TY - JOUR
T1 - Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl-CaCl2) solutions
AU - Bourg, Ian C.
AU - Sposito, Garrison
N1 - Funding Information:
This research was performed under the auspices of the Center for Nanoscale Control of Geologic CO 2 , an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-AC02-05CH11231. It used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Data interpretation benefitted from discussions between the first author and Alejandro Fernández-Martínez (LBNL) and Laura Nielsen (UC Berkeley).
PY - 2011/8/15
Y1 - 2011/8/15
N2 - 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.
AB - 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.
KW - Adsorption
KW - Clay mineral
KW - Diffusion
KW - Electrical double layer
KW - Molecular dynamics simulation
KW - Montmorillonite
KW - Smectite
KW - Surface complex
KW - Surface complexation model
KW - Triple layer model
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U2 - 10.1016/j.jcis.2011.04.063
DO - 10.1016/j.jcis.2011.04.063
M3 - Article
C2 - 21571296
AN - SCOPUS:79958799210
SN - 0021-9797
VL - 360
SP - 701
EP - 715
JO - Journal of Colloid And Interface Science
JF - Journal of Colloid And Interface Science
IS - 2
ER -