Membrane characterization

Characterization of filtration
membranes
Course of Bioreactors and Downstream Processes M
cristiana.boi@unibo.it
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Membrane characterization
Cristiana Boi
 The characterization of membrane properties helps not only
to choose the right membrane for a given application, but
also to gain a better understanding of their preparation
methods and on the selectivity and fouling mechanisms.
 The methods used give access to macroscopic or
microscopic quantities, characteristic of the membrane
structure and the chemistry of the material.

Membrane choice
Cristiana Boi
Structural and transfer characteristics (hydraulic permeability
and selectivity curve) provide information on how the
membrane will perform in the intended separation process:
the permeate flow that can be expected and the size of
molecules likely to be rejected by the membrane.
Surface physicochemical and chemical properties (charge,
hydrophilic–hydrophobic nature, and chemical composition),
which allow fouling and interactions among the different types
of molecules at the membrane surface to be predicted to
some extent.

Characterization methods
Cristiana Boi
Membrane characterization methods can be divided in two
categories:
 for the chemical and structural properties of the membrane;
 for the functional properties, such as selectivity or
permeability.

Structural Characteristics
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The characterization methods developed for obtaining
information on the membrane structure can be divided into
three types:
 microscopy techniques;
 liquid intrusion or displacement techniques;
 techniques that measure tracer molecule retention.

Microscopy Techniques
Cristiana Boi
These techniques provide information on surface topology,
roughness, and pore size.
Several microscopic observation methods are used, which
differ by their implementation and their resolution.
 Electron microscopy (SEM, TEM);
 Near Field Microscopy (STM, AFM);
 X-ray synchrotron microtomography.

Principles of Electron Microscopes
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Applications of SEM
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SEM is be an effective method for:
1. characterizing flat membranes;
2. assessing the effect of preparation conditions on the
structure of hollow fiber membranes;
3. observing the evolution of membrane morphology after
contact with washing solutions;
4. measuring the thickness of deposits (0.2–0.4 µm) formed
during membrane fouling.

SEM image of a polysulfone hollow fiber
membrane (MWCO: 40 kDa)
Cristiana Boi
C Causserand and P Aimar in Comprehensive Membrane Science and Engineering, Drioli and Giorno Eds, 2010 Elsevier.

Schematic description of a (STM)
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Example of STM of a membrane
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Image of a 100 kDa polysulfone membrane obtained by scanning
tunneling microscopy (STM); (b) Cross sections corresponding to the
lines drawn in the picture 5 (middle-bottom).
Bessieres, A., Meireles, M., Coratger, R., Beauvillain, J., Sanchez, V. J. Membr. Sci. 1996, 109, 271–284.

Example of an AFM of a membrane
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(a) Example of an image of a tracked etched polycarbonate membrane
(Nuclepore) – nominal pore size : 0.2 µm; (b) same sample – blow-up
(1000x1000)nm2; and (c) cross section along the line drawn in picture (a).

Comparison STM and AFM
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X-ray synchrotron
microtomography
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3D reconstruction volume of a part of
a PVDF hollow fiber microfiltration
membrane from SRµCT.
Dimensions of the observed volume
are 420 µm x 320 µm x 190 µm.
Remigy, J.C., Meireles, M., Thibault, X. J. Membr. Sci. 2007, 305, 27–35.

Displacement Techniques
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These methods consist of wetting the membrane with a wetting
agent (phase 1) of known surface tension and contact angle
with the membrane, then expelling it from the membrane pores
by displacing it with a second phase (phase 2), which is
generally air or an immiscible liquid.
 bubble point;
 liquid/gas displacement;
 liquid–liquid porometry.

Bubble point
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In the bubble point method, the pores of the membrane are
completely wet with a liquid (phase 1) which is then displaced
with air (phase 2). The Young–Laplace equation allows the
maximum pore radius to be calculated from the value of the
transmembrane pressure measured when the first bubbles
are detected in the permeate compartment:
where γL is the surface tension of the liquid (N m-1) and ϑ the
contact angle between the liquid and the membrane surface.

