Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5441_Библиотеки_им_академика_М_И_Перельмана.pdf
229
9.5 Stability
The term “emulsion stability” describes an emulsion’s capacity to withstand long-
term alterations to its physicochemical characteristics. Differences in the formula-
tion circumstances affect the properties even if the concentrations of the compositions
are identical. Additionally, an emulsion has various collapse processes, including
creaming, aggregation, Ostwald ripening, and coalescence. These processes are
dependent on the ingredients and preparation circumstances as well as their concen-
trations (Fig.9.3). Creaming and sedimentation are mostly due to the differences in
density between the two liquids, but the other phenomena are related to the charac-
teristics of the liquid-liquid interface. Flocculation is the development of droplet
aggregates. It is caused by the interaction of adsorbed layers on the surface of the
emulsion droplets. The coalescence is the process by which two droplets merge to
become one. Ostwald ripening is a mass transfer process from small to large drop-
lets which occurs due to partial breakdown of the dispersed liquid phase brought on
by capillary pressure. Though primarily determined by the solubility of the dis-
persed liquid and mostly dependent on droplet size, this phenomenon is also
impacted by the presence of surfactants and the rheological properties of the inter-
facial layers. The aforementioned destabilization mechanisms exhibit interdepen-
dence and have the potential to impact one another as the emulsion ages. For
instance, occulation and creaming tend to compact the droplet population, which
may speed up the occulation and coalescence processes. But they have no effect on
the droplet size distribution, which is determined by Ostwald ripening and coales-
cence. However, occulation and the rise in average droplet sizes brought on by
Ostwald ripening and coalescence serve to accelerate the process of creaming and,
ultimately, the emulsion destabilization (Huck-Iriart et al. 2016). The various
aspects of emulsion stability are discussed in the following sections.
Fig. 9.3 Various destabilization process in an emulsion
9 Liquid andPolydisperse Systems: Emulsions
230
9.5.1 Gravitational Separation
9.5.1.1 Creaming
Gravitational separation can be categorized into two mechanisms such as creaming
and sedimentation (Hu etal. 2017). The phenomenon known as “creaming” is one
of the collapse processes, in which the droplets of an emulsion are transferred to the
upper or lower regions due to the variations in density between the continuous and
dispersed phase. The creaming rate of an emulsion is affected by the viscosity and
droplet size of the dispersed phase and so on (Fig.9.4).
9.5.1.2 Sedimentation
Sedimentation is a process in which the dispersed phase’s droplets move down-
wards due to the higher density as compared to the continuous phase. Creaming and
sedimentation are reversible in nature as droplets can be redispersed with simple
shaking, but coalescence results into permanent damage of emulsion. The sedimen-
tation rate of an emulsion is regulated by Stoke’s law (Eq.9.1).
rg
s
cd
c
=
−
()
2
9
2
ρρ
η
(9.1)
where V
s
represents the terminal velocity of settling drop, g is the acceleration
due to gravity, ρ
d
and ρ
c
are individual densities of the dispersed phase and continu-
ous phase, respectively, η
c
is viscosity of continuous phase, and r is the droplet’s
radius, which is used to monitor its settling velocity. According to Stoke’s law, the
settling velocity of drops is directly proportional to the square of the radius of drop-
lets. The lower radius of emulsion droplets is resistant to destabilizing phenomena
such as sedimentation (Singh etal. 2017).
9.5.1.3 Flocculation
The stability of an emulsion against coalescence and occulation is especially
important for emulsions that are stabilized by macromolecules such as
Fig. 9.4 Schematic representation of gravitational separation
K. Shubham et al.
231
biomolecules, polymers, and, most notably, proteins (Huck-Iriart etal. 2016).
Proteins are often good O/W emulsion stabilizers because they may form dense,
impermeable adsorption layers at the oil drop surface. These interactions prevent
the oil droplets from coalescing by generating repulsive steric and electrostatic
interactions. Nonetheless, occulation is a relatively typical event for these sys-
tems, particularly when emulsions are stored for a long period of time with poten-
tial environmental disturbances, as in food applications (Ravera etal. 2021). The
occulation behaviour of emulsion stabilized by protein is often explained by
using theoretical descriptions of colloidal interactions (Delahaije et al. 2013).
