Dispersion and Stabilization in Water of Droplets of Hydrophobic

The addition of PS was found to remarkably improve the dispersion stability of Bz SFE, and the turbid dispersion state and 200−300-nm droplet sizes ...
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Langmuir 2003, 19, 4063-4069

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Articles Dispersion and Stabilization in Water of Droplets of Hydrophobic Organic Liquids with the Addition of Hydrophobic Polymers Keiji Kamogawa,†,‡ Naoko Kuwayama,§ Toshiyuki Katagiri,§ Hidetaka Akatsuka,§ Toshio Sakai,§ Hideki Sakai,‡,§ and Masahiko Abe*,‡,§ El. & Sec. Ed. Bureau, Ministry of Education, Sports, Culture, Science and Technology, 3-2-2 Kasumigaseki, Chiyoda, Tokyo 100-8959, Japan, Institute of Colloid and Interfacial Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku, Tokyo 162-8601, Japan, and Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received August 28, 2002. In Final Form: February 4, 2003 An attempt was made to improve the dispersion stability of surfactant-free oil-in-water emulsions (SFEs) with the addition of hydrophobic polymers on the basis of a new hydrophobic approach for dispersion. Benzene (Bz) and cyclohexane (Cy), both with added polystyrene (PS), were dispersed in water. The addition of PS was found to remarkably improve the dispersion stability of Bz SFE, and the turbid dispersion state and 200-300-nm droplet sizes remained unchanged for more than 1 year. Contrary to a preliminary droplet growth observed for PS/Bz, the initial droplet size in PS/Cy SFE was 100 nm, which could not be attained in Cy SFE and showed no change for a long time. The data on the viscosity of the oil phase and the ζ potential of the oil droplets suggested that the increase in the oil viscosity of the PS/Bz droplets and coverage of the surface of the Cy droplets with PS reduce the rates of droplet coalescence and flocculation, respectively. When polyisobutylene (Pib), a viscous liquid polymer, was used instead of PS to disperse alkanes such as Cy and n-hexane as fine droplets in water, the droplets formed could be observed for 1 month. However, both the droplet-stabilizing and ζ-potential deepening effects of Pib varied irregularly with the change in the oil viscosity. On the basis of the stabilization effect at various Pib weight ratios and average Pib molecular weight, this irregularity was suggested to arise from Pib adsorption on the droplet surface, which depresses the droplet flocculation rate by making the surface more hydrophobic and sterically repulsive.

1. Introduction Oil-in-water (O/W) emulsion is a metastable system in which oil droplets grow and coalesce with one another, and it eventually separates into two phases, oil and water.1,2 Because the process of oil dispersion into water inevitably causes a large increase in the interfacial free energy, emulsion preparation has so far been conducted on the principle of lowering the interfacial free energy by using a surfactant. Surfactants, despite their usefulness, arouse serious concerns about environmental pollution as a result of their use in large amounts and face requests for their removal from waste water3 and synthesized polymer products for a better quality,4,5 etc. * To whom correspondence should be addressed. † Ministry of Education, Sports, Culture, Science and Technology. ‡ Institute of Colloid and Interfacial Science, Tokyo University of Science. § Faculty of Science and Technology, Tokyo University of Science. (1) Clayton, W. Theory of Emulsions, 4th ed.; Blackston Co.: New York, 1943. (2) Friberg, S. E.; Yang, J. In Emulsions and Emulsion Stability; Sjoblom, J., Ed.; Surfactant Science Series 61; Marcel Dekker: New York, 1996; Chapter 1, pp 1-40. (3) In Detergents in the Environment; Schwuger, M. J., Ed.; Surfactant Science Series 65; Marcel Dekker: New York, 1997. (4) Zhang, G.; Niu, A.; Peng, S.; Jiang, M.; Tu, Y.; Li, M.; Wu, C. Acc. Chem. Res. 2001, 34, 249. (5) Appell, J.; Porte, G.; Rawiso, M. Langmuir 1998, 14, 4409.

