Cation effect in the ionic solution optical Kerr effect measurements: a simulation study

We carried out the modeling of the Optical Kerr effect (OKE) signals of several metallic chloride ionic solutions to study the microscopic origin of the cation effect on the OKE spectral features. The modeling was based on molecular dynamic simulation and the dipole-induced-dipole method for induced polarization calculation. Decent agreement was achieved between the simulation and the experiment. An extended projection method was adapted to decompose the OKE signals into the contributions from the reorientational and the collision-induced motions of the bulk and the shell water. Further analysis suggested that the different cation effects on the OKE measured relaxation time constant originate from their different water affinities. The weak water affinity of Na(+) causes the water in its first solvation shell to be only insignificantly perturbed in dynamics and frequently exchanges with water in bulk, which results in an negligible concentration dependence of the OKE time constant. The OKE time constants of Mg(2+) and Al(3+) have much stronger dependences on concentration due to their stronger water affinities, which create the more stable first solvation shells and slower water motion in the shell. Compared with Mg(2+), Al(3+) can more significantly retard the water motion outside of the shell, which causes an even stronger concentration dependence of the OKE time constant. Our study provided a microscopic picture on how the cation effect on the water dynamics is reflected in the OKE measurements.