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Journal of Colloid and Interface Science 333 (2009) 635–640
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Journal of Colloid and Interface Science
www.elsevier.com/locate/jcis
Variation in emulsion stabilization behavior of hybrid silicone polymers with
change in molecular structure: Phase diagram study
Somil C. Mehta a ,P.Somasundaran a,∗ ,RaviKulkarni b
a NSF Industry/University Cooperative Research Center for Advanced Studies in Novel Surfactants, Columbia University, New York, NY 10027, USA
b Elkay Chemicals Pvt Ltd, Pune, India
article info abstract
Article history:
Received 26 November 2008
Accepted 12 January 2009
Available online 18 January 2009
Keywords:
Silicone emulsion
Phase diagram
Hybrid silicone surfactants
Silicone copolyol
Ionic silicone
Stability
Silicone oils are widely used in cosmetics and personal care applications to improve softness and
condition skin and hair. Being insoluble in water and most hydrocarbons, a common mode of delivering
them is in the form of emulsions. Currently most applications use polyoxyethylene (non-ionic) modified
siloxanes as emulsifiers to stabilize silicone oil emulsions. However, ionically grafted silicone polymers
have not received much attention. Ionic silicones have significantly different properties than the nonionic
counterpart. Thus considerable potential exists to formulate emulsions of silicones with different
water/silicone oil ratios for novel applications. In order to understand the mechanisms underlying
the effects of hydrophilic modifications on the ability of hybrid silicone polymers to stabilize various
emulsions, this article focuses on the phase diagram studies for silicone emulsions.
The emulsifying ability of functional silicones was seen to depend on a number of factors including
hydrophilicity of the polymer, nature of the functional groups, the extent of modification, and the method
of emulsification. It was observed that the region of stable emulsion in a phase diagram expanded
with increase in shear rate. At a given shear rate, the region of stable emulsion and the nature of
emulsion (water-in-oil or oil-in-water) was observed to depend on hydrophilic–hydrophobic balance of
the hybrid silicone emulsifier. At a fixed amount of modification, the non-ionically modified silicone
stabilized an oil-in-water emulsion, whereas the ionic silicones stabilized inverse water-in-oil emulsions.
This was attributed to the greater hydrophilicity of the polyoxyethylene modified silicones than the ionic
counterparts. In general, it is postulated that with progressive increase in hydrophilicity of hybrid silicone
emulsifiers, their tendency to stabilize water-in-oil emulsion decreases with corresponding increase in
oil-in-water emulsion. Further, this behavior is hypothesized to depend on the nature of modifying
functional groups. Thus a hybrid silicone polymer can be tailored by selecting the nature and degree
of hydrophilicity to obtain a desired silicone emulsion.
© 2009 Elsevier Inc. All rights reserved.
1. Introduction
Silicone oils are used as emollients (skin softeners), lubricants,
thickeners, and volatile liquids in many applications including textile
treatment [1], personal [2] and home care [3], cosmetics [4],
drug delivery [5] and printing ink formulations [6]. Mostofthese
applications are water-based. Thus in order to make silicone oil
compatible with water, hybrid silicone emulsifiers are used [7]. The
choice of hybrid silicones is very important to achieve the most efficient
formulation of emulsions of silicones with water [8].
Silicone emulsions have historically been made using nonionically
modified silicone emulsifiers, also known as silicone
polyethers or silicone copolyols [7]. In most cases, the hydrophilic
moiety of choice has been polyoxyethylene (EO) or polyoxypropy-
*
Corresponding author. Fax: +1 212 854 8362.
E-mail address: ps24@columbia.edu (P. Somasundaran).
lene (PO) [9,10]. They have been studied in some detail for their
emulsifying properties to stabilize water/silicone oil [7,11–13] and
hydrocarbon oil/silicone [14] emulsions. Microstructures of emulsions
stabilized using non-ionic silicones have also be studied in
past [15,16].
In the case of ionic silicone surfactants, cationic [17,18] and
anionic [19] trisiloxanes have been found to behave as excellent
foaming agents [20,21]. There has been some work done on the
solution properties of cationic [22], anionic [23] and zwitterionic
[21] silicone surfactants. However, very little is known about the
emulsifying properties of these ionic compounds. As opposed to
non-ionic silicones, the area of liquid–liquid interfacial behavior
of ionic silicones has remained mostly unexplored. Ionic silicones
show very different properties than the non-ionic counterparts.
