Bilayers at High pH in the Fatty Acid Soap Systems ... - ACS Publications

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Bilayers at High pH in the Fatty Acid Soap Systems and the Applications for the Formation of Foams and Emulsions Wenlong Xu, Heng Zhang, Yingping Zhong, Liwen Jiang, Mengxin Xu, Xionglu Zhu, and Jingcheng Hao*

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 14, 2015 | http://pubs.acs.org Publication Date (Web): August 10, 2015 | doi: 10.1021/acs.jpcb.5b04553

Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, P.R. China ABSTRACT: In our previous work, we reported bilayers at high pH in the stearic acid/CsOH/H2O system,24 which was against the traditional viewpoint that fatty acid (FA) bilayers must be formed at the pKa of the fatty acid. Herein, the microstructures at high pH of several fatty acid soap systems were investigated systematically. We found that palmitic acid/ KOH/H2O, palmitic acid/CsOH/H2O, stearic acid/KOH/ H2O, and stearic acid/CsOH/H2O systems can form bilayers at high pH. The bilayer structure was demonstrated by cryogenic transmission electron microscopy (cryo-TEM) and deuterium nuclear magnetic resonance (2H NMR), and molecular dynamics simulation was used to confirm the formation of bilayers. The influence of fatty acids with different chain lengths (n = 10, 12, 14, 16, and 18) and different counterions including Li+, Na+, K+, Cs+, (CH3)4N+, (C2H5)4N+, (C3H7)4N+, and (C4H9)4N+ on the formation of bilayers was discussed. The stability of foam and emulsification properties were compared between bilayers and micelles, drawing the conclusion that bilayer structures possess a much stronger ability to foam and stronger emulsification properties than micelles do. formation of bilayers.4,13,14 However, at high pH, micelles of fatty acid soap are mainly existent.14−21 A few papers also reported gels of fatty acid with Na+ as counterions at high pH; the gels contain fibers.22,23 Bilayers are rarely observed at high pH in the fatty acid soap systems; however, in the SA/CsOH/ H2O system we observed bilayers at pH ≫ pKa.24 We assumed that Cs+ is the vital element for the formation of bilayers. Based on the previous observations for the SA/CsOH/H2O system,24 in the present work we reported a systematic investigation on different fatty acid soap systems. The purpose of the study could provide a better understanding of the formation of bilayers in fatty acid systems at high pH. In fact, we have studied the phase behavior of lauric acid (LA) with different counterions of Na+, Cs+, and (C2H5)4N+.25 We found that micelles form at pH ≫ pKa for different counterions. In our primary observations of fatty acid cesium salts with different chain lengths from 10 to 18, we found that at much higher concentration of CsOH, i.e., pH ≫ pKa, two L1/Lα phases after a homogeneous L1 phase was observed in both PA/CsOH/H2O and SA/CsOH/H2O systems. Therefore, we investigated the mixture of different fatty acids with chain lengths of n = 12, 14, 16, and 18 with different counterions including Li+, Na+, K+, Cs+, (CH3)4N+, (C2H5)4N+, (C3H7)4N+, and (C4H9)4N+. We found that four systems, PA/KOH/H2O, PA/CsOH/H2O,SA/KOH/H2O, and SA/CsOH/H2O, can form bilayers at high pH. The phase diagrams including pH

1. INTRODUCTION Fatty acids (FAs) are a kind of cheap and valuable biosurfactant in organisms. The applications of fatty acids originated thousands of years ago; at that time, our ancestor used soaps as detergents. Nowadays, soaps of fatty acids are still widely used in industry and our daily lives, covering the fields of cosmetics, detergents, oil exploitation, etc. Fatty acids possess low surface tension1 and high surface activity,2,3 and soaps of fatty acids can be used as excellent foaming agents4−10 and emulsifiers.7,9,10 The study of phase behavior is of importance for the application of fatty acids. Bilayers are reported to have an obvious advantage over micelles in the realm of foam stability.6,9 Thus, the systematic investigation of the phase behavior of fatty acid soaps can help us choose appropriate concentrations, ratios of fatty acid, and counterions for different applications in industry and our daily lives. The first-phase behavior of fatty acid was investigated as early as the 1920s.11 However, due to the limitation of the characterization techniques, the microstructure was first observed for the publication of the paper titled ‘‘Ufasomes Are Stable Particles Surrounded by Unsaturated Fatty Acid Membranes’’ in 1973,12 in which the authors provided the morphology of fatty acid vesicles by freeze-fracture transmission electron microscopy (FF-TEM) for the first time. After that, scientists prepared bilayers with different fatty acid soap systems. The reports demonstrated that pH plays an important role in the formation of bilayers in fatty acid systems. When the pH is equal to the pKa of the fatty acid assemblies, the amount of protonated and deprononated fatty acid molecules is almost the same, and the hydrogen bonds between them facilitate the © 2015 American Chemical Society