Meanwhile, emulsion preparation has also been achieved with the limited use of a surfactant, as referred to in soapfree polymerization based on a moderately hydrophobic oil phase.6,7 We have been interested in the surfactantfree O/W emulsions (SFEs) of hydrophobic oils and have started to study such emulsions to develop their kinetic control and application.8-15 First, we examined the behavior of SFEs of oils with a carbon atomic number of 6 and found that the emulsions are slightly metastable, characterized by two discrete droplet-size distributions and a stepwise droplet growth.8 While these droplets (6) Okubo, M.; Kusano, T. Colloid Polym. Sci. 1994, 272, 1521. (7) Xu, J.; Lie, P.; Wu, C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2069. (8) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. J. Jpn. Oil Chem. Soc. 1998, 47, 159. (9) Kamogawa, K.; Matsumoto, M.; Kobayashi, T.; Sakai, T.; Sakai, H.; Abe, M. Langmuir 1999, 15, 1913. (10) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Yokoyama, S.; Sakai, T.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 80, 41. (11) Sakai, T.; Kamogawa, K.; Harusawa, F.; Momozawa, N.; Sakai, H.; Abe, M. Langmuir 2001, 17, 255. (12) Sakai, T.; Kamogawa, K.; Abe, M. Oleoscience 2001, 1, 33 (in Japanese). (13) Kamogawa, K.; Abe, M. Encyclopedia of Surface and Colloid Science; Marcel Dekker: New York, 2002; pp 5214-5229. (14) Sakai, T.; Kamogawa, K.; Kwon, K. O.; Sakai, H.; Abe, M. Colloid Polym. Sci. 2002, 280, 99. (15) Sakai, T.; Kamogawa, K.; Nishiyama, K.; Sakai, H.; Abe, M. Langmuir 2002, 15, 1985.

10.1021/la020749i CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003

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retained the submicron size only within 0.5-1 h, a much higher dispersion stability was found for SFEs of long n-alkanes, branched-chain hydrocarbons, and their mixtures.9 In particular, SFEs of n-hexadecane and squalane were most stable, and these compounds were found to act as good stabilizers when added to SFEs of aromatic oils9,13 as if they were hydrophobic emulsifiers. It suggests a new approach for emulsification by the use of oils and additives, both being sufficiently hydrophobic. Polymers are also promising for emulsion preparation at higher oil contents because the entropy loss upon mixing will be lower than that for smaller molecules. They are added to emulsion as a stabilizer or coemulsifier, and most of them are amphiphilic.16-18 Amphiphilic block copolymers have also been studied in relation to their selfassembly formation.5,16,19,20 Furthermore, hydrophobic polymers can be good stabilizers for SFE from the viewpoint of a new hydrophobic strategy. On this basis, we will investigate the dispersing and stabilizing abilities of hydrophobic polymers for SFE in the present study. 2. Experimental Section 2.1. Materials. Benzene (Bz; Wako Pure Chemicals, reagent grade 99.5% purity) and cyclohexane (Cy; Tokyo Kasei Kogyo, reagent grade, 99.5% purity) were used as primary oils as supplied. The secondary oil used for comparison with the polymer was styrene (St; Tokyo Kasei Kogyo, reagent grade 99% purity), and the water used was distilled water purchased from Ohtsuka Pharm. Co. Poly(ethylene glycol) (PEG; Tokyo Kasei Kogyo, molecular wt 4000) as an amphiphilic polymer and polystyrene (PS; Scientific Polymers Inc., molecular wt 45 000) and polyisobutylene (Pib; Scientific Polymers Inc., molecular wts 2800 and 85 000; kind gifts from Kuraray, molecular wts 1900, 3330, 4460, 6970, 8830, and 10 850) as hydrophobic polymers were used as the additives. 2.2. Preparation of Oil-Droplet Dispersion in Water. The polymer was dissolved in Bz or Cy oil at polymer/oil weight ratios of 1:500, 1:300, 1:100, and 1:10, followed by agitation in a vortex mixer for 5 min at room temperature (MS Instrument, Vortex-2 GENIE). The polymer/oil mixture or the pure oil was added dropwise to 50 mL of water in a conical flask at the oil volume that corresponded to 100 and 10 mmol/L oil content in water, respectively, to obtain an O/W mixture. This crude O/W mixture was emulsified for 8 min with an ultrasonic washer (Smith-Kline Co., Bransonic 220, 125 W) immediately after being shaken. 2.3. Measurements. The oil-droplet-size distribution was measured by the dynamic light-scattering method (homodyne method) using a NICOMP380 ZLS (Particle Sizing Systems Co.). The light source was a diode-pump solid-state laser, and the wavelength and scattering angle were 535 nm and 90°, respectively. The measurements on the sample were performed at 30 °C in both stoppered glass and quartz cells after allowing the sample cells to stand in a thermostated holder for a certain period of time. Scattered light was detected statically for solubility measurements at 30 °C using a Submicron Particle Analyzer System 4700 (Malvern Instrument Co.), equipped with an argonion laser (Coherent Co., maximum output power 5 W, 488 nm). The turbidity of the sample was recorded at 30 °C with a Shimadzu model MPS-2000 UV-visible spectrophotometer. A portion of the sample was taken into a stoppered cubic quartz cell prior to the measurement. The interfacial tension was measured with a Kyowa model CBVP-Z Wilhelmy-type tensiometer. The densities of the oil and water were measured with a model DA-210 densitometer (Kyoto Electronic Ind. Co.), and the specific-gravity difference calculated using the density data was used to evaluate the interfacial (16) Reference 2; pp 20-21. (17) Garti, N.; Aserin, A. Adv. Colloid Interface Sci. 1996, 65, 37. (18) Blythe, P. J.; Morrison, B. R.; Mathauer, K. A.; Sudol, E. D.; El-Aasser, M. S. Langmuir 2000, 16, 898. (19) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (20) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903-1907.