Therefore, occulation might be considered as a form of aggregation. The occur-
rence of occulation frequently affects the structure, stability, and rheology of
O/W emulsions that contain food proteins (Dickinson 2019). Flocculation is inu-
enced by pH and concentration and is often seen above a critical concentration of
the stabilizing polymer (Huck-Iriart etal. 2016). This has been detected in the
emulsion gel stabilized by chitosan polymer. It has been observed that the degree
of droplet occulation in the emulsions with pH6.8 and 7.0 increases. The major-
ity of droplets were signicantly occulated but did not coalesce. The aforemen-
tioned observation implies that the emulsion structuring and gel network
strengthening at pH values higher than 6.5 are mostly due to the occulation of
aggregated chitosan-coated droplets and their sustained connection over extended
storage periods (Wang etal. 2020).
9.5.2 Non-gravitational Separation
9.5.2.1 Coalescence
The process of droplet coalescence, or fusion, in an emulsion is a crucial destabili-
zation mechanism that eventually causes increase in droplet size with time
(Langevin 2019). This phenomenon is caused by the thinning and rupture of the
liquid lm between them and is greatly inuenced by the behaviour of adsorbed
layers. Emulsions may undergo coalescence as a result of occulating droplets or
when the droplets come in contact and merge with each other. In an emulsion,
droplets’ movement is relative to each other due to Brownian motion, gravity, and
external mechanical forces such as centrifugation. Coalescence of droplets occurs
because of their collisions with each other during the movement. However, only a
fraction of all collisions may result into a coalescence event (Ho etal. 2024). There
are three stages of ow-induced coalescence of emulsied liquid droplets. In the
rst stage, the droplets approach under a continuous ow. The second stage
includes the collision of droplets resulting in the creation of a thin liquid lm,
which further thins due to uid drainage between the two droplets. The last stage
is the rupturing of thin lm that results in coalescence (Narayan etal. 2020). Within
a protein-stabilized emulsion, coalescence is a crucial stability parameter. This
phenomenon is inuenced by both pH and protein concentration in an emulsion. It
is generally accepted that stability against coalescence during emulsion formation
is largely dependent on the protein concentration. At low protein concentration, the
9 Liquid andPolydisperse Systems: Emulsions
232
coalescence takes place because of insufcient amounts of protein to completely
cover the interface. At high protein concentration, coalescence does not take place
because they completely cover the interface during the emulsication timescale
(Delahaije etal. 2017).
9.5.2.2 Droplet Aggregation
The droplet aggregation is typically triggered by several interactions between drop-
lets such as electrostatic interaction, hydrophobic interaction, etc. This results in an
increase in droplet size (Shiraki etal. 2020). This phenomenon affects the stability
of emulsions specially in nanoemulsions where the loss of all unique properties is at
a nanoscale level. This increased aggregation may eventually lead to phase separa-
tion, which damages the system irreversibly. An empirical method for understand-
ing the energetics underlying the aggregation process has been developed, utilizing
classical Newtonian mechanics (Eq.9.2). Overall interaction (I
E
) between droplets
can be characterized as the sum of electrostatic (I
el
), hydrophobic (I
h
), van der Waals
(I
VDW
), and steric (I
s
) interactions occurring in between droplets. However, there are
various other factors which affect the level of interactions.
I IIII
EelhVDW S
++ +
(9.2)
Meanwhile, developing a nanoemulsion, the interaction between droplets should
be minimized by calibrating all the parameters listed above and plotting a potential
energy curve against inter-droplet distance. The distances at which any two droplets
might approach one another without interacting are represented by the minimum in
this potential energy curve, which gives an approximate idea of the nanoemulsion’s
overall stability (Singh etal. 2017).
9.5.2.3 Ostwald Ripening
Ostwald ripening is also a major stability problem in emulsions. This is typically due
to the coarsening of small droplets into large droplets, i.e. disappearing of small drop-
lets which is energetically favourable (Zwicker etal. 2015). This process results in a net
mass transfer of small to large droplets. This is caused by the difference in the Laplace
pressure of the droplets, which is inversely proportional to their radius (Llamas etal.
