Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl-CaCl2) solutions

Ian C. Bourg, Garrison Sposito

Research output: Contribution to journalArticle

130 Scopus citations

Abstract

We report new molecular dynamics results elucidating the structure of the electrical double layer (EDL) on smectite surfaces contacting mixed NaCl-CaCl2 electrolyte solutions in the range of concentrations relevant to pore waters in geologic repositories for CO2 or high-level radioactive waste (0.34-1.83molcdm-3). Our results confirm the existence of three distinct ion adsorption planes (0-, β-, and d-planes), often assumed in EDL models, but with two important qualifications: (1) the location of the β- and d-planes are independent of ionic strength or ion type and (2) " indifferent electrolyte" ions can occupy all three planes. Charge inversion occurred in the diffuse ion swarm because of the affinity of the clay surface for CaCl+ ion pairs. Therefore, at concentrations ≥0.34molcdm-3, properties arising from long-range electrostatics at interfaces (electrophoresis, electro-osmosis, co-ion exclusion, colloidal aggregation) will not be correctly predicted by most EDL models. Co-ion exclusion, typically neglected by surface speciation models, balanced a large part of the clay mineral structural charge in the more concentrated solutions. Water molecules and ions diffused relatively rapidly even in the first statistical water monolayer, contradicting reports of rigid " ice-like" structures for water on clay mineral surfaces.

Original languageEnglish (US)
Pages (from-to)701-715
Number of pages15
JournalJournal of Colloid And Interface Science
Volume360
Issue number2
DOIs
StatePublished - Aug 15 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Biomaterials
  • Surfaces, Coatings and Films
  • Colloid and Surface Chemistry

Keywords

  • Adsorption
  • Clay mineral
  • Diffusion
  • Electrical double layer
  • Molecular dynamics simulation
  • Montmorillonite
  • Smectite
  • Surface complex
  • Surface complexation model
  • Triple layer model

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