Langmuir 1999, 15, 1913-1917
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Dispersion and Stabilizing Effects of n-Hexadecane on Tetralin and Benzene Metastable Droplets in Surfactant-Free Conditions Keiji Kamogawa,† Mitsufumi Matsumoto,‡ Tatsuya Kobayashi,‡ Toshio Sakai,‡ Hideki Sakai,†,‡ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278-8510, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162, Japan Received June 10, 1998. In Final Form: November 30, 1998 The size distribution of metastable oil droplets in water was investigated by the dynamic light-scattering method for tetralin (1,2,3,4-tetrahydronaphthalene), benzene, n-hexadecane, and their mixtures under surfactant-free conditions. For pure tetralin, droplets appeared with multiple peak distributions, first at sizes around 5 × 102 to 2 × 103 nm, and shortly coalesced to grow discretely to 3 µm within 30 min. On the contrary, for pure n-hexadecane, droplets appeared with a single location at 40-120 nm and their sizes remained unchanged for several hours. The addition of a small amount of n-hexadecane, even at 1:1000 and 1:100 molar ratios, to tetralin significantly improved the dispersion and stability of tetralin droplets. Peculiarly, the interfacial tension of the mixed oils with water was raised with the n-hexadecane concentration. The dispersed solutions of benzene could be further stabilized (for as long as 1 month) with the addition of hexadecane. Expulsion of n-hexadecane onto the droplet surfaces and the related surface modification seem to be responsible for the stabilization effects.
Introduction Oils do not mix with water in the liquid state spontaneously. Such oil drops can be deformed and disrupted into small droplets with sufficient input of energy. This has been expected to generate smaller droplets for oils with lower interfacial tension or higher viscosity.1,2 Formation of such droplets is accompanied by an increase in the interfacial energy between these liquids.2 Hence, surfactant as an emulsifying agent is required to attain prolonged and/or steady-state stabilization of such droplets. Emulsions and microemulsions can frequently be prepared as well dispersed systems in the presence of aliphatic alcohol,3 amine, or hydrocarbon.3-9 They are called cosurfactant or coemulsifier though the role of these compounds has not been understood well at present. n-Hexadecane is also added as the third component in other colloidal assemblies such as emulsions10,11 and vesicles12 and so forth. Introduction of such an alkanewater interface is thought to provide the assembly with * To whom correspondence should be addressed. † Institute of Colloid and Interface Science, Science University of Tokyo. ‡ Faculty of Science and Technology, Science University of Tokyo. (1) Gopal, E. S. R. In Emulsion Science; Sherman, P., Ed.; Academic Press: London, 1969; Chapter 1. (2) Walstra, P. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, Chapter 2. (3) Miller, C. M.; Venkaatesan, J.; Silebi, C. A.; Sudol, E. D.; ELAsser, M. S. J. Colloid Interface Sci. 1994, 162, 11. (4) Clements, D. J.; Dungan, S. R. Colloid Surf. 1995, 104, 127. (5) Engels, T.; Forster, T.; von Rybbinski, W. Colloids Surf., A 1995, 99, 141. (6) Oh, S. G.; Shah, D. O. J. Dispersion Sci. Technol. 1994, 15, 297. (7) Adamy, S. T. J. Dispersion Sci. Technol. 1994, 15, 727. (8) Weiss, J.; Coupland, J. N.; Brathwaite, D.; McClements, D. J. Colloids Surf. 1997, 121, 53. (9) Labes-Carrier, C.; Dumas, J. P.; Mendiboure, B.; Lachaise, J. J. Dispersion Sci. Technol. 1995, 16, 607. (10) Buscall, R.; Davis, S. S.; Potts, D. C. Colloid Polym. Sci. 1979, 257, 636. (11) Davis, S. S.; Round, H. P.; Purewell, T. S. J. Colloid Interface Sci. 1981, 80, 508.
