Phase Behavior and Nano-emulsion Formation by the Phase

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Phase Behavior and Nano-emulsion Formation by the Phase Inversion Temperature Method Paqui Izquierdo,† Jordi Esquena,† Tharward F. Tadros,‡ Joseph C. Dederen,§ Jin Feng,† Maria J. Garcia-Celma,†,| Nu´ria Azemar,† and Conxita Solans*,† Departament de Tecnologia de Tensioactius, Institut d’Investigacions Quı´miques i Ambientals de Barcelona, CSIC, Jordi Girona 18-26, 08034-Barcelona, Spain, 89 Nash Grove Lane, Wokingham, Berkshire, RG40 4HE, United Kingdom, ICI Belgium, Uniqema Health and Personal Care, Dorpsstraat 144A, B-3078 Meerbeek, Belgium, and Departament de Farmacia i Tecnologia Farmace` utica, Universitat de Barcelona, Avda. Joan XXIII s/n, 08028-Barcelona, Spain Received February 19, 2004. In Final Form: April 22, 2004 Formation of oil-in-water nano-emulsions has been studied in the water/C h 12E h 4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 °C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (LR). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.

Introduction Nano-emulsions are a class of emulsions with small droplet diameter (typically in the range of 20-200 nm) and narrow droplet size distribution.1-3 The high kinetic stability, low viscosity, and optical transparency make them very attractive for many industrial applications, for example, in the pharmaceutical field as drug delivery systems,4-7 in cosmetics as personal care formulations,8-12 in agrochemicals for pesticide delivery,13,14 in the chemical * To whom correspondence should be addressed. Phone: 3493-4006159. Fax: 34-93-2045904. E-mail: [email protected]. † Departament de Tecnologia de Tensioactius, Institut d’Investigacions Quı´miques i Ambientals de Barcelona, CSIC. ‡ 89 Nash Grove Lane. § ICI Belgium, Uniqema Health and Personal Care. | Departament de de Farmacia i Tecnologia Farmace ` utica, Universitat de Barcelona. (1) Nakajima, H.; Tomomasa, S.; Okabe, M. Preparation of Nanoemulsions. In Proceedings of First Emulsion Conference, Paris, 1993; EDS: Paris, 1993; Vol. 1, p 1-11/162. (2) Forgiarini, A.; Esquena, J.; Gonza´lez, C.; Solans, C. Langmuir 2001, 17 (7), 2076-2083. (3) Solans, C.; Esquena, J.; Forgiarini, A.; Uso´n, N.; Morales, D.; Izquierdo, P.; Azemar, N.; Garcı´a, M. J. In Adsorption and Aggregation of Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Marcel Dekker: New York, 2002; pp 525-554. (4) Benita, S.; Levy, M. Y. J. Pharm. Sci. 1993, 82 (11), 1069-1079. (5) Calvo, P.; Alonso, M. J.; Vila-Jato, J. J. Pharm. Sci. 1996, 85 (5), 530-536. (6) Amselem, S.; Friedman, D. In Submicron emulsions in drug targeting and delivery; Benita, S., Ed.; Harwood Academic: Amsterdam, 1998; pp 153-173. (7) Bhalani, V. T.; Satishchandra, S. P. U.S. Patent 5,858,401A, 1999. (8) Sagitani, H. J. Am. Oil Chem. Soc. 1981, 58, 738. (9) Petrescu, M.; Lupulet, M.; Pintilie, G. V.; Paraschiv, S. Romanian Patent RO 108842 B1 30, 1994. (10) Simonnet, J. T. European Patent Application EP 780114 A1, 1997. (11) Nakajima, H. In Industrial Applications of Microemulsions; Solans, C., Kunieda, H., Eds.; Marcel Dekker: New York, 1997; pp 175-197. (12) Miller, D.; Henning, T.; Johannpeter, W.; Wiener, E. European Patent Application EP 1174180 A1 23, 2002.

