ISO 13099-2:2012
(Main)Colloidal systems — Methods for zeta-potential determination — Part 2: Optical methods
Colloidal systems — Methods for zeta-potential determination — Part 2: Optical methods
This part of ISO 13099 specifies two methods of measurement of electrophoretic mobility of particles suspended in a liquid: video microscopy and electrophoretic light-scattering. Estimation of surface charge and determination of zeta-potential can be achieved from measured electrophoretic mobility using proper theoretical models, which are described in detail in ISO 13099-1.
Systèmes colloïdaux — Méthodes de détermination du potentiel zêta — Partie 2: Méthodes optiques
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INTERNATIONAL ISO
STANDARD 13099-2
First edition
2012-06-15
Colloidal systems — Methods for zeta-
potential determination —
Part 2:
Optical methods
Systèmes colloïdaux — Méthodes de détermination du potentiel zêta —
Partie 2: Méthodes optiques
Reference number
©
ISO 2012
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ii © ISO 2012 – All rights reserved
Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 2
4 Principles . 3
5 Microscopic methods . 4
6 Electrophoretic light-scattering (ELS) method. 5
6.1 General . 5
6.2 Cell design . 5
6.3 Reference beam optical arrangement . 6
6.4 Cross-beam optical arrangement . 6
6.5 Signal processing . 7
6.6 Determination of electrophoretic mobility . 9
7 Calculation of zeta-potential . 9
8 Operational procedures .10
8.1 Requirements .10
8.2 Verification .12
8.3 Sources of measurement error .13
8.4 Test report .15
Annex A (informative) Electroosmosis within capillary cells .16
Bibliography .19
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 13099 was prepared by Technical Committee ISO/TC 24, Particle characterization including sieving,
Subcommittee SC 4, Particle characterization.
ISO 13099 consists of the following parts, under the general title Colloidal systems — Methods for zeta-
potential determination:
— Part 1: Electroacoustic and electrokinetic phenomena
— Part 2: Optical methods
The following part is under preparation
— Part 3: Acoustic methods
iv © ISO 2012 – All rights reserved
Introduction
Zeta-potential is a parameter that can be used to predict the long term stability of suspensions and emulsions
and to study surface morphology and adsorption on particles and other surfaces in contact with a liquid. Zeta-
potential is not a directly measurable parameter. It can be determined using appropriate theoretical models
from experimentally determined parameters, such as electrophoretic mobility. Optical methods, especially
electrophoretic light scattering, have been widely used to determine electrophoretic mobility of particles or
macromolecules in suspension or in solution. The purpose of this part of ISO 13099 is to provide methods for
measuring electrophoretic mobility using optical means and for calculating zeta-potential.
INTERNATIONAL STANDARD ISO 13099-2:2012(E)
Colloidal systems — Methods for zeta-potential
determination —
Part 2:
Optical methods
IMPORTANT This part of ISO 13099 shall be read in conjunction with ISO 13099-1, which gives a
comprehensive overview of the theory.
1 Scope
This part of ISO 13099 specifies two methods of measurement of electrophoretic mobility of particles
suspended in a liquid: video microscopy and electrophoretic light-scattering. Estimation of surface charge
and determination of zeta-potential can be achieved from measured electrophoretic mobility using proper
theoretical models, which are described in detail in ISO 13099-1.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 13099-1, Colloidal systems — Methods for zeta-potential determination — Part 1: Electroacoustic and
electrokinetic phenomena
ISO Guide 30: Terms and definitions used in connection with reference materials
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
Brownian motion
random movement of particles suspended in a liquid cause by thermal movement of medium molecules
3.1.2
Doppler shift
change in frequency and wavelength of a wave for an observer moving relative to the source of the wave
3.1.3
electric surface potential
difference in electric potential between the surface and the bulk liquid
NOTE Electric surface potential is expressed in volts.
3.1.4
electrokinetic potential
zeta-potential
ζ-potential
ζ
difference in electric potential between that at the slipping plane and that of the bulk liquid
NOTE Electrokinetic potential is expressed in volts.