Figure 1. Plots of the cube of droplet size as a function of time. (b) PEG/Bz ) 1:10. (2) PEG/Bz ) 1:100. (1) PEG/Bz ) 1:300. ([) PEG/Bz ) 1:500. (9) Bz. tension. Measurement of the oil viscosity was conducted at 30 °C with a Tokimec Visconic ED viscometer. The ζ-potential measurement was performed at 30 °C by the Doppler laser method using a NICOMP380 ZLS (Particle Sizing Systems Co.). The light source was similar to that used in the particle-sizedistribution measurement, and the scattering angle and field strength were respectively 19.8° and 5.0 V/cm. The intensity of the laser light was 10-20 mW.

3. Results and Discussion 3.1. Dispersion Characteristics of Reference Systems. First, the characteristics of Bz SFE were examined as a control for the polymer-added SFEs at the same oil content (100 mmol/L). Rapid creaming was visually observed for this SFE during the initial 3 h. Its dropletsize distribution changed with time, during which the peak shifted from ∼30 to ∼300 nm in 1 hr and then to the micrometer region within 1 day. While the peak locations were almost similar to those at low Bz content,8 the growth speed was a few times slower. Droplets of such watersoluble oils are supposed to grow continuously by molecular diffusion from smaller to larger droplets, that is, the Ostwald ripening (OR).2,21,22 Nevertheless, the size shift occurred like a transition where the shrinkage of the smaller droplets necessitated in OR was not obvious. Amphiphilic polymers have been used in emulsification, like detergents, according to the conventional strategy to decrease interfacial tension. Poly(ethylene oxide), that is, PEG of a high molecular weight, was also used to stabilize SiO2 colloidal particles, despite its short alkyl side chain.23 In the present work, we took up PEG as the simplest form of an amphiphilic water-soluble polymer and added it to the Bz/water system to compare it with the hydrophobic polymers to be studied. While the droplet peak shifted upward to around 40-100 nm upon the addition of PEG at a weight ratio of PEG/BZ of 1:10, it moved so slowly that it reached the micrometer region in 1 day. According to the Lifshitz-Slyozov-Wagner (LSW) theory on OR, the number-averaged volume 〈r(t)3〉 is expected to increase linearly with time.2,24 Figure 1 represents a rise of 〈r(t)3〉 with time (t) for Bz and PEG/Bz SFEs, where the (21) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, U.K., 1989; Vol. 1, pp 346-348. (22) Taylor, P. Adv. Colloid Interface Sci. 1998, 75, 107. (23) Killman, E.; Adolph, H. Colloid Polym. Sci. 1995, 273, 1071. (24) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35.

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Table 1. Physicochemical Properties of the Oil Phase for PEG/Bz Dispersion oil

