Estudio de films nanoestructurados obtenidos mediante técnicas de autoensamblado capa por capa

Los electrodos modificados químicamente son sustratos conductores recubiertos con una película delgada que les confiriere propiedades funcionales de interés: electrocatálisis, electrocromismo, protección de la corrosión y otras. El método de autoensamblado capa por capa es una técnica para preparar películas de polielectrolitos de espesor nanométrico sobre todo tipo de sustratos. En esta Tesis se estudian las propiedades fisicoquímicas y la respuesta electroquímica de electrodos modificados mediante este método y su dependencia de las condiciones de deposición y medición y de la arquitectura molecular de la película. Los polielectrolitos redox empleados en esta Tesis son una poli(alilamina) modificada con el complejo Os(bpy)2pyCl2+/+ (PAH-Os) y nuevos polímeros conteniendo el complejo [Os(CN)5py]3-/2- . Se estudió la estructura, composición y propiedades electroquímicas de equilibrio de multicapas de polielectrolitos redox empleando una combinación de técnicas espectroscópicas y electroquímicas y modelado teórico. A parir del modelo de Donnan se relacionó el potencial de interface medido por voltametria cíclica con la composición interna y las propiedades de intercambio iónico. Se propuso un conjunto de reglas de diseño para controlar las permselectividad iónica de películas multicapa redox y no-redox controlando la relación entre el pH de ensamblado y de medición y la capa terminal. El caso de una única capa de PAH-Os sobre un electrodo tiolado se estudió con una teoría molecular que considera en forma explícita el tamaño, forma, carga y conformaciones de las todas las moléculas en la interfase, sus interacciones y la presencia de los equilibrios químicos. La teoría describe cuantitativamente la respuesta electroquímica del sistema bajo distintas condiciones experimentales y el espesor de la capa y puede usarse para estudiar propiedades no accesibles a los experimentos. La teoría predice que la estructura, el potencial electrostático y el potencial redox aparente de la cupla redox dependen de la distancia al electrodo y que los equilibrios redox y ácido-base se encuentran acoplados debido a la regulación de carga. Las propiedades de transporte y transferencia de carga en las multicapas se estudiaron con distintas técnicas electroquímicas. Para películas terminadas en polianión, se observa un impedimento al proceso redox. Mediante un análisis con voltametria cíclica y un modelo de difusión modificado, se mostró que el coeficiente de difusión para el transporte de carga cae aproximadamente 3 órdenes de magnitud al adsorber el polianión y que es posible revertir este fenómeno aumentando la fuerza iónica de la solución. Se atribuyó el efecto a cambios en los movimientos segmentales del polímero. Se empleó la técnica de microbalanza de cristal de cuarzo con control electroquímico para estudiar el transporte de iones y solvente en las películas redox. Se observó un aumento de la masa de la película durante la oxidación y una disminución durante la reducción. Este proceso es afectado por la última capa ensamblada y efectos específicos de aniones. Se discute el origen del aumento en la masa de la película que se observa con el número de ciclos de oxidaciónreducción. Se prepararon electrodos modificados con nanopartículas de Pd para reducción electrocatalítica mediante la reducción electroquímica de iones Pd confinados en el interior de una multicapa depositada sobre un electrodo de tela de carbono. El análisis de los materiales preparados mediante XPS, elipsometría y microscopía SEM demostró que las nanopartículas se forman preferencialmente en la interfase electrodo/película. La selectividad y eficiencia de los nanocatalizadores superan con creces la de los electrodos depositados por electroreducción directa de sales de Pd, tradicionalmente usados en la literatura de electrosíntesis. Las películas electroactivas nanoestructuradas preparadas por el método capa por capa presentan algunos comportamientos que no se observan en películas desordenadas depositadas por métodos tradicionales. Estas diferencias