A model for foreign exchange markets based on glassy Brownian systems.

In this work we extend a well-known model from arrested physical systems, and employ it in order to efficiently depict different currency pairs of foreign exchange market price fluctuation distributions. We consider the exchange rate price in the time range between 2010 and 2016 at yearly time intervals and resolved at one minute frequency. We then fit the experimental datasets with this model, and find significant qualitative symmetry between price fluctuation distributions from the currency market, and the ones belonging to colloidal particles position in arrested states. The main contribution of this paper is a well-known physical model that does not necessarily assume the independent and identically distributed (i.i.d.) restrictive condition.

Diffusive and Arrestedlike Dynamics in Currency Exchange Markets.

This work studies the symmetry between colloidal dynamics and the dynamics of the Euro-U.S. dollar currency exchange market (EURUSD). We consider the EURUSD price in the time range between 2001 and 2015, where we find significant qualitative symmetry between fluctuation distributions from this market and the ones belonging to colloidal particles in supercooled or arrested states. In particular, we find that models used for arrested physical systems are suitable for describing the EURUSD fluctuation distributions. Whereas the corresponding mean-squared price displacement (MSPD) to the EURUSD is diffusive for all years, when focusing in selected time frames within a day, we find a two-step MSPD when the New York Stock Exchange market closes, comparable to the dynamics in supercooled systems. This is corroborated by looking at the price correlation functions and non-Gaussian parameters and can be described by the theoretical model. We discuss the origin and implications of this analogy.

Permeability of Soft Particles at Electrical Fields.

The influence of the charge on the permeability of microgel particles is studied in the presence of electric fields. Electrophoresis experiments performed on pH-ionizable pNIPAM-AA microgels show that particles behave as permeable spheres when the network is ionized. However, they keep non-permeable in the absence of charge. The ionic nature of the network thus controls the permeability of the soft particles. A salt-dependent local viscosity explains these permeability changes. This is confirmed by NMR as alternative independent technique. Strongly hydrated counterions located around fixed charges on the network are considered responsible for the local viscosity variations.

Structure factor scaling in colloidal charge heteroaggregation.

The structure factor, S(q), of a system composed of a 1:1 mixture of oppositely charged colloids undergoing heteroaggregation is studied by Browninan dynamics simulations. A peak develops in S(q) at low wave vectors, which can be scaled for different times to overlap using the scaling of spinodal decomposition, as shown for DLCA. The same master function is obtained for different interaction ranges. The origin of the peak can be traced back to a depletion layer of clusters surrounding every aggregate. At those long distances, cluster-cluster interaction is negligible and the aggregation is diffusion limitted, as deduced from the evolution of peak position, and the S(q) scaling at different interaction ranges. The interaction is, nevertheless, strong enough to affect the internal cluster structure.

Ionic correlations in highly charge-asymmetric colloidal liquids.

We use electrophoretic mobility (mu) measurements of charged colloidal particles under the presence of multivalent counterions as a probe of the electrostatic correlations between them; they become important for sufficiently high surface charge densities of the colloid (sigma) and result in a decreasing mu upon increasing sigma. The physics of this decrease is the same as that giving rise to charge inversion. We account qualitatively for the observations by considering recent theoretical arguments that assume the counterions next to the colloid surface as a strongly correlated liquid of properties similar to that of a Wigner crystal.

Static light scattering from microgel particles: model of variable dielectric permittivity.

We perform static light scattering experiments on a dilute suspension of microgel particles and model the resultant form factors Pq by assuming an exponentially decaying dielectric permittivity. The result is that Pq is a Lorentzian function of the scattering wavevector q for length scales greater than the particle size; the width approximately corresponding to twice the particle radius. This simple model reasonably accounts for scattered light from both swollen and shrunken microgel phases.

Internal structure of clusters from charge heteroaggregation.

The internal structure of clusters formed by colloidal heteroaggregation of particles with opposite signs of charge is studied by means of computer simulations. Every particle is surrounded by a layer of particles of opposite sign, a second neighbors shell of particles mainly with the same sign, a third one of opposite sign, etc. As the distance from the particle increases, the system becomes more homogeneous and no difference between the numbers of particles with similar or opposite signs of charge can be noticed for distances larger than ten times the particle radius. For low ionic concentrations the local environment of particles is formed by quasi-straight branches, where the sign of charge alternates, and at high concentrations the structure of the cluster is typical of DLCA and the alternation is restricted to very short distances. However, this effect is not responsible for the low fractal dimensions observed in charge heteroaggregates.

