Langmuir 1997, 13, 6069-6073
6069
Protective Colloidal Effects of Hydroxypropyl Methyl Cellulose on the Stability of Silicone Oil Emulsions Kazuhisa Hayakawa,*,† Masami Kawaguchi,*,‡ and Tadaya Kato‡ Specialty Chemicals Research Center, Shin-Etsu Chemical Co., Ltd., 28-1 Nishifukushima, Kubiki, Niigata 942, Japan, and Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan Received March 21, 1997. In Final Form: August 21, 1997X Oil in water emulsions were prepared by dispersion of silicone oil in aqueous solutions of hydroxypropyl methyl cellulose (HPMC) with various molecular weights. The resulting emulsions were characterized by a combination of size measurement and rheology techniques as a function of time elapsed after the preparation. The size and size distribution of the oil droplets showed little changes with the elapsed time, indicating the resulting emulsions are stable. The size of the oil droplet increased with the molecular weight of HPMC at a fixed concentration of HPMC, whereas it decreased with an increase in HPMC concentration at a constant molecular weight HPMC. All emulsions showed pseudoplastic flow and solidlike viscoelastic responses; their steady-state shear viscosity coefficients and dynamic moduli increased with an increase in the effective volume fraction of oil droplets in the emulsion.
1. Introduction Emulsifier agents, such as surfactants, act to lower interfacial tension between immiscible fluids and form stable emulsions. When emulsions are oil and water mixtures and are stabilized by a surfactant, they consist of either oil droplets dispersed in water or water droplets in oil. Though emulsions are metastable and nonequlibrium systems, they are extremely important for various applications, including oil recovery, food processing, and cosmetology. Thus, the stability of the prepared emulsion is a critical property for all applications, several approaches have been performed to improve the emulsion stability, and studies on highly concentrated oil in water emulsions were deeply investigated.1-9 On the other hand, water-soluble polymers10 were used as emulsifiers instead of surfactant, since they could form a thick adsorbed layer, which plays a role in a protective layer. Furthermore, when polymers are present in the dispersion phase, its viscosity increases and the coalescence of droplets would be deduced. For the preparation of oil in water emulsions, a variety of water-soluble polymers, such as homopolymers,10-12 block copolymers,7,8,10 and proteins,10,13 were used and they should contain amphiphilic characters, namely, they adsorb and create a hydrophilic layer on the surface of oil droplets. Since water-soluble cellulose derivatives have an amphiphilic character because of substitution of hydroxyl groups, they †
Shin-Etsu Chemical Co., Ltd. Mie University. X Abstract published in Advance ACS Abstracts, October 1, 1997. ‡
(1) Princen, H. M. J. Colloid Interface Sci. 1979, 71, 55. (2) Princen, H. M.; Aronson, M. P.; Moser, J. C. J. Colloid Interface Sci. 1980, 75, 246. (3) Princen, H. M. J. Colloid Interface Sci. 1985, 105, 150. (4) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427. (5) Bibette, J.; Morse, D. C.; Witten, T. A.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 2439. (6) Mason, T. G.; Bibette, J.; Weitz, D. A. Phys. Rev. Lett. 1995, 75, 2051. (7) Pons, R.; Solans, C.; Tadros, Th. T. Langmuir 1995, 11, 1966. (8) Pons, R.; Rossi, R.; Tadros, Th. T. J. Phys. Chem. 1995, 99, 12624. (9) Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1996, 179, 439. (10) Dickinson, E. Pure Appl. Chem. 1992, 64, 1721 and therein references. (11) Cheng, J. T. J. Macromol. Sci.-Phys. 1981, B20, 365. (12) Meller, A.; Stavans, J. Langmuir 1996, 12, 301. (13) Fang. Y.; Dalgleish, D. G. J. Colloid Interface Sci. 1993, 156, 329.
