CO2-Controllable Foaming and Emulsification Properties of the

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CO2‑Controllable Foaming and Emulsification Properties of the Stearic Acid Soap Systems Wenlong Xu, Hongyao Gu, Xionglu Zhu, Yingping Zhong, Liwen Jiang, Mengxin Xu, Aixin Song, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China S Supporting Information *

ABSTRACT: Fatty acids, as a typical example of stearic acid, are a kind of cheap surfactant and have important applications. The challenging problem of industrial applications is their solubility. Herein, three organic aminesethanolamine (EA), diethanolamine (DEA), and triethanolamine (TEA)were used as counterions to increase the solubility of stearic acid, and the phase behaviors were investigated systematically. The phase diagrams were delineated at 25 and 50 °C, respectively. The phase-transition temperature was measured by differential scanning calorimetry (DSC) measurements, and the microstructures were vesicles and planar sheets observed by cryogenic transmission electron microscopy (cryo-TEM) observations. The apparent viscosity of the samples was determined by rheological characterizations. The values, rcmc, for the three systems were less than 30 mN·m−1. Typical samples of bilayers used as foaming agents and emulsifiers were investigated for the foaming and emulsification assays. CO2 was introduced to change the solubility of stearic acid, inducing the transition of their surface activity and further achieving the goal of defoaming and demulsification.

1. INTRODUCTION As the most ancient detergents, fatty acid soaps are often used in the industry and our daily life. Although they have a long history in the applications of fatty acid soaps, the microstructures in the solution have been widely investigated since the first observation of fatty acid vesicles in 1973.1 Since then, the formation mechanism of the fatty acid bilayers has caused great interest and argument.2 After exploration for several decades, people found hydrogen bonds between the protonated molecules, and deprotonated molecules were assumed to play a vital role in the formation of bilayers.3−7 Hydrogen bonds are easy to form at the pKa of the fatty acids, at which the ratio of the protonated molecules and deprotonated molecules is almost 1:1. In our previous study, fatty acid bilayers at high pH were discovered with the introduction of Cs+, which should promote the application scopes of fatty acid bilayers with a wide pH range.2 The investigation of the phase behavior and microstructure is a platform for the further application of the fatty acid soap systems. It was reported that fatty acid bilayers possess excellent foaming8−11 and emulsification properties.9,10 Foams are formed by gas dispersing into liquid, and they can be widely used in the fields of food, detergents, and oil recovery.12 However, it is a thermodynamically metastable system and separates with time.13 Fatty acid soap systems combining the surface density, compactness to avoid gas diffusion, and © 2015 American Chemical Society

mechanical strength altogether can effectively increase the foam stability.12 On the other side, both foaming and deforming are important in industrial manufacturing and daily life.14 Therefore, stimuli-responsive foams have attracted more interest in recent years,11,14,15 and the study of emulsification and demulsification is equally valuable.16,17 For example, emulsification is of great importance in the pipeline transport of crude oil, and when the crude oil arrives at its destination, demulsification is needed to achieve the separation of crude oil.16 CO2 has been reported as a smart gas to control the transition in the literature.18,19 The main problem of using fatty acids is their insolubility in aqueous solution. Common alkalis such as NaOH and KOH usually dissolve fatty acids at high temperature with a high Krafft point. Organic counterions can solve this problem.9,12,20−25 A comparison between organic and inorganic counterions has been studied in our previous study, in which we also found that the organic counterion could decrease the Krafft point of the fatty acid effectively.26 Therefore, the advantage of a fatty acid soap system with organic counterions rather than inorganic counterions is that fatty acids can be applied at room temperature. Received: April 9, 2015 Revised: May 9, 2015 Published: May 11, 2015 5758

DOI: 10.1021/acs.langmuir.5b01295 Langmuir 2015, 31, 5758−5766

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(cryo-TEM) and rheological characterizations. The foaming and emulsification properties of the typical samples were investigated, and the defoaming and demulsification processes were controlled simply by the introduction of CO2 . Considering the excellent CO2-controllable defoaming and demulsification properties, we hope our investigation may make a contribution to the study of self-assembly of fatty acid and their applications.