favorable surface modification through changes in dielectric properties13,14 and molecular orientation related to hydrophobic hydration15 as well as with entire improvements in hydrophobic-hydrophilic balance and so forth. Preparation of metastable oil droplets without surfactant has some advantages in the field of emulsification chemistry. The properties of the alkane-water interface will significantly affect the dynamic behavior. Such droplets are likely to eventually bring about a macroscopic phase separation, and their surface should be highly hydrophobic due to the absence of a hydrophilic layer. Recently, we have reported the transient generation of submicron droplets of some C6 molecular oils dispersed in water by ultrasonication in surfactant-free conditions.16 Although the droplets grew to particles at the micron order within several tens of minutes due to the lack of surfactant, they revealed some characteristic behavior. Their particle size distribution was not broad but multiply populated, revealing classified profiles including 101, 102, and 103 nm regions. For convenience, they are called S, M, and L classes, respectively. Benzene and fluorobenzene were able to provide all of these three classes, while hexane and cyclohexane gave only M and L classes. Cyclohexane did not generate smaller sizes of droplets than benzene, in spite of the fact that the former is more viscous than the latter. During the growing process, the population transferred from a smaller class to a larger one, instead of a gradual drift of the peak locations. From a fluorescence probe study, S-class droplets of benzene were found to continue successive evolution after dispersion.17 Particle (12) Tajima, K.; Imai, Y.; Nakamura, A.; Koshinuma, M. J. Jpn. Oil. Chem. Soc. 1997, 46, 283. (13) Dunstan, D. E. Langmuir 1992, 8, 1507. (14) Dunstan, D. E.; Saville, D. A. J. Chem. Soc, Faraday Trans. 1992, 88, 2031. (15) Michael, D.; Benjamin, I. J. Phys. Chem. 1995, 99, 1530. (16) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. J. Jpn. Oil. Chem. Soc. 1998, 47, 159.
10.1021/la9806799 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/24/1999
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Table 1. Physicochemical Properties of Oil Phase, and Change in Time Course of Averaged Drop Size Measured by Dynamic Light Scatteringa av drop size/nm chemical system
interfacial tension at 30 °C/mN‚m-1
viscosity at 30 °C/cP
solubility at 30 °C/wt %
dielectric constant
benzene tetralin n-hexadecane tetralin/n-hexadecane (10/1 molar fraction) tetralin/n-hexadecane (100/1) tetralin/n-hexadecane (1000/1)
33.2 25.8 52.9 39.5 35.3 34.3
0.564 1.715 2.427 1.685 1.565 1.603
1.79 × 10-1 16 3.97 × 10-3 22 2.92 × 10-14 8
2.27 2.67 2.05
a
the initial drop size
after 30 min
after 60 min
after 180 min
618.0 90.4 390.1 308.6 618.0
3040 90.8 440.3 331.6 629.0
91.6 540.0 386.5 676.0
94.3 576.5 265.3 1630
Concentration of dispersed oil: 1 mmol dm-3.
growth for such slightly water-soluble aromatic oils may be interpreted in terms of enhanced molecular diffusion, that is, Ostwald ripening18 prior to creaming, while that for water-insoluble aliphatic oils may be interpreted in terms of coalescence. From our observations, aliphatic oil droplets are presumed to resist growing even at the size of the M class. This behavior would be related with the stability of visible droplets characteristically known for hexadecane19,20 and the coemulsifying effects. Therefore, it is of importance to ascertain whether the droplet formation of higher aliphatic oils such as n-hexadecane prevents coalescence and involves the above-mentioned coemulsifying effect or not. Experimental Section Materials. Tetralin (1,2,3,4-tetrahydronaphthalene), benzene, and n-hexadecane at GR grade from Tokyo Kasei Co., Ltd. were used as received. Distilled and deionized water at injection grade purchased from Ohtsuka Pharm. Co., Ltd. was also used without further purification. Methods. Tetralin, benzene, and hexadecane were volumetrically weighed at the molar ratios 10:1, 100:1, and 1000:1 to hexadecane and mixed before dispersion. A volumetrically weighed amount of single or mixed oil at the fixed concentration of tetralin or benzene 1 or 40 mmol/L, respectively, was gently mixed with water in a 50 mL flask, and then the mixture was irradiated in an ultrasonic cleaning bath (Bransonic 220, 125W, Smith Kline Company) for 8 min at 30 °C. The exposure time was selected upon comparison with other data to satisfy a sufficient yield of smaller droplets. Immediately after dispersion, the sample was transferred to a cuvette for dynamic lightscattering measurements. A submicron particle analyzer (System 4700, Malvern Instrument) was used to evaluate the size distribution of oil droplets. Dispersed samples were not diluted further for the measurement. The particle size distribution was analyzed and represented in the number percentage mode. This is because the mode was able to give the diameters of standard latex particles, consistent with those determined with an electron microscope.16 The mean diameter was calculated from the number percent distribution. Interfacial tension values were also recorded for mixtures of tetralin and n-hexadecane, prepared at several volume fractions. The mixture was subjected to further treatment with a Vortex mixer before the measurement with a Wilhelmy type interfacial tension meter (Model CBVP-A3, Kyowa Interfacial Science Co., Ltd.). Viscosity was measured for the mixed oils with a Viscomate VM-150 (TOKIMEC INC.) which was thermostated at 30 °C. Samples were treated with a Vortex mixer and then kept at 30 °C for 30 min before measurements. Data were averaged for 10 runs.
