Extracting information from the ionic strength dependence of electrophoretic mobility by use of the slope plot

The effective mobility (μ(ep)) is the main parameter characterizing the electrophoretic behavior of a given solute. It is well-known that μ(ep) is a decreasing function of the ionic strength for all solutes. Nevertheless, the decrease depends strongly on the nature of the solute (small ions, polyelectrolyte, nanoparticles). Different electrophoretic models from the literature can describe this ionic strength dependence. However, the complexity of the ionic strength dependence with the solute characteristics and the variety of analytical expressions of the different existing models make the phenomenological ionic strength dependence difficult to comprehend. In this work, the ionic strength dependence of the effective mobility was systematically investigated on a set of different solutes [small mono- and multicharged ions, polyelectrolytes, and organic/inorganic (nano)particles]. The phenomenological decrease of electrophoretic mobility with ionic strength was experimentally described by calculating the relative electrophoretic mobility decrease per ionic strength decade (S) in the range of 0.005-0.1 M ionic strength. Interestingly, the "slope plot" displaying S as a function of the solute electrophoretic mobility at 5 mM ionic strength allows for defining different zones that are characteristic of the solute nature. This new representative approach should greatly help experimentalists to better understand the ionic strength dependence of analyte and may contribute to the characterization of unknown analytes via their ionic strength dependence of electrophoretic mobility.