Atomistic computer simulation of the clay–fluid interface in colloidal laponite

Abstract

Monte Carlo and molecular dynamics computer simulations have been used to study the structure and dynamics of the interlayer aqueous solution in a colloidal sodium laponite clay at 277 K. The system studied has a clay-clay spacing of 34.06 Å, and contains 1200 interlayer water molecules and 24 sodium counterions. The density profiles for interlayer species show two distinct layers of surface water as one moves away from the clay particles. The innermost of these layers is strongly oriented to form hydrogen bonds to the surface oxygen atoms. Radially averaged pair distributions have been calculated as a function of distance from the clay surfaces, and show that throughout our system the water structure is significantly perturbed from the bulk. In particular, we observe an increase in the second nearest-neighbor oxygen-oxygen distance, similar to that reported for low-density water at 268 K ͓A. K. Soper and M. A. Ricci, Phys. Rev. Lett. 84, 2881 ͑2000͔͒. The majority of the sodium counterions are fully hydrated by six water molecules. These hydrated ions have a strong tendency to remain close to the solid surfaces, as so-called ''outer-sphere'' complexes. However, we also observe cations further from the clay sheets, in the diffuse layer. Diffusion of water and cations in the plane of the clay sheets is comparable to that in the bulk, but is significantly reduced normal to the clay sheets.

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

“…3). Our results show that the pronounced water density layering at the mineral surface dissipates within about 9 Å (three times the diameter of a water molecule), in agreement with previous studies of water on flat solid surfaces [30,37,38,94,95]. The z coordinates of the first shoulder and first four peaks of the water density profile are reported in [44,53] of the first shoulder and first two peaks of the water density profile to water molecules that form hydrogen bonds with two (z* ≈ 1.8 Å), one (z* = 2.7 Å), or no surface O atoms (z* = 4.0 Å), respectively (Fig.…”

“…simulations have provided insight into the behavior of water and ions near flat charged surfaces [2,18,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. In particular, these simulations have confirmed that ions confined between planar charged surfaces do form ISSC, OSSC, and diffuse swarm species [35] in agreement with most EDL models.…”

“…Following Tournassat et al [52], we focus on smectites because these ubiquitous clay mineral nanoparticles play important roles in soil chemistry [55,56], soil mechanics [57], fault friction [58,59], materials chemistry [60], landfill and contaminated site isolation [61,62], high-level radioactive waste storage [63,64], CO 2 sequestration [65,66], and gas hydrate stability in marine sediments [67,68]. Their adsorption properties for small inorganic ions have been extensively studied using macroscopic [55,56,69], spectroscopic [24,25], and MD simulation techniques [18,35,37,45,49,52].…”

The CS/K Exchange in Muscovite Interlayers: An Ab Initio Treatment

“…Empirical potential molecular dynamics or Monte Carlo simulations of aspects of cation exchange and interlayer water structure have only recently become available (e.g. Skipper et al, 1991;Chang et al, 1995;Bridgeman and Skipper, 1997;Greathouse and Sposito, 1998;Smith, 1998;Young and Smith, 2000;de Carvalho and Skipper, 2001;Wang et al, 2001). The DFT total energy calculations have been applied successfully to phyllosilicates elsewhere (e.g.…”

Diffusion with micro-sorption in bentonite: evaluation by molecular dynamics and homogenization analysis

“…Local properties of smectite clays have been treated mainly in a classical sense based on a continuum theory such as an electrochemical double layer theory (see, for example, Robinson and Stokes 1970;Tamamushi 1991; Clay Science Society of Japan 1987). Nevertheless, the modern concerns are keenly related to their molecular and atomic-based characteristics (Greathouse, Refson and Sposito 2000;Leote de Carvalho and Skipper 2001). This is because the interlayer space is so narrow that the direct continuum-based theory is not directly applicable.…”