Received: May 12, 2015 Revised: July 31, 2015 Published: August 3, 2015 10760

DOI: 10.1021/acs.jpcb.5b04553 J. Phys. Chem. B 2015, 119, 10760−10767

Article

The Journal of Physical Chemistry B

spectrometer equipped with a pulsed field gradient module (zaxis). The samples were prepared in a 3 mL vial and stirred until the samples became homogeneous phases, and then the samples were transferred into NMR tubes. Finally, NMR tubes were plunged into the spectrometer at the resonance frequency of 61.42 MHz. Typically, 128 scans were accumulated for each spectrum, and a recycle delay of 1.0 s was used. 2.7. Rheological Measurements. The rheological experiments were operated on a HAAKE Rheo Stress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). In oscillatory measurements, an amplitude sweep at a fixed frequency of1 Hz was performed prior to the following frequency sweep to ensure the selected stress was in the linear viscoelastic region. 2.8. Molecular Dynamics Simulation. Molecular dynamics simulations were performed using the GROMACS free software package (version 4.5). The all-atom optimized performance for liquid systems (OPLS-AA) force field was adopted for all of the potential function terms to calculate the interatomic interactions. In the simulation box, 40 stearate anions, 400 K+, and 360 Cl− ions were solvated in as many water molecules (4675) as possible to mimic the surfactant concentration in the experiment. The system was initialized by minimizing the energies of the initial configurations using the steepest descent method. Following the minimization, a 33 ns MD simulation under isothermal−isobaric ensemble (NPT) was carried out for each system, using a time step of 1 fs. Bond lengths were constrained using the LINCS algorithm, and periodic boundary conditions were applied in all directions. Short-range nonbonded interactions were cut off at 12 Å, with long-range electrostatics calculated using the particle mesh Ewald method. Trajectories were stored every 100 ps and visualized using VMD 1.9.1. The last 3 ns trajectories were used for further analysis. 2.9. Foaming Formation. The foaming properties and their stability were determined by a HT FOAMSCAN apparatus (TECLIS, France). An initial sample volume of 60 mL was foamed by sparging N2 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 foams reached a certain volume (200 mL); after that, the stability of the foam was analyzed. 2.10. Emulsification. Emulsions were prepared with different oil/water ratios (rO/W) from 1:9−9:1. Hexadecane was applied as the oil phase. SA was dissolved in the hexadecane first and then mixed with alkali aqueous solutions, and the emulsions were formed simply by vortexing at room temperature.


