B. Janczuk et al., THE CONTRIBUTION OF DOUBLE-LAYERS TO THE FREE-ENERGY OF INTERACTIONS IN THE CASSITERITE-SDS SOLUTION SYSTEM, Colloids and surfaces. A, Physicochemical and engineering aspects, 100, 1995, pp. 93-103
From the values of contact angles for water, glycerol and diiodomethan
e taken from the literature, the Lifshitzvan der Waals, electron-accep
tor and electron-donor parameters corresponding to the polar head of t
he surface free energy of sodium dodecyl sulphate (SDS) were determine
d 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. Th
e electrostatic component of the free energy was estimated from the ps
i value for SDS found in the literature. These calculations allow the
estimation of the total free energy of interaction between the SDS apo
lar and polar moieties through water on the basis of two models for in
teractions: plane-plane and cylinder-cylinder, The results were compar
ed with data from the literature, showing good agreement (for electrol
yte of 0.1 M ionic strength) for both models, and with the value obtai
ned from CMC data. However, for other ionic strengths some differences
are observed. The free energy of interaction between SDS and cassiter
ite through water was evaluated from the components of the surface fre
e 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 o
f the surface free energy of cassiterite with different surface treatm
ents, taken from the literature. On the basis of these calculations, i
t was possible to infer the main influence of pH on the SDS-cassiterit
e 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 interfa
ce, SDS adsorption onto cassiterite is possible only by acid-base inte
ractions between the apolar chain of SDS and the cassiterite surface.