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Accepted Manuscript

Title: Rheological and Interfacial Properties of Silicone OilEmulsions Prepared by Polymer Pre-adsorbed onto Silica

Particles

Authors: Noriaki Sugita, Masami Kawaguchi

PII: S0927-7757(08)00428-7

DOI: doi:10.1016/j.colsurfa.2008.06.044

Reference: COLSUA 15402

To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: 10-9-2007

Revised date: 17-6-2008

Accepted date: 17-6-2008

Please cite this article as: N. Sugita, M. Kawaguchi, Rheological and Interfacial

Properties of Silicone Oil Emulsions Prepared by Polymer Pre-adsorbed onto Silica

Particles, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2007),

doi:10.1016/j.colsurfa.2008.06.044

This is a PDF file of an unedited manuscript that has been accepted for publication.

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http://dx.doi.org/doi:10.1016/j.colsurfa.2008.06.044http://dx.doi.org/10.1016/j.colsurfa.2008.06.044http://dx.doi.org/10.1016/j.colsurfa.2008.06.044http://dx.doi.org/doi:10.1016/j.colsurfa.2008.06.044


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Abstract

Emulsions stabilized by colloidal particles, namely Pickering emulsions were

prepared by mixing silicone oil with silica particles pre-adsorbed hydroxypropyl methyl

cellulose (HPMC) in the continuous water phase as functions of added amount of HPMC

and silicone oil viscosity. Characteristics of the resulting oil dispersed in water (O/W)

emulsions were determined by the measurements of adsorbed amounts of the silica

particles, oil droplet size, and some rheological responses, such as hysteresis loop,

stress-strain sweep curve, and dynamic viscoelastic moduli. These results were

compared with those prepared by silica particles without PHIC or PHIC. The

adsorbed amounts of the silica particles pre-adsorbed HPMC were increased with an

increase in the amount of added HPMC. However, no adsorption of the silica particles

without pre-adsorbed HPMC occurred. The size of oil droplets prepared by the silica

suspensions pre-adsorbed HPMC decreased with an increase in the adsorbed amount of

HPMC and it increased with increasing the viscosity of the silicone oil at the fixed

amount of adsorbed HPMC. The emulsions prepared by evey emulsifier showed thattheir stress-strain sweep curves were satisfied with Hookes law at the smaller

deformation, whereas at the larger deformation they showed thixotropic behavior,

irrespective of the silicone oil. An increase in the viscosity of the silicone oil gives the

larger difference between the up and down curves at lower shear rates for the hysteresis

loops. Moreover, dynamic viscoelastic moduli measurements showed that storage

moduli of the emulsions were increased by one order of magnitude by adsorption of

HPMC, where the elastic responses was controlled by the silica suspensions pre-adsorbed

HPMC at the interface.

Keywords: Pickering emulsions; Silica particles pre-adsorbed hydroxypropyl methyl

cellulose; Silicone oil; Rheological properties; Interfacial properties


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1. Introduction

Preparation of Pickering emulsions [1] has been performed by using various particles,

such as carbon [2, 3] silica [4, 5], clay [5], latex [5, 6], and layered double hydroxides [7].

Sometimes the system contains both particle and amphiphilic molecule [8-12], and

pre-adsorbed polymer [13-19] onto various particles. Moreover, some interesting

reviews concerning with Pickering emulsions have recently reported [20-24]. The

amphiphilic molecule could modify wettability of the particles and thus influence the

type and stability of the prepared emulsions. Advances have been made in developing

Pickering emulsions prepared by polymer-grafted particles. Some pH-responsive

Pickering emulsions were prepared with polystyrene latex particles that were sterically

stabilized by block copolymers and statistical copolymer and with lightly cross-linked

poly(4-vinylpyridine)-silica microgel particles [19]. Furthermore, highly charged

polyelectrolyte-grafted silica particles were used to prepare Pickering emulsions and they

were highly efficient emulsifiers and were able to prepare Pickering emulsions as little asapproximately 0.04 wt% [18].

On the other hand, Midmore found that highly stable paraffin oil emulsions were able

to be formed by silica particles that had been flocculated by adsorption of hydroxypropyl

cellulose in water: neither silica nor polymer was an emulsifier for the corresponding

paraffin oil by itself [14]. Midmore subsequently found that the formation of oil

dispersed in water emulsions prepared by silica and polyoxyethylene surfactants was

caused by the synergy between them, namely, 1) flocculation of the silica particles, 2)

rendering the silica particle partially wettable, 3) decreasing of the interfacial tension [15].

