Article pubs.acs.org/JPCC
Beyond the Diffuse Layer Theory: A Molecular Analysis of the Structural, Dynamical, and Mechanical Properties of Charged Solid/ Liquid Interfaces A. Delville* Centre de Recherche sur la matière Divisée, CNRS − Université d’Orléans, 1B rue de la Férollerie, 45071 Orléans Cedex 02, France ABSTRACT: This work focuses on the long-range gap detected during the swelling of charged solid/liquid interfaces neutralized by monovalent counterions. A molecular model is used to investigate the influence of the long-range electrostatic coupling on the thermodynamical properties of charged solid/ liquid interfaces at separations varying between 20 and 40 Å, corresponding to the transition between discontinuous crystalline swelling, detected below 20 Å, and the continuous osmotic swelling, detected at separations larger than 40 Å. While the primitive model perfectly reproduces the stability of these charged interfaces in the osmotic swelling regime, it becomes inappropriate in the crystalline swelling regime, since it neglects the organization of the confined water molecules which is responsible for the discontinuous crystalline swelling. In that context, we study the influence of the valence of the neutralizing counterions and the location of the electric charges embedded within the charged lamellae on the structural, dynamical, and mechanical properties of these interfacial systems. This molecular approach clearly identifies two different environments accessible to the neutralizing sodium counterions, corresponding respectively to solvent separated and contact ionic pairing. The mobility of the confined counterions and water molecules is quantified by numerical simulations of molecular dynamics. Finally, the interfacial free energy provides, without introducing any additional parameter, the first theoretical interpretation of the coexistence between swollen and collapsed charged interfaces neutralized by monovalent counterions, in agreement with experimental evidence.
I. INTRODUCTION In the last decades, charged colloids were the subject of numerous experimental1−20 and numerical8,21−36 studies since their long-range electrostatic coupling is responsible for various properties (swelling, gelling, ion adsorption, and exchange capacity) exploited in numerous applications (civil engineering, drilling, paint and cosmetic industry, water treatment). In the framework of the diffuse layer theory, the overlap between the diffuse layers of condensed counterions1,2,37−39 was early identified as the source of the long-range repulsion between charged colloids occurring at separations larger than 40 Å and monitored by entropy-driven swelling mechanisms. At small separations (i.e.,