Application of the colloidal stability of TiO2 particles for recovery and reuse in solar photocatalysis.

TiO2-catalyst suspensions work efficiently in photocatalysis for wastewater treatment. Nevertheless, once photocatalysis is complete, separation of the catalyst from solution becomes the main problem. Catalyst recovery has been enhanced through charge neutralisation and coagulation with electrolytes at lab and pilot-plant scale (40 L) to evaluate the potential for its separation after photocatalytic degradation of pollutants. Zeta-potential analysis showed that the isoelectric point (IEP) of TiO2 suspensions is near pH 7. Settling rates and hydrodynamic diameter of TiO2 particles are maximum at the IEP. However, suspensions are stable at different pH. TiO2 was reused in solar photocatalysis pilot-plant (40 L) for treatment of tetrachloroethylene (C2Cl4) comparing two procedures: reuse of the entire suspension after destruction of the organics without separation of the catalyst, and reuse of the catalyst after it had settled to the bottom and clear water had been removed. Photocatalytic efficiency worsens with successive runs when catalyst and water are reused without separation, whereas, when TiO2 is separated, the photocatalyst is not deactivated.

Induced asymmetries in the heteroaggregation of oppositely charged colloidal particles.

Heterocoagulation of cationic and carboxylated polystyrene latexes is studied for a wide range of salt concentrations by static light scattering. The weak character of the surface groups providing the charges allows variation of the relative charge of the systems. Two situations are studied: both latexes with similar surface charges and with very different ones. In both cases at low ionic concentration pure heteroaggregation takes place, whereas diffusive aggregation is observed at high kappa, above the critical coagulation concentration (C.C.C.) of both latexes. The overall rate of aggregation describes a minimum at intermediate salt concentrations when both latexes bear similar charges. The heterocoagulation rate constant decreases continuously to reach the diffusive value at high salt. An interesting behavior is observed when the latexes have very different charge. The heterocoagulation kinetic constant becomes diffusive above the C.C.C. of the less charged latex.

Colloidal stability of a temperature-sensitive poly(N-isopropylacrylamide/2-acrylamido-2-methylpropanesulphonic acid) microgel.

In this work, a study of the thermally induced flocculation of poly(N-isopropylacrylamide/2-acrylamido-2-methylpropanesulphonic acid) microgel, that is, poly(NIPAM/aMPS) microgel samples is presented. The aggregation process is monitored in continuous by dynamic light scattering measurements. Theoretical explanations are considered and discussed in terms of interaction potentials, paying special attention to osmotic and elastic contributions due to particle interpenetration. The rate of aggregation is studied as a function of temperature, and theoretical calculations of kinetic constants based on model core-shell particle interactions provide very good agreement with experimental results. The reversibility of aggregation is also analyzed. It is found that aggregation is completely reversible only under certain conditions: the temperature of an aggregated sample must decrease under 20 degrees C. These results are experimental evidence of the secondary minimum in the interaction potential where aggregation occurs.

Structure formation from mesoscopic soft particles.

In this work, the aggregation of mesoscopic gel particles (soft colloids) has been experimentally investigated. The interaction between particles was controlled through the addition of salt, above the critical coagulation concentration, resulting in aggregation with finite bond energies. Attention has been paid to the structure of the clusters formed in the process as well as to the aggregation kinetics. The results indicate that the clusters are fractal and the kinetics of aggregation can be described through the dynamic scaling solution of the Smoluchowski equation. As the energy minimum increases in depth the resultant clusters pass from a very compact structure to typical diffusion-limited cluster aggregation (DLCA) fractal dimension values. In addition, the kinetics of growth change from those observed in reaction controlled aggregation to DLCA. These results can be explained within the framework of a reversible growth model, arising from the fact that aggregation takes place in an energy minimum of restricted depth. Moreover, they show that structure and kinetics decouple for such a soft sphere system, in contrast to what is encountered for DLCA and reaction-limited processes. Finally, an unexpected return to a reaction controlled aggregation kinetics was observed for sufficiently deep energy minima, which could be due to the polymerlike particularities of the soft particles considered in this work.

Nonlinear effects in the stability of highly charged colloidal suspensions.