surgen de las nuevas variables de proceso disponibles, que pueden ser ajustadas para optimizar las propiedades funcionales.Chemically modified electrodes are conductive substrates coated with a thin layer that introduces functional properties of interest: electrocatalysis, electrochromism, corrosion protection and so on. Layer by layer self-assembly is a novel method for the preparation of nanometer-thick polyelectrolyte films. In this thesis we address the physicochemical properties and electrochemical response of layer by layer modified electrodes and their dependence on the assembly, testing conditions and the molecular architecture of the film. The redox polyelectrolytes used in this Thesis are a poly(allylamine) modified with the complex Os(bpy)2pyCl2+/+ (PAH-Os) and new polymers bearing the complex [Os(CN)5py]3-/2- . The structure, composition and equilibrium electrochemical properties of redox polyelectrolyte multilayers were studied with a combination of spectroscopic and electrochemical techniques and theoretical models. The Donnan partition model was used to link the interfacial potential measured by cyclic voltammetry with the inner composition and ionic exchange. We proposed a set of design rules to control ionic permselectivity in redox and non-redox multilayers by means of the relationship between assembly and testing pH and the polyelectrolyte in the outmost layer. In order to study a single layer of PAH-Os on a thiolated electrode we developed a molecular theory which explicitly considers the size, shape, charge and conformations of all the molecular species at the interface, their interactions and the chemical equlibria. It quantitatively describes the electrochemical response under different experimental conditions as well as the thickness of the layer. Moreover, the theory can be used to study properties that are difficult to address experimentally. It predicts that the structure, electrostatic potential and apparent redox potential of the couple depend on the distance to the electrode. The theory shows the coupling between the redox and acid-base equilibria that arises due to charge regulation, as well as its experimental manifestations. Charge transfer and transport in multilayer films were studied with different electrochemical techniques. For polyanion capped multilayers, we observe a hindrance for the redox process. Based on cyclic voltammetry measurements and a modified diffusion model, we showed that the diffusion coefficient for charge transport decreases by 3 orders of magnitude upon the assembly of the polyanion and that this hindrance for the redox process can be eliminated by increasing solution ionic strength. The effect was attributed to changes in polymer segmental mobility. In order to study ion and solvent fluxes in the films we used electrochemical quartz crystal microbalance. An increase of film mass during oxidation and a decrease upon reduction was observed. This process is affected by the outmost layer and specific anionic effects. We discuss why the mass of the film increases with the number of oxidation-reduction cycles. We prepared electrodes modified with Pd nanoparticles for electrocatalytical hydrogenation. This material was prepared by electroreduction of Pd ions confined inside a multilayer deposited on a carbon felt electrode. The as prepared electrodes were characterized with XPS, ellipsometry and SEM. These techniques showed that the nanoparticles preferentially grow at the electrode/film interface. The selectivity and performance of the nanocatalyzers greatly surpasses those of the electrodes prepared by direct electroreduction of Pd salts in solutions, which are a standard in the electrosynthesis literature. Electroactive nanostructured multilayer fims can behave differently to random films prepared by traditional techniques. The differences arise from the new process variables available in the layer by layer technique, which can be tuned to optimize film functional properties.Fil:Tagliazucchi, Mario Eugenio. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Nanopore gatesviareversible crosslinking of polymer brushes: a theoretical study