and conductivity data were presented. The microstructures of bilayers were demonstrated by cryogenic transmission electron microscopy (cryo-TEM) observations and deuterium nuclear magnetic resonance (2H NMR) measurements. Molecule dynamics simulation supported our finding of bilayers at high pH. The rheological data indicated the viscoelasticity of the bilayers samples. Finally, the foam and emulsification properties were compared between bilayers and micelles. We found that bilayers, which can stabilize the foams and emulsions, have much better properties than micelles.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Lauric acid (LA, > 99 wt %) was purchased from Fluka. Myristic acid (MA, > 98 wt %) and palmitic acid (PA, analytical reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Stearic acid (SA, > 98 wt %), NaOH (>98 wt %), CsOH(>99 wt %), (CH3)4NOH (25 wt % aqueous solution), (C2H5)4NOH (25 wt % aqueous solution), (C3H7)4NOH (25 wt % aqueous solution), and (C4H9)4NOH (40 wt % aqueous solution) were purchased from J & K Scientific Co., Ltd. (China). LiOH(>99 wt %) and KOH (>96 wt %) were purchased from Aladdin Chemistry Co., Ltd. Hexadecane (>95 wt %) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Ultrapure water was used with a resistivity of 18.25 MΩ·cm from a UPH-IV ultrapure water purifier. Other agents were of analytical purity. 2.2. Phase Behavior Study. The phase diagrams of the investigated systems were delineated at 25.0 °C. The samples were prepared with the following procedures: a series of concentration of fatty acids and alkalis were added into several tubes, and with the supply of ultrapure water, the total volume of the samples was 5 mL. The concentrations of fatty acids were all fixed at 20 mM, and those of the alkalis were changed from 20 mM to 1.0 M. The homogeneous solutions were obtained by ultrasonic treatment at 50 °C and transferred into an incubator at 25 °C for further study. The phase diagrams were established by visual inspection with the help of crossed polarizers and conductivity measurements. 2.3. Conductivity and pH Measurements. The conductivity measurements were performed on a DDSJ-308A (China) conductivity meter with a DJS-10C glass electrode at room temperature. The values of pH were determined on a PHS-3C pH meter (China) with an E-201-C glass electrode at room temperature. The two-phase samples were detected under stirring. 2.4. ζ Potential Measurements. The ζ potential was measured with a ZetaPALS potential analyzer instrument (Brookhaven) with parallel-plate platinum black electrodes spaced 0.5 cm apart and a rectangular organic glass cell with a 10 mm path length. All samples were measured using a sinusoidal voltage of 80 V with a frequency of 3 Hz. 2.5. Cryogenic Transmission Electron Microscopy Observations. A drop of sample solution (approximately 4 μL) was dropped on a grid in a high-humidity environment (>90%). The excess sample was blotted up by two pieces of blotting paper, leaving a thin film sprawling on the grid. The grid was then plunged into liquid ethane that was frozen by liquid nitrogen. The vitrified sample was transferred into a sample holder (Gatan626) and observed on a JEOL JEM-1400 TEM (120 kV) at about −174 °C. The images were recorded on a Gatan multiscan CCD. 2.6. Deuterium Nuclear Magnetic Resonance Spectra. 2 H NMR measurements were operated on a Bruker Avance 400

3. RESULTS AND DISCUSSION 3.1. Phase Behaviors and the Stability of the Lα-Phase. In our previous study,24 the bilayers of stearic acid with Cs+ as counterion at high pH were found. We proposed that Cs+ ions play a vital role in the formation of the bilayers. Cs+ can screen the electrostatic repulsion between the deprotonated stearic acid molecules and decrease the distance between the polar head groups. According to the packing parameter theory,26 p = v/a0l, the average headgroup area decreases, and the packing parameter p increase, resulting in the formation of bilayers. In the present study, the phase behaviors of different fatty acids and alkalis were investigated at cOH− ≫ cFA to detect the influence on the formation of bilayers. As shown in Table 1, only PA and SA with counterions of K+ and Cs+ can form bilayers. 10761


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systems can stay stable for more than 1 year. Derjaguin and Landau, Verwey, and Overbeek (DLVO) theory27,28 proposes that the stability of a solution is dependent on the balance of the repulsion energy and attraction energy. If the repulsion energy is larger than the attraction energy, the aggregates repel each other, and the solution will stay stable. On the contrary, the solution will form larger aggregates and eventually deposit as time goes on. ζ-potential can reflect the surface charge of the aggregate and predict the stability of the solution.29−32 Counterions penetrate into the electrical double layer inducing the decrease of ζ-potential, which decreases the repulsion energy, and finally, the solution can not stay stable. Generally, a solution with a larger ζ-potential value is considered to be stable. The ζ-potential of the typical samples in the three systems were measured as follows: −46.6 ± 2.38 mV for the sample 20 mM PA and 200 mM KOH, −34.28 ± 3.46 mV for the sample 20 mM SA and 200 mM KOH, and −7.43 ± 1.99 mV for the sample 20 mM PA and 400 mM CsOH. These values are in agreement with our observation that the Lα-phase samples in the PA/CsOH/H2O system are unstable. Therefore, the PA/CsOH/H2O system was not in the consideration in the following experiments. 3.2. Microstructures of the Lα Phase. 2H NMR measurements are powerful characterizations to identify the aggregate morphology in the solution by replacing H2O into D2O. Due to the interactions of the deuteron quadrupole moment with the electric field gradient active on the nucleus, 2 H NMR spectra can recognize different microstructures according to the appearance of the splitting peak.33,34 Several samples in the Lα phase of the PA/KOH/H2O and SA/KOH/ H2O systems were characterized by 2H NMR measurements to identify the microstructures in the solution. From Figure 2, one can observe the incompletely splitting doublets for the Lα-phase samples, corresponding to planar sheet structure, which is

Table 1. Phase Behavior of Fatty Acid with Excess Alkalis (cFA = 20 mM)a LiOH NaOH KOH CsOH (CH3)4NOH (C2H5)4NOH (C3H7)4NOH (C4H9)4NOH


a

LA

MA

PA

SA

P G L1 L1 L1 L1 L1 L1

P G L1 L1 L1 L1 L1 L1

P G Lα Lα L1 L1 L1 L1

P G Lα Lα L1 L1 L1 L1

P: precipitation. G: gel. Lα: bilayers. L1: micelle.

We systematically focused on the investigation of the phase behaviors and microstructures for the three systems, PA/KOH/ H2O, PA/CsOH/H2O, and SA/KOH/H2O. The phase diagrams, including pH and conductivity data, are exhibited in Figure 1. At a fixed fatty acid concentration of cFA = 20 mM with the addition of KOH or CsOH, the phase behaviors change from L1/Lα phase to homogeneous Lα phase and L1/Lα phase finally, agreeing with our previous observations in the SA/CsOH/H2O system.24 The pH of the Lα phase is near 13 or larger than 13 for the three systems. It is much higher than the pKa values of palmetic acid assemblies (pKa = 8.7) and stearic acid assemblies (pKa = 10).10,13 The conductivity data can identify the phase boundaries of the Lα phase. When it is in the L1/Lα phase, the conductivity increases sharply, but it shows a decrease or a slow increase in the Lα phase with the increase of alkali, indicating the binding of counterions to bilayers.24 Given enough time, the homogeneous Lα-phase samples in the PA/CsOH/H2O can separate into the L1/Lα phase. The Lαphase samples in the PA/KOH/H2O and SA/KOH/H2O

Figure 1. Phase diagrams of the PA/KOH/H2O, SA/KOH/H2O, and PA/CsOH/H2O systems, including conductivity and pH data (cFA = 20 mM). 10762


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Figure 2. 2H NMR spectra of the Lα-phase samples of the PA/KOH/H2O (a) and SA/KOH/H2O (b) systems (cFA = 20 mM).

consistent with the previous observation.20,24 The typical samples in the Lα phase for the two systems were characterized by cryo-TEM observations to detect the microstructures. The unclosed planar sheets structures can be observed in Figure 3, which is the direct evidence for the planar sheet structure of the bilayers.24

As we referred to in the SA/CsOH/H2O system in the previous work,24 the counterions played the vital role in the formation of bilayer at high pH. A precipitation phase for FA/ LiOH/H2O systems and a gel phase for FA/NaOH/H2O systems can be observed. These are easy to understand because of the weak alkalinity of LiOH for the FA/LiOH/H2O systems, and many papers have reported the gel formation for the FA/ NaOH/H2O systems.22,23 Only the micelle phase can be observed at high pH in the FA/organic alkali/H2O systems. A clear comparison of the appearance between bilayers and micelles is shown in Figure 5. The bilayer solutions exhibit a bluish appearance by the naked eye and birefringent textures in the crossed polarizers, while the micelle samples are colorless and transparent, and the birefringent textures cannot be observed in the crossed polarizers. The viscoelasticity and the apparent viscosity (η) of the typical samples were determined by rheological characterizations. As shown in Figure 6a, for both of the Lα-phase samples in the PA/KOH/H2O and SA/KOH/H2O systems, the elastic modulus G′ and the viscous modulus G″ are both independent of the shear frequency, and G′ is an order of magnitude larger than G″, indicating the elasticity dominating properties. From Figure 6b, the apparent viscosity (η) of the Lα-phase samples is 2−3 orders of magnitude larger than that of the L1-phase samples. In addition, η of the Lα-phase samples shows shear thinning in the whole shear rate range, meaning that the transition of the bilayers structures with the increase of shear rate. However, it shows this independent of shear rate for the L1-phase samples at high shear rate, exhibiting the Newtonian fluid feature. According to the literature, the counterions are attached to the bilayers in the form of hydrated ions.39 Counterions with smaller hydration radii are easier to penetrate into the stern layer of the bilayers, possessing a stronger ability for screening electrostatic repulsion. It has been reported that the hydration radius is in the order of Cs+ (0.329 nm) < K+ (0.331 nm) < (CH3)4N+ (0.347 nm) < Na+ (0.358 nm) < Li+ (0.382 nm) < (C2H5)4N+ (0.400 nm) < (C3H7)4N+ (0.452 nm) < (C4H9)4N+ (0.494 nm).40 Thus, K+ and Cs+ have larger screen effects in the electrostatic repulsion interaction, resulting in the dense stacking of the surfactant molecules. However, the fatty acids with larger chain lengths can provide stronger hydrophobic forces. Combining these two factors, the packing parameter, p, increases, and bilayers can be formed at high pH. The proposed mechanism is shown in Scheme 1. Usually, bilayers can be formed at the pKa of the fatty acid with the help of hydrogen

