Introduction
Many studies up to now on poulticing have focused on the salt
and moisture transport solely in terms of advection and
diffusion. However in salt extraction, as
there are salt gradients present so too will there be osmotic
pressure gradients which can have a large influence.For a single
capillary pore the capillary pressure pc is
given by:
where γ (N/m) is the surface tension of
the liquid/vapor interface, φ the
contact angle between the liquid/air and liquid/solid interface
and r the pore size. During drying, the pattern of liquid water
migration is very much affected by the different capillary
forces. Where micropores are interconnected with macropores,
water is preferentially drawn into the micropores due to
capillary pressure gradients. Thus, the surface macropores begin
to empty of liquid water, while the micropores remain full.
Hence when two different saturated porous materials are placed
in hydraulic contact with each other and dried, the material
with the largest pores will start to drain first. Therefore,
during drying water is removed first from the material with the
largest pores. Consequently, in the case of poultice/substrate
systems, the poultice will dry first if it has larger pores than
the substrate.Hence there will be moisture flow, and thereby an
advection of ions, from the substrate into the poultice.In this
case there will be an high efficiency of removal of salts from
the substrate.
In most porous materials the pores are not uniform, and
therefore there is a pore size distribution. Thus the overall
macroscopic capillary pressure ψc of a material is a function of its pore size
distribution. When the substrate/poultice system contains
a saline solution there is an additional contribution to the
macroscopic capillary pressure of each porous material due to
the osmotic pressure, i.e.:
where the osmotic pressure, ψo, is given by:
where R is the universal gas constant, T the absolute temperature
and aw the water activity (for pure water aw = 1 and
hence the osmotic pressure is zero). When the well known Pitzer’s
activity coefficient model is applied the osmotic pressure can be
calculated.
Experiment showing the effect of osmotic
pressure
In order to see purely the osmotic effect in an experiment a
Bentheimer saturated with 5 M NaCl solution (representing the
poultice) was put on top of a
2 MNaCl solution saturated Bentheimer (representing the substrate).
In Fig. 1 the measured moisture profiles are given. Based purely on
capillary pressure one does not expect any effect.
Fig. 1. The water profiles in the Bentheimer/Bentheimer system
plotted several times during drying. Initially the Bentheimer on the top was saturated with 5 M NaCl solution and the
substrate Bentheimer was saturated with 2 M NaCl solution
In Fig. 2 both the corresponding theoretically derived and measured
moisture content relation at the interface are given. In this case
due to the higher salt concentration the effective pore size of the
Bentheimer with 5 M NaCl solution will be smaller.
Fig. 2 The water saturation of the 2 M NaCl solution saturated
Bentheimer at the interface as a function of water saturation of the 5 M NaCl solution saturated Bentheimer at
the interface (see also Fig. 1). The solid line represents the
relation when both materials are water saturated whereas the dashed
line represents the relation as determined from the measured capillary pressure curves and the calculated osmotic
pressure
As can be seen indeed the Bentheimer with 5 M NaCl solution dried
more slowly indicating that its effective pore size is smaller.
After some time there is a deviation from the relationship as
predicted on basis of the calculated osmotic pressure. This is due
to the salt transport in the Bentheimer saturated with 2 M NaCl
solution towards the interface, as a result of which the
concentration difference decreased. In this case the difference in
drying of the two Bentheimer parts is purely due to the osmotic
pressure effect.
Conclusion and discussion
These drying experiments demonstrate the effect that osmotic
pressure has on salt and moisture transport within porous materials.
Essentially, a saline solution in a porous material will exert an
osmotic pressure that acts to reduce the effective pore size of the
material. These findings have potential practical implications for
the optimisation of poulticing treatments. While the pore size
requirements for advection to take place at the start of the process
remain, nevertheless, these constraints need not be quite so severe,
as they are gradually overcome by the build up of an osmotic
pressure due to the ongoing migration of salt from the substrate to
the poultice. Moreover, it is clear that the longer a poultice stays
in contact with the substrate, the more it will accumulate salt, and
thereby the osmotic pressure is increased and its effective pore
size will become smaller.
An extensive description can be found in:
Leo Pel, Alison
Sawdy, Victoria Voronina, Physical principles and efficiency of
salt extraction by poulticing, Journal of Cultural
Heritage11
59–67 (2010)
A. Sawdy, B. Lubelli, V. Voronina , and L.
Pel, Optimizing the extraction of soluble salts from porous
materials by poultices, Studies
in Conservation55
26-40 (2010)
V. Voronina , L. Pel, A. Sawdy,
K. Kopinga,The
influence of osmotic pressure on poulticing treatments for
cultural heritage objects, Materials and Structures,
2012 (DOI
10.1617/s11527-012-9896-0)
V. Voronina, Salt extraction by
poulticing: an NMR study
, Eindhoven University of Technology (2011). (Download
2.4 Mb)
V. Voronina, L. Pel, K. Kopinga, Effect of osmotic pressure on
salt extraction by a poultice, Construction and Building Materials
53 432–438 (2014)