engleski

Protolytic equilibria at ‘inert’ — electrolyte interfaces

In major physical chemistry journals, the question whether hydroxide ions interact with inert surfaces is currently a hot issue. Macroscopic measurements suggest that such interfaces have isoelectric points of about 2 to 4. This is explained by a strong affinity of hydroxide ions for the interfacial water present at these surfaces. Consequently it is suggested that hydroxide ions can be found at these interfaces and are strongly preferred say at pH 5 compared to protons although the hydroxide ion concentration in bulk solution is 4 orders of magnitude lower than the proton concentration. Molecular Dynamic simulations would predict that protons have a stronger affinity to such surfaces. Also experimental data with proton and hydroxide, at concentrations above millimolar, clearly confirms the prediction of the Molecular Dynamics simulations. The contradiction of the Molecular Dynamics simulations to the experimental work at lower proton and hydroxide concentrations remains and is probably difficult to resolve and thus the origin of the unexpected pH-dependent behaviour of a wide variety of surfaces that bear no surface functional groups is not clear. Whatever the origin of the pH dependent charging of these inert interfaces might be, it should be important to consider it in mineral surface – electrolyte studies of geochemical interest. Thus for example the isoelectric points of many alumina single crystal samples have been reported to be rather low (actually around pH 4) and they presently remain largely unexplained. More interestingly, available isoelectric points are within or close to the range reported for the inert interfaces. This would indicate that the work on the alumina single crystals might involve reactions that have so far not been considered in the geochemical literature. We report some experimental results on a range of inert surfaces to show how they all agree with each other and relate them to data on alumina samples. Our data confirm the previously reported experimental data at the millimolar scale. One possible explanation for the continuing discrepancy might be that the extrapolation of results for the above millimolar to the below millimolar concentration range is not appropriate. In particular we show that in some of the systems, we see clear changes in patterns like the salt dependency of zeta-potential at about millimolar concentrations.

surface potential; electrical interfacial layer; surface comlexation; counterion association

doi: 10.1016/j.gca.2010.04.037