3.1.5
electroosmosis
motion of liquid through or past a charged surface, e.g. an immobilized set of particles, a porous plug, a
capillary or a membrane, in response to an applied electric field, which is the result of the force exerted by the
applied field on the countercharge ions in the liquid
3.1.6
electroosmotic velocity
υ
eo
uniform velocity of the liquid far from the charged interface
NOTE Electroosmotic velocity is expressed in metres per second.
3.1.7
electrophoretic mobility
µ
electrophoretic velocity per electric field strength
NOTE 1 Electrophoretic mobility is positive if the particles move toward lower potential (negative electrode) and
negative in the opposite case.
NOTE 2 Electrophoretic mobility is expressed in metres squared per volt second.
3.1.8
electrophoretic velocity
υ
e
particle velocity during electrophoresis
NOTE Electrophoretic velocity is expressed in metres per second.
3.1.9
slipping plane
shear plane
abstract plane in the vicinity of the liquid/solid interface where liquid starts to slide relative to the surface under
influence of a shear stress
3.2 Symbols
a particle radius
D diffusion coefficient
E electric field strength
k Boltzmann constant
B
I light intensity
N Avogadro’s number
A
n medium refractive index
R capillary radius
cap
2 © ISO 2012 – All rights reserved
S(ω) frequency power spectrum of scattering
Γ characteristic Lorentzian half peak width
ε medium permittivity
ζ electrokinetic potential (zeta-potential)
η medium viscosity
θ angle between incident light and scattered light
θ’ angle between two cross-beams
κ reciprocal Debye length
λ wavelength
µ electrophoretic mobility
µ electroosmotic mobility of liquid
eo
ν frequency
ξ angle between scattered light and electric field direction
τ delay time in autocorrelation function
ϕ volume fraction
ω rotational frequency (= 2πν)
4 Principles
A suspension of particles having a given electrokinetic charge is placed in a cell which has a pair of electrodes
placed some distance apart (Figure 1). This cell can be in the form of either a cylindrical or rectangular capillary
with electrodes at either end, or a pair of electrodes at a known fixed distance apart that are dipped into a
cuvette or other vessel. A potential is applied between the electrodes. Due to the process of electrophoresis,
particles carrying a net negative charge are drawn towards the electrode of opposite sign and vice versa. In
addition, if the capillary walls are charged, then an effect called electroosmosis causes the liquid to stream
along the capillary walls. The direction and velocity of this flow depends on the sign and magnitude of the wall
charge. The resulting velocity of the particle in the frame of references associated with the cell is superposition
of the electrophoretic velocity and the velocity of electroosmotic flow. Here it should be noted that the time
taken for the particle to reach the terminal electrophoretic velocity after the application of the electric field is
much shorter than the period of time needed to fully establish the electroosmosis flow throughout the whole
cell. This difference is exploited in some implementations. The velocity of the particles measured at a specific
position can be determined using either video microscope or electrophoretic light scattering through a laser
Doppler arrangement. Both the velocity and the direction of the moving particles in the frame of references
associated with the cell are determined. Provided that the distance between the electrodes is known together
with the applied electric potential, then the electrophoretic mobility can be established, from which a zeta-
potential can be calculated using established theories. Alternatively, calibration with particles having a known
zeta-potential can be used to eliminate the need to determine the unknown cell constant of a particular cell.
There are two distinctively different approaches to monitor particle motion in the electric field. Historically, the
first deals with particle images observed through a microscope. It is referred to as the “microscopic method”,
or alternatively as “microelectrophoresis”. The second relies on measuring light scattered by particles and
extracting information on electrophoretic mobility from the Doppler frequency shift of the scattered light.
This method is called the “electrophoretic light-scattering method”. For optical techniques, a cell constant for
many types of cells has to be determined, through either calculation or measurement of a solution of known
conductivity.
a
d
Key
d distance
a
Measurement zone.
Figure 1 — Schematic diagram of electrophoresis measurement
5 Microscopic methods
The main principles of these methods can be traced back over two centuries (Reference [1]) following the
development of microelectrophoresis. A light source illuminates particles migrating under the influence of a
d.c. or a.c. electric field. The illuminated particles can be observed due to scattering. This illumination can
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