However, such synergy effects have not been quantitatively estimated.

Our recent preliminary work on preparation of emulsion by mixing silicone oil and

fumed hydrophilic silica particles dispersed in water showed that silicone oil droplets are

emulsified by the silica particles dispersed in the continuous water phase surrounding the

oil droplets. An increase in the silica concentration decreased the oil droplet size and


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increased the amount of oil emulsified. The resulting emulsions showed thixotropic

behavior.

Here we report on emulsifying characteristics of the corresponding fumed

hydrophilic silica particles modified with pre-adsorption of hydroxypropyl methyl

cellulose (HPMC). When HPMC was adsorbed on surfaces of the fumed silica

particles, flocculation of the silica particles occurred, they were gradually precipitated at

their concentrations lower than 2.5 wt%, and beyond the 2.5 wt% silica concentration a

gel-like silica suspension was formed [25, 26]. In this study, since the silica

concentration is fixed at 1.5 wt %, silica suspensions are flocculated by adsorption of

HPMC. HPMC also played a role in an emulsifier of silicone oil and the interfacial

and rheological properties of the resulting silicone oil emulsions were investigated as

functions of oil viscosity and molecular weight of HPMC [27-29]. The present work

is specifically focused on the interfacial and rheological properties of silicone oil

emulsions prepared by the fumed silica suspensions containing different adsorbed

amounts of HPMC in terms of the quantitative estimation of the synergy effects, such asan amount of the silica particles pre-adsorbed HPMC and a decrease in the interfacial

tension, in comparison with those of the corresponding silicone oil emulsions prepared

by the silica particles without HPMC or HPMC.

2. Experimental Section

2.1. Samples

Four silicone oils were kindly supplied by Shin-Etsu Chemical Co. Ltd. and their

viscosities of KFL96-1, KF96-10, KF96-100, and KF96-1000 are 1, 10, 100, and 1000

cSt at 25oC, respectively.

Aerosil 130 silica powder supplied from Nippon Aerosil Co. was treated as described

previously before use [30]. From the manufacturer of the Aerosil 130, the primary


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silica has an average diameter of 16 nm, a surface area of 130 m2/g, and a silanol density

of 2.0/nm2, but in air the silica particles tend to form aggregates due to the hydrogen

bonding between the silanol groups.

An HPMC sample obtained from Shin-Etsu Chemical Co. Ltd. was purified by the

same method as previously reported [26-29]. The molecular weight of HPMC was

determined to be 38.8 104

and its molecular weight distribution was 2.47. The

degrees of the substitution of methoxy and hydroxypropoxyl groups were measured to be

1.8 and 0.25, respectively [27].

Water was purified by a Milli-Q Academic A10 ultra-pure water system.

2.2. Preparation of Emulsions

The respective silicone oils of 15 g were mixed with 0.45 g silica dispersed in 30 g

water to prepare silicone oil emulsions in a 100 mL glass bottle and agitated for 30 min

under 8000 rpm at 25

o

C, using a Yamato Ultra Disperser with an S-25N-25F agitationshaft.

Silica suspensions pre-adsorbed HPMC were prepared as follows: 30 g water

dissolved 0.015, 0.030, and 0.05 g HPMC, which is less than the overlapping

concentration of HPMC, 0.172 g/100 mL, where HPMC chains in water start to contact

each other, in a 50 mL glass bottle were mixed with 0.45 g Aerosil silica powder at 25 oC

for 24 hr, where the added amounts of HPMC should almost adsorb on the silica surfaces

according to the previous our study [26]; the resulting silica suspensions were

sedimented using a Kubota 6500 centrifuge, the separated silica suspensions were three

times rinsed with water, and then the resulting separated silica suspensions were

re-dispersed in water to maintain at the same silica concentration as 0.45 g silica

dispersed in 30 g water; and the re-dispersed silica suspensions are named the silica

suspensions pre-adsorbed HPMC as follows. Since the silica suspensions pre-adsorbed

HPMC were flocculated as mentioned above, they were agitated to well disperse in water


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at ca. 500 rpm by a Tokyo-Rikaki CM1000 mixer until they are used for emulsifiers, and

their pH was 5.5 [26].

To prepare an emulsion stabilized by the silica suspensions pre-adsorbed HPMC, they

were mixed with 15 g the KFL96-1 silicone oil by the same method as described above.

To understand the effects of oil viscosity on the formation of emulsion, 15 g other

silicone oils of KF96-10, KF96-100, and KF96-1000 were also mixed with the silica

suspensions pre-adsorbed HPMC, i.e., an adsorbed amount of 0.03 g HPMC.

