LU und Exkursion Chemische Technologie CHE.170
Polymerisation of Emulsions
based on the PhD work of Sebastijan Kovacic
Florian Preishuber-Pflügl, Eva Pump and Christian Slugovc
1. Emulsions
Many situations in different area of food industry, petroleum production (drilling
fluids), pharmaceutics (mainly cosmetics), and environmental applications require
mixing oil and water. For that reason the production and use of stable emulsions had
been extensively examined. Emulsions are heterogeneous dispersions in which
drops of liquid are dispersed in continuous immiscible liquid phase of different
composition. The dispersed phase is sometimes referred to as internal phase and the
continuous phase as the external phase. In most cases one of the liquid is water,
while the other liquid is hydrocarbon. One of the most important characteristic of an
emulsion is its morphology (the spatial organization of matter in the emulsions). From
that point of view two types of emulsions can be distinguished in principle, depending
upon which liquid forms the continuous phase:
1
oil-in-water (O/W) for oil droplets dispersed in water
water-in-oil (W/O) for water droplets dispersed in oil
Also double emulsion can be prepared, oil-in-water-in-oil (O/W/O) or water-in-oil-in-
water (W/O/W). O/W/O denotes double emulsion containing oil droplets dispersed in
aqueous medium that are in turn dispersed in a continuous oil phase (or vice versa
for W/O/W type).
2
Emulsions with no aqueous phase, so-called anhydrous or oil-in-oil
emulsions can also be prepared where the selection of the phases depends largely
on the polarity of the solvents. Non-aqueous emulsions could replace regular
aqueous emulsions wherever the presence of water is undesirable; for example, in
cleaning systems that are sensitive to formation of rust and other mechanical
systems.
3
The type of emulsion that is preferred depends upon a number of factors including
temperature, electrolyte concentration, proportions of components, nature of
components and surfactant structure. The simple emulsion structures are droplets of
water, in case of water-in-oil (W/O) emulsions or droplets of oil, in case of oil-in-water
(O/W) emulsions. Interface between phases in both emulsion types lining the
stabilizer molecules which differ in chemical structure. Most meta-stable emulsions
that are used in practice contain oil, water and stabilizers which are usually
surfactants (relying on charge and attractive forces stabilisation) or macromolecules
(relying by steric stabilisation). The surfactant is needed to make emulsion easier to
form by creating protective film around droplets and keeping the emulsion from
1
Schramm, L. L. Emulsion, foams and suspensions: Fundamentals and applications.
Weinheim, Wiley-VCH, 2005.
2
Davis, S.S., Hadgraft, J., Palin K.J.: in P.Becher (ed): Medical and Pharmaceutical
application of emulsions. Encyclopaedia of Emulsion technology, Vol.2, Marcel Dekker, New
York, 1985.
3
Imhof, A., Pine, J., Stability of nonaqueous emulsions, J. Coll. Interf. Sci., 1997, 192, 368-
374.
breaking. The liquid in which the surfactant has a higher solubility tends to be the
continuous phase, meaning that W/O emulsions persists in the presence of oil
soluble surfactant and are rapidly destroyed in the presence of a water soluble
surfactant. Conversely, O/W emulsions persist in the presence of water soluble
surfactants. This is a model which predicts emulsion morphology based on surfactant
affini
ty to the continuous phase and is named Bancroft’s rule.
4
Assumption of
emulsion morphology by Bancroft’s model seems to be unlikely because it leaves us
with question: which properties of surfactant determine its affinity for particular phase.
The first who has quantified surfactant affinity for aqueous or oil phase of the
emulsion and gave answer to the preferred emulsion morphology was Griffin by
introducing the hydrophilic-lipophilic balance concept (HLB).
5
Drawback of Griffin’s
method is that contains only certain classes of surfactants where a direct link
between structure of the surfactant and its HLB can be found. Concept which obeys
emulsion as a whole system and interprets surfactant affinity to the both phases is
the phase inversion temperature (PIT) concept. The PIT concept was proposed by
Shinoda and Kunieda as a method for the classification of non-ionic surfactants and
to predict correlation between emulsion morphology and temperature.
6
While HLB is
a surfactant property, PIT is an emulsion property. The intermediate temperature, at
which surfactant affinity for both phases in the emulsion is equal, is the phase
inversion temperature. Deficiency of PIT, despite of its importance during preparation
of a stable emulsion, is its inapplicability to ionic surfactants.
Besides temperature and surfactant affinity, there are other variables that influence
surfactant distribution at the interface and therefore the emulsion morphology: the
salinity of the aqueous phase, the polarity of the oil phase, the HLB value of the
surfactant and the co-surfactant. Salager suggested merging all variables into one
framework which would yield preferred emulsions morphology.