industry as polymerization reaction media,15,16 and so forth. Although the appearance of nano-emulsions may resemble microemulsions,17,18 for example, both being transparent, there are essential differences between the two systems, namely, the thermodynamic stability of microemulsions versus the kinetic stability of nanoemulsions. One important advantage of nano-emulsions over microemulsions, from an applied viewpoint, is the lower surfactant concentration required for their formation. Nano-emulsions have also advantages over conventional emulsions or macroemulsions. Due to their small droplet size, nano-emulsions may possess higher stability against sedimentation or creaming19 because the diffusion rate can be faster than the sedimentation rate. They also can be stable against flocculation and coalescence.20 Although the key factors that allow nano-emulsion formation are not completely understood yet, it has been found that the phase transitions produced during the emulsification process are of great importance to obtain emulsions with low droplet size. Several authors have reported that emulsion properties depend on the emulsification pathway, due to the different phase transitions produced.21-25 For instance, it has been reported23 that in (13) Lee, G. W. J.; Tadros, Th. F. Colloids Surf. 1982, 5, 105-115. (14) Jon, D. I.; Prettypaul, D. I.; Benning, M. J.; Narayanan, K. S.; Ianniello, R. M. In Pesticide Formulations and Application Systems; Nalewaja, J. D., Goss, G. R., Tann, R. S., Eds.; ASTM STP 1347; American Society of Testing and Materials: West Conshohocken, PA, 1998; Vol. 18, pp 228-241. (15) Ugelstad, J.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci. 1973, 11, 503. (16) Sudol, E. D.; El-Aasser, M. S. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; Wiley & Sons: New York, 1997; pp 699-722. (17) Hoar, T. P.; Schulman, J. H. Nature 1943, 152, 102. (18) Overbeek, J. Th. G. Faraday Discuss. Chem. Soc. 1978, 65, 7. (19) Stokes, G. G. Phylos. Mag. 1857, 1, 337. (20) Deminiere, B. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, 1998; pp 261291.

10.1021/la049566h CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

Phase Behavior and Nano-emulsion Formation

order to obtain fine emulsions in the water/C16-18E12/ glycerol monostearate/cetearyl isononanoate system, the transition through a bicontinuous microemulsion (D) and/ or lamellar liquid crystalline phase was necessary. Emulsification in the above report was performed by the phase inversion temperature (PIT) method.26-28 It has been also reported that to obtain oil-in-water (O/W) nanoemulsions, with very small and uniform droplet size, by this method, it is necessary to achieve a complete solubilization of the oil phase in the microemulsion (D) phase at the starting emulsification temperature,25 an indication that droplet formation would be mainly controlled by the structure of the microemulsion phase. In a previous work,29 the influence of surfactant concentration on the formation of nano-emulsions by the PIT method, as well as on their stability, was reported. The selected system was water/C h 12E h 4/hexadecane, and it was found that nano-emulsion droplet size decreased with the increase in surfactant concentration due to the decrease in the hydrophilic-lipophilic balance (HLB) temperature and consequently interfacial tension. However, nano-emulsion stability decreased due to the increase in the Ostwald ripening rate. This instability mechanism30-32 is governed by oil molecular diffusion from the small to the large droplets as a result of the difference in solubility between different size droplets. In the present work, to establish the relationship between nano-emulsion formation and stability with the nature of the phases formed during the emulsification process, we have studied the phase behavior of the water/ C h 12E h 4/isohexadecane system. Nano-emulsions have been prepared using the PIT method, and the stability has been assessed by determining the changes in droplet size as a function of time. Materials and Methods Materials. Branched C16 alkane, isohexadecane (commercial name Arlamol HD), was obtained from UNIQEMA (Belgium) and used as received. Technical grade polyoxyethylene lauryl ether (Brij 30) was purchased from Sigma. The average ethylene oxide (EO) content, determined by proton nuclear magnetic resonance, H NMR,33 is 4.1 EO mol/mol.34 The hydrophobic chain composition, determined by matrix-assisted laser desorption/ ionization time-of-flight, MALDI-TOF, mass spectrometry,35 is 1% C8, 9% C10, 63% C12, 21% C14, and 6% C16.34 The molecule is abbreviated as C h 12E h 4 (the bars identicate that the alkyl is mainly C12 and the number of ethoxylated units is mainly 4). NaCl (purity (21) Lin, T. J.; Kurihara, H.; Ohta, H. J. Soc. Cosmet. Chem. 1975, 26, 121-139. (22) Sagitani, H. In Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker: New York, 1992; pp 259-271. (23) Wadle, A.; Fo¨rster, T.; von Rybinski, W. Colloids Surf., A 1993, 76, 51-57. (24) Esquena, J.; Solans, C. Prog. Colloid Polym. Sci. 1998, 110, 235239. (25) Morales, D.; Gutie´rrez, J. M.; Garcı´a-Celma, M. J.; Solans, C. Langmuir 2003, 19, 7196-7200. (26) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 70. (27) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1969, 30, 258. (28) Shinoda, K.; Kunieda, H. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, pp 337367. (29) Izquierdo, P.; Esquena, J.; Tadros, Th. F.; Dederen, C.; Garcı´a, M. J.; Azemar, N.; Solans, C. Langmuir 2002, 18, 26-30. (30) Lifshitz, I. M.; Slezov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (31) Wagner, C. Z. Elektrochem. 1961, 65, 581. (32) Kabalnov, A. S.; Shchuckin, E. D. Adv. Colloid Interface Sci. 1992, 38, 69. (33) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy; VCH: Weinheim, Germany, 1991. (34) Izquierdo, P. Ph.D. Thesis, Universitat de Barcelona, Barcelona, Spain, 2002. (35) Schmitt, T. Analysis of Surfactants; Surfactant Science Series, Vol. 40; Marcel Dekker: New York, 1992.