Polymer-brush-modified nanopores are synthetic structures inspired by the gated transport exhibited by their biological counterparts. This work theoretically analyzes how the reversible crosslinking of a polymer network by soluble species can be used to control transport through nanochannels and pores. The study was performed with a molecular theory that allows inhomogeneities in the three spatial dimensions and explicitly takes into account the size, shape and conformations of all molecular species, considers the intermolecular interactions between the polymers and the soluble crosslinkers and includes the presence of a translocating particle inside the pore. It is shown than increasing the concentration of the soluble crosslinkers in bulk solution leads to a gradual increase of its number within the pore until a critical bulk concentration is reached. At the critical concentration, the number of crosslinkers inside the pore increases abruptly. For long chains, this sudden transition triggers the collapse of the polymer brush to the center of the nanopore. The resulting structure increases the free-energy barrier that a translocating particle has to surmount to go across the pore and modifies the route of translocation from the axis of the pore to its walls. On the other hand, for short polymer chains the crosslinkers trigger the collapse of the brush to the pore walls, which reduces the translocation barrier.Fil: Pérez Sirkin, Yamila Anahí. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Szleifer, Igal. Northwestern University; Estados Unido

Ionic Conductance of Polyelectrolyte-Modified Nanochannels: Nanoconfinement Effects on the Coupled Protonation Equilibria of Polyprotic Brushes

A theoretical methodology is introduced to calculate the low-bias conductance, structure, and composition of long polyelectrolyte-modified nanochannels of arbitrary geometry. This methodology is applied to explore the coupling between acid-base equilibrium and geometry in cylindrical, conical, and trumpet-shaped nanochannels modified by end-grafted layers of poly(2-(methacryloyloxy)ethyl-phosphate) (PMEP), a diprotic polyacid. The ionic conductance and speciation curves (i.e., the fraction of deprotonated, monoprotonated, and diprotonated acid segments) for this system were predicted as a function of the solution pH. The apparent pKa's and widths of the transitions between the different acid-base states determined from the speciation curves depend on the diameter and shape of the nanochannel and the bulk salt concentration. In the limit of wide channels, the apparent pKa's and widths can be estimated by a simplified analytical model derived from the more general molecular theory. Both the general and the simplified theory predicts that, due to charge-regulation effects, the first acid-base transition (0/-1 transition) is wider than the second one (-1/-2), and both transitions are wider than the ideal one expected for an isolated acid-base group in the bulk. It is also shown that the inflection points of the conductance versus pH curves provide a very good estimation of the apparent pKa's of the polyelectrolyte for cylindrical channels, but the quality of the estimation decreases for noncylindrical geometries.Fil: Gilles, Facundo Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Azzaroni, Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Szleifer, Igal. Northwestern University; Estados Unido

Self-assembly of model short triblock amphiphiles in dilute solution

In this work, a molecular theory is used to study the self-assembly of short diblock and triblock amphiphiles, with head-tail and head-linker-tail structures, respectively. The theory was used to systematically explore the effects of the molecular architecture and the affinity of the solvent for the linker and tail blocks on the relative stability of the different nanostructures formed by the amphiphiles in dilute solution, which include spherical micelles, cylindrical fibers and planar lamellas. Moreover, the theory predicts that each of these nanostructures can adopt two different types of internal organization: (i) normal nanostructures with a core composed of tail segments and a corona composed of head segments, and (ii) nanostructures with a core formed by linker segments and a corona formed by tail and head segments. The theory predicts the occurrence of a transition from micelle to fiber to lamella when increasing the length of the tail or the linker blocks, which is in qualitative agreement with the geometric packing theory and with experiments in the literature. The theory also predicts a transition from micelle to fiber to lamella as the affinity of the solvent for the tail or linker block is decreased. This result is also in qualitative agreement with experiments in the literature but cannot be explained in terms of the geometric packing theory. The molecular theory provides an explanation for this result in terms of the competition between solvophobic attractions among segments in the core and steric repulsions between segments in the corona for the different types of self-assembled nanostructures.Fil: Zaldivar, Gervasio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Samad, M. B.. University of Nebraska; Estados UnidosFil: Conda Sheridan, Martin. University of Nebraska; Estados UnidosFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Molecular orientation in self-assembled multilayers measured by Second Harmonic generation using femtosecond pulses

