Species Surface Distribution and Surface Tension of Aqueous Solutions of MIBC and NaCl Using Molecular Dynamics Simulations

Abstract

Methyl isobutyl carbinol (MIBC) is a high-performance surfactant with unusual interfacial properties much appreciated in industrial applications, particularly in mineral flotation. In this study, the structure of air–liquid interfaces of aqueous solutions of MIBC-NaCl is determined by using molecular dynamics simulations employing polarizable and nonpolarizable force fields. Density profiles at the interfaces and surface tension for a wide range of MIBC concentrations reveal the key role of polarizability in determining the surface solvation of Cl− ions and the expulsion of non-polarizable Na+ ions from the interface to the liquid bulk, in agreement with spectroscopic experiments. The orientation of MIBC molecules at the water liquid–vapor interface changes as the concentration of MIBC increases, from parallel to the interface to perpendicular, leading to a well-packed monolayer. Surface tension curves of fresh water and aqueous NaCl solutions in the presence of MIBC intersect at a reproducible surfactant concentration for a wide range of salt concentrations. The simulation results for a 1 M NaCl aqueous solution with polarizable water and ions closely capture the MIBC concentration at the intercept. The increase in surface tension of the aqueous MIBC/NaCl mixture below the concentration of MIBC at the intersection seems to originate in a disturbance of the interfacial hydrogen bonding structure of the surface liquid water caused by Na+ ions acting at a distance and not by its presence on the interface.

Alkali Halide and MIBC Interaction at Typical Flotation Interfaces in Saline Water as Determined by Molecular Dynamics Simulations

“…MIBC molecules prefer the interface with the oxygen atoms remaining on average in the liquid phase and the carbon atoms in the vapor phase. As the concentration of MIBC in the system increases, its density at the interface increases, and its density profile shifts more toward the water vapor phase, as shown by a previous study [18]. In other words, since the constituent O atoms, in the polar end of the chain, remain anchored in the water, the MIBC chain is stretched so that C atoms at the opposite end of oxygen settle in the vapor phase.…”

“…For sodium iodides, Nguyen et al reach the same conclusion with non-polarizable force fields, although they do not report surface tension [17]. In addition, a recent study evaluated the effect of including polarization in simulations of aqueous NaCl solutions [18]. With polarization, the ion distribution obtained coincides with other simulations in the literature and with the experimental results already mentioned, and also the prediction of surface tension improves significantly.…”

“…1.2 mM, referred to by Castro et al [19] as a surface tension switch point, is very close to the experimental value [5,20,21] and effectively marks the change in surface tension behavior. Below this point, the tension is controlled by the ions at the interface and is higher than that of freshwater; above this point, the tension is controlled by MIBC and is lower than freshwater, a behavior first reported by Ozdemir et al [22] and later by Castro et al [19] and Alvarado et al [18].…”

“…The system used is a box of dimensions 𝐿𝐿 𝑥𝑥 = 𝐿𝐿 𝑦𝑦 = 3 nm and 𝐿𝐿 𝑧𝑧 = 10 nm as in Alvarado et al [18]. The size of the box is large enough for the results to be reproducible and small enough for the computational cost to be affordable.…”

“…These data determined averaged density profiles along the two liquid-vapor interfaces. The parameters used are the same as in Alvarado et al [18]. The position of each interface, z = 0, was defined as the point where the density of water equals its mean value along the liquid-vapor interface.…”