Moreover, the respective silicone oils of 15 g were mixed with 0.015, 0.030, and 0.050 g

HPMC dissolved in 30 g water to emulsify silicone oil by HPMC. The resulting

emulsions were kept at 25oC in an incubator after preparation to separate into two or

three phases. The code of 1-45-1.5 was designed for an emulsion prepared by mixing

of 1 cSt silicon oil, 0.45 g silica, and 0.015 g HPMC. The applied shear rate in the

preparation of emulsions was calculated to be approximately 2200 s-1

from the diameters

of the shaft and bottle and the speed of 8000 rmp.

2.3. Interfacial tension measurements

The values of interfacial tension of the KFL96-1 silicone oil against water, aqueous

solutions prepared by dissolution of 0.015, 0.030, and 0.050 g HPMC into 30 g water, the

silica suspensions pre-adsorbed HPMC dispersed in 30 g water, and the silica suspension

dispersed in 30 g water were measured using a Du No y tensiometer at 25oC.

2.4. Measurements of adsorbed amounts of emulsifiers

To determine quantitatively the adsorbed amounts of the emulsifiers, such as HPMC,

the silica particles, and the silica particles pre-adsorbed HPMC at the interfaces between

water and the silicone oil of the emulsified phase for the elapsed time of one week after

preparation of the corresponding emulsions, 5 mL of the bottom phase parts were


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extracted, evaporation of water was carried out by heating and the residue was weighed

after drying in vacuum.

This gravimetric analysis gives the concentrations of the respective emulsifiers that

are suspended in the continuous part of the emulsion phase. In order to determine their

actual adsorbed amounts, the calculated amounts are subtracted from the initially added

amounts of the emulsifiers. The gravimetric analysis for the adsorbed amounts of the

respective emulsifiers was performed at least twice and the experimental errors were less

than 5 %. From the sensitivity of a Mettler AT250 electronic balance used, this

method allows us to determine the lowest concentration of 2 10-6 g/mL.

2.5. Optical microscopy measurements

Optical microscopic observation of the emulsified phase as a function of the elapsed

time after preparation was carried out using an Olympus STM5-UM light microscope to

estimate their droplets and changes in the appearances of the emulsions after the additionof water or silicone oil. An aliquot of the emulsified phase was placed in the hollow of

a depth of 0.5 mm in the center of a slide glass and covered with a cover glass.

Furthermore, optical microscopic observation was performed using a Thermo Haake

Rheo Scope 1 with the cone-plate geometry (diameter, 70 mm; cone angle, 1o), which is

designed by the concept of rheo-optics consisting of microscopic and rheological

techniques, with and without shear flow [29].

2.6. Rheological measurements

Measurements of hysteresis loop, stress-strain (S-S) sweep, and dynamic viscoelastic

moduli of the emulsified phase for the elapsed time of one week after preparation were

carried out at 25oC using the same Rheo Scope 1 with the same cone-plate geometry as

optical microscopic measurements. The hysteresis loop measurements were


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performed by increasing shear rate from 0 to 300 s-1

and by decreasing it from 300 to 0

s-1 for 1 min, respectively, and the S-S sweep curves were done when shear stresses were

applied from 0.1 to 100 Pa. Moreover, the dynamic viscoelastic modulus

measurements in the linear responses were performed at the angular frequency of 0.1 to

100 rad/s. Respective measurements were repeated at least three times and their

experimental errors were within 10 %.

3. Results and Discussion

3.1. Appearances of emulsions

Most emulsified mixtures prepared by using the silica suspensions pre-adsorbed

HPMC or HPMC as an emulsifier separated into an upper emulsified phase and a lower

aqueous phase after preparation, except for the 1-45-1.5, 1-45-3.0, and 1-45-5.0

emulsions, which separated into three phases: an upper silicone oil phase, a middleemulsified phase, and a bottom silica aqueous suspension phase. Moreover, the 1-45-0

and 10-45-0 emulsions prepared by the silica suspensions also separated into three phases.

The relative amounts rel of oil emulsified for all emulsions are summarized in Table 1.

The emulsified phases for the respective emulsified mixtures were collected before the

measurements.

On the other hand, little emulsions were obtained by mixing the KF96-100 and

KF96-1000 silicone oils and the silica suspensions containing 0.45 g silica particles.

Thus, it is found that the silica suspensions pre-adsorbed HPMC play a more effective

role in emulsifying silicone oil than the silica suspension without HPMC.