S0743-7463(97)00306-5 CCC: $14.00
can be expected to be of benefit to emulsifier agents. Among cellulose derivatives, hydroxypropyl methyl cellulose (HPMC) was used for the suspension polymerization of vinyl chloride to stabilize the monomer droplets and control the particle structure by HPMC layer coated onto the droplets.10 Thus, an HPMC adsorbed layer is useful for protective action for oil droplets. Furthermore, aqueous HPMC solution is surface active and its surface tension is reduced. In this study, we present results of the protective colloidal effect of HPMC in oil in water emulsions as a function of elapsed time after preparation in terms of the adsorption behavior of HPMC, the stability, and the rheological properties. These emulsions were prepared using silicone oil and aqueous solutions of HPMC with different molecular weights. 2. Experimental Section Sample. HPMC samples were obtained from Shin-Etsu Chemical Co. Ltd. They were purified by precipitation of their aqueous solutions into acetone and then freeze-dried from their aqueous solutions. The molecular weight and the molecular weight distribution were measured by gel permeation chromatography according to the method reported in a previous paper.14 The degrees of substitution of methoxy and hydroxypropoxyl groups were measured using the Zeisel-GC method.15 Silicone oil used in this experiment was a Shin-Etsu KF-96 silicone oil with the viscosity of 1 mPa‚s at 25 °C. Distilled water was used as a solvent for HPMC samples. Preparation of Emulsions. To prepare an emulsion, 70 g of the oil and 140 g of aqueous solution of HPMC with a given concentration were mixed in a 200 mL bottle and agitated for 20 min with 8000 rpm at 25 °C, using a Yamato Ultra Disperser with a S-25N-25F agitation shaft. Observation of Stability of Emulsions. Just after the preparation of emulsions, they were replaced in a 200 mL measuring cylinder of an inside diameter of 3.0 cm. With an increase in the elapsed time, the resulting emulsions were separated into two phases: an upper oil-rich (emulsified) phase and a lower transparent oil-poor phase, i.e. aqueous HPMC solution. Changes in the volume faction in the upper phase per the total volume were measured as a function of the time elapsed after the preparation of the emulsions. Droplet Size Measurements. A Coulter Counter Multisizer ZM was used in conjunction with 400 µm aperture at 25 °C. An aliquot of the emulsions was dispersed into Isoton II in the measuring cell dropwise. (14) Kato, T.; Tokuya, T.; Takahashi, A. Kobunshi Ronbunshu 1982, 39, 293. (15) Gobler, J. G.; Samsel, E. P.; Beader, G. H. Talanta 1962, 9, 473.
© 1997 American Chemical Society
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Figure 1. Double-logarithmic plots of viscosities of HPMC solutions and emulsions with HPMC as a function of shear rate: HPMC solutions of 60SH-5 (9), 60SH-50 ([), 60SH-400 (b), and 60SH-4000 (2); emulsions for just after the preparation of the emulsions with 60SH-5 (0), 60SH-50 (]), 60SH-400 (O), and 60SH-4000 (4); emulsions for 72 h after the preparation with 60SH-5 (*), 60SH-50 (diamond, solid right), 60SH-400 (x), and 60SH-4000 (3). Table 1. Molecular Characteristics of HPMC sample codes of HPMC
Mw × 10-4
Mw/Mn
DS
MS
60SH-5 60SH-50 60SH-400 60SH-4000
7.14 19.8 38.8 101.2
2.13 2.45 2.47 3.77
1.8 1.8 1.8 1.8
0.25 0.25 0.25 0.25
Optical Microscopy Measurements. Optical microscopic observation of the emulsions was made using an Olympus BH-2 Nomarski differential interference microscope attached with a scale. An aliquot of the emulsions 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. Rheological Measurements. Rheological measurements were carried out using a controlled strain Advanced Rheometrics Expansion System (ARES) instrument with a Couette type cylinder (outer diameter, 34 mm; inner diameter, 32 mm) and with a parallel plate (diameter, 50 mm) at 25 °C. The steadystate shear viscosity measurements were performed at a shear rate of 1.0-1260 1/s, and the