arXiv subjects

A. Gama Goicochea

Publications and source records attributed to A. Gama Goicochea.

26 records · Page 2Linked to original sources

arXiv2014

Hyperscaling relation between the interfacial tension of liquids and their correlation length near the critical point

Interfaces involving coexisting phases in condensed matter are essential in various examples of soft matter phenomena such as wetting, nucleation, morphology, phase separation kinetics, membranes, phase coexistence in nanomaterials, etc. Most analytical theories available use concepts derived from mean field theory which does not describe adequately these systems. Satisfactory numerical simulations for interfaces at atomistic to mesoscopic scales remains a challenge. In the present work, the interfacial tension between mixtures of organic solvents and water is obtained from mesoscopic computer simulations. The temperature dependence of the interfacial tension is found to obey a scaling law with an average critical exponent mu = 1.23. Additionally, we calculate the evolution of the correlation length, defined as the thickness of the interface between the immiscible fluids, as a function of temperature and find that it obeys also a scaling law with the average critical exponent being nu = 0.67. Lastly, we show that the comparison of mu and nu for these binary mixtures constitutes the first test of Widom hyperscaling relation between these exponents in 3d, expressed as mu = nu (d - 1). Based on these values and those for the 3d Ising model it is argued that both systems belong to the same universality class, which opens up the way for the calculation of new scaling exponents.

cond-mat.stat-mech↗

arXiv2014

Competitive adsorption of surfactants and polymers on colloids by means of mesoscopic simulations

The study of competitive and cooperative adsorption of functionalized molecules such as polymers, rheology modifiers and surfactant molecules on colloidal particles immersed in a solvent is undertaken using coarse grained, dissipative particle dynamics simulations. The results show that a complex picture emerges from the simulations, one where dispersants and surfactants adsorb cooperatively up to certain concentrations, on colloidal particles, but as the surfactant concentration increases it leads to dispersant desorption. The presence of rheology modifying agents in the colloidal dispersion adds complexity through the association of surfactant micelles to hydrophobic sites of these agents. Analysis of the simulation results reported here point clearly to the self association of the hydrophobic sites along the different polymer molecules as the mechanism responsible for their competitive and cooperative adsorption.

cond-mat.soft↗

arXiv2014

Designing biodegradable surfactants and effective biomolecules with dissipative particle dynamics

The design of a biodegradable, environmentally friendly surfactant is carried out, taking the structure of a known surfactant that lacks these qualities as the starting point, using mesoscopic computer simulations. The newly designed surfactant is found to perform at least as well as its predecessor, without the latter inimical characteristics. In the second part of this work, a comparative study of model proteins with different amino acid sequence interacting with surfaces is undertaken. The results show that, all other aspects being equal, this sequence is the key factor determining the optimal activity of the proteins near surfaces. These conclusions are found to be in agreement with recent experiments from the literature.

cond-mat.soft↗

arXiv2014

Folding of the apolipoprotein A1 driven by the salt concentration as a possible mechanism to improve cholesterol trapping

The folding of the cholesterol trapping apolipoprotein A1 in aqueous solution at increasing ionic strength is studied using atomically detailed molecular dynamics simulations. We calculate various structural properties to characterize the conformation of the protein, such as the radius of gyration, the radial distribution function and the end to end distance. Additionally we report information using tools specifically tailored for the characterization of proteins, such as the mean smallest distance matrix and the Ramachandran plot. We find that two qualitatively different configurations of this protein are preferred, one where the protein is extended, and one where it forms loops or closed structures. It is argued that the latter promote the association of the protein with cholesterol and other fatty acids.

