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Rheological and interfacial properties of silicone oil emulsions prepared by polymer pre-adsorbed onto silica particles
Colloids and Surfaces A: Physicochem. Eng. Aspects 328 (2008) 114–122
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Rheological and interfacial properties of silicone oil emulsions prepared by polymer pre-adsorbed onto silica particles Noriaki Sugita, Shinji Nomura, Masami Kawaguchi ∗ Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan
a r t i c l e
i n f o
Article history: Received 10 September 2007 Received in revised form 17 June 2008 Accepted 17 June 2008 Available online 4 July 2008 Keywords: Pickering emulsions Silica particles pre-adsorbed hydroxypropyl methyl cellulose Silicone oil Rheological properties Interfacial properties
a b s t r a c t 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 HPMC or HPMC. 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 preadsorbed 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 every emulsifier showed that their stress–strain sweep curves were satisfied with Hooke’s 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. © 2008 Elsevier B.V. All rights reserved.
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
∗ Corresponding author at: Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan. Tel.: +81 59 231 9432; fax: +81 59 231 9471. E-mail address: [email protected] (M. Kawaguchi). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.06.044
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 as approximately 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 and (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
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the oil droplet size and 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 as an 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 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 25 ◦ C, 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 silica has an average diameter of 16 nm, a surface area of 130 m2 /g, and a silanol density of 2.0 nm−2 , 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 ◦ C, using a Yamato Ultra Disperser with an S-25N-25F agitation shaft. 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 ◦ C for 24 h, 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
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HPMC as follows. Since the silica suspensions pre-adsorbed HPMC were flocculated as mentioned above, they were agitated to well disperse in water 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 preadsorbed 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 25 ◦ C 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 rpm. 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 preadsorbed HPMC dispersed in 30 g water, and the silica suspension dispersed in 30 g water were measured using a Du Noüy tensiometer at 25 ◦ C. 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 1 week after preparation of the corresponding emulsions, 5 mL of the bottom phase parts were 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 addition of 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, 1◦ ), which is designed by the concept of rheo-optics consisting of microscopic and rheological techniques, with and without shear flow [29].
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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 1 week after preparation were carried out at 25 ◦ C using the same Rheo Scope 1 with the same cone-plate geometry as optical microscopic measurements. The hysteresis loop measurements were 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 middle emulsified 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 1 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 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 as the 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 0.050 g HPMC were almost adsorbed at the 0.45 g silica particles. The measured values of the corresponding silica suspensions preadsorbed HPMC against the silicone oil were displayed in Table 1. It is noticed that the values of for the two silica suspensions preadsorbed HPMC of 0.015 and 0.030 g 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 0.050 g, 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 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
Table 1 Relative amount rel of 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)
S.D. of D3,2 (m)
(mN/m)
1-45-1.5 1-45-3.0 1-45-5.0 10-45-3.0 100-45-3.0 1000-45-3.0 1-0-1.5 1-0-3.0 1-0-4.5 10-0-3.0 100-0-3.0 1000-0-3.0 1-45-0 10-45-0
0.95 0.89 0.82 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.88 0.84
0.57 0.50 0.45 0.49 0.48 0.69 0.71 0.65 0.69 0.66 0.66 0.66 0.69 0.66
81.4 27.0 14.5 27.7 38.3 94.5 50.4 46.7 41.1 46.2 78.9 128 113 136
12.0 6.2 3.9 7.9 12.8 28.9 11.7 10.9 7.35 11.5 18.9 34.3 21.1 16.9
36.3 36.6 20.5 – – – 17.6 17.2 17.1 – – – 36.8 –
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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 1 week after preparation. The solid bar in the figure corresponds to the length of 100 m.
weakly stabilized by the partial flocculated silica particles through hydrophobic interactions between silica particles 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 1 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 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
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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 the lower aqueous phase. 3.4. Optical microscopic images Optical microscopic images of the KFL96-1 silicone oil droplets in the emulsified phase are shown in Fig. 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
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 1 week after preparation. The solid bar in the figure corresponds to the length of 100 m.
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elapsed time of 1 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 1 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 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
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.
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energy consumption is enough to break up oil droplets due to the lower interfacial tension [29,33]. Fig. 2 shows that an increase in the viscosity of silicone oil gives larger oil droplets 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 parti-
Fig. 4. (a) Flow curves of the 10-45-0 (diamonds), 10-0-3.0 (circles), and 10-453.0 (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.