2018). Ostwald ripening is more challenging to manage and can be seen in both macro-
and nanoemulsions, but it typically appears in those emulsions where water is continu-
ous phase (Koroleva and Yurtov 2021). The Ostwald ripening rates of emulsions are
determined by the characteristics of the dispersed oils (solubility, composition, etc.)
and their diffusion rates throughout the aqueous phase. The diffusion rate of oil mole-
cules across oil droplet surfaces is a signicant component in inuencing the Ostwald
ripening rates of vital oil emulsions (Park etal. 2020). The Kelvin equation illustrates
how varying droplet sizes can affect the solubility of dispersed uids (Eq.9.3).
c
rc e
V
rRT
()
=∞
()
2
γ
m
(9.3)
where, for O/W emulsion, c(r) denotes the water solubility of the oil (dispersed
phase) contained in a droplet of radius r, whereas c(∞) represents the
K. Shubham et al.
233
corresponding solubility at a planar interface. V
m
denotes molar volume of the dis-
persed phase, γ is the interfacial tension between the two liquids, R is the gas con-
stant, and T is the absolute temperature (Khedr and Striolo 2019). Based on the
above-mentioned equation, as the droplet radius falls, solubility increases; the uid
in the smaller droplets dissolves predominantly into the continuous phase and dif-
fuses into the bigger droplets. The overall size of the emulsion droplets grows as the
uid condenses on the larger droplet surfaces, ultimately resulting in a reduction of
total interfacial area (Khedr and Striolo 2019).
9.5.2.4 Phase Inversion
The process of inter-converting two forms of simple emulsions, W/O and O/W
emulsions, is known as emulsion phase inversion (Kumar etal. 2015). The industry
has utilized this method extensively to create a range of emulsions, and it has sig-
nicant interest due to its underlying mechanism (Chen etal. 2020b). Traditional
phase inversion, which occurs when the emulsier afnity shifts from one phase to
the other, can cause phase inversion. Traditional phase inversion is started by chang-
ing the way the emulsier and the oil-water combination system interact. For exam-
ple, temperature can alter the hydrophilic-lipophilic balance (HLB) of non-ionic
surfactants, which can result in emulsion inversion (Chen etal. 2020b). In contrast
to W/O emulsions, which form at higher temperatures, non-ionic surfactants are
more hydrophilic at low temperatures and generate O/W emulsions. In addition,
HLB varies in response to light irradiation, pH, and brine salinity. Catastrophic
phase inversion is caused by a shift in the emulsion’s water-to-oil ratio (Kumar etal.
2015). The coalescence of internal phases brought on by the presence of excess
internal phases leads to catastrophic phase inversion. Phase inversion of pickering
emulsions has also drawn a lot of interest. The primary component inuencing these
types of emulsions is particle wettability, which functions similarly to surfactant
HLB.In simple terms, particles that are hydrophilic tend to stabilize O/W emul-
sions, while those that are hydrophobic help W/O emulsions form. Therefore, by
altering the proportion of hydrophilic to hydrophobic particles, phase inversion can
be accomplished in a pickering emulsion (Chen etal. 2020b).
9.6 Evaluation
9.6.1 Macroscopic Evaluation
The macroscopic evaluation of an emulsion is performed through naked eyes or
under a microscope. One of the macroscopic evaluations is the determination of
creaming index. Creaming changes the qualities of an emulsion product over time
and temperature (Hong etal. 2018). The creaming index of an emulsion is deter-
mined by placing the system in a test tube. After a particular time, there is a separa-
tion of the system into an opaque cream layer and a transparent serum layer at the
bottom (Hosseini etal. 2015). The creaming index is then determined by the follow-
ing formula (Eq.9.4):
9 Liquid andPolydisperse Systems: Emulsions
234
Cr
eamingindex%
[]
=×100
(9.4)
where H
s
is the height of a serum layer and H
t
represents the total height of an
emulsion sample.