Thus ionic silicones are expected to demonstrate completely different
emulsion stabilization behavior, which might lead to the development
of novel applications. Thus a comparative phase diagram
study of hybrid silicones as a function of hydrophilic modification
0021-9797/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2009.01.028

636 S.C. Mehta et al. / Journal of Colloid and Interface Science 333 (2009) 635–640
was continuously stirred under low shear or high shear. At low
shear rate, the emulsification was achieved by imparting shear using
a magnetic stir bar (L = 10 mm, φ = 4mm,1500rpm).High
shear rate was achieved by homogenization using Ultraturrax T-18
homogenizer at 22000 rpm. The emulsification was achieved by
homogenizing for 2 min after the addition of water. The nature of
emulsion (oil-in-water or water-in-oil) was determined by conductivity
measurements. Emulsion stability was monitored by visual
observation of oil or water separation. The emulsion was considered
to be stable if there was no separation of oil or water after
allowing it to stand under gravity for 24 h.
2.2.2. Microscopy
The oil–water interface was observed using a Nikon Eclipse microscope
(Model ME 600) under transmitted light. Spot Insight CCD
camera (Model #4.2) was utilized to capture images of emulsions
to measure the dispersed phase droplet sizes [25].
3. Results and discussion
3.1. Nature of emulsion
Fig. 1. Structural representation of functionally modified silicone polymer. The grafting
ratio is denoted by m:n.
is undertaken in the present work. It is intended to establish the
effect of hydrophilic modifications on the ability of hybrid silicone
polymer to stabilize silicone emulsions.
2. Experimental
2.1. Materials
Various polymeric silicone surfactants with non-ionic, cationic,
anionic and amphoteric functional modifications were synthesized
at Elkay Chemical Pvt. Ltd. (Pune, India) and used for the current
study without further purification. Amino modified polymeric silicone
was synthesized from decamethyl cyclopentasiloxane (D5),
tetramethyl ammonium hydroxide, and amino siloxane via the
equilibration process. The cationic methylated amino silicone was
synthesized by mono-methylation of the amino modified polymeric
siloxane using p-tolyl methyl tosylate and the anionic silicone
polymer by reacting the amino modified silicone polymer
with itaconic acid [24], while the non-ionically modified silicones
were synthesized by hydrosilation of silanic hydrogen moiety with
poly(allyl alkoxylate). In addition, in some cases 50% of the amino
groups in aminosilicone were modified to obtain 50% acid and
methylated amino silicones. The ratio of the reactants was selected
to give m:n ratio of approximately 1:7.5 (Fig. 1, where m = a + b).
In a separate series, acid modified silicones with m:n ratio of
1:4.5 and 1:99 were also synthesized. The approximate molecular
weight of all the compounds as measured from viscosity was
5000 and a volatile content below 1%. The details on the synthesis
of each polymer has been reported earlier [23]. The general structures
of the above mentioned silicone surfactants are illustrated in
Fig. 1.
Decamethyl cyclopentasiloxane was purchased from Aldrich Co.
and used without any purification as silicone oil. Triple distilled
water was used as the aqueous phase.
2.2. Methods
2.2.1. Emulsion preparation
Emulsions were prepared by first dissolving the functionalized
polymeric silicone surfactants in cyclic silicone oil followed by
dropwise addition of water. No pH adjustment was made in water,
and thus all the emulsions were prepared at pH ∼6. The mixture
Unlike the non-ionic silicone surfactants, most of their ionic
counterparts stabilized an inverse water-in-oil emulsion. The nonionic
silicone surfactant has hydrophilic ethylene oxide chains (10
EO units), which make them hydrophilic and water soluble. On
the other hand, under identical percentage modification, the ionic
silicone surfactants are relatively less hydrophilic due to shorter
graft chains, and are more compatible with the cyclic silicone oil.
Emulsions were prepared at pH 6, which leads to partial ionization
of ionic functional groups. This also will play a role in making
ionic silicone polymers less soluble in water than non-ionic silicone.
Due to these properties, the emulsions formed with ionic
silicone emulsifiers were water-in-oil emulsions. It is noteworthy
that the inverse emulsion stabilized by one of the acid modified
silicone co-polymer with low shear rate was stable up to as high
as 80% of water as the inner phase. Due to very high concentration
of dispersed water droplets these emulsion appeared very creamy.
