Ion-Dipole Polarization Explains How Salinity Effects the Variation in the Dielectric Constant of Aqueous Solutions.

Careful dielectric measurements of aqueous solutions allowed us to determine the effect of salinity on dielectric constant values. The effect is due to ion-dipole polarization that adds to the usual orientation polarization which has a relaxation frequency of 19.2 GHz at room temperature. Ion-dipole relaxation frequencies are salinity dependent and the effect is explained using Falkenhagen’s ion atmospheres [1] where ions in solution are hydrated by water shells which form polarizable units. Ion-dipole relaxation frequencies ranges from 2 kHz for de-ionized water with an electrical conductivity of 0.0013 dS/m to 500 MHz in 100 dS/m water. In addition, our measurements revealed that not only is the relaxation frequency salinity dependent but also the dielectric constants of both polarization mechanisms are salinity dependent. For the orientation polarization, Robinson and Stokes [2] reported that the molar depression of the dielectric constant is 10 at 100 dS/m for KCl solutins; whereas, we observed a depression of 40 at only 1 dS/m for KCl solutions. For the ion-dipole polarization, its dielectric constant is also depressed by an increase in ion concentration. At 0.001 dS/m its dielectric constant is 970 and decreases to 500 at 1 dS/m for KCl solutions. 

In this paper we will describe our experimental approach that uses insulated electrodes which are critical in making the observations. In our measurements we have eliminated interferences such as probe end effects and air capacitances so we can measure the dielectric properties of our solutions with a resolution of 0.001 pF. In developing the methodology for determining the dielectric constants, we characterized six solutions with different dielectric constants that ranged from 2.4 to 78 and six water solutions with different KCl conductivities that ranged from 0.001 to 1.4 dS/m. A mathematical model was developed and used to simultaneously fit all twelve data sets. We gained confidence in our approach by fitting the experimental data to a 2D finite element model. This model revealed another important consideration, the lateral extent of the electric fields. We modeled the stream lines, as opposed to equipotential lines, which revealed the volume being sampled by our measurements. We found the sampling volume to be geometrically non-linear and dependent on the dielectric constant of the sample. 

References:

1. H. Falkenhagen, Electrolytes, Oxford (London, 1934), translated by R. P. Bell.
2. R. A. Robinson and R. H. Stokes, Electrolyte Solutions, 2nd Edition, Dover Publications, Inc. (Mineola, New York, 1959).