ISO/TR 13097:2013
(Main)Guidelines for the characterization of dispersion stability
Guidelines for the characterization of dispersion stability
ISO/TR 13097:2013 addresses the stability characterization of liquid dispersions (suspensions, emulsions, foams and mixtures thereof) for applications, such as new product design, optimization of existing products, quality control during processing and during usage of the product. The stability of a dispersion in the sense of ISO/TR 13097:2013 is defined in terms of the change in one or more physical properties over a given time period.
Lignes directrices pour la caractérisation de la stabilité des dispersions
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 13097
First edition
2013-06-15
Guidelines for the characterization of
dispersion stability
Lignes directrices pour la caractérisation de la stabilité des dispersions
Reference number
ISO/TR 13097:2013(E)
©
ISO 2013
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ISO/TR 13097:2013(E)
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ISO/TR 13097:2013(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Terms and definitions . 1
3 Basics of stability . 3
3.1 Stability — Summary . 3
3.2 Characteristic features with regard to dispersion stability . 4
3.3 Alteration of the state of a dispersion . 4
4 Characterizing the change of the state of a dispersion . 6
4.1 General comments . 6
4.2 Direct methods . 7
4.3 Correlative methods . 8
4.4 Procedures to accelerate the evaluation of long-term stability . 8
5 Prediction of the shelf life of a dispersion .10
5.1 General comments .10
5.2 Comparative analysis .10
5.3 Predictive analysis .10
Annex A (informative) A compilation of relevant international and national standards .12
Bibliography .14
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ISO/TR 13097:2013(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
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.
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. www.iso.org/directives
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received. www.iso.org/patents
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
The committee responsible for this document is ISO/TC 24, Particle characterization including sieving,
Subcommittee SC 4, Particle characterization.
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ISO/TR 13097:2013(E)
Introduction
Stability with respect to changes in relevant product specifications and product performance is
important in industry and for end users.
Various terminologies are used to reflect different phenomena as well as different user perspectives.
In the literature and in practice, one frequently finds terms such as dispersion, suspension or emulsion
stability, demixing or separation stability, sedimentation or creaming stability, physical stability,
colloidal stability, and kinetic stability.
This Technical Report focuses on instability driven by thermodynamics and does not include phenomena
1)
that are due too, e.g., radiation, chemical or enzymatic reactions or are related to the growth/metabolism
of biological organisms like bacteria. These phenomena are often described as photo, UV or irradiation
stability, thermal or chemical stability of one or the other constituent, enzymatic or microbial stability, etc.
The Technical Report concerns general aspects of stability test methods, acceleration procedures and
data evaluation. In addition, recommendations of instrument manufacturer, information from the
scientific or user community as well as from regulatory bodies are intended to be taken into account.
1) Chemical and physical properties are often interrelated.
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TECHNICAL REPORT ISO/TR 13097:2013(E)
Guidelines for the characterization of dispersion stability
1 Scope
This Technical Report addresses the stability characterization of liquid dispersions (suspensions,
emulsions, foams and mixtures thereof) for applications, such as new product design, optimization
of existing products, quality control during processing and during usage of the product. The stability
of a dispersion in the sense of this Technical Report is defined in terms of the change in one or more
physical properties over a given time period. Stability can be either monitored (determined) in real time
or predicted on the basis of physical quantities related to stability. In the case of very stable dispersions,
procedures that accelerate the changes under consideration or accelerated aging tests administered
over a shorter time scale can be appropriate. Shelf life can be estimated based on the observed rate of
the change in the physical property and the user-required specifications for the product. Guidelines
are given for choosing relevant measurements that can be used for the ranking, identification and
quantification of instability.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
agglomeration
assembly of particles in a dispersed system into loosely coherent structures that are held together by
weak physical interactions
Note 1 to entry: Agglomeration is a reversible process.
Note 2 to entry: Synonymous with coagulation and flocculation.
1 2
[SOURCE: ISO 14887:2000, 3.1, modified — text altered; IUPAC Gold Book, modified]
2.2
aggregation
assembly of particles into rigidly joined structures
Note 1 to entry: Aggregation is an irreversible process.