The more soluble non-ionic silicone surfactant formed an oilin-water
emulsion. These emulsions could not reach as high a
dispersed phase concentration as the emulsions stabilized by the
ionics and therefore appeared like milk. The maximum amount of
oil that could be stabilized as dispersed phase in water continuum
with non-ionic silicones was about 20%.
3.2. Phase diagrams
Phase diagrams were measured under low and high shear in
order to understand the effect of method of emulsification on the
stabilization behavior of emulsions.
3.2.1. Low shear rate
Under low shear conditions, amino silicone and methylated
amino silicone were unable to stabilize any emulsions. However,
the acid modified silicone co-polymer was observed to give a stable
emulsion in a very narrow region of 80% water and 20% oil
at most emulsifier concentrations. The phase diagram for 1% acid
modified silicone is presented in Fig. 2. Below 80% water an excess
oil layer was observed at the top of the formulation and
above 80% water, excess water drops were observed at the bottom.
Such behavior could be better understood by correlating them with
Winsor emulsions [26]. Below 80% water content, the emulsions
behaved similar to Winsor I, and above 80% water the emulsions
behaved like Winsor II.
Droplet size of the dispersed phase of the emulsions changed
with the change in emulsion formulation. Though the emulsions

S.C. Mehta et al. / Journal of Colloid and Interface Science 333 (2009) 635–640 637
Fig. 2. Phase diagram of silicone oil–water emulsion stabilized by 1% acid modified silicone polymeric emulsifier at low shear rate. (") Stableemulsion,(2) emulsion+ excess
water, (!) emulsion+ excess oil.
were turbid and macro-sized in the entire concentration range of
components, the size of water droplets (as observed under microscope)
decreased with increase in the amount of emulsifier.
Emulsion was stable for more than a day with as low as 0.2%
emulsifier (80% water and 20% silicone oil) with the droplet size of
dispersed phase at about 250 μm. There were several oil pools in
the emulsions as observed in Fig. 2 due to inefficient mixing. With
increase in emulsifier concentration, the droplet size decreased to
about 35 μm at 10% emulsifier concentration.
3.2.2. High shear rate
The phase diagrams of emulsions stabilized with various functional
silicone surfactants with 11.7% modification measured at
high shear rates are presented in Fig. 3. Itshouldbenotedthat
out of 11.7%, about 6% constituted amino group and rest 6% modification
was various functional groups (e.g. acid modified polymer
is a ter-polymer of PDMS, amino silicone and acid modified silicone).
It can be seen that the shape and size of the stable phase
region change with the nature of the functional group modification.
Compared to the other functional modifications, the emulsion
made with the acid modified silicone was relatively more stable
and more viscous. This is reflected by the wider region of stable
emulsion in the phase diagram for acid modification as compared
to the others. The maximum amount of water that could be incorporated
in this water-in-oil (W/O) emulsions was about 50%.
Above 50%, though a stable emulsion could be obtained, the period
of stability was less than 24 h. Emulsions were obtained even
with as low as 0.2% surfactant concentration, but they remained
stable for maximum of 4 h and hence were not considered stable
for the phase diagram study.
Amino silicones, though showed a comparable wide region of
stable emulsion as acid modified silicones, the emulsions formed
with it was observed to be relatively less stable than the emulsions
of acid modified silicone ter-polymer. Most of the emulsions
destabilized within a week, whereas some of the emulsions made
with acid modified silicones were stable up to 3 months (the phase

638 S.C. Mehta et al. / Journal of Colloid and Interface Science 333 (2009) 635–640
Fig. 3. Phase diagram of silicone oil–water emulsions stabilized by 11.7% functional silicone polymeric emulsifiers at high shear rate. (") Stable emulsion, (!) phase separation,
( ) turbid solution, ( ) water-in-oil emulsion, ( ) oil-in-water emulsion.
diagrams drawn here represent stability for 24 h and thus this behavior
is not captured here).
The emulsifying ability of methylated amino silicone terpolymer
was found to be negligible and no region of stable emulsion
was observed. This in part can also be due to bulky tosylate
counter ion. Effect of counter ion on phase stability is not considered
in the present work.
On the other hand, the EO modified silicone is very hydrophilic,
and therefore is more compatible with water rather than the oil. It
stabilizes an oil-in-water emulsion, especially in the low oil content
region. The emulsions formed were translucent with the particle
size of the dispersed phase in miniemulsion range, i.e. about
100 to 400 nm [27]. Slight haze in the emulsions indicated that
the drop size has not reached the level of a microemulsion (less
than 100 nm).