Langmuir, Vol. 20, No. 16, 2004 6595 > 99.5%) was obtained from Merck. Water was deionized and Milli-Q filtered. Methods. Phase Diagrams. The phase behavior of the water/ C h 12E h 4/isohexadecane system has been studied at a constant temperature of 25 °C and at a constant 20 wt % oil concentration, as a function of temperature. The phase diagram at 25 °C was constructed by the titration method and also by weighing all components at the final composition. In the first method, samples were prepared by weighing appropriate amounts of surfactant and oil (or surfactant and water) and adding water (or oil) gradually. After each addition, the samples were homogenized with a vibromixer and then kept at constant temperature, 25 °C. In the second method, the samples were prepared by weighing all components in ampules and after sealing them, they were homogenized with a vibromixer and then kept at 25 °C. To detect liquid crystalline phases, samples were observed under polarized light and polarized optical microscopy. The effect of temperature on the phase behavior was investigated at a constant oil concentration of 20 wt %. Oil, water, and surfactant were weighed in ampules, which were sealed. The samples were homogenized and stored at -18 °C before starting the phase behavior study. During this study, the temperature of the samples was gradually increased from 10 to 90 °C. HLB Temperature Determination. Emulsions with an oil concentration of 20 wt % and different nonionic surfactant and water (10-2 mol dm-3 NaCl was added for the conductivity determinations) concentrations were prepared by manual shaking at room temperature (∼25 °C). The conductivity of the resulting emulsions was measured as a function of temperature using a Crison 525 conductivity meter and a dipping cell (with a Pt/platinized electrode) with a cell constant of 1.02 cm-1 (25 °C). The latter was determined using standard KCl solutions. Preparation of Nano-emulsions by the PIT Method. Emulsions were produced, according to Shinoda,26-28 when the samples were brought, rapidly and with agitation, from the corresponding HLB temperature (or a slightly higher temperature) to 25 °C. To achieve fast cooling, a water/ice bath was used. The resulting systems were kept at 25 °C. Droplet Size and Stability Determinations. The mean droplet size and size distribution of the nano-emulsions were determined by dynamic light scattering (DLS), by using a Malvern 4700 photon correlation spectrometer (Malvern Instruments, Malvern, U.K.). An argon laser (λ ) 488 nm) with variable intensity was used to cover the wide size range involved. The hydrodynamic radius measurements were always carried out at a scattering angle of 90°. After the measurements, the DLS data were processed. The values of droplet size given in this work are those obtained according to the REPES analysis.36,37 Stability was assessed by measuring the droplet size as a function of time.