We present measurements of the optical second-harmonic generation in self assembled multilayer films of PAZO/PAH polymers with the aim to investigate molecular order in the layer-by-layer architecture. The experiments are performed in transmission, using a femtosecond Ti:Sa pulsed laser, which allows a more accurate determination of the amplitude of the second harmonic signal, without interference fringes usually present in nanosecond experiments. We found that the first bilayer, in contact with the substrate, presents a broad distribution of the orientation of the molecules, while the addition of successive bilayers (up to 12) produces ordering of the molecules with a small tilt angle respect to the surface normal. This result, together with the monotonic increment of the second harmonic signal with the number of layers indicates that the molecules grow orderly assembled in these films.Fil: Von Bilderling, Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; ArgentinaFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Calvo, Ernesto Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Bragas, Andrea Veronica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Laboratorio de Electrónica Cuántica; Argentin

Nanocompartmentalization of the Nuclear Pore Lumen

The nuclear pore complex (NPC) employs the intrinsically disordered regions (IDRs) from a family of phenylalanine-glycine-rich nucleoporins (FG-Nups) to control nucleocytoplasmic transport. It has been a long-standing mystery how the IDR-mediated mass exchange can be rapid yet selective. Here, we use a computational microscope to show that nanocompartmentalization of IDR subdomains leads to a remarkably elaborate gating structure as programmed by the amino acid sequences. In particular, we reveal a heterogeneous permeability barrier that combines an inner ring barrier with two vestibular condensates. Throughout the NPC, we find a polarized electrostatic potential and a diffuse thermoreversible FG network featuring mosaic FG territories with low FG-FG pairing fraction. Our theoretical anatomy of the central transporter sheds light into the sequence-structure-function relationship of the FG-Nups and provides a picture of nucleocytoplasmic mass exchange that allows a reconciliation of transport efficiency and specificity.Fil: Huang, Kai. Northwestern University; Estados UnidosFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Park, Sung Hyun. Northwestern University; Estados UnidosFil: Rabin, Yitzhak. Bar-Ilan University; IsraelFil: Szleifer, Igal. Northwestern University; Estados Unido

Self-Assembled Nanostructures of Peptide Amphiphiles: Charge Regulation by Size Regulation

Self-assembled nanostructures of peptide amphiphiles (PAs) with molecular structures C16K2 and C16K3 (where C indicates the number of carbon atoms in the alkyl chain and K is the lysine in the head group) were studied by a combination of theoretical modeling, transmission electron and atomic force microscopes, and acid-base titration experiments. The supramolecular morphology of the PAs (micelles, fibers, or lamellas) was dependent on the pH and ionic strength of the solution. Theoretical modeling was performed using a molecular theory that allows determining the equilibrium morphology, the size, and the charge of the soft nanoassemblies as a function of the molecular structure of the PA, and the pH and salt concentration of the solution. Theoretical predictions showed good agreement with experimental data for the pH-dependent morphology and size of the nanoassemblies and their apparent pKa's. Two interesting effects associated with charge regulation mechanisms were found: first, ionic strength plays a dual role in the modulation of the electrostatic interactions in the system, which leads to complex dependencies of the aggregation numbers with salt concentration; second, the aggregation number of the nanostructures decreases upon increasing the charge per PA. The second mechanism, charge regulation by size regulation, tunes the net charge of the assemblies to decrease the electrostatic repulsions. A remarkable consequence of this behavior is that adding an extra lysine residue to the charged region of the PAs can lead to an unexpected decrease in the total charge of the micelles.Fil: Zaldivar, Gervasio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Vemulapalli, Sridhar. University Of Nebraska Medical Center; Estados UnidosFil: Udumula, Venkatareddy. University Of Nebraska Medical Center; Estados UnidosFil: Conda Sheridan, Martin. University Of Nebraska Medical Center; Estados UnidosFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Sustainable electrochemical extraction of lithium from natural brine for renewable energy storage

