F. Galisteo-González

Hydration forces between silica surfaces: Experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.

The role played by hydration forces in the stability of protein-coated particles: non-classical DLVO behaviour

Abstract An anomalous colloidal stability is observed when protein-covered particles are exposed to high salt concentrations, in opposite to the classical theory (DLVO) predictions. In hydrophilic systems some other discrepancies with respect to this theory has also been described in the literature and hydration forces are invoked to rationalize these phenomena. In our case, the dependence of the anomalous behaviour with pH and electrolyte ion concentration points to the specific adsorption of cations as responsible. An extension to the DLVO theory including hydration forces and its dependence with salt concentration is proposed. From the practical point of view, this stabilization mechanism is of great interest in the development of clinical latex immunoassays, which often suffer from colloidal stability problems.

Systematic study on the preparation of BSA nanoparticles.

Albumins, in the form of nanoparticles, are increasingly used as drug carriers in the medical field, and the size effect of these nanomaterials is of major importance since it may affect their bioavailability and the in vivo behaviour after intravenous injection. This research provides a comprehensive study on the preparation of BSA nanoparticles, based on a simple coacervation method, with suitable size, size distribution, and surface charge for drug-delivery applications. Numerous experimental variables were examined in order to characterize their impact on nanoparticle size, distribution, electrophoretic mobility, and yield. Particle size was controlled by adjusting self-assembly phenomena of the protein molecules, which was affected by preparation conditions including BSA content, pH, and ionic strength (a parameter that strongly influences nanoparticle formation but surprisingly has not been previously studied in detail). Small particles with a narrow size distribution were obtained under experimental conditions where the repulsion between BSA molecules was high, i.e. at pH values far from the isoelectric point of the protein and low salt concentration. Changes in temperature, volume, and rate of addition of the dehydrating agent (ethanol) also affect nanoparticle characteristics, as they influence the nucleation rate and particle growth. The effect of these experimental conditions on the quantity of protein still dissolved in the aqueous phase after desolvation (i.e. the yield of BSA nanoparticles) was also studied. Nanoparticles surface charge was modulated with the extension of cross-linking. Finally, long-term colloidal stability of samples was evaluated after 2 months of storage.

Looking into overcharging in model colloids through electrophoresis: Asymmetric electrolytes

Some theories claim that the Poisson–Boltzmann approach could fail to describe the electric double layer of colloids under certain conditions as a result of neglecting ion size correlations. For instance, if the surface charge density and/or the electrolyte concentration are high enough, the counterion local density in the vicinity of charged surface could become so large that the particle charge would be overcompensated. This phenomenon is theoretically known as overcharging and, sometimes, should involve a ζ-potential reversal. Accordingly, this work looks into overcharging through electrophoresis experiments. The electrophoretic mobility has been measured for latex particles with moderate and large surface charge density in solutions of asymmetric electrolytes z:1 (symmetric electrolytes have been studied in a previous work). In order to find out the relevance of ion size correlations, results are analyzed within the so-called hypernetted-chain/mean-spherical approximation (HNC/MSA) as well as a Poisso...

Probing charge inversion in model colloids: electrolyte mixtures of multi- and monovalent counterions

Under certain conditions ion?ion correlations play a crucial role in the description of the electrical double layer of colloidal particles. In fact, in many instances, the inclusion of the short range correlations between ions in the study of the ionic distribution leads to quite different results with respect to the classical treatment (where ions are assumed to be points). In particular, these discrepancies become more noticeable for highly charged particles in the presence of moderate or highly multivalent counterion concentrations. Moreover, it can be shown that the existence of an electrolyte mixture consisting of multi-?and monovalent counterions may cause that system to become overcharged, a feature that cannot be predicted from a classical point of view based on the Boltzmann distribution function. Precisely this aspect has recently produced an enormous interest in the field of biophysics since small variations in the physiological conditions of biocolloidal systems (e.g.?the addition of a multivalent salt) can induce important changes in their behaviour. In order to determine the relevance of ion correlations in electrolyte mixtures, we present some experimental results on the electrophoretic mobility of latex particles in the presence of different 1:1 and 3:1 salt mixtures. Likewise, these results are analysed within the so-called hypernetted-chain/mean spherical approximation where ion size correlations are taken into account.