Results and Discussion Phase Behavior of the Water/C h 12E h 4/Isohexadecane System. Figure 1 shows the pseudoternary phase diagram at 25 °C, the temperature at which nano-emulsion properties were studied. The main features are the presence of three singlephase regions: Om, a micellar solution or water-in-oil (W/ O) microemulsion; LR, a lamellar liquid crystalline phase, as detected by polarizing optical microscopy; and D, a bicontinuous-type microemulsion that appears in a narrow range of surfactant concentrations. The rest of the diagram consists of multiphase regions, for which equilibrium was not determined, except for three two-phase regions: LR + Om, Wm + O, and Wm + LR, Wm being a micellar solution or O/W microemulsion and O, an oil phase. The phase behavior observed for this technical grade surfactant C h 12E h4 system is similar to that described for pure surfactant systems at temperatures near to the HLB temperature.38,39 (36) Jakes, J. Czech. J. Phys. 1988, B38, 1305-1316. (37) Johnsen, R. M. In Light Scattering in Biochemistry; Harding, S. E., Sttelle, D. B., Bloomfield, V. A., Eds.; The Royal Society of Chemistry: Cambridge, 1992; pp 77-91.

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Figure 1. Pseudoternary phase diagram at 25 °C of the system water/C h 12E h 4/isohexadecane. Wm, isotropic bluish liquid phase (micellar solution or O/W microemulsion); LR, anisotropic phase (lamellar liquid crystalline phase); O, isotropic liquid colorless oil phase; Om, isotropic liquid colorless phase (inverse micellar solution or W/O microemulsion); D, isotropic bluish liquid phase (microemulsion); MLC, multiphase region (equilibrium not determined).

Figure 2. Phase diagram of the system water/C h 12E h 4/isohexadecane as a function of temperature and surfactant concentration at 20 wt % oil concentration. W, isotropic liquid aqueous phase; D, isotropic bluish liquid phase (microemulsion); LR, anisotropic phase (lamellar liquid crystalline phase); O, isotropic liquid colorless oil phase.

Comparison of the phase diagram shown in Figure 1 with those of the corresponding systems with hydrocarbons with different alkyl chain lengths at the same temperature2,40 revealed that its features were closer to those of the systems with decane and dodecane than to those of the hexadecane system, an indication that isohexadecane behaves as a shorter alkyl chain length hydrocarbon. The phase behavior was also studied as a function of temperature to obtain information about the phase transitions that take place during the emulsification process by the PIT method. Figure 2 shows the phase diagram for the system at constant oil concentration (20 wt %) as a function of temperature and surfactant concentration. The three-phase region, consisting of water (W), microemulsion (D), and oil (O) phases, is formed at low surfactant concentrations and temperatures above 40 °C. This region is shifted to lower temperatures as the (38) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3 (3), 233-244. (39) Kunieda, H.; Nakamura, K.; Davis, H. T.; Evans, F. Langmuir 1991, 7, 1915-1919. (40) Forgiarini, A. Ph.D. Thesis, Universitat de Barcelona, Barcelona, Spain, 2001.

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Figure 3. Conductivity as a function of temperature in the system aq. 10-2 M NaCl/C h 12E h 4/isohexadecane at different concentrations of surfactant, S, and constant oil concentration (20 wt %). Table 1. Surfactant Concentrations and HLB Temperatures from Phase Behavior and Conductivity Determinations of the Samples in the System Water/ C h 12E h 4/Isohexadecane at 20 wt % Oil Concentration C h 12E h 4, wt %

THLB, °C (phase behavior)

THLB, °C (conductivity)