The volume fraction of the silicone oil in the emulsified phase for the elapsed

time of one week after preparation was calculated from the volumes of the emulsified oil

and the emulsion phase in a glass bottle and it is summarized in Table 1. The values of

for the emulsions prepared by the silica suspensions pre-adsorbed HPMC were smaller


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than those prepared by HPMC or the silica suspension except for the 1000-45-3.0

emulsion, and their magnitudes decrease with an increase of the added HPMC amount

and they are much less than the volume fraction of randomly closed-packed spheres,

0.635. The observation that the oil volume fraction in the emulsion is less than the

random close packing limit in the emulsion may be attributed to not only greater steric

repulsions between HPMC adsorbed silica particles but also larger sizes of the silica flocs

by adsorption of HPMC.

In order to confirm what kind emulsion can be prepared in the present study, the

emulsified phase was mixed with water or the corresponding silicone oil. Every

emulsion was able to dilute by water and this means that the resulting emulsions

correspond to oil dispersed in water (O/W) emulsion.

3.2. Interfacial tensions

The interfacial tension of water against the KFL96-1 silicone oil was the same asthe silica suspension containing 0.45 g silica against the corresponding silicone oil and it

was determined to be 36.8 mN/m. This means that Aerosil 130 silica particles do not

behave like a surface active agent for the interface between water and the silicone oil.

Similar result was obtained when monodisperse spherical polystyrene particles were

covered at octane/water interface [31].

As mentioned above, the added amounts of 0.015, 0.030, and 0.050g HPMC were

almost adsorbed at the 0.45g silica particles. The measured values of the

corresponding silica suspensions pre-adsorbed HPMC against the silicone oil were

displayed in Table 1. It is noticed that the values of for the two silica suspensions

pre-adsorbed HPMC of 0.015 and 0.030g is near to that between water and the silicone

oil, a clear decrease in the value is observed at the highest added amount of HPMC of

0.050g, and its magnitude is higher than those between the aqueous HPMC solutions

dissolved 0.015, 0.030, and 0.050 g HPMC in 30 g water against the silicone oil as seen


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from Table 1. However, the values of water, the silica suspensions, and the silica

suspensions pre-adsorbed HPMC against other silicone oils of KF96-10, KF96-100, and

KF96-1000 were unable to reproductively determine because of the higher viscosities of

the corresponding silicone oils.

3.3. Adsorbed amounts of emulsifiers

The measurements of the concentrations of the silica particles in the lower aqueous

phases of the 1-45-0 and 10-45-0 emulsions prove to be the same as the added silica

concentrations for the preparation of the corresponding emulsions. This means that no

adsorption of Aerosil silica particles occurs at all to the interface between oil and water.

Therefore, stabilization of the oil droplets by the silica suspensions could not be

guarantied by the formation of a dense film of the silica particles adsorbed around the

dispersed droplets. The dispersed oil droplets in water could be weakly stabilized by

the partial flocculated silica particles through hydrophobic interactions between silicaparticles and silicone oil. Moreover, it was found that the silicone oil emulsions

prepared by the silica particles gradually became unstable for the elapsed time of one

month after preparation and the portions of the emulsified phase decreased with an

increase in time.

The added amounts of HPMC were almost adsorbed on the silica particles as

previously reported [26]. The adsorption interaction of HPMC on the silica surface

could be dominated by hydrogen bonding between ether groups in HPMC and silanol

groups on the silica surface due to the silica being hydrated at pH = 5.5. In addition,

desorption of HPMC from the silica particles was not observed to occur when the silica

suspensions pre-adsorbed HPMC were washed with water. Similar results have been

reported for some systems [30, 32].

On the other hand, since the concentrations of the silica suspensions pre-adsorbed

HPMC in the lower aqueous phase were determined to be lower than those before the


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preparation, adsorption of the silica suspensions pre-adsorbed HPMC occurred at the

interface between oil and water. The adsorbed amounts of the silica suspensions

pre-adsorbed HPMC per gram of the silicone oil were calculated to be 17.2, 22.4, and

30.4 mg/g in the order of the amount of the added HPMC, in which 73.6, 81.5, and

91.0 % of the silica suspensions pre-adsorbed HPMC were adsorbed at the interface,

respectively. Thus, modification of the silica particles by adsorption of HPMC

enhances the wettability of the silica particles and then causes adsorption of the modified

silica suspensions at the interface between water and the silicone oil unless the value is

decreased. Moreover, it can be expected that an increase in the adsorbed amounts of

the silica suspensions pre-adsorbed HPMC improve the stability of the silicone oil

droplets due to the much more steric repulsion of the silica particles by adsorption of

HPMC.

