ISO/TR 19997:2018
(Main)Guidelines for good practices in zeta-potential measurement
Guidelines for good practices in zeta-potential measurement
This document addresses the zeta-potential measurement operation for applications such as new product design, optimization of existing products, quality control during processing and/or during usage of the product. It does not provide a complete procedure for zeta-potential measurements. The instructions and key points addressed in this document are considered useful for performing zeta-potential measurements as specified in ISO 13099-1 and ISO 13099-2.
Lignes directrices relatives aux bonnes pratiques pour la mesure du potentiel zéta
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 19997
First edition
2018-09
Guidelines for good practices in zeta-
potential measurement
Lignes directrices relatives aux bonnes pratiques pour la mesure du
potentiel zéta
Reference number
ISO/TR 19997:2018(E)
©
ISO 2018
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ISO/TR 19997:2018(E)
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ISO/TR 19997:2018(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 1
5 Principle . 1
6 Sample preparation . 2
6.1 General . 2
6.2 Sampling and sample inspection . 2
6.3 Sample dilution procedures. 3
6.4 Sample stability test . 3
7 Measurement uncertainty and sources of error . 4
7.1 General . 4
7.2 Carryover contamination from previous samples . 4
7.3 Inappropriate sample preparation procedures . 4
7.4 Inappropriate samples for electrophoretic light scattering measurement . 5
7.5 Inappropriate liquid medium . 5
7.6 Incorrect entries of parameters by the operator . 5
7.7 Air bubbles . 6
7.8 Inappropriate theory for calculating zeta-potential from the measured
electrophoretic mobility . 6
7.9 Carbon dioxide . 6
7.10 Effect of the applied electric field on susceptible samples . 6
Annex A (informative) Zeta-potential measurement for particles in non-polar media .8
Bibliography . 9
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ISO/TR 19997:2018(E)
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
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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on the ISO list of patent declarations received (see www .iso .org/patents).
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 24, Particle characterization including
sieving, Subcommittee SC 4, Particle characterization.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
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ISO/TR 19997:2018(E)
Introduction
Zeta-potential is often used to investigate the isoelectric point (IEP) and surface adsorption for
particles in liquid media, and as an indicator in comparing different samples regarding electrostatic-
dependent dispersion stability. Zeta-potential is not a directly measurable quantity, but is established
using an appropriate theory. Furthermore, zeta-potential is not an intrinsic property of suspended
particles; it depends on both particle and medium properties, and how they interact at the interface.
Any variation in the liquid chemical and ionic composition affects this interfacial equilibrium and,
consequently, zeta-potential. Therefore, sample preparation and measurement procedures can both
affect the measurement result. Incorrect conclusions often result from artefacts in sample preparation
and issues arising from measurement procedures, or incorrect application of theoretical models for
calculating zeta-potential from measurement results.
This document provides general guidelines for sample preparation and measurement procedures for
the determination of zeta-potential by optically-based electrophoretic mobility or electroacoustic
methods.
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TECHNICAL REPORT ISO/TR 19997:2018(E)
Guidelines for good practices in zeta-potential
measurement
1 Scope
This document addresses the zeta-potential measurement operation for applications such as new
product design, optimization of existing products, quality control during processing and/or during
usage of the product. It does not provide a complete procedure for zeta-potential measurements. The
instructions and key points addressed in this document are considered useful for performing zeta-
potential measurements as specified in ISO 13099-1 and ISO 13099-2.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
4 Symbols
μ electrophoretic mobility
ε relative permittivity of the medium
m
ζ electrokinetic potential, zeta-potential
η medium viscosity
o
5 Principle
Zeta-potential (ζ) is the electric potential at a hypothetical shear plane that separates the mobile solvent
from solvent molecules that associate with the particle surface. Zeta-potential is frequently used to
predict the stability of a suspension or the adhesion of suspended particles onto macroscopic surfaces
(e.g. cellulose fibres, membranes). This is because interaction between particles or between particles
and surfaces or between particles and proteins is often governed by the ion distribution in the diffuse
layer, which is closely related to zeta-potential. Whenever electrostatic forces dominate interactions
between particles or between particles and surfaces, zeta-potential is the principal system parameter
[1]
to evaluate these interactions . Repulsion requires high surface charges of equal sign, whereas
attraction occurs in the absence of surface charge or for oppositely charged or “patchy” surfaces
containing both negative and positive domains. High zeta-potential absolute values cause strong
repulsion between dispersed particles and, thus, favour the stabilization of colloidal suspensions.