7
All of these factors in
some way affect the interaction between surfactant molecule and each phase of the
emulsion and this interaction can be thermodynamically expressed as chemical
potential of the surfactant in the water or oil phase
µ
w(o)
= µ
w(o)
* + RT ln x
w(o)
γ
w(o)
Equation 1
where µ
i
is the chemical potential, µ
i
* is the standard chemical potential, x
i
the molar
fraction and γ
i
the activity coefficient of the surfactant in water phase (subscript w) or
oil phase (subscript o). With this equation it is easier to define surfactant affinity to the
individual emulsion phase where a low standard chemical potential corresponds to a
high affinity and vice versa.
8
As mentioned before, the simplest emulsion structures are droplets which are
isolated and spherical if emulsion solution is diluted while during concentrating, the
4
Bancroft, W.D., The theory of emulsiofication I, J.Phys.Chem. 16, 1912, 177-233.
5
(a) Griffin, W.C., Classification of Surface-active Agents by “HLB”, J. Soc. Cosmet. Chem.
1949, 1, 311-326; (b) Griffin, W.C., Calculation of HLB values of nonionic surfactants, J.
Soc. Cosmet. Chem. 1954, 5, 249-256.
6
Shinoda, K., Kunieda, H. in P. Becher, (ed.): Encyclopedia of Emulsion technology, Volume
1, Marcel Dekker, New York, chapter 5, 1983.
7
Salager J.L., Phase behavior of amphiphile-oil-water systems related to the butterfly
catastrophe, J. Coll. Interf. Sci., 1985, 105, 21-26.
8
Salager J.L. in P. Becher (ed): Encyclopedia of Emulsion technology, Volume 3. Marcel
Deckker, New York, chapter 3, 1988.
emulsion drops become packed as dense as possible and changing their spherical
shape into the rhombic dodecahedral shapes.
9
Figure 1. Rhombic dodecahedron shape
Emulsions exhibit classical “milky” look with which they are usually associated after
preparation. All kinds of appearances are possible, depending upon droplet size and
the difference in refractive index between phases. An emulsion can be transparent if
either the refractive index of both phases are the same, or if the dispersed phase is
made up of droplets that are sufficiently small compared to the wavelength of the
light. Emulsions that overcome emulsification treatments may be stable for days,
weeks or even months if prepared with appropriate combination of variables needed
for kinectic stability.
1.1 Surface active agents (surfactants)
Surfactants are amphiphilic molecules which exhibit a polar/apolar duality. A typical
amphipilic molecule contains polar group which contains hetero atoms such as O, S,
P or N included in different functional groups and on the other hand apolar group
which is usually hydrocarbon chain of alkyl or alkylbenzene type (Figure 2).
O S
O
O
O
Na
Sodium Dodecyl (ester) Sulfate
Na
Sodium Dodecyl Benzene Sulfonate
S
O
O
O
C
12
H
25
Figure 2. Amphiphilic molecules
For a surfactant to act as an emulsifier it must satisfy certain conditions. It has to
show good surface activity and produce a low interfacial tension at interface. This
means that it must have tendency to migrate into the interface rather than stay
dissolved in either one of the bulk phases. Next feature that must be considered prior
to selection is the ability to form as highly as possible condensed interfacial film,
either by itself or with other adsorbed molecules that are present, with lateral
interactions between molecules comprising the film. The most condensed interfacial
film between oil and water is formed when maximum number of surfactant molecules
is packed and are oriented vertically.
10
Surfactant concentration at a boundary
depends upon the surfactant structure and the chemistry of the two phases that meet
at the interface. Surfactant must migrate to the interface at such rate that interfacial
tension is reduced to a level of stability during the emulsion preparation. Rate of
migration depends on whether the surfactant is placed into the oil or water phase.
9
Neil C.R., Sherrington D.C., High internal phase emulsions (HIPEs)-structure, properties
and use in polymer preparation, Adv. in Polym. Sci., 1996, 124, 163-214.
10
Israelachvili J., The science and applications of emulsions - an overview, Coll. and Surf. A,
1994, 91, 1-8.
Surfactant simultaneously serves three functions in stabilizing an emulsion. They play
an essential role in determining whether oil in water is the continuous phase. They
control many of the attractive and repulsive forces between droplets that govern the
flocculation and coagulation. They determine the mechanical properties of the
interfacial films that control coalescence rates.
