Unexpected Presence of Solute-Free Zones at Metal-Water Interfaces

Abstract

Solute–free zones, termed “exclusion zones” are routinely seen next to hydrophilic surfaces in aqueous solution. Here we report similar zones next to various metals. The largest, approximately 200 µm in width, was found adjacent to zinc. Other reactive metals, including aluminum, tin, lead, and tungsten exhibited distinct but smaller exclusion zones, while precious metals such as platinum and gold did not produce any. Electrical potential measurements showed positive potentials within the exclusion zones, while pH measurements revealed an abundance of OH− groups in the aqueous regions beyond the exclusion zones. A correspondence was found between exclusion-zone size and the respective metal’s position within the galvanic series. The presence of these interfacial exclusion zones is unexpected, and may shed new light on electrochemical processes taking place at metal interfaces.

Capillarity ion concentration polarization for spontaneous biomolecular preconcentration mechanism

“…57,58 Although this exclusion had occurred near metal-water or Nafion-water interfaces, our mechanism involves the infiltration of water through the nanopores, with the ion depletion zone remaining near the interface which has not been observed in surface charge based exclusion.…”

Long-range repulsion of colloids driven by ion exchange and diffusiophoresis

“…Such peculiar behavior has been observed by researchers from different disciplines for a wide range of materials, including biological tissues such as rabbit cornea (1), white blood cells (2), polymer gels (3), ion-exchange membranes (4), or metals (5). Depending on the field of research, different terms have been used to refer to the behavior.…”

“…The observed EZ formation is highly surprising, as the forces acting on the colloidal particles can extend over distances of hundreds of micrometers (1)(2)(3)(4)(5). Long-range interactions acting on colloidal particles are generally of electrostatic nature (6,7), with a range set by the thickness of the electrical double layer surrounding a charged colloidal particle, the Debye length λ D .…”

“…Recently, a chemotaxis hypothesis similar to diffusiophoresis has been suggested and theoretically investigated to account for the EZ formation (11,12). Effects assuming a long-range structuring of water near hydrophilic surfaces have also been suggested as a possible origin of the behavior (3)(4)(5). However, to date it is still unclear whether any of these hypotheses, individually or in combination, can fully account for the observed behavior, as existing comparisons between experiments and theoretical predictions are still unable to clearly discern between the different hypotheses.…”

Exclusion-Zone Dynamics Explored with Microfluidics and Optical Tweezers

“…This theory has been debated [5] and needs experimental testing. Although the EZ has been characterized by numerous experimental approaches [1,[6][7][8][9][10][11][12][13][14][15][16][17][18], key features related to its thermodynamics and kinetics still need to be investigated. In the present work we measured the short-and long-time-scale kinetics of EZ formation in specially designed microfluidic devices that reduce unwanted local perturbation effects, estimated the magnitude of the exclusion force, and tested key predictions of the most plausible microscopic mechanisms of EZ formation.…”