It has been observed that creaming index is directly proportional to the gum
concentration. Recently the creaming behaviour has also been evaluated using vari-
ous methods like ultraviolet-visible spectroscopy and multiphoton ionization time-
of-ight mass spectrometry (MPI-TOFMS). The MPI-TOFMS offers information
about the oil component concentration in an emulsion. The change in the oil com-
ponent caused by creaming can be evaluated by determining the mass signals.
Evaluation of creaming by spectrophotometer involves the measurement of degree
of turbidity. The UV-visible transmittance is measured across a wavelength (λ
max
)
from 200 to 800nm (Shinoda and Uchimura 2018).
9.6.2 Microscopic Evaluation
The most common method for the structural analysis of the emulsions involves
visualization under a microscope. As emulsions are multiphase systems, imaging
under a normal light microscope provides a clear understanding about the micro-
structure. The other techniques that have been used in the imaging of emulsions are
uorescence and the confocal microscopy. Advanced microscopic imaging using
electron beams such as transmission electron microscope (TEM) and scanning elec-
tron microscope (SEM) are also used to investigate the structure of the emulsion
gels. Fluorescence imaging is a simple and non-destructive method. The light
sources that have been used in uorescence microscope are the xenon arc lamp or a
mercury vapour lamp. The major principle underlying the imaging of the substances
in this technique is based on the processes occurring during the electronic transi-
tions. In this technique, the uorophores present in the samples absorb photons and
are excited to a higher energy state. When the electrons return to the ground state,
they emit another light in the visible region. This phenomenon can be well under-
stood by Jablonski diagram (Fig.9.5).
In situ uorescence imaging of miniemulsion revealed that the droplet size of
emulsion inuences the particle size and macroscopic reaction rate during miniemul-
sion polymerization (Liu etal. 2020). Confocal microscopy works on the same prin-
ciple as that of uorescence microscope. The difference in the two techniques lies
in the use of light source. The source of light in a confocal microscope is laser. Laser
of a particular wavelength is used for the imaging of the specimen. In this micros-
copy, the light which is not from the microscope’s focal plane is excluded during
imaging. This technique overcomes the difculty of uorescence and conventional
microscopy by creating more detailed sharp 3D image. These two methods help
identify the location of adsorbed molecule and encapsulated drug/microorganism
(Zhang et al. 2022). SEM and TEM are not quantitative methods but these
K. Shubham et al.
235
Fig. 9.5 Jabonski diagram showing the phenomenon of uorescence and phosphorescence
Table 9.2 Some basic techniques for the visualization of gel microstructure
Technique Advantages
Optical microscopy Direct method requires no sample preparation
Scanning electron microscopy (SEM) Sample preparation is easy
Transmission electron microscopy (TEM) Easier visualization of small crystal structure
Cryo-TEM Samples resistant to temperature
techniques are helpful in studying the microstructure morphology of the emulsion
droplets. These methods provide an insight about the superstructures of nano- or
microdimension formed in these systems (Tian etal. 2022).
Cryogenic-temperature transmission electron microscopy (cryo-TEM) is a
method that can be used for the micrometre to nanometre characterization of emul-
sion droplets. The high-resolution images obtained from this technique enable in-
depth analysis of structures. The basic building blocks of structures can be elucidated
by the direct images. However, a major limitation of cryo-TEM is that the sample
thickness should be approximately 300nm. Another disadvantage of this technique
is that it avoids the imaging of the strands in the gel network placed on the same
focal plane. Table9.2 shows some basic techniques and their advantages for visual-
ization of emulsions.
9 Liquid andPolydisperse Systems: Emulsions
236
9.6.3 Droplet Size Analysis
One of the most important factors affecting an emulsion’s stability and physico-
chemical qualities is droplet size. Typically, small-angle X-ray scattering (SAXS),
dynamic light scattering (DLS), electrical pulse counting, or ultrasonic spectrome-
try techniques are used to examine the size and dispersion of emulsion droplets
(Kale and Deore 2017; Hong etal. 2018). The smaller particles move more quickly
than larger ones and produce a greater rate of intensity uctuation. The DLS devices
measure the intensity uctuations of light scattered by the droplets of emulsions.
