542 research outputs found
On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water
By means of atomistic molecular dynamics simulations we investigate the
behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures
below and above the lower critical solution temperature (LCST), including the
undercooled regime. The transition between water soluble and insoluble states
at the LCST is described as a cooperative process involving an intramolecular
coil-to-globule transition preceding the aggregation of chains and the polymer
precipitation. In this work we investigate the molecular origin of such
cooperativity and the evolution of the hydration pattern in the undercooled
polymer solution. The solution behaviour of an atactic 30-mer at high dilution
is studied in the temperature interval from 243 to 323 K with a favourable
comparison to available experimental data. In the PNIPAM water soluble states
we detect a correlation between polymer segmental dynamics and diffusion motion
of bound water, occurring with the same activation energy. Simulation results
show that below the coil-to-globule transition temperature PNIPAM is surrounded
by a network of hydrogen bonded water molecules and that the cooperativity
arises from the structuring of water clusters in proximity to hydrophobic
groups. Differently, the perturbation of the hydrogen bond pattern involving
water and amide groups occurs above the transition temperature. Altogether
these findings reveal that even above the LCST PNIPAM remains largely hydrated
and that the coil-to-globule transition is related with a significant
rearrangement of the solvent in proximity of the surface of the polymer. The
comparison between the hydrogen bonding of water in the surrounding of PNIPAM
isopropyl groups and in bulk displays a decreased structuring of solvent at the
hydrophobic polymer-water interface across the transition temperature, as
expected because of the topological extension along the chain of such
interface
Mode-coupling theory predictions for a limited valency attractive square-well model
Recently we have studied, using numerical simulations, a limited valency
model, i.e. an attractive square well model with a constraint on the maximum
number of bonded neighbors. Studying a large region of temperatures and
packing fractions , we have estimated the location of the liquid-gas
phase separation spinodal and the loci of dynamic arrest, where the system is
trapped in a disordered non-ergodic state. Two distinct arrest lines for the
system are present in the system: a {\it (repulsive) glass} line at high
packing fraction, and a {\it gel} line at low and . The former is
essentially vertical (-controlled), while the latter is rather horizontal
(-controlled) in the plane. We here complement the molecular
dynamics results with mode coupling theory calculations, using the numerical
structure factors as input. We find that the theory predicts a repulsive glass
line -- in satisfactory agreement with the simulation results -- and an
attractive glass line which appears to be unrelated to the gel line.Comment: 12 pages, 6 figures. To appear in J. Phys. Condens. Matter, special
issue: "Topics in Application of Scattering Methods for Investigation of
Structure and Dynamics of Soft Condensed Matter", Fiesole, November 200
A closer look at arrested spinodal decomposition in protein solutions
Concentrated aqueous solutions of the protein lysozyme undergo a liquid solid
transition upon a temperature quench into the unstable spinodal region below a
characteristic arrest temperature of Tf=15C. We use video microscopy and
ultra-small angle light scattering in order to investigate the arrested
structures as a function of initial concentration, quench temperature and rate
of the temperature quench. We find that the solid-like samples show all the
features of a bicontinuous network that is formed through an arrested spinodal
decomposition process. We determine the correlation length Xi and demonstrate
that Xi exhibits a temperature dependence that closely follows the critical
scaling expected for density fluctuations during the early stages of spinodal
decomposition. These findings are in agreement with an arrest scenario based on
a state diagram where the arrest or gel line extends far into the unstable
region below the spinodal line. Arrest then occurs when during the early stage
of spinodal decomposition the volume fraction phi2 of the dense phase
intersects the dynamical arrest threshold phi2Glass, upon which phase
separation gets pinned into a space-spanning gel network with a characteristic
length Xi
Numerical study of the glass-glass transition in short-ranged attractive colloids
We report extensive numerical simulations in the {\it glass} region for a
simple model of short-ranged attractive colloids, the square well model. We
investigate the behavior of the density autocorrelation function and of the
static structure factor in the region of temperatures and packing fractions
where a glass-glass transition is expected according to theoretical
predictions. We strengthen our observations by studying both waiting time and
history dependence of the numerical results. We provide evidence supporting the
possibility that activated bond-breaking processes destabilize the attractive
glass, preventing the full observation of a sharp glass-glass kinetic
transition.Comment: 15 pages, 9 figures; Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted to J. Phys.: Condens. Matt.
