Exploring Solid/Aqueous Interfaces with Ultradilute Electrokinetic Analysis of Liquid Microjets

We describe a novel method that exploits electrokinetic streaming current measurements for the study of ion-interface affinity. Through the use of liquid microjets and ultradilute solutions (<1 μM), we are able to overcome inherent difficulties in electrokinetic surface measurements engendered by changing double-layer thicknesses. Varying bulk KCl concentrations produce statistically significant changes in streaming current down at picomolar concentrations. Because the attending ion concentrations are below that from water autoionization, these data are compared with those from ultradilute HCl and KOH solutions assuming that the K and Cl− introduce no new counterions. This permits comparison of the individual effects of K and Cl− on the interface, evidencing a cooperative effect between these ions at silica surfaces. Altogether, these results establish the effectiveness of this experimental approach in revealing new ion−surface phenomena and indicate its promise for the general study of aqueous interfaces.