density/ g cm-3

interfacial tension/ mN m-1

viscosity/ cP

Bz PEG/Bz ) 1:500 PEG/Bz ) 1:300 PEG/Bz ) 1:100 PEG/Bz ) 1:10

0.8685 0.8688 0.8692 0.8704 0.8874

32.0 15.8 14.7 14.2 13.0

0.594 0.520 0.532 0.571 0.746

water

0.9956

0.797

volume average is adopted instead of the number average. Volume averaging was necessary to treat 〈r(t)3〉 continuously in such a discrete growth over several size ranges. The plots for Bz SFE showed a crude linearity, giving a growth rate of about 1 × 102 nm3 s-1. This value is much smaller than the value of 3.1 × 106 nm3 s-1 obtained for a Bz/water emulsion stabilized with sodium dodecyl sulfate.25 On the other hand, the growth rate was accelerated by about 10 times up to 2 × 103 nm3 s-1 for PEG/Bz SFE at a weight ratio of 1:500, while it was decelerated at ratios of 1:100-1:10. The LSW equation predicts the growth rate to increase in proportion to the interfacial tension (γ) as long as the medium viscosity (ηm) is invariant. In the present study, the viscosity of the oil phase (ηo) was used instead of ηm to analyze the acceleration-deceleration effect. As summarized in Table 1, ηo decreased at a PEG/ Bz ratio of 1:500 and increased at a ratio of PEG/Bz of 1:10. On the other hand, γ decreased at a ratio of PEG/Bz of 1:500, though the growth rate was accelerated, while γ remained unchanged at ratios of 1:100 and 1:10, with the growth being decelerated. This suggests that ηo contributes to the activation energy of the coalescing droplet pair.26 Because PEG might be redistributed from the droplets to the medium, a comparison was also made for the preparation in which the Bz droplets are dispersed in a PEG/water solution. Opaque turbulence could be monitored for 1 day so that the overall stability was improved for the preparation. Nevertheless, the dispersion stability was not as high as was expected. 3.2. PS/Bz System. On the basis of the new strategy of hydrophobic dispersion, a higher stability will be established with the addition of hydrophobic polymers to SFEs. Among these polymers, PS would be one choice recommended for Bz because Bz is a good solvent for PS.27 A crude evaluation was completed photographically for PS/Bz SFE against Bz SFE, as shown in Figure 2. An improvement in the dispersion stability was obtained for 1:10 PS/Bz droplets so remarkably that the dispersion kept its opaque appearance for more than 1 year. Shortterm stability was also obtained with a weak diminution in the turbidity (τ), although τ decayed to 1/20 within 3 h for Bz SFE. However, the addition of the monomer St caused a negligible effect on the τ decay for St/Bz at a weight ratio of 1:100, at which PS stabilized the Bz droplets. The stabilization effects of PS demonstrate that the polymer chain plays a more important role in stabilizing the oil droplets than does the molecular affinity to Bz. The droplet size was initially 50-100 nm in PS/Bz emulsion at all weight ratios, and this was larger than that for pure Bz droplets. The droplets then grew to 200 (25) Kabalnov, A. S.; Makarovov, K. N.; Petzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1990, 138, 98. (26) Buscall, R.; Davis, S. S.; Potts, D. C. Colloid Polym. Sci. 1979, 257, 636. (27) Hahnfeld, J. L.; Dalke, B. D. Encyclopedia of Polymer Science and Engineering; Wiley: New York, 1989; Vol. 16.

Figure 2. Appearance of PS/Bz mixtures dispersed in water at a Bz concentration of 100 mM. Table 2. Physicochemical Properties of the Oil Phase for PS/Bz Dispersion oil

density/ g cm-3

interfacial tension/ mN m-1

viscosity/ cP

Bz PS/Bz ) 1:500 PS/Bz ) 1:300 PS/Bz ) 1:100 PS/Bz ) 1:10

0.8685 0.8676 0.8686 0.8703 0.8833

32.0 30.6 30.5 30.5 30.4

0.594 0.608 0.652 0.715 2.277

water

0.9956

0.797

nm and stayed unchanged for more than 1 week, and at a 1:10 ratio, they stayed unchanged for more than 1 year. Such preliminary growth can be made clearer with the averaged diameter 〈d(t)〉, calculated using the same data as above. While the average reached 340 nm at a ratio of 1:100 after a growth period of about 0.5 days, it converged more quickly to 430 nm at a ratio of 1:10. Addition of PS, therefore, accelerates the preceding growth until the averaged size reaches an invariant level. 3.2.1. ζ-Potential and Viscosity Changes in the Early Stage. The first analysis consists of correlating the droplet size with the properties of the oil or droplet surface. The droplet size obtained with mechanical disruption is known to decrease depending on the term of γmηmn, where γ is the oil/water interfacial tension and ηm is the viscosity of water.28-30 Such an empirical correlation was found between γ/ηo (m ) 1; n ) -1) and the initial droplet size in the pure-oil SFEs so far measured, though it holds with the oil-phase viscosity ηo instead of ηm.13 Because γ is almost independent of the oil species,10 oils with a higher viscosity would tend to give smaller values of γ/ηo and droplet sizes. As summarized in Table 2, PS addition raised ηo up to 4 times while it lowered γ slightly, thus leading to a γ/ηo in the range of 15-55 (mN m-1)/cP. Despite the decrease in γ/ηo, the plots of droplet size versus γ/ηo at the early stage gave a line with a (28) Gopal, E. S. R. In Emulsion Science; Sherman, P., Ed.; Academic Press: London, U.K., 1976; Chapter 1. (29) Walstra, P. In Encyclopedia of Emulsion Technology; Bechap, P., Ed.; Marcel Dekker: NewYork, 1983; Vol. 1, Chapter 2. (30) Horiuchi, T. Research and Development for Novel Emulsifiers and New Emulsifier Technology; CMC Co., Ltd.: Tokyo, 1998; Chapter 1 (in Japanese).

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Figure 5. Correlations between the reciprocal mean diameter and ζ potential of the oil droplets at PS/Bz ) 1:10.

Figure 3. Relations between the initial droplet size and γ/ηo. (9) Bz. ([) PS/Bz ) 1:500. (1) PS/Bz ) 1:300. (2) PS/Bz ) 1:100. (b) PS/Bz ) 1:10.

