Measurement of Ion-Pairing Interactions in Buffer Solutions with Microwave Microfluidics

Microwave microfluidic spectroscopy is an emerging technique for quantifying the frequency-dependent electrical response of fluids. This technique can access important physical properties, including ion mobility and hydration, which are directly applicable to biochemistry. One critical step toward quantifying these effects is to develop accurate models for the behavior of buffer solutions containing mobile ions. Here, we show that ions in buffer solutions produce a weak ion-pairing response. We used microfluidic channels integrated with coplanar waveguides in combination with a hybrid microwave calibration protocol to extract the broadband microwave admittance spectra of a standard TAE-Mg2+ buffer solution between 100 kHz and 67 GHz. To characterize the ion-pairing response, we fit the calibrated admittance data with two models: a conventional model without ion-pairing and with a water relaxation described by a “Cole-Cole” function, to our alternative model that includes ion-pairing and a single Debye-type water relaxation. Including ion-pairing improved the goodness of fit across the entire frequency range. In the higher concentration buffer solution, we saw a reduction in the max systematic error in the fit residuals from 10% to less than 4%. The measurement and fitting techniques are widely applicable, providing critical information about the behavior of solvated ions.