Herein, three organic alkalis were chosen to investigate the phase behavior of the stearic acid and their applications as foaming agents as well as emulsifiers. Ethanolamine (EA), diethanolamine (DEA), and triethanolamine (TEA) are cheap organic solvents, and they can induce the structure transition by forming hydrogen bonds with the carboxyls of the surfactant molecules.27 As shown in Scheme 1, EA and DEA both possess Scheme 1. Chemical Structures of EA, DEA, and TEA

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Stearic acid (SA, >98%, mass fraction), ethanolamine (EA, >99%, mass fraction), and diethanolamine (TEA, >99%, mass fraction) were purchased from J&K Scientific Co. Ltd. (China). Triethanolamine (TEA, >99%, mass fraction) was purchased from Acros Organics (USA). Hexadecane (analytically pure) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All chemicals were used as received. Ultrapure water was used with a resistivity of 18.25 MΩ·cm from a UPH-IV water purifier. Other reagents were of analytical purity. 2.2. Phase Behavior. The phase diagrams of SA/EA/H2O, SA/ DEA/H2O, and SA/TEA/H2O systems were delineated at 25.0 and 50.0 °C, respectively. The samples were prepared in tubes. Different amounts of SA were weighed into tubes. Different volumes of a 200 mmol·L−1 organic amine (EA, DEA, or TEA) aqueous solution were added to these tubes, and the total volume of the solution for each

O−H and N−H groups, whereas TEA possesses only the O−H group, which may induce different polymorphisms. The phase behaviors of the three systems were investigated systematically at 25 and 50 °C with the help of conductivity and differential scanning calorimetry (DSC) measurements. The microstructures and the apparent viscosity of the samples were determined by cryogenic transmission electron microscopy

Figure 1. Phase diagrams of the SA/EA/H2O, SA/DEA/H2O, and SA/TEA/H2O systems at 25 and 50 °C. 5759


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Figure 2. Photographs of typical samples. SA/EA/H2O system at 25 °C (a) and at 50 °C (b); SA/DEA/H2O system at 25 °C (c) and at 50 °C (d); SA/TEA/H2O system at 25 °C (e) and at 50 °C (f) with cSA = 50 mmol·L−1. 2.6. Conductivity and pH Measurements. The conductivity measurements were performed on a DDSJ-308A (China) conductivity meter with a DJS-1C glass electrode at 25.0 and 50 °C. The values of pH were determined on a PHS-3C pH meter (China) with an E-201C glass electrode at 25 and 50 °C. The two-phase samples were detected under stirring. 2.7. Surface Tension Measurements. Surface tension measurements were performed on a Krüss K100 (Germany) surface tensiometer using the plate method. The temperature was controlled at 25 °C with a Haake K10 (Germany) superconstant temperature trough. All of the surface tension was measured after stirring and equilibration and was repeated at least twice until the error was negligible. 2.8. Foaming and Defoaming. The foaming properties and their stability were determined by an HT FOAMSCAN apparatus (TECLIS, France). An initial sample volume of 60 mL was foamed by sparging N2 or CO2 through a porous disk (pore sizes = 40−100 μm) at a constant gas flow rate of 75 mL/min. The gas flow was stopped when the foams reached a certain volume, and then the stability of the foam was analyzed. 2.9. Emulsification and Demulsification. Emulsions were prepared with different oil/water ratios (rO/W) from 1:9 to 9:1. The oil phase was hexadecane. SA was dissolved in the hexadecane first, and then it was mixed with amine aqueous solutions. Different methods were used to prepare the emulsions according to the different rO/W values. For the rO/W between 1:9 and 8:2, ultrasonication at 50 °C