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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 straight line of the slope of unity.
cles 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 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. Fig. 3a and b shows 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 low shear 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. Fig. 4a–c shows the flow curves of the KF96-10, KF96-100, and KF96-1000 silicone oil emulsions prepared by the added HPMC amount of 0.30 g, 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 pre-
pared 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. 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
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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 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 Hooke’s 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 particles themselves, 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 in 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 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. Fig. 6a and b shows 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 145-0 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 prepared by 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 sili-
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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 as 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 as function of angular frequency.
cone 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. 6b. 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 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
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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 occurred. 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 preadsorbed 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. References [1] [2] [3] [4] [5]
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Rheological and interfacial properties of silicone oil emulsions prepared by polymer pre-adsorbed onto silica particles Noriaki Sugita, Shinji Nomura, Masami Kawaguchi ∗ Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan
a r t i c l e
i n f o
Article history: Received 10 September 2007 Received in revised form 17 June 2008 Accepted 17 June 2008 Available online 4 July 2008 Keywords: Pickering emulsions Silica particles pre-adsorbed hydroxypropyl methyl cellulose Silicone oil Rheological properties Interfacial properties
a b s t r a c t 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 HPMC or HPMC. 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 preadsorbed 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 every emulsifier showed that their stress–strain sweep curves were satisfied with Hooke’s 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. © 2008 Elsevier B.V. All rights reserved.
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
∗ Corresponding author at: Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan. Tel.: +81 59 231 9432; fax: +81 59 231 9471. E-mail address: [email protected] (M. Kawaguchi). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.06.044
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 as approximately 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 and (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
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the oil droplet size and 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 as an 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 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 25 ◦ C, 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 silica has an average diameter of 16 nm, a surface area of 130 m2 /g, and a silanol density of 2.0 nm−2 , 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 ◦ C, using a Yamato Ultra Disperser with an S-25N-25F agitation shaft. 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 ◦ C for 24 h, 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
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HPMC as follows. Since the silica suspensions pre-adsorbed HPMC were flocculated as mentioned above, they were agitated to well disperse in water 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 preadsorbed 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 25 ◦ C 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 rpm. 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 preadsorbed HPMC dispersed in 30 g water, and the silica suspension dispersed in 30 g water were measured using a Du Noüy tensiometer at 25 ◦ C. 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 1 week after preparation of the corresponding emulsions, 5 mL of the bottom phase parts were 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 addition of 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, 1◦ ), which is designed by the concept of rheo-optics consisting of microscopic and rheological techniques, with and without shear flow [29].
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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 1 week after preparation were carried out at 25 ◦ C using the same Rheo Scope 1 with the same cone-plate geometry as optical microscopic measurements. The hysteresis loop measurements were 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 middle emulsified 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 1 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 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 as the 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 0.050 g HPMC were almost adsorbed at the 0.45 g silica particles. The measured values of the corresponding silica suspensions preadsorbed HPMC against the silicone oil were displayed in Table 1. It is noticed that the values of for the two silica suspensions preadsorbed HPMC of 0.015 and 0.030 g 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 0.050 g, 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 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
Table 1 Relative amount rel of 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)
S.D. of D3,2 (m)
(mN/m)
1-45-1.5 1-45-3.0 1-45-5.0 10-45-3.0 100-45-3.0 1000-45-3.0 1-0-1.5 1-0-3.0 1-0-4.5 10-0-3.0 100-0-3.0 1000-0-3.0 1-45-0 10-45-0
0.95 0.89 0.82 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.88 0.84
0.57 0.50 0.45 0.49 0.48 0.69 0.71 0.65 0.69 0.66 0.66 0.66 0.69 0.66
81.4 27.0 14.5 27.7 38.3 94.5 50.4 46.7 41.1 46.2 78.9 128 113 136
12.0 6.2 3.9 7.9 12.8 28.9 11.7 10.9 7.35 11.5 18.9 34.3 21.1 16.9
36.3 36.6 20.5 – – – 17.6 17.2 17.1 – – – 36.8 –
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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 1 week after preparation. The solid bar in the figure corresponds to the length of 100 m.
weakly stabilized by the partial flocculated silica particles through hydrophobic interactions between silica particles 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 1 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 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
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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 the lower aqueous phase. 3.4. Optical microscopic images Optical microscopic images of the KFL96-1 silicone oil droplets in the emulsified phase are shown in Fig. 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
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 1 week after preparation. The solid bar in the figure corresponds to the length of 100 m.
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elapsed time of 1 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 1 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 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
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.
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energy consumption is enough to break up oil droplets due to the lower interfacial tension [29,33]. Fig. 2 shows that an increase in the viscosity of silicone oil gives larger oil droplets 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 parti-
Fig. 4. (a) Flow curves of the 10-45-0 (diamonds), 10-0-3.0 (circles), and 10-453.0 (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.
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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 straight line of the slope of unity.
cles 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 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. Fig. 3a and b shows 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 low shear 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. Fig. 4a–c shows the flow curves of the KF96-10, KF96-100, and KF96-1000 silicone oil emulsions prepared by the added HPMC amount of 0.30 g, 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 pre-
pared 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. 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
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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 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 Hooke’s 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 particles themselves, 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 in 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 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. Fig. 6a and b shows 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 145-0 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 prepared by 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 sili-
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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 as 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 as function of angular frequency.
cone 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. 6b. 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 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
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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 occurred. 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 preadsorbed 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. References [1] [2] [3] [4] [5]
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