Amino group is the least hydrophilic among the selected functionalities,
leading to better solubility in silicone oil than others.
But due to the relatively low polarity of amino groups, amino silicones
do not have good surfactant properties. Consequently amino
silicones stabilize emulsions but only for a relatively short time.
On methylation, the nitrogen atom of amino silicone acquires a
positive charge, because of which the hydrophilicity of the polymer
increases and the polymer becomes less compatible with the
silicone oil (hydrophilicity will also be affected by tosylate counter
ion). Though this increase in the hydrophilicity should increase the
compatibility with water; it was seen that 11.7% modification was
not enough to make the methylated silicone water soluble. Consequently,
the emulsifying capability of methylated amino silicone
polymer ter-polymer was significantly low. The hydrophilicity of
acid modified silicone ter-polymer is higher than that of the amino
silicone, yet the degree of modification of the tested acid modified
silicone, was such that it retained its solubility in the oil phase.
Therefore, because of the presence of strong hydrophilic group,
acid modified silicone possessed higher water stabilizing capacity
than the amino silicone. The acid modified silicones are thus
better emulsifiers than the amino and methylated amino silicone.
The non-ionically modified silicone co-polymer is very hydrophilic
at similar grafting ratio because of the presence of long ethylene
oxide side chains. As a result, the non-ionic silicone is water soluble
and oil incompatible, and stabilizes oil-in-water emulsion. In
general, it can be stated that with increase in hydrophilicity, the
tendency to stabilize water-in-oil emulsion decreases and the oilin-water
emulsion increases.
The above hypothesis was tested by constructing the phase diagrams
of acid modified silicones with different degree of acid
modifications. The phase diagrams of 1%, 11% and 18% acid modified
silicone co-polymer emulsifiers are presented in Fig. 4. Again
it can be seen that as the percentage acid modification is increased,
the hydrophilicity of the emulsifier increases, making it
less compatible with the silicone oil. As a result, increase in hydrophilicity
of the acid modified silicone leads to shrinking and
eventual disappearance of the stable emulsion region. Based on
these observations, it is postulated that with progressive increase
in hydrophilicity of hybrid silicone emulsifier, its tendency to stabilize
water-in-oil emulsion decreases and oil-in-water emulsion
increases. Further, this tendency to switch from water-in-oil emul-

S.C. Mehta et al. / Journal of Colloid and Interface Science 333 (2009) 635–640 639
Fig. 4. Phase diagram of silicone oil–water emulsions stabilized by various % modification of acid functional silicone polymeric emulsifiers at high shear rate. (") Stable
emulsion, (!) phase separation, (
) turbid solution.
sion stabilizer to oil-in-water emulsion stabilizer depends on the
nature of modifying functional group. The given EO modified silicone
(11.7% modification) stabilizes oil-in-water emulsion, whereas
the acid modified silicone at 11.7% modification stabilizes water-inoil
emulsion.
When Figs. 2 and 4(a) are compared for the phase diagrams
of 1% acid modified silicone co-polymer, it can be seen that the
increased shear rate helps to broaden the stable emulsion region.
This can be explained as due to better spreading of the emulsifying
polymer into the oil–water interfaces with higher shear rate, which
is consistent with the mechanism of emulsion stabilization [29].
Another interesting observation can be made by comparing the
phase diagram of 11% acid modified ter-polymer in Fig. 4(b) with
the phase diagram of 11.7% acid modified co-polymer in Fig. 3(c).
Though the % modifications are similar, the region of stable emulsion
in Fig. 4(b) is much smaller than that for the region of stable
emulsion in Fig. 3(c). This can be due to the presence of 50% amine
groups in the 11.7% modified silicone of Fig. 3(c). The presence
of amino groups changes the solubilization and consequently the
emulsification properties of the emulsifier. There is also a possibility
for weak non-bonding interactions among the acid and amine
groups of the same (intramolecular) or different (intermolecular)
polymer chains which will aid the formation and stabilization of
emulsions.
3.3. Stability of emulsions
Stability of emulsions is an important parameter that is indicative
of the mechanism of emulsion stabilization. The stability of
emulsions can be monitored by measuring the time required for
separation of the two phases. Since it will take a long time for sep-
Table 1
Time required for phase separation of an emulsion made with 1% emulsifier, 70%
water and 30% cyclic silicone oil.
