© Copyright 1998 American Chemical Society
JUNE 9, 1998 VOLUME 14, NUMBER 12
Letters Preparation of Stable Silicone Oil Emulsions in the Presence of Hydroxypropyl Methyl Cellulose Kenji Yonekura, Kazuhisa Hayakawa,† Masami Kawaguchi,* and Tadaya Kato Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan, and Specialty Chemical Research Center, Shin-Etsu Chemical Co., Ltd, 28-1 Nishifukushima, Kubiki, Niigata 942, Japan Received December 22, 1997. In Final Form: March 9, 1998 Oil-in-water emulsions were prepared by dispersion of various silicone oils in an aqueous solution of hydroxypropyl methyl cellulose (HPMC) as a function of mixing time. The mixing time of 10 s leads to a stable emulsion, indicating that adsorbed HPMC chains act as a good protective agent for the preparation of silicone oil emulsions. The resulting emulsions were characterized by combining size measurement and rheology techniques. The size of the oil droplets showed much change with the mixing time: the size decreased with an increase in the mixing time, and after 20 min of agitation the size showed little change. The respective emulsions showed pseudoplastic flow and solid-like viscoelastic responses; their steadystate shear viscosities and dynamic moduli increased with an increase in the effective volume fraction of oil droplets in the emulsion.
Introduction In general emulsions are metastable and nonequilibrium systems, and they are commonly encountered in various fields. The stability of the resulting emulsion is a critical property for all applications. Oil-in-water emulsions are stabilized by two main types of emulsifier agents, namely low molecular weight surfactants and water-soluble polymers.1 Among water-soluble polymers, proteins,1-3 block copolymers,4,5 polysaccharides,6,7 and surface active homopolymers8,9 should be suitable for emulsifier agents to prepare oil-in-water emulsions, since †
Specialty Chemical Research Center.
(1) Dickinson, E. Pure Appl. Chem. 1992, 64, 1721 and references therein. (2) Fang, Y.; Dalgleish, D. G. J. Colloid Interface Sci. 1993, 156, 329. (3) Dickinson, E.; Golding, M. J. Colloid Interface Sci. 1997, 191, 166. (4) Pons, R.; Solans, C.; Tadros, Th. T. Langmuir 1995, 11, 1966. (5) Pons, R.; Rossi, R.; Tadros, Th. T. J. Phys. Chem. 1995, 99, 12624. (6) Cheng, J. T. J. Macromol. Sci., Phys. 1981, B20, 365. (7) Hayakawa, K.; Kawaguchi, M.; Kato, T. Langmuir 1997, 13, 6069. (8) Meller, A.; Stavans, J. Langmuir 1996, 12, 301. (9) Howe, A. M.; Clarke, A.; Whitesides, T. H. Langmuir 1997, 13, 2617.
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 could be deduced. Thus, some of the most significant aspects of the oil-in-water emulsions stabilized by polymer chains are polymer adsorption and their rheological properties. In a previous paper,7 we reported that oil-in-water emulsions prepared by dispersion of silicone oil into aqueous solutions of hydroxypropyl methyl cellulose (HPMC) were stable and no changes in the average size and the distribution of the oil droplets was observed with the time elapsed after the preparation. The substituted methoxy and hydroxypropoxyl groups in HPMC strongly adsorb on the surface of oil droplets, and HPMC acts to lower the interfacial tension between its aqueous solution and the oil. Furthermore, such a strong adsorption can be expected to form stable oil-inwater emulsions in a short mixing time. The purpose of this letter is to form stable emulsions of silicone oil dispersed in aqueous HPMC solution within 10 s at constant agitation by changes in silicone oil viscosity. We
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3146 Langmuir, Vol. 14, No. 12, 1998
Letters Table 1. Characteristics of Emulsions
for 10 s of mixing
for 20 min of mixing
oil viscosity (mPa s)
Dp (µm)
std dev of Dp (µm)
meas G′ at 1 rad/s (Pa)
calc G′ (Pa)
Dp (µm)
std dev of Dp (µm)
meas G′ at 1 rad/s (Pa)
calc G′ (Pa)
1 10.0 100.0 1000.0
74.8 84.3 95.6 102.1
17.2 18.2 24.2 28.4
87.6 60.9 36.5 13.3
31.5 28.8 26.0 28.1
37.6 45.3 60.7 85.9
6.4 9.0 14.4 22.4
123 74.1 49.7 34.2
64.5 53.7 40.9 31.5
