Transitional Phase Inversion of Emulsions Monitored by in Situ Near

Article pubs.acs.org/Langmuir

Transitional Phase Inversion of Emulsions Monitored by in Situ NearInfrared Spectroscopy R. M. Charin,† M. Nele,†,‡ and F. W. Tavares*,†,‡ †

Programa de Engenharia Química, COPPE, Universidade Federal do Rio de Janeiro, CEP 21945-970, Rio de Janeiro, RJ, Brazil Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, CEP 21949-900, Rio de Janeiro, RJ, Brazil

‡

ABSTRACT: Water−heptane/toluene model emulsions were prepared to study emulsion transitional phase inversion by in situ near-infrared spectroscopy (NIR). The first emulsion contained a small amount of ionic surfactant (0.27 wt % of sodium dodecyl sulfate) and n-pentanol as a cosurfactant. In this emulsion, the study was guided by an inversion coordinate route based on a phase behavior study previously performed. The morphology changes were induced by rising aqueous phase salinity in a “steady-state” inversion protocol. The second emulsion contained a nonionic surfactant (ethoxylated nonylphenol) at a concentration of 3 wt %. A continuous temperature change induced two distinct transitional phase inversions: one occurred during the heating of the system and another during the cooling. NIR spectroscopy was able to detect phase inversion in these emulsions due to differences between light scattered/absorbed by water in oil (W/O) and oil in water (O/W) morphologies. It was observed that the two model emulsions exhibit different inversion mechanisms closely related to different quantities of the middle phases formed during the three-phase behavior of Winsor type III.

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INTRODUCTION Generally, emulsions are formed when two immiscible liquids are subjected to a strong shear stress in presence of surface active agents. If the emulsion formed is normal, it can be classified into two types: oil in water (O/W) or water in oil (W/O). The emulsion can be inverted; emulsion phase inversion is most of times characterized by the continuous phase of emulsion changing from water phase to oil phase or vice versa. According to known rules,1−7 one of the types of inversion is characterized by surfactant affinity shift. It is called transitional phase inversion. Recent works8−13 investigated this type of phase inversion as a low-energy emulsification method for nanoemulsion production. A usual method of detecting emulsion phase inversion is to monitor emulsion conductivity. While conductivity measurement is simple and versatile, it does not provide additional information about emulsion properties. It is well-known that the magnitude of some properties such as viscosity,14 droplet size,15 and stability16 changes sharply during phase inversion. Some authors17−19 demonstrated that coupling a torque sensor to a stirrer is useful to detect phase inversion and obtain rheological information simultaneously. Regarding the transitional phase inversion, continuous monitoring of both emulsion conductivity and viscosity enables the identification of several phenomena which take place during the process.17 It includes droplet size variations near inversion and information about intermediate morphologies exhibited around inversion. Other authors20,21 investigated the applicability of light backscattering (using Turbiscan equipment22) to pinpoint transitional phase inversion and obtain information about droplet size. © 2013 American Chemical Society

Near-infrared spectroscopy (NIR) is a type of vibrational spectroscopy which uses a wavelength range from 750 to 2500 nm (wavenumbers of 13 300 to 4000 cm−1). For a long period of time, while mid-infrared spectroscopy (IR) was largely used, near-infrared spectroscopy was neglected because its spectral range was not able to provide additional information comparing to IR possibilities.23 Results of NIR were usually difficult to interpret because of band overlapping. Nevertheless, the development of electronic/optical components and the advent of computers which are capable of effectively processing the information contained in NIR spectra allowed the expansion of this technique in an increasing number of fields.24 The success of NIR is usually related to some mathematical/statistical procedures which correlate spectra with standard experimental methods. One of the main advantages of this technique is the possibility of using probes that can perform the measurement in situ. The signal of NIR is transmitted by optical fibers, and it is a nondestructive and noninvasive technique for process monitoring. Furthermore, the NIR spectrum contains not only chemical information but also can be used to measure physical properties of the sample. For colloidal processes, in addition to light absorption, NIR spectra display a baseline change due to different sizes and numbers of aggregates, drops, and particles that are dispersed in the continuous media. Therefore, information about the state of the colloid can be obtained;25 this includes dispersed phase concentration, sizes, and even Received: February 25, 2013 Revised: April 21, 2013 Published: April 22, 2013 5995

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H2O) is increased. The oil phase of emulsion was a mixture of the HepTol 1:1 mole ratio. Phase behavior experiments were performed in order to determine inversion route using microemulsion phase equilibrium regions. In this stage of the work, the “ionic systems” were prepared according to the unidimensional formulation scan presenting Winsor types I, II, and III behaviors. In such experiments, only one of the formulation variables is changed, while all the others, such as compositional variables, i.e., surfactant concentration and water/oil ratio (WOR), are held constant.33 In this study, water salinity was the scanned variable. The experiments were executed in test tubes (maximum volume of 20 mL) covered with screw caps. The water-to-oil ratios (WOR) investigated were 1:1.5 (40 vol % of water), 1:1 (50 vol % of water), and 1:0.67 (60 vol % of water). The SDS and n-pentanol compositions were fixed at 0.27 wt % and 5.88 vol %, respectively. The addition sequence of liquids put inside the test tubes was first npentanol, then HepTol, and finally the aqueous solution. The test tubes were gently shaken for 30 s and left to rest in a metal support at room temperature (20 ± 1 °C) until no more visual changes is observed. Figure 1 shows the unidimensional mapping of the “ionic

growing rate. NIR technology application in polymer reaction engineering26 is an example of the NIR probe usage which allows obtaining different process variables during emulsion27−29 and suspension30−32 polymerization reactions. In this article, phase inversions of the two model emulsions are studied using NIR and conductometer probes immersed in a mixing vessel. Once the properties of emulsions change along the phase inversion, it is reasonable to expect that the NIR probe can detect the phenomena involved. Differences in sizes and numbers of droplets and differences in emulsion morphologies, including multiple morphology types, may change the NIR light extinction during inversion. The goal of this work is to test the NIR probe to investigate transitional phase inversion. Furthermore, the paper provides phase behavior/inversion experimental data of two model emulsions discussing the transition region of inversion.

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EXPERIMENTAL SECTION

Two model emulsions were prepared in order to study transitional phase inversion. The first emulsion contains an ionic surfactant plus a cosurfactant (“ionic system”). In this emulsion, water salinity increment causes phase inversion. The second emulsion contains nonionic surfactant (“nonionic system”). For this case, phase inversion was achieved by temperature change. It is important to emphasize that both model systems contain sodium chloride; nevertheless, the nomenclature, “ionic system” and “nonionic system”, highlights the type of surfactant used. All experiments were carried out three times, in independent experimental procedure (triplicates). Emulsion Phase Inversion Experimental Apparatus. The emulsion phase inversion was observed using a Gehaka/CG2000 conductometer. At the same time, the process was monitored by a near-infrared spectrophotometer (FTNIR) (ABB/FTLA-154). A transflectance probe (Solvias) was connected to the NIR instrument to perform in situ measurements. The NIR optical path was 2 × 0.5 mm (1 mm). GRAMS software collected spectra in absorbance mode (wavenumbers from 10 000 to 4000 cm−1). The results of wavenumbers 7500−4200 cm−1 are presented in this work because this spectral range provided better results, i.e. less instrumental noise. The reported spectra were the average of 128 scans for emulsions formed by the “ionic system” and 64 scans for emulsions by the “nonionic system”. Emulsion inversion in the “ionic system” was carried out in a jacketed glass vessel (maximum volume of 1 L) with a four-blade stirrer (IKA/Labortechinik) operated at 500 rpm. All experiments were run at room temperature (20 °C). The experimental apparatus for the emulsion inversion in the “nonionic system” was comprised of a stainless steel lid, a Buchiglasuster/bmd075 magnetic coupling, and a glass condenser to prevent emulsion evaporation. A submersible pump (Little Giant Pump) transferred water from a cold tank (4 ± 2 °C) to the condenser continuously. A Julabo F32 circulation bath controlled the temperature inside the mixing vessel during experiments which was measured by a platinum sensor PT-1000. Materials. The ionic and nonionic surfactants were sodium dodecyl sulfate (purity of 95%) manufactured by Vetec S.A, Brazil, and ethoxylated nonylphenol (purity of 98%) with an average of 9.5 ethylene oxide group per molecule supplied by Oxiteno S.A, Brazil, respectively. Additionally, sodium chloride (purity of 99%), n-pentanol (purity of 98%), n-heptane (purity of 99.5%), and toluene (purity of 99.5%) were supplied by Vetec S.A, Brazil. All chemicals were used without any further purification. Inversion of the “Ionic System” Emulsion. The “ionic system” was composed of sodium dodecyl sulfate (SDS)/n-heptane + toluene (HepTol)/brine/n-pentanol. SDS is a widely used ionic surfactant characterized by high hydrophilicity; the addition of a lipophilic alcohol such as n-pentanol allows surfactant affinity to change from water to oil when the salinity of aqueous solution (g NaCl/100 g

Figure 1. Phase equilibrium test tubes of “ionic system”, WOR 1:1, HepTol 1:1, SDS 0.27 wt %, and n-pentanol 5.88 vol %. The “optimum formulation” is close to 2.5 g/100 g H2O. system” (WOR 1:1). According to this picture, it is possible to obtain the three phase limits (end points) and the tube which is the closest to the so-called “optimum formulation”.34 This balanced system presents a minimum interfacial tension, and similar amounts of both water and oil are solubilized by the middle phase. According to the phase behavior observed, a formulation variable (in this case salinity) versus water volume fraction map35 was used as a framework to create an inversion route (Figure 2); the numbers from 1 to 11 indicate the inversion route that crosses the three-phase region. Thus, the phase in which microemulsions were formed indicates the continuous phase of emulsion according to Bancroft’s rule. In inversion route, the salinity is varied from 2 g/100 g H2O to 3 g/ 100 g H2O with increases of 0.1 g/100 g H2O per step, while water volume fraction is varied from 0.4 to 0.6 with a 0.02 increment. The inversion of the “ionic system” emulsion was carried out using a “steady state” protocol characterized by periodic additions of aqueous solutions containing NaCl, SDS, and n-pentanol at constant stirring. The steady state was achieved when no more significant conductivity and NIR spectra baselines changes were observed. The quantity and concentration of the solutions were calculated in such way to modify the formulation variable (salinity), and the WOR, while keeping constant the mass fraction of SDS and the volume fraction of npentanol. Thereby, the compositions of inversion route follow the phase equilibrium study. According to the calculations, initial and final overall volumes inside mixing vessel were 531.2 and 796.9 mL, respectively. All the experiments were performed at room temperature (20 ± 1 °C). The initial mixture of surfactant + oil phase + water phase (SOW) + cosurfactant was prepared 1 day before the experiment in order to archive pre-equilibrium. The procedure started by the insertion of probes (conductivity and NIR transflectance) and impeller into the 5996

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Figure 2. Inversion route. The diagram contains phase equilibrium data (optimum formulation and three-phase equilibrium end points). The experimental fluctuations are indicated by the standard deviation (black bars). mixing vessel. The initial salinity corresponds to the point number 1 labeled in inversion route (Figure 2). According to the Bancroft rule, the initial emulsion is O/W type. However, a special care must be taken about the position of impeller inside the vessel because this system can form unstable multiple or “abnormal”35 emulsions depending on the impeller position. After the initial emulsion was formed, three NIR spectra were collected, and then, the first salt solution was added in order to raise the salinity and water volume fraction according to inversion route (Figure 2). This procedure continues until the end of inversion route. Therefore, 33 NIR spectra were collected at each experiment at 11 different salinity−WOR relations. Figure 3a shows the experimental setup. Inversion of the “Nonionic System” Emulsion. The “nonionic system” consists of a SOW composed by ethoxylated nonylphenol (EON = 9.5)/n-heptane + toluene (HepTol 1:1 mole ratio)/brine (1 g NaCl/100 g H2O). The concentration of surfactant was 3 wt %, and the WOR investigated was 1:1. Emulsion inversion was obtained by a “continuous” protocol in which temperature was continuously varied to cross the temperature phase inversion (PIT) twice. Thus, each experiment was characterized by two phase inversions: one during the heating process and another during the cooling. One day before the experiment, the SOW system was prepared for pre-equilibrium at a temperature of 20 °C. The total volume of liquid phases inside the mixing vessel was 500 mL. At the beginning of experimental procedure, the probes (conductivity, PT 1000, and NIR transflectance) and the impeller were positioned properly within the mixing vessel. Subsequently, a stainless steel cover and a condenser were installed. Then, the agitation and the circulating bath were turned on. The temperature of emulsion was raised from 20 to 40 °C during 5 min. The spectra collection started at 40 °C. Temperature variation rate for the heating and cooling processes was 0.6 °C/min. This variation rate was related to the time it takes to the spectrophotometer to collect 70 spectra during heating and cooling. From this procedure we collected several spectra from 40 °C to nearly 63 °C; then the cooling process started, and additional spectra were collected until 43 °C. Figure 3b shows the experimental setup.

Figure 3. Experimental setups. (a) Experimental setup of the “ionic system” inversion; the scheme shows the conductometer and NIR probes inside the mixing vessel. The numbers indicate the solutions that should be periodically added to change the formulation/WOR according to inversion route. (b) Experimental setup of the “nonionic system” inversion; the inversion is observed by changing the emulsion temperature.

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RESULTS “Ionic System” Emulsion Inversion. Figure 4 shows experimental conductivity versus salinity of “ionic system” emulsion for three replicate inversion experiments; it is possible to observe that the results are very reproducible. The label numbers in Figure 4 indicate the formulation versus WOR relation of the inversion route (Figure 2). Up to the salinity indicated by number 4, the emulsion is O/W type. Emulsions

Figure 4. Conductivity versus salinity results of the “ionic system” emulsion inversion. Number labels indicate the inversion route, as shown in Figure 2.

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indicated by numbers 5 and 6 are clearly in transition between O/W type and W/O type, even though low conductivities are observed. Emulsions indicate by number 7 or larger are of W/O type. Figure 5 shows absorbance NIR spectra of experiment 1. Three NIR spectra were collected at each formulation/WOR

Figure 6. NIR’s validation for the “ionic system” emulsion inversion. The diagram shows the morphologies of three replicates comparing absorbance (at 5200 cm−1, open labels) and conductivity (black labels).

Figure 5. NIR spectra for the experiment 1 of “ionic system” emulsion inversion. O/W (continuous black lines) and W/O (dashed gray line). Transition conditions are shown as dashed black lines.

relation, during the inversion route, but the first spectrum is discarded in order to ensure that the system reached steady state. Therefore, NIR results presented are an average of the last two spectra collected at each formulation/WOR relation. It was observed that steady state is reached very quickly (2 or 3 s) mainly because of the low interfacial tension between phases which facilitates the emulsification process. An examination of Figure 5 shows the progressive shift of the spectra baseline during inversion process. It is worth note that the NIR probe is able of detect emulsion phase transition between the O/W and W/O types of morphologies. The absorbance around 5200 cm−1 showed the most reproducible results during the emulsion inversion, and it was chosen to compare NIR and conductometer results (Figure 6). In this validation diagram, there is an evident correlation between measurements. “Nonionic System” Emulsion Inversion. Figure 7 shows the experimental conductivity versus temperature curve of three independent emulsion inversion experiments using the “nonionic system”; it is possible to observe that the experiment showed a good reproducibility and that the conductometer was able to detect transition morphologies emulsions. Figure 8 shows the NIR spectra for the experiment III (Figure 8a for heating and Figure 8b for cooling). The obtained NIR spectra showed a clear baseline shift between the two normal morphologies, which pinpoints the phase inversion. Different from the other model emulsion studied (“ionic system”), NIR spectra presented other characteristics during transition region between the two normal emulsion morphologies (W/O and O/W). The absorbance of 5200 cm−1 was chosen to compare results obtained using conductometer and NIR (Figure 9). The diagram includes the absorbance (heating and cooling) for the three experiments. As the temperature varies continuously, each temperature indicated on diagram is an average value between the initial and final temperature measured during one sample of

Figure 7. Conductivity versus temperature results for the emulsion inversion in the “nonionic system”. Lines show the morphology limits of three replicates.

the NIR spectrum collected. According to Figure 9, there is a clear correlation between the measurements using both NIR and conductometer.

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DISCUSSION Two model emulsions were used to study the transitional phase inversion by means of different formulation variables, salinity and temperature, and by means of different inversion protocols, “steady state” and “continuous”. The in situ NIR spectroscopy proved to be able to detect the emulsion phase inversion. The NIR spectra baselines shifted significantly, as expected, and the experiments presented good reproducibility. The emulsion inversion of the “ionic system” presented an intermediary conductivity and NIR spectra baseline shift between the two normal emulsion morphologies (W/O and O/W) in the transition region (labeled in Figure 6). It is wellknown that three phase emulsions formed in the transition region are unstable. The behavior of these multiple emulsions is important for some practical applications such as in control of multiphase flow mobility in underground porous media.36 When the system presents a three-phase behavior under 5998

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Figure 10. Nonionic system at 50 °C. The test tube was kept at 50 °C for 30 min. After 30 min, the test tube is moderately shaken and rested at 50 °C. The picture shows a system which was equilibrated along 2 days.

stirring, a phase separation occurs between middle phase (bicontinuous microemulsion) and water/oil excess phases, increasing the number of possible emulsion morphologies considerably. 37 According to the three-phase behavior presented in Figure 1, the bicontinuous microemulsion (middle phase) cannot be the continuous phase of the multiple emulsion formed by this system under stirring due to volumetric restriction. Figure 10 shows a “nonionic system” which equilibrates at 50 °C. It was found that the inversion of “nonionic system” emulsion under stirring occurs during the three-phase behavior. The volumetric quantity of the “nonionic system” middle phase is larger compared to volume of the “ionic system” middle phase (Figure 1). This suggests that the intermediate phase shown in Figure 10 is the continuous phase of the multiple emulsion formed during inversion. The results presented by other authors17,38 also support that. Therefore, despite the fact that the conductivity curves of “ionic” and “nonionic” emulsions under stirring show the same nontrivial conductivity in transition region, it can be suggested that the mechanisms of morphologies changes during these phase inversions are different and depend on phase equilibrium. It seems that the difference between NIR spectra of these two model emulsions during transition region is closely linked to phase behavior although this difference could be caused by liquid crystal formation in “nonionic” emulsion. It is important to report that the “ionic” emulsion indicated by number 7 (inversion route) presents a slightly superior conductivity than others W/O emulsions. Although the graph scale does not allow viewing, while emulsion number 7 presented conductivity on the order of 40 μS, the others W/O emulsions presented conductivity of ∼15 μS. This suggests that emulsion number 7 is formed by three phases, and it is consistent with the phase behavior results. Figure 11 shows the curves of NIR absorbance (wavenumber of 5200 cm−1) and conductivity versus temperature of the three replicates emulsion inversion experiments using the “nonionic system”. According to NIR results, it is proposed a transition region mapping. In these diagrams, transition region boundaries (vertical dotted line) are located where the absorbance spectrum changes abruptly (consistent with the features

Figure 8. Experiment III: NIR spectra for emulsion inversion of “nonionic system”. (a) Heating process. (b) Cooling process. Black continuous lines for O/W. Gray dashed lines for W/O. Absorbances of transition region are shown in black dashed lines.

Figure 9. NIR’s validation for the “nonionic system” inversion. Morphologies of the three replicates are compared using absorbance (wavenumber of 5200 cm−1) obtained by in situ NIR spectroscopy (open points) and conductivity measures (dark points).

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Figure 11. Transition region mapping of the emulsion inversion in the “nonionic system” (wavenumber of 5200 cm−1). The vertical dotted lines (···) are the transition region limits of phase inversion. The vertical dashed lines (- - -) are the minimum droplet size observed. The vertical continuous line () is the maximum droplet size formed between the two droplet-size minima; it corresponds to the PIT. The stability of emulsion is minimum at PIT.

shown in Figure 8). It occurs because the third phase is formed. According to Figure 11, it can be suggested that spectra baseline changes are related to the change in the droplet size, in agreement with two works.15,39 They reported that the drop size near inversion exibits a complex behavior as the result of a dynamic equilibrium between two strong and opposite effects. As optimum formulation is approached from both sides, the

interfacial tension decrease favors the breaking process and, thus, tends to produce a smaller drop size. However, the emulsion stability decreases strongly as well, and drops are likely to coalesce instantly upon contact very near of optimum formulation, which favors the opposite trend, that is, the attainment of a larger drop size.17 This means that NIR’s baseline changes could provide scattering information directly 6000

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According to results, the phase inversion occurs at temperature of 52.96 °C with 0.4 °C of standard deviation. The use of NIR helps to detect the inversion point with good accuracy. The transition region mappings by NIR and conductivity were similar, within the experimental error. Nevertheless, the absorbance results contain more information in the transition region. It is important to note that it is not possible to associate droplet size changes of emulsion formed by “ionic system” because the WOR changes during this process. Figure 11 also shows that NIR spectra of the O/W emulsion present baseline differences in the heating and cooling processes; these differences should be related to droplet size changes. In the heating process, the emulsion was prepared directly, while in the cooling process the O/W was prepared by phase inversion temperature emulsification method (PIT) that produces small droplets.6,8−13 The developments of experimental methods which perform in situ measurements are important owing to the pronounced instability of emulsions near inversion. Often, the lightscattering theories explain NIR’s behavior in colloidal solutions. Several recent works40−42 employ these theories for diluted colloidal solutions. Nevertheless, multiple light scattering effects are expected to become important when emulsions contain high disperse phase concentration.43 The multiple light scattering effects hinders the development of theoretical

Table 1. Absoption Band Table Characteristic of Chemical Bondsa

a

absorption band

wavenumber (cm−1)

O−H first overtone O−H combinations C−H second overtone C−H combinations first overtone C−H first overtone C−H combinations

7143−6896 5263−5063 8888−8163 7404−6896 6154−5634 5128−4081

Adapted from ref 25.

related to droplet sizes, despite the fact that it should also be sensitive to differences in the morphologies and dispersion instabilities. The presence of two minima (vertical dashed lines) inside transition region should be associated with the occurrence of a minimum droplet size resulted from the compromise between low interfacial tension, which supports small droplet size formation, and the “not too large” coalescence rate.15,39 In a mapping similar to this one,17 the droplet size changes are correlated with an indirect measure of emulsion viscosity. Exactly at inversion point, which corresponds to the maxima indicated (vertical dark line), the coalescence rate grows up rapidly, producing larger droplets.15,39

Figure 12. Effect of the wavenumber on the absorbance for mapping of the emulsion inversion of “nonionic system”, Experiment II. Wavenumbers: (a) 4500, (b) 5200, (c) 6000, and (d) 7000 cm−1. 6001

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(8) Spernath, L.; Levi-Kalisman, Y.; Magdassi, S. Phase Transitions in O/W Lauryl Acrylate Emulsions during Phase Inversion, Studied by Light Microscopy and Cryo-TEM. Colloids Surf., A 2009, 332, 19−25. (9) Mei, Z.; Xu, J.; Sun, D. O/W Nano-emulsions with Tunable PIT Induced by Inorganic Salts. Colloids Surf., A 2011, 375, 102−108. (10) Mei, Z.; Liu, S.; Wang, L.; Jiang, J.; Xu, J.; Sun, D. Preparation of Positively Charged Oil/Water Nano-emulsions with a Sub-PIT Method. J. Colloid Interface Sci. 2011, 361, 565−572. (11) Solans, C.; Solé, I. Nano-emulsions: Formation by Low-Energy Methods. Curr. Opin. Colloid Interface Sci. 2012, 17, 246−254. (12) Machado, A. H. E.; Lundberg, D.; Ribeiro, A. J.; Veiga, F. J.; Lindman, B.; Miguel, M. G.; Olsson, U. Preparation of Calcium Alginate Nanoparticles Using Water in Oil (W/O) Nanoemulsions. Langmuir 2012, 28, 4131−4141. (13) Roger, K.; Cabane, B.; Olsson, U. Formation of 10−100 nm Size Controlled Emulsions through a Sub-PIT Cycle. Langmuir 2010, 26, 3860−3867. (14) Salager, J.-L.; Miñana-Pérez, M.; Andérez, J.; Grosso, J.; Rojas, C. Surfactant-Oil-Water Systems near the Affinity Inversion Part II: Viscosity of Emulsified Systems. J. Dispersion Sci. Technol. 1983, 4, 161−173. (15) Pérez, M.; Zambrano, N.; Ramirez, M.; Tyrode, E.; Salager, J.-L. Surfactant-Oil-Water Systems near the Affinity Inversion Part XII: Emulsion Drop Size versus Formulation and Composition. J. Dispersion Sci. Technol. 2002, 23, 55−63. (16) Salager, J.; Loaiza-Maldonado, I.; Minaña-Perez, M.; Silva, F. Surfactant-Oil-Water Systems near the Affinity Inversion Part I: Relationship between Equilibrium Phase Behavior and Emulsion Type and Stability. J. Dispersion Sci. Technol. 1982, 3, 279−292. (17) Allouche, J.; Tyrode, E.; Sadtler, V.; Choplin, L.; Salager, J. L. Simultaneous Conductivity and Viscosity Measurements as a Technique to Track Emulsion Inversion by the Phase-InversionTemperature Method. Langmuir 2004, 20, 2134−2140. (18) Song, D.; Zhang, W.; Gupta, R. K.; Melby, E. G. Relating Viscosity Changes to Phase Inversion during the Synthesis of Tackifier Emulsions. Int. Congr. Rheol. 2008, 866−868. (19) Moradpour, H.; Chapoy, A.; Tohidi, B. Phase Inversion in Water-Oil Emulsions with and without Gas Hydrates. Energy Fuels 2011, 25, 5736−5745. (20) Pizzino, A.; Rodriguez, M. P.; Xuereb, C.; Catté, M.; Hecke, E. V.; Aubry, J.-M.; Salager, J.-L. Light Backscattering as an Indirect Method for Detecting Emulsion Inversion. Langmuir 2007, 23, 5286− 5288. (21) Pizzino, A.; Catté, M.; Hecke, E. V.; Salager, J.-L.; Aubry, J.-M. On Line Light Backscattering Tracking of the Transitional Phase Inversion of Emulsions. Colloids Surf., A 2009, 338, 148−154. (22) Mengual, O.; Meunier, G.; Cayré, I.; Puech, K.; Snabre, P. Turbiscan MA 2000: Multiple Light Scattering Measurement for Concentrated Emulsion and Suspension Instability Analysis. Talanta 1999, 50, 445−56. (23) Pasquini, C. Near Infrared Spectroscopy: Fundamentals, Practical Aspects and Analytical Applications. J. Braz. Chem. Soc. 2003, 14, 198−219. (24) Blanco, M.; Villarroya, I. NIR Spectroscopy: A Rapid-Response Analytical Tool. Trends Anal. Chem. 2002, 21, 240−250. (25) Sjoblom, J.; Aske, N.; Auflem, I. H.; Brandal, O.; Havre, T. E.; Saether, O.; Westvik, A.; Johnsen, E. E.; Kallevik, H. Our Current Understand of Water-in-Crude Oil Emulsions: Recent Characterization Techniques and High Pressure Performance. Adv. Colloid Interface Sci. 2003, 100−102, 399−473. (26) Santos, A. F.; Silva, F. M.; Lenzi, M. K.; Pinto, J. C. Monitoring and Control of Polimerization Reactors Using NIR Spectroscopy. Polym.-Plast. Technol. 2005, 44, 1−61. (27) Reis, M. M.; Araújo, P. H. H.; Sayer, C.; Giudici, R. Correlation between Polymer Particle Size and in-Situ NIR Spectra. Macromol. Rapid Commun. 2003, 24, 620−624. (28) Silva, W. K.; Chicoma, D. L.; Giudici, R. In-Situ Real Time Monitoring of Particle Size, Polymer and Monomer Content in

relations between spectra and droplet sizes because it is not possible to treat absorption and scattering separately.42 Therefore, calibration techniques which correlate spectra data with standard methods directly should be used to obtain further information.44 Table 1 presents some absorption bands in the NIR spectral region. Figure 12 shows selected absorbances from experiment II. Comparing these data, presented in Figure 12 and in Table 1, it is possible to conclude that the baseline changes are pronounced for wavenumbers not related to water signal that presents high absorption effects. The scattering effects are more evident for the less pronounced absorption effects. This occurs because the NIR light extinction depends on both light scattering and light absorption.

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CONCLUSIONS In situ near-infrared spectroscopy (NIR) detected phase inversion in two different model emulsions. The validation was performed by comparing results obtained with NIR to those with conductometry. The different scattered/absorbed near-infrared radiation between the two normal morphologies indicated inversion by spectra baseline changes. In addition to inversion detection, the main advantage of using in situ NIR spectroscopy is to capture further details about the phenomenon which take place during inversion. According to results obtained here, droplet size changes and transition region boundaries are examples of information that can be obtained from the NIR spectra.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected] (F.W.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CNPq, CAPES, and FAPERJ for scholarships and financial support. The authors dedicate this paper to Professor Alberto Luiz Coimbra, on the 50th anniversary of COPPE (1963−2013), the Graduate School of Engineering of the Federal University of Rio de Janeiro.

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REFERENCES

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dx.doi.org/10.1021/la4007263 | Langmuir 2013, 29, 5995−6003