Two other general guidelines have to be taken into account during surfactant
selection. Surfactants that are preferentially oil soluble form W/O emulsions and
those which are water soluble form O/W emulsions. A mixture of oil soluble surfactant
and water soluble surfactant often produce better and more stable emulsions than an
individual surfactant.
11
A frequently used method for selection of appropriate surfactant is known as the HLB
(hydrophobic-lipophobic balance) method. It is a rule-of-thumb to predict a preferred
emulsion type produced by a certain surfactant. In this method, a number (0-40) is
related to the balance between the hydrophilic and lipophilic (hydrophobic) portions
of the molecule and has been assigned to many commercial surfactants. When a
combination of surfactant with different HLB values is used, the HLB number of the
mixture is calculated as the weight average of the individual HLB numbers:
13
HLB
mix
= ∑ x
i
·HLB Equation 2
Typical W/O surfactants have an HLB number in the range of 4-8, an O/W surfactant
lie in the range 12-16.
1.2 Stability of the emulsions
Emulsions are heterogeneous systems which will eventually phase separate into two
clear, homogeneous layers. Liquid/liquid immiscibility creates an interfacial tension at
the contact area between the two liquids, which disturbs the emulsion stability. As a
consequence emulsions are thermodynamically unstable systems which mean that
the increase of interfacial contact area is accompanied by the increase of chemical
potential which causes the free energy penalty (incre
asing of ΔG), meaning phase
separation. The free energy penalty of emulsion formation can be reduced through
the surfactants additions which sufficiently reduce the interfacial tension.
From thermodynamics it is know that spontaneous processes occur in the direction of
decreasing Gibbs free energy (
ΔG<0). In the case of emulsions the spontaneous
trend leads towards diminishing of interfacial area between the dispersed phase and
the dispersed medium. For the dispersed systems, such as emulsions, the degree of
kinetic stability is very important. Minimizing the interfacial area is reached mainly by
two mechanisms, coalescence and Ostwald ripening (the most studied
mechanism).
12,13
The former is the process by which larger droplets grow at the
expense of the higher solubility of smaller ones in the continuous phase. There is no
fusion of droplets involved. Ostwald ripening can be minimized by decreasing the
solubility of the dispersed phase in the continuous phase by decreasing the
interfacial tension between the two phases.
11
Atwood, D., Florence, A.T. Surfactant systems. New York: Chapman and Hall, 1985.
12
De Smet, Y., Deriemaeker, L., Finsy R., Ostwald
r
ipening of
a
lkane
e
mulsions in the
p
resence of
s
urfactant
m
icelles, Langmuir, 1999, 15, 6745-6754.
13
Bhakta, A.; Ruckenstein, E., Ostwald ripening: A stochastic approach, J. Chem. Phys.
1995, 103, 7120-7136.
Kinetic stability. Emulsion stability is a kinetic concept. A freshly prepared emulsion
changes its properties with time due to series of processes that occurs at the
microscopic level. Emulsion will always spontaneously change into a smaller number
of larger droplets. A complete characterisation of emulsion stability requires
consideration of the different processes through which dispersed droplets can
encounter each other. The four main ways in which an emulsion may become
unstable are creaming (or sedimentation), aggregation (flocculation), coalescence
and Ostwald ripening (Figure 3).
1
1
2
3
3
4
3
3
Figure 3. Degradation of emulsion: 1 phase separation (creaming, sedimentation); 2 Ostwald ripening;
3 aggregation processes (flocculation, coagulation, coalescence); 4 phase inversion
Coalescence is the formation of larger droplets by merging of smaller ones where
small droplets come into the contact by thinning and finally dissolution of the liquid
surfactant film that covers them. While aggregation is process where smaller droplets
clump together like a grape and form one bigger aggregate. The major difference
between aggregation and coalescence is that aggregation process does not form a
new bigger droplet. For that reason there is no reduction of the total surface area in
the case of aggregation process while in the case of coalescence process the total
surface area is reduced. The emulsion that is stable against coalescence and
aggregation is called kinetically stable emulsion.
In this case word “stable” describes the extent, to which smaller droplets can remain
uniformly distributed in emulsion. Furthermore can be kinetically unstable emulsions
termed as dynamically stable under agitation where droplets break up counteracts
coalescence. Emulsion instability can be also described by other processes which
encounter to the changing of emulsion properties. Creaming results from density
difference between the dispersed and continuous phase and produced two separate
layers. In other words creaming is the process by which emulsion droplets tend to
rise to the top of a container. The sedimentation is the same process as creaming,
but in the opposite direction. If one of the layers (upper or lower) will contain an
enhanced concentration of dispersed phase, this may promote aggregation. In
aggregation droplets retain their identity but lose their kinetic independence since
aggregate moves as a single unit. So, kinetic stability could thus have different
starting points. However, a system can be kinetically stable with respect, for example
to the coalescence, and is unstable with respect to the aggregation. Or, system could
be kinetically stable with respect to aggregation, but unstable with respect to
sedimentation
14
.