Figure 4. ζ potential for SFEs as a function of the mean diameter at 30 °C. (9) Bz. (b) PS/Bz ) 1:10.

negative slope, as shown in Figure 3. This tendency is similar to that for droplets of oleic acid esters obtained around 5 (mN m-1)/cP,10 but the range of γ/ηo is different. This implies that droplets of a higher PS content might have grown larger than those of a lower PS content before the size measurement. To analyze the preliminary droplet growth from the viewpoint of the interface, ζ-potential measurement was carried out. As shown in Figure 4, the addition of PS at a 1:10 ratio decreased the ζ potential of the Bz droplets to -45 mV and remained unchanged for 3 h. Thus, such a negative potential was neither effective in nor responsible for stabilizing the droplet size at the early stage. Meanwhile, the ζ potential of the pure Bz droplets rose stepwise from the initial value of -36 mV, as did that of oleic acid SFE. In the latter, the ζ-potential value approached 0 almost in proportion to 〈d(t)〉-1.10 Such reciprocal dependence implies a certain electrostatic effect geometrically induced by the droplet curvature. Following the case of oleic acid SFE, a hyperbolic curve was fitted to the ζ-potential data for pure Bz droplets:

ζ ) a/d + b

(1)

where the term b indicates a contribution from the infinitely large interface. Figure 5 reveals that this equation can be best fitted to the potential changes with constants of a ) -4.5 × 10-7 m and b ) -29.3 mV, the

latter making a dominant contribution. From the dominant contribution of the b term, the droplet surface in the micrometer region is suggested to much resemble the oil/ water interface on the macroscopic scale. If droplets of pure Bz and PS/Bz have surfaces similar to each other, their ζ-potential values would be on the same curve of eq 1, even at different diameters. Contrary to this expectation, the ζ potential of -45 mV for PS/Bz droplets clearly deviates from the fitted line in Figure 5, indicating a big difference in the surface state between the two. Negative ζ-potential values have been found for droplets of hydrocarbon oils such as tetradecane.31 Also, solid particles of docosane exhibited a value of -60 mV, which was ascribed to the adsorption of the OH- ion to the particle surface.32 An alternative idea has been presented in relation to the dielectric properties of the interface.33 According to this idea, the dielectric constant decreases at the O/W interface to destabilize the ions, but the OHion is destabilized to a lesser extent because of its ionic radius. This may result in the preferential adsorption of OH- ions to the surface layer. The evidence is given by the fact that droplets of longer-chain oils above C9 showed ζ potentials of -70 to -80 mV, while those of shorterchain oils gave ζ potentials of -40 mV.34 Because the difference parallels the ζ-potential lowering for PS/Bz SFE, the decrease in the ζ potential would be ascribable to the enrichment of the OH- ions on the PS/Bz droplet surface. At the present stage, OH- adsorption is a plausible explanation, and the mechanism is still under investigation. 3.2.2. Mechanisms of the Preliminary Growth and Long-Term Stability. The small growth rate obtained from Figure 1 suggests a certain difficulty in the application of the OR scheme to Bz SFE. Buscall et al.35 argued that the volume-averaged droplet growth rate in emulsion linearly increases with time, irrespective of whether the growth proceeds through OR or the collision and coalescence of droplets, if it involves a second-order reaction. This means there is a certain difficulty in the mechanism analysis based on the proportionality of volume growth and time. Another comparison with the shrink effect on the droplet size can be made. According to the OR scheme,36 molecular oil diffusion would take place from smaller droplets to larger ones, and the former would shrink until the Kelvin effect is balanced with the osmotic pressure depression (31) Wiacek, A.; Chibowski, E. Colloids Surf., A 1999, 159, 255. (32) Dunstan, D. E. Langmuir 1992, 8, 1507. (33) Schecter, R. S.; Graciaa, A.; Lachaise, J. J. Colloid Interface Sci. 1998, 204, 398. (34) Stachurski, J.; Michalek, M. J. Colloid Interface Sci. 1996, 184, 433. (35) Buscall, R.; Davis, S. S.; Potts, D. C. Colloid Polym. Sci. 1979, 257, 636. (36) Kabalnov, A. S.; Petzov, A. V.; Shchukin, E. D. Colloids Surf. 1987, 24, 19.