dynamic modulus measurements were done at a frequency of 1.0-102 rad/s. Interfacial Tensions. Interfacial tensions of 0.5 wt % aqueous HPMC solution against oil were measured using a Shimadzu Du Nouy tensiometer at 25 °C. Adsorption of HPMC. The resulting emulsions were kept at 25 °C for 72 h after the preparation. Parts of the lower phase were extracted using a syringe and dried for 48 h at 105 °C to evaporate water in a vacuum oven. The dried residue was weighed to calculate the concentration of HPMC in the lower phase. By subtraction of the calculated concentration of HPMC from the dosing concentration of HPMC, the amount of HPMC adsorbed onto the oil surfaces was obtained. Furthermore, the adsorbed amount of HPMC was conducted after verification of the amount of oil contained in the lower phase by analysis of the dried residue using the Zeisel-GC method.15
3. Results and Discussion Molecular characteristics, such as the molecular weight (Mw), the molecular weight distribution (Mw/Mn), and the degree of substitution (DS) of the methoxy group and
Figure 2. Double-logarithmic plots of storage (G′) and loss (G′′) moduli of HPMC solutions and emulsions with HPMC as a function of frequency (ω): HPMC solutions of 60SH-5 (9), 60SH-50 ([), 60SH-400 (b), and 60SH-4000 (2); emulsions for just after the preparation of the emulsions with 60SH-5 (0), 60SH-50 (]), 60SH-400 (O), and 60SH-4000 (4); emulsions for 72 h after the preparation with 60SH-5 (*), 60SH-50 (diamond, solid right), 60SH-400 (x), and 60SH-4000 (3). Table 2. Volume Fraction of the Upper Phase sample codes of HPMC
0h
4h
24 h
72 h
60SH-5 60SH-50 60SH-400 60SH-4000
1.00 1.00 1.00 1.00
0.677 0.767 0.858 0.95
0.53 0.532 0.548 0.735
0.527 0.527 0.527 0.527
the molar substitution (MS) of the hydroxypropoxyl group, for the purified HPMC samples are summarized in Table 1. The interfacial tensions of the 0.5 wt % aqueous solution of HPMC against the silicone oil were determined to be 19.0 mN/m, irrespective of the HPMC molecular weight. Figure 1 shows double-logarithmic plots of steady-state shear viscosities of various HPMC solutions with 0.5 wt % concentration as a function of shear rate. For the corresponding solutions, storage (G′) and loss (G′′) moduli are double-logarithmically plotted as a function of frequency (ω) in Figure 2. The highest molecular weight HPMC solution shows a shear-thinning response at higher shear rates since the 0.5 wt % concentration is well beyond the overlapping concentration. Frequency dependence of G′ and G′′ deviated from the simple fluid behavior in lower frequency regions, i.e., G′ ∼ ω2 and G′′ ∼ ω, and G′ and G′′ values in the figure almost scale with ω1.5 and ω0.7, respectively. This indicates that aqueous HPMC solutions show somewhat elastic responses, which may be responsible for the presence of the weak internal structure, attributed to the hydrophobic interactions between the substituted groups. Changes in the volume fraction of the upper phase are displayed as a function of time elapsed after the prepara-
Protective Colloidal Effects of HPMC
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Figure 3. Photomicrographs of emulsions with different molecular weight HPMC samples for just after preparation and 72 h after the preparation of the emulsions. Table 3. Preparation Conditions and Characteristics for Emulsions just after preparation sample codes of HPMC 60SH-5 60SH-50 60SH-400 60SH-4000 a
72 h after preparation
concn of HPMC (wt %)
viscositya (mPa‚s)
wt av diameter of oil droplets (µm)
std dev (µm)
wt av diameter of oil droplets (µm)
std dev (µm)
adsorbance of HPMC onto oil droplets (g/g)
0.50 4.00 0.50 1.50 1.90 0.50 0.90 1.10 0.50 0.65
1.96 41.1 3.86 28.0 54.6 9.25 32.3 62.1 33.7 75.2
26.9 18.9 35.9 32.2 28.3 43.5 40.6 35.5 48.0 42.8
7.32 4.16 9.73 9.02 5.49 12.5 11.6 8.95 12.2 12.5
31.8 20.5 37.3 31.4 27.7 43.7 40.6 38.0 44.6 42.1
7.31 3.79 9.51 5.93 5.18 9.61 8.93 8.47 12.0 11.3
1.19 × 10-4 1.18 × 10-4 1.03 × 10-4 1.04 × 10-4
At the shear rate of 12.6 s-1.