Results and Discussion n-Hexadecane and tetralin are interesting oils with regard to preparation of oil droplets of small sizes under (17) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. In preparation for publication. (18) Tayler, P. Colloids Surf., A 1995, 99, 175. (19) Birdi, K. S. Colloids Surf., A 1997, 123, 543. (20) Deshikan, S. R.; Papadopoulos, K. D. J. Colloid Interface Sci. 1995, 174, 302.
surfactant-free conditions. In our previous report,16 aliphatic oils such as n-hexane and cyclohexane provided only M-class droplets in the 102 nm region. The reason for this was thought to be due to their hydrophobicity being higher than that of aromatic oils. Higher hydrophobicity related with lower solubility of oil in water leads to an increase in the interfacial tension between the oil and water. This brings about an increase in the Gibbs free energy for surface area change, according to eq 1.2
∆Gs ) γ∆A
(1)
where ∆A denotes the increment of surface area. As can be seen from eq 1, a higher ∆Gs is expected for oils having higher γ. For instance, the γ values of n-hexane and cyclohexane are 50.80 and 50.59 mN/m, respectively, while the value for benzene is as low as 35.0 mN/m. For the formation of spherical droplets of radius r with invariant γ, the surface energy ∆Gs would compete with the expense of internal energy -Vm∆P, where Vm is the molar volume of oil. An unstable equilibrium can be established at the force balance as follows
∂(∆Gs - V∆P)/∂r ) 0
(2a)
∂2(∆Gs - V∆P)/∂2r < 0
(2b)
Equations 2a and 2b give the Laplace pressure ∆P ) 2γVm/r. This means that the droplets are thermodynamically unstable to grow larger via fusion on coalescence or molecular dissipation on Ostwald ripening and so forth. The droplet size after homogenization has been considered to be smaller for fluids with higher viscosity (η) and lower γ,1 where η varies much from η ) 0.313 of n-hexane to η ) 0.9 of cyclohexane while γ does not.21 Cyclohexane and benzene (η ) 0.564) may generate smaller droplets than n-hexane in that order. Apparently, however, cyclohexane and n-hexane gave submicron-sized droplets larger than those of benzene.16 Other factors, such as molecular shape, solubility, and so forth, should also be responsible for the initial size. Table 1 shows the physicochemical properties of various oils and the stability of their dispersions. From the above, oils with low interfacial tension and appreciable solubility are expected to give smaller droplets in the crude extent. Evolution is a process of gradual change that takes place over a long time. If a lower value of γ is the reason for the generation of S-class droplets, then n-hexadecane with γ ) 53.77 mN/m would not give S-class droplets in the present conditions. Tetralin also has a higher hydrophobicity, and the solubility was found to be ∼0.3 mM with the spectrophotometric method,22 (21) Dean, J. A. Lange’s Handbook of Chemistry, 14th ed.; McgrawHill Inc.: New York, 1992. (22) Kobayashi, T.; Kamogawa, K.; Sakai, H.; Abe, M. In preparation.