However, it was impossible to accurately measure changes in the concentration of

the silica suspensions pre-adsorbed HPMC for the KF96-10, KF96-100, and KF96-1000

silicone oils since a little portion of the corresponding silicone oils is remained in thelower aqueous phase.

3.4. Optical microscopic images

Optical microscopic images of the KFL96-1 silicone oil droplets in the emulsified

phase are shown in Figure 1 for the 1-45-0, 1-0-1.5, 1-0-3.0, 1-0-5.0, 1-45-1.5, 1-45-3.0,

and 1-45-5.0 emulsions for the elapsed time of one week after preparation, respectively.

The common volume-surface average size, D3,2, namely the Sauter mean diameter of the

oil droplet is calculated for over 200 individual oil droplets in the respective emulsions

and the D3,2 values are summarized in Table 1, together with their standard deviations for

the elapsed time of one week after preparation. The magnitude of the D3,2 value and

the standard deviation decreases with an increase in the concentration of HPMC,

suggesting that the size distribution becomes narrower with increasing the HPMC


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concentration. Similar results for the silicone oil emulsions prepared by HPMC at the

concentrations higher than the overlapping concentration were obtained [27].

Moreover, the value of D3,2 was also calculated after dilution by water and it is almost the

same as before dilution and this means that there are no changes in the size of oil droplets

due to dilution of water.

Adsorption of HPMC on the silica particles causes a decrease in the D3,2 value as

shown in Table 1 and an increase in the adsorbed amount of HPMC also decreases the

value of D3,2. Such a tendency of the D3,2 value could be related to not only an

increase in the viscosity of the dispersion medium but also a decrease in the interfacial

tension between the silicone oil and water. The effect of the dispersion medium

viscosity on the oil droplet size is in qualitative agreement with our previous studies [27,

28]. On the other hand, the dependence of the interfacial tension on the oil drop size is

also coincident with the previous experimental results since smaller energy consumption

is enough to break up oil droplets due to the lower interfacial tension [29, 33].

Figure 2 shows that an increase in the viscosity of silicone oil gives larger oildroplets in the size, irrespective of the emulsifier since the higher the viscosity of the

dispersed oil is, the harder the dispersion of oil is. Similar results were obtained in our

previous experiments [27-29]. We notice that reduction in the oil droplet size one

order of the magnitude occurs by adsorption of HPMC on the silica particles from a

comparison of the 10-45-0 emulsion and the 10-45-3.0 one as shown in Table 1. The

D3,2 values and their size distributions of the emulsions prepared by the silica suspensions

pre-adsorbed HPMC are somewhat wider with an increase in the silicone oil viscosity as

displayed in Table 1, and their D3,2 values are almost smaller than those prepared by

HPMC.

3.5. Rheological properties

Hysteresis loop measurements are often performed to distinguish between


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Newtonian flow and non-Newtonian flow, such as thixotropy behavior of dispersion

systems. Thixotropy behavior is displayed when the shear stress measured by

progressively increasing the shear rate is larger than that measured when one

progressively decreases it. Moreover, thixotropy behavior observed in dispersion

systems is mainly originated from partial breakdown of their microstructures under shear

flow.

Figures 3-a and 3-b show the shear rate dependences of the shear stresses, namely

the flow curves of the KFL96-1 silicone oil emulsions prepared by different

concentrations of HPMC and those by the silica suspension or the silica suspensions

pre-adsorbed HPMC under increasing and under decreasing shear rate, respectively.

No emulsions exhibit Newtonian behavior and the flow curves of the emulsions prepared

by HPMC are almost superimposed when the shear rate is increased and decreased.

The flow curves for the emulsions prepared by the silica suspension or the silica

suspensions pre-adsorbed HPMC show typical thixotropic behavior, namely the up and

down flow curves are not superimposed and the discrepancy is mainly observed at lowshear rates. Adsorption of HPMC on the silica particles causes much difference

between the up and down flow curves and the discrepancy increases with an increase in

the adsorbed amount of HPMC. Moreover, the apparent viscosity at a given fixed

shear rate is pronounced when the adsorbed amount of HPMC increases.

