Measuring Ion-Pairing in Buffer Solutions with Microwave Microfluidics

Microwave microfluidics is an emergent technique for characterizing conductivity and permittivity of fluids and has wide-ranging applications in the materials science and biomedical fields. The electrical properties of fluids as a function of frequency can be leveraged to characterize interface effects such as electrical double layers (EDL), solvent-mediated ion interactions, and bound water molecules. However, extraction of quantitative electrical properties over a wide range of frequencies (100 kHz-67 GHz) is nontrivial, and calibrations are required. Here, we utilize a microfluidics device with incorporated coplanar waveguides to characterize buffer solutions in situ and non-destructively. With a two-step fitting procedure, we fit relaxations associated with the EDL, water molecules, and ion-pairing in solution. We compare the three-Debye relaxation (water loss, ion-pairing and EDL relaxations fit to a Cole-Cole/Debye (water loss Cole-Cole and EDL Debye relaxations) model which does not include the ion-pairing relaxation, and find improved goodness of fit. This technique is broadly applicable to ionic solutions, and provides critical information about solvated ions in biological systems.