8,042 research outputs found
Open chain pseudopeptides as hydrogelators with reversible and dynamic responsiveness to pH, temperature and sonication as vehicles for controlled drug delivery
A new family of open chain-pseudopeptidic compounds displaying a pendant carboxylic group have been prepared with excellent yields. Their self-assembly has been studied under different conditions and in different media. Some of the compounds obtained have revealed to act as very efficient hydrogelators at low concentrations (CGC 1 mg mL−1). The resulting hydrogels show some interesting properties, including a high thermal stability, with the hydrogels maintaining their structure at temperatures above 65 °C, and their reversible dynamic sol–gel behavior being responsive to thermal and sonochemical inputs and to changes in the basic/acidic properties of the medium. Preliminary studies for controlled drug delivery have been carried out using a Franz Cell and employing a skin pig membrane, confirming that these Low Molecular Weight Gelators (LMWGs) can be appropriate vehicles for the controlled transdermal delivery of small-molecule drugs.Funding for open access charge: CRUE-Universitat Jaume
Time Scales for transitions between free energy minima of a hard sphere system
Time scales associated with activated transitions between glassy metastable
states of a free energy functional appropriate for a dense hard sphere system
are calculated by using a new Monte Carlo method for the local density
variables. We calculate the time the system,initially placed in a shallow
glassy minimum of the free energy, spends in the neighborhood of this minimum
before making a transition to the basin of attarction of another free energy
minimum. This time scale is found to increase with the average density. We find
a crossover density near which this time scale increases very sharply and
becomes longer than the longest times accessible in our simulation. This scale
shows no evidence of dependence on sample size.Comment: 25 pages, Revtex, 6 postscript figures. Will appear in Phys Rev E,
March 1996 or s
On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits
(abriged) Stellar clusters with the same general physical properties (e.g.,
total mass, age, and star-formation mode) may have very different stellar mass
spectra due to the incomplete sampling of the underlying mass function; such
differences are especially relevant in the high-mass tail due to the smaller
absolute number of massive stars. The dispersion in the number of massive stars
also produces a dispersion in the properties of the corresponding ionising
spectra. In this paper, we lay the bases for the future analysis of this effect
by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an
important consequence, we found that the intensities of synthetic fluxes at
different ionisation edges are strongly correlated, a fact suggesting that no
additional dispersion will result from the inclusion of sampling effects in the
analysis of diagnostic diagrams; this is true for HII regions on all scales.
Additionally, we find convincing suggestions that the He II lines are strongly
affected by sampling, and so cannot be used to constrain the evolutionary
status of stellar clusters. We also establish the range of applicability of
synthesis models set by the Lowest Luminosity Limit for the ionising flux, that
is the lowest limit in cluster mass for which synthesis models can be applied
to predict ionising spectra. This limit marks the boundary between the
situations in which the ionising flux is better modeled with a single star as
opposed to a star cluster; this boundary depends on the metallicity and age,
ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models
should not be used to try to account for the properties of clusters with
smaller masses.Comment: Replaced with accepted versio
Nonlinear Hydrodynamics of a Hard Sphere Fluid Near the Glass Transition
We conduct a numerical study of the dynamic behavior of a dense hard sphere
fluid by deriving and integrating a set of Langevin equations. The statics of
the system is described by a free energy functional of the
Ramakrishnan-Yussouff form. We find that the system exhibits glassy behavior as
evidenced through stretched exponential decay and two-stage relaxation of the
density correlation function. The characteristic times grow with increasing
density according to the Vogel-Fulcher law. The wavenumber dependence of the
kinetics is extensively explored. The connection of our results with
experiment, mode coupling theory, and molecular dynamics results is discussed.Comment: 34 Pages, Plain TeX, 12 PostScript Figures (not included, available
on request
Resgate de germoplasma de espécies de gramíneas de elite da flora forrageira dos estados do Rio Grande do Sul e Santa Catarina
Supramolecularly assisted synthesis of chiral tripodal imidazolium compounds
A strong preference for the formation of tripodal systems over the related monotopic and ditopic compounds is observed for the reaction between tris(halomethyl)benzenes and imidazoles derived from
amino acids and containing an amide fragment. This preference allows the formation of the tripodal
derivative as the major product even when an equimolar mixture of the tris(halomethyl)benzene and the
imidazole is reacted (1 : 1 ratio instead of the stoichiometric 1 : 3 ratio). The reactions were monitored
using 1
H NMR spectroscopy and ESI mass spectrometry and kinetically characterized. Computational
studies were also performed in order to rationalize the observed preference of the tri-substituted
product. The results reveal the existence of well-defined supramolecular interactions between the imidazolium groups and the reacting imidazoles that facilitate the formation of the multitopic systems once the
first imidazolium group is formed. Analysis of the different structural components shows that the presence
of the amide group from the amino acid moiety is the key structural requirement for such supramolecular
assistance to take place. The preorganization of the supramolecular intermediates formed through hydrogen bonding interactions involving amide-NH fragments in imidazoles and bromide anions in imidazolium
groups seems to be also present at the corresponding TSs, decreasing the associated energy barriers
Entropic Origin of the Growth of Relaxation Times in Simple Glassy Liquids
Transitions between ``glassy'' local minima of a model free-energy functional
for a dense hard-sphere system are studied numerically using a
``microcanonical'' Monte Carlo method that enables us to obtain the transition
probability as a function of the free energy and the Monte Carlo ``time''. The
growth of the height of the effective free energy barrier with density is found
to be consistent with a Vogel-Fulcher law. The dependence of the transition
probability on time indicates that this growth is primarily due to entropic
effects arising from the difficulty of finding low-free-energy saddle points
connecting glassy minima.Comment: Four pages, plus three postscript figure
Proceedings of the First Annual Workshop of the HORIZON 2020 CEBAMA Project (KIT Scientific Reports ; 7734)
The Proceedings of the First Annual Workshop of the Collaborative Project CEBAMA addresses key scientific questions related to the use of cement-based materials in nuclear waste disposal applications. Progress beyond the state-of-the-art is achieved by providing basic knowledge, new experimental data, improved modeling and arguments for the Nuclear Waste Disposal Safety Case. CEBAMA is funded by the European Commission under the Horizon 2020 frame of EURATOM
Renormalization Group Study of the Intrinsic Finite Size Effect in 2D Superconductors
Vortices in a thin-film superconductor interact logarithmically out to a
distance on the order of the two-dimensional (2D) magnetic penetration depth
, at which point the interaction approaches a constant. Thus,
because of the finite , the system exhibits what amounts to an
{\it intrinsic} finite size effect. It is not described by the 2D Coulomb gas
but rather by the 2D Yukawa gas (2DYG). To study the critical behavior of the
2DYG, we map the 2DYG to the massive sine-Gordon model and then perform a
renormalization group study to derive the recursion relations and to verify
that is a relevant parameter. We solve the recursion relations
to study important physical quantities for this system including the
renormalized stiffness constant and the correlation length. We also address the
effect of current on this system to explain why finite size effects are not
more prevalent in experiments given that the 2D magnetic penetration depth is a
relevant parameter.Comment: 8 pages inRevTex, 5 embedded EPS figure
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