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due to PS enrichment. Contrary to this view, the size of the PS/Bz droplets was found to increase to 200 nm in the preliminary growth, instead of shrinking. The initial droplets are, therefore, hardly assigned as those in OR that promote the molecular diffusion of oil to larger droplets. Another criterion is the solubility enhancement stated by the Ostwald-Freundlich rule.37 Such enhancement was previously noticed with density and refractiveindex changes for Bz SFE at a lower oil content.8 In the present study, the oil solubility was determined more accurately with the static light-scattering method. The scattered-light intensity abruptly increased at 4 mmol/L for PS/Bz droplets, while it started to increase at 30 mmol/L for Bz SFE, in agreement with the result in the previous report. The Ostwald-Freundlich rule, however, predicts no decrease in the solubility at any droplet size. A plausible explanation for the exceedingly low solubility is that a number of Bz molecules escape from the drops, thereby depositing into the water during emulsification. Because of the difficulties mentioned above, it is worth examining the droplet growth-and-stabilization profiles in terms of the collision-fusion reaction of two oil droplets:38 k1

k2

M1 + M 1 9 8 M1-M1 98 M2 k -1

(2)

where k1 and k-1 are the rate constants of the droplet flocculation and deflocculation and k2 is the rate constant of the fusion of the flocculated droplets. This fundamental picture is convenient to describe the switching of the two extremes: the flocculation-restricted and coalescencerestricted processes. The overall reaction rate is determined by the flocculation (collision) rate if the fusion rate is high or by the fusion rate if it is low. Switching to k2 would require a variation in the activation energy for coalescence. The activation energy has been regarded as the energy necessary to break up the thin water film between two adjacent hemispherical oil droplets, and hence much attention has been paid to the aqueous phase instead of to the oil phase. However, the growth rate for the oleic acid ester droplets decreases when plotted semilogarithmically against the viscosity of the oil.10 This suggests that the activation energy is not only solely controlled by the medium but also closely related to the oil phase, possibly through a viscoelastic deformation of the oil hemispheres. The viscosity of the PS/Bz solution at a ratio of 1:10 was 4 times higher than that of pure Bz, and hence coalescence would occur as slowly as it did for n-hexadecane.13 The activation-energy scheme is in agreement with the stabilization effect of PS over a long period of time. On the other hand, because the preliminary growth was essentially independent of the oil-phase viscosity, it should correspond to a fast flocculation process (flocculationrestricted). Faster rise of the averaged diameter at higher PS content implies that PS accelerates the flocculation rate k1 but does not depress the coalescence rate k2. The acceleration would be somehow related with the dropletsurface properties. Because Bz is a good solvent for PS, the polymer would be dissolved homogeneously in a droplet, and the surface density of PS would be as low as that of the bulk PS content. This would lead to an insufficient coverage of the surface by PS, thereby inducing an attractive effect, as is observed with polymers bridging two solid particles. After the preliminary growth, however, (37) Reference 21; pp 10-11. (38) Lawrence, A. S. C.; Milles, O. S. Discuss. Faraday Soc. 1954, 18, 98.

Figure 6. ζ potential of SFEs as a function of time. (9) Cy. (b) PS/Cy ) 1:10. Table 3. Physicochemical Properties of the Oil Phase for PS/Cy Dispersion oil

viscosity/cP

oil

viscosity/cP

Cy PS/Cy ) 1:500 PS/Cy ) 1:300

0.825 0.454 0.690

PS/Cy ) 1:100 PS/Cy ) 1:10 water

0.743 1.236 0.797

the surface density of PS will increase for the reduced surface area, and hence such a bridging effect will be less favorable. 3.3. PS/Cy System. The droplet stabilizing action of PS is brought about by the high affinity of Bz to the polymer. PS would then be worth applying to stabilize the droplets of poor solvents for PS such as Cy. A rough evaluation was conducted with turbidimetry on Cy and PS/Cy SFEs. While the turbidity diminished monotonously in 3 h for the Cy system, it decayed little in 2 h, followed by a monotonous decay for the PS/Cy (1:500) system. This slow decaying stage lasted longer at a 1:10 ratio during the entire time period. Although a bimodal size distribution was observed for PS/Cy SFE at weight ratios of 1:100 and 1:10, the two peaks at 70-100 and 200-300 nm were both smaller than those for pure Cy droplets. The size distribution remained unchanged for 2 weeks even at a 1:100 PS/Cy ratio and for more than 10 months at a 1:10 ratio. Consequently, PS can be said to stabilize Cy droplets for a long time, accompanied by a decrease in the peak height. The characteristics of the PS/Cy droplets will be analyzed in terms of the surface electric properties. The ζ potential of the 1:10 PS/Cy droplets initially dropped to -59 mV and thereafter remained unchanged, while that of Cy droplets rose to -37 mV in 3 h, as shown in Figure 6. The value of -59 mV is more negative than the -44 mV for PS/Bz droplets of similar sizes, suggesting a higher hydrophobicity of the PS/Cy surface due to a satisfactory shielding by PS. As was mentioned previously, the oil-phase viscosity (ηo) can contribute to disruption of the droplets as well as depression of the coalescence rate. Although ηo varied with the PS/Cy ratio, as shown in Table 3, the variations in ηo had no relation with changes in the initial droplet size. Such a lack of correlation between ηo and the initial droplet size was also found for the long-term stability. While the droplets grew fast in the micrometer region at a 1:300 ratio and slowly at a 1:10 ratio in accordance with changes in the viscosity in PS/Cy SFE, the stabilizing action of PS was still observed at a 1:100 ratio despite the decrease in