Compounds % modification X-component
of 3D-HLB
PDMS 0 0 0
Amino silicone 11.70% 0.46 9
Acid modified silicone 5.8% + 5.8% amino 1.91 55
Methylated amino silicone 5.8% + 5.8% amino 2.76 0
1% acid modified silicone 1% 0.44 9
Time
(min)
aration of the two phases of a stable emulsion under free settling
conditions, a method commonly used in the industry to assess the
stability of emulsions is by monitoring the phase separation under
centrifugation. Emulsions with 70% water, 30% oil and 1% emulsifier
were made with emulsifiers as indicated in Fig. 3 and were
discretely centrifuged at 1000 rpm for increasing durations until
phase separation was observed. The times required for phase separation
of various emulsions are presented in Table 1.
In order to efficiently formulate an emulsion, it is useful to understand
dependence of emulsion stability on the emulsifier structure.
Toward this purpose, an attempt was made to correlate the
time required for emulsion destabilization with the 3D-HLB concept
[28]. 3D-HLB concept is based on the representation of emulsion
type on a ternary diagram made up of hydrophilic, lipophilic
and silophilic moieties of the molecule represented by 3 coordinates.
In the selected emulsifiers, there is a regular variation in the
functional group (or hydrophilicity) with the remaining molecule
maintained constant. Since only the functional group is changing,
the X-component of 3D HLB, which represents the hydrophilicity
of the silicone emulsifier, was correlated with the emulsifier per-

640 S.C. Mehta et al. / Journal of Colloid and Interface Science 333 (2009) 635–640
emulsion stabilities quantitatively, the stability of emulsions was
correlated with the structure of the emulsifier using the 3D-HLB
concept. It was observed that the emulsion stability at a centrifugation
speed of 1000 rpm (∼100g) follow a linear profile
with the hydrophilic component of 3D-HLB concept, but the correlation
brakes at higher hydrophilicity when the phase inverses
from water-in-oil to oil-in-water. The observed phenomenon can
be used to predict and adapt the emulsion stabilization performance
of hybrid silicone by selecting the nature and degree of
hydrophilic modification.
Acknowledgments
Fig. 5. Correlation between the stability of emulsions under centrifugation at
1000 rpm with the X-component of 3D HLB.
formance. Fig. 5 shows a plot of the stability of the emulsions as a
function of the hydrophilicity of the silicone emulsifier. A linear relation
can be observed between the stability of emulsions and the
hydrophilicity of the emulsifier. However the trend breaks when
the emulsifier becomes incompatible with the cyclic silicone oil.
Thus, it is clear, that the ability to stabilize water-in-oil and oil-inwater
emulsion can be tailored by adjusting the hydrophilicity of
the hybrid silicone polymers. This behavior, except for the phase
inversion behavior, can be predicted with 3D-HLB model.
4. Summary
In order to effectively formulate silicones oil/water emulsions,
it is vital to have an accurate knowledge of the mechanisms by
which modified silicone polymeric emulsifiers act and their required
properties. Phase diagrams studies of silicone oil and water
were undertaken in this work to understand the effect of functional
modifications on the emulsifying ability of hybrid silicone
polymers. It was observed that the emulsion stability was dependent
on the functional modification as well as method of emulsification.
Under low shear rate conditions, the tested acid modified
silicone co-polymer stabilized an inverse water-in-silicone oil
emulsioninaverynarrowwindowofaround80%waterand20%
oil. The aminosilicone and quaternary amino silicone co-polymers
were unable to stabilize the emulsion. With increase in shear rate,
the region of stable emulsion expanded. The amino and acid modified
silicone ter-polymer stabilized inverse emulsions, whereas the
quaternary amino silicone ter-polymer was unable to stabilize an
emulsion under tested conditions. Conversely, the tested EO modified
silicone stabilized an oil-in-water emulsion with up to 20%
water as the dispersed phase. The observed behavior of emulsion
stabilization is explained based on the hydrophilic–hydrophobic
balance of the hybrid silicone polymers. It is postulated that with
increase in hydrophilicity of hybrid silicones, its tendency to stabilize
water-in-oil emulsion decreases and oil-in-water emulsion
increases. Further, this tendency for switching from water-in-oil
emulsifier to oil-in-water emulsifier depends on the nature and
extent of functional modification. In order to be able to predict
Theauthorswouldliketoacknowledgethesupportfromthe
National Science Foundation (Grant #0328614) and industry sponsors
of the Industry/University Center for Novel Surfactants at
Columbia University.
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