  Zeta potentials of several polar protic (water, ethylene glycol, formamide)
as well as polar aprotic (dimethyl sulfoxide) liquids were measured in contact
with three non-polar surfaces using closed-cell electro-osmosis. The test
surfaces were chemisorbed monolayers of alkyl siloxanes, fluoroalkyl siloxanes
and polydimethylsiloxanes (PDMS) grafted on glass slides. All these liquids
exhibited substantial electrokinetics in contact with the non-polar surfaces
with these observations: the electrokinetic effect on the fluorocarbon-coated
surface is the strongest; and on a PDMS grafted surface, the effect is the
weakest. Even though these hygroscopic liquids contain small amounts of water,
the current models of charging based on the adsorption of hydroxide ions at the
interface or the dissociation of preexisting functionalities (e.g., silanol
groups) appear to be insufficient to account for the various facets of the
experimental observations. The results illustrate how ubiquitous the phenomenon
of electro-kinetics is with polar liquids contacting such apparently passive
non-polar surfaces. We hope that these results will inspire further
experimental and theoretical studies in this important area of research that
has potential practical implications.
