1126
Langmuir 2004, 20, 1126-1129
Rheo-Optical Properties of Silicone Oil Emulsions in the Presence of Polymer Emulsifiers Masami Kawaguchi* and Kenji Kubota Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan Received July 3, 2003. In Final Form: December 1, 2003 Oil in water emulsions prepared by dispersion of silicone oils into an aqueous solution of hydroxylpropyl methyl cellulose (HPMC) or poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPOPEO) block copolymers were characterized by measurements of steady-state shear viscosities, dynamic moduli, and stress-strain sweep curves coupled with optical microscopic observation. The emulsions prepared with HPMC showed solidlike viscoelastic responses and a clear yield stress, whereas the emulsions emulsified by PEO-PPO-PEO block copolymers indicated liquidlike viscoelastic behavior. The difference should be attributed to stronger protective colloidal effects, that is, the former emulsifiers form a more viscoelastic polymer layer adsorbed on the oil droplets than the latter ones. Moreover, the simultaneous optical microscopic observation showed that the emulsions stabilized by HPMC do not flow below the yield stress and beyond the yield stress the movements of oil droplets occur first.
Introduction Oil in water emulsions are basically obtained by dispersing oil into an aqueous solution of emulsifier. They are produced by fragmenting oil phase into aqueous solution, accompanied with adsorption of the emulsifier on the surfaces of the oil phase, leading to an oil droplet stabilized by the emulsifier in the aqueous phase. The persistence of the emulsifier is entirely governed by its surface activity to prevent coalescence of the dispersion oil droplets. Emulsifier agents such as water-soluble polymers are often used to stabilize emulsions against coalescence since it can be expected that the polymers form a thicker and viscoelastic polymer layer adsorbed on the surface of oil droplets, leading to strong protective colloidal effects.1-5 Among water-soluble polymers, proteins, block copolymers, polysaccharides, and surface active homopolymers are suitable to prepare oil in water emulsions. In previous papers,6,7 we prepared oil in water emulsions by dispersion of silicone oil into aqueous solutions of hydroxypropyl methyl cellulose (HPMC) as functions of the HPMC molecular weight6 and the silicone oil viscosity,7 and the resulting emulsions were stabilized by the protective colloidal effects of the HPMC layer adsorbed on the oil droplets, leading to viscoelastic properties. Since poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers can exhibit amphiphilic properties in their aqueous solutions, they are often used not only in basic research but also in many applications.8-10 They could be used as emulsifiers for their surfactant properties due to variation of copolymer composition and molecular weight; the hydrophilic PEO blocks extend into the water, and the hydrophobic (1) In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, 1998. (2) Bibette, J.; Leal-Calderson, F. Curr. Opin. Colloid Interface Sci. 1996, 1, 746. (3) Kabalnov, A. Curr. Opin. Colloid Interface Sci. 1998, 3, 270. (4) Lequeux, F. Curr. Opin. Colloid Interface Sci. 1998, 3, 408. (5) Dickinson, E. Curr. Opin. Colloid Interface Sci. 1998, 3, 633. (6) Hayakawa, K.; Kawaguchi, M.; Kato, T. Langmuir 1997, 13, 6069. (7) Yonekura, K.; Hayakawa, K.; Kawaguchi, M.; Kato, T. Langmuir 1998, 14, 3145. (8) Alexandridis, P.; Hatton, T. A. Colloids Surf. 1995, 1, 96. (9) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490; 1997, 2, 478. (10) Sedev, R.; Exerowa, D. Adv. Colloid Interface Sci. 1999, 83, 111.