Mercury intrusion
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Variant of the bubble point method in which phase 1 is air and
phase 2 is mercury that is gradually forced into the membrane
pores.
Mercury has a very small contact angle with most materials.
Both bubble point and mercury intrusion have the drawback
that they require very high pressures (from 1 to several
kilobars) to analyze pores of the order of 10 nm like those
found in ultrafiltration membranes.

Liquid–liquid porometry
Cristiana Boi
This method consists of wetting the membrane with a wetting
liquid (phase 1) then expelling the liquid from the membrane
pores by displacing it with another liquid (phase 2) that is
immiscible with the former. The pressure that has to be
applied depends directly on the interfacial tension between
the two liquids. Thus, by a appropriate choice of the liquid 1-
liquid 2 pair (based on a water–isobutanol–methanol
combination), the pressures to be applied for the analysis of
small-radius pores need not exceed 10 bar or so.

Liquid–liquid porometry
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Example of a bi-modal pore-size distribution obtained on an ultrafiltration membrane by
bi-liquid porometry (liquid mixture: methanol, isobutanol, 1 butanol and water; interfacial
tension between organic phase and aqueous phase: 0.35mN-1m).

Water permeability
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In the presence of a pure solvent, any increase in
transmembrane pressure leads to a proportional increase in
the permeate volume flux density which allows the membrane
to be considered as an ideal porous medium.
The coefficient of proportionality, Lp (assumed constant), is
called the hydraulic permeability of the membrane and
depends on its intrinsic characteristics: porosity, pore-size
distribution, thickness, and hydrophilic nature. This parameter
(Lp) thus represents the volume of solvent that passes through
the membrane per unit of pressure, filtering area, and time.

Water permeability
Cristiana Boi
From tests with pure water, the permeability of the membrane
can be determined by means of Darcy’s law:
Deviations from linearity may arise from the compressibility of
the membrane, or from the retention of some impurities
remaining in the retentate compartment.
J : flux density (m s-1),
Q : pure solvent filtration rate (m3 s-1),
A : filtering surface area (m2),
Lp : hydraulic permeability of membrane (m),
µ : dynamic viscosity of pure solvent (Pa s),
P : transmembrane pressure (Pa)
Rm: hydraulic resistance of membrane (m-1)

Tracer Retention Techniques
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Tracer retention techniques measure the transfer of
macromolecules (ultrafiltration) of calibrated particles called
tracers, and these measurements are then compared with a
transport model.
The principle is to measure the retention of a series of tracers
of different sizes so as to obtain a curve of selectivity versus
molar mass (or size) of the molecules

Choice of Tracers
Cristiana Boi
The tracers used must fulfill several criteria:
 have few specific interactions with the membrane material
or the macromolecules that may be absorbed on to the
membrane to account for its intrinsic characteristics;
 have molar masses (or sizes) that cover the broadest
retention range possible, especially in the region close to
100%;
 be detectable even at very low concentrations in the
collected permeate.

Examples of Tracers
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Applicability of tracer
retention methods
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 Method is suitable for UF membranes: dextrans, PEGs and
proteins are used as tracers;
 NF membranes: the tracer can be small PEGs, sugars or
inorganic salts;
 RO: NaCl as a tracer. Membranes for water desalination
have 99% NaCl rejection, while membranes for brackish
water have 96% rejection.
For MF membranes is very difficult apply since there are only
a few suitable tracers of sufficient size (polyethylene oxide,
high MW dextrans).

Selectivity curve
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Selectivity curve for dextran molecules: MWCO (molecular weight of a
molecule rejected at 90% by the membrane) is 85 kDa

Microbiological Tracers
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Microbiological tracers are used because membranes have
been considered as potential screens against microbiological
contamination of waters or air.
 Bacteria;
 Viruses;
 Surrogates.

