Thermodynamic and Transport Properties of H₂/H₂O/NaB(OH)₄ Mixtures Using the Delft Force Field (DFF/B(OH)₄^–)

Abstract

Sodium borohydride (NaBH4) has a high hydrogen (H2 ) gravimetric capacity of 10.7 wt %. NaBH4 releases H2 through a hydrolysis reaction in which aqueous NaB(OH)4 is formed as a byproduct. NaB(OH)4 strongly influences the thermophysical properties of aqueous solutions (i.e., densities, viscosities, and electrical conductivities) and the hydrolysis reaction kinetics and conversion of NaBH4. Here, molecular dynamics (MD) simulations are performed to compute viscosities, electrical conductivities, and self-diffusivities of H2 , Na+, and B(OH)4 – for a temperature and concentration range of 298–353 K and 0–5 mol NaB(OH)4/kg water, respectively. Continuous fractional component Monte Carlo (CFCMC) simulations are used to compute the solubilities of H2 and activities of water in aqueous NaB(OH)4 solutions for the same temperature and concentration range. A new force field is developed (Delft force field of B(OH)4 –: DFF/B(OH)4 –) in which B(OH)4 – is modeled as a tetrahedral structure with a scaled charge of −0.85. The OH group in B(OH)4 – is modeled as a single interaction site. This force field is based on TIP4P/2005 water and the Madrid-2019 Na+ force field. The MD simulations can accurately capture the densities and viscosities within 2.5% deviation from available experimental data at 298 K up to a concentration of 5 mol NaB(OH)4/kg water. The computed electrical conductivities deviate by ca. 10% from experimental data at 298 K for the same concentration range. Based on the molecular simulations results, engineering equations are developed for shear viscosities, self-diffusivities of H2, Na+, and B(OH)4 –, and solubilities of H2, which can be used to design and model NaBH4 hydrolysis reactors.

Diffusivity of CO2 in H2O: A Review of Experimental Studies and Molecular Simulations in the Bulk and in Confinement

“…Alternatively, approaches such as MD simulations can provide detailed physical insight; , the downside being that they are significantly more computationally demanding compared to engineering models. During the past three decades years, MD has become a reliable and widely used approach for obtaining diffusivities of pure components and mixtures. ,,− This development is the direct result of a number of factors including: (i) the increase of available computational power, (ii) the availability and wide use of optimized open-source software, , and (iii) the development of accurate force fields. − The data obtained from MD simulations can be further used to devise engineering models and validate the semiempirical approaches. ,, Macro-scale modeling approaches involving an equation-of-state such as PC-SAFT coupled with Stokes–Einstein equation or entropy scaling to compute self-diffusivities have been reported in literature, although, to the best of our knowledge, such methods have not been used to compute diffusivity of CO 2 in H 2 O. − …”

Accurate Free Energies of Aqueous Electrolyte Solutions from Molecular Simulations with Non-polarizable Force Fields

“…On the basis of TIP4P/2005 water, different force fields for salts (e.g., NaCl, KCl, and KOH) have been developed. ,,− The charges of ion force fields are commonly scaled down (usually by a factor of 0.85 or 0.75 , ) , to account for the effective charge screening that occurs in the aqueous medium. , Charge scaling follows from the Electronic Continuum Correction and accounts for polarizability of ions in a mean-field way. , Using the “scaled charge” force fields of Madrid-2019 (scaled charges of +0.85/–0.85), Madrid-Transport (scaled charges of +0.75/–0.75), and the Delft Force Field of OH – (DFF/OH – ) (scaled charge of −0.75), many of the properties of aqueous NaCl, KCl, NaOH, and KOH solutions, such as densities, viscosities, and interfacial tensions, and their temperature dependence can be accurately computed. ,,,, Force fields with integer charges of ions (e.g., +1/–1 for Na + /Cl – ), such as the Joung–Cheatam force field, significantly overestimate the change in liquid-phase viscosities and ion diffusivities in concentrated solutions (i.e., close to the solubility limit) with respect to the pure solvents . The infinite dilution free energies of hydration of salts can be accurately captured using available integer charge force fields, whereas scaled charge force fields of ions deviate by ca.…”

“…This also explains the ca. 30% lower viscosities for TIP4P/μ at 298 K and 1 bar compared to experiments . Using the ECS approach for TIP4P/2005, we accurately model the excess chemical potentials of pure liquid water compared to experiments for a wide temperature range (i.e., 300–500 K).…”

“…Rigid non-polarizable water force fields that accurately capture experimental μ w ex and the vaporization enthalpy of water, e.g., SPC, TIP4P, and TIP4P/μ , (defined in the Supporting Information of ref ), poorly predict other important properties of the liquid phase (e.g., transport properties) compared to TIP4P/2005. ,,,, It becomes clear that modeling both transport properties of water and μ w ex is not possible using available non-polarizable force fields. , Already in 1987, Berendsen et al discussed this issue: to obtain effective interactions between water molecules in the liquid phase (thereby capturing experimental transport properties), the absolute value of the charges in the water force field needs to be enhanced to account for the polarization energy of water (i.e., “the missing term” in non-polarizable force fields mentioned in the title of the famous paper by Berendsen and co-workers). Explicitly accounting for “the missing term” in non-polarizable force fields automatically results in an overestimation of the heat of vaporization and, hence, poor predictions of the VLE of water. ,, Some polarizable force fields (e.g., BK3 and HBP) capture the VLE of water without compromising the transport properties of the liquid phase but at the cost of higher complexity, significantly higher computational time (usually by a factor of ca.…”