Geoscience Reference
In-Depth Information
Neutral silica surface
Neutral bulk solution
Double layer + acidic pore water
A
-
A
-
A
-
SiOH
SiO-M
+
M
+
H
+
A
-
A
-
SiOH
M
+
SiOH
M
+
M
+
A
-
SiOH
SiO
-
M
+
M
+
M
+
H
+
SiOH
SiO
-
M
+
A
-
H
+
A
-
A
-
SiO-M
+
SiOH
M
+
M
+
M
+
A
-
A
-
=
+
SiOH
SiOH
A
-
Diffuse
layer
A
-
SiOH
SiO-M
+
A
-
A
-
M
+
M
+
M
+
SiOH
M
+
SiOH
M
+
A
-
A
-
H
+
A
-
SiOH
SiOH
SiO
-
M
+
M
+
M
+
A
-
M
+
M
+
A
-
SiOH
A
-
H
+
A
-
H
+
o-plane
OHP
o-plane
A
-
A
-
Figure 1.2
Formation of the electrical double layer in the case of silica. In the present case, a neutral silica surface is brought in
contact with a neutral pore water solution composed of cations M
+
and anions A
−
. The silanol surface groups at the surface of silica
release a certain number of protons in the pore water, making the solution slightly acidic. Some of the cations from the pore water
are adsorbed in the Stern layer. The surface charge density and the Stern layer charge density are compensated in the diffuse layer. In a
sandstone, the bulk pore water is neutral (no net charge density), and only the diffuse layer is not neutral and more precisely
characterized usually by an excess of (positive) charges.
> SiO
−
+H
+
> SiOH+M
+
> SiO
−
M
+
+H
+
,
K
M
> SiOH
K
−
1 2
1 3
where
K
M
corresponds to the equilibrium constant for this
reaction. Sorption is distinct from precipitation, which
involves the formation of covalent bonds with the mineral
surface. This sorption can be strong (formationof an inner-
sphere complexes with no mobility along the mineral sur-
face) or weak. In the
where
K
±
are the two equilibrium constants associated
with the surface sorption and desorption of protons. This
2-p
K
model considers that two charged surface species,
namely, >SiO
−
and >SiOH
2
+
, are responsible for the sur-
face charge density of silica. That said, the reaction in
Equation (1.1) is often neglected in a number of studies
because the occurrence of the positive sites, >SiOH
2
+
, can
only happen at low pH values (typically below pH <3 as
mentioned briefly previously).
Wealsoassume that theporewater contains a completely
dissociated monovalent salt (e.g., NaCl providing the same
amount of cations Na
+
and anions Cl
−
). In the following, a
“
the formation of the
Stern layer is a kind of condensation effect demonstrated
by molecular dynamics. A weak sorption example is the
case of a hydrated sodium. In this example, the sorbed
counterion Na
+
keeps its hydration sphere, and it forms a
so-called outer-sphere complex with the mineral surface
(e.g., Tadros & Lyklema, 1969). Such counterions are
expected to keep some mobility along the mineral surface,
responsible (as briefly explained in Section 1.3) for a low-
frequency polarization of themineral grains in an alternat-
ing electrical field. The layer of ions formed by the sorption
of these counterions directlyonthemineral surface is called
the Stern layer. The Stern layer is therefore located
between the o-plane (mineral surface) and the d-plane,
which is the inner plane of the electrical diffuse layer
(Figures 1.1 and 1.2). The sorption of counterions occurs
at the
“
weak case,
”
isanionthat is characterizedbyachargeoppo-
site to the charge of the mineral surface, while a
counterion
”
has
a charge of the same sign as themineral surface. The typical
case for silica is tohave anegative surface charge, and there-
fore, the counterions are the Na
+
cations and the coions are
the Cl
−
anions. Note however that the sorption of cations is
characterized by a high valence and a strong affinity for the
silica surface (for instance, Al
3+
) and can reverse the charge
of themineral surface (surface andStern later together) and
therefore can reverse the sign of the charge of the diffuse
layer. The sorption is described by the following reaction:
“
coion
”
which is located in between the o- and
d-planes shown in Figure 1.1.
“
β
-plane
”
