The mechanism of adsorption of sodium dodecylsulfonate on fluorite and its surface free energy

The interaction mechanism of sodium dodecyl sulfonate (NaDS) with mineral fluorite has been investigated by Infrared Attenuated Total Reflection (ATR). Measurements have been done using a modified ATR cell which allows the study of powdered samples without previous manipulations. Adsorption experiments were carried out at 25°C and pH 6.5. Also measurements of the contact angle for water, glycerol, formamide and diiodomethane on the fluorite covered with NaDS film were made, and from the results the Lifshitz-van der Waals, electron-acceptor and electron-donor parameters of the surface free energy of the fluorite/NaDS film were determined. The comparison of the spectra obtained with samples of fluorite, both pure and after adsorption and further washings with water at room temperature and with hot water, respectively, and the spectrum of the adsorbed phase after extraction with CCl4, suggests that chemisorption and physisorption processes take place on the fluorite surface. This is also viewed from the surface free energy components of bare fluorite and fluorite with SDS film washed with water at room temperature and hot water. A proposed mechanism suggests that the DS− ions are firstly chemisorbed and a CaFDS compound is formed. Then, a physisorption process takes place between the fluorite with CaFDS and DS− ions, by acid-base interactions. These results confirm those obtained from flotation and adsorption isotherms for this system.