30 research outputs found
Image charges in spherical geometry: Application to colloidal systems
The effects of image charges (i.e., induced surface charges of polarization)
in spherical geometry and their implication for charged colloidal systems are
investigated. We study analytically and exactly a single microion interacting
with a dielectric sphere and discuss the similarities and discrepancies with
the case of a planar interface. By means of extensive Monte Carlo (MC)
simulations, we study within the framework of the primitive model the effects
of image charges on the structure of the electrical double layer. Salt-free
environment as well as salty solutions are considered. A remarkable finding of
this study is that the position of the maximum in the counterion density
(appearing at moderately surface charge density) remains quasi-identical,
regardless of the counterion valence and the salt content, to that obtained
within the \textit{single}-counterion system.Comment: 14 pages, 12 figures, RevTeX4, to appear in J. Chem. Phy
Adsorption of Oppositely Charged Polyelectrolytes onto a Charged Rod
The adsorption of highly charged flexible polycations and polyanions on a
charged cylindrical substrate is investigated by means of Monte Carlo (MC)
simulations. A detailed structural study, including monomer and fluid charge
distributions, is provided. The influence of a short range attraction between
the polycations and the negatively charged substrate is also considered. We
demonstrate that the building up of multilayer structures is highly prohibited
mainly due to the high entropy penalty stemming from the low dimensionality of
the substrate at strong curvature.Comment: 18 pages - 7 (main) EPS figs - revtex4 - to appear in J. Chem. Phy
Like-Charge Colloid-Polyelectrolyte Complexation
We investigate the complexation of a highly charged sphere with a long
flexible polyelectrolyte, \textit{both negatively charged} in salt free
environment. Electroneutrality is insured by the presence of divalent
counterions. Using molecular dynamics (MD) within the framework of the
primitive model, we consider different Coulomb coupling regimes. At strong
Coulomb coupling we find that the adsorbed chain is always confined to the
colloidal surface but forms different conformations that depend on the linear
charge density of the chain. A mechanism involving the polyelectrolyte
\textit{overcharging} is proposed to explain these structures. At intermediate
Coulomb coupling, the chain conformation starts to become three-dimensional,
and we observe multilayering of the highly charged chain while for lower charge
density the chain wraps around the colloid. At weak Coulomb coupling,
corresponding to an aqueous solvent, we still find like-charge complexation. In
this latter case the chain conformation exhibits loops.Comment: 18 pages, 13 (main) eps figures, RevTeX4, submitted to J. Chem. Phy
Effect of colloidal charge discretization in the primitive model
The effect of fixed discrete colloidal charges in the primitive model is
investigated for spherical macroions. Instead of considering a central bare
charge, as it is traditionally done, we distribute \textit{discrete} charges
randomly on the sphere. We use molecular dynamics simulations to study this
effect on various properties such as overcharging, counterion distribution and
diffusion. In the vicinity of the colloid surface the electrostatic potential
may considerably differ from the one obtained with a central charge. In the
strong Coulomb coupling, we showed that the colloidal charge discretization
qualitatively influences the counterion distribution and leads to a strong
colloidal charge-counterion pair association. However, we found that
\textit{charge inversion} still persists even if strong pair association is
observed.Comment: 16 pages, 16 ps figures, REVTEX, accepted for publication in EPJ
Polyelectrolyte Multilayering on a Charged Sphere
The adsorption of highly \textit{oppositely} charged flexible
polyelectrolytes onto a charged spherical surface is investigated by means of
Monte Carlo simulations in a fashion which resembles the layer-by-layer
deposition technique introduced by Decher. Electroneutrality is insured at each
step by the presence of monovalent counterions (anions and cations). We study
in detail the structure of the \textit{equilibrium} complex. Our investigations
of the first few layer formations strongly suggest that multilayering in
spherical geometry is not possible as an equilibrium process with purely
electrostatic interactions. We especially focus on the influence of specific
(non-electrostatic) short range attractive interactions (e.g., Van der Waals)
on the stability of the multilayers.Comment: 24 Pages - 15 (main) Figs - RevTeX 4 - published versio
Monte Carlo simulation of abnormal grain growth in two dimensions
Abnormal grain growth in the presence of second phase particles is
investigated with the help of a two-dimensional Monte Carlo simulation. An
aggregate of equiaxed grains is considered with constant grain boundary energy
and mobility. The only driving force accounted for stems from the grain
boundary curvature. The process of abnormal grain growth is investigated as a
function of two governing parameters, the initial degree of pinning of the
matrix grains by the particles and the initial size advantage of the anomalous
grain. In such conditions, moderate growth is obtained whose specific features
are discussed with respect to the available models. It is shown that it is
possible to obtain drastic grain growth by introducing the thermally activated
unpinning of grain boundaries from particles. For this purpose, a simplified
but effective procedure is proposed and discussed that includes the influence
of the capillary force on the height of the local energy barrier for grain
unpinning.Comment: 16 pages, 15 eps figures, Latex. Accepted for publication im Mat.
Sci. Eng.
Packing confined hard spheres denser with adaptive prism phases
[EN] We show that hard spheres confined between two parallel hard plates pack denser with periodic adaptive prismatic structures which are composed of alternating prisms of spheres. The internal structure of the prisms adapts to the slit height which results in close packings for a range of plate separations, just above the distance where three intersecting square layers fit exactly between the plates. The adaptive prism phases are also observed in real-space experiments on confined sterically stabilized colloids and in Monte Carlo simulations at finite pressure. © 2012 American Physical Society.We thank Elvira Bonet, Moises Garin, and Kevin Mutch for helpful discussions. This work was partially supported by the DFG within the SFB TR6 (Project D1), and by the Spanish CICyT Projects FIS2009-07812 and PROMETEO/2010/043. F. R.-M. acknowledges the support from the EU Marie Curie Project APPCOPTOR-275150 (FP7-PEOPLE-2010-IEF).Oguz, EC.; Merechal, M.; Ramiro Manzano, F.; RodrĂguez, M.; Messina, R.; Meseguer Rico, FJ.; Loewen, H. (2012). Packing confined hard spheres denser with adaptive prism phases. Physical Review Letters. 109(21):218301-1-218301-5. https://doi.org/10.1103/PhysRevLett.109.218301S218301-1218301-51092
Mitochondrial physiology
As the knowledge base and importance of mitochondrial physiology to evolution, health and disease expands, the necessity for harmonizing the terminology concerning mitochondrial respiratory states and rates has become increasingly apparent. The chemiosmotic theory establishes the mechanism of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force provides the framework for developing a consistent theoretical foundation of mitochondrial physiology and bioenergetics. We follow the latest SI guidelines and those of the International Union of Pure and Applied Chemistry (IUPAC) on terminology in physical chemistry, extended by considerations of open systems and thermodynamics of irreversible processes. The concept-driven constructive terminology incorporates the meaning of each quantity and aligns concepts and symbols with the nomenclature of classical bioenergetics. We endeavour to provide a balanced view of mitochondrial respiratory control and a critical discussion on reporting data of mitochondrial respiration in terms of metabolic flows and fluxes. Uniform standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of data repositories of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery