Water Droplet Formation by Temperature Change in the Water

Feb 28, 2006 - Droplet sizes of oil/water (O/W) nanoemulsions prepared by the phase inversion temperature (PIT) method, in the water/C16E6/mineral oil...
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Langmuir 2006, 22, 3014-3020

Oil/Water Droplet Formation by Temperature Change in the Water/ C16E6/Mineral Oil System D. Morales,*,†,‡ C. Solans,† J. M. Gutie´rrez,‡ M. J. Garcia-Celma,§ and U. Olsson⊥ Institut d’ InVestigacions Quı´miques i Ambientals de Barcelona (IIQAB), Consell Superior d’ InVestigacions Cientı´fiques (CSIC), Jordi Girona, 18-36 Barcelona 08034 Spain, Departament d’Enginyeria Quı´mica, Facultat de Quı´mica UniVersitat de Barcelona (UB), Martı´ i Franque´ s 1, Barcelona 08028 Spain, Departament de Gale` nica i Quı´mica Farmace` utica, Facultat de Farma` cia UniVersitat de Barcelona (UB), AVenida Joan XXIII sn, Barcelona 08028 Spain, and Physical Chemistry 1, Chemical Center, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden ReceiVed August 25, 2005. In Final Form: October 7, 2005 Droplet sizes of oil/water (O/W) nanoemulsions prepared by the phase inversion temperature (PIT) method, in the water/C16E6/mineral oil system, have been compared with those given by a theoretical droplet model, which predicts a minimum droplet size. The results show that, when the phase inversion was started from either a single-phase microemulsion (D) or a two-phase W+D equilibrium, the resulting droplet sizes were close to those predicted by the model, whereas, when emulsification was started from W+D+O or from W+D+LR (LR ) lamellar liquid crystal) equilibria, the difference between the measured and predicted values was much higher. The structural changes produced during the phase inversion process have been investigated by the 1H-PFGSE-NMR technique, monitoring the selfdiffusion coefficients for each component as a function of temperature. The results have confirmed the transition from a bicontinuous D microemulsion at the hydrophile-lipophile balance (HLB) temperature to oil nanodroplet dispersion in water when it is cooled to lower temperatures.

Introduction The production of small droplet-sized and monodisperse emulsions (e.g., nanoemulsions) using minimum surfactant concentrations and a low-energy input is attractive from both theoretical and industrial points of view. For this reason, there is a strong need for studies on low-energy emulsification methods. These methods, also called physicochemical methods,1,2 take advantage of the “spontaneous” emulsification produced when the system undergoes certain phase transitions. For instance, it is well-known that a temperature change can promote an emulsion phase inversion in water/nonionic ethoxylated surfactant/ hydrocarbon systems, which, in turn, produces very fine emulsions. This method is known as the phase inversion temperature (PIT) method3 and is based on the temperaturedependent solubility of the nonionic ethoxylated surfactants in water. At low temperatures, nonionic ethoxylated surfactants are mainly water soluble, whereas, at high temperatures, they become water insoluble. At an intermediate temperature, called the hydrophile-lipophile balance (HLB) temperature, surfactant affinity for water and oil phases is balanced, and a surfactant phase can appear.4-6 This surfactant phase (microemulsion (D) or lamellar liquid crystal (LR)) solubilizes high amounts of water (W) and oil (O) in a thermodynamically stable and transparent * Corresponding author. E-mail: [email protected]. † Consell Superior d’ Investigacions Cientı´fiques. ‡ Facultat de Quı´mica Universitat de Barcelona. § Facultat de Farma ` cia Universitat de Barcelona. ⊥ Lund University. (1) Becher, P. In Encyclopedia of Emulsion Technology; Marcel Dekker Inc.: New York, 1983; Vol. 1. (2) Binks, B. P. In Modern Aspects of Emulsion Science; The Royal Society of Chemistry: Cambridge, U.K., 1998. (3) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 70. (4) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233-244. (5) Kahlweit, M.; Strey, R.; Schoma¨cker, R.; Hasse, D. Langmuir 1989, 5, 305-315. (6) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomaecker, R. Langmuir 1988, 4, 499-511.

phase.7,8 Surfactant phases are usually employed as starting points for emulsification purposes because the two immiscible liquids are intimately dispersed in a single phase.9,10 However, the phase behavior observed at the HLB temperature is very rich, and, depending on the system composition, other phase equilibria can be found (e.g., W+D+O, W+D, D+O, W+LR+D, etc.).4 The effect of the phase equilibrium at the starting point in PIT emulsification has not been sufficiently studied.11-15 During emulsion phase inversion, changes in the solution state of the system components and changes in the microstructure occur. Small variations in the molecular solubility of the components in bulk liquids or in the system topology can produce important changes in the self-diffusion behavior of each component. It has been reported that in water/continuous droplet dispersions, (e.g., oil/water (O/W) nanoemulsions), the water self-diffusion coefficient is similar to that of pure water (on the order of 10-9 m2/s), whereas those of surfactant and oil molecules, which from droplets, are much lower (on the order of 10-14 m2/s) than those obtained by measuring these components as bulk phases. This value essentially corresponds to droplet selfdiffusion. However, in bicontinuous structures, the self-diffusion (7) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391. (8) Kahlweit, M., Strey, R.; Haase, D.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borkovec, M.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funck, Th.; Richmann, H.; Magid, L.; So¨derman, O.; Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W. Colloid Interface Sci. 1987, 118, 436-453. (9) Fo¨rster, T.; Rybinski, W. V.; Tesmann, H.; Wadle, A. Int. J. Cosmet. Sci. 1993, 16, 86-94. (10) Nakajima, H.; Tomomasa, S.; Okabe, M. In Proceedings of the 1st World Emulsion Congress, Paris, France, 1993; Vol. 1, Worshop 11, Communication no. 162. (11) Friberg, S.; Solans, C. J. Colloid Interface Sci. 1978, 66, 367-368. (12) Friberg, S. E.; Solans, C. J. Colloid Interface Sci. 1978, 66, 367-368. (13) Izquierdo, P.; Esquena, J.; Tadros, Th. F.; Dederen, C.; Garcia, M. J.; Azemar, N.; Solans, C. Langmuir 2002, 18, 26-30. (14) Min˜ana-Pe´rez, M.; Gutron, C.; Zundel, C.; Anderez, J. M.; Salager, J. L. J. Dispersion Sci. Technol. 1999, 20, 893-905. (15) Forgiarini, A.; Esquena, J.; Gonzalez, C.; Solans, C. Prog. Colloid Polym. Sci. 2000, 115, 36-40.

10.1021/la052324c CCC: $33.50 © 2006 American Chemical Society Published on Web 02/28/2006

O/W Droplet Change by Temperature Change

Langmuir, Vol. 22, No. 7, 2006 3015 R)

3(Φo + RΦo + fs(Φs - RΦo))‚ls (Φs - RΦo)

(1)

where Φo and Φs correspond to the oil and surfactant volume fractions, respectively; ls is the so-called surfactant length (the ratio between the volume of the surfactant molecule (vs) and its area per molecule (as)); R is the surfactant solubility in oil, expressed in volume fraction; and fs corresponds to the volume fraction of the lipophilic chain in the surfactant molecule, calculated by eq 2: Figure 1. Droplet model consisting of an spherical oil droplet covered by a surfactant monolayer. The oil pool solubilizes some surfactant molecules. This droplet and the C16E6 surfactant molecules at the interface are drawn in scale, so this figure represents an ∼30 nm droplet.

of oil and water molecules is similar to that of their respective pure liquids.16,17

fs )

(V h s - (nV h EO + V h OH)) V hs

(2)

The surfactant molar volume (V h S) can be obtained by knowing its density (FS) and molecular weight (MS), according to eq 3: V hS )

MS FS

(3)

In a previous paper, a systematic study on nanoemulsion formation by the PIT method in the water/C16E6/mineral oil system was reported.18 Emulsification was started from different phase equilibria at the HLB temperature, and a correlation between the phase behavior at the HLB temperature and the resulting emulsion droplet sizes was found. It was shown that the lowest droplet sizes (∼40 nm) and narrow size distributions were achieved when, at the HLB temperature, the phase behavior consisted of either a single-phase D microemulsion or a two-phase W+D equilibrium. A nanoemulsion formation mechanism was postulated consisting of a bicontinuous microemulsion being disrupted into droplets by lowering temperature. However, the droplet size values were not deeply analyzed, nor was the proposed mechanism confirmed.

The molar volumes for each of the ethoxy (V h EO) and hydroxyl (V h OH) units have been reported to be 38.8 and 8.8 cm3/mol, respectively.22 The average number of ethylene oxide units (n) is six for the surfactant used in this paper. Methods. Small-Angle X-ray Scattering (SAXS). The SAXS spectra were obtained with a KRATKY camera built with a linear detector (MBraun, Graz), an X-ray source SEIFERT ID 3000 (3.5 Kw), a vacuum pump, and a temperature control system ((0.1 °C). Samples were placed between thin mica films. The lamellar phase was identified by the characteristic pattern of the diffraction peak ratios (1:1/2:1/3...). Assuming equal weight fractions of water and surfactant, the interlayer spacing (d) of the lamellar phase is related to the surfactant volume fraction (Φs) by eq 4, which has been adapted from a previously reported equation23 to fulfill the condition of equal weight of water and surfactant:

In this work, a droplet model is considered to describe nanoemulsion structure at an interfacial level and to confirm which phase equilibria are more convenient to start emulsification to produce the smallest droplet sizes. In addition, the 1H-PFGSENMR technique was used to confirm the mechanism proposed, based on the transition from a bicontinuous microemulsion to an O/W nanoemulsion by lowering temperature.

1 1 + 1.95R R ) ‚Φs d 2ls 2ls

Experimental Section Materials. Mineral oil (F(40°C) ) 0.84 g/mL, η(20°C) ) 25-80 mPa‚s) was supplied by Merck. Technical-grade hexaethyleneglycol monohexadecyl ether surfactant, abbreviated as C 16E6, was provided by Huntsman Surface Science. Both products were used without further purification. Water was deionized and Milli-Q filtered. Droplet Model. This model considers an O/W emulsion as being formed by monodisperse spherical oil droplets stabilized by a compact surfactant monolayer (Figure 1). In this model, surfactant solubility in the oil component is also taken into account. According to the model assumptions, a theoretical droplet size can be estimated by using the following expression (eq 1), which improves a similar equation previously reported:19-21 (16) Lindman, B.; Olsson, U.; So¨derman, O. In Handbook of Microemulsion Science and Technology; P. Kumar, K. L. Mittal, Eds.; Marcel Dekker Inc.: New York, 1999; pp 309-356. (17) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344363. (18) Morales, D.; Gutie´rrez, J. M.; Garcı´a-Celma, M. J.; Solans, C. Langmuir 2003, 19, 7196-7200. (19) Evilevitch, A.; Jo¨nsson, B.; Olsson, U.; Wennerstro¨m, H. Langmuir 2001, 17, 6893-6904. (20) Leaver, M. S.; Olsson, U.; Wennerstro¨m, H.; Strey, R. J. Phys. II 1994, 4, 515-531. (21) Morris, J.; Olsson, U.; Wennerstro¨m, H. Langmuir 1997, 13, 606-608.

(4)

Density Measurements. Oil and surfactant densities were measured at 40 °C using a digital densitometer ANTON PAAR K.G. DMA 46. Determination of Surfactant Monomeric Solubility in Mineral Oil at 40 °C by Direct Phase BehaVior ObserVation. A very small water amount (0.1 wt %) was added to several surfactant/oil mixtures with surfactant concentrations between 1 and 10 wt %. Samples with surfactant concentrations below the critical micelle concentration (cmc) appear turbid, whereas those above this value were completely transparent. The surfactant monomeric solubility in the oil was taken as the average between the turbid sample with a higher surfactant concentration and the transparent sample with the lower surfactant concentration. Self-Diffusion Coefficient Determination. Self-diffusion coefficients (D) of water, surfactant, and oil molecules were obtained simultaneously by the measurement of the attenuation of their 1H NMR echo signals at several gradient field strengths, using a BRUKER DMX200 spectrometer. The NMR peaks selected were 4.7 ppm (water hydroxyl group), 3.8 ppm (surfactant methylene group in the ethoxylated chain), and 0.8 ppm (oil methyl group). An stimulated-echo pulse sequence consisting of two gradient field pulses of intensity G (0-9 T/m) and a duration δ (0.5-3 ms) separated by a time ∆ (50-100 ms), between three 90° radio frequency pulses separated by τ1 and τ2, was used. Assuming Gaussian diffusion, (i.e., random diffusion), the intensity of the spin-echo signal (I) follows an exponential decay that depends on the diffusion coefficient (D) and on the experimental constant (k) (eqs 5 and 6).24 Thus, by plotting ln(I/I0) against k, diffusion coefficients can be directly (22) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831-838. (23) Olsson, U.; Wu¨rz, U.; Strey, R. J. Phys. Chem. 1993, 97, 4535-4539. (24) Stilbs, P. Prog. Nucl. Magn. Res. Spectrosc. 1987, 19, 1-45.

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Morales et al.

Table 1. Interlayer Spacings (d) Obtained at 40 °C as a Function of Oil Concentration in the Water/C16E6/Mineral Oil System.a mineral oil concentration (wt %)

d (Å)

5 10 15 20 25 35

84.8 89.5 94.9 103.5 106.3 135.1

a

The water/C16E6 weight ratios have been fixed to 1:1.

obtained from the slope of the curves. Self-diffusion coefficients were measured at several temperatures between 40 and 60 °C. The measurements were carried out starting from an O/W nanoemulsion at 40 °C prepared by the PIT method as described in a previous paper.18 I ) I0‚exp(-kD)

(

k ) (γδG)2‚ ∆ -

(5)

δ 3

)

(6)

The self-diffusion coefficient (D) for spheres dispersed in a medium as a function of the sphere volume fraction (φspheres) follows the Quemada equation:25

(

)

φspheres 0.63

D ) D0 1 -

2

(7)

where D0 is the infinite dilution diffusion coefficient. In very diluted sphere dispersions, diffusion coefficients are related to the sphere hydrodynamic radius (Rh), the viscosity of the medium (η), absolute temperature (T), and the Boltzmann constant (kb) by the StokesEinstein equation:26 D)

kbT 6πηRh

(8)

If sphere size remains unchanged between two temperatures, the measured diffusion coefficients can be predicted by an expression obtained from a modification of the Stokes-Einstein equation:

( )

ηT1 T2 ‚ T1 ηT2

DT2 ) DT1‚

(9)

Results Determination of the Parameters Required in the Droplet Model. The surfactant solubility in oil (R) and the surfactant length (ls) parameters were obtained by SAXS measurements by monitoring the swelling by oil addition of the lamellar liquid crystalline phase formed when mixing equal weights of water and C16E6 at 40 °C.27 As the local curvature “experienced” by a surfactant molecule at the 30 nm droplet interface is essentially negligible, as schematically shown in Figure 1, it is reasonable to assume that surfactant packing will be very similar to that measured in the lamellar phase. Several samples with equal amounts of water and surfactant at different oil concentrations were prepared. The interlayer spacings (d) obtained for each first reflection peak are shown in Table 1. An increase in d values is observed when increasing the oil concentration because of the swelling of the oily domains in the lamellar phase. (25) Quemada, D. Rheol. Acta 1997, 16, 82. (26) Evans, D. F.; Wennerstro¨m, H. In The Colloidal Domain where Physics, Chemistry, Biology and Technology Meet; VCH Publishers Inc.; New York, 1994. (27) Morales, D. Ph.D. Thesis. Universitat de Barcelona (UB), 2003.

Figure 2. Inverse of the interlayer spacing at 40 °C as a function of the surfactant volume fraction in the water/C16E6/mineral oil system at different mineral oil concentrations. The water-to-surfactant weight ratio was 1:1.

According to eq 4, by plotting 1/d versus Φs (Figure 2), the model parameters R ) 0.028 and ls ) 1.96 nm have been estimated. The R value is reasonable compared to the oil solubilities of other alcohol ethoxylates.28 On the other hand, when determining the monomeric solubility of C16E6 in mineral oil at 40 °C by direct phase behavior studies, an R value of 0.031 was obtained. Although both values are very similar, the latter value is not an extrapolated value, and it is supposed to be more accurate. For this reason, it has been used in the model calculations. The density value at 40 °C found for the C16E6 surfactant, determined by densitometry, was 0.95 g/cm3. This value, together with an averaged surfactant molecular weight of 506 g/mol, gives, according to eq 3, a surfactant molar volume of 533 cm3/mol. As a result, the volume fraction of the C16E6 lipophilic moiety (fs) parameter has been estimated to be 0.55, according to eq 2. Comparison between Experimental and Theoretical Droplet Sizes. In this section, the experimental droplet sizes at 40 °C determined by dynamic light scattering (DLS) in a previous work18 have been compared with those given by the model presented above. The results are shown in Figures 3-5. In Figure 3, the experimental droplet sizes as a function of surfactant concentration at a fixed oil/water weight ratio Row of 0.2, defined as O/(O+W), are compared to those predicted by the model. Two theoretical curves have been plotted together with the experimental droplet sizes. One of them takes into account surfactant solubility in oil (i.e., R ) 0.031), while the other neglects it (i.e., R ) 0). According to the model, an increase in droplet size is expected when decreasing surfactant concentration. Experimental droplet sizes clearly follow this trend, but a good fit to the model is only observed at surfactant concentrations higher than 4 wt %. The fit is much better when taking into account the surfactant solubility in oil. However, at low surfactant concentrations (below 4 wt %), experimental droplet sizes are significantly different from the predicted ones. Moreover, when surfactant concentration decreases, theoretical sizes become more sensitive to the value of R selected. It is noteworthy that a change in the observed phase behavior at the HLB temperature is also produced close to 4 wt % surfactant concentration (Figure 3). Three-liquid phases of W+D+O are observed below 4 wt % surfactant, whereas a single D phase microemulsion appears at higher surfactant concentration. These results show that emulsification by the PIT is the most effective when the entire oil (28) Kunieda, H.; Yamagata, M. Langmuir 1993, 9, 3345-3351.

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Figure 3. Droplet size at 40 °C as a function of surfactant concentration at a constant Row ratio of 0.2 in the water/C16E6/mineral oil system. Circles: experimental values; triangles: predicted values assuming R ) 0; diamonds: predicted values assuming R ) 0.031. The corresponding phase behavior at their respective HLB temperatures is shown on the top. Dashed lines are used as a guide to the eye. The experimental droplet sizes and phase behavior data have already been published in a previous work.18

Figure 5. Droplet sizes at 40 °C as a function of water concentration at a constant Ros ratio of 0.67 in the water/C16E6/mineral oil system. Circles: experimental values; triangles: predicted values assuming R ) 0; diamond: predicted values assuming R ) 0.031. The corresponding phase behavior at their respective HLB temperatures is shown on the top. Dashed lines are used as a guide to the eye. The experimental droplet sizes and phase behavior data have already been published in a previous work.18

Figure 4. Droplet sizes at 40 °C as a function of the Ros ratio at a constant water concentration of 95 wt % in the water/C16E6/mineral oil system. Circles: experimental values; triangles: predicted values assuming R ) 0; diamond: predicted values assuming R ) 0.031. The corresponding phase behavior at their respective HLB temperatures is shown on the top. Dashed lines are used as a guide to the eye. The experimental droplet sizes and phase behavior data have already been published in a previous work.18

