The potentials on the gibbsite and the silica and edge planes are 20 and mV, respectively, with little salt dependence in the range of ionic strength that was studied except for the silica plane at very low salt content. As pointed out before, the overall zeta-potential estimation requires a recalculation of the diffuse layer charge densities from the measured potentials and area-weighted summation of the contribution.
With the assumption that the gibbsite and silica planes have roughly the same area and within the experimental errors given, the charge from those two planes cancel each other at approximately 10 mM. Scaled to as the overall surface, this would be much lower resulting in an overall diffuse layer potential below mV in 10 mM salt. The zeta-potential measurement under these conditions is not available for the very same sample.
For 1 mM background electrolyte concentration, a value between and mV can be inferred from the thesis of Gupta [ 50 ]. Therefore, the range appears reasonable, although a self-consistent comparison again is not possible for lack of data. Nothing can be further inferred from this result though. Finally, it has also to be acknowledged that the recalculation of diffuse layer potentials from overall charges involves geometry.
The same is true for the calculation of zeta-potentials from electrophoretic mobility. Consequently, much can be done to improve the present attempts to gain insight in the effect of crystal plane-specific charging on overall charge. Interestingly, the silica and the edge planes are very similar again. The salt dependencies of the two basal planes are compared to related surfaces in Figure 5b gibbsite face and Figure 7 silica face.
We first discuss the gibbsite face and some additional data on pure gibbsite particles. Figure 5b shows that there is quite close agreement between gibbsite samples for pH approximately 5.
True gibbsite particles that are dominated by the basal plane exhibit high zeta-potentials and a characteristic maximum at millimolar concentration. This is reminiscent of published AFM charge data on the basal plane of gibbsite [ 40 ] We note that it is not possible to directly compare the trends in the charges and potentials.
In the cited work, the potentials were not reported. Figure 6c shows a comparison between data at the lower end of conductance and compares the trend to a suspension freshly prepared in 0. Clearly, the zeta-potential is higher in the case of NaCl addition, supporting the idea that the increase in conductivity arises from dissolution of gibbsite.
The overall trend in zeta-potential of the particles shown in Figure 5b appears to be more difficult to understand and would be the consequence of the various processes that occur. In themselves, these processes involving dissolution, re-adsorption, or even re-precipitation are complex, and these processes and the effect of added salt could also cause changes in pH.
The situation with respect to the silanol face is shown in Figure 7. Interfacial potentials are negative, but the range can vary widely. The data in Figure 7 are by no means exhaustive. There is a wealth of data available both from force curves Silica is a popular tip material in AFM force distance studies either directly or as a consequence of Si3N4 oxidation. Interfacial potentials of silica relevant to the kaolinite silica plane as a function of salt concentration equivalent to ionic strength in the case of the monovalent salts at pH 5.
Kaolinite and fused silica data are from Liu et al. Data for silica AFM tip are from Siretanu et al. Data for silica particles are those cited by Liu et al. Despite the huge step forward to characterizing individual surface planes of distinct particles in terms of charging, there is ample space for improving the experimental data basis.
Also, the kaolinite study has opened a plethora of new questions, one being why the observed kaolinite edge surface chemistry with an IEP below pH 4 disagrees with all previous estimates of the point of zero net proton charge of above pH 5. It also disagrees with the theoretical estimates which suggest a pK of 5.
Noteworthy, the determination of the edge face IEP was done by excluding the basal planes. The same could be said about the determination of the properties of the basal planes themselves, in which case, however, the edge planes were not excluded. Ideally measurements were made sufficiently far from the edges. The study of different sizes of the otherwise identical particles could shed light on possible surface potential effects.
The mica basal plane is probably one of the most frequently studied single crystal surfaces. Because it can be prepared as an atomically flat sample, it is suitable for a range of sophisticated methods that are not applicable to rougher surfaces, including X-ray reflectivity [ 38 , 39 , 63 ], high-resolution AFM that is able to show hydration layers [ 64 ], or ions on mica [ 65 ] or nano-ultrasonics [ 66 ], to name a few such approaches.
The literature on macroscopic data on mica is also abundant. The present work is therefore not citing all the available work and comparing all available data. Figure 12 shows data for mica at constant pH for variable salt levels of KCl. The data for the higher pH deviate, whereas the remainder of the data is in a common cloud independent of the method.