3.0 4.0 5.0 6.0 7.0

50 43 37 34 32

51 43 39 37 33

surfactant concentration increases, a typical feature for systems with technical grade surfactants or surfactant mixtures.41 A microemulsion (D) phase region exists at surfactant concentrations higher than 4.1 wt %, which is separated by a two-phase region (D + LR) at surfactant concentrations higher than 5.5 wt %. According to the bibliography,41 the HLB temperature or PIT at each surfactant concentration falls in the middle of the (W + D + O) region for the lower surfactant concentrations and in the middle of the (D + LR) region for higher surfactant concentrations. Table 1 shows the HLB temperature values at each surfactant concentration. As shown in Figure 2, the system evolves from a W/O to an O/W structure by decreasing the temperature. The transition takes place through the three-phase region (W + D + O) below 4.1 wt % surfactant concentration, while at higher surfactant concentrations, transitions through either D or D-(D + LR)-D phases take place. The transitions during the emulsification by the PIT method are reflected by the variation of conductivity as a function of temperature (Figure 3). This figure shows the plots for samples with different C h 12E h 4 concentrations. As expected, at all surfactant concentrations, the conductivity of the emulsion initially increases with the increase of temperature, reaching a maximum, and then it suddenly decreases. A second maximum in conductivity appears in samples with a concentration of surfactant above 5.0 wt %. As can be observed in the phase diagram (Figure 2), the presence of the second maximum in conductivity corresponds to the transition through the region with lamellar liquid crystalline phase present. The HLB temperature values assessed from the conductivity plots, which consist of the average temperature values at which the maximum and the minimum values of conductivity are obtained (i.e., neglecting the intermediate well-defined maximum), are also shown in Table (41) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107 (1), 107-121.

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1. The HLB temperature values determined by conductivity agree with those assessed from the phase behavior for samples with 3-4 wt % surfactant concentrations (samples belonging to the three-phase region (Figure 2)) but are slightly higher for higher surfactant concentrations. The HLB temperature decreases with the increase in C h 12E h 4 concentration. This reduction can be attributed to the different partition of surfactant homologues at the O/W interface depending on the total surfactant concentration.41,42 By increasing surfactant concentration, there will be more accumulation of surfactant homologues with low EO content in the oil phase, and this will result in a reduction in the HLB temperature. HLB temperature values for the system with isohexadecane are lower than those for the system with the linear alkane and closer to those for dodecane29 (the HLB temperature decreases when the oil solubility in water increases).43 Formation of Nano-emulsions in the Water/C h 12E h 4/ Isohexadecane System by the PIT Method. As mentioned in the experimental section, the nano-emulsions were prepared by rapid cooling of the samples with constant isohexadecane concentration (20 wt %) from the corresponding HLB temperature to 25 °C. Figure 4 shows the experimental droplet radii (r), obtained immediately after emulsification, as a function of surfactant concentration. The droplet size of the emulsions decreased from 80 to 29 nm with the increase in surfactant concentration from 3.0 to 7.0 wt %, respectively. This is the expected trend since the amount of surfactant determines the total interfacial area (i.e., the average size of the emulsion droplets). Figure 4 also shows the theoretical radii (solid line), r, obtained by geometrical considerations, assuming that emulsion droplets are spherical and that all surfactant molecules are at the O/W interface (eq 1).

r)

3φoilMw XFoANA

(1)

where the oil weight fraction (φoil) is 0.2, the surfactant molecular weight (Mw) is 362.6 g/mol, the density of the oil (Fo) is 0.79 g/cm3, and the area occupied by a single surfactant molecule at the oil/water interface (A), obtained from interfacial tension measurements,34 is 0.63 nm2/ molecule. X is the weight fraction of surfactant necessary for the formation of an emulsion with a droplet radius r, and NA is the Avogadro number. The experimental radii are higher than those predicted theoretically, which means that not all the surfactant is placed at the O/W interface (i.e., there is surfactant in excess). Indeed, simple calculations, from the results shown in Figure 4, indicate that the surfactant excess is 2.1 and 4.5 wt % when the surfactant concentration is 3 and 7 wt %, respectively. Considering the phase behavior (Figure 1), it is readily seen that the samples under study belong to the two-phase region (Wm + O). Therefore, it is reasonable to assume that the excess surfactant forms micellar aggregates in the aqueous phase. The difference between theoretical and experimental radii decreases with the increase in surfactant concentration. This means that the emulsification process is more effective at higher surfactant concentrations, which may (42) Marszall, L. In Nonionic Surfactants; Shick, M. J., Ed.; Surfactant Science Series, Vol. 23; Marcel Dekker: New York, 1987; pp 493-547. (43) Shinoda, K.; Kunieda, H. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, pp 337367.

