María José Gálvez-Ruiz

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.

Langmuir-Blodgett films of biopolymers: a method to obtain protein multilayers

In this work, we present a methodology for choosing the best experimental conditions for transferring protein Langmuir films onto solid substrates. As an example of applying the proposed methodology, we used monolayers of the protein bovine serum albumin, which is a very stable protein and is of great interest in the development of immunosensors. Langmuir-Blodgett (LB) films of this protein, on different solid substrates, were obtained and characterized as a function of pH, surface pressure, temperature, and contact angle. The compressibility modulus, the spreading entropy, and the fraction of desorbed protein sections were used as control parameters to find these conditions. A careful analysis of these parameters shows that there is a window on the values of these experimental parameters in which the LB films are best formed. Our methodology can be applied to other biomacromolecules to find the best conditions to form LB films from isotherm measurements.

Controlling lipolysis through steric surfactants: new insights on the controlled degradation of submicron emulsions after oral and intravenous administration.

In this work we have investigated how steric surfactants influence the metabolic degradation of emulsions (lipolysis). To do so, we have prepared submicron emulsions stabilized with Pluronic F68, Pluronic F127, Myrj 52 or Myrj 59, four non-ionic surfactants with key differences on their structure. Submicron emulsions have been prepared also with mixtures of these surfactants with different proportions between them. Then, in vitro methods have been applied to analyze the lipolysis of these emulsions, both under duodenal and intravenous conditions, to simulate lipolysis after oral and intravenous administration. Our results show that the properties of the surfactant influence dramatically the lipolysis rates observed both under duodenal and intravenous conditions, e.g., intravenous lipolysis was completely blocked when Pluronic F127 was used, while it was almost complete within 6h when using Myrj 52. The reason for this seems to be the steric hindrance that the surfactant produces around the droplet and at the interface. As a result, we can modify the lipolysis patterns by changing some characteristics of the surfactant, or by varying the proportion between two surfactants in a mixture. These findings may be applied in the development of novel strategies to rationally design submicron emulsions as lipophilic drug carriers.

Block copolymers at interfaces: interactions with physiological media.

Triblock copolymers (also known as Pluronics or poloxamers) are biocompatible molecules composed of hydrophobic and hydrophilic blocks with different lengths. They have received much attention recently owing to their applicability for targeted delivery of hydrophobic compounds. Their unique molecular structure facilitates the formation of dynamic aggregates which are able to transport lipid soluble compounds. However, these structures can be unstable and tend to solubilize within the blood stream. The use of nanoemulsions as carriers for the lipid soluble compounds appears as a new alternative with improved protection against physiological media. The interfacial behavior of block copolymers is directly related to their peculiar molecular structure and further knowledge could provide a rational use in the design of poloxamer-stabilized nanoemulsions. This review aims to combine the new insights gained recently into the interfacial properties of block copolymers and their performance in nanoemulsions. Direct studies dealing with the interactions with physiological media are also reviewed in order to address issues relating metabolism degradation profiles. A better understanding of the physico-chemical and interfacial properties of block copolymers will allow their manipulation to modulate lipolysis, hence allowing the rational design of nanocarriers with efficient controlled release.

Adsorption of antibody onto Pluronic F68-covered nanoparticles: link with surface properties

The use of nanoparticles as drug delivery systems is an emerging application to improve intravenous therapy. Controlling the biocompatibility of the nanoparticles is a crucial step towards the optimal implementation of these systems. Adsorption of serum components onto the nanoparticles is driven mainly by hydrophobic forces. Thus, incorporation of hydrophilic polymers such as polyethylene oxide (PEO) derivatives to the nanostructure surface reduces the interaction of nanoparticles with blood stream components (IgG). The effectiveness of the poloxamer for reducing protein adsorption depends on the resistance of this coating layer. A fundamental understanding of the properties of this surface coating is crucial towards the rational design of these systems. Here, we have used an innovative combination of experimental techniques to evaluate the properties of the nanoparticles and more specifically, the mechanical properties of the coating. Electrophoretic mobility and colloidal stability data suggest similar surface characteristics between IgG–Pluronic–polystyrene (PS) and IgG–PS complexes, indicating that the protein adsorption is just slightly reduced by the presence of poloxamer. Nevertheless, the biological activity of the adhered antibodies suggests that the Pluronic F68 significantly altered their immunoactivity. The decrease in the activity might indicate a partial denaturation of the protein and/or changes in the preferential orientation when adsorbing caused by the surfactant–protein interactions. The surface characterisation of the IgG layers adsorbed onto a Pluronic covered surface importantly provides evidence of the conformational change undergone by the protein, supporting the partial protein denaturation suggested by the loss of immunoreactivity in the IgG–Pluronic–PS particles. The use of surface tension to obtain structural and mechanical information about the coating procedure is a novel approach to understand generic features of the biocompatibility of colloidal systems. These results may help to understand why drug nanocarriers coated by poloxamers improve their long-circulating properties in comparison with uncoated particles.