Similar data are shown for a constant salt level and pH variation in Figure 9a. An important difference occurs at the lower pH range, where the IEP would be obtained. Depending on the source, the slope of the interfacial potential with decreasing pH differs a lot, resulting in different IEPs for the samples under investigation.
Additional data on ionic strength dependence are shown in Figure 9b involving different salts. SFA data are from Pashley [ 67 ]. AFM data are from Jiang et al. Streaming potential data are from Scales et al. Data in Figure 9a are from Jiang et al. The possible origins of the differences include aging according to Lyons et al. Clearly, aging in both cases strongly increases the IEP. Figure 10b also shows that freshly cleaved basal planes do not necessarily yield unique IEPs.
There is significant scatter, and there is some discussion in the literature about how to prepare these surfaces. Figure 10c shows similar results for uncleaved mica basal planes. Again, there is clear scatter for mica samples from the same source.
Also, the IEP is higher than for the freshly cleaved surfaces. This is consistent with the results obtained on aging of freshly cleaved samples. Besides these preparation-related differences, the origin of the mica itself has also been shown to affect the results as discussed in some detail by Lyons et al. Therefore, mica data should best be compared if the source of the sample and the preparation of the surface under investigation were identical.
Figure 11a shows a comparison for results from SFA measurements. The cations are chosen such that no aqueous hydrolysis species occur, keeping the speciation simple and avoiding solution reactions that would also affect the pH. Clearly, the surface remains negative for the mono- and divalent salts. With the SFA, it was reported that measurements above 10 mM cation concentration resulted in too weak forces to measure. At very high concentration, measurements show very interesting features that are related to hydration forces triggered by the hydration shells of adsorbed cations.
The divalent cations [here Ca, but the results are generic according to Pashley and Israelachvili for [ 73 ] alkaline earth elements, except Be, which strongly hydrolyzes and was not studied] generate less negative interfacial potentials than the monovalent cations.
For the trivalent cation La, there is charge reversal. The surface turns positive at submillimolar concentrations. Figure 11b shows that the same kind of results has been independently obtained by zeta-potential measurements.
The comparison shows good agreement. DLVO potentials from SFA experiments a and zeta-potentials from streaming potential measurements b of mica basal planes as a function of cation concentration at near-neutral pH for mono-, di-, and trivalent cations in chloride solutions.
The conclusion to be drawn from the above discussion of available data note that we do not claim that the compilation of data is complete, nor do we claim that we have covered all available sources of measurements is that tendencies are well retrieved for comparable samples and that even a quantitative agreement is probably achieved in particular for the divalent and trivalent cations at near-neutral pH.
The data also clearly prove that the valence of the cations strongly affects the interfacial potentials. This will be further discussed in Chapter 4 in the context of electrolyte titrations. Despite the good agreement in some cases, caution is required when studying mica basal planes because widely varying results can be obtained depending on sample pre-treatment, mica origin i. For mica, some data sets exist showing that the IEP of the edge plane is at pH approximately 8.
Figure 12 shows such data for different runs. As pointed out before, the measurements on the edge planes are difficult. However, the absolute values away from the IEP may strongly differ for data taken at different locations thus varying in roughness in particular. Figure 13 shows that even the action of divalent cations at pH 8. The data from Yan et al. This clearly will generate anisotropy, and for the overall particle, it will require electrostatic charge regulation.
It is not clear whether and how pH will be affected by such regulation. Various data sets on talc are available from the literature.
Zeta-potential measurements by Nalaskowski et al. The streaming potential measurements are in qualitative agreement with force distance curves by the same authors while showing distinct differences between the behavior of the edge and basal planes. The zeta-potential curves of the two planes are likewise not identical, so that the data appear self-consistent.
These data sets, however, are in conflict with force measurements reported by Yan et al. Figure 14a illustrates that the IEP of the basal plane is below pH 3 i. The origin of the differences is not clear.
The latter study shows the anisotropy, which is further supported by data for divalent cations at pH 8. The results are similar to those obtained with mica shown in Figure Again, the edge planes may change sign, whereas the basal plane remains negative.
Similarly data for chlorite also exist and confirm the data sets discussed here, in general, and clearly exhibit anisotropy [ 70 ]. Overall, the examples shown in this part again suggest very complex situations when basal and edge surfaces are present simultaneously. Such situations are discussed in the following section, where first we discuss the amount of solid clay particles is buffering a suspension with respect to pH yielding the overall point of zero net proton condition for the clay particles, whereupon, in the second step, the action of adding electrolyte to the buffered system can be investigated.
