la6b02493_si_001.pdf (2.51 MB)
Molecular Origins of the Zeta Potential
journal contribution
posted on 2016-09-19, 00:00 authored by Milan Předota, Michael
L. Machesky, David J. WesolowskiThe zeta potential (ZP) is an oft-reported
measure of the macroscopic
charge state of solid surfaces and colloidal particles in contact
with solvents. However, the origin of this readily measurable parameter
has remained divorced from the molecular-level processes governing
the underlying electrokinetic phenomena, which limits its usefulness.
Here, we connect the macroscopic measure to the microscopic realm
through nonequilibrium molecular dynamics simulations of electroosmotic
flow between parallel slabs of the hydroxylated (110) rutile (TiO2) surface. These simulations provided streaming mobilities,
which were converted to ZP via the commonly used Helmholtz-Smoluchowski
equation. A range of rutile surface charge densities (0.1 to −0.4
C/m2), corresponding to pH values between about 2.8 and
9.4, in RbCl, NaCl, and SrCl2 aqueous solutions, were modeled
and compared to experimental ZPs for TiO2 particle suspensions.
Simulated ZPs qualitatively agree with experiment and show that “anomalous”
ZP values and inequalities between the point of zero charge derived
from electrokinetic versus pH titration measurements both arise from
differing co- and counterion sorption affinities. We show that at
the molecular level the ZP arises from the delicate interplay of spatially
varying dynamics, structure, and electrostatics in a narrow interfacial
region within about 15 Å of the surface, even in dilute salt
solutions. This contrasts fundamentally with continuum descriptions
of such interfaces, which predict the ZP response region to be inversely
related to ionic strength. In reality the properties of this interfacial
region are dominated by relatively immobile and structured water.
Consequently, viscosity values are substantially greater than in the
bulk, and electrostatic potential profiles are oscillatory in nature.
History
Usage metrics
Categories
Keywords
macroscopic charge stateoft-reported measureSimulated ZPsviscosity valueselectroosmotic flowHelmholtz-Smoluchowski equationsalt solutionsSrCl 2Molecular OriginsZP response regionmolecular-level processesmacroscopic measureelectrokinetic phenomenapH titration measurementssurfacecounterion sorption affinitiesTiO 2pH valuescontinuum descriptionsTiO 2 particle suspensionsdynamics simulations15 Å
Licence
Exports
RefWorks
BibTeX
Ref. manager
Endnote
DataCite
NLM
DC