No abstract

The binding affinities of polyanions for bovine serum albumin in NaCl solutions from I ) 0.01-0.6 M, were evaluated on the basis of the pH at the point of incipient binding, converting each such pH c value into a critical protein charge Z c . Analogous values of critical charge for mixed micelles were obtained as the cationic surfactant mole fraction Y c . The data were well fitted as Y c or Z c ) KI a , and values of K and a were considered as a function of normalized polymer charge densities (τ), charge mobility, and chain stiffness. Binding increased with chain flexibility and charge mobility, as expected from simulations and theory. Complex effects of τ were related to intrapolyanion repulsions within micelle-bound loops (seen in the simulations) or negative protein domainpolyanion repulsions. The linearity of Z c with I at I < 0.3 M was explained by using protein electrostatic images, showing that Z c at I < 0.3 M depends on a single positive "patch"; the appearance of multiple positive domains I > 0.3 M (lower pH c ) disrupts this simple behavior.

The complex formation between a weak polyelectrolyte chain and an oppositely charged nanoparticle is investigated using Monte Carlo simulations. Global structural parameters such as the polyelectrolyte length, nanoparticle size, solution pH, and ionic concentration as well as local features, such as the nanoparticle surface charge density and polyelectrolyte intrinsic stiffness influences, are systematically investigated. Phase states of the polyelectrolyte/nanoparticle complexes are presented, and to bridge the gap with experiments, titration curves are calculated. It is shown that the presence of one oppositely charged nanoparticle significantly modifies the acid/base properties of the weak polyelectrolyte as well as the charge distribution along the polymer backbone and that the solution pH and ionic concentration largely control the polyelectrolyte conformation at the nanoparticle surface. Chain stiffness promotes the polyelectrolyte expansion as well as ionization but penalizes the polyelectrolyte adsorption at the nanoparticle surface, hence affecting its acid/base behavior.

Various models of the complex formation between polyelectrolyte chains and oppositely charged macroions are reviewed. In recent years, a great deal of knowledge of the multitude of possible polyelectrolyte conformations at the macroion surface has been accumulated, which consequently has led to increasing interest in using such complexes in the design of nanomaterials. This review focuses on key studies relating to the effects of various physico-chemical parameters on complex formation and areas for future research are identified.

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.

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.

The electrostatic driven complex formation between a weak polyampholyte chain and one positively charged nanoparticle is investigated using Monte Carlo simulations. The influence of parameters such as the polyampholyte contour length, number and size of blocks, nanoparticle surface charge density, and solution properties, such as the pH and ionic concentration, on the PA titration curves is investigated. It is shown that the presence of one positively charged nanoparticle significantly modifies the acid/base properties of the weak polyampholyte by, on the one hand, promoting the formation of negatively charged monomers and, on the other hand, limiting the number of positively charged monomers. The electrostatic interactions of this system can be modified by pH, ionic concentration, and nanoparticle surface charge. The competition between attractive and repulsive, intramolecular and intermolecular electrostatic interactions leads to a wide range of possible PA conformations at the nanoparticle surface, which have a direct impact on the nanoparticle stabilized or destabilized solutions. Extended conformations, electrostatic rosettes, and dense multiplayer structures are observed. Nonetheless, the intramolecular interactions between the positively and negatively charged PA monomers, in particular at the isoelectric point, are found to play important and subtle roles for both the isolated and adsorbed chain conformations. It is also found that nanoparticle charge inversion is an important ingredient for the formation of multilayer structures at the nanoparticle surface.

Understanding the deposition of liquid droplets on surfaces is essential in many environmental and industrial processes. In this paper, we describe computer simulations of homogeneous deposition of liquid droplets on an ideal, smooth, and horizontal solid surface. The statistical evolution of droplet deposition and growth processes are investigated. It is found that three basic events, namely, deposition, incorporation, and coalescence, produce droplets of different sizes and that the droplet size polydispersity is continuously increasing with time leading to a bimodal distribution. By considering the total number of droplets N tot on the surface versus time, we demonstrate that four growth regimes must be considered. These regimes reflect the relative influence of the three events during the deposition process. Variation with time of the surface coverage Γ as a function of the contact angle θ between the droplets and the surface is also investigated. The effect of a variable contact angle on the value of surface saturation and dynamical growth is calculated. Finally, by considering the total mass of the deposited material and surface coverage, some guidelines to achieve an efficient droplet deposition process and surface coverage versus total droplet mass are proposed.