This effect is even more pronounced for thick double layers in low electrolyte content. In contrast, low
zeta-potential absolute values (+ or −), zeta-potentials of opposite sign (polarity), or high electrolyte
concentrations, can promote agglomeration. Hence, zeta-potential can be principally employed to
predict the suspension stability, which is frequently determined as a function of the pH and/or the
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ISO/TR 19997:2018(E)
concentration of indifferent electrolytes and surface active ionic species (e.g. ionic surfactants, multi-
[2][3]
valent ions and polyelectrolytes) .
Zeta-potential can be probed by imposing a relative motion between bulk solvent and particle. Zeta-
potential measurements on colloidal suspensions are frequently conducted via either electrophoresis
or electroacoustics for applications under different sample conditions (e.g. diameter range from
−4 [4]
nanometre up to tens of micrometres with particle volume fraction from roughly 10 % to 40 %) .
Zeta-potential is not an intrinsic particle property; it depends on the chemical equilibrium between the
particle surface and the liquid phase in which it is dispersed. Any variation of the liquid chemical and
[5][6][7]
ionic composition may affect this equilibrium and, consequently, affects zeta-potential .
The perception that there is a universally valid critical zeta-potential value that defines the transition
from unstable to stable suspensions has been proven only in limited applications. Zeta-potential
values need to be used carefully when evaluating the suspension stability. It is recommended that in
formulation work to predict stability, a second measurand (e.g. size distribution, turbidity, viscosity,
etc.) can be monitored and correlated to verify the conclusion derived from zeta-potential measurement.
Information on Zeta-potential measurement for particles in non-polar media can be found in Annex A.
6 Sample preparation
6.1 General
As the particle zeta-potential depends on particles as well as the dispersion medium, simple dilution can
change the chemical composition of the medium and then affect the particle zeta-potential. Therefore,
[8]
in addition to general practice in sample preparation for particle systems , special measures need
to be taken. Dilution can also induce dissolution, which alters both the surface and the medium. The
sample preparation needs to follow a procedure such that zeta-potential is not changed from the
original system to the diluted sample.
The sample preparation procedure requires that upon dilution not only do particles and their surfaces
remain identical between the original system and the diluted system, but also that the medium
remains electrochemically identical. Particle surface charge is wholly dependent upon the chemical
characteristics of the dispersion medium. Both the pH and specific ion concentration of the dispersing
medium are vital characteristics to be controlled if concentrated suspensions are to be diluted for
measurement. The conditions that particles undergo within a concentrated suspension need to be
entirely matched by the diluent.
This condition is not easy to satisfy if both dilution and surfactant stabilization of the sample are
involved. The sample preparation procedures can affect liquid composition tremendously. It is rather
difficult to adjust the particle concentration for electrokinetic measurements without impacting the
physico-chemical properties of the dispersion medium and the interface. For instance, dispersing
amorphous silica in KNO solution results in a suspension that will be different from the same material
3
in de-ionized water with respect to both pH and ionic strength. These differences have a considerable
impact on the interfacial properties, such as the zeta-potential.
6.2 Sampling and sample inspection
The electrophoretic mobility (μ) measured in a sample is only valid for a batch of material if the test
sample is representative of that batch and has been sampled adequately.
The material to be analysed should be inspected to ensure the particles have been dispersed adequately
without any sedimentation occurring upon standing for a period relevant to the measurement time. If
particles sediment during measurement, measurement using optical methods may not be appropriate
since particles remaining in the laser beam may not represent the whole sample (e.g. with polydisperse
samples, large particles will settle differentially resulting in a biased measurement of the smaller size
particles).