cond-mat.soft↗

arXiv2014

On the potential of mean force of a sterically stabilized dispersion

The potential of mean force (PMF) of a colloidal dispersion under various circumstances of current interest, such as varying solvent quality, polymer coating thickness, and addition of electrostatic interaction is obtained from radial distribution functions available from the literature. They are based on an implicit solvent, molecular dynamics simulation study of a model titania dispersion that takes into account three major components to the interaction between colloidal particles, namely van der Waals attraction, repulsion between polymer coating layers, and a hard core particle repulsion. Additionally, a screened form of the electrostatic interaction was included also. It is argued that optimal conditions for dispersion stability can be derived from a comparative analysis of the PMF under the different situations under study. This thermodynamics based analysis is believed to be more accessible to specialists working on the development of improved titania formulations than that based on the more abstract, radial distribution functions.

cond-mat.soft↗

arXiv2013

Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in theta solvent

We undertake the investigation of sheared polymer chains grafted on flat surfaces to model liposomes covered with polyethylene glycol brushes as a case study for the mechanisms of efficient drug delivery in biologically relevant situations, for example, as carriers for topical treatments of illnesses in the human vasculature. For these applications, specific rheological properties are required, such as low viscosity at high shear rate to improve the transport of the liposomes. Therefore non - equilibrium, DPD simulations of polymer brushes of various length and shear rates are performed to obtain the average viscosity and friction coefficient of the system as functions of the shear rate and polymerization degree under theta solvent conditions, and find that the brushes experience shear thinning at large shear rates.The viscosity and the friction coefficient are shown to obey scaling laws at high shear rate in theta solvent, irrespective of the brushes degree of polymerization. These results confirm recent scaling predictions and reproduce very well trends in measurements of the viscosity at high shear of red blood cells in a liposome containing medium.

physics.bio-ph↗

arXiv2012

Dissipative particle dynamics simulations of weak polyelectrolyte adsorption on charged and neutral surfaces as a function of the degree of ionization

The influence of the chain degree of ionization on the adsorption of weak polyelectrolytes on neutral and on oppositely and likely charged surfaces is investigated for the first time, by means of Monte Carlo simulations with the mesoscopic interaction model known as dissipative particle dynamics. The electrostatic interactions are calculated using the three-dimensional Ewald sum method, with an appropriate modification for confined systems. Effective wall forces confine the linear polyelectrolytes, and electric charges on the surfaces are included. The solvent is included explicitly also and it is modeled as an athermal solvent for the polyelectrolytes. The number of solvent particles is allowed to fluctuate. The results show that the polyelectrolytes adsorb both onto neutral and charged surfaces, with the adsorption regulated by the chain degree of ionization, being larger at lower ionization degrees, where polyelectrolytes are less charged. Furthermore, polyelectrolyte adsorption is strongly modulated by the counterions screening of surface charge. These findings are supported with predictions of adsorption isotherms with varying ionization degree. We obtain also the surface force mediated by adsorbed polyelectrolytes, which is calculated for the first time as a function of ionization degree. The adsorption and surface force isotherms obtained for weak polyelectrolytes are found to reproduce main trends in experiments, whenever those results are available, and provide additional insight into the role played by the competitive adsorption of the counterions and polyelectrolytes on the surfaces.

physics.bio-ph↗

arXiv2010

Simulation of Polyelectrolytes in Solution Using Dissipative Particle Dynamics in the Grand Canonical Ensemble: Interaction Strength and Salt Effects

We have studied a bulk electrolyte, and polyelectrolyte solutions with surfactants or multivalent salt with the explicit presence of counterions and solvent molecules by means of the mesoscopic dissipative particle dynamics (DPD) method in the Grand Canonical ensemble. The electrostatic interactions are calculated using the Ewald sum method and the structure of the fluid is analyzed through the radial distribution function between charged particles. The results are in very good agreement with those reported in the literature using a different method for the calculation of the electrostatic forces, and with those obtained using DPD in the canonical ensemble. We also studied the salt dependent conformation of polyelectrolyte solutions as a function of the solvent quality, and analyzed the electrostatic interaction strength dependence of dilute flexible polyelectrolytes in solution. For the complex systems mentioned above, the electrostatic interactions and the solvent quality play a key role in understanding phenomena that do not occur in noncharged systems.

cond-mat.soft↗