Colloidal aggregation in energy minima of restricted depth

Coagulation rates of bare and protein-covered colloidal particles show a different dependence on experimental conditions. While the rapid coagulation rate for the bare particles obeys the modified Smoluchowski theory and is independent of pH and the nature of the cation and the anion, the value for the coated particles is lower and depends on pH and ions’ nature. The variation in the Hamaker constant and the existence of a shallow primary minimum of the interparticle potential for the latex–protein complex, both attributed to the layer of water molecules and ions adsorbed on protein, may explain these results. Coagulation rates were measured with a low angle light scattering apparatus, and the experimental curves of stability fitted using Fuchs’ equation and the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory. In the case of covered particles, a modified expression of the van der Waals attraction was used. This attraction depends on the Hamaker constant for the protein in the vacuum, whose value was estima...

Adsorption of monoclonal IgG on polystyrene microspheres

In this paper the adsorption of a monoclonal antibody IgG-1 isotype against HBsAg onto positively and negatively charged polystyrene beads has been studied. To determine the role played by electrostatic forces in the adsorption process different pH values were used. It was confirmed that the affinity of adsorption isotherms depends on the electrostatic interaction between protein and polymer surface. The maximum adsorption amount is located around the i.e.p. of the dissolved protein, and decreases markedly as pH moves away. Thus, the major driving force for adsorption of monoclonal antibodies on polystyrene beads comes from the hydrophobic interaction between the antibody molecules and the adsorbent surface. Desorption of preadsorbed IgG molecules by increasing ionic strength has shown that the positively charged polystyrene is also more hydrophobic in character than the negatively charged one. Finally, electrokinetic experiments have determined that the electric double layer (e.d.l.) of monoclonal antibody changes as the consequence of adsorbing on charged polymer surfaces.

Influence of electrostatic forces on IgG adsorption onto polystyrene beads

Abstract Some electrostatic aspects of the interaction between rabbit immuno gamma globulins (IgG) and positively and negatively charged polystyrene (PS) beads are reported. Adsorption and electrophoresis experiments have been performed as a function of pH and ionic strength. Both polystyrene beads reached a maximum adsorption around pH 6.0. The isoelectric points of the IgG-PS complexes were 6.0 and 8.0 for the anionic and cationic latices, respectively. This difference indicates that the charge of the PS beads at least partially compensates the net charge of the IgG molecules. The influence of electrostatic forces on protein adsorption has been studied by investigating the ability of adsorbed proteins to be displaced by an increase in ionic strength. Finally, the ionic strength effect on electrophoretic mobility of the sensitized cationic polystyrene beads has been analyzed.

Primitive models and electrophoresis: an experimental study

Several theories (developed in the context of the so-called primitive models) argue that the classical Poisson/ Boltzmann (PB) approach could fail to describe the electrical double layer in the case of high surface charge densities and/or ionic strengths. Under such conditions, the concentration of counterions in the vicinity of the surface would be larger than expected. What is more, the particle charge could be overcompensated (which is known as overcharging). This would lead to a considerable reduction in the magnitude of the z -potential (as compared with PB predictions) and, even, a reversal in its sign. Although these predictions are quite remarkable, these new approaches have been applied just scarcely in practice. Obviously, electrophoresis offers an excellent possibility of testing these theories. Accordingly, we have measured the electrophoretic mobility for latex particles with moderate and large surface charge densities at high ionic strengths. A commercial setup specifically designed for measuring extremely low mobilities has been used. The results have been interpreted with the help of the so-called hyper-netted-chain/mean-spherical approximation (HNC/MSA), a theory that includes ion size effects. In this way, overcharging has been experimentally investigated. Although the HNC/MSA is not a completely successful approach, our analysis suggests that the role of ion size correlations is important, particularly in electrolytes with a counterion valency of 2 or greater. # 2003 Elsevier B.V. All rights reserved.

Stabilization of protein-latex complexes at high ionic strength

Abstract When studying the colloidal stability of an immunolatex formed by F(ab′)2 protein fragments and a polystyrenepolychloromethylstyrene copolymer, striking results were obtained. After the expected particle aggregation induced by an increase in the electrolyte concentration (KBr), subsequent restabilization occurred when this electrolyte concentration was increased even more. This non-DLVO behaviour was studied under different experimental conditions (ionic strength, pH and surface coverage) and with a different solid support (polystyrene) and covering protein (bovine serum albumin) in order to generalize to other systems. An explanation based on ion hydration layer, similar to that proposed for hydrophilic surfaces, is assumed.