Adsorption on nanoparticles with surface defects: mean field and energy level approaches.

In this work two theoretical approximations, the so-called theoretical approach of energy levels and an extension of the modified mean field approach (TAEL and MMFA, respectively) are applied to the study of surface decoration of modified nanostructures like crystalline nanoparticles. The surface of the nanoparticles is modified by the irreversible random deposition of defects consisting in isolated atoms. Such deposition is carried out until a certain surface density is reached, leaving the rest of the sites available for a second species to adsorb. Through the formulation of the integral equation, the theoretical approaches permit obtaining the adsorption isotherms and the compressibility of the adlayer. The main difference between the two approaches is the degree of details considered in their mathematical formulations: TAEL takes in account all the energy levels meanwhile MMFA only an average. The degree of precision and usefulness of both theories were evaluated in comparison with Monte Carlo simulations in the grand canonical assembly. Several cases were studied: attractive and repulsive lateral interactions and different fraction of defects. The effects of the nanoscale were considered for different types and sizes of nanoparticles. By calculating an integral error, we are able to affirm that TAEL reproduces all the properties of the analyzed quantities from the reference simulated curves. On the other hand, the MMFA performance is good only for a certain limited range of the parameters, however the strength is in the mathematical simplicity compared to TAEL.

Theoretical approach to energy levels applied to modified surfaces.

The main objective of this work is to present a new theoretical basis to describe surface deposition on a modified electrode surface. The surface is modified via the irreversible deposition of fixed particles or impurities that can block a fraction of the adsorption sites. An electroactive species was allowed to adsorb to the accessible sites and transfer electric charge. Energetics interactions between the electroactive particles and impurities were considered. The theoretical approach of energy levels (TAEL) was presented, through the integral equation formalism, where for its formulation the binomial distribution of energy levels and the standard Langmuir isotherm were considered. Adsorption isotherms and the compressibility of the adsorption layer were compared with Monte Carlo simulations and the recently published modified mean field approach (MMFA). Various conditions were studied: attractive and repulsive lateral interactions, and different quantities of impurities in one- and two-dimensional lattices. The performance of the theoretical approximations was analyzed by calculating an integral error.

Mean field approach applied to surface deposition on a modified electrode.

In this work we study the deposition phenomena on a modified electrode in the framework of the mean field theory. The electrode surface is modified by irreversible deposition of impurities which can block a fraction of the adsorption sites. Then, an electroactive species is allowed to adsorb on the accessible sites, transferring electric charge and generating a current that can be calculated and measured. Nearest-neighbor lateral interactions are considered both between electroactive particles and between particles and impurities. A modified Bragg-Williams theoretical approach considers both the blocking effects of impurities and the lateral interactions, through different concentrations of impurities and particles. The analysis is based on the study of adsorption isotherms and voltammograms, considering different interaction energies and impurity concentrations. The potentialities and limitations of the analytical approximation are discussed by comparing theoretical predictions with Monte Carlo simulations and experimental measurements in which artificial clay represents the impurity and a [Fe(CN)6]4 redox probe is the species that transfers the charge.

Addressing the surface coverage of Au nano-agglomerates and the electrochemical properties of modified carbon paste electrodes: Experimental and theoretical studies on ascorbic acid oxidation.

This article shows the formation of Au nano-agglomerates when increasing amounts of gold nanoparticles (AuNP) are incorporated into carbon paste electrodes. The surface coverage by this agglomerates is related to the electro-oxidation of a widely studied redox compound, ascorbic acid (AA); by analyzing the effect on the oxidation peak potential (Ep,a) and oxidation peak current (ip,a). The effects of pH and scan rate on the Ep,a and ip,a were investigated by cyclic voltammetry, and enabled to estimate the transfer coefficient and the number of electrons involved in the rate determining step (αnα), the standard heterogeneous rate constant (ks), and the diffusion coefficient of the redox compound, being 0.52 and 3.5 × 10-3 cm s-1 and 6.3 × 10-6 cm2 s-1, respectively. On the other hand, the sensing ability of the modified electrode was evaluated, obtaining a sensitivity of (63.2 ± 2.5) µA mM-1, a detection limit of 2.7 µM and a quantification limit of 8.9 µM. Additionally, a computational model based on lattice-gas model and Monte Carlo simulations in the Grand Canonical Ensemble was proposed in order to reproduce the behavior of the system, in terms of ip,a and Ep,a shift with increasing surface coverage by Au nano-agglomerates.