tube was 5 mL by supplying ultrapure water. The concentration range of SA is 10−100 mmol·L−1, and that of organic amine is 0−200 mmol· L−1. The mixtures were dissolved to homogeneous solutions under ultrasonication at 50 °C. The phase diagram was established by visual inspection with the help of crossed polarizers. All samples were kept for at least 1 month to observe the phase behavior. 2.3. Cryogenic Transmission Electron Microscopy (CryoTEM) Observations. Within a high-humidity environment (>90%), the sample (∼4 μL) was dropped on a grid. The excess sample was blotted up with two pieces of blotting paper, leaving a thin film sprawling on the grid. Then the grid was plunged into liquid ethane which was frozen by liquid nitrogen. The vitrified sample was transferred to a sample holder (Gatan 626) and observed on a JEOL JEM-1400 TEM (120 kV) at about −174 °C. The images were recorded on a Gatan multiscan CCD. 2.4. Rheological Characterization. The rheological experiments were carried out on a HAAKE RheoStress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep in order to ensure that the selected stress was in the linear viscoelastic region. 2.5. Differential Scanning Calorimetry (DSC) Measurements. The phase-transition temperature and chain melting temperature were measured on a DSC-Q10 (TA Instruments, New Castle, PA, USA). The measuring range was from 25 to 50 °C at a rate of 10 °C/min. 5760


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Figure 3. DSC data of the typical samples in the three systems at cSA = 50 mmol·L−1 with different amounts of EA, DEA, and TEA. and vortex mixing at 25 °C must be applied. For the rO/W of 9:1, just vortex mixing at 25 °C was applied. Demulsification was achieved by sparging CO2 into the emulsions at a rate of 0.15 L·min−1.

SA/TEA/H2O system. However, when cEA,DEA > cSA, the solutions change to the clear micelle phase and the birefringence disappears at 50 °C. The temperature effects were investigated by DSC characterization. From Figure 3, one can observe the obvious differences among the three systems. For the SA/TEA/H2O system, no obvious changes can be observed, but for the SA/EA/H2O and SA/DEA/H2O systems, broad peaks can be observed when the concentration of alkali was larger than that of SA, indicating the phase transition of the solutions. This is accordance with the visual observation. From Figure 3a,b, one can find that the phase-transition temperature decreases with the addition of amines. 3.2. Microstructures and Formation Mechanism. CryoTEM is a direct method of detecting the microstructure of the solution. Through the quick-freezing process, the morphology of the microstructure can be fixed. The microstructures of the typical samples are shown in Figure 4. Vesicles and planar sheets coexist in the Lα phase sample, and only planar sheets exists in the Lα′ phase in all three systems. The microstructures of the aggregates for the surfactant solutions can be estimated according to the packing parameter (p) theory,28,29 where p = v/al, v = 0.0274 + 0.0269n (nm3), l = 0.154 + 0.1265n (nm), v and l are the volume and length of the hydrophobic chain, respectively, a is the average headgroup area, and n is the chain length of the surfactant. In this work, the fatty acid is stearic acid, indicating fixed v and l values. The value of a can be changed by the distance between the polar headgroups, which can be regulated by the weak interactions among the molecules.30,31 In our previous study of the SA/ CsOH/H2O system, the weak interactions for the formation mechanism of the bilayers were proposed, including electrostatic forces, hydrogen bonds, and hydrophobic forces.32 Hydrogen bonds play a vital role in the formation of fatty acid bilayers. The pH values of the bilayer solutions are mostly