We investigate the nonlinear effects related to the formation of particle-counterion clusters in highly charged asymmetric colloidal suspensions. The ocurrence of such clustering is experimentally probed by studying the stability of the colloidal system. The results demonstrate that a renormalized charge is needed in order to explain the observed critical coagulation concentrations. This renormalization is predicted by an extension of the Debye-Hückel-Bjerrum liquid state theory [A. Diehl, M. C. Barbosa, and Y. Levin, Europhys. Lett. 53, 86 (2001)]. Therefore, counterion condensation seems to become apparent in particle aggregation processes through control of the repulsive barrier that keeps the system stable. As a consequence of the agreement, new insights into the microscopic state of highly charged complex fluids follow.

Electrokinetic Characterization of Poly(N-isopropylacrylamide) Microgel Particles: Effect of Electrolyte Concentration and Temperature.

A fundamental study about the characterization of a poly(N-isopropylacrylamide), poly(NIPAM), microgel, cross-linked with N,N'-methylene bisacrylamide (BA), has been carried out. The anionic charge of this system was increased by copolymerization with 2-acrylamido-2methylpropanesulphonic (AMPS) acid. The electrokinetic behavior of these aMPS/NIPAM microgel particles has been investigated. First, the surface charge density was determined by conductimetric titration, and the particle size was obtained by transmission electron microscopy under several conditions. Electrophoretic mobility measurements were carried out as a function of electrolyte concentration and temperature. The hydrodynamic diameter of the particles was also measured by photon correlation spectroscopy as a function of electrolyte concentration and temperature. These size data have been taken into account in order to explain the mobility behavior by using Ohshima's theory for soft particles, with very good agreement between experiments and theoretical predictions. Copyright 2001 Academic Press.

Particle-counterion clustering in highly charge-asymmetric complex fluids.

The formation of particle-counterion clusters through electrostatic interaction is studied in this work for a highly charge asymmetric colloidal suspension. The occurrence of such clustering is probed by the particle electrophoretic mobility, i.e., with the aid of a transport property. The results show that the effective charge manifesting under the presence of an external electric field is the renormalized charge predicted by an extension of the Debye-Hückel-Bjerrum theory to the fluid state of highly charged colloids.

Coadsorption of IgG and BSA onto sulfonated polystyrene latex: I. Sequential and competitive coadsorption isotherms.

In this work the sequential and competitive coadsorption of IgG and BSA proteins on a sulfonate polystyrene latex with high surface charge density have been studied. For sequential coadsorption the IgG/a-CRP was first adsorbed and then the free surface of the particle was saturated by redispersion of the pellet in a solution with a high concentration of monomeric BSA (m-BSA). The competitive coadsorption experiments were carried out in two separate experiments by changing the initial concentration of one protein when the concentration of other protein was high and constant. During the incubation the pH was 5 or 6, and the ionic strength 2 mM, as in previous studies the adsorption of BSA was very low at neutral or basic pH regardless of the amount of adsorbed IgG. From these coadsorption experiments it was possible to obtain latex-protein complexes with a similar degree of coverage by each protein, high adsorption of IgG and different amounts of BSA, or high adsorption of BSA and a low, but significant, amount of IgG. The latex-protein complexes were electrokinetically characterized by measuring the electrophoretic mobility of each complex vs the pH of redispersion. In that way we can detect the i.e.p. of the complexes and the pH range in which the electrostatic repulsion can make them colloidally stable.

Coadsorption of IgG and BSA onto sulfonated polystyrene latex: II. Colloidal stability and immunoreactivity.

The present work deals with the study of the colloidal stability and immunoreactivity of sulfonated polystyrene latex particles covered by different amounts of m-BSA and IgG/a-CRP. These proteins have been previously adsorbed onto a sulfonated latex by sequential and competitive coadsorption experiments and it was possible to obtain latex-protein particles with different degrees of coverage by each protein. The latex particles, fully or partially covered by each protein (termed latex-protein complexes), were resuspended under several conditions (different pH and ionic strength values) and their colloidal stability, vs the addition of the electrolyte was studied using turbidity measurements. This stability appeared at a high degree of coverage by BSA and at a pH in which the BSA was negatively charged. At a high degree of coverage by IgG, the latex particles were unstable at all pHs. As a final part of this work, the immunoreactivity of several complexes was studied following the changes in the turbidity after the addition of CRP antigen. Only the complexes which were colloidally stable gave detectable reactivity. However, the complexes with a relatively low degree of coverage by IgG/a-CRP gave good immunoreactivity. Therefore, the latex-protein complex properties depended on the percentage of BSA or IgG adsorbed and on the electric state of the proteins at the redispersion pH. Under specific incubation conditions, sulfonated latex covered by significant IgG/BSA percentages was obtained, which showed a high colloidal stability and good immunoreactivity.