Structural surface and thermodynamics analysis of nanoparticles with defects.

In this work, we analyze the surface structure and thermodynamics regarding the decoration of nanoparticles with defects, using statistical calculations and Monte Carlo simulations in a complementary way. The main objective is to design and analyze a simple model as a general tool that can help the interpretation of results from more specific and complex models. In particular, we show how the presence of surface defects of the same nature as the nanoparticle induces different site distributions depending on different factors such as the density of defects, and the geometry and size of the considered nanoparticle. These distributions are analyzed for icosahedron nanoparticles of different sizes and densities of defects, and then are linked with Monte Carlo simulations to interpret the thermodynamic effects of the modified surfaces. Under low temperature or strong attractive interaction conditions, the details emerging from the defective surfaces were manifested as wide plateaus in the isotherm and peaks in the compressibility of the adlayer. Different situations were observed as the temperature increases, since the structural details gradually disappear from the thermodynamic measurements, until plateaus and compressibility peaks completely merge under high enough temperature conditions. Adsorption site distribution, adsorption isotherms, energy per site, compressibility of the adlayer, and other relevant properties are analyzed as a function of the number of defects and the chemical potential. In addition to the icosahedron, cuboctahedron and truncated octahedron geometries are also analyzed.

Fractional and integer stages of lithium ion-graphite systems: the role of electrostatic and elastic contributions.

In the present work, we analyze the hot topic of integer and fractional stages of lithium-ion batteries by using Monte Carlo simulations. While fractional stages have been demonstrated through several experimental, simulation and theoretical measurements, in other experimental techniques, such as electrochemical ones, there is no evidence for them. In previous work, we have analyzed the thermodynamics and kinetics of lithium-ion intercalation using a potential based on empirical parameterization, where multiple stages (integer and fractional) were found and analyzed. The present simulations suggest that if we consider repulsive elastic interactions in addition to electrostatic ones, the Hamiltonian symmetry is broken and there is no evidence for fractional stages. The physical origin of these repulsive interactions is assigned to the increasing graphite layer separation during lithium-ion intercalation. In the light of these simulations, selected experimental data are revisited, validating the presented novel parameterization. The parametrization used here can be used for other kinds of intercalation compounds, such as those involving Na or K.

A new model for lipid monolayer and bilayers based on thermodynamics of irreversible processes.

Lipid monolayers are used as experimental model systems to study the physical chemical properties of biomembranes. With this purpose, surface pressure/area per molecule isotherms provide a way to obtain information on packing and compressibility properties of the lipids. These isotherms have been interpreted considering the monolayer as a two dimensional ideal or van der Waals gas without contact with the water phase. These modelistic approaches do not fit the experimental results. Based on Thermodynamics of Irreversible Processes (TIP), the expansion/compression process is interpreted in terms of coupled phenomena between area changes and water fluxes between a bidimensional solution of hydrated head groups in the monolayer and the bulk solution. The formalism obtained can reproduce satisfactorily the surface pressure/area per lipid isotherms of monolayer in different states and also can explain the area expansion and compression produced in particles enclosed by bilayers during osmotic fluxes. This novel approach gives relevance to the lipid-water interaction in restricted media near the membrane and provides a formalism to understand the thermodynamic and kinetic response of biointerphases to biological effectors.

Interaction of semiochemicals with model lipid membranes: A biophysical approach.

Unravelling the chemical language of insects has been the subject of intense research in the field of chemical ecology for the past five decades. Insect communication is mainly based on chemosensation due to the small body size of insects, which limits their ability to produce or perceive auditory and visual signals, especially over large distances. Chemicals involved in insect communication are called semiochemicals. These volatiles and semivolatiles compounds allow to Insects to find a mate, besides the oviposition site in reproduction and food sources. Actually, insect olfaction mechanism is subject to study, but systematic analyses of the role of neural membranes are scarce. In the present work we evaluated the interactions of α-pinene, benzaldehyde, eugenol, and grandlure, among others, with a lipid membrane model using surface pressure experiments and Monte Carlo computational analysis. This allowed us to propose a plausible membranotropic mechanism of interaction between semiochemicals and insect neural membrane.