Figure 3. Cryo-TEM images of the typical Lα-phase samples in the systems of PA/KOH/H2O (a) and SA/KOH/H2O (b) (cFA = 20 mM, cKOH = 200 mM, scale bar =1 μm).

The molecular dynamics simulation was performed to support the bilayer structures.35,36 As shown in Figure 4a, the bilayer structures can be observed. The hydrophilic groups of −COO− (red) are located at the terminals of the bilayers, and the hydrophobic groups (cyan and white) are in the center of the bilayers, with K+ (purple) surrounding the terminal of the bilayers. The close combination of K+ and −COO− can be clearly seen in the top view (Figure 4b). The distance between OCOO- and the binding K+ is as short as 0.268 nm (Figure 4c), indicating the strong interaction between K+ and the polar head groups. The distance between the hydrophilic groups is shortened to 0.456 nm (Figure 4d), resulting in the decrease of a and the increase of p; therefore, the bilayers are formed. 3.3. Influence of Chain Length and Counterions. According to Table 1, fatty acids with longer chain length (PA and SA) can form the Lαphase, indicating that hydrophobic forces play a vital role in the formation of the Lα phase. In our previous paper,37 we investigated the lyotropic liquid crystal phases of perfluorinated fatty acid lithium salts in aqueous solutions. The perfluorinated fatty acid lithium salts with a larger chain length (n = 12 and 14) can form lyotropic liquid crystal phases, but a shorter chain length (n = 8 and 10) cannot. As we well know, with the increase of chain length, hydrophobic forces increase. The surfactant molecules should stack more densely and induce a larger parameter, p, which is easier to form bilayer structures.26,38 10763


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Figure 4. Snapshots of bilayer fragment of the SA/KOH/H2O system in the molecular dynamics simulation, front view (a) and top view (b). Carbon (C), oxygen (O), hydrogen (H), and potassium (K) are presented by cyan, red, white, and purple spheres, respectively. Water molecules were omitted for clarity. Also shown: radial distribution functions of K+−OCOO− (c) and OCOO−−OCOO− (d).

bonds between protonated and deprotonated fatty acid molecules. When the pH is higher than pKa, micelles are the general structure due to the strong electrostatic repulsion forces between the deprotonated fatty acid molecules. However, if the fatty acid is PA or SA and the counterion is K+ or Cs+, the strong hydrophobic forces of the hydrophobic chains and the electrostatic screening effect result in the formation of bilayers at high pH. 3.4. Foaming and Emulsification Properties. The foaming and emulsification properties were compared between the bilayers and micelles of fatty acids. It was reported that the foam stability of the bilayer solution is higher than that of the micelle solution. Thus, we chose the typical samples from the FA/KOH/H2O system as the bilayer solution and the FA/ (C4H9)4NOH/H2O system as the micelle solution to perform the foaming test. From Figure 7a, we can observe that the foam stability of the bilayer solution of PA/KOH/H2O system is much higher than that of the micelle solution of the PA/ (C4H9)4NOH/H2O system. The foaming time for reaching the

Figure 5. Photographs of the typical samples in different systems with (down) and without (up) polarizers. (a) PA/KOH/H2O, (b) PA/ (CH 3 ) 4 NOH/H 2 O, (c) PA/(C 2 H 5 ) 4 NOH/H 2 O, (d) PA/ (C3H7)4NOH/H2O, (e) PA/(C4H9)4NOH/H2O, (f) SA/KOH/ H2O, (g) SA/(CH3)4NOH/H2O, (h) SA/(C2H5)4NOH/H2O, (i) SA/(C3H7)4NOH/H2O, and (j) SA/(C4H9)4NOH/H2O; cSA = 20 mM and calkali = 200 mM.