Note 2 to entry: The forces holding an aggregate together are strong, for example covalent bonds or those resulting
from sintering or complex physical entanglement.
Note 3 to entry: In common use, the terms aggregation and agglomeration are often applied interchangeably.
1 3
[SOURCE: ISO 14887:2000, 3.2, modified — text has been altered; ISO 26824 ]
2.3
coalescence
disappearance of the boundary between two particles (usually droplets or bubbles) in contact, or between
one of these and a bulk phase followed by changes of shape leading to a reduction of the total surface area
Note 1 to entry: The flocculation of an emulsion, namely the formation of aggregates, may be followed by coalescence.
2
[SOURCE: IUPAC Gold Book ]
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2.4
creaming
rise (separation) of the dispersed phase in an emulsion due to the lower density of the dispersed phase
(droplets) compared to the continuous phase
Note 1 to entry: Creaming velocity has a negative sign as particle movement is opposite to the acting force.
2.5
dispersion
in general, microscopic multi-phase system in which discontinuities of any state (solid, liquid or gas:
discontinuous phase) are dispersed in a continuous phase of a different composition or state
Note 1 to entry: If solid particles are dispersed in a liquid, the dispersion is referred to as a suspension. If the
dispersion consists of two or more liquid phases, it is termed an emulsion. A suspoemulsion consists of both solid
and liquid phases dispersed in a continuous liquid phase.
4 2
[SOURCE: Hackley et al. ; IUPAC Gold Book, modified]
2.6
dispersion stability
ability to resist change or variation in the initial properties (state) of a dispersion over time, in other
words, the quality of a dispersion in being free from alterations over a given time scale
Note 1 to entry: In this context, for instance agglomeration or creaming represents a loss of dispersion stability.
2
[SOURCE: IUPAC Gold Book ]
2.7
flocculation
assembly of particles in a dispersed system into loosely coherent structures that are held together by
weak physical interactions
Note 1 to entry: The term flocculation is used frequently to denote agglomeration facilitated by the addition of a
flocculating agent (e.g. a polyelectrolyte).
Note 2 to entry: See 2.1.
2.8
flotation
migration of a dispersed solid phase to the top of a liquid continuous phase, when the effective particle
density is lower relative to the continuous phase density
Note 1 to entry: It may be facilitated by adhering gas bubbles, for example dissolved air flotation, or the application
of lipophilic surfactants (e.g. in ore processing).
2.9
particle
minute piece of matter with defined physical boundaries
Note 1 to entry: A physical boundary may also be described as an interface.
Note 2 to entry: A particle may move as a unit.
5
[SOURCE: ISO 14644-5:2004, 3.1.7, modified — Note 1 is different and Note 2 has been added;
6
ISO/TS 27687:2008, modified — Notes 1 and 2 have been altered and Note 3 has been deleted.]
2.10
Ostwald ripening
dissolution of small particles and the redeposition of the dissolved species on the surfaces of larger particles
Note 1 to entry: The process occurs because smaller particles have a higher surface energy, hence higher total
Gibbs energy, than larger particles, giving rise to an apparent higher solubility.
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2
[SOURCE: IUPAC Gold Book ]
2.11
phase inversion
phenomenon whereby the phases of a liquid-liquid dispersion (emulsion) interchange such that the
dispersed phase spontaneously inverts to become the continuous phase and vice versa, under conditions
determined by the system properties, volume ratio and energy input
7
[SOURCE: Yeo et al. ]
2.12
phase separation
process by which a macroscopically homogeneous suspension, emulsion or foam separates into two or
more new phases
7
[SOURCE: Yeo et al. ]
2.13
sedimentation
settling (separation) of the dispersed phase due to the higher density of the dispersed particles compared
to the continuous phase. The accumulation of the dispersed phase at the bottom of the container is
evidence that sedimentation has taken place
Note 1 to entry: In the case of a dispersed liquid (emulsion), droplets can sediment if their density is higher than
that of the continuous liquid phase (e.g. water in oil emulsion).