Figures 4-a, 4-b, and 4-c show the flow curves of the KF96-10, KF96-100, and

KF96-1000 silicone oil emulsions prepared by the added HPMC amount of 0.30g, the

silica suspension, and the silica suspensions pre-adsorbed HPMC under increasing and

under decreasing shear rate, respectively. The flow curves of the silicone oil emulsions

prepared by HPMC indicate weak thixotropic behavior, irrespective of the silicone oil,

and the difference between the up and down flow curves at low shear rates increases with

an increase in the viscosity of silicone oil. Adsorption of HPMC on the silica particles

also induces the pronounced thixotropic behavior similar to the KFL96-1 silicone oil

suspensions prepared by the silica suspensions pre-adsorbed HPMC as displayed in Fig.


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3.

We notice that the shear stress steeply increases at low shear rates and then

gradually decreases with an increase in shear rate for all emulsions prepared by the silica

particles as displayed in Figs. 3 and 4. Such a flow behavior could be attributed to a

partial breakdown of the silica suspensions pre-adsorbed HPMC adsorbed at the interface

between silicone oil and water. However, no coalescence and no deformation of the

silicone oil droplet in the shape are detective after shear cessation or hysteresis loop

measurements for every emulsion from optical microscopic observation and it can be

concluded that the emulsions in this study are stable under flow. We also notice that

the difference between the up and down flow curves increases with a decrease in the

droplet size, namely a decrease in the volume fraction of the silicone oil in the

emulsified phase as displayed in Figs. 3 and 4. This means that the droplets can mare

easily make a rearrangement of their positions under shear flow rather than the

deformation of them in the shape [34, 35] at lower value of. The reason why droplet

deformation is not taken account of in this study is responsible to the emulsifiers used,which can be expected to form a viscoelastic layer adsorbed on the silicone oil surface.

Moreover, the power-law exponent, namely nPL-1 calculated from the plots of the shear

viscosity against the shear rate for various silicone oil emulsions prepared by HPMC, the

silica suspensions, and the silica suspensions pre-adsorbed HPMC are ranged from -0.62

to -0.54, indicating that the corresponding emulsions behave as shear thinning. The

resulting nPL values from 0.38 to 0.46 at > 0.57 are similar to those for concentrated

suspensions of hard particles at moderate volume fraction [34] and they are larger than

those of emulsions with high deformability of droplets, which were prepared by SDS.

Furthermore, changes in the difference between the up and down flow curves are

well correlated to the emulsifiers used, irrespective of the silicone oil: the larger changes

are caused by the silica suspensions pre-adsorbed HPMC than the silica suspension or

HPMC and the former emulsifiers should form a more viscoelastic adsorbed layer on the

droplets than the latter ones. This will be confirmed by elastic responses of the


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resulting emulsions described below.

The S-S sweep curves of the 1-0-5.0, 1-45-0, and 1-45-5.0 emulsions, together

with the optical microscopic images of their silicone oil droplets for the 1-45-0 and

1-45-5.0 emulsions under given strains are displayed in Fig. 5. The respective S-S

sweep curves show that the shear stress tends to be nearly proportional to the shear strain

for the shear strain ranges lower than 1%. This proportionality relation provides

Hookes law and Hooke elastic modulus determined from the slope of a linear plot of the

shear stress against the shear strain are 30, 75, and 125 Pa in the order of the 1-45-0,

1-0-5.0, and 1-45-5.0 emulsions. Moreover, we can obtain the yield stress and the

yield shear strain at which the linear response ends. The resulting yield stress shows

the same trend as the Hooke elastic modulus; whereas the yield shear strain is opposite to

the order of the yield stress. Similar dependences were obtained for the FK96-10

silicone emulsions.

The elastic properties of the emulsions prepared by silica suspensions with or without

HPMC should be originated from the aggregated structure of the fumed particlesthemselves, which is partially broken at the large deformation. The partial breaking of

such aggregated structure of silica particles should cause thixotropic behavior mentioned

above. On the other hand, the elastic responses of the emulsions prepared by HPMC

could be governed by the chain entanglements between the adsorbed HPMC chains and

the free ones in the dispersion medium [27-29]. Since a matter starts to flow under

shear beyond the yield shear strain, where the weakest connections in the corresponding

matter break, the yield shear strain corresponds to a measure of the brittleness of the

matter. Since adsorption of HPMC on the silica particles could partially break down a

hydrogen bonding connection between the aggregated particles in their sintered structure

in water, the yield shear strain of the emulsion prepared by the silica suspension should

be smaller than that by the silica suspension pre-adsorbed HPMC.

