential controlled by a 20x20 random matrix. Good folding
sequences, characterized by a low native energy, display three main
thermodynamical phases, namely a coil-like phase, an unfolded globule and a
folded phase (plus other two phases, namely frozen and random coil, populated
only at extremes temperatures). Interestingly, the unfolded globule has some
regions already structured. Poorly designed sequences, on the other hand,
display a wide transition from the random coil to a frozen state. The
comparison with the analytic theory of heteropolymers is discussed
Density matrix renormalisation group study of the correlation function of the bilinear-biquadratic spin-1 chain
Using the recently developed density matrix renormalization group approach,
we study the correlation function of the spin-1 chain with quadratic and
biquadratic interactions. This allows us to define and calculate the
periodicity of the ground state which differs markedly from that in the
classical analogue. Combining our results with other studies, we predict three
phases in the region where the quadratic and biquadratic terms are both
positive.Comment: 13 pages, Standard Latex File + 5 PostScript figures in separate (New
version with SUBSTANTIAL REVISIONS to appear in J Phys A
Elastic energy of proteins and the stages of protein folding
We propose a universal elastic energy for proteins, which depends only on the
radius of gyration and the residue number . It is constructed using
physical arguments based on the hydrophobic effect and hydrogen bonding.
Adjustable parameters are fitted to data from the computer simulation of the
folding of a set of proteins using the CSAW (conditioned self-avoiding walk)
model. The elastic energy gives rise to scaling relations of the form
in different regions. It shows three folding stages
characterized by the progression with exponents , which we
identify as the unfolded stage, pre-globule, and molten globule, respectively.
The pre-globule goes over to the molten globule via a break in behavior akin to
a first-order phase transition, which is initiated by a sudden acceleration of
hydrogen bonding
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