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ISO/TR 19997:2018(E)
Considerable care needs to be exercised during sample preparation to avoid changing the electrophoretic
[3]
mobility of the sample to be tested. Labware that comes into direct contact with the sample, such as a
glass beaker or syringe, may adsorb specific ions from the medium or add residual contaminants to the
sample that remain from a prior cleaning process or from production of the labware itself. Disposable
plastic preparation beakers and pipettes are generally preferred, as long as they are chemically
compatible with the sample.
A detailed report describing precisely how the sample was handled and how the diluent was prepared
is to accompany the result. Several complete dilutions and measurements of the sample can be made to
demonstrate that the method adopted is stable and reproducible.
6.3 Sample dilution procedures
In electroacoustic measurements, little or no sample preparation is typically required and instruments
convert the raw measurement data to zeta-potential using a theory and calibration procedure that
[9]
account for the finite particle concentration , as well as for the effect of particle size. In systems relying
on electrophoresis with optical detection, particle-particle interactions are minimized by diluting the
sample to an appropriate concentration. In this case, care needs to be taken that solvent shock or other
dilution effects do not alter the electrokinetic properties of the sample.
Sample dilution can follow the so-called equilibrium dilution approach, wherein the same liquid as that
in the original system is utilized as the diluent. When done properly, equilibrium dilution results in
a sample where the only parameter modified is the particle concentration. Only sample preparation
based on equilibrium dilution yields zeta-potential values that are theoretically identical between
the original system and the diluted sample. Simple dilution using deionized water, for example, is a
misleading and generally incorrect way to prepare samples for zeta-potential measurement.
There are two approaches to the collection of the liquid used for equilibrium dilution. The first consists
of extracting a supernatant using gravitational sedimentation or centrifugation. This supernatant or
“mother liquor” can then be used for diluting the initial sample to the degree that is optimal for the
chosen measurement technique. This method is suitable for large particles with sufficient density
contrast. It is not very convenient for nanoparticles and biological systems with low density contrast.
For emulsions, where a third phase (an emulsifier) stabilizes the normally immiscible oil and aqueous
phases, dilution into a matched ionic background is typical, due to the difficulty of using centrifugation
in this case. Ideally, this preserves the same ionic background in both the concentrated and more dilute
forms. This diluent can be obtained by knowledge of the ionic composition (ions, ionic surfactants) in
the dispersant phase. However, this will not account for species released by the particle phase itself.
A third approach, perhaps more suitable for nano- and bio-colloids is to employ dialysis. Dialysis
membranes are required that are permeable for ions and molecules, but not for colloidal particles, and
the process needs to be validated to avoid artefacts such as particle or surfactant loss to the membrane.
In some rare cases, there is a need to prepare samples at higher concentrations than the native material.
This can be achieved by initially separating particles from the medium and then re-dispersing them
into the same medium, but at a higher volume fraction. It may also be possible to gently centrifuge the
particles to obtain a more concentrated fraction, after removal of the supernatant phase. This process
needs to be optimized to mitigate particle loss or agglomeration effects.
Any medium utilized for dilution or for preparing samples is required to be initially free of particles (at
least to the extent that residual particles can impact the zeta-potential measurement). For relatively
“clean” media, one can use membrane filters (e.g. syringe filters) with a mean pore size smaller than the
smallest particles to be analysed. The hydrophobicity or chemical resistivity of the membrane should
also be considered. More complex media may prove more difficult to process. Centrifugation can also
be used to achieve this objective.
6.4 Sample stability test
It is advisable to conduct a series of measurements, sequenced in time, to demonstrate that the sample
[8]
is stable . For example, the disassociation of ionic species from the particles in suspension may result
in a change of pH or conductivity over time together with the mobility value. It is recommended that
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ISO/TR 19997:2018(E)
a determination of pH and/or conductivity of the suspension be conducted before and after each
measurement, if possible. Thi
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