at an Ros ratio of 0.67, both experimental and theoretical values are the closest. Considering the phase behavior, the best fit of the model is obtained when a W+D equilibrium appears at the HLB temperature. On the other hand, when excess oil or lamellar liquid crystalline phases appear, the emulsification leads to higher droplet sizes because self-emulsification becomes more difficult. The existence of an optimum Ros in the W+D equilibrium region (i.e., Ros ) 0.67) indicates that not only the phase behavior, but also the D-phase structure and composition play a role in the most efficient emulsification. In Figure 5, the experimental droplet sizes have been compared with those given by the model in samples with a constant Ros ratio of 0.67 and different water concentrations (between 75 and 95 wt %). The model predicts no dependence between droplet size values and water concentration (horizontal lines). Experimental results followed the predicted trend. In all samples, highly monodisperse O/W nanoemulsions were obtained, showing similar droplet sizes and were very close to the predicted ones. As the spontaneous emulsification is produced into the D microemulsion phase, the final droplet sizes are not significantly affected when crossing the phase boundary between a singlephase D microemulsion and a two-phase W+D equilibrium (above 80 wt % water). A general conclusion for this section is that, when using the PIT emulsification method, the most effective emulsification (i.e., the lowest droplet sizes) is observed when the oil and surfactant components are completely solubilized at the HLB temperature in D microemulsion, independently of whether the phase behavior corresponds to a single-phase or a two-phase (W+D) equilibrium. The monodisperse O/W nanoemulsions obtained could be described by a droplet model that assumes a monodisperse spherical droplet dispersion covered by a single surfactant monolayer. On the other hand, this model does not describe emulsion structure when excess oil or lamellar liquid crystalline phases are observed at the HLB temperature, mainly because of the high polydispersity obtained. 1H-PFGSE-NMR Self-Diffusion Studies. The structural changes produced during the PIT emulsification process were studied by determining the self-diffusion of each system component as a function of temperature in a sample with 70 wt

component solubilizes in a single-phase D microemulsion before the temperature decreases. However, when the system is cooled from a W+D+O equilibria, the emulsification becomes less effective. It is reasonable to expect that the system is much more difficult to self-emulsify when an oil excess phase is present (i.e., W+D+O equilibrium). In this situation, the proposed model cannot describe the final emulsion structure. In Figure 4, experimental and theoretical droplet sizes at 40 °C have been plotted as a function of the Ros weight ratio, defined as O/(O+S), in samples with a constant water concentration of 95 wt %. The model predicts a monotonic decrease in droplet sizes when decreasing the Ros ratio. Instead, a U-shaped curve has been obtained experimentally. Droplet sizes increase either by increasing or decreasing the Ros ratio from a minimum value of 40 nm found at an Ros ratio of 0.67. Although experimental droplet size values are higher than those predicted by the model,

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Morales et al. Table 2. Diffusion Coefficients at Different Temperatures for Water (Dw), Surfactant (Ds), and Oil (Do) Molecules in the Water/C16E6/Mineral Oil System at 70 wt % Water and an Ros Ratio of 0.67

Figure 6. Phase behavior as a function of temperature for a sample with an Ros ratio of 0.67 and 70 wt % water in the water/C16E6/ mineral oil system. (Data taken from ref 18.)

temperature (°C)

Dw (m2/s)

Ds (m2/s)

Do (m2/s)

40 42 44 46 48 50 52 54 56 58 60

2.37 × 10-9 2.50 × 10-9 2.60 × 10-9 2.70 × 10-9 2.75 × 10-9 1.98 × 10-9a 1.86 × 10-9a 7.57 × 10-11a 1.00 × 10-9 4.60 × 10-10 6.04 × 10-11

2.76 × 10-12 2.88 × 10-12 3.41 × 10-12 2.80 × 10-12 3.10 × 10-13 2.80 × 10-13a 2.50 × 10-13a 1.50 × 10-13a 3.13 × 10-11 2.00 × 10-11 7.00 × 10-12

2.84 × 10-12 3.10 × 10-12 3.21 × 10-12 2.57 × 10-12 2.59 × 10-13 2.46 × 10-13a 2.60 × 10-13a 1.17 × 10-13a 4.41 × 10-11 4.04 × 10-11 5.23 × 10-11

a Not an accurate result because the obtained ln(I/I0) vs K curve is not linear.

Figure 7. Relative echo attenuation for the water signal I/I0 as a function of K at different temperatures: 40 °C (×); 42 °C (0); 44 °C (]); 46 °C (4); 48 °C (O); 50 °C ([); 52 °C (9); 54 °C (2); 56 °C (+); 58 °C (/); 60 °C (b).

