2 research outputs found
Measuring the Isoelectric Point of the Edges of Clay Mineral Particles: The Case of Montmorillonite
The isoelectric point (IEP) of the
edge surface of a montmorillonite
sample was determined by using electrophoretic mobility measurements.
This parameter, which is fundamental for the understanding of the
charging behavior of clay mineral surfaces, was never measured so
far because of the presence of permanent negative charges within the
montmorillonite structure, charges that mask the electrokinetic behavior
of the edges. The strategy was to block or neutralize the structural
charges with two different cations, methylene blue (MB<sup>+</sup>) and tetraethylenepentaminecopperÂ(II) ([CuÂ(tetren)]<sup>2+</sup>), so that the charging behavior of the particles becomes that of
the edge surfaces. Adsorption isotherms of MB<sup>+</sup> and [CuÂ(tetren)]<sup>2+</sup> at different ionic strengths (NaCl) were performed to establish
the uptakes that neutralize the cation exchange capacity (CEC, 0.96
meq g<sup>–1</sup>) of the sample. At high adsorptive concentrations,
there was a superequivalent adsorption of MB<sup>+</sup> (adsorption
exceeding the CEC) and an equivalent adsorption of [CuÂ(tetren)]<sup>2+</sup> (adsorption reaching the CEC). In both cases, structural
charges were neutralized at uptakes very close to the CEC. Zeta potential
(ζ) vs pH data at different ionic strengths of montmorillonite
with adsorbed MB<sup>+</sup> allowed to estimate an upper limit of
the edge’s IEP, 5.3 ± 0.2. The same kind of data obtained
with adsorbed [CuÂ(tetren)]<sup>2+</sup> provided a lower limit of
the IEP, 4.0 ± 0.2. These values are in agreement with previously
informed IEP and point of zero charge of pyrophyllite, which is structurally
analogous to montmorillonite but carries no permanent charges. The
importance of knowing the IEP of the edge surface of clay minerals
is discussed. This value characterizes the intrinsic reactivity of
edges, that is, the protonating capacity of edge groups in absence
of any electric field generated by structural charges. It also allows
us to correct relative edge charge vs pH curves obtained by potentiometric
titrations and to obtain the true edge charge vs pH curves at different
electrolyte concentrations