2
[SOURCE: IUPAC Gold Book ]
2.14
shelf life
recommended time period during which a product (dispersion) can be stored, throughout which the
defined quality of a specified property of the product remains acceptable under expected (or specified)
conditions of distribution, storage, display and usage
8
[SOURCE: Gyeszly ]
3 Basics of stability
3.1 Stability — Summary
Stability is the capacity of a dispersion to remain unchanged with respect to predefined stability criteria
over a given time under stated or reasonably expected conditions of storage and use. It depends therefore
on the application. For instance a cosmetic emulsion may be considered stable if no oil phase formation
is observed during a period of three years. On the other hand, natural fruit juice can exhibit pulp settling
without any reduction in quality. There is no universal method or technique to quantify all stability
aspects due to the complexity of stability related phenomena. Therefore, it is always necessary to define:
a) stability metrics: properties of the state or behaviour of a dispersion which should be monitored
according to the demanded specific product qualities.
b) stability criteria: deviations from the initial properties at production date, which are acceptable.
Shelf life is defined in terms of the alteration of stability metrics. In general, faster alteration leads to
shorter shelf life.
In order to meet the predefined stability criteria of very stable products, analytical techniques having
high resolution/sensitivity need to be used and procedures can be required in order to accelerate the
alteration. However, because of the interrelated physical, physico-chemical and chemical properties of
a liquid dispersion, adequate acceleration methods should be chosen and validated in the context of a
specific product.
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3.2 Characteristic features with regard to dispersion stability
[9]
Generally speaking, dispersions are thermodynamically unstable. However, for a dispersed system,
the rate of change in its state may be acceptably low and therefore it exhibits kinetic stability. Kinetic
stability may be improved by electrostatic, steric or electrosteric stabilization, or particle coating, as
well as by pickering or rheological additives to the continuous phase.
The state of dispersion stability depends upon numerous interrelated physical, physico-chemical and
chemical parameters, and its nature is therefore complex. The parameters may be categorized as follows:
a) volume or mass concentration of dispersed phase (e.g. spatial homogeneity, diluted or concentrated);
b) state of the continuous phase (e.g. density, viscosity, surface tension, chemical potential, quality of
solvent);
c) state of the dispersed phase (e.g. size, shape and density distribution, as well as viscosity of droplets,
deformability of particles, structure of particulate surface);
d) interaction between particles/droplets (e.g. electrostatic and van der Waals force, steric and
depletion force);
e) interaction between dispersed and continuous phase (e.g. wettability, interfacial tension, surface
and volume rheology, solubility, dissolvability, network formation).
The volume concentration of the dispersed phase of a dispersion is one of the primary requirements of
any product design and it should be homogenous within the entire product during the entire life span. In
general, the higher the volume concentration, the higher the physical stability (e.g. less phase separation).
Formulators have to achieve product specifications and sufficient dispersion stability as demanded
by the application or customer. This is accomplished by choosing the state of the dispersed phase
(e.g. size distribution, shape, density match, restrictions to oversize, surface charge and coating)
and the appropriate behaviour of the continuous phase. Traditionally, electrostatic stabilization has
been principally used. Today, polymeric additives are commonly employed to tailor properties of the
continuous phase of innovative products. Two essential aspects with regard to dispersion stability are
particle-particle interactions and interactions between the dispersed and continuous phase. Tuning of
particle interactions is an important tool to stabilize a dispersion. Electrostatic, steric and depletion
stabilization or combinations of these are the most commonly used approaches. The theoretical
foundation of these approaches is based on the classic DLVO (Derjaguin, Landau, Verwey, Overbeek)
[10] [11]
theory (see Overbeek ) and more recently, the extended DLVO theory. In general, any specific
interaction energy between two particles (e.g. double layer interaction, van der Waals attraction, steric
interaction) is calculated as a function of the particle distance. The dependence on the distance is
interaction specific. The different interaction energies are additive and the resulting energy-distance
[11]
curve allows for stabilization evaluation. It should be emphasized that products today (e.g. paints,
nutritional suspoemulsions, cosmetic multiple emulsions) often consist of several dispersed phases, and
that the continuous phase may contain many constituents.