As shown Fig. 5, the optical microscopic images of the 1-45-0 and 1-45-5.0

emulsions show that the oil droplets below the yield shear strain are almost the same


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position and beyond that they flow and faster flow with increasing strain neither changes

in the packing state nor deformation of their shapes. Moreover, at a strain larger than

1000 % the flow rate is so fast that it is impossible to adjust the focus of a CCD camera.

The 1-0-5.0 emulsion shows the similar optical microscopic images (as not shown) to

those reported previously [29], that is the same trend for the emulsions prepared by the

silica suspensions.

Figures 6-a and 6-b show the double-logarithmic plots of the storage moduli (G)

of the KFL96-1 silicone oil emulsions and other silicone oil emulsions prepared by

HPMC, silica suspensions, and the silica suspensions pre-adsorbed HPMC as a function

of angular frequency, respectively. All data for G were obtained for the linear

response regions and they are one order of magnitude larger than the loss modulus (G)

over the angular frequency ranges examined in this study, irrespective of the emulsion.

The G values of the 1-45-0 and 10-45-0 emulsions are almost independent of the angular

frequency, showing that the emulsions behave a solid matter. However, the G values

of other emulsions show weak angular frequency dependence and the emulsions preparedby the silica suspensions pre-adsorbed HPMC have a little stronger angular frequency

dependence of G than those prepared by HPMC. Moreover, at the fixed angular

frequency the G values of the KFL96-1 silicone oil emulsions prepared by the silica

suspensions pre-adsorbed HPMC increase with an increase in the added HPMC amount.

Other silicone oil emulsions prepared by the silica suspensions pre-adsorbed HPMC give

larger G than the KF96-10 silicone oil emulsion by the silica suspension as shown in Fig.

6-b. Moreover, the G values are comparable to the Hooke elastic moduli calculated

the slopes of the respective S-S sweep curves for the emulsions prepared by the silica

suspensions pre-adsorbed HPMC, the silica suspensions, and HPMC.

In addition, the elastic stress should be related to the strength of the interparticle

attraction, the particle volume fraction, the particle size, and the microstructure of the

particles. Adsorption of HPMC on the silica suspensions should cause changes in the

four factors mentioned above: the first factor is somewhat weaken since HPMC


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flocculates silica, the second and third factors are somewhat strengthen, and the

aggregated structure of the silica particles is reinforced. Thus, it can be concluded that

the final factor strongly influences on changes in the elastic responses of the silicone oil

emulsions prepared by the silica suspensions pre-adsorbed HPMC.

4. Conclusions

When the silica suspensions pre-adsorbed HPMC were mixed with silicone oils to

prepare emulsions, the adsorption of the silica suspensions pre-adsorbed HPMC occurred

at the interface between silicone oil and water and its adsorbed amount was increased

with an increase in the amount of the pre-adsorbed HPMC. This caused a decrease in

the oil droplet size, a decrease in the volume fraction of the emulsified oil in the

emulsified phase, an increase in the emulsified oil volume, and an increase in the elastic

responses, in comparison with the silicone oil emulsions prepared by the silica

suspensions without HPMC, which no adsorption of the silica suspensions ocurred.Thus, the adsorption of the silica suspensions pre-adsorbed HPMC reinforces the

aggregated structure of silica particles and it provides more steric hindrance to

coalescence between the silicone oil droplets than the silica suspension or HPMC. The

enhanced steric stabilization of the silicone oil emulsions can be confirmed by the

measurements of rheological responses at the smaller deformation, such as the S-S sweep

curve and the dynamic viscoelastic modulus. Moreover, at the larger deformation the

emulsions prepared by the silica suspensions pre-adsorbed HPMC showed thixotropic

behavior and the difference of the flow curves between increasing and decreasing shear

rate increased with an increase in the adsorbed amounts of the silica particles. The

effect of oil viscosity was also observed: an increase in the oil viscosity led to not only

the larger oil droplet size and but also the larger differences discrepancy of the negative

hysteresis curves.


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References

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Figure Captions

Fig. 1. Optical microscopic images of the 1-45-0, 1-0-1.5, 1-0-3.0, 1-0-5.0,

1-45-1.5, 1-45-3.0, and 1-45-5.0 emulsions for the elapsed time of one week after

preparation. The solid bar in the figure corresponds to the length of 100 m.

Fig. 2. Optical microscopic images of the 10-45-0, 10-0-3.0, 100-0-3.0,

1000-0-3.0, 10-45-3.0, 100-45-3.0, and 1000-45-3.0 emulsions for the elapsed time of

one week after preparation. The solid bar in the figure corresponds to the length of

100 m.

