THE CONTRIBUTION OF DOUBLE-LAYERS TO THE FREE-ENERGY OF INTERACTIONS IN THE CASSITERITE-SDS SOLUTION SYSTEM

From the values of contact angles for water, glycerol and diiodomethane taken from the literature, the Lifshitzvan der Waals, electron-acceptor and electron-donor parameters corresponding to the polar head of the surface free energy of sodium dodecyl sulphate (SDS) were determined and compared with those found in the literature. From these values, the Lifshitz-van der Waals and acid-base components of the free energy of interaction between SDS molecules through water were evaluated. The electrostatic component of the free energy was estimated from the psi value for SDS found in the literature. These calculations allow the estimation of the total free energy of interaction between the SDS apolar and polar moieties through water on the basis of two models for interactions: plane-plane and cylinder-cylinder, The results were compared with data from the literature, showing good agreement (for electrolyte of 0.1 M ionic strength) for both models, and with the value obtained from CMC data. However, for other ionic strengths some differences are observed. The free energy of interaction between SDS and cassiterite through water was evaluated from the components of the surface free energy for both the apolar and the polar moieties of SDS and its psi value, and from the Lifshitz-van der Waals and acid-base components of the surface free energy of cassiterite with different surface treatments, taken from the literature. On the basis of these calculations, it was possible to infer the main influence of pH on the SDS-cassiterite interactions: they are predominantly electrostatic forces at low pH, and acid-base forces at a pH corresponding to the ZPC. Also, at high pH, when negative charges are present at the cassiterite-water interface, SDS adsorption onto cassiterite is possible only by acid-base interactions between the apolar chain of SDS and the cassiterite surface.