PPO block binds to the surface of the oil droplets.1 In this study, we present results of the rheological properties and microscopic observation of silicone oil emulsions stabilized by two HPMC samples and two PEO-PPO-PEO block copolymers in water as a function of viscosity of the silicone oil. Moreover, we illustrate how the stability of the emulsions under shear flow may be influenced by changes in the emulsifiers through the optical microscopic observation. Experimental Section Samples. Two HPMC samples obtained from Shin-Etsu Chemical Co. Ltd. are the same as previously reported.7 The molecular weights of HPMC-15 and HPMC-400 were determined to be 15.0 × 103 and 388 × 103, respectively. Two Pluronic PEO-PPO-PEO samples of F-68 and F-108 were kindly supplied by Asahidenka Co. Ltd. and used without further purification. According to the manufacturer of PEOPPO-PEO, the PEO composition is 80 wt % and the total molecular weights of F-68 and F-108 are 8.35 × 103 and 15.5 × 103, respectively. Four silicone oils were kindly supplied by Shin-Etsu Chemical Co. Ltd., and the viscosities of KFL96-1, KF96-10, KF96-100, and KF96-1000 are 1, 10, 100, and 1000 mPa s at 25 °C, respectively. Water purified with a Millipore MQ Academic-Elix-UV 5 system was used as a solvent for polymer emulsifiers. Preparation of Emulsions. To prepare an emulsion, 25 g of the silicone oil and 50 g of 0.5 g/100 mL aqueous solution of HPMC or PEO-PPO-PEO block copolymer were mixed 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. The applied shear rate was calculated to be approximately 2200 s-1 from the diameters of the shaft and bottle. After the agitation, the resulting emulsions were separated into two phases: an upper emulsion phase and a lower transparent phase. Optical Microscopy Measurements. Optical microscopic observation of the emulsion was performed using a Rheo Scope 1 (Thermo Haake, Karlsruhe, Germany), which is designed by the concept of rheo-optics consisting of microscopic and rheological techniques, with and without shear flow at 25 °C. The microscope is embedded under the plate sensor with a diameter of 70 mm; it can be moved by a ray of the cone sensor with a cone angle of 1°. The image of the sample can be observed with a 20 times magnification objective, and it can be captured at 15 frames per second. Each image size is 190 µm × 140 µm with a resolution of 1 µm.
10.1021/la035187x CCC: $27.50 © 2004 American Chemical Society Published on Web 01/17/2004
Rheo-Optical Properties of Silicone Oil Emulsions
Langmuir, Vol. 20, No. 4, 2004 1127
Figure 1. Optical microscopic images of the emulsions stabilized by (I) HPMC-400 and (II) F-108 for different silicone oils: (a) KFL96-1, (b) KF96-10, (c) KF96-100, and (d) KF96-1000. The bar in the figure corresponds to 50 µm.
Figure 2. Steady-state shear viscosities of the emulsions stabilized by (a) HPMC-400 and (b) F-108 as a function of shear rate: KFL96-1 (O), KF96-10 (0), KF-100 (4), and KF-1000 (]). Rheological Measurements. Measurements of steady-state viscosity, dynamic moduli, and stress-strain sweep of the emulsions were carried out at 25 °C using the same Rheo Scope 1 with the same cone and plate geometry as described above. The steady-state viscosity measurements were performed at the shear rate of 10-2 to 103 s-1, and the dynamic modulus measurements in the linear responses were done at the frequency of 10-1 to 102 rad/s. Moreover, the yield stress measurements were performed from stress-strain (S-S) sweep curves when shear stresses were applied from 0.009 to 50 Pa.
Results and Discussion Since there is little trace of silicone oil in the lower phase after evaporation of water, all added silicone oils
Figure 3. Double-logarithmic plots of storage (G′) and loss (G′′) moduli of the emulsions prepared by (a) HPMC-400 and (b) F-108 as a function of angular frequency: G′ (O) and G′′ (b) for KFL96-1, G′ (0) and G′′ (9) for KF96-10, G′ (4) and G′′ (2) for KF96-100, G′ ()) and G′′ (() for KF96-1000.
are emulsified. Thus, the volume fraction φ of the oil in the emulsified phase could be calculated to be 0.72; it is larger than the volume fraction of randomly closed-packed spheres, 0.635, and it is near to 0.74 for the volume fraction in the hexagonal close-packed structure. Appearances of the packing of oil droplets in the upper phase are revealed in typical optical microscopic images under no applied shear stress as shown in Figure 1 for the emulsions prepared in the presence of HPMC-400 and F-108, respectively, and the oil droplets in the respective emulsions are randomly close-packed rather than hexagonal. The size of the oil droplet decreases with an
1128
Langmuir, Vol. 20, No. 4, 2004
Kawaguchi and Kubota
Figure 4. S-S sweep curves for the emulsions prepared by HPMC-400 (open symbols) and HPMC-15 (filled symbols): KFL96-1 (circle), KF96-10 (square), KF-100 (triangle), and KF-1000 (diamond), together with the optical microscopic images of KFL96-1 stabilized by HPMC-400 at given strains. A circle in each image corresponds to the same oil droplet.