Figure 6. Oscillatory (a) and steady (b) shear rheograms of the typical samples with cFA = 20 mM and calkali = 200 mM. 10764


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Scheme 1. Microstructures at Different pH of the Fatty Acid Soap System

Figure 7. Foaming and the stability of the foams for the PA/alkali/H2O system (a) and the SA/alkali/H2O system (b); cFA = 20 mM and calkali = 200 mM.

preset foaming volume (200 mL) is 202 and 114 s for the bilayer solution and the micelle solution, respectively. However, the half-life time of the foams stabilized by bilayers is 1700 s, while that of the micelles is about 400 s, and the foams disappear completely in 700 s. It is the same for the SA/alkali/ H2O system (Figure 7b), with the half-life times of 1540 and 367 s corresponding to bilayer and micelle solutions, respectively. These data indicate that the foams stabilized by bilayers possess much better stability than those stabilized by micelles. The emulsification properties were also tested by the four sample solutions. With FA predissolved in hexadecane, the emulsions were formed simply by mixing the oil phase and the alkali aqueous solution. The comparison was made between bilayers and micelles in the formation of homogeneous emulsions from rO/W = 1:9 to 9:1. We found that the bilayerstabilized emulsions with rO/W = 1:9 to 6:4 could stay stable as homogeneous white solutions for long time, while phase separation could be observed in a few minutes for all the emulsion stabilized by micelles. An obvious contrast can be clearly seen from Figure 8. The emulsions stabilized by bilayers of PA/KOH/H2O and SA/KOH/H2O can stay homogeneous, while those stabilized by the micelles of PA/(C4H9)4NOH/ H2O and SA/(C4H9)4NOH/H2O separate into three phases: water phase in the bottom, oil phase on the top, and emulsion in the middle. In literature, they also reported that the bilayers stabilized emulsions; however, those only can be stable for 2 h9 or several days,7 while in our system the samples of emulsions can stay stable for several months. From the above analysis, we

Figure 8. Photographs of the emulsions (rO/W = 5:5) stabilized by the four systems: PA/KOH/H 2 O (a), SA/KOH/H 2 O (b), PA/ (C4H9)4NOH/H2O (c), and SA/(C4H9)4NOH/H2O (d) (cFA = 20 mM and calkali = 200 mM).

concluded that the bilayers possessed much better foaming and emulsification properties.

4. CONCLUSIONS In summary, the bilayers at high pH were investigated systematically in several fatty acid soap systems, including fatty acids with different chain length (n = 10, 12, 14, 16, and 18) and different counterions including Li+, Na+, K+, Cs+, (CH3)4N+, (C2H5)4N+, (C3H7)4N+, and (C4H9)4N+. We found that four systems of PA/KOH/H2O, SA/KOH/H2O, PA/ CsOH/H2O, andSA/CsOH/H2O can form bilayers at high pH. The phase behaviors were investigated and the phase 10765


