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Measuring Ion-Pairing in Buffer Solutions with Microwave Microfluidics



Angela C. Stelson, Charles A. Little, Nathan D. Orloff, Christian J. Long, James C. Booth


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 elec-trical 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 fit to a Cole-Cole/Debye model which does not include the ion-pairing relaxation, and find improved goodness of fit. This technique is broadly applicable to ionic solu-tions, and provides critical information about solvated ions in bio-logical systems.
Conference Dates
June 14-15, 2018
Conference Location
Philadelphia, PA
Conference Title
IEEE-MTT-S International Microwave Biomedical Conference


Permittivity measurement, Microfluidics, Transmission line measurements, Coplanar waveguides, Circuit testing
Created June 14, 2018, Updated April 4, 2019