Fig. 3. (a) Flow curves of the 1-0-1.5 (circles), 1-0-3.0 (squares), and 1-0-5.0

(triangles) emulsions under increasing (filled symbols) and under decreasing (open

symbols) shear rate; (b) flow curves of the 1-45-0 (diamonds), 1-45-1.5 (circles),

1-45-3.0 (squares), and 1-45-5.0 (triangles) emulsions under increasing (filled symbols)and under decreasing (open symbols) shear rate.

Fig. 4. (a) Flow curves of the 10-45-0 (diamonds), 10-0-3.0 (circles), and

10-45-3.0 (squares) emulsions under increasing (filled symbols) and under decreasing

(open symbols) shear rate; (b) flow curves of the 100-0-3.0 (circles) and 100-45-3.0

(squares) emulsions under increasing (filled symbols) and under decreasing (open

symbols) shear rate; (c) flow curves of the 1000-0-3.0 (circles) and 1000-45-3.0 (squares)

emulsions under increasing (filled symbols) and under decreasing (open symbols) shear

rate.

Fig. 5. S-S sweep curves for the 1-0-5.0 (triangle), 1-45-0 (square), and 1-45-5.0

(circle) emulsions, together with the optical microscopic images of the 1-45-0 and

1-45-5.0 emulsions at given strains. A dashed line in the figure corresponds to the


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straight line of the slope of unity.

Fig. 6. (a) Double-logarithmic plots of storage modulus (G) of the 1-45-0 (closed

diamond), 1-0-3.0 (open square), 1-0-5.0 (open triangle), 1-45-1.5 (closed circle),

1-45-3.0 (closed square), and 1-45-5.0 (closed triangle) emulsions a function of angular

frequency; (b) double-logarithmic plots of G of the 10-45-0 (closed diamond), 10-0-3.0

(open circle), 10-45-3.0 (closed circle), 100-0-3.0 (open square), 100-45-3.0 (closed

square), 1000-0-3.0 (open triangle), and 1000-45-3.0 (closed triangle) emulsions a

function of angular frequency.


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Table 1

Relative amount relof the silicone oil emulsified, volume fraction of the silicone oil in

the emulsified phase, volume-surface average size D3,2 of oil droplets, its standard

deviation, and interfacial tension for silicone oil emulsions prepared by silica particles

pre-adsorbed HPMC, HPMC, and silica particles

Emulsions rel D3,2 (m) Std dev of D3,2 (m) (mN/m)

1-45-1.5 0.95 0.57 81.4 12.0 36.3

1-45-3.0 0.89 0.50 27.0 6.2 36.6

1-45-5.0 0.82 0.45 14.5 3.9 20.5

10-45-3.0 1.0 0.49 27.7 7.9 __

100-45-3.0 1.0 0.48 38.3 12.8__

1000-45-3.0 1.0 0.69 94.5 28.9__

1-0-1.5 1.0 0.71 50.4 11.7 17.6

1-0-3.0 1.0 0.65 46.7 10.9 17.2

1-0-4.5 1.0 0.69 41.1 7.35 17.1

10-0-3.0 1.0 0.66 46.2 11.5__

100-0-3.0 1.0 0.66 78.9 18.9__

1000-0-3.0 1.0 0.66 128 34.3__

1-45-0 0.88 0.69 113 21.1 36.8

10-45-0 0.84 0.66 136 16.9__


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1-45-0

1-0-1.5 1-45-1.5

1-0-3.0 1-45-3.0

1-0-5.0 1-45-5.0

Fig. 1 N. Sugita et al.


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10-45-0

10-0-3.0 10-45-3.0

100-0-3.0 100-45-3.0

1000-0-3.0 1000-45-3.0



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0

10

20

30

0 100 200 300

Shearstress

(Pa)

Shear rate (1/s)

a

0

10

20

30

40

50

0 100 200 300

S

hearstress

(Pa)

Shear rate (1/s)

b



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0

10

20

30

40

50

0 100 200 300

Shearstress(P

a)

Shear rate (1/s)

a

0

10

20

30

40

50

0 100 200 300

Shearstress

(Pa)

Shear rate (1/s)

b

0

10

20

30

40

50

0 100 200 300

She

arstress

(Pa)

Shear rate (1/s)

c

.



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10-1

100

101

102

10-2

100

102

104

106

Shearstress

(Pa)

Strain (%)



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101

102

103

10-1

100

101

102

G'

(Pa)

Angular frequency (rad/s)

a

101

102

103

10-1

100

101

102

G'

(Pa)

Angular frequency (rad/s)

b


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