Droplet size from 3nm to 5μm, concentration, size distribution, and polydispersity
index (PDI) can all be computed with it. The uniformity of droplet size is described
by the PDI, which has values ranging from 0 to 1. A value of 0 indicates a system
that is entirely monodisperse (Gurpreet and Singh 2018). Higher emulsion stability
and a more uniform distribution of droplet sizes are indicated by a lower PDI value,
and vice versa. The droplet size changes greatly in emulsions where occulation
and coalescence take place, which raises the PDI.Similar to the DLS approach,
small-angle X-ray scattering (SAXS) techniques use a monochromatic X-ray beam
that is size-dependently scattered by droplets of emulsion in place of a light beam.
The detection of changes in the sample’s electrical conductivity brought on by the
droplets of emulsion moving between two electrodes is the foundation of electrical
pulse counting techniques. Electrical pulses are produced when droplets moving
between the electrodes alter the electrical current going through the emulsion
because oils have a far lower electrical conductivity than water. The assumption
used to calculate droplet size is that larger particles move more slowly and produce
larger electrical pulses. This method works well for measuring droplets with diam-
eters between 0.4 and 1200μm. Prior to any analysis, all emulsion samples must be
diluted (Hu etal. 2017). Dilution can disturb the collected droplets and can provide
erroneous interpretations of emulsion stability data. Although this approach works
well for droplet size analysis, it is not optimal for research in the occulation pro-
cess. The use of ultrasonic spectrometry provides a dilution-free way to determine
droplet size in emulsions with greater droplet concentrations up to 50% of the total
volume (Silva etal. 2022). This method calculates the size and concentration of
emulsion droplets by analysing the scattering of ultrasonic waves by the emulsion.
Droplet sizes from 10 nm to 1000 μm can be measured using ultrasonic
spectrometry.
9.6.4 Rheology andFlow Behaviour
Emulsions are a class of complex uid that behaves like both solids and liquids
(Dekker etal. 2022). Rheology is the study of science of deformation. These studies
give an idea about the ow behaviour and rigidity of the emulsion. It is crucial to
understand the ow behaviour of these systems from both an industrial and a basic
standpoint. An O/W emulsion exhibits Newtonian behavior for low volume frac-
tions (ϕ<0.1) of oil droplets. When the droplet interacts, the oil percentage rises
K. Shubham et al.
237
(ϕ>0.1) and the emulsions are non-Newtonian (Fuhrmann etal. 2022). Newtonian
ow is exhibited by low molecular weight substances such as organic and inorganic
liquids, salts, molten metal, and gases. In these types of systems, the shear stress (σ)
is proportional to the rate of shear (γ˙) at constant temperature and pressure. The
constant of proportionality is the dynamic viscosity (η). Multiphase systems such as
foams, emulsions, dispersions, suspensions, slurries, solutions (both natural and
synthetic), and polymeric melts do not follow the Newtonian postulate of the linear
relationship between (γ˙) and (σ); such materials are thus known as non-Newtonian
or rheologically complex systems. The non-Newtonian behaviour of the emulsions
is classied as plastic, pseudoplastic, and dilatant ow. Plastic systems are also
known as Bingham bodies. These systems exhibit a ow after a particular stress
value called the yield value. Emulsions exhibiting pseudoplastic ow are known as
shear thinning systems, that is, the apparent viscosity decreases as the shear rate
increases (Santos etal. 2017). Another type of ow behaviour is known as the dilat-
ancy. This behaviour is characterized by an increase in the viscosity with the shear
rate; some examples include sand/water mixtures, candy compounds, clay slurries,
corn starch in water, etc. Dilatancy is also referred to as shear thickening systems.
The major ow behaviour exhibited by the multiphase systems is the pseudoplastic
or plastic ow (Tao etal. 2020). The “power law” model can be used to calculate the
viscosities of emulsions (Eq.9.5):
= K
n
Ù
(9.5)
where σ is shear stress, ˙γ is shear rate, K is consistency index, and n is power law
index (Chalapud etal. 2018). A straight line obtained from the log-log shear rate/
shear-stress plot signies the shearing behaviour of the system. The above equation
gives an indication about the ow of the system. For Newtonian uids “n” and “K”
values are 1, for a pseudoplastic (shear thinning) system, n<1, and for dilatant or
shear thickening systems, n>1. The rheological analysis of an emulsion is carried
out using a rheometer (Anton Paar) equipped with a cone plate or a parallel plate
geometry.