tion of the respective emulsions in Table 2. The volume fraction of the upper oil-rich (emulsified) phase in the total volume decreases with an increase in time and becomes a constant value, 0.527 for the elapsed time of 72 h, irrespective of the molecular weight of HPMC, leading to the volume fraction, φ ) 0.72 of oil in the upper phase. The value of φ ) 0.72 is larger than the volume fraction of randomly close-packed spheres, 0.635, and it is near to 0.74 for the volume fraction of the rigid spheres in the hexagonal close-packed structure. Appearances for the packing of oil droplets in the upper phase are revealed in the optical microscopic images as shown in Figure 3 and the droplets are randomly close packed rather than hexagonal. The images show little interdrop coalescence even if an increase in the time elapsed after the preparation, indicating the resulting emulsions are stabilized by the presence of adsorbed HPMC layer. The adsorbed amount of HPMC on the silicone oil as shown in Table 3 is comparable to the adsorption of HPMC onto various solid surfaces.16-18 The concentration of HPMC in the adsorbed layer should be
much higher than the dosing concentration HPMC, leading to the protective colloidal effects for the oil droplets. The higher stability of the emulsions formed in the presence of HPMC can be also confirmed by the droplet size measurements. As seen from Table 3, there are no significant changes in the weight average diameter or in the overall distribution for the oil droplets with an increase in time elapsed after the preparation of the emulsions. Thus, the resulting emulsions are stable and the only processes of importance are droplet breakage during preparation and drainage of the fluid (HPMC solution in this study) between the drops until they are close packed and the repulsion holds them apart. Thus, coalescence does not occur to any significant extent. The average diameter of the oil droplet increases with an increase in the molecular weight of HPMC at a fixed (16) Kawaguchi, M.; Kimura, Y.; Tanahashi, T.; Takeoka, J. Kato, T.; Suzuki, J.; Funahashi, S. Langmuir 1995, 11, 563. (17) Kawaguchi, M.; Naka, R.; Imai, M.; Kato, T. Langmuir 1995, 11, 4323. (18) Yonekura, K.; Kawaguchi, M. Unpublished data.
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HPMC concentration, whereas the average size of the oil droplets prepared in the presence of the same molecular weight of HPMC decreases with an increase in HPMC concentration. The effect of viscosity of the dispersion medium on the oil droplet size is quite opposite. Tanaka and Oshima obtained a power law relation between the droplet size and the viscosity of the dispersion medium for suspension polymerization of styrene19
Dp ∼ ηc-0.4
(1)
where Dp is the diameter of the droplet and ηc is the viscosity of the dispersion medium. Equation 1 indicates that the droplet size decreases with an increase in the viscosity. A similar result has been reported by Taylor, who predicted that the more viscous medium is able to bring smaller dispersed drops based on fluid mechanics for the deformation of droplets dispersed in a dispersion medium with a constant shear rate gradient.20 These results are in agreement with the HPMC concentration dependence of oil droplet size. However, the molecular weight dependence of the droplet size cannot be interpreted and we do not have any good explanation for the difference in the present time. Steady-state shear viscosities and dynamic moduli of the emulsions are displayed in Figures 1 and 2, respectively. The steady-state shear viscosities of the emulsions just after the preparation are more than 1 order of magnitude larger than those of the HPMC solutions and the emulsions show weak shear-thinning behavior. On the other hand, dynamic responses of the emulsions are different from those of the HPMC solutions: the G′ values at lower frequency ranges for the emulsions in the presence of 60SH-5 and 60SH-400 give a constant value, indicating the presence of the internal structure in the emulsions. However, the G′ is smaller than the G′′ in the entire frequency range and this shows that the emulsions just after the preparation behave in a liquid-like viscoelastic matter, which is attributed to the fact that the oil droplets are dispersed in the entire volume without phase separation. An increase in the time elapsed after the preparation of the emulsions proceeds drainage of the HPMC solution and should lead to changes in rheological properties of the emulsions. The rheological properties depend on the drainage rates which affect the time to get to a given volume fraction. As seen from Figure 1, the molecular weight of HPMC does not vary with the steady-state shear viscosities of the emulsions for the elapsed time of up to 72 h, and these emulsions show stronger shear-thinning behavior than those just after the preparation. To make clear the molecular weight dependence of viscosities of the emulsions, the relative viscosities are often taken into account. For the calculation of the relative viscosity, it is necessary to obtain the viscosity in the dispersion medium. From the amount of HPMC adsorbed on the oil as seen in Table 3, only less than 1 wt % of the dosing HPMC concentration is adsorbed. Thus, the steady-state viscosity measurements of the lower phase give almost the same shear viscosity as that of the aqueous 0.5 wt % HPMC solution, irrespective of the HPMC molecular weight. Figure 4 shows the relative viscosities as a function of shear rate. Effects of the HPMC molecular weight on the relative viscosities of the emulsions are clearly observed, and the presence of smaller molecular weight HPMC gives a higher relative viscosity. The difference in the relative (19) Tanaka, M.; Oshima, E. Kagaku Kogaku Ronbunshu 1982, 8, 734. (20) Taylor, G. I. Proc. R. Soc. London 1879, 29, 71.
