Molecular dynamics studies of aqueous silica nanoparticle dispersions: salt effects on the double layer formation

Abstract

The ion distribution around hydroxylated silica nanoparticles (NP-H) dispersed in brine was investigated by fully atomistic molecular dynamics. The NP-H dispersions in aqueous electrolyte media are simulated in solutions of varying salinity (NaCl, CaCl2, and MgCl2), salt concentration (0.06 × 10(-3) to 3.00 × 10(-3) mole fraction [Formula: see text]), and temperature (300 and 350 K) at 1 atm. The NP-H models reproduce the observed experimental concentration of silanol and geminal surface sites, which are responsible for local charge variations on the nanoparticles' surface. Interestingly, under certain salt concentration conditions, the formation of an electrical double layer (DL) around the overall neutral NP-H occurs. The resulting DLs are attenuated with increasing temperature for all evaluated salts. With increasing salt concentration, a sign inversion of the effective charge at the first ion layer is observed, which modifies the electrostatic environment around the nanoparticle. The minimum salt concentration that leads to a DL formation at 300 K is 1.05 × 10(-3), 0.37 × 10(-3), and 0.06 × 10(-3) χs for NaCl, CaCl2, and MgCl2, respectively. The width of the DL decreases sequentially in ionic strength from NaCl to CaCl2 to MgCl2, which is similar to that found for highly charged surfaces. These results are in line with our previous experimental data for negative charged NP-H. All together, these observations suggest an interplay mechanism between the formation and narrowing of electric double layers on the stability of NP dispersions in both neutral and negatively charged NP-H.

Alkali binding by magnesium silicate hydrates

“…The observed increase of surface sites at higher pH values agrees with the observation in plain M–S–H, where a continuous increase of CEC was measured from pH 8 to pH 10. These observations are consistent with a higher negative surface charge at higher pH values due to the deprotonation of the silanol surface groups at increasing pH as observed for M–S–H and for silica …”

“…In NaNO 3 solution, a CEC of 31 meq/100 g, 30 meq/100 g, and 34 meq/100 g was measured in the 10, 50, and 100 mmol/L NaNO 3 samples, respectively, comparable to the 35 meq/100 g of pure M–S–H (Figure A). At 10 mmol/L NaNO 3 where 1 mmol/L magnesium and 9 mmol/L sodium were present in the solution, 20 meq/100 g of magnesium and 10 meq/100 g of sodium were found on the cation exchange sites indicating again a higher preference of the silanol surface groups for the bivalent magnesium than the monovalent sodium, in agreement with observations on silica . At higher sodium concentration, most of the cation exchange sites are occupied by sodium due to its very large concentration compared to magnesium.…”

The Activation Mechanism of Bi3+ Ions to Rutile Flotation in a Strong Acidic Environment

“…The species distribution diagrams for Bi 3+ , Pb 2+ , Ti 4+ ions and SPA are shown in Figure 9 and help explain the results obtained from zeta potential experiments [19]. On one hand, Pb 2+ ions adsorbed on the rutile surface through electrostatic forces [8], resulting in the increase of zeta potential; on the other hand, because of the presence of salt solution, the thickness of the electric double layer of the rutile surface was compressed [26][27][28], resulting in the decrease of the zeta potential. In the pH range of 1-2.5, which was the optimal pH range for flotation, the rutile surface charge was positive.…”

“…The presence of Bi 3+ ions increased the surface potential, and facilitated the large specific adsorption in the form of the hydroxyl compounds, which greatly increased the adsorption capacity of SPA on the rutile surface, thus improving the hydrophobicity of the rutile surface, resulting in an increase in flotation recovery. on the rutile surface through electrostatic forces [8], resulting in the increase of zeta potential; on the other hand, because of the presence of salt solution, the thickness of the electric double layer of the rutile surface was compressed [26][27][28], resulting in the decrease of the zeta potential. In the pH range of 1-2.5, which was the optimal pH range for flotation, the rutile surface charge was positive.…”

A computational approach to evaluation of nanoporous zeolitic membranes for reverse osmosis desalination

“…The potentials for hydroxylated zeolites were adopted from the studies on SiO2 nanostructures in aqueous environment in order to obtain the interactions with the aqueous solution (i.e. bulk water and NaCl ions) (Cruz-Chu et al, 2006, de Lara et al, 2015. These potentials for the ions and the hydroxylated zeolites were described by Lennard-Jones (LJ) function (see equation 4-9 in Chapter 4).…”