Interplay between Spinodal Decomposition and Glass Formation in Proteins Exhibiting Short-Range Attractions
We investigate the competition between spinodal decomposition and dynamical
arrest using aqueous solutions of the globular protein lysozyme as a model
system for colloids with short-range attractions. We show that quenches below a
temperature Ta lead to gel formation as a result of a local arrest of the
proteindense phase during spinodal decomposition. The rheological properties of
these gels allow us to use centrifugation experiments to determine the local
densities of both phases and to precisely locate the gel boundary and the
attractive glass line close to and within the unstable region of the phase
diagram
The Development of an Ethical Strategy for Managers of International Hotels in Third World Countries
This article discusses some of the dilemmas of conscience that greet the international hotel manager in the Third World. It offers realistic and ethical guidelines for decision-making and problem-solving. Some of these guidelines require only common sense and good will to implement; others require a great deal of creativity, sensitivity, effort, and care; and a few will demand the courage to stand up for what is right in the face of competitive pressure
Aging in short-ranged attractive colloids: A numerical study
We study the aging dynamics in a model for dense simple liquids, in which
particles interact through a hard-core repulsion complemented by a short-ranged
attractive potential, of the kind found in colloidal suspensions. In this
system, at large packing fractions, kinetically arrested disordered states can
be created both on cooling (attractive glass) and on heating (repulsive glass).
The possibility of having two distinct glasses, at the same packing fraction,
with two different dynamics offers the unique possibility of comparing --
within the same model -- the differences in aging dynamics. We find that, while
the aging dynamics of the repulsive glass is similar to the one observed in
atomic and molecular systems, the aging dynamics of the attractive glass shows
novel unexpected features.Comment: 8 pages, 11 figures, submited to Journal of Chemical Physic
Numerical modelling of non-ionic microgels: an overview
Microgels are complex macromolecules. These colloid-sized polymer networks
possess internal degrees of freedom and, depending on the polymer(s) they are
made of, can acquire a responsiveness to variations of the environment
(temperature, pH, salt concentration, etc.). Besides being valuable for many
practical applications, microgels are also extremely important to tackle
fundamental physics problems. As a result, these last years have seen a rapid
development of protocols for the synthesis of microgels, and more and more
research has been devoted to the investigation of their bulk properties.
However, from a numerical standpoint the picture is more fragmented, as the
inherently multi-scale nature of microgels, whose bulk behaviour crucially
depends on the microscopic details, cannot be handled at a single level of
coarse-graining. Here we present an overview of the methods and models that
have been proposed to describe non-ionic microgels at different length-scales,
from the atomistic to the single-particle level. We especially focus on
monomer-resolved models, as these have the right level of details to capture
the most important properties of microgels, responsiveness and softness. We
suggest that these microscopic descriptions, if realistic enough, can be
employed as starting points to develop the more coarse-grained representations
required to investigate the behaviour of bulk suspensions
Take the Con Out of Consulting in the Food Service Industry
Consultants can help a food service operator with almost any problem which needs solving. Howeve6 the manager must manage the consultant. The author offers a design for planning for hiring and evaluating the work of anyone given the job of analyzing existing systems and diagnosing problems
Mode-Coupling Theory of Colloids with Short-range Attractions
Within the framework of the mode-coupling theory of super-cooled liquids, we
investigate new phenomena in colloidal systems on approach to their glass
transitions. When the inter-particle potential contains an attractive part,
besides the usual repulsive hard core, two intersecting liquid-glass transition
lines appear, one of which extends to low densities, while the other one, at
high densities, shows a re-entrant behaviour. In the glassy region a new type
of transition appears between two different types of glasses. The complex
phenomenology can be described in terms of higher order glass transition
singularities. The various glass phases are characterised by means of their
viscoelastic properties. The glass driven by attractions has been associated to
particle gels, and the other glass is the well known repulsive colloidal glass.
These correspondences, in associations with the new predictions of glassy
behaviour mean that such phenomena may be expected in colloidal systems with,
for example, strong depletion or other short-ranged attractive potentials.Comment: 17 pages, 8 figure
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