Tetralin and Benzene Metastable Droplets
Figure 1. Size distribution of oil droplets at various times at 30 °C: (a) pure tetralin (1 mM); (b) pure n-hexadecane (1 mM).
much smaller than that for benzene, as suggested from the molecular structure. Therefore, S-class droplets are also hardly expected for tetralin. The size distribution of oil droplets was measured by the dynamic light-scattering method. As shown in Figure 1a, just after ultrasonication, the distribution of tetralin droplets peaked at 6 × 102 to 7 × 102 nm, which is for M-class droplets, being accompanied by an envelope from 5 × 102 to 2 × 103 nm. The envelope then disappeared to give a new peak at 3 × 103 nm for L-class droplets in 20 min. Therefore, the multiple peak locations are consistent with our previous data22 and analogous with those of C6 oils.16 On the other hand, as seen in Figure 1b, nhexadecane showed a single distribution around 40-120 nm for at least 1 day (1440 min). Moreover, the droplet size was as small as nanosize, observed for the benzene/ water mixture.16 The generation of S-class droplets of n-hexadecane is contrary to the expectation from the C6 oils. Instead of smaller γ, the larger viscosity might be essential, as has been derived for viscous force.2 At present, we feel that there are two types of oils giving S-class droplets, one with low γ (such as benzene) and the other with higher η (such as n-hexadecane). The superior dispersity seems to be essential for the alkane-water interface. Dunstan et al. found induced eletrophoretic mobility for solid docosane particles suspended in an aqueous salt solution.13,14 They proposed the idea of ion capture at the surface, resulting from a dielectric interaction at the hairy interface. Recently, surface charge on the gas-water interface was explained by Schechter et al.,23 in terms of (23) Schechter, R. S.; Graciaa, A.; Lachase, J. In preparation for publication.
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Figure 2. Size distribution of oil droplets at various times at 30 °C: (a) tetralin (1 mM) + n-hexadecane (0.01 mM); (b) tetralin (1 mM) + n-hexadecane (0.001 mM).
Figure 3. Interfacial tension (γ) for tetralin-water boundary at various contents of n-hexadecane (30 °C).
dielectrically induced unbalance between H+ and OH- on the interface. These effects may also be responsible for the noncoalescence behavior of the hexadecane drops.20 The dielectric constant of n-hexadecane is estimated to be 2.04-2.06 from the data for the octane-dodecane series21 or 2.05 from the ) n2 relationship. It is smaller than 2.67 of tetralin and 2.27 of benzene, and hence, it suggests a negative surface charge on the n-hexadecane droplets and their excellent stabilization. Figure 2a indicates the size distribution of droplets of tetralin (1 mmol/L) mixed with n-hexadecane (0.01 mmol/ L; molar ratio is 100:1) in water, as a function of the time elapsed after ultrasonic dispersion. The size distribution of droplets of the mixed oil remained in the region 1 × 102 to 1 × 103 nm for 3 h, differing from that of pure tetralin
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Figure 4. Photographic recording of droplet stabilization for n-hexadecane/benzene mixture in water at 30 °C.
droplets (Figure 1a). To some extent the same behavior was observed for tetralin (1 mmol/L)/hexadecane (0.001 mmol/L) at a molar ratio of 1000:1, as shown in Figure 2b. For the addition of a small amount of n-hexadecane, the particle size remained at 400-600 nm for 50 min, and then L-class particles appeared later. Temporal changes in the mean droplet size were summarized in Table 1 for various oil droplets and times. The size of the tetralin droplets grew quickly. In the case of the 1000:1 mixture, although S-class droplets were not generated under these conditions, the size of the droplets
remained constant at around 600 nm for 50 min. In the cases of 100:1 and 10:1 mixtures, the particle size was kept below 500 nm for a long time. Apparently, the viscosity differences among the oils are negligibly small compared to the size changes. These results indicate that highly hydrophobic nhexadecane serves as a kind of stabilizer for M-class droplets of tetralin even at the 1000:1 ratio and in the absence of surfactant. In addition, at higher fractions n-hexadecane stabilized S-class tetralin droplets, which are hardly distinguished in the size distribution due to
Tetralin and Benzene Metastable Droplets
