High ionic strength electrokinetics of clay minerals

doi:10.1016/j.colsurfa.2006.06.037

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 291, Issues 1–3, 15 December 2006, Pages 212-218

https://doi.org/10.1016/j.colsurfa.2006.06.037 Get rights and content

Abstract

The electrokinetic potential of kaolin and montmorillonite at high ionic strengths (up to 2 M of 1-1 electrolyte) was measured as a function of pH by means of three instruments. The present results indicate that the electrokinetic potential does not cease at high ionic strengths. Potassium and cesium salts induced a shift in the IEP of kaolin to high pH. Similar shifts were observed for montmorillonite, but only in the presence of cesium. The increase in 1-1 electrolyte concentration resulted in depression of the absolute value of the ζ potential of kaolin and montmorillonite at neutral and basic pH. However, ζ potentials at high ionic strengths were surprisingly high in the absolute value. Namely, the negative ζ potentials up to −20 mV were measured in 1 M solutions of Li and Na salts.

Introduction

When the ionic strength is not too high, addition of 1-1 electrolyte (alkali nitrate V, chlorate VII or halide) at otherwise the same experimental conditions induces a decrease in the absolute value of the electrokinetic potential [1]. Then addition of even more electrolyte would depress the electrokinetic potential to zero. Till very recently direct measurements of the electrokinetic potential at high ionic strength were not possible, and it was believed that the electrokinetic potential at electrolyte concentrations of about 1 M is about zero. Indeed, popular models of the electric double layer (triple layer model, Stern) produce ζ potentials in the range of ±1 mV in 1 M 1-1 electrolyte (with model parameters, which give reasonable ζ potentials and surface charge densities at lower ionic strengths). Recently, the electroacoustic method made it possible to perform measurements of the electrokinetic potential at electrolyte concentrations of 1 M and even more. More recently, new equipment made it possible to perform measurements at high ionic strengths also by means of electrophoresis [2]. These measurements indicated surprisingly high absolute values of the electrokinetic potential at high ionic strengths. Moreover, certain 1-1 electrolytes (lithium salts and certain sodium salts) induced a shift in the IEP of metal oxides to high pH, which indicates specific adsorption of cations. The driving force for adsorption of metal cations on metal oxides at high ionic strengths is at least partly due to ion–ion interactions in solution. However, the difference in high ionic strength behavior between particular metal oxides suggests, that specific interactions between the solid and the ions also play an important role.

The electrokinetic behavior of metal oxides and silica in highly concentrated 1-1 electrolytes is well-documented [3], but there are no data for other materials. We are interested if the surprisingly high electrokinetic potentials at high ionic strength occur also with materials other than sparingly soluble oxides. The aim of this study is to generalize the previous observations, and to verify the hypotheses regarding the mechanism of the observed phenomena. We are especially interested if the salt-specificity, which was observed for metal oxides occurs also for other materials.

Kaolinite and montmorillonite were selected for this study as examples of clay minerals. Both have well-defined chemical formulas and crystalline structures, but actual samples vary in composition. This is because in contact with aqueous electrolytes clay minerals show cation-exchange properties and certain degree of dissolution (or rather selective leaching of components). The shape of the particles is far from spherical, and the charge is not evenly distributed over the surface. Moreover, part of the charge of the particles is located in the bulk. The charge of clay mineral particles consists of negative structural charge, which is rather insensitive to the pH and ionic strength and of variable (pH and ionic strength dependent) charge of the edges. This results in a negative electrophoretic mobility over the entire pH range [4], [5], [6], [7], [8], [9], [10], although a few studies report an IEP at low pH [11], [12]. Probably the electrokinetic behavior is affected by the morphology of the particles (aspect ratio). Strictly speaking, for clay minerals one should use a term “apparent ζ potential” for the value that is calculated from the measured quantity (e.g., electrophoretic mobility) by the instrument software, since the physical sense of this quantity is vague.

Section snippets

Materials

Kaolin (Sigma-Aldrich, particle size 0.1–4 μm, according to the manufacturer) was used as obtained. Montmorillonite was a fine fraction separated from montmorillonite K 10 (Fluka, surface area 200 ± 20 m²/g). The original product contained coarse grains which are not suitable for electrophoretic or electroacoustic measurements. In order to separate a fine fraction, the commercial product was dispersed in water (1:10). After 10 min the dispersion was separated from the sediment, and after further

Kaolin

Electrokinetic curves for kaolin are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 (Malvern), Fig. 8 (Acoustosizer), and Fig. 9 (DT). The results obtained by means of various instruments were qualitatively consistent with very few exceptions. Only negative ζ potentials at a low ionic strength were observed by means of Malvern and DT. A few titrations carried out by means of Acoustosizer (not shown here) suggest an IEP at pH about 3.6. A few publications report similar IEP of

Discussion

The electrokinetic behavior of clay minerals and silica show many common features. The IEP at low ionic strengths falls at pH below 4 or does not exist at all. This is due to common physical properties (relatively low dielectric constant [16]) and to similar character of the surface groups responsible for the surface charging. The preference for the alkali metal cations decreases in the series Cs > Rb > K > Na > Li. In this respect silica and clay minerals can be categorized as “soft” in contrast with

Conclusions

The results obtained by means of three different instruments based on different principles are qualitatively consistent. Thus, the observed phenomena are likely to be real effects rather than instrument artifacts. The present study confirmed that the ζ potentials in 1 M 1-1 electrolyte solutions can assume values in excess of ±20 mV. The shifts in the IEP induced by a 1-1 electrolyte when its concentration exceeds 0.1 M are a common phenomenon. These shifts are salt-specific. For “soft” materials,

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High ionic strength electrokinetics of clay minerals

Abstract

Introduction

Section snippets

Materials

Kaolin

Discussion

Conclusions

J. Colloid Interf. Sci.

Adv. Colloid Interf. Sci.

J. Colloid Interf. Sci.

Microporous Mesoporous Mater.

Colloids Surf. A

J. Colloid Interf. Sci.

J. Colloid Interf. Sci.

Geochim. Cosmochim. Acta