Surface and zeta-potentials of silver halide single crystals: pH-dependence in comparison to particle systems

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

Nanomolar Surface-Active Charged Impurities Account for the Zeta Potential of Hydrophobic Surfaces

The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ potential of hydrophobic surfaces for varying pH and salt concentration by solving the Poisson-Boltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the experimentally measured surface tension of the air-electrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H₃O⁺, OH^-, HCO₃^-, and CO₃^2- cannot account for the ζ potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impurities' acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pK_a = 5-7) and weak bases (pK_b = 12) at a concentration of the order of 10^-7 M.

The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP

A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the individual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.

The pH dependent surface charging and points of zero charge. VII. Update

The pristine points of zero charge (PZC) and isoelectric points (IEP) of metal oxides and IEP of other materials from the recent literature, and a few older results (overlooked in previous searches) are summarized. This study is an update of the previous compilations by the same author [Surface Charging and Points of Zero Charge, CRC, Boca Raton, 2009; J. Colloid Interface Sci. 337 (2009) 439; 353 (2011) 1; 426 (2014) 209]. The field has been very active, but most PZC and IEP are reported for materials, which are very well-documented already (silica, alumina, titania, iron oxides). IEP of (nominally) Gd₂O₃, NaTaO₃, and SrTiO₃ have been reported in the recent literature. Their IEP were not reported in older studies.

How test vessel properties affect the fate of silver nitrate and sterically stabilized silver nanoparticles in two different test designs used for acute tests with Daphnia magna

The relation between test conditions such as medium composition or pH on silver nanoparticle (AgNP) behavior and its link to toxicity is one of the major topics in nanoecotoxicological research in the last years. In addition, the adaptation of the ecotoxicological standard tests for nanomaterials is intensely discussed to increase comparability and reliability of results. Due to the limitation of test material production volumes and the need for high-throughput screening, miniaturization has been proposed for several test designs. In the present study, the effect of a miniaturization of the acute Daphnia immobilization test on AgNP behavior was investigated. For this purpose, available, adsorbed, and dissolved silver fractions were measured using AgNP and silver nitrate in the following two test designs: a standard test (ST) design and a miniaturized test (MT) design with reduced test volume and less animals. Despite the increase in surface area in relation to the test volume in MT, more AgNP attached to the ST vessel surface, so that in this case, exposure concentrations were significantly lower compared to the MT assessment. Ionic silver concentrations resulting from AgNP dissolution were similar in both test designs. The same was observed for ionic silver concentrations in silver nitrate (AgNO₃) treatments, but adsorbed silver was also higher in ST treatments. Assessing the structure-activity relationships revealed that surface properties such as hydrophobicity, potential binding sites, or surface roughness were of higher importance than surface:volume ratios for both test substances.

Adsorption of dissolved aluminum on sapphire-c and kaolinite: implications for points of zero charge of clay minerals

We have studied the impact of dissolved aluminum on interfacial properties of two aluminum bearing minerals, corundum and kaolinite. The effect of intentionally adding dissolved aluminum on electrokinetic potential of basal plane surfaces of sapphire was studied by streaming potential measurements as a function of pH and was complemented by a second harmonic generation (SHG) study at pH 6. The electrokinetic data show a similar trend as the SHG data, suggesting that the SHG electric field correlates to zeta-potential. A comparable study was carried out on kaolinite particles. In this case electrophoretic mobility was measured as a function of pH. In both systems the addition of dissolved aluminum caused significant changes in the charging behavior. The isoelectric point consistently shifted to higher pH values, the extent of the shift depending on the amount of aluminum present or added. The experimental results imply that published isoelectric points of clay minerals may have been affected by this phenomenon. The presence of dissolved aluminum in experimental studies may be caused by particular pre-treatment methods (such as washing in acids and subsequent adsorption of dissolved aluminum) or even simply by starting a series of measurements from extreme pH (causing dissolution), and subsequently varying the pH in the very same batch. This results in interactions of dissolved aluminum with the target surface. A possible interpretation of the experimental results could be that at low aluminum concentrations adatoms of aluminum (we will refer to adsorbed mineral constituents as adatoms) can form at the sapphire basal plane, which can be rather easily removed. Simultaneously, once the surface has been exposed to sufficiently high aluminum concentration, a visible change of the surface is seen by AFM which is attributed to a surface precipitate that cannot be removed under the conditions employed in the current study. In conclusion, whenever pre-treatment or the starting point of an experiment favor the dissolution of aluminum, dissolved Al may remain in the experimental system and interact with the target surfaces. The systems are then no longer pristine and points of zero charge or sorption data are those of aluminum-bearing systems.