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the viscosity. Another stabilizing mechanism seems to work independently of the oil viscosity at this PS content, in addition to the depression of the coalescence at the other PS content. The ζ-potential lowering suggests that PS modifies the droplet surface to reduce the flocculation rate k1, in contrast to the case of PS/Bz. The poor affinity between PS and Cy would expel PS onto the surface, causing PS-enriched shell formation even at low PS contents. Such a surface shell should induce repulsion among PS chains adsorbed on the different droplets. 3.4. Pib/Hydrocarbon Systems. The difference in the PS effect on Bz and Cy droplets seems to come from the difference in the coat structure, which is related to the affinity of PS to the oils. If this is the case, aliphatic polymers are expected to mix with aliphatic oils such as Cy and n-hexane (Hx) and stabilize the droplets, as found for PS/Bz SFE. In addition, viscous liquid polymers would stabilize the oil droplets as long as they make the oil viscosity higher to decelerate the rate of droplet coalescence. Pib with a high viscosity (250 cP for Pib2800; molecular wt ) 2800) was chosen as such a polymer that satisfies the above two requisites and was tested to see if it stabilizes Cy, Hx, n-decane (Dc), and n-dodecane (Dd) droplets. 3.4.1. Primary Oil Dependence. A comparison was made with respect to the viscosity among the stock Pib2800 solutions at a 1:10 polymer ratio. An increase in the viscosity was achieved for the solutions in Bz, Hx, and Dc, which gave a viscosity 2 times higher than that of the pure oil as expected, but the viscosity decreased by 20% for the Pib2800/Cy solution. These three SFEs exhibited a unimodal size distribution around 200-300 nm, though the peak size was not well correlated with the change in γ/ηo. While a preliminary growth was noticed only for Pib2800/Cy and Pib2800/Hx, the size distributions generally were stable for 1 month, except for that of Pib2800/Dc. ζ-potential measurements were performed on these SFEs to see if any irregular response to changes in the viscosity would be from the surface electric viewpoint. The initial ζ-potential value decreased from -37 mV for the Bz droplets to -50 mV for the Pib2800/Bz droplets, a change suggesting an increased hydrophobicity of the surface due to an adsorbed Pib2800 layer. On the contrary, the ζ potential remained unchanged for Pib2800/Cy droplets during the preliminary growth. Furthermore, the ζpotential value rose slightly when Pib2800 was present in the oil droplets for Dc and Dd. This ζ-potential rise is similar to the change from -40 mV for n-alkanes to -35 mV for Bz, and hence the formation of a weakly hydrated surface on the droplet is suggested. The principal role of Pib2800 would be modification of the droplet surface to induce nonviscous and nonelectrostatic stabilization. Thus, the steric repulsion among the polymer chains adsorbed on the droplet surface39 is the possible mechanism we propose. Steric repulsion consists of two contributions: one due to the osmotic repulsion among the solvated polymer chains at a moderate distance and another due to the entropic repulsion among the desolvated polymer chains at close contact.40,41 Steric repulsion will be more significant for the Pibenriched surface that results from the poor miscibility and large sizes of the polymer. It turns out to be an attractive effect for Cy and Hx droplets in the preliminary growth. Pib2800 is miscible with Cy and Hx in the droplet (39) Reference 21; pp 454-453. (40) Ortega-Vinuesa, J. L.; Martin-Rodriguez, A.; Hidalgo-Alvarez, R. J. Colloid Interface Sci. 1996, 184, 259. (41) Reference 21; pp 471-473.

Kamogawa et al.

