Measurement of the surface free energy of calcium-montmorillonite

Abstract

The surface free energy of Ca-montmorillonite has been determined from measurement of the contact angle of water on an oriented deposit immersed in n-alkanes; the oriented deposits were equilibrated at different relative humidities. These experiments provided the polar, ~s, and dispersive, ~, components of the surface free energy 7s. The results indicate that: (I) ?~s is mainly due to dispersive forces (7~ -~ 145 mJ.m 2) and, to a lesser degree, polar forces (~ _~ 35 mJ.m z). (2) The value of the apparent surface free energy of the clay, y,, tends towards that measured for water when the solid water content exceeds 50 wt%. (3) The sorption of water molecules substantially modifies the surface free energy of the solid; ?s decreases from 180 mJ.m -2 to 60 mJ.m z when the water content increases from 0 to 50%. These results agree with other observations made on solids whose surface characteristics are similar to montmorillonite, i.e. micas, silicates and glasses. In addition, variations of ?s with water content can be related to the hydration processes of smectites. Finally, these results show that the silicate no longer influences the surface properties when the montmorillonite water content exceeds 60%, In order to analyse the extent to which organic compounds control clay wettability, we have measured the surface free energy of organo-mineral complexes. The organic and mineral phases were first investigated separately and then in association. In this paper we describe the surface free energy properties of calcium-montmorillonite, which is often used as a model material. THEORY Measurement of the surf ace free energy of a solidUnlike liquids, the surface free energy of solids can not be determined directly and indirect methods have had to be developed (Adamson, 1982). We chose to estimate the surface free energy of Ca-montmorillonite from contact-angle measurements of water on the clay. Since this material is a high-energy solid, the usual method in a liquidsolid-vapour system cannot be used; the solid surface is completely wetted by the liquid, the contact angle 0 then being zero. We used the alternative method developed by Tamai et al. (1967), where the vapour phase is replaced by a second liquid phase. This method has been tested by Schultz et al. (1977a,b) on high-energy solid surfaces. They used saturated hydrocarbons as a second liquid phase, which enabled them to determine finite contact angles.

Surface Free Energy Components of Clay-Synthetic Humic Acid Complexes from Contact-Angle Measurements

“…Montmorillonite surfaces are extremely hygroscopic, and the adsorption of liquid or vapor on them severely decreases the surface free energy components of the solid (Chassin et al, 1986;Chibowski and Staszczuk, 1988). Furthermore, because of the high surface free energy of montmorillonite, a finite contact angle for water on that mineral cannot be measured.…”

“…According to Young (1958) and as was reported by Chassin et aL (1986), dispersive forces on Ca-montmorillonite originate from the silicate tetrahedra. Increasing the amount of adsorbate on the montmorillonite decreased the 3' SO component of ~s.…”

“…This method was used to determine the dispersive and polar components of the surface free energy of solids equilibrated at 0% RH. For each solid, water contact angles were determined by using the same technique described by Chassin et al (1986). Contact angles…”

“…The hydrophobic action of humic substances was suggested by Robinson and Page (1950) on the basis of contactangle determinations on clay fractions extracted from high organic-matter content soils. Chassin et al (1986) determined % for Ca-montmorillonite. In the present study, changes in 3's were measured after adsorption of a synthetic humic acid on Ca-montmorillonite, and the extent by which the amount of adsorbate influenced the measured values was noted.…”

“…The <2.0-~tm fraction of this Crook County, Wyoming, montmorillonite was purified according to the method given by Chassin et aL (1986) and washed three times with a 1.0 M CaCI2 solution to transform the clay to a homoionic form. The suspension was then extensively washed with deionized water on a tangential ultrafiltration device Figure 2.…”