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. The phase behaviors at 25 and 50 °C were investigated for the three systems. The phase diagrams are shown in Figure 1. SA does not dissolve in water at 25 and 50 °C. With the addition of organic amines, SA can be partially dissolved in water and forms a mixture of precipitation and bluish solution (P/Lα phase). More organic amines can completely dissolve SA, and the solution exhibits a homogeneous bluish appearance, whose microstructure was determined as bilayers in the cryo-TEM images. However, differences occur for the three systems with many more organic amines added. At 25 °C, for the SA/TEA/H2O system, no changes can be observed, whereas another homogeneous phase (the Lα′ phase) occurs for the SA/EA/H2O and SA/DEA/H2O systems. These solutions exhibit a textured appearance and higher viscoelasticity. At 50 °C, after the Lα phase, it is the same with the phase behavior at 25 °C for the SA/TEA/H2O system, whereas it is a separated Lα/L1 phase for the SA/EA/H2O and SA/DEA/H2O systems. With many more alkalis, a transparent micelle phase (L1) can be observed in the SA/EA/H2O and SA/DEA/H2O systems whereas the SA/TEA/H2O system retains no changes. It was reported that the Krafft point of fatty acid soaps was mostly higher than room temperature,12 which largely limits their applications. Herein, in the three systems, expansive areas in the phase diagrams at 25 °C belong to the Lα phase and the Lα′ phase, indicating that the Krafft point is lower than room temperature. The appearance of typical sample solutions of stearic acid mixed with EA, DEA, and TEA is shown in Figure 2. We can clearly observe stable solutions at 25 and 50 °C. At 25 °C, all of the samples are bluish solutions and birefringent under crossed polarizers. At 50 °C, no obvious changes can be observed in the 5761


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Hydrogen bonds can form between the N−H group of the amine and the −COO− of the deprotanated fatty acid. These hydrogen bonds decrease the distance between the molecules further, inducing a decrease in a and an increase in p. As a result, only planar sheets can be observed when excess EA and DEA exist. However, this change cannot be observed in the SA/TEA/H2O system because of the absence of the N−H group in TEA molecules. 3.3. Rheological Characterizations. The rheological properties were characterized to detect the viscoelasticity changes in the solutions. Figure 5 shows the steady shear rheograms for the samples in the three systems at 25 and 50 °C. The apparent viscosity of all of the samples decreases with the increase in shear rate at 25 °C. The shear-thinning property indicates the destruction of the bilayers. In general, the apparent viscosity of the Lα′-phase samples is at a high level compared to that of the Lα-phase samples. However, at 50 °C, most of the samples exhibit the same property as at 25 °C, whereas the samples with more amines in the SA/EA/H2O and SA/DEA/H2O systems are independent of the shear rate, which is the typical property of a Newtonian fluid. This change indicates the transition from a viscoelastic bilayer solution to a low-viscosity micelle solution. From the analysis above, we can conclude that no phase transition occurs in the SA/TEA/H2O system from 25 to 50 °C. However, the other two systems should be divided into two cases: when cEA,DEA < cSA, it is the same as that in the SA/TEA/H2O system, whereas bilayers change into micelles with the increase in temperature when cEA,DEA > cSA. 3.4. Conductivity Measurements. The conductivity data can also estimate the microstructures. At 25 °C, with the increase in alkalis, all of the conductivity values (Figure 6a) increase initially. After that, a difference occurs among the three systems: For the SA/TEA/H2O system, with the increase in alkalis, the conductivity increases gradually over the whole concentration range and finally reaches a plateau. For the SA/ EA/H2O and SA/DEA/H2O systems, when it reaches a maximum, the conductivity decreases promptly and then levels off. To our knowledge, SA is difficult to dissolve in water, and the conductivity itself is close to zero. With the addition of alkalis, part of the SA is deprotonated and dissolves in water in the form of carboxylate (S−). Thus, with the addition of alkalis, the conductivity should increase as shown in the SA/TEA/H2O system. However, the viscosity data of the samples increases obviously with excess alkalis in the SA/EA/H2O and SA/DEA/ H2O systems (Figure 5a,c). According to the conductivity changes in Figure 6a, one can imagine that the microstructures in these solutions can trap the excess ions and decreases the conductivity. In other words, the excess ions participate in the formation of the microstructures. At 50 °C, these solutions become clear micelle phases, the excess ions are released into the water, and the conductivity increases to a high level (Figure 6b). In the SA/TEA/H2O system, no phase transition occurs from 25 to 50 °C, thus the conductivity increases a little due to the temperature effect. 3.5. Surface Tension. The organic amines (EA, DEA, and TEA) are miscible with water at any ratio, and the surface tension of their aqueous solutions is similar to that of water, indicating that the organic amines have no surface activity. Three samples in the SA/EA/H2O, SA/DEA/H2O, and SA/ TEA/H2O systems at the same concentration were prepared to investigate the influence of the organic amines on the surface activity of SA. SA is difficult to dissolve in water, and its surface

