Measurement of the surface free energy of calcium-montmorillonite

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, γ S P , and dispersive, γ S D , components of the surface free energy γ s . The results indicate that: (1) γ s is mainly due to dispersive forces ( γ S D ≃ 145 mJ.m −2 ) and, to a lesser degree, polar forces ( γ S P ≃ 35 mJ.m −2 ). (2) The value of the apparent surface free energy of the clay, γ α , 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; γ s decreases from 180 mJ.m −2 to 60 mJ.m −2 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 γ 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%.