This complex structure of dispersions implies that a single parameter is generally insufficient to
characterize or predict the stability state of a dispersion.
3.3 Alteration of the state of a dispersion
Figure 1 and Figure 2 schematically display a selection of primary and secondary mechanisms,
respectively, which, over time, change the state of the dispersed phase and/or homogeneity of the
dispersion. They are indicators of loss of stability. Additionally, aged dispersions may undergo phase
separation that is obvious by visual observation. Destabilization mechanisms are sequenced simply for
the sake of clarity and cannot be distinguished in most practical cases.
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ISO/TR 13097:2013(E)
AB
CD
EF
Key
A creaming, flotation D flocculation, agglomeration
B sedimentation E Ostwald ripening
C coalescence F phase inversion
[12][13]
Modified from source.
Figure 1 — Primary destabilization phenomena of the state of liquid-solid or liquid-liquid
dispersions
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CD E
Key
C coalescence
D flocculation, agglomeration
E Ostwald ripening
In case C, coalescence may progress to build up a continuous liquid phase again on top of the creaming layer for
oil-in-water emulsion.
Figure 2 — Secondary alteration phenomena due to destabilization of a dispersion
There are three main categories that provoke an alteration of the state of a dispersion over time.
a) Mechanical-driven processes: phase separation, i.e. creaming or flotation (see key item A of Figure 1)
and sedimentation (see key item B of Figure 1) are driven by gravity, but other forces, such as
mechanical, electric or magnetic can also give rise to concentration inhomogeneity within the product.
b) Thermal-driven phenomena: Ostwald ripening (see key item E of Figure 1) and phase inversion (see
key item F of Figure 1) are purely diffusion or thermally driven processes. The extent depends on
the given physico-chemical properties of the two phases.
c) Interaction-force-driven processes: in case of coalescence (see key item C of Figure 1), flocculation,
agglomeration and aggregation (see key item D of Figure 1), the particles need to collide with each
other or at least come within range of a mutual attractive force. For such processes, thermal and/or
mechanical energy are necessary to initiate destabilization. Network formation or gelation may
take place at particle volume concentrations above some threshold levels.
Stability analysis consists of monitoring, over an appropriate time, the alteration of the dispersion
state (e.g. loss of homogeneity via sedimentation, creaming, particle size alteration, flocculation or
coalescence). These are the stability metrics. On the other hand, stability criteria define the acceptable
deviations in the metrics (e.g. sediment height, cream layer thickness, shift of particle size distribution,
flocculation/agglomeration rate, coalescence rate, and appearance of oversized particles or the extent
of developed new phases).
4 Characterizing the change of the state of a dispersion
4.1 General comments
In practice, after determining the stability metric(s), it is necessary to select an appropriate test method.
It is recommended, if possible, to select a method that does not require sample preparation, so that the
sample is analysed in its original state. The state of a dispersion is complex, and any sample preparation
can alter the native state.
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4.2 Direct methods
4.2.1 Visual observation
This approach has been applied for centuries to examine the time behaviour of a dispersion and to
estimate its shelf life. The dispersion is placed in a test-tube, a test-bottle or into the container in which
the product is delivered to the end-user and placed into storage. Alterations are visually observed
at appropriate time intervals of days, weeks or months. Qualitative results are reported as “yes/no”
alterations or “more/less” than a preset threshold or reference sample. Visual tests are simple and low
cost, but require substantial storage space; they are time consuming, subjective (result depends on
operator) and not traceable (unless linked to photographic images). It should be mentioned that often
not only the stability in the sense of this Technical Report, but also the general appearance of a stored
product is evaluated, and that some regulatory bodies demand such procedures be performed in the
final containment vessel.
