Abstract
We studied the charging of inert surfaces (polytetrafluoroethylene, i.e., PTFE; graphite; graphene; and hydrophobic silica) using classical colloid chemistry approaches. Potentiometric titrations showed that these surfaces acquired less charge from proton-related reactions than oxide minerals. The data from batch-type titrations for PTFE powder did not show an effect of ionic strength, which was also in contrast with results for classical colloids. In agreement with classical colloids, the electrokinetic results for inert surfaces showed the typical salt level dependence. In some cases, the point of zero net proton charge as determined from mass and tentatively from acid–base titration differed from isoelectric points, which has also been previously observed, for example by Chibowski and co-workers for ice electrolyte interfaces. Finally, we found no evidence for surface contaminations of our PTFE particles before and after immersion in aqueous solutions. Only in the presence of NaCl-containing solutions did cryo-XPS detect oxygen from water. We believe that our low isoelectric points for PTFE were not due to impurities. Moreover, the measured buffering at pH 3 could not be explained by sub-micromolar concentrations of contaminants. The most comprehensive explanation for the various sets of data is that hydroxide ion accumulation occurred at the interfaces between inert surfaces and aqueous solutions.
Highlights
Inert surfaces, i.e., surfaces that do not expose any surface functional groups to the surrounding electrolyte solution, develop a pH-dependent charge, the origin of which has been at the center of some fierce debates [1,2]
Such charge separation has been related to electrets and the water ions again have been a preferred interpretation of charge separation on all kinds of surfaces that we experience in dry environments sometimes [13]
Measurements at the Zagreb laboratory were conducted on flat surfaces (PTFE, graphite, and graphene) and colloidal particles (graphite and carbon nanotubes (CNTs))
Summary
I.e., surfaces that do not expose any surface functional groups to the surrounding electrolyte solution, develop a pH-dependent charge, the origin of which has been at the center of some fierce debates [1,2]. In the absence of a better explanation, we believe that currently this buffering can only be explained by involving the adsorption of the water ions (i.e., in particular the hydroxide ions, which lowers the pH) Overall, this explanation still appears to be able to phenomenologically account for all observations. We focus of this introduction on how Emil Chibowski has been contributing to this field His interest in contact angles [19,20,21], surface energies [22,23,24], and electrokinetics [25,26,27] brought him into contact with discussions about the origin of the charge that was measured in electrolyte solutions on, for example, oil droplets [26,27,28], paraffin [25], or ice [29,30]. We doubt that on the basis of our results with high concentrations of powder, which we show is not contaminated before and after experiments (within what is detectable by XPS), that contaminants at low concentrations can cause pH values as low as 3, which we observed in suspensions of inert particles
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