% water and an Ros ratio of 0.67. A bluish transparent O/W nanoemulsion at 40 °C was prepared by the PIT method.18 Therefore, temperature was increased gradually from 40 to 60 °C. The phase behavior as a function of the temperature of this sample is shown in Figure 6. A bluish transparent O/W nanoemulsion is observed between 40 and 47 °C. The equilibrium phases observed at this temperature range consisted of a water microemulsion and excess oil phases (Wm+O). This nanoemulsion shows a high kinetic stability, remaining homogeneous during the diffusion measurement. As a result, a nanoemulsion droplet diffusion coefficient could be obtained. Upon further heating, a shear-birefringent D microemulsion region is formed between 47 and 49 °C. Next to this microemulsion region, a turbid emulsion region between 49 and 55 °C appears. Furthermore, another shear-birefringent D microemulsion is found between 55 and 57 °C, and, at higher temperatures, a turbid W/O emulsion is formed consisting of a W+Om equilibrium (almost pure water together with an oil microemulsion). The diffusion behavior in multiphase and nonhomogeneous regions (e.g., between 50 and 54 °C and between 58 and 60 °C) is very complex, and the interpretation of the diffusion curves is not straightforward. Water Self-Diffusion as a Function of Temperature. The relative echo attenuations as a function of k for the water peak between 40 and 60 °C are plotted in Figure 7. A linear fit has been obtained between 40 and 48 °C, indicating Gaussian diffusion. From their slopes, values for the water diffusion coefficients have been determined (Table 2). Within this temperature range, an increase in the water self-diffusion coefficient is observed. These values are of the same order of magnitude as those reported for pure water but slightly lower, probably due to the obstruction produced by the dispersed oil aggregates. In contrast, between 50 and 54 °C, a pronounced curvature is observed, indicating complex diffusion behavior, as previously mentioned. In addition, water diffusion decreases sharply. The presence of inhomogeneities affecting the sample viscosity can explain this decrease. At 56 °C, the water diffusion coefficient

Figure 8. Relative echo attenuation for the oil signal I/I0 as a function of K at different temperatures: 40 °C (×); 42 °C (0); 44 °C (]); 46 °C (4); 48 °C (O); 50 °C ([); 52 °C (9); 54 °C (2); 56 °C (+); 58 °C (/); 60 °C (b).

suddenly increases, again achieving the same order of magnitude as that of pure water, and then decreases at higher temperatures. Oil Self-Diffusion. The self-diffusion curves for the oil component are presented in Figure 8. Between 40 and 44 °C, a Gaussian model is followed, indicating that oil molecular diffusion is free and unrestricted. However, the obtained values are 1 order of magnitude lower than the diffusion coefficient of pure oil. This result, together with the high diffusion values of water molecules at this temperature range, suggests that oil molecules are diffusing as aggregates in a water continuous media. The increase in the diffusion coefficient of the oil aggregates when temperature is increased from 40 to 44 °C will be described below. Between 44 and 48 °C, oil diffusion coefficients are reduced 1 order of magnitude, while water diffusion coefficients remain

O/W Droplet Change by Temperature Change

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Figure 10. Oil droplet diffusion coefficients (at 40 °C) as a function of the droplet volume fraction using the Quemada equation.

Figure 9. Relative echo attenuation for the surfactant signal I/I0 as a function of K at different temperatures: 40 °C (×); 42 °C (0); 44 °C (]); 46 °C (4); 48 °C (O); 50 °C ([); 52 °C (9); 54 °C (2); 56 °C (+); 58 °C(/); 60 °C (b).

