Size and pH effect on electrical and conformational behavior of poly(acrylic acid): Simulation and experiment

Monte Carlo simulations, experimental titrations and fluorescence correlation spectroscopy experiments were used to investigate the conformational and electrical properties of polyacrylic acids (PAA). On the one hand, titration curves were calculated to get an insight into the role of pH on the degree of ionization and conformation of PAA chains. On the other hand, experimental potentiometric titrations of PAA were also achieved for different PAA molecular weights and compared to the calculated titration curves obtained by Monte Carlo coarse grained simulations. It was found that for a large range at intermediate PAA ionizations, a good correlation is obtained between experimental and simulations data thanks to the prominence of electrostatic interactions in this domain. The effect of ionic concentration and PAA molecular weight on the titration curves was also investigated. In order to get a better understanding of PAA conformational behavior, we also investigated PAA diffusion properties in aqueous solutions as a function of pH and ionic strength by fluorescence correlation spectroscopy (FCS), thanks to its high sensitivity to measure diffusion coefficients of tracer solutes. Good qualitative agreements were observed between experimental diffusivities and polymer properties calculated from MC simulations. It was shown that the high molecular weight PAA chains display more significant changes in diffusivity in agreement with the ionization degrees and conformational changes observed in the simulations.     

Complex formation between a nanoparticle and a weak polyelectrolyte chain: Monte Carlo simulations

Understanding the complexation processes between nanoparticles and polyelectrolytes is an essential aspect in many branches of nanotechnology, nanoscience, chemistry, and biology to describe processes such as nanoparticle stabilization/destabilization and dispersion, water treatment, microencapsulation, complexation with biomolecules for example, and evolution of the interface of many natural and synthetic systems. In view of the complexity of such processes, applications are often based on empirical or semiempirical observations rather than on predictions based on theoretical or analytical models. In this study, the complex formation between an isolated weak polyelectrolyte and an oppositely charged nanoparticle is investigated using Monte Carlo simulations with screened Coulomb potentials in the grand canonical ensemble. The roles of the nanoparticle surface charge density , solution pH and ionic concentration C_i are systematically investigated. The phase diagrams of complex conformations are also presented. It is shown that the polyelectrolyte conformation at the surface of the nanoparticle is controlled by the attractive interactions with the nanoparticle but also by the repulsive interactions between the monomers. To bridge the gap with experiments titration curves are calculated. We clearly demonstrate that an oppositely charged nanoparticle can significantly modify the acid/base properties of a weak polyelectrolyte.     

Nanoparticle adsorption on a weak polyelectrolyte. Stiffness, pH, charge mobility, and ionic concentration effects investigated by Monte Carlo simulations

Monte Carlo simulations have been used to study two different models for a weak linear polyelectrolyte in the presence of nanoparticles: (i) a rodlike and (ii) a flexible polyelectrolytes. The use of simulated annealing has made it possible to simulate a polyelectrolyte chain in the presence of several nanoparticles by improving conformation sampling and avoiding multiple minima problems when dense conformations are produced. Nanoparticle distributions along the polymer backbone were analyzed versus the ionic concentration, polyelectrolyte stiffness, and nanoparticle surface charge. Titration curves were calculated and the influences of the ionic concentration, solution pH, and number of adsorbed nanoparticles on the acid/base polyelectrolyte properties have been systematically investigated. The subtle balance of attractive and repulsive interactions has been discussed, and some characteristic conformations are presented. The comparison of the two limit models provides a good representation of the stiffness influence on the complex formation. In some conditions, overcharging was obtained and presented with respect to both the polyelectrolyte and nanoparticle as the central element. Finally, the charge mobility influence along the polyelectrolyte backbone was investigated by considering annealed and quenched polyelectrolyte chains.     

Simulations and scattering functions of polyelectrolyte–macroion complexes

Using Monte Carlo simulations of complex formation between a polyelectrolyte chain and an oppositely charged macroion, we calculated the scattering function of the polyelectrolyte chain. We investigated the case of the isolated polyelectrolyte chain and studied the effect and influence of key parameters such as the ionic concentration of the solution, polyelectrolyte length and intrinsic rigidity on the scattering function. Then, we focused on the polyelectrolyte–macroion complex by calculating the structure factor S(q) of the adsorbed polyelectrolyte chain. Typical conformations ranging from coils, extended chains to solenoids are revealed and the corresponding S(q) analysed. The effects of ionic concentration, chain length and intrinsic rigidity and relative size ratio between the polyelectrolyte and the macroion are investigated. Important effects on the structure factor of the adsorbed polyelectrolyte are observed when the macroion is partially or totally wrapped by the polyelectrolyte. Distance correlations between the polyelectrolyte monomer positions at the surface of the macroion induce the formation of peaks in the fractal regime of S(q). For semiflexible chains, when solenoid conformations are observed, the position of the peaks in the fractal regime corresponds directly to the separation distance between the turns. The formation of a protruding tail in solution is also observed through the formation in the fractal regime of a linear domain.This revised version was published online in November 2004 with corrections to the authors.     

Titration of hydrophobic polyelectrolytes using Monte Carlo simulations

The conformation and titration curves of weak (or annealed) hydrophobic polyelectrolytes have been examined using Monte Carlo simulations with screened Coulomb potentials in the grand canonical ensemble. The influence of the ionic concentration pH and presence of hydrophobic interactions has been systematically investigated. A large number of conformations such as extended, pearl-necklace, cigar-shape, and collapsed structures resulting from the subtle balance of short-range hydrophobic attractive interactions and long-range electrostatic repulsive interactions between the monomers have been observed. Titration curves were calculated by adjusting the pH-pK(0) values (pK(0) represents the intrinsic dissociation constant of an isolated monomer) and then calculating the ionization degree alpha of the polyelectrolyte. Important transitions related to cascades of conformational changes were observed in the titration curves, mainly at low ionic concentration and with the presence of strong hydrophobic interactions. We demonstrated that the presence of hydrophobic interactions plays an important role in the acid-base properties of a polyelectrolyte in promoting the formation of compact conformations and hence decreasing the polyelectrolyte degree of ionization for a given pH-pK(0) value.     

Size effect of complexing microcapsules on copper ion extraction

In a previous work [J. Microencapsulation, in press], polyamide microcapsules containing a poly(acrylic acid) gel as a macromolecular ligand (PAA-CAPS) with a mean diameter of 210 μm were prepared using an original two-step polymerization process combining interfacial polycondensation and radical polymerization in a water in oil inverse emulsion system. Extractions of many divalent cations were examined. In this work, we proposed to synthesise by the same process, smaller microcapsules with a mean diameter of 10 μm (PAA-μCAPS). Reference polyamide microcapsules, i.e. without ligand were also synthesized (μCAPS) and (CAPS) [J. Microencapsulation, in press]. Microcapsule wall thickness was evaluated by SEM and TEM observations of microcapsule cross-section cuts, microcapsule water content was determined by thermogravimetric experiments. Specific surface area and total volume of the pore of microcapsules were determined by BET method based on N₂ adsorption/desorption. The comparison of the extractabilities and the stripping of Cu(II) into the various kind of microcapsules were examined.