Molecular dynamics study of perchloric acid using the extended Madrid-2019 force field

Abstract

Perchloric acid (HClO$_4$) is widely used to prepare perchlorate salts with applications in propellants, industry, environmental chemistry, and biology. In this work, we used the intermolecular parameters from the extended Madrid-2019 force field for the perchlorate anion (ClO$_4^-$) and the oxonium cation (H$_3$O$^+$) together with TIP4P/2005 water to model perchloric acid solutions. The force field uses scaled charges of $\pm0.85e$ for monovalent ions and has been widely applied for aqueous ionic systems. We used the model to predict thermodynamic properties [densities and temperatures of maximum in density (TMD)], structural features (ion-water correlations: ion-hydrogen and ion-oxygen), and transport properties (self-diffusion coefficients and viscosity) of perchloric acid solutions at several concentrations. Experimental densities are predicted in excellent agreement up to 10 $m$. We also performed molecular simulations over a wide range of temperatures in order to determine the TMD of perchloric acid at different molalities. Predicted viscosities at 298.15 K and 1 bar are in good agreement with experimental data for concentrations below 4 $m$. Results are discussed in terms of model strengths and limitations.

Figures (5)

• Figure 1: (Colour online) Density as a function of molality for aqueous solutions of perchloric acid (HClO$_4$) at three temperatures (namely 298.15, 283.15, and 323.15 K) and 1 bar. Solid symbols represent the MD results, while the continuous line corresponds to the experimental data. Experimental data are taken from reference Brickwedde1949properties. The MD simulations were performed using the Madrid-2019 force field ($q = \pm0.85e$) Blazquez2025perchlorateGamez2025_H3O. For better visualization, the densities of perchloric acid at 298.15 K and 283.15 K are shifted upward by 100 and 200 units, respectively.
• Figure 2: (Colour online) Panel (a): number-density profiles of ClO$_4^-$ and H$_3$O$^+$ ions as a function of the box length along the three Cartesian coordinates ($x$, $y$, and $z$) for the 8.48 $m$ HClO$_4$ solution. The profiles are nearly flat and overlap in all directions, indicating that the system remains homogeneous, with no evidence of aggregation. Only small statistical fluctuations ($\approx 1\%$) are observed across the simulation box. For clarity, the $y$-axis and $x$-axis values in the number density were shifted upward by one and two number density units, respectively. Panel (b): representative snapshot of the system at the highest molality studied in this work (8.48 $m$), showing the spatial distribution of perchlorate (green and yellow) and hydronium (blue and red) ions.
• Figure 3: (Colour online) Panel (a): density as a function of temperature for aqueous solutions of perchloric acid (HClO$_4$) at 0.30 $m$, 0.49 $m$, 0.69 $m$, and 0.98 $m$ at 1 bar. Solid squares denote MD simulation results, dashed lines represent third-order polynomial fits to the MD data obtained with the extended Madrid-2019 model Blazquez2025perchlorateGamez2025_H3O, and stars indicate the temperature of maximum density. Panel (b): variation of $\Delta$ as a function of $m$ for perchloric acid solutions. The dashed line corresponds to a linear fit to the MD simulation results (filled symbols).
• Figure 4: (Colour online) Radial distribution functions for aqueous solutions of perchloric acid (HClO$_4$) at low (0.98 $m$, solid black line) and high (5.42 $m$, dotted red line) concentrations at 298.15 K and 1 bar. Panel (a): RDFs for H$_w$--Cl$_p$ and H$_w$--O$_p$ interactions. Panel (b): RDFs for O$_w$--Cl$_p$ and O$_w$--O$_p$ interactions. Panel (c): RDFs for O$_{\text{H}_3 \text{O}^+}$--Cl$_p$ and H$_{\text{H}_3 \text{O}^+}$--O$_p$, interactions. The vertical dotted lines correspond to the experimental values for the position of the first maximum of the RDFs.
• Figure 5: (Colour online) Panel (a): self-diffusion coefficients of water and perchloric ion (ClO$_4^{-}$) as a function of molality at 298.15 K and 1 bar. Filled squares represent simulation results, while dashed lines are included as visual guides. The experimental was obtained from reference Malhotra1993. Panel (b): shear viscosity as a function of molality for aqueous solutions of perchloric acid (HClO$_4$) at 298.15 K and 1 bar. Filled squares represent simulation results, while continuous lines correspond to experimental data from reference Brickwedde1949properties.