9.6.5 Determination ofEmulsion Optical Properties
Transparency, opacity, turbidity, and colour are examples of optical properties of
emulsions that are dependent on the amount of light that is absorbed and scattered
by the sample (Rahn-Chique etal. 2012). The refractive index of an emulsion is
determined using refractive indexometers (Moghaddam etal. 2023). Refractive
index is dened as the product of the phase velocity of light in the evaluated medium
and the speed of light in a vacuum. An emulsion is deemed transparent if its refrac-
tive index is either the same as or nearly the same as that of water, i.e. 1.333
(Kupikowska-Stobba etal. 2024). Since droplet size, distribution, and concentration
have a signicant impact on how an emulsion appears, tracking the optical charac-
teristics of an emulsion over time might yield important insights into potential
9 Liquid andPolydisperse Systems: Emulsions
238
destabilizing processes in the system (Mirhosseini and Tan 2010). Any variation in
the droplet size distribution during the emulsion’s coalescence changes the overall
appearance of an emulsions. The coalescence process causes larger droplets to
develop, which makes the emulsion less clear, opaque, and turbid. Furthermore, the
merging of the droplets may result in an increase in colour intensity. Floc formation
or droplet aggregates, during occulation, result in increased light scattering and
decreased emulsion transparency. The emulsion appears hazy or unclear due to its
higher turbidity, which is a result of the increase in light scattering. The colour
intensity can also vary because the optical density or hue of ocs differs from that
of the individual droplets. Similar changes are also observed in Ostwald ripening.
Ostwald ripening causes changes in light scattering and absorption as well as a
reduction in transparency because the average droplet size in the emulsion rises. All
these optical changes can be detected quantitatively or visually by UV-vis spectro-
photometers, colorimeters, and refractometers (Kupikowska-Stobba etal. 2024).
Spectrophotometers detect the absorption and reection of light by emulsions over
a wide range of wavelengths. Colorimeters quantify parameters such as brightness
and saturation to determine the appearance of emulsions.
9.6.6 Determination ofZeta Potential
When ionizable or charged emulsier molecules are adsorbed to the surface of
emulsion droplets, the droplets become electrically charged. An electric double
layer is formed by two layers that encircle the droplets. The outer diffuse layer is
made up of loosely connected ions, and the inner layer is composed of strongly
bound ions. Ions inside the boundary follow the moving emulsion droplet, whereas
those outside the boundary stay in the bulk dispersion. The shear plane is the inter-
face between diffuse and stationary layers of counterions, and the zeta potential
represents the potential of this interface (Kamble etal. 2022). The factors that con-
tribute to the droplet charge include the type and quantity of molecules that have
been adsorbed. The composition of the continuous phase and its pH, particularly the
existence of other charged species like ions or macromolecules, also affect the
charge distribution. The zeta potential is the most frequently examined parameter
that describes the charge on the droplet surface. It is dened as the potential differ-
ence between the continuous phase and charged droplet surface. The zeta potential
is an important indicator of emulsion stability. A stronger repulsive electrostatic
interaction between the droplets hinders the droplet occulation and coalescence.
The zeta potential eventually improves the stability of the emulsion (Kasprzak etal.
2023). A stable emulsion is generally indicated by a high negative or positive zeta
potential of around ±30mV (Gurpreet and Singh 2018). The zeta potential of an
unstable emulsions is around zero (Pinto and Buss 2020). It is important to remem-
ber that molecular mass distributions in nanoemulsions are also affected by the zeta
potential; higher zeta potential nanoemulsions resist occulation and coagulation
and are electrically stabilized, exhibiting improved physical stability (Ravindran
etal. 2018). Zeta potential of an emulsion is calculated by a Zetasizer (Nano-ZSP,
K. Shubham et al.