Figure 4. Microstructures in the three systems at 25 °C. (a) cEA = 20 mmol·L−1, (b) cEA = 200 mmol·L−1, (c) cDEA = 20 mmol·L−1, (d) cDEA = 200 mmol·L−1, (e) cTEA = 40 mmol·L−1, and (f, g) cTEA = 200 mmol· L−1. For all samples, cSA = 50 mmol·L−1.

near the pKa of the fatty acids, at which the amounts of protonated and deprotonated fatty acids are almost the same and hydrogen bonds are easy to form between them. It was reported that the pKa of SA is about 10.0.33 From Figure S1, one can observe that the pH ranges of the three systems are all near or below 10.0, indicating the coexistence of protonated and deprotonated fatty acids and the existence of hydrogen bonds. In this work, the counterions are changed from Cs+ to organic amine ions, for which the radius of the counterions is larger. As a result, the steric hindrance increases largely, resulting in a decrease in the screening electrostatic repulsion effect. Thus, the distance between surfactants decreases, causing an increase in a and a decrease in p. According to the reports in the literature, with the increase in p, the aggregates change from spherical micelles, wormlike micelles, vesicles, and planar sheets to planar lamella.26,32,34 In our previous work on the SA/ CsOH/H2O system, only planar sheets can be observed. Herein, the decrease in p induces the planar sheets to change to the coexistence of the vesicles and planar sheets (Lα phase). However, with the increase in amine, only planar sheets can be observed in the Lα′ phase. We assumed that this was attributed to the hydrogen bonds between amine molecules and the fatty acid molecules. EA and DEA both possess O−H and N−H groups, whereas TEA possesses only the O−H group. 5762


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Figure 5. Viscosity data of the samples for the three systems at 25 and 50 °C with cSA = 50 mmol·L−1.

Figure 6. Conductivity data with the increase in camine at 25 and 50 °C with cSA = 50 mmol·L−1.

different organic amines contribute the same to the surface activity of SA. 3.6. Foaming Properties. It was reported that bilayer samples possessed better foaming stability than micelle samples.35 Therefore, typical Lα samples in the three systems were chosen to assess the foaming properties and their stability. As shown in Figure 8, a fixed foaming volume (200 mL) was set for N2 foaming, and similar foaming times (about 110 s) are

tension cannot be measured directly. By adding organic amines, SA molecules were deprotonated to be anionic S−, increasing its solubility. The surface tension, as shown in Figure 7, can be largely decreased with the addition of the samples, which can be attributed to the long hydrophobic chain of SA. Comparing the three samples, there is no obvious difference in either the critical micelle concentration (cmc) or γcmc. That is to say, 5763


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volume for all samples, and only a small amount of foams exist stably after 300 s. 3.7. Emulsification and Demulsification. Emulsification was performed using hexadecane as oil phase. Similar to the foaming experiments, we chose the samples from the phase diagrams in two considerations: the samples must locate in the Lα phase and close to the phase boundary between Lα phase and P/Lα phase. Emulsions with different oil/water ratios (rO/W) were prepared via two methods, and their stability was determined, which is shown in Table 1. All of the O/W Table 1. Emulsions Prepared at Different rO/W Values Figure 7. Surface tension of the three samples with rSA/amine = 1:40 and the pure amines.

rO/W

method

1:9, 2:8, 3:7, 4:6, 5:5 6:4, 7:3, 8:2

ultrasonication at 50 °C and vortex mixing at 25 °C ultrasonication at 50 °C and vortex mixing at 25 °C vortex mixing at 25 °C

9:1

needed for all three systems, indicating similar foaming ability. After the N2 flow is stopped, a slow decrease in foaming volume can be observed. However, after being reduced to about 160 mL, the foam volumes remain unchanged for the SA/EA/H2O and SA/DEA/H2O samples for up to 10 000 s whereas the halflife time appears at about 8500 s for the SA/TEA/H2O sample (insets of Figure 8). These samples that we chose to foam are located in the Lα phase, near the phase boundary between the Lα and P/Lα phases (Figures 1). Therefore, the precipitate can separate out by adding a small amount of acid, and the samples must undoubtedly lose their surface activity. Herein, CO2 was sparged as a smart way to induce the phase transition and achieve the defoaming process. Though the CO2 foaming process was similar to that of N2 foaming, the foaming volume was regulated to low values (100 and 80 mL) due to the weak foaming ability. One can observe a sharp decrease in foam

stability stable unstable stable

emulsions with different rO/W values were determined by the dilution method. For the emulsions with rO/W = 1:9−5:5, the interface between oil and water shifted up when ultrasonicating at 50 °C, and the lower water phase became white, indicating the emulsion formation process. The mixture was vortex mixed at room temperature and ultrasonicated again and again. The mixture finally became a homogeneous white solution after several treatments. The sample can remain stable for several months without separating. For the emulsions with rO/W = 6:4− 8:2, the same method can be applied to form homogeneous emulsions; however, phase separation occurs in 2 days. For the emulsions with rO/W = 9:1, vortex mixing at room temperature without ultrasonication can form homogeneous emulsions in 1 min because the emulsion possess rather high viscosity with an

Figure 8. Foaming by N2 and CO2 and their stability in the typical samples in the three systems. cSA = 50 mmol·L−1 with cEA,DEA = 20 mmol·L−1 and cTEA = 40 mmol·L−1. 5764


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ACKNOWLEDGMENTS

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REFERENCES

(1) Gebicki, J. M.; Hicks, M. Ufasomes are Stable Particles Surrounded by Unsaturated Fatty Acid Membranes. Nature 1973, 243, 232−234. (2) Xu, W.; Zhang, H.; Dong, S.; Hao, J. 133Cs NMR and Molecular Dynamics Simulation on Bilayers of Cs+ Ion Binding to Aggregates of Fatty Acid Soap at High pH. Langmuir 2014, 30, 11567−11573. (3) Haines, T. H. Anionic Lipid Headgroups as a Proton-Conducting Pathway along the Surface of Membranes: A Hypothesis. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 160−164. (4) Apel, C. L.; Deamer, D. W.; Mautner, M. N. Self-Assembled Vesicles of Monocarboxylic Acids and Alcohols: Conditions for Stability and for the Encapsulation of Biopolymers. Biochim. Biophys. Acta 2002, 1559, 1−9. (5) Caschera, F.; de la Serna, J. B.; Löffler, P. M. G.; Rasmussen, T. E.; Hanczyc, M. M.; Bagatolli, L. A.; Monnard, P.-A. Stable Vesicles Composed of Monocarboxylic or Dicarboxylic Fatty Acids and Trimethylammonium Amphiphiles. Langmuir 2011, 27, 14078−14090. (6) Kanicky, J. R.; Shah, D. O. Effect of Premicellar Aggregation on the pKa of Fatty Acid Soap Solutions. Langmuir 2003, 19, 2034−2038. (7) Smith, R.; Tanford, C. Hydrophobicity of Long Chain N-Alkyl Carboxylic Acids, as Measured by Their Distribution between Heptane and Aqueous Solutions. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 289− 293. (8) Novales, B.; Riaublanc, A.; Navailles, L.; Houinsou, H. B.; Gaillard, C.; Nallet, F.; Douliez, J. P. Self-Assembly and Foaming Properties of Fatty Acid−Lysine Aqueous Dispersions. Langmuir 2010, 26, 5329−5334. (9) Novales, B.; Navailles, L.; Axelos, M.; Nallet, F.; Douliez, J.-P. Self-Assembly of Fatty Acids and Hydroxyl Derivative Salts. Langmuir 2008, 24, 62−68. (10) Fameau, A.-L.; Houinsou, H. B.; Ventureira, J. L.; Navailles, L.; Nallet, F.; Novales, B.; Douliez, J.-P. Self-Assembly, Foaming, and Emulsifying Properties of Sodium Alkyl Carboxylate/Guanidine Hydrochloride Aqueous Mixtures. Langmuir 2011, 27, 4505−4513. (11) Fameau, A.-L.; Saint-Jalmes, A.; Cousin, F.; Houinsou, H. B.; Novales, B.; Navailles, L.; Nallet, F.; Gaillard, C.; Boué, F.; Douliez, J.P. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angew. Chem., Int. Ed. 2011, 50, 8264−8269. (12) Fameau, A.-L.; Zemb, T. Self-Assembly of Fatty Acids in the Presence of Amines and Cationic Components. Adv. Colloid Interface Sci. 2014, 207, 43−64. (13) Vignes-Adler, M.; Weaire, D. New Foams: Fresh Challenges and Opportunities. Curr. Opin. Colloid Interface Sci. 2008, 13, 141−149. (14) Zhu, Y.; Jiang, J.; Cui, Z.; Binks, B. P. Responsive Aqueous Foams Stabilised by Silica Nanoparticles Hydrophobised in Situ with a Switchable Surfactant. Soft Matter 2014, 10, 9739−9745. (15) Balasuriya, T. S.; Dagastine, R. R. Interaction Forces between Bubbles in the Presence of Novel Responsive Peptide Surfactants. Langmuir 2012, 28, 17230−17237. (16) Liang, C.; Harjani, J. R.; Robert, T.; Rogel, E.; Kuehne, D.; Ovalles, C.; Sampath, V.; Jessop, P. G. Use of CO2-Triggered Switchable Surfactants for the Stabilization of Oil-in-Water Emulsions. Energy Fuels 2012, 26, 488−494. (17) Liang, C.; Liu, Q.; Xu, Z. Surfactant-Free Switchable Emulsions Using CO2-Responsive Particles. ACS Appl. Mater. Interfaces 2014, 6, 6898−6904. (18) Su, X.; Robert, T.; Mercer, S. M.; Humphries, C.; Cunningham, M. F.; Jessop, P. G. A Conventional Surfactant Becomes CO2Responsive in the Presence of Switchable Water Additives. Chem. Eur. J. 2013, 19, 5595−5601. (19) Jiang, J.; Zhu, Y.; Cui, Z.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ

observe the homogeneous white emulsion before sparging with CO2. With the introduction of CO2, a clear oil phase separates from the upper layer gradually, and this preparation is finished in almost 2 h. However, the water phase is still white, and further CO2 induces the water phase to become transparent. When the sample is held for 1 day without any operation, the water phase becomes completely transparent.

4. CONCLUSIONS In this work, we introduced three organic amines to decrease the Krafft point of stearic acid effectively. The phase behaviors were investigated systematically by delineating the binary phase diagrams at 25 and 50 °C. Vesicles and planar sheets coexisted in the Lα phase, and only planar sheets existed in the Lα′ phase, which was demonstrated by cryo-TEM. DSC measurements determined the phase-transition temperature, and the apparent viscosity of the typical samples was demonstrated by rheological characterizations. The foaming and defoaming processes were simply controlled by sparging N2 and CO2. The emulsification process was performed using hexadecane as the oil phase, proving that stearic acid was an excellent emulsifier. CO2 induced the demusification process because CO2 decreased the solubility of stearic acid and resulted in the loss of surface activity. Considering that fatty acid is cheap and useful, we hope our work will provide valuable information to the fields of cosmetics and oil exploitation. ASSOCIATED CONTENT

S Supporting Information *

pH values with the increase in amines in the three systems at 25 and 50 °C with cSA = 50 mmol·L−1. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01295.

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This work is financially supported by the National Nature Science Foundation (Grant Nos. 21420102006 & 21273134).

Figure 9. Demulsification process by sparging CO2 at different time intervals (0−4.5 h) and holding at 25 °C without any operation for 1 day.

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Corresponding Author

*Tel: +86-531-88366074. Fax: +86-531-88564750. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5765


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