Oil-in-Water Emulsions Stabilized by Acylglutamic AcidâAlkylamine

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O/W Emulsions Stabilized by Acylglutamic Acid-Alkylamine Complexes as Noncovalent-Type Double-Chain Amphiphiles Toru Tojinbara, Masaaki Akamatsu, Kenichi Sakai, and Hideki Sakai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03468 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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O/W Emulsions Stabilized by Acylglutamic Acid-Alkylamine Complexes as Noncovalent-Type Double-Chain Amphiphiles Toru Tojinbara,1 Masaaki Akamatsu,1 Kenichi Sakai,1,2* and Hideki Sakai1,2 1. Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. 2. Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. * To whom correspondence should be addressed: [email protected]

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Abstract We have studied the preparation and stabilization mechanism of oil-in-water (O/W) type emulsions in the presence of amphiphilic 1:1 stoichiometric complexes of acylglutamic acids (CnGlu) with tertiary alkylamines (CnDMA). Relatively stable emulsions were obtained when C16Glu-C16DMA (or C18Glu-C18DMA), hexadecane, and water were homogenized at 80 °C and then stored at room temperature. The gel–liquid crystal phase transition temperature (Tc) of C16Glu-C16DMA and C18Glu-C18DMA dispersed in water was determined to be ca. 39 and 53 °C, respectively. This indicates that the complexes form an adsorbed layer at the oil/water interface during the homogenization process above the Tc, and then change into a gel during storage at room temperature. The gel phase formed at the oil/water interface prevents the oil droplets from coalescing. In contrast, shorter chain analogues (C10Glu-C10DMA and C12Glu-C12DMA) did not yield stable emulsions since their adsorption layers were not able to prevent coalescence of the oil droplets (i.e., the Tc of these analogues was below the room temperature). We have also demonstrated that the dispersion stability of these emulsion systems can be controlled by changing the aqueous pH.


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1. Introduction Dimeric or gemini surfactants are promising candidates as stabilizers for liquid-based dispersion systems owing to their excellent adsorption capability even at low concentrations. Thus, these surfactants are categorized as environmentally friendly materials. One important issue to consider from the industrial standpoint is the difficult synthesis and purification of such high-performance surfactants, leading to high selling prices in the market.1 We envisioned that the use of acid–base reactions, e.g., between alkyl carboxylic acid and alkyl amine compounds, would be a possible cost-effective method to obtain such high-performance surfactants. Recently, a series of amphiphilic compounds obtained by acid–base complex formation have been reviewed by Wang et al.2 These amphiphilic compounds are sometimes called “pseudo-gemini”,3-6 “counterion-coupled gemini (cocogem)”,7,8