Microbiological Tracers
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In general, filtration reduces the content in microorganisms,
but does not guarantee their complete elimination. The
reduction in microorganisms is often measured by the log
removal value (LRV), defined as
The maximum LRV that can be claimed depends on the
concentration of bacteria in the challenging suspension. For
example, if the concentration is 106 cfu/ml in the retentate and
zero in the permeate, one counts 1 cfu/ml in the latter and the
LRV is then 6.
P
F
c
c
LRV 10
log


Bacteria
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The first discrimination was done between filters rated 0.2 and
0.45 µm, by using Brevundimonas diminuta; several bacteria
are used for membrane characterization, as for example

Virus
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Most viruses are two orders of magnitude smaller than
bacteria.
Differences in membrane characterization:
 higher diffusivity;
 surface interactions in their behavior.
In particular, viruses have a strong tendency to adsorb onto
surfaces. For this reason, the apparent retention of viruses by
membranes may be much higher than the actual filter
capacity.
Experiments of virus retention are performed in the worst-
case conditions: this will guarantee that when in operation, the
membrane will show either the same or a better retention of
the viruses.

Surrogates
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Surrogates of the microorganisms, which would mimic their
behavior and allow simpler and faster membrane
characterization. Latex or silica particles, and gold sols have
therefore been used. Thus far, two major drawbacks have
been pointed out:
 the much lower sensitivity of the particle detection methods,
which limits the range of LRV that can be explored;
 the difference in surface properties (charge, stiffness,
hydrophilic character, etc..) between such surrogates and
bioparticles, which induces a difference in retention, as, for
example, by charge exclusion, by adsorption on or in the
membrane material.

Determination of Hydrophilic/
Hydrophobic Nature of Membranes
Cristiana Boi
The hydrophilic nature of a material is a very important
parameter as it conditions the solute-membrane and solvent-
membrane interactions.
In many applications, hydrophilic membranes are more
efficient than hydrophobic ones, the latter being confronted
with more serious fouling whenever hydrophobic molecules or
particles (proteins, colloids, etc.) are present in the fluid to be
filtered. The same is true for surfactants, the adsorption of
which is more pronounced on hydrophobic materials.
The hydrophilic/hydrophobic nature of a membrane is
determined by measuring the contact angle or by using the
capillary elevation balance. The liquid normally used for this
purpose is water.

Contact Angle Measurement
Cristiana Boi
Sessile drop method
This method consists of placing a drop of liquid (generally water)
on the surface of the membrane.
The wettability of the surface is then characterized by the contact
angle between the solid surface and the tangent to the liquid
surface at the contact point.
• ϑ=0 solid is perfectly wettable by
the liquid (hydrophilic if the liquid is
water);
• ϑ <π/2 more or less wettable;
• ϑ >π/2 more or less not wettable
(hydrophobic if the liquid is water).

Cristiana Boi
Captive bubble method
Used when membrane properties do not allow the contact angle
to be measured by placing a drop on the surface.
The membrane is immersed in water with the surface to be
analyzed facing downward. A micro-syringe is then used to trap an
air bubble on the lower surface of the membrane.

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Capillary Elevation Balance
For membranes with nonplanar geometry, such as hollow
fibers, another method can be envisaged. It consists of
measuring the weight gained by a material placed in contact
with a liquid as a function of time. The speed at which a given
liquid spontaneously penetrates a porous solid by capillarity
depends directly on the porous structure of the material and
the affinity of the liquid for the material.

Cristiana Boi
 The values obtained for contact angles depend on the
measuring technique used;
 Moreover, they also depend on many factors connected
with how the sample is prepared and the characteristics of
the zone analyzed (local roughness, porosity, heterogeneity
in the surface chemical composition, etc.).
 It has been shown that the more porous a membrane is,
the more hydrophilic it appears to be.
It is preferable to use these methods to establish a
comparative classification of membranes, all other things
being equal.

Membrane characterization

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