Simulation of electrokinetic and colloidal stability properties with the Primitive Model of Electrolyte

Abstract

Colloidal suspensions are important in various areas of technology ranging from medica! applications in therapy treatments and drug design to the synthesis of biocompatible complex molecules like polymer or polyelectrolyte gels. A profound understanding of the rules behind the electrokinetic and colloidal stability properties of colloids is important for the development of more applications with greater accuracy. At the molecular leve!, computar simulations within the Primitiva Model (PM) of electrolyte appear as a very useful tool. Although this model omits an explicit representation of solvent (water), it provides valuable information about ionic correlations (caused by coulombic interactions and/or ionic size) beyond the classical theories. In the present thesis, different aspects of colloidalsuspensions and polyelectrolyte solutions are studied with the help of Monte Cario (MC) simulations within the PM of electrolyte. The role of ionic correlations in the Electrical Double Layer (EDL) is tested contrasting computar simulations with a modified classical theory that includes ionic size effects. After the inclusion of dispersion torces in the PM, salt specificity is tested for colloidalsuspensions in the presence of mono- and divalent electrolytes. The EDL interactions are studied through net pressure calculations for a pair of equally and oppositely charged platas in the presence of mono- and divalent electrolyte solutions.The inclusion of bonding interactions between monomers in the PM makes possible to modal polyelectrolyte networks. The dependence on charge and temperatura of thermo-shrinking polyelectrolyte gels is analyzed with the help of a hydrophobic modal for the interaction between uncharged monomers of unbounded polymer units. The basic PM can also be modified to modal ions with interna! charge distribution. A model of spheroidal ions with two elementary charges is proposed. Thus, the quadrupolar interactions arising from the added orientational degrees of freedom and their effects on the EDL are analyzed providing further insight into the role of ionic correlations of coulombic and non-coulombic origins. The different analyses carried out exhibit a remarkable impact of ionic correlations. Through the consideration of realistic hydrated ionic diameters in the MC simulations, important disagreements appear with the classical EDL theories and even with previous computar simulation studies where the impact of ionic size was underestimated.Moreover, the importance of the salt specificity is proved to depend strongly on the size of the ions. The behavior of polyelectrolyte gels is found to depend on the competition between the length of the polyelectrolyte chains (together with the hydrophobic interaction they tend to shrink the network) and the charge of the polyelectrolyte chains (that tends to swell the network). In addition, discontinuous phase transitions arise in the case of weakly hydrophobic polyelectrolyte gels.