find that only 10 s of agitation forms an emulsion stabilized by the protective effect of adsorbed HPMC chains. This work describes the adsorption behavior of HPMC and the rheological properties of the emulsions prepared by 10 s and 20 min of mixing as a function of oil viscosity. Experimental Section Sample. One HPMC sample was obtained from Shin-Etsu Chemical Co. Ltd. It was 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 to be 388 × 103 and 2.47 by gel permeation chromatography, according to the method reported in a previous paper.10 The degrees of substitution of methoxy and hydroxypropoxyl groups were measured using the Zeisel-GC method.11 The silicone oils used in this experiment were Shin-Etsu KF96L-1, KF96-10, KF96-100, and KF96-1000, and the respective viscosities were 1, 10, 100, and 1000 mPa s at 25 °C. Water purified with a Millipore Q-TM system was used as a solvent for HPMC samples. Preparation of Emulsions. To prepare an emulsion, 12.5 g of the oil and 25 g of an aqueous solution of HPMC with 0.5 g/100 mL were mixed in a 50-mL glass bottle and agitated for desired times under 8000 rpm at 25 °C, using a Yamato Ultra Disperser with a S-25N-25F agitation shaft. The applied shear rate was calculated to be approximately 2800 s-1 from the diameters of the shaft and the bottle. With an increase in the elapsed time after the mixing, the resulting emulsions were separated into two phases: an upper emulsified phase and a lower transparent phase. The emulsions were kept at 25 °C for 72 h after the preparation. Parts of the lower phase were extracted using a syringe, and their HPMC concentrations were measured with an Ohtsuka Denshi DRM1021 refractometer. 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. The errors in the adsorbed amount were less than 10%. Optical Microscopy Measurements. Optical microscopic observation of the emulsions was performed for 3 days after preparation using a Zeiss LSM 510 confocal scanning laser microscope. An aliquot of the emulsion 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 of the emulsions were carried out for 3 days after preparation using a controlled strain Advanced Rheometrics Expansion System (ARES) instrument using a cone and plate geometry (plate diameter, 50 mm; cone angle, 0.04 rad) at 25 °C. The steadystate shear viscosity measurements were performed at the shear rate 10-2 to 103 s-1, and the dynamic modulus measurements were performed at the frequency 10-1 to 102 rad/s. Interfacial Tensions. Interfacial tensions of the 0.5 wt % aqueous HPMC solution against the oils were measured using a Shimadzu Du Nouy tensiometer at 25 °C.
Results and Discussion The interfacial tensions of the 0.5 wt % aqueous solution of HPMC against the silicone oils with the viscosities 1, 10, 100, and 1000 mPa s were determined to be 19.0, 19.6, (10) Kato, T.; Tokuya, T.; Takahashi, A. Kobunshi Ronbunshu 1982, 39, 293 (in Japanese). (11) Gobler, J. G.; Samsel, E. P.; Beader, G. H. Talanta 1962, 9, 473.
Figure 1. Volume-surface average size Dp of oil droplets of silicone oils with different viscosities as a function of mixing time: 1 mPa s (]); 10 mPa s (4); 100 mPa s (3); 1000 mPa s (O).
20.1, and 22.0 mN/m, respectively. The difference may be attributed to an increase in the surface tension of the silicone oil alone as a function of viscosity. The volume fraction of the upper phase versus the total volume for each emulsion decreases with an increase in time elapsed after the preparation of the emulsion and becomes a constant volume, 0.53, for the elapsed time of 3 days. Since there is little trace of silicone oil in the lower phase after evaporation of water, the volume fraction φ of oil in the emulsified phase can be calculated to be 0.72. 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, the volume fraction of the rigid spheres in the hexagonal close-packed structure. The resulting φ value is the same as that described in a previous paper.7 The common volume-surface average size (Dp) of oil droplets is displayed as a function of agitation time in Figure 1. The droplet size steeply decreases with an increase in the agitation time and approaches a plateau size for 20 min of agitation, irrespective of the silicone oil. The plateau droplet size increases with an increase in the oil viscosity, indicating that the higher the viscosity of the dispersed oil is, the harder the dispersion of oil is. The Dp value and its standard deviation are summarized in Table 1. On the other hand, the size-distribution of the oil droplet was evaluated as a plot of the diameter of the oil droplet in the optical microscopic image as a function of the number of droplets with the same size range. The size distribution is wider with increasing viscosity of oil, and it becomes narrower with increasing mixing time. The optical microscopic images showed little interdrop coalescence even for 10 s of mixing time, indicating the resulting emulsions are stabilized by the presence of an adsorbed HPMC layer. Furthermore, we found that (1) there is no change in the size distribution during the storage period of 3 days before the rheological measurements and (2) after the measurements there is little change in the Dp values and the size distribution, and we believe
Letters
Langmuir, Vol. 14, No. 12, 1998 3147
Figure 2. Adsorbance A of HPMC onto oil droplets prepared from silicone oils with different viscosities as a function of mixing time: 1 mPa s ([); 10 mPa s (2); 100 mPa s (1); 1000 mPa s (b).