4.2.2 Instrumental methods
Results of instrumental methods are, in contrast to visual observations, objective and traceable. They
also exhibit higher sensitivity, reproducibility and data storage. Appropriate measuring techniques have
to be chosen in accordance with stability metrics according to the application. Scanning and spatially
resolving techniques are appropriate in particular to detect phase separation. In addition, position
resolved data help to discriminate between phase separation and phase changes. Different techniques
are available, which are sensitive to alterations in the state or behaviour of the dispersion.
a) Optical measuring principles are designed to monitor the changes of the state of dispersion by
recording transmission and/or back scattering intensities. They are applicable to dispersions such
as emulsions, suspensions and foams. Recorded intensities depend on instrumental design (e.g.
[14]
wavelength, optical path length (see ISO 10934-1:2002 ) and the properties of the sample (e.g.
volume concentration, particle size, shape, refractive index). A wide range of volume concentrations
may be analysed by choosing the analysis method with appropriate optical path for low concentration
transmission or back scattering with long path length and for high concentration, back scattering
or transmission with short path length. If near-infrared sources are used, in most cases measured
intensities do not depend on the optical properties (absorbance) of the continuous or dispersed
phase. Instruments are available using transmission and/or backscattering sensors combined with
spatial resolution from a few micrometres up to tens of micrometres, and measuring intervals from
seconds to days.
b) X-ray transmission methods including industrial computed tomography are tools for high volume
concentration, but they are practically restricted to dispersed phases of atomic numbers >12
(carbon). Transmitted X-ray intensity depends only on the mass concentration and not on the particle
size itself, as in case of light waves. Spatial resolution and measuring intervals are comparable to
optical methods.
c) Acoustic and electroacoustic spectroscopies (i.e. ultrasonic methods) offer another penetrating
wave approach. Similar to light, attenuation of acoustic waves or in some cases changes of sound
velocity are analysed to characterize the state of dispersion. Generally, these methods require
somewhat higher volume concentrations (e.g. of order a few percent volume fraction). Quantitative
analysis requires appropriate equations and material specific parameters. Scanning instruments
are used because the current spatial resolution of sensors is limited.
d) Measurements of electrical properties, such as conductivity or permittivity, can be used to
characterize inhomogeneity (e.g. particle concentration) within the dispersion.
Direct methods operate in real time and require minutes to months to identify nascent destabilization
phenomena. Nevertheless, sensitivity and accuracy of instruments provides the capacity to detect
alterations in the state of samples far earlier than visual observations. The techniques described above
do not require any sample preparation to measure the kinetics of dispersion state alterations. These
methods can be used for measuring shelf life.
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4.3 Correlative methods
Correlative methods focus on determination of a single physical parameter of the state of a dispersion
that is known to correlate with stability of the dispersion. For example one of the following parameters
may be measured and compared with pre-defined acceptable values:
a) density differences;
b) mean particle size;
c) particle size distribution;
d) electrophoretic mobility, zeta potential;
e) concentration of particles/droplets larger than a stated size value;
f) rheological parameters.
NOTE Various standard methods have been developed for each of these measurements, and the details can
vary according to industry, type of material and application. It is up to the user to determine which of the available
standards is most relevant and to adhere to that standard when making the measurement.
The advantage of this approach is that the evaluation may be performed immediately after the
formulation of a new dispersion or after processing the product. A representative sample is needed, and
the measurement technique may require sample preparation, which can alter the state of dispersion.
Care should be taken to validate such procedures.
Correlative measurements provide quantitative values corresponding to one physical measurand. It
characterizes the corresponding property at the time of the measurement and does not yield kinetic
information. Due to the complexity of the state of dispersion (cf. 4.2), at the time of publication of this
Technical Report, no theory exists to calculate or predict the time course of the dispersion state alteration
or even predict shelf life based on a single physical parameter obtained at any single point in time.
4.4 Procedures to accelerate the evaluation of long-term stability
4.4.1 Purpose
Shorter evaluation time in research and development (R&D) and pre-shipping quality control (QC) is
a challenge for highly stable dispersions (e.g. cosmetics, dispersions for construction, agrochemicals).
Although instrument manufacturers have increased the sensitivity to detect minor alterations of the
dispersion state, the acceleration of the naturally occurring, thermodynamically bas
...
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