Dynamic and Static Properties of Aqueous NaCl Solutions at 25°C as a Function of NaCl Concentration: A Molecular Dynamics Simulation Study
Song Hi Lee
Abstract
We present the result of molecular dynamics (MD) simulations to calculate the molar conductivity <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" id="M1"> <a:msub> <a:mi mathvariant="normal">Λ</a:mi> <a:mrow> <a:mi>m</a:mi> </a:mrow> </a:msub> <a:mfenced open="(" close=")" separators="|"> <a:mrow> <a:mo>=</a:mo> <a:mtext> </a:mtext> <a:msub> <a:mrow> <a:mi>λ</a:mi> </a:mrow> <a:mrow> <a:mi>N</a:mi> <a:msup> <a:mrow> <a:mi>a</a:mi> </a:mrow> <a:mrow> <a:mo>+</a:mo> </a:mrow> </a:msup> </a:mrow> </a:msub> <a:mo>+</a:mo> <a:mtext> </a:mtext> <a:msub> <a:mrow> <a:mi>λ</a:mi> </a:mrow> <a:mrow> <a:mi>C</a:mi> <a:msup> <a:mrow> <a:mi>l</a:mi> </a:mrow> <a:mrow> <a:mo>−</a:mo> </a:mrow> </a:msup> </a:mrow> </a:msub> </a:mrow> </a:mfenced> </a:math> of NaCl in SPC/E water at 25°C as a function of NaCl concentration (c) using Ewald sums employing a velocity Verlet algorithm. It is found that the MD result for Λm with Ewald sum parameter κ = 0.10 Å−1 gives the closest one to the experimental data and that the obtained radial distribution functions <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" id="M2"> <g:msub> <g:mrow> <g:mi>g</g:mi> </g:mrow> <g:mrow> <g:mi>i</g:mi> <g:mi>i</g:mi> </g:mrow> </g:msub> </g:math> (r) with κ = 0.10 Å−1 show a dramatic change with a very deep minimum of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" id="M3"> <i:msub> <i:mrow> <i:mi>g</i:mi> </i:mrow> <i:mrow> <i:mtext>NaCl</i:mtext> </i:mrow> </i:msub> </i:math> (r) and, as a result, sharp maxima of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" id="M4"> <k:msub> <k:mrow> <k:mi>g</k:mi> </k:mrow> <k:mrow> <k:mtext>NaNa</k:mtext> </k:mrow> </k:msub> </k:math> (r) and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" id="M5"> <m:msub> <m:mrow> <m:mi>g</m:mi> </m:mrow> <m:mrow> <m:mtext>ClCl</m:mtext> </m:mrow> </m:msub> </m:math> (r) at the distance 9.95 Å, which indicates a characteristic of ionic atmosphere, the basis of the Debye–Hückel theory of ionic solutions. The static and dynamic properties of NaCl (aq) solutions are analyzed in terms of radial distribution functions, hydration numbers, coordination numbers around Na+ and Cl−, residence times of water around Na+ and Cl−, water diffusion, and ion-ion electrostatic energies to explain the behavior of the molar conductivity Λm of NaCl obtained from our MD simulations.