Figure 5. S-S sweep curves for the emulsions prepared by F-108 (open symbols) and F-68 (filled symbols), together with the optical microscopic images of KFL96-1 stabilized by F-108 at given strains. The symbols are the same as in Figure 4. A circle in each image corresponds to the same oil droplet.
increase in the viscosity of the silicone oil, irrespective of the polymer emulsifier used and the polymer molecular weight, and the size distribution of the oil droplet becomes wider with increasing silicone oil viscosity. The PEOPPO-PEO block copolymers can more easily emulsify silicone oils than HPMC samples, due to the lower surface tension of their aqueous solutions, leading to the smaller size of oil droplets for the silicone oils with the lower viscosities: the former solutions give a surface tension of 38 mN/m, whereas the latter solutions show a surface
tension of 47 mN/m, and the respective surface tensions were determined by the Wilhelmy plate method. All aqueous solutions of 0.5 wt % HPMC and PEOPPO-PEO block copolymers show Newtonian flow behavior, and the zero-shear viscosities of the HPMC-15 and HPMC-400 solutions are obtained to be 2.0 and 10.0 mPa s, respectively, whereas those of the block copolymer solutions are determined to be 1.0 mPa s, irrespective of the molecular weight. Typical steady-state shear viscosities of the emulsions prepared in the presence of HPMC-
Rheo-Optical Properties of Silicone Oil Emulsions
Langmuir, Vol. 20, No. 4, 2004 1129
400 and F-108 are displayed as a function of shear rate in panels a and b of Figure 2, respectively. All emulsions show shear thinning responses, and the emulsions prepared in the presence of HPMC-400 display stronger shear thinning behavior than those in the presence of F-108. The plot of the steady-state shear viscosity against the shear rate for the emulsions prepared in the presence of F-108 becomes similar to those of the polymer solutions having entanglements of polymer chains with an increase in the silicone oil viscosity. When the molecular weight of the emulsifier was changed, the shear viscosity behavior of the emulsion was similar to that described above. Moreover, the steady-state shear viscosity decreases with an increase in the viscosity of silicone oil, namely, an increase in oil droplet size, regardless of the constant volume fraction, irrespective of the emulsifier. This may be attributed to an increase in the effective volume fraction with decreasing oil droplet size. To determine the effective volume fraction, it is necessary to account for both the volume of the oil droplets and the volume of the water film with thickness h, which is responsible for the emulsified polymer layer adsorbed on the oil droplets. Thus, an effective volume fraction φeff can be given by using the thickness h and the oil droplet size Dp.11-13
φeff ) φ(1 + 3h/Dp)
(1)
The polymer adsorbed layer was determined to be close to the diameter of a free polymer chain in the bulk solutions for many systems.14 If the value of h is assumed to be independent of the silicone oil viscosity, eq 1 yields that the effective volume fraction should increase with a decrease in the oil droplet size at constant φ. Panels a and b of Figure 3 show double-logarithmic plots of storage (G′) and loss (G′′) moduli of the emulsions prepared in the presence of HPMC-400 and F-108 as a function of angular frequency, respectively. All data for G′ and G′′ were obtained for the linear response regions. The dynamic responses of the emulsions strongly depend on the emulsifiers used. The emulsions prepared in the presence of HPMC-400 behave as solidlike viscoelastic matter, and the values of G′ at lower frequency ranges give almost a constant value, indicating that the internal structure is present in the emulsions. Similar results were obtained for the emulsions prepared in the presence of HPMC-15. The presence of the internal structure for the emulsions stabilized by HPMC causes the stronger shear thinning behavior described previously. On the other hand, except for the emulsion of the lowest viscosity in the silicone oil, the emulsions prepared in the presence of F-108 behave as liquidlike viscoelastic matter and all emulsions prepared in the presence of F-68 behave as liquidlike viscoelastic matter. Moreover, we notice that the values of G′ and G′′ increase with a decrease in oil viscosity, irrespective of the emulsifier. Figure 4 shows the plots of shear stress as a function of strain (S-S sweep curves) for the emulsions prepared in the presence of HPMC-15 and HPMC-400, together with the optical microscopic images of the emulsion of KFL96-1 stabilized by HPMC-400 under given strains. The respective emulsions prepared in the presence of the HPMC samples show a clear yield stress in their S-S (11) Princen, H. M.; Aronson, M. P.; Mosen, J. C. J. Colloid Interface Sci. 1980, 75, 246. (12) Mason, T. G.; Bibette, J.; Weitz, D. A. Phys. Rev. Lett. 1995, 75, 2051. (13) Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1996, 179, 439. (14) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.