Criticality of the phase transition on stage two in a lattice-gas model of a graphite anode in a lithium-ion battery.

Herein, a Monte Carlo study within the canonical assembly has been applied to elucidate the lithium-ion phase transition order of a stage II lithium-graphite intercalation compound (LiC12) around the critical point. The results reveal a weakly first-order phase transition at 354.6 ± 0.5 K via measurements that follows the power laws with effective exponents. The graphite-lithium system was emulated within a lattice-gas model, comprising specific insertion sites arranged in four parallel planes with a triangular geometry. Moreover, two different types of energetic interactions were used: a Lennard-Jones potential, for particle interactions in the same plane, and a power law potential that decreased with distance, for particles in different planes. The energy per site and order parameter distribution were used to classify the order of the transition. Furthermore, the order parameters, susceptibility, and heat capacity were computed and analyzed.

Simulation of selective thermodynamic deposition in nanoholes.

The deposition of particles in nanoholes is analyzed, taking into account the curvature of their inner walls. Different lattice-gas models of the nanoholes are considered. The heterogeneous surface are shaped from a (100)-surface where a nanohollow are incorporated with parallelepiped or polyhedral geometry. Several deposition stages are identified as a function of the degree of curvature of the inner walls of the nanoholes. The Monte Carlo technique in the grand canonical ensemble is used to calculate isotherms, isosteric heats, energies per site and other thermodynamic properties. This study is based on different magnitudes of the interaction energies between the particles being deposited and those surrounding the nanohole.

Monomolecular adsorption on nanoparticles with repulsive interactions: a Monte Carlo study.

In the present work, we study the adsorption of different monomolecular species on nanoparticles with different sizes and geometries using a grand canonical Monte Carlo method. These species are characterized by repulsive lateral interactions between themselves, as takes place in the case of the adsorption of partially charged atoms or molecules. Nanosize effects are analyzed in terms of adsorption on edge and facet sites. The energy minimization in these systems comes out as a complex conjugation of the repulsive lateral interactions between the adsorbates and the attractive interactions of the adsorbates with the nanoparticle. The phenomenon is analyzed as a function of the occurrence of different ordered structures being formed on the surface of the nanoparticle. We find that layers with different structures may coexist on different facets of the nanoparticle. Finally, a discussion of deposition on flat surfaces and in finite systems is given.

Functional role of water in membranes updated: A tribute to Träuble.

The classical view of a cell membrane is as a hydrophobic slab in which only nonpolar solutes can dissolve and permeate. However, water-soluble non-electrolytes such as glycerol, erythritol, urea and others can permeate lipid membranes in the liquid crystalline state. Moreover, recently polar amino acid's penetration has been explained by means of molecular dynamics in which appearance of water pockets is postulated. According to Träuble (1971), water diffuses across the lipid membranes by occupying holes formed in the lipid matrix due to fluctuations of the acyl chain trans-gauche isomers. These holes, named "kinks" have the molecular dimension of CH2 vacancies. The condensation of kinks may form aqueous spaces into which molecular species of the size of low molecular weight can dissolve. This molecular view can explain permeability properties considering that water may be distributed along the hydrocarbon chains in the lipid matrix. The purpose of this review is to consolidate the mechanism anticipated by Träuble by discussing recent data in literature that directly correlates the molecular state of methylene groups of the lipids with the state of water in each of them. In addition, the structural properties of water near the lipid residues can be related with the water activity triggering kink formation by changes in the head group conformation that induces the propagation along the acyl chains and hence to the diffusion of water.

The adsorption of a mixture of particles with non-additive interactions: a Monte Carlo study.

The adsorption of binary mixtures with non-additive lateral interactions has been studied through grand canonical Monte Carlo simulations in the framework of the lattice-gas model. The traditional assumption of additive lateral interactions is replaced with a more general one including non-pairwise interactions. It is assumed that the energy linking a certain atom with any of its nearest neighbors strongly depends on the state of occupancy in the first coordination sphere of such an adatom. The process has been monitored through the adsorption isotherms and the differential heats of adsorption. Different combinations of both inter- and intra-species interactions have been considered in the analysis. Interesting behaviors were observed and discussed in terms of the low temperature phases formed in the system.