their small weight. Such S-class droplets of tetralin seem to be generated during the dispersion process, even though they should later undergo fast coagulation and coalescence. These facts raise a question whether hexadecane plays the role of an emulsifier-like surfactant or not. To answer this question, it is necessary to investigate the surface activity of n-hexadecane at the tetralin-water interface. If the molecule is attractively adsorbed at the interface just like surfactant, the interfacial tension should decrease by the addition of n-hexadecane to tetralin. Figure 3 shows the interfacial tension (γ) for the tetralin-water boundary at various contents of hexadecane. Starting from the value of 25.8 mN/m in the absence of n-hexadecane, the interfacial tension increased by 4.0 mN/m with the addition of 4 × 10-4 mol/L n-hexadecane. The value continued to increase until 0.1 mol/L nhexadecane was mixed with 1 mmol/L tetralin. Beyond this concentration, the interfacial tension was almost invariant at around 40 mN/m to give a plateau level. Peculiarly enough, the value is still smaller than the value of 72.6 mN/m reported for water.24 Finally it approached the value for n-hexadecane with its further addition. The initial rise in the interfacial tension at low content of hexadecane is quite peculiar and characteristic. It is in contrast to the current view of energetic stabilization of an o/w interface. As is found everywhere, addition of surfactant inevitably and profoundly decreases γ for the oil-water interface.25 Therefore, the observed increase of γ upon addition of n-hexadecane would mean expulsion of the molecule from the interface, and the value of γ would stay at the level of pure tetralin. This is not the case in our observation. Thus, n-hexadecane seems to be concentrated on the surface but not as surfactant. The S-shaped increase and the plateau for γ in Figure 3 indicate reconstruction of the interface at a certain contribution of n-hexadecane molecules. The expulsion of hexadecane onto the surface may result from the insufficient miscibility between them. Mixing of benzene and alkane oils such as benzene-hexadecane was often endothermic, giving positive free energy at these conditions.26 This would not necessarily apply for the (24) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1986; Vol. 1, p 233. (25) Ito, A.; Kamogawa, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935.
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interfacial region, where the low molecular density and specific orientation26 would reduce tetralin-tetralin and water-water attraction. Since the viscosity change upon mixing was small, the effects of n-hexadecane should be significant for Ostwald ripening or droplet-droplet repulsion. Kabal’nov et al. discussed Ostwald ripening for two-component droplets,27 for which a decrease in the growth rate has been found.10 According to his theory, ideally mixed two-component droplets including one soluble oil with solubility in water c01 and one poorly soluble oil with solubility in water c02 can reach an equilibrium as to the competition between Ostwald ripening and Raoult’s effect, as long as the initial fraction of the insoluble component in the droplet, χ02, is larger than c02/c01. For a tetralin-hexadecane droplet, the stabilizing effect was observed above 1 × 10-3, which is agreeable in tendency with c02/c01 ∼ 3 × 10-10. The discrepancy may be related with acceleration of molecular diffusion in the aggregated droplets16 and the inhomogeneous structure of the mixed droplets, which are not included in Kabal’nov’s model.28 From the above, excellent stabilization is expected for benzene droplets, since benzene has higher solubility in water than tetralin and poor miscibility with n-hexadecane. Figure 4 is a typical photographic recording of droplet stabilization for n-hexadecane/benzene mixtures in water. Although benzene dispersed in water underwent complete creaming within 5 h, the 100:1 mixture (40 mmol/L benzene/0.40 mmol/L n-hexadecane) remained dispersed well for at least 1 month. Since the stabilization was effective for as long as 1 month, the effect seems to be significant also in the ordinary mixed-oil emulsions stabilized with surfactants. In the present procedure, however, the stabilizer is dissolved in the dispersed oil phase; that is, n-hexadecane cannot be classified as a surfactant from the phase view as well as the hydrophobic nature and the viscosity rise. A future report will deal with the effects in more detail. LA9806799 (26) Aveyard, R. J. J. Colloid Interface Sci. 1975, 52, 621. (27) Jain, D. V. S.; Gupta, V. K.; Lark, B. S. J. Chem. Thermodyn. 1973, 5, 451. Jain, D. V. S.; Lark, B. S. J. Chem. Thermodyn. 1973, 5, 455. Diaz Pena, M.; Menduina, C. J. Chem. Thermodyn. 1974, 6, 387. (28) Kabalnov, A. S.; Peertzov, A. V.; Schukin, E. D. Colloid Surf. 1987, 24, 19.