Figure 4. Double-logarithmic plots of relative viscosities of emulsions as a function of shear rate: emulsions for 72 h after the preparation with 60SH-5 (0), 60SH-50 (]), 60SH-400 (O), and 60SH-4000 (4).
viscosity could relate to an effective volume fraction of oil droplets in emulsions. To determine the effective volume fraction, it is necessary to account for both the volume of the oil droplets as well as the volume of the thin film of water between the oil droplets due to the stabilizing repulsion of the adsorbed stabilizer. When such a water thin film is present, an effective volume fraction, φeff, can be given by using a water film thickness, h, and the oil droplet size, Dp2,6,9
φeff ) φ(1 + 3h/Dp)
(2)
For the oil droplets stabilized by charged surfactant solutions in which the film thickness is consistent with the Debye length of the double-layer repulsion, some values for the water film thickness were determined as a function of φ. Since HPMC is not a polyelectrolyte and no electrostatic repulsive forces occur, the water layer may be responsible for the hydration effect in this study. However, no data for the water film layer thicknesses are available for the emulsions of silicone oil dispersed in aqueous solutions of HPMC. If the value of h is fixed and is unlikely to exceed 0.1 µm, irrespective of the molecular weight of HPMC, the effective volume fraction increases with a decrease in the mass of HPMC. Thus, molecular weight dependence of the relative viscosity may be qualitatively interpreted by taking into account the effective volume fraction containing the water layer. On the other hand, emulsions for the elapsed time of 72 h show solid-like viscoelastic responses as shown in Figure 2, since the G′ value is larger than the G′′ value in the entire frequency range. Both G′ and G′′ values are almost independent of the frequency. The G′ value is 3 orders of the magnitude larger than that for the emulsions just after the preparation, whereas the G′′ value increases by 2 orders of the magnitude larger than that for the emulsions just after the preparation. Furthermore, the G′ value at a fixed frequency increases with an increase in the effective volume fraction. Though emulsions consist solely of fluids, they often behave as elastic solids. The key to the origin of the elasticity of the emulsions is considered to result from the energy stored by additional deformation of the shape
Protective Colloidal Effects of HPMC
induced by an applied strain. Several approaches have been performed to obtain the expressions of the elastic modulus as functions of φeff and the internal or Laplace pressure of the oil droplets, 2σ/Dp, where σ is the interfacial tension. The measured modulus for the polydisperse emulsions was found to vary as G′ ∼ φeff1/3(φeff - φc)2σ/Dp, where φc ) 0.712, calculated from the volume fraction of hexagonal close-packing, by Princen and Kiss.4 On the other hand, Mason, Bibette, and Weitz obtained the following relationship, G′ ∼ φeff(φeff - φc)2σ/Dp, where φc ) 0.635, the value of the random close-packing, for monodisperse emulsions.9 The emulsions in the this study, however, are not monodisperse and the value of φeff is not known, the value of G′ from the relation of G′ ∼ φeff(φeff - 0.635)2σ/Dp is approximately determined to be 58 Pa by using φeff ) 0.72, σ ) 19 mN/m, and Dp ) 40 µm. The calculated value of G′ is 1 order of magnitude less than the experimental
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value of G′, whereas the order of the G′ value with the oil droplet size is reasonably interpreted. 4. Conclusions Oil in water emulsions prepared by dispersion of silicone oil into aqueous solutions of HPMC were stable due to the protective effects of HPMC layer adsorbed on oil droplets. No changes in average size and size distribution of the oil droplets show with the time elapsed after the preparation, indicating the formation of the stable emulsions. These results indicate the difficulty of the coalescence of the oil droplet. The emulsions for 72 h elapsed after the preparation showed solid-like viscoelastic responses, and their viscosities and storage moduli increased with an increase in the effective volume fraction. LA970306S