Figure 4. Nano-emulsion droplet r at 25 °C as a function of C h 12E h 4 concentration in the system water/C h 12E h 4/isohexadecane, at constant oil concentration (20 wt %).

be attributed to several factors. One is the lower interfacial tension, γ, at 25 °C due to the proximity of the HLB temperature of the system to 25 °C.44,45 The different phase transitions produced during the emulsification process, as can be observed in Figure 2, can be another factor. While in samples with low surfactant concentrations (below 5 wt %) emulsification started at the three-phase region (W + D + O), experiencing phase transition to the two-phase (Wm + O) region where the O/W structure is formed, above 5.0 wt %, emulsification started in the upper D phase or in the (D + LR) region, experiencing phase transition through the lower D phase and finally to the two-phase (Wm + O) region. The emulsification from, or phase transition through, the D phase could account for the effectiveness of the emulsification process. It has been reported that the spontaneous emulsification produced within the bicontinuous microemulsion (D) phase is the key factor for the formation of small and uniform droplets.25,46,47 Nano-emulsion Stability. Droplet size was measured as a function of time to assess stability. Droplet radii increased with time, the increase being faster at higher surfactant concentrations. To determine if the main breakdown process was Ostwald ripening, the cube of the droplet radius, r3, was plotted as a function of time (Figure 5). The Lifshitz-Slezov (1961) and Wagner (1961) (LSW) theory30-32 gives the following expression (eq 2) for the rate of Ostwald ripening:

ω ) dr3/dt ) (8/9)[(C∞γVmD)/FRT]

(2)

where C∞ is the bulk phase solubility (the solubility of the oil in an infinitely large droplet), γ is the interfacial tension, Vm is the molar volume of the oil, D is the diffusion coefficient of the oil on the continuous phase, F is the density of the oil, R is the gas constant, and T is the absolute temperature. Equation 2 predicts a linear relationship between the cube of the radius, r3, and time, t. This linear dependence was observed experimentally, as shown in Figure 5, indicating that the main driving force for the instability (44) Kunieda, H.; Friberg, S. Bull. Chem. Soc. Jpn. 1981, 54 (4), 1010-1014. (45) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Surfactant Science Series, Vol. 30; Marcel Dekker: New York, 1988. (46) Friberg, S.; Solans, C. J. Colloid Interface Sci. 1978, 66, 66. (47) Solans, C.; Garcı´a-Domı´nguez, J. J.; Friberg, S. E. In Proceedings of the Jornadas del Comite´ Espan˜ ol de la Detergencia XII; Asocicio´n de Investigacio´n de Detergentes, Tensioactivos y Afines: Barcelona, 1981; pp 509-525.

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Figure 5. Nano-emulsion droplet r3 as a function of time at 25 °C in the system water/C h 12E h 4/isohexadecane at different concentrations of surfactant, S, and constant oil concentration (20 wt %). Table 2. Surfactant Concentrations and Ostwald Ripening Rates, ω, at 25 °C, of Nano-emulsions in the System Water/C h 12E h 4/Isohexadecane at 20 wt % Oil Concentration C h 12E h 4, wt %