that there is no change caused by stress exerted during the measurements. Thus, the resulting emulsions are very stable. The amount of HPMC adsorbed on the silicone oil as shown in Figure 2 increases with an increase in the mixing time and approaches an equilibrium value within 20 min, at which the oil droplet size also tends to a constant value. The smaller adsorbed amount of HPMC for 10 s of mixing should be attributed to the smaller total surface area of the oil droplets because of the larger size of the oil droplets. The magnitude of the adsorbance of HPMC expressed in mass per unit area is slightly smaller than the equilibrium one, 0.23 mg/m2, which is comparable to the adsorption of HPMC onto various solid surfaces.12-14 On the other hand, the HPMC concentrations in the lower phases ranged from 0.476 to 0.492 g/100 mL from the refractometry measurements, and these concentrations were near the dosing concentration of HPMC. The 0.5 wt % HPMC solution shows Newtonian flow behavior, and the zero-shear viscosity is obtained to be 1.0 mPa s. Steady-state shear viscosities and dynamic moduli of the emulsions prepared by 10 s and 20 min of mixing are displayed in Figures 3 and 4, respectively. The steady-state shear viscosities of the emulsions prepared by 10 s of mixing are slightly less than those for emulsions prepared in 20 min. The shear thinning exponent n obtained from the slope (n - 1) of a double-logarithmic plot of steady-state shear viscosity against shear rate could be increased with an increase in the mechanical strength of the internal structure. From Figure 3, the shear thinning exponent for the emulsions prepared by 20 min of mixing is larger than that for those prepared by 10 s of mixing. Thus, the longer mixing time is useful to prepare the more stable and stronger emulsions, leading to the larger values of G′ for the emulsions prepared by 20 min of mixing than those for the one prepared by 10 s of mixing, as shown in Figure 4. The steady-state shear viscosity at lower shear rates increases with a decrease in oil droplet size, regardless of the constant φ value, and 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 (12) Kawaguchi, M.; Kimura, Y.; Tanahashi, T.; Takeoka, J.; Kato, T.; Suzuki, J.; Funahashi, S. Langmuir 1995, 11, 563. (13) Kawaguchi, M.; Naka, R.; Imai, M.; Kato, T. Langmuir 1995, 11, 4323. (14) Yonekura, K.; Kawaguchi, M. Unpublished data.
Figure 3. Double-logarithmic plots of steady-state shear viscosities of emulsions prepared with different viscosities for 10 s (a) and 20 min (b) as a function of shear rate (γ). Symbols are the same as those in Figure 1.
droplets and the volume of the water film between the oil droplets due to the stabilizing repulsion of the adsorbed stabilizer. When such a water film is present, an effective volume fraction, φeff, can be given by using the water film thickness h and the oil droplet size Dp.15-17
φeff ) φ(1 + 3h/Dp)
(1)
The water layer in this study may be responsible for the hydration layer of the adsorbed HPMC chains. Since the polymer adsorbed layer was determined to be the diameter of a free polymer chain in the bulk solution in many systems,18 it can be assumed that the value of h is larger than the diameter of a free HPMC chain, 0.02 µm calculated from the Flory-Fox relation. The effective volume fraction should increase with a decrease in the oil droplet size from eq 1 when φ is constant. On the other hand, the dynamic responses of the emulsions are different from those of the HPMC solutions: the resulting emulsions behave as a solid-like viscoelastic matter, and the G′ values at lower frequency (15) Princen, H. M.; Aronson, M. P.; Moser, J. C. J. Colloid Interface Sci. 1980, 75, 246. (16) Mason, T. G.; Bibette, J.; Weitz, D. A. Phys. Rev. Lett. 1995, 75, 2051. (17) Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1996, 179, 439. (18) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymer at Interfaces; Chapman & Hall: London, 1993.
3148 Langmuir, Vol. 14, No. 12, 1998
Letters
increases with a decrease in oil viscosity. The G′ value for the emulsions can be expressed as a function of φeff and the Laplace pressure of the oil droplets, 2σ/Dp, where σ is the interfacial tension.16,17,19,20 According to Mason, Bibette, and Weitz,16 the following relation was obtained
G′ ∼ φeff(φeff - φc)2σ/Dp
(2)
where φc ) 0.635, the value of the random close-packing, for monodisperse emulsions. The emulsions in this study, however, are not monodisperse, and the value of G′ was determined from eq 2 by using the φeff value calculated from eq 1 by putting h ) 0.02 µm and the measured values of σ and Dp. Table 1 shows a comparison of the calculated and measured values of G′; the calculated G′ value is the same order of magnitude as the experimental value, and the order of the G′ value with the oil droplet size is reasonably interpreted. Conclusions
Figure 4. Double-logarithmic plots of storage (G′) and loss (G′′) moduli of emulsions prepared with different viscosities for 10 s (a) and 20 min (b) as a function of shear rate (ω): G′ ([) and G′′ (]) for 1 mPa s; G′ (2) and G′′ (4) for 10 mPa s; G′ (1) and G′′ (3) for 100 mPa s; G′ (b) and G′′ (O) for 1000 mPa s.
ranges give a constant value, indicating the presence of the internal structure in the emulsions. The G′ value
Only 10 s of mixing with silicone oil and an aqueous solution of HPMC leads to a stable emulsion due to the protective effects of the HPMC layer adsorbed on oil droplets. With an increase in the mixing time, the size of the oil droplets became smaller and its distribution became narrower; however, the volume fraction of oil in the emulsion showed no changes. The difference in the rheological responses between the emulsions prepared by changing the mixing time was interpreted by the effective volume fraction due to a decrease in the size of the oil droplets. LA9714014 (19) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160. (20) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427.
