Biomedical subjects

A B Jódar-Reyes

Publications and source records attributed to A B Jódar-Reyes.

6 recordsLinked to original sources

PubMed2006

Can linear micelles bridge between two surfaces?

Surfactants are extensively used as stabilizers of colloidal particles, even though the use of high surfactant concentrations can induce a loss of the stability of the dispersion. The depletion mechanism is believed to be responsible for this instability. In this paper, we show that there exists an alternative interpretation, namely that wormlike micelles can bridge between two surfaces. Such a stalk-like object connecting two adsorbed bilayers is (in first order) stable when the endcap (free) energy of the wormlike micelle (in solution) is higher than the connection (free) energy of the stalk with the surface layer. As an example, we consider an aqueous solution of nonionic C(12)E(6) surfactants and use a molecularly realistic self-consistent field approach to evaluate the free-energy of bridge formation. It appears impossible to connect linear micelles to hydrophobic surfaces onto which a monolayer of surfactants exists, and stalks only occur with an exponentially low probability for very hydrophilic surfaces. However, at a wide regime of moderately hydrophilic surfaces the stalks are thermodynamically stable. In this regime, the adsorbed bilayers are typically only marginally stable. We identify a range of parameters for which such adsorbed bilayer ruptures around the stalk and then the wormlike micelle essentially connects (head-on) to the bare surface. The strength of interaction is of the order of the endcap energy which easily exceeds 10 k(B)T. The range of interactions is expected to be large as it is set by the characteristic size of the linear micelles in solution. The regime of moderately hydrophilic surfaces is relevant experimentally, and we conclude that surfactant-induced flocculation may well be the result of stalks. The depletion mechanism is only expected for systems with extremely hydrophobic and with very hydrophilic particles.

Adsorption↗

PubMed2006

Colloidal stability of pluronic F68-coated PLGA nanoparticles: a variety of stabilisation mechanisms.

Poloxamers are a family of polypropylene oxide (PPO) and polyethylene oxide (PEO) tri-block copolymers that are usually employed in the micro- and nanoparticulate engineering for drug delivery systems. The aim of this work is to study the electrophoretic mobility (mu(e)) and colloidal stability of complexes formed by adsorbing a poloxamer (Pluronic F68) onto poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles. A variety of stabilisation mechanisms have been observed for the Pluronic-coated PLGA nanoparticles, where DLVO interactions, solvent-polymer segment interactions and hydration forces play different roles as a function of the adsorbed amount of Pluronic. In addition, the mu(e) and stability data of these complexes have been compared to those obtained previously using a PLGA-Pluronic F68 blend formulation. As both the mu(e) and the stability data are identical between the two systems, a phase separation of both components in the PLGA-Pluronic blend formulation is suggested, being the PLGA located in the core of the particles and the Pluronic in an adsorbed shell.

Adsorption↗

PubMed2006

Self-consistent field modeling of linear nonionic micelles.

A self-consistent field theory is used to predict structural, mechanical, and thermodynamical properties of linear micelles of selected nonionic surfactants of the type C(n)()E(m). Upon increase in surfactant concentration the sudden micelle shape transition from spherical to cylindrical (second critical micelle concentration (cmc)) is analyzed. The cylindrical micelles consist of a body (with radius R(c) and length L) and two slightly swollen endcaps. For small L, the shape resembles a dumbbell. With increase in the length of the body, an oscillatory behavior in the grand potential of the micelle is found. The wavelength of the oscillation (lambda(d)) is proportional to the surfactant tail length n. The amplitude of these oscillations decreases exponentially with a decay length xi. In the limit of very long micelles, the grand potential converges to the endcap (free) energy E(c). This endcap energy increases approximately quadratic with the tail length and diminishes by increasing the headgroup size m. The micelle size distribution is generated showing non-monotonic features due to the presence of short dumbbells and becomes exponential when L >> 8R(c). It is also shown that the endcap energy can be estimated in first order by the grand potential of the spherical micelle that coexists with infinitely long cylindrical micelles. The persistence length l(p) of these linear micelles is evaluated to estimate the relative importance of conformational entropy for these micelles.

Journal Article↗

PubMed2005

Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants.

This work is focused on analyzing the electrokinetic behavior and colloidal stability of latex dispersions having different amounts of adsorbed ionic surfactants. The effects of the surface charge sign and value, and the type of ionic surfactant were examined. The analysis of the electrophoretic mobility (mu(e)) versus the electrolyte concentration up to really high amounts of salt, much higher than in usual studies, supports the colloidal stability results. In addition, useful information to understand the adsorption isotherms was obtained by studying mu(e) versus the amount of the adsorbed surfactant. Aggregation studies were carried out using a low-angle light scattering technique. The critical coagulation concentrations (ccc) of the particles were obtained for different surfactant coverage. For latex particles covered by ionic surfactants, the electrostatic repulsion was, in general, the main contribution to the colloidal stability of the system; however, steric effects played an important role in some cases. For latices with not very high colloidal stability, the adsorption of ionic surfactants always improved the colloidal stability of the dispersion above certain coverage, independently of the sign of both, latex and surfactant charge. This was in agreement with higher mobility values. Several theoretical models have been applied to the electrophoretic mobility data in order to obtain different interfacial properties of the complexes (i.e., zeta potential and density charge of the surface charged layer).

Adsorption↗

PubMed2005

Hofmeister effects on the colloidal stability of an IgG-coated polystyrene latex.

The effect of various ions related to the Hofmeister series (HS) on different properties of a cationic latex covered with a protein (IgG) is analyzed in this study. NaNO3, NH4NO3, and Ca(NO3)2 were used to compare the specificity of the cations, and NaCl, NaSCN, NaNO3, and Na2SO4, to compare the specificity of the anions. Two pH values, 4 and 10, were chosen to analyze the behavior of these ions acting as counter- and co-ions. At pH 4, the total surface charge is positive, whereas at pH 10 it is negative. Three different phenomena have been studied in the presence of these Hofmeister ions: (1) colloidal aggregation, (2) electrophoretic mobility, and (3) colloidal restabilization. The specific effect of the ions was clearly observed in all experiments, obtaining ion sequences ordered according to their specificity. The most important parameter for ion ordering was the sign of the charge of the colloidal particle. Positively charged particles displayed an ion order opposite that observed for negatively charged surfaces. Another influential factor was the hydrophobic/hydrophilic character of the particle surface. IgG-latex particle surfaces at pH 10 were more hydrophilic than those at pH 4. The SCN- ion had a peculiar specific effect on the phenomena studied (1)-(3) at pH 10. With respect to the restabilization studies at high ionic strengths, new interesting results were obtained. Whereas it is commonly known that cations may provoke colloidal restabilization in negative particles when they act as counterions, our experiments demonstrated that such restabilization is also possible with positively charged particles. Likewise, restabilization of negative surfaces induced by the specific effect of chaotropic anions (acting as co-ions) was also observed.

Adsorption↗

PubMed2005

Adsorption of different amphiphilic molecules onto polystyrene latices.

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain-surface and chain-chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.

Adsorption↗