Electrokinetic effects in poly(ethylene glycol)-coated capillaries induced by specific adsorption of cations

Capillaries coated with poly(ethylene glycol) (PEG) exhibit unique electrokinetic properties because of the specific adsorption of cations at the solid/liquid interface. The adsorption of cations present in methanol-or acetonitrile-containing electrolytes can induce a positive surface charge on PEG-coated capillaries. This results in an adjustable anodic electroosmotic flow (EOF) in nonaqueous electrolytes, whereas a reduced cathodic EOF is observed in aqueous electrolytes. The EOF is dependent on the electrolyte constitution, namely, on the type of solvent used and especially on the abilities of the background cations to interact with polyethers such as PEG. The dependency of the EOF on the electrolyte concentration in nonaqueous electrolytes can be explained by two counterbalancing mechanisms: (i) the increase in surface charge density and (ii) the decrease in double-layer thickness. Using different alkali metal and alkaline earth metal cations dissolved in methanol the EOF can be varied over a wide range. The magnitude of the induced anodic EOF was found to vary in the order Ba2+ > Ca2+ > Mg2+ for alkaline earth metal ions and K+ > Cs > Na for alkali metal ions, whereas with lithium ions, a cathodic EOF was observed. The magnitude of the anodic EOF of PEG-coated capillaries in nonaqueous electrolytes was found to be dependent on the coating thickness.