Emulsion stability is not necessarily a function of dispersed droplets size, although
there may be an optimum size for an individual emulsion type. Most emulsions
contain droplets which are polydisperse. Stirring and vigorous mixing tend to lead to
uncontrolled and wide droplet size distribution. Several methods for the preparation
of monodisperse emulsions exist, of which the simplest is extrusion of a dispersed
phase through a pipette into a continuous phase. The size distribution has an
important influence on the viscosity. The viscosity of all emulsions will tend to be
higher when the dispersed droplets sizes are relatively homogeneous.
1.3 Microemulsions, macroemulsions and high internal phase emulsions (HIPE)
From thermodynamical point of view we can distinguish between two emulsion
systems. Thermodynamically stable systems are known as microemulsions, while
thermodynamically unstable or metastable systems are known as macroemulsions,
where can also be counted high internal phase emulsions or HIPEs. These three
types of the emulsions differ in structural length scale; microemulsions involve
droplets in range between 5 and 100 nm while macroemulsions and high internal
phase emulsions in range between 100 nm and 10 µm.
Microemulsions are thermodynamically stable liquid dispersions of an oil phase or a
water phase, where dispersion phase is included in aggregates of surface active
agents the concentration of which is generally much higher compared with coarse
emulsions.
15,16
Microemulsions have nothing in common with coarse emulsions
except that both can be considered as liquid-liquid dispersions. Droplets involved in
microemulsions are in a size range of 5 nm - 100 nm, and has very low oil/water
interfacial tension. Because the droplets sizes are less than the wavelength of visible
light, microemulsions are transparent. Formation of microemulsions is readily and
sometimes spontaneously, generally without high energy input. Microemulsions tend
to decrease Gibbs free energy (
ΔG<0). Small droplet sizes make the specific area
larger which increase surface tension and increase Gibbs free energy which is in
agreement with equation,
dG = γ · dA Equation 3
where γ is surface tension and A is surface area. This excess free energy per unit
area that exists in the surface molecule is defined as the surface tension (γ). Surface
14
Tadros, T.F., Vincent, B.: Emulsion stability, in P. Becher (ed): Encyclopedia of Emulsion
technology, Volume 1. Marcel Decker, New York, 129-285, 1983.
15
Ruckenstein, E., The Thermodynamics of Microemulsions revisited, Langmuir 1994, 10,
1777-1779.
16
Ruckenstein, E., In Progress in Microemulsions; Martelluci, S., Chester, A. N., (Eds.),
Plenum Press, New York, p.3, 1989.
tension is a thermodynamic property and can be measured under constant T and p
and its value represents the amount of minimum work required per unit area to create
a greater surface area.
Furthermore, Young-Laplace equation predict an inverse relationship between
pressure and droplets radius which means the smaller the droplets radius the higher
the surface tension, which again consecutively increase Gibbs free energy by the
equation,
∆p = γ · (1/R
1
+ 1/R
2
) Equation 4
for spherical objects R
1
=R
2
= Rs, and
∆p = 2γ/Rs Equation 5
where ∆p is pressure difference inside and outside of the curvature drop’s surface, γ
is surface tension and R stands for droplet radii.
17
Arguments which are in
accordance with Young-Laplace predictions (eq. 3) and surface Gibbs free energy
(eq. 1), predict a coalescence of the droplets and for that reason there is no prove
about thermodynamically stable system.
However, microemulsion formation can be considered to depend on the extent to
which surfactant lowers the surface tension of the oil
–water interface or depend on
the change of entropy of the system by equation,
dG =
γdA - TdS Equation 6
where dG
is the Gibbs free energy of formation, γ is the surface tension of the oil–
water interface, dA is the change in interfacial area, dS is the change in entropy of
the system, and T is the temperature. When a microemulsion is formed the change in
dA is very large due to the large number of very small droplets formed. The work
done by lowering the interfacial tension is for that reason base
d on system’s entropy
(∆S) due to a creation of a large number of nano-sized droplets. Thus a negative free
energy of formation is achieved through large reductions in surface area which is
accompanied by significant favourable entropic change. Microemulsions form
spontaneously only when interfacial tension is small (in the order of 10
-3
mN/m).
18,19
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