For the interpretation of such systems, assuming a full data set from force measurements or distinct measurements on specific planes were available, it would be very important to know whether there is a strong pH effect on the charge of a given plane or not.
This is not always clear and may depend on many things as has been discussed in much detail for mica basal planes. As already discussed in the introduction, the macroscopic measurements that are applied to investigate the charging behavior have mainly involved electrokinetic methods, force measurements, and all kinds of titrations.
The outcome of these measurements needs to be associated further with the properties of the EIL electric interfacial layer. The relation of the measurements to the interfacial potentials has been described in detail elsewhere [ 12 ] and will only be shortly summarized here.
Electrokinetic methods probe the overall charge within the so-called shear plane, which is at the head end of the immobile part of the EIL. In classical electrophoretic mobility experiments, where the velocity of a particle in an applied electric field is measured, the resulting interfacial potential the so-called zeta-potentials is the result of the interfacial species that are present within the immobile part i.
The zeta-potential is unspecific in the sense that it gives the overall result from a multitude of processes that may occur on complex particles as would be the case for clays.
Electrokinetic potentials of clay particles are typically negative over the full pH range investigated. The isoelectric point IEP, at which the electrokinetic mobility is zero is usually at low pH, sometimes not accessible to measurement. This result suggests that the negative, permanent charge is controlling the overall electrokinetic behavior of the clays. The application of the method has become standard and many experimental set-ups are available to determine the electrokinetic properties of particles.
Titration methods may probe specific kinds of interactions. Potentiometric acid-base titrations [ 19 , 81 ] will include all those interactions that involve reactions of the complete system with protons and hydroxide ions This also includes dissolved carbon dioxide, which is a substantial complication due to the aqueous-phase reactions but also because carbon dioxide-related species can adsorb on surfaces.
In all these reactions, protons are typically involved, which is why titrations are usually carried out in an inert atmosphere such as purified argon. With natural clays, it is conceivable that calcite is present as a secondary mineral contained in a given sample, which makes the performance of reasonable acid-base titrations more or less senseless. Mass titrations are a variant of potentiometric titrations [ 21 , 23 , 82 ]. They involve adding a known amount of solid to a solution of known composition and follow the evolution of the pH with increasing solid content.
At a sufficient amount of solid in the system, the surface acid-base reactions buffer the system yielding the point of zero net proton charge for pure oxide systems.
In such systems, the end point also coincides with the isoelectric point from electrokinetics. In the case of clays again, the situation is more complex. Yet, a further development of this technique is electrolyte titration [ 10 ]. From the effect of adding electrolyte to a surface buffered system on the resulting pH, the specific role of the electrolyte can be inferred.
For well-behaved oxides, usually no effect is observed if inert electrolytes such as NaClO 4 are used. For clays as pointed out earlier, there are multiple effects of electrolyte ions that could play a role in such measurements.
One would be the ion-exchange part, which could lead to reactions not related to classical acid-base equilibria, but yet they might cause interactions. Soils with a higher clay fraction tend to have a higher CEC. Organic matter has a very high CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil. Clay has a great capacity to attract and hold cations because of its chemical structure.
However, CEC varies according to the type of clay. It is highest in montmorillonite clay, found in chocolate soils and black puggy alluvials. Water, which is two hydrogen atoms and one oxygen atom, also is made up of charged particles, with the two hydrogen atoms having a positive charge. A clay mineral consists 2 silica tetrahedral sheets and 1 aluminum octahedral sheet. The clay mineral consists 2 silica tetrahedral sheets and 1 aluminum octahedral sheet with a magnesium hydroxide sheet in the interlayer space between sheets.
Isomorphous substitution is the most important source and variable charge at broken edges or hydroxyl surfaces is another. Describe the relative concentrations of CO 2 and O 2 at the root surface compared to pores far from the root.
At the root surface, the CO2 concentration will be higher and the O2 concentration will be lower than in the bulk atmosphere. What processes are taking place that move gas from the root into pores away from the root and gas to the root from pores away from the root.
Diffusion will move CO2 away from the root and O2 towards the root. Mass flow or convection will move O2 and CO2 towards the root as the root consumes oxygen and water. Ion exchange is stiochiometric and reversible. Soil Type. Very strongly acidic. Strongly Acidic. Mildly Alkaline. Moderately Acidic. Moderately Alkaline.
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