Abstract
Clays are not only ubiquitous in nature, but they are also used in huge quantities in a broad range of industrial applications, such as thixotropic drilling fluids, ore pelletizers, waste disposal sealants, or fillers in polymer nanocomposites. In order to model environmental processes or to design new materials on a rational base, it is of prime importance to determine and possibly modify the interfacial properties of clay platelets at the solid/electrolyte interface. In this context, the fundamental question rises how far the stoichiometric interlayer charges as determined by the composition of the silicate layer correlates with the diffuse double-layer properties. Here, this question is addressed by means of a series of purposely synthesized sodium 2:1 layered silicates with defined composition and hence interlayer charge densities, respectively. Platelets of layered silicates of large enough diameter to perform AFM colloidal probe measurements were produced by melt synthesis. For comparison also, a natural muscovite mica has been included in this study. The diffuse layer properties in electrolyte solution have been determined by direct force measurements using the colloidal probe AFM technique and by electrokinetic measurements, respectively. We find that the diffuse layer potential decreases with increasing interlayer charge of the 2:1 layered silicates. This counterintuitive finding is attributed to ion adsorption and was further corroborated by determining the quantitative adsorption of polyelectrolytes, namely poly(amidoamine) dendrimers.Graphical abstract
Highlights
Volodymyr Kuznetsov and Katharina Ottermann contributed to this work.Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Layered silicates are omnipresent in nature and have a wide range of important applications, such as for thixotropic drilling fluids, ore pelletizers, waste disposal sealants, or fillers in polymer nanocomposites to improve their properties as gas barriers and flame retardants [1,2,3,4,5,6,7]
In order to exclude that the dimensions of the platelets of the different layered silicates have a significant influence on the electrophoretic mobility, we utilized an additional electrokinetic technique, the so-called streaming current detection
It should be noted that due to the different ionic strength used for the various analytical techniques, such a deviation would be expected on base of the Grahame equation, which already in its linearized form would stipulate a dependence of the potential on ionic strength by means of ψ = 2πκσ, where σ is the diffuse layer charge density on the silicate platelet
Summary
This determination provides the diffuse layer potential and charge regulation parameter of the probe, which is a prerequisite for a quantitative evaluation of force profiles as determined between different silicates and comparable colloidal probes (asymmetric combination). A set of measurements comparing the intra-platelet variation with the inter-platelet variation indicates that the observed variations have to be attributed to a large extend to variations within each individual platelet (cf. Fig. 6b)
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