sweep curves, and the yield stress can be regarded as a broken point in the S-S sweep curve of the corresponding emulsions. The yield stress is observed at higher strain, and its magnitude increases with a decrease in the viscosity of silicone oil, irrespective of the molecular weight of HPMC. Moreover, the resulting yield stress of the emulsion in the presence of HPMC-400 is larger than that in the presence of HPMC-15 for the same silicone oil. Two optical microscopic images observed below at the yield stress are quite the same, indicating that any motions of the oil droplets in the corresponding emulsion do not occur under shear flow. This may be due to the stronger protective colloidal effects by the viscoelastic HPMC layer adsorbed on the oil droplet. However, beyond the yield stress the emulsion has just started to flow, the flow rate increases with increasing strain, and at a strain larger than 4000% the flow rate is so fast that it is impossible to adjust the focus of a CCD camera. During the S-S sweep measurements, any coalescence and breakup of the oil droplets in the emulsion and any changes in the packing state were not observed. Other emulsions prepared in the presence of HPMC-400 and HPMC-15 display flow behavior similar to that of the emulsion of KFL96-1 stabilized by HPMC-400. This is the first direct observation of the motion of oil droplets in emulsions stabilized by polymer emulsifiers under shear flow. Figure 5 shows S-S sweep curves of the emulsions in the presence of F-108 and F-68, together with the optical microscopic images of the emulsion of KFL96-1 stabilized by F-108 under given strains. The S-S sweep curve of KFL96-1 dispersed in an aqueous solution of F-108 only shows an obscure broken point; this is relevant to the slightly larger storage modulus than the loss one as mentioned above. On the other hand, other emulsions prepared in the presence of the block copolymers do not display any broken points in the corresponding S-S sweep curves due to the liquidlike viscoelastic matter. The shear strain at which the shear stress is observed for the first time increases with an increase in the viscosity of the silicone oil, irrespective of the molecular weight. The optical microscopic images for the emulsion using KFL96-1 show that the oil droplets flow even below the yield stress and faster flow with increasing strain shows neither changes in the packing state nor deformation of the oil droplets in the emulsion. Similar optical microscopic images are observed for other emulsions prepared in the presence of the block copolymers. The difference in the optical microscopic images below the yield stress between the emulsions prepared with HPMC and the block copolymer should be attributed to the protective colloidal effects. Conclusions Oil in water emulsions prepared by dispersion of silicone oils into an aqueous solution of HPMC samples or PEOPPO-PEO block copolymers are stable due to the protective effects of the respective polymers. The emulsions prepared in the presence of HPMC samples show stronger shear thinning behavior than those in the presence of the block copolymers. The former emulsions are solidlike viscoelastic matter, whereas the latter ones are liquidlike viscoelastic matter. The differences in the rheological properties are well correlated with the optical microscopic images under shear flow. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area (A) “Dynamic Control of Strongly Correlated Soft Materials” (No. 413/13031045) from the Ministry of Education, Science, Sports, Culture, and Technology. LA035187X