ω, m3 s-1 × 1027

C h 12E h 4, wt %

ω, m3 s-1 × 1027

3.0 4.0

4.1 8.0

5.0 6.0

22.6 50.7

is Ostwald ripening. Table 2 shows the Ostwald ripening rate constants, ω, calculated from the slope of the linear plots. The Ostwald ripening rate, ω, increases with the increase in surfactant concentration from 4.1 × 10-27 m3 s-1 (at 3.0 wt %) to 50.7 × 10-27 m3 s-1 (at 6.0 wt %). The nano-emulsion with 7.0 wt % surfactant concentration (not plotted in Figure 5) showed the lowest stability, experiencing a creaming within 8 h, and its Ostwald ripening rate could not be determined. The theoretical Ostwald ripening rate of the system is 0.96 × 10-27 m3 s-1. It was calculated assuming the following: (a) the molecular solubility of isohexadecane was that of dodecane (C∞ ) 5.2 × 10-9 mL mL-1)48 because in a previous work it was shown that isohexadecane behaves close to dodecane;29 (b) a correction coefficient49 equal to 2, for the volume fraction φ ) 0.2; (c) the diffusion coefficient D reported for dodecane,50 6.1 × 10-10 m2 s-1; and (d) γ equal to γCMC ) 2 mN m-1 (from interfacial tension measurements in the isohexadecane system).34 The experimental rates are higher than but of the same order as the theoretical value at lower surfactant concentrations and 1 order of magnitude higher at higher concentrations. (48) McAuliffe, C. J. Phys. Chem. 1966, 70, 1267. (49) Enamoto, Y.; Kawasaki, K.; Tokuyama, M. Acta Metall. 1987, 35, 907. (50) Taylor, P.; Ottewill, R. H. Colloids Surf., A 1994, 88, 303-316.

The difference between theoretical and experimental rates could be due to factors not taken into account in the LSW theory (eq 2), such as oil transport due to the presence of surfactant aggregates, that is, micelles,51,52 and/or microemulsion droplets53 in the continuous phase, increase in Brownian motion of the droplets,54 and lowering of the interfacial Gibbs elasticity.55 The increase in the number of micelles or other aggregates in the aqueous phase may result in an enhancement of the Ostwald ripening process. In a work reported recently on the mechanisms of transport in the micellar solubilization of alkanes in oil-in-water emulsions belonging to water/SDS/aliphatic hydrocarbon systems,56 the solubilization process, controlled by interfacial transport at the droplet interface, was considered to be the main mechanism of alkane transport. In this work, the surfactant used, C h 12E h 4, is of technical grade. Therefore, the partition of the surfactant homologues at the O/W interface depends on the total surfactant concentration.41,42 The decrease in HLB temperature found with the increase in surfactant concentration indicates higher lipophilicity of the O/W interface. This means that the concentration of surfactant homologues with high EO content in the aqueous phase will increase. Therefore, the number of micelles in the aqueous phase may increase, favoring the increase in the Ostwald ripening rate. On the other hand, the decrease in droplet size with the increase in surfactant concentration may favor Brownian motion, which may result in an enhancement of the Ostwald ripening process.54 Conclusions Nano-emulsions with a droplet radius in the range of 29-80 nm have been obtained in the system water/C h 12E h 4/ isohexadecane by the PIT emulsification method at 20 wt % oil concentration and at surfactant concentrations between 3 and 7 wt %. The lowest radii were obtained for compositions in which the emulsification started in the D or (D + LR) phase regions. The decrease in nano-emulsion stability with the increase in surfactant concentration has been attributed to the increase in the oil diffusion rate, as a consequence of higher surfactant excess. Acknowledgment. Financial support by UNIQEMA is gratefully acknowledged. The authors also acknowledge the support by MCYT (Grant PPQ2002-04514-CO3-03) and “Comissionat per a Universitats i Recerca, Generalitat de Catalunya” (Grant 2001SGR-00357). LA049566H (51) Kabalnov, A. S. Langmuir 1994, 10, 680. (52) Taylor, P. Colloids Surf., A 1995, 99, 175-185. (53) Taisne, L.; Cabane, B. Langmuir 1998, 14, 4744-4752. (54) Kabalnov, A. S.; Makarov, K. N.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1990, 138 (1), 98-104. (55) Walstra, P. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1996; Vol. 4, pp 1-62. (56) Dugan, S. R.; Tai, B. H.; Gerhardt, N. I. Colloids Surf., A 2003, 216, 149-166.