3 orders of magnitude higher. This has been interpreted as an aggregate growth phenomenon. Therefore, the suggested structure for the microemulsion observed at 48 °C corresponds to a water continuous phase with anisotropic oil aggregates. The shearbirefringence observed is produced by the shear-induced alignment of nonspheric aggregates, most probably prolates.29 The oil diffusion between 50 and 54 °C decreases dramatically, showing complex diffusion behavior similar to that which occurred with the water molecules. However, at 56 °C, a sudden increase in the oil molecular diffusion coefficient is observed. In this case, the obtained value is on the order of pure mineral oil. This result, together with the high diffusion found for water molecules at 56 °C, confirms the bicontinuous nature of the microemulsion. In the temperature range between 56 and 60 °C, oil diffusion follows a Gaussian behavior, and the diffusion coefficients are similar to those of the pure oil. These values, together with the restricted diffusion of the water molecules, suggest that the phase inversion has been produced. Surfactant Self-Diffusion. The relative echo attenuations for surfactant molecules as a function of temperature are shown in Figure 9. Between 40 and 48 °C, self-diffusion coefficients are very similar to those obtained by the oil, suggesting that they are diffusing in the same aggregate. As described in previous sections, the interpretation of the diffusion behavior in the multiphase region between 50 and 54 °C is not straightforward. A phase inversion to a water-in-oil emulsion is produced between 56 and 60 °C. Considering a homogeneous water droplet dispersion in oil medium, similar diffusion coefficients for water and surfactant molecules should be found. However, experimental results revealed that surfactant molecular diffusion is lower than that (29) Olsson, U. Lund University, Lund, Sweden. Personal communication, 2003.

Figure 11. Diffusion coefficient for the oil droplets as a function of temperature. The dotted line corresponds to the values predicted by eq 9, assuming a constant droplet structure.

observed for the water. The reason for that could be the inhomogeneity of the emulsion sample. A concentrated emulsion starts to form (70% internal phase), and no sample agitation occurs inside the NMR magnet. Oil Droplet Diffusion at 40 °C as a Function of Droplet Volume Fraction. The diffusion of nanoemulsion droplets has been studied as a function of droplet volume fraction at a constant temperature (i.e., 40 °C). The reduction of the droplet diffusion coefficient is related to the droplet volume fraction by the Quemada equation (eq 7). In Figure 10, oil diffusion coefficients at 40 °C have been plotted as a function of the droplet volume fraction in samples with an Ros ratio of 0.67 and water concentrations of 60, 70, and 90 wt %. The slope corresponds to an infinite dilution diffusion coefficient D0 of 1.58 × 10-11 m2/s, which, when introduced in the Stokes-Einstein equation (eq 8), gives a hydrodynamic droplet size of 44 nm. This value is consistent with the droplet size values obtained by DLS (∼40 nm). Oil Droplet Diffusion between 40 and 48 °C. On the other hand, the aggregate growth observed between 40 and 48 °C has been studied using a modification of the Stokes-Einstein equation. If droplet geometry and size is conserved, the dependence between the diffusion coefficient and the temperature should follow eq 9, described in the Experimental Section. In

3020 Langmuir, Vol. 22, No. 7, 2006

Figure 11, the oil diffusion coefficients in a sample with Ros ) 0.67 and 90% water have been plotted as a function of temperature together with the values predicted by eq 9. Between 40 and 44 °C, the obtained values are very close to the predicted ones. Nanoemulsion droplets remain with the same size and geometry and diffuse faster when temperature is increased by a thermal effect. However, from 46 to 48 °C, a very strong deviation is observed. The aggregate diffusion is much lower than that expected, indicating a droplet growth. Moreover, the observed shear-birefringence also indicates that spherical shape is not conserved.

Conclusions When using the PIT method, monodisperse O/W nanoemulsions with the most effective use of surfactant molecules (lowest droplet sizes) were obtained when, at the HLB temperature, either a single-phase D microemulsion or a two-phase W+D equilibrium

Morales et al.

was observed. In these cases, the O/W nanoemulsion structure has been described according to a droplet model based on spherical monodisperse oil droplets stabilized by a surfactant monolayer. Self-diffusion measurements have confirmed the bicontinuous structure of the D microemulsion observed at the HLB temperature. As a result, the nanoemulsion formation mechanism is produced by a thermal-induced disruption of a bicontinuous microemulsion into nanoemulsion droplets. Acknowledgment. Financial support from CYCYT QUI99-0997-CO2, the Generalitat de Catalunya 1999SGR-00, and from a Ph.D. grant from the University of Barcelona (UB) is gratefully acknowledged. D.M. also acknowledges Karin Malmborg’s expertise in the diffusion experiments performed in Lund University. LA052324C