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Composed of Monocarboxylic or Dicarboxylic Fatty Acids and Trimethylammonium Amphiphiles. Langmuir 2011, 27, 14078−14090. (15) Feinstein, M. E.; Rosano, H. L. The Influence of Micelles on Titrations of Aqueous Sodium and Potassium Soap Solutions. J. Phys. Chem. 1969, 73, 601−607. (16) Cistola, D. P.; Hamilton, J. A.; Jackson, D.; Small, D. M. Ionization and Phase Behavior of Fatty Acids in Water: Application of the Gibbs Phase Rule. Biochemistry 1988, 27, 1881−1888. (17) 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, Biomembr. 2002, 1559, 1−9. (18) Morigaki, K.; Walde, P. Fatty Acid Vesicles. Curr. Opin. Colloid Interface Sci. 2007, 12, 75−80. (19) Dowling, M. B.; Lee, J. H.; Raghavan, S. R. pH-Responsive Jello: Gelatin Gels Containing Fatty Acid Vesicles. Langmuir 2009, 25, 8519−8525. (20) Xu, W.; Song, A.; Dong, S.; Chen, J.; Hao, J. A Systematic Investigation and Insight into the Formation Mechanism of Bilayers of Fatty Acid/Soap Mixtures in Aqueous Solutions. Langmuir 2013, 29, 12380−12388. (21) Fameau, A. L.; Arnould, A.; Saint-Jalmes, A. Responsive SelfAssemblies Based on Fatty Acids. Curr. Opin. Colloid Interface Sci. 2014, 19, 471−479. (22) Yuan, Z.; Lu, W.; Liu, W.; Hao, J. Gel Phase Originating from Molecular Quasi-Crystallization and Nanofiber Growth of Sodium Laurate-Water System. Soft Matter 2008, 4, 1639−1644. (23) Doscher, T. M.; Vold, R. D. The Stability of Sodium Stearate Gels. J. Am. Oil Chem. Soc. 1949, 26, 515−519. (24) 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. (25) Xu, W.; Wang, X.; Zhong, Z.; Song, A.; Hao, J. Influence of Counterions on Lauric Acid Vesicles and Theoretical Consideration of Vesicle Stability. J. Phys. Chem. B 2013, 117, 242−251. (26) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of SelfAssembly of Hydrocarbon Amphiphiles into Micelles and Bilayers. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525−1568. (27) Derjaguin, B.; Landau, L. Theory of the Stability of Strongly Charged Lyophobic Sols and of the Adhesion of Strongly Charged Particles in Solutions of Electrolytes. Acta Physico Chemica URSS 1941, 14, 633−662. (28) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, The Netherlands, 1948. (29) Stachurski, J.; Michałek, M. The Effect of the ζ Potential on the Stability of a Non-Polar Oil-in-Water Emulsion. J. Colloid Interface Sci. 1996, 184, 433−436. (30) Celik, M. S.; Yasar, E.; El-Shall, H. Flotation of Hetero coagulated Particulates in Ulexite/SDS/Electrolyte System. J. Colloid Interface Sci. 1998, 203, 254−259. (31) Hunter, R. J. ζ Potential in Colloid Science: Principle and Applications; Academic Press Inc: Salt Lake City, Utah, 1981. (32) Zhang, Y.; Yang, M.; Portney, N. G.; Cui, D.; Budak, G.; Ozbay, E.; Ozkan, M.; Ozkan, C. S. Zeta Potential: A Surface Electrical Characteristic to Probe the Interaction of Nanoparticles with Normal and Cancer Human Breast Epithelial Cells. Biomed. Microdevices 2008, 10, 321−328. (33) Firouzi, A.; Schaefer, D. J.; Tolbert, S. H.; Stucky, G. D.; Chmelka, B. F. Magnetic-Field-Induced Orientational Ordering of Alkaline Lyotropic Silicate- Surfactant Liquid Crystals. J. Am. Chem. Soc. 1997, 119, 9466−9477. (34) Mohanty, A.; Dey, J. Effect of the Headgroup Structure on the Aggregation Behavior and Stability of Self-Assemblies of Sodium N-[4(n-Dodecyloxy)benzoyl]- L-aminoacidates in Water. Langmuir 2007, 23, 1033−1040. (35) Morrow, B. H.; Koenig, P. H.; Shen, J. K. Atomistic Simulations of pH-Dependent Self-Assembly of Micelle and Bilayer from Fatty Acids. J. Chem. Phys. 2012, 137, 194902.

boundaries were identified with the help of conductivity. The bilayer microstructures were demonstrated by cryo-TEM, 2H NMR measurements, and molecular dynamics simulation. The influence of chain length and counterions on the microstructures was discussed, and a formation mechanism was presented. Finally, the foaming and emulsification properties were compared between bilayers and micelles, and we concluded that the bilayers possessed much better foaming and emulsification properties than micelles did.

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AUTHOR INFORMATION

Corresponding Author

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


The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the NSFC (grant nos. 21273134 and 21420102006). REFERENCES

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Bilayers at High pH in the Fatty Acid Soap Systems ... - ACS Publications

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