Microthermodynamic interpretation of fluid states from FTIR measurements in lipid membranes: a Monte Carlo study.

Fourier transform infrared spectroscopy (FTIR) is usually employed to obtain transition temperatures of lipids and lipid mixtures and the effect on it of several effectors, such as cholesterol. However, no interpretation of the molecular information provided by the frequency shift to higher values observed at Tc is available. In this article, we demonstrate that data obtained by means of FTIR measurements contain information about the microscopic thermodynamics of the lipid-phase transition. By means of Monte Carlo simulation, we have been able to show that the frequency shift from low to high values can be taken as a two-state transition of molecular constituents in a lattice rearrangement. According to the model, at temperatures below Tc all of the groups are defined in the lowest-energy state defined by the lowest frequency value and therefore they are all connected in a gel lattice. Above Tc, some groups may reach different energy states depending on the restrictions imposed on the groups. Ideally, when all of the groups are able to reach the highest frequency, a fully "fluid" state is reached, which is a disordered state. If we take this hypothetical state as a reference, it is possible to show that the higher states become less accessible. The model is suitable for describing the effect of cholesterol, which is able to dump the phase transition and is congruent with previous data denoting that in the so-called fluid phase the first four to five methylene groups remain in the gel state even above Tc. The frequency value attained above Tc depends on the nature of the lipid acyl chain.

Computer simulation and detailed mean-field approximation applied to adsorption on nanoparticles.

Adsorption thermodynamics of interacting particles adsorbed on icosahedral and truncated octahedral nanoparticles was studied by a detailed mean-field approximation and Monte Carlo simulations. The nanoparticle is tackled as a multivariate surface, where different types of adsorption sites occur according to coordination with nearest neighbors. In addition, lateral couplings between the adsorbed particles are considered. The analysis covers a wide range of interactions, extending from physical to strong chemical bonds, and different sizes and shapes of nanoparticles.

Manejo quirúrgico del síndrome de silla turca vacía primaria con compromiso visual campimétrico y sin evidencia de herniación del sistema visual / Surgical treatment of primary empty sella syndrome with visual compromise and no evidence of visual system herniation

Presentamos el caso de una mujer de 36 años con síndrome de silla turca vacía primaria (STVP) caracterizado por cefalea, estrechamiento concéntrico periférico progresivo de la visión y oligomenorrea, quien fue sometida a remodelamiento selar con colocación de un autoinjerto intraselar. La evolución postoperatoria fue con mejoría importante del defecto campimétrico, en ambos ojos.

We report the case of a 36 year old woman with primary empty sella syndrome (PESS) and symptoms consisting in headache, progressive concentric peripheral narrowing of vision and oligomenorrhea, who underwent sellar remodeling with placement of an intrasellar autograft. Post operative course showed bilateral improvement in campimetric defect.

Computer simulation of adsorption on nanoparticles: the case of attractive interactions.

A lattice-gas model describing adsorption on nanoparticles of different sizes and shapes is proposed and the adsorption thermodynamics is studied. The nanoparticle is modeled assuming different geometries, and Monte Carlo simulations are performed in the grand canonical ensemble. Adsorption isotherms, differential heats of adsorption, and other relevant thermodynamic properties are analyzed as a function of nanoparticle sizes. The simulations cover a wide range of interactions, ranging from physical to strong chemical bonds.

Lattice-gas model of nonadditive interacting particles on nanotube bundles.

In the present paper, the adsorption thermodynamics of a lattice-gas model which mimics a nanoporous environment is studied by considering nonadditive interactions between the adsorbed particles. It is assumed that the energy linking a certain atom with any of its nearest neighbors strongly depends on the state of occupancy in the first coordination sphere of such an adatom. By means of Monte Carlo (MC) simulations in the grand canonical ensemble, adsorption isotherms and differential heats of adsorption were calculated. Their striking behaviors were analyzed and discussed in terms of the low temperature phases formed in the system. Finally, the results obtained from MC simulations were compared with the corresponding ones from Bragg-Williams approximation.