Investigation of Potential Distribution and the Influence of Ion Complexation on Diffusion Potentials at Aqueous−Aqueous Boundaries within a Dual-Stream Microfluidic Structure

Abstract

The occurrence of reactions at boundaries between adjacent miscible but unmixed aqueous streams coflowing in a microfluidic channel structure has been studied by simulation of the diffusion potentials that develop between the two coflowing aqueous electrolyte streams and by measurement of the effects of aqueous ion complexation on diffusion potentials. The microfluidic structure consisted of a Y-shaped microchannel with off-chip electrodes immersed in electrolyte reservoirs connected by capillaries to the Y-microchannel. The time-dependent, one-dimensional Nernst-Planck equation employing the electroneutrality condition was solved numerically to calculate the diffusion potentials established at the boundary between the two coflowing aqueous streams. Under the experimental conditions (channel length and width, flow rate) employed, it was shown that the use of the Henderson equation was appropriate. It was also shown that the cross-channel diffusion potential remained constant from the entrance of the channel to the exit. The influence of cation complexation by a neutral ionophore was investigated by experimentally measured diffusion potentials. It was found that potassium complexation by the cyclic polyether 18-crown-6 altered the experimental diffusion potential, whereas the interaction of sodium or lithium cations with the ionophore did not perturb the diffusion potential. The results are consistent with the literature data for aqueous-phase complexation of these cations by this ionophore. The results of these investigations demonstrate that relatively simple diffusion potential measurements between coflowing streams in microchannels may be used as a basis for study of ion complexation reactions occurring at boundaries between miscible fluids.

Pharmaceutical modulation of diffusion potentials at aqueous–aqueous boundaries under laminar flow conditions

“…As described previously [30,32], the measured open circuit potential, j oc , is a combination of the diffusion potential, j d , and the potential difference between the electrodes, j e , caused by the difference in chloride concentrations in the opposing reservoirs.…”

“…This enabled in situ measurement of potentials in the microfluidic cell using off-chip electrodes and hence simplified the fabrication procedure compared to a previous device we studied that employed integrated electrodes [31]. We recently extended this to the measurement of chemical interactions occurring at the aqueous-aqueous boundary [32,33].…”

Potentiometric Investigation of Protonation Reactions at Aqueous−Aqueous Boundaries within a Dual-Stream Microfluidic Structure

“…The one-dimensional space model presented involves several simplifications, as mentioned before. 16 These are as follows: (i) The system was assumed to be homogeneous along the channel height. This assumption has been discussed by Kamholz and Yeager who presented a theoretical investigation of the effect of the nonuniform velocity profiles associated with pressure-driven flux on the apparent diffusion coeffcient.…”

“…If both streams contain equimolar concentrations of K þ ions, but a complexing agent (the crown ether) is added to one of the solutions prior to injection into the microchannel, an increase of diffusion potential with increasing crown ether concentration is observed, generated by the lower mobility of the ion-ionophore complex compared to the uncomplexed potassium ion. 16 This approach is similar to the premixed diffusion-based assay used by Hatch et al 17 to study biomolecular interactions, such as biotin with avidin. In this case, the reactants were premixed before injection into one of the inlets of a Y-shaped microchannel.…”

Several ways to simulate time dependent liquid junction potentials by finite differences

“…It could be a membrane between two well stirred solutions. Experimentally, the situation of a watery boundary can also be be realised in microchannels, involving dual-stream laminar flow of two aqueous solutions along each other carrying different electrolyte salts at various concentrations [18][19][20][21][22] forming a flowing junction [17]. potential using the Henderson equation [20] and numerical solutions of the Nernst-Planck equation [21,22] and the measured open-cicuit potentials has been found.…”