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3
4
1
2
Fig. 8.1
A model of surface diffuse layer near the wall of a capillary filled with groundwater.
1
—Stern layer which consists of two sublayers;
2
—A region of mobile ions;
3
—An adsorbed ion;
4
—hydrated ion
M .OH/
n
$
ŒM .OH/
n1
O
C
H
C
:
(8.1)
In this model the positive charge of the solution consists of the bound H
C
ions fixed
at the surface and of the diffuse layer of mobile H
C
ions. A model of this layer is
displayed in Fig.
8.1
.
The layer of bound charges referred to as Stern layer consists of two sub-layers
(Overbeek
1952
). The first sub-layer contains the positive ions connected with the
negatively charged wall by an absorption force. The sub-layer of hydrated/aquated
ions, which weakly interact with the solid phase, joints the flank of the first sub-
layer. In the region of mobile ions there are the diffusion flow of excess ions and
electric current flowing in opposite direction. At the state of dynamical equilibrium
there is a balance between these two flows.
The diffusion layer has a thickness of the order of Debye screening radius of ions.
Figure
8.2
shows the approximate dependence of electric potential on distance near
the capillary wall. Usually the Stern layer has a negative potential with respect to
that of bulk electrolyte as shown with curve 1 in Fig.
8.2
. However, if the number of
positive ions in the Stern layer is greater than that of the negative charges induced on
the surface of capillary, then the potential of Stern layer becomes positive as shown
in Fig.
8.2
with the curve 2.
In the presence of pressure gradient the capillary fluid becomes to move.
However, the fluid flow is not excited in the boundary layer which consists of
ions and molecules absorbed at the capillary walls. This layer is separated from
the moving fluid by the so-called slip plane which approximately coincides with the
external boundary of the Stern layer shown in Fig.
8.1
. It is customary to introduce
the electrokinetic potential or &-potential as the potential difference between the slip
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