Effective charges and virial pressure of concentrated macroion solutions

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-07, Vol.112 (30), p.9242-9246
• Main Authors: Boon, Niels, Guillermo Ivan Guerrero-García, Reníé van Roij, Monica Olvera de la Cruz
• Format: Article
• Language: English
• Subjects: cell model; colloids; DLVO; Effectiveness; electrolytes; electrostatic interactions; Ions; macroions; MATERIALS SCIENCE; Molecular biology; osmotic pressure; Physical properties; Physical Sciences; Pressure; Salt; society
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1511798112

The stability of colloidal suspensions is crucial in a wide variety of processes, including the fabrication of photonic materials and scaffolds for biological assemblies. The ionic strength of the electrolyte that suspends charged colloids is widely used to control the physical properties of colloidal suspensions. The extensively used two-body Derjaguin−Landau−Verwey−Overbeek (DLVO) approach allows for a quantitative analysis of the effective electrostatic forces between colloidal particles. DLVO relates the ionic double layers, which enclose the particles, to their effective electrostatic repulsion. Nevertheless, the double layer is distorted at high macroion volume fractions. Therefore, DLVO cannot describe the many-body effects that arise in concentrated suspensions. We show that this problem can be largely resolved by identifying effective point charges for the macroions using cell theory. This extrapolated point charge (EPC) method assigns effective point charges in a consistent way, taking into account the excluded volume of highly charged macroions at any concentration, and thereby naturally accounting for high volume fractions in both salt-free and added-salt conditions. We provide an analytical expression for the effective pair potential and validate the EPC method by comparing molecular dynamics simulations of macroions and monovalent microions that interact via Coulombic potentials to simulations of macroions interacting via the derived EPC effective potential. The simulations reproduce the macroion−macroion spatial correlation and the virial pressure obtained with the EPC model. Our findings provide a route to relate the physical properties such as pressure in systems of screened Coulomb particles to experimental measurements. Colloids constitute the basic components of many everyday products and are integrated into the fabric of modern society. Understanding their assembly is key for nanotechnological and biotechnological advances. At the single-particle level, colloids commonly possess electric charge. Consequently, the structure to which they conform is strongly influenced by electrostatic interactions. In solution, these interactions are modified by the presence of ions. We have developed a model for computing the corresponding effective electrostatic interactions as well as the osmotic pressure. Our model extends the applicability of Derjaguin−Landau−Verwey−Overbeek theory to dense systems in which many-body effects are cruci