Interactions, desorption and mixing thermodynamics in mixed monolayers of β-lactoglobulin and bovine serum albumin

The interactions between milk proteins, beta-Lactoglobulin (beta-Lg) and bovine serum albumin (BSA), at the air-water interface have been evaluated. The surface pressure (pi), molecular area (a) isotherms were obtained by compression of the monolayers at different pH and temperature. In the method used to calculate the interactions, the desorbed segments of the proteins into the aqueous subphase have been considered. Earlier, the desorbed segments have been estimated from the compressibility factor, z, as a function of the surface pressure (virial state equation). The main conclusion from this study is that for biopolymers it is not possible to apply only the mixing thermodynamics to evaluate the intermolecular forces. It is necessary to include the desorption phenomenon. From these results, we can conclude that the main interaction between both proteins is of electrostatic character.

Evaluation of the interactions between lipids and γ-globulin protein at the air–liquid interface

The interactions between lipids (cholesterol, distearoylphosphatidylcholine, distearoylphosphatidylethanolamine and sphingomyelin) and the g-globulin protein have been analyzed using the monolayer technique at the air‐liquid interface. The analysis has been carried out using both state equations and an adequate thermodynamic formulation for the surface pressure (p)‐molecular area (a) isotherms. Different parameters as the virial coefficients, have been estimated. For the uncharged lipid monolayers, the interactions between the molecules are of an attractive nature, at medium and long distance, and of a steric repulsive nature at short distance. At low surface pressures the lipid molecules form small domains. The net force between g-Globulin molecules in the monolayers has been found to be attractive. Finally, it can be concluded that when the lipid monolayers are uncharged, there is practically no interaction between the protein and lipid molecules at the mentioned interface.

The effect of polymeric surfactants on the rheological properties of nanoemulsions

We have investigated the rheological properties of submicron emulsions and how they are affected by the structure of polymeric surfactants. We have prepared oil-in-water emulsions stabilized with five steric surfactants, two of them belonging to the Myrj family and three belonging to the Pluronic family, with key differences on their structures. Droplet size and volume fraction have been kept constant to analyze only the influence of the surfactant. The viscoelasticity has been characterized by dynamic oscillatory shear experiments, while the shear viscosity was measured during steady shear flow tests. The results show a qualitatively similar gel-like behavior for all the emulsions, but with remarkable quantitative differences. Surfactants with longer hydrophilic tails produced emulsions with higher viscoelasticity. Pluronics, having a central hydrophobic part between two hydrophilic tails, produced emulsions with notably higher viscoelasticity and yield stress than Myrjs with comparable hydrophilic tails. The reason for this seems to be a more efficient steric barrier at the interface, induced by this central hydrophobic part.

Measurement of interactions between protein layers adsorbed on silica by atomic force microscopy

The present work, using an atomic force microscope and the colloid probe technique, investigates the interaction forces between bovine serum albumin (BSA) layers and between apoferritin layers adsorbed on silica surfaces. The measurements have been carried out in an aqueous medium at different pH values and NaCl concentrations. Similar behaviours have been found with both proteins. Electrostatic and steric forces dominate the interactions between the protein layers at low NaCl concentrations. However, a very strange behaviour is found as a function of pH at high NaCl concentrations. The results obtained under these conditions could be explained if the presence of hydration forces in these systems is assumed.

Preparation, characterization and in vivo evaluation of nanoemulsions for the controlled delivery of the antiobesity agent N-oleoylethanolamine

UNLABELLED > AIMS N-oleoylethanolamine (OEA) is a lipid mediator that acts as a satiety factor. The main limiting factor for its administration is its poor water solubility. We designed and characterized new nanoemulsions as delivery system for hydrophobic compounds such as OEA. MATERIALS & METHODS The nanoemulsion components and preparation methods were selected in order to achieve the desired final properties. Then, we evaluated the in vivo properties of the nanoemulsions as drug-delivery systems testing the anorectic effects of OEA in rats after both intragastric and intraperitoneal administration. The in vivo toxicity of the nanoemulsions was evaluated after a 3-week treatment. RESULTS Nanoemulsions proved to be stable, nontoxic and had no effect on feeding behavior when administered without OEA. The effects of OEA were observable after its oral and parenteral administration with the nanoemulsions to 24-h fasted rats, finding a better efficacy compared with a vehicle containing Tween(®) 20 (Sigma-Aldrich, MO, USA) after oral administration. CONCLUSION These results support the efficacy of these nanoemulsions to deliver highly hydrophobic bioactive drugs.