M. Tirado-Miranda

The aggregation behaviour of protein-coated particles: a light scattering study

The different mechanisms involved in the aggregation of spherical latex particles coated with bovine serum albumin (BSA) have been studied using static and dynamic light scattering. These techniques assess the fractal dimension of the aggregates and their mean hydrodynamic radius. Particles with different degrees of surface coverage have been prepared. The net charge of the covered particles has been modified by varying the pH of the aqueous phase. The aggregation rate was measured and used to determine the importance of the different aggregation mechanisms that are responsible for these types of flocculation processes. At low and intermediate degrees of surface coverage, bridging flocculation is the principal aggregation mechanism irrespective of the electrical state of the protein-particle complexes. At high degree of surface coverage, however, weak flocculation is important only when the BSA molecules are at their isoelectric point.

Effective charges of colloidal particles obtained from collective diffusion experiments

In this work, the collective diffusion coefficient of highly charged colloidal particles in dilute dispersions has been measured by means of dynamic light scattering. The possibility of obtaining valuable information about the particle charge from these data is looked into with the help of electrophoresis experiments. Our results suggest that this is possible in the case of slight or moderately interacting particles as long as experimental data are properly treated. For highly interacting colloids, however, such information could not be so reliable, presumably due to certain shortcomings of the experimental technique at low angle. The role of charge renormalization is also discussed in this work.

Dynamic scaling and fractal structure of small colloidal clusters

Abstract We studied the aggregation of small colloidal polystyrene spheres during the limiting growth regime of diffusion-limited aggregation. The number-average mean cluster size and the fractal dimension of the aggregates were obtained by dynamic and static light scattering (DLS and SLS). The experimental results are interpreted using the spatial and dynamic scaling formalism. The homogeneity exponent λ was used to identify the aggregation mechanism. An unexpected cluster size scaling was observed for early aggregation stages. This feature allowed the use of spatial and dynamic scaling concepts as an alternative method for determining the Smoluchowski rate constant k 11 .

Aggregation of protein-coated colloidal particles: Interaction energy, cluster morphology, and aggregation kinetics

The salt-induced aggregation of polystyrene particles, partially coated with monomeric bovine serum albumin molecules, has been studied. For this purpose, the cluster fractal dimension df and the van Dongen–Ernst homogeneity parameter λ have been measured by means of static and dynamic light scattering techniques. The theoretical analysis indicates that aggregation takes places at an interaction energy minimum predicted by a modified Derjaguin–Landau–Verwey–Overbeeck (DLVO) theory when additional osmotic and elastic terms are considered. It could be shown that the depth of the energy minimum is directly related to df and λ. Moreover, the particle–particle distance within the cluster could also be assessed and compared with the interparticle distance at which the DLVO-predicted minimum appears.

Primary and secondary bonds in field induced aggregation of electric double layered magnetic particles.

Magnetic filaments able to survive on removal of the magnetic field have led to new applications in biotechnology and microfluidics. In this work, the stability of linear field induced aggregates composed of electric double layered magnetic particles has been studied in the framework of the DLVO theory. A suitable system of differential equations is proposed in order to determine how the percentage of bonds formed in a primary or a secondary energy minimum depends on the exposure time to the magnetic field. The theoretical predictions were compared with experimental data obtained by means of dynamic light scattering. The aggregation experiments were performed at different electrolyte concentrations and exposure times to the magnetic field. Fitting the experimental results according to the proposed theory allowed us to determine the rate at which bonds formed in a secondary energy minimum turn into stable bonds.

A simpler derivation of the integral formula of electrical resistance

The proportionality constant in Ohms law is the electrical resistance. The resistance of conducting objects depends on their geometrical features such as cross-sectional area and length. Since the cross-sectional area of resistors usually varies as its length, the calculation of resistances is a non-trivial task. In this letter, a closed-form integral expression for resistance is just derived for resistors with parallel curved terminals. The derivation proposed here, equally applicable to the calculation of capacitances, uses the properties of an electric field and the mathematical concept of parallel surfaces. This new teaching strategy can be useful for first undergraduate courses of science and engineering studies.

Infuence of surface characteristics on fast-aggregating protein-coated polymer colloids

A study of fast aggregating surface modified colloidal particles is presented. The surface characteristics of the particles were modified by adsorbing different amounts of bovine serum albumin. Since the protein charge depends on the electrochemical properties of the suspension medium, the influence of the protein net charge on both the cluster structure and the aggregation mechanisms could be studied by changing the pH of the aqueous phase. Physical parameters, such as the fractal dimension, the scaling exponent and the aggregation rate, were determined by laser light scattering. The experimental results show that the adsorbed protein molecules modify the aggregation mechanism and allow structural rearrangement within the clusters.