An electrochemical reactor for the extraction of lithium from natural brine has been designed. It comprises two 3D porous packed bed electrodes and a porous separator filled with electrolyte. The electrodes are filled with conducting petroleum coke particles covered respectively with LiMn 2 O 4 selective to lithium ions and polypyrrole selective to anions. It operates in two steps: First, the porous electrodes and the separator are filled with natural brine to extract Li + and Cl - by intercalation and adsorption. Then, after rinsing with water the reactor is filled with a dilute LiCl recovery solution and LiCl is recovered by reversing the electrical current. A mathematical model for the reactor comprising the Nernst-Planck equation and the battery intercalation model has been developed. The model was solved using the finite element method under the COMSOL Multiphysics environment in order to obtain the electrostatic potential and the ion currents and concentrations across the system. Unlike the asymmetric LiMn 2 O 4 /activated carbon super-capacitor, in the lithium extracting reactor the total LiCl concentration decreases in the extraction step and increases in the recovery step. A good agreement between the experimental and simulated potential difference vs. time at constant current validates the model of the reactor.Fil: Romero, Valeria Carolina Estefanía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Jujuy. Universidad Nacional de Jujuy. Centro de Investigaciones y Transferencia de Jujuy; ArgentinaFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Flexer, Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Jujuy. Universidad Nacional de Jujuy. Centro de Investigaciones y Transferencia de Jujuy; ArgentinaFil: Calvo, Ernesto Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Modulation of Polyelectrolyte Adsorption on Nanoparticles and Nanochannels by Surface Curvature

This paper presents theoretical results on the adsorption of polyelectrolyte chains on surfaces with opposite charge and nanoscale curvature. The theory predicts that increasing the surface curvature can either increase or decrease the amount of adsorbed polyelectrolyte, depending on the type of curvature (convex or concave) and whether the polyelectrolyte undercompensates or overcompensates the initial charge of the substrate. For small bulk salt concentration (10-4 M), increasing the curvature of the surface displaces the adsorption equilibrium of the polyelectrolyte in order to decrease the absolute value of the effective charge density for concave surfaces (nanochannels) or to increase it for convex surfaces (nanoparticles). This behavior is traced back to the dependence of the total free energy as a function of the curvature of the surface. For intermediate salt concentrations (0.01-0.1 M), the magnitude of the effect is larger than that for low salt concentrations, although the general picture becomes more complex due to the fact that the added salt competes with the polycation to screen the negative charge of the substrate. It is argued that the effect under discussion will be relevant for nano-objects that have different radii or type of curvature at different locations (i.e. conical nanochannels or cylindrical nanorods with hemispherical tips) as our theory predicts inhomogeneous polyelectrolyte adsorption on their surfaces.Fil: Gilles, Facundo Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Boubeta, Fernando Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Azzaroni, Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Szleifer, Igal. Northwestern University; Estados UnidosFil: Tagliazucchi, Mario Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Modulation of Polyelectrolyte Adsorption on Nanoparticles and Nanochannels by Surface Curvature

This paper presents theoretical results on the adsorption of polyelectrolyte chains on surfaces with opposite charge and nanoscale curvature. The theory predicts that increasing the surface curvature can either increase or decrease the amount of adsorbed polyelectrolyte, depending on the type of curvature (convex or concave) and whether the polyelectrolyte undercompensates or overcompensates the initial charge of the substrate. For small bulk salt concentration (10-4 M), increasing the curvature of the surface displaces the adsorption equilibrium of the polyelectrolyte in order to decrease the absolute value of the effective charge density for concave surfaces (nanochannels) or to increase it for convex surfaces (nanoparticles). This behavior is traced back to the dependence of the total free energy as a function of the curvature of the surface. For intermediate salt concentrations (0.01-0.1 M), the magnitude of the effect is larger than that for low salt concentrations, although the general picture becomes more complex due to the fact that the added salt competes with the polycation to screen the negative charge of the substrate. It is argued that the effect under discussion will be relevant for nano-objects that have different radii or type of curvature at different locations (i.e. conical nanochannels or cylindrical nanorods with hemispherical tips) as our theory predicts inhomogeneous polyelectrolyte adsorption on their surfaces.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasConsejo Nacional de Investigaciones Científicas y Técnica