Figure 7. Changes in the droplet peak diameter as a function of time. (9) Cy. (1) Pib/Cy ) 1:300. ([) Pib/Cy ) 1:500. (b) Pib/Cy ) 1:10. (2) Pib/Cy ) 1:100.

and remains on the droplet surface, sparsely covering it. This satisfies the condition for accelerated flocculation through polymer bridging between two approaching droplets, as discussed for PS/Bz droplets. 3.4.2. Pib/Cy Weight-Ratio Dependence. The combination of Pib2800 and Cy yielded a nonviscous feature of the droplet stabilization in a long period. As long as Pib2800 adsorption on the Cy surface is responsible for this kind of stabilization, the effect should depend on the Pib2800/Cy weight ratio. The dependence of the droplet size on the Pib2800/Cy ratio was actually found, as shown in Figure 7. The initial size became as small as 50-70 nm at the ratios above 1:300, and hence the polymer addition is effective in emulsification. The droplet stabilization after the preliminary growth was evident at the ratios above 1:100, a fact showing that the surface of the droplets is readily saturated with Pib2800 and the rate of droplet coalescence is depressed. The minimum ratio of 1:100 indicates the saturation of the surface coverage with Pib2800 at this polymer content. 3.4.3. Pib Molecular-Weight Dependence. Another test is available for variations in the molecular weight of Pib because it will also affect the affinity of Pib to Cy. From viscosity measurements, a reduction to 85% in ηo was found for the Pib2800/Cy system, while the other mixtures raised ηo depending on the polymer’s molecular weight. In photographic monitoring, Pib2800/Cy dispersion gave the highest initial turbidity, and Pib1900/Cy with a 1.2 times higher ηo yielded the most stable dispersion for 4 months. Pib85 000/Cy dispersion showed only slight turbidity, decaying fast by rapid creaming, despite its 5.3 times higher ηo. Pib2800 and Pib1900 are, therefore, the most effective in keeping the dispersion state of the Cy droplets. Thus, the nonviscous modification mechanism of the droplet surface seems to work, together with the viscousdepression mechanism, in the growth of the Pib1900/Cy droplets. Then, a ζ-potential comparison was made between the Pib2800/Cy and Pib1900/Cy droplets. While the potential stayed at -45 mV for the Pib2800/Cy droplets as shown previously, that for the Pib1900/Cy droplets remained unchanged at -34 mV. The less negative potential for the latter implies the emergence of a new surface on the Pib1900/ Cy droplets, a surface which would be hydrophilic, similar to the surfaces of the Pib2800/Dc and Pib2800/Dd droplets in the previous section. For the Pib85 000/Cy droplets, the ζ

Droplets of Hydrophobic Organic Liquids

potential lowered initially to -53 mV and then rose with time. This behavior might be related to the nonuniform mixing of the Pib85 000 molecules in the Cy droplets. 3.5. Summarizing Discussion. As revealed in the results obtained so far, the stabilization of hydrophobic oil droplets by hydrophobic polymers is achieved through two mechanisms: flocculation-rate suppression by dropletsurface modification and coalescence-rate suppression through a viscosity rise. Because the two mechanisms have different time scales for the earlier and later periods, respectively, each may oppose the other in the same dispersion. The coalescence-depression mechanism works in the PS/Bz and PS/Cy systems in which the polymer is well-dissolved and, remarkably, increases the viscosity of the oil phase, thereby retarding the droplet growth in proportion to the viscosity increase. The main retarding factor would be the activation energy for coalescence that increases with the viscosity of the internal oil phase, as schematically shown in Figure 8A. The flocculationdepression mechanism is effective in poorly miscible systems with low viscosities, such as the Pib2800/Bz system in which the originally rapid flocculation process is suppressed independently of the oil-phase viscosity. The main scheme for this would be Pib2800 enrichment on the Bz droplet surface, which makes the surface more hydrophobic and repulsive, as shown in Figure 8B. A moderate degree of affinity and a suitable weight ratio of the polymer to the oil seem to be essential for such a protective coating. Both types of droplet stabilization are kinetically given through the energy barrier toward flocculation and coalescence, instead of the energy lowering at the dispersed state. There will be mutual interference between the two mechanisms, especially when the preliminary growth observed for PS/Bz and Pib2800/Cy SFE is regarded as an acceleration effect that counters the depression effect in a long period. The polymer frames are rather sparsely distributed on the droplet surface and partially exposed to water so that they can bridge the approaching droplets

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Figure 8. Proposed mechanisms for hydrophobic polymer stabilization. (A) Accelerated flocculation through polymer bridging and the activated state of the encountered droplets upon coalescence. (B) Decelerated flocculation due to steric repulsion among the polymer chains attached on the droplet surface.

by the strong affinity of the polymer to the oil molecules. Such a bridging effect should be more significant for smaller oil droplets and polymer-oil pairs with a high affinity. While these mechanisms still remain to be studied in more detail, hydrophobic polymers including PS and Pib are, therefore, promising as a primary emulsifier for hydrophobic oils, up to a certain oil content, as long as their affinities to primary oil and the mixing weight fraction are adequately controlled. Acknowledgment. This research was supported by a Grant-in-Aid for Scientific Research (no. 12640565) from the Ministry of Education, Sports, Culture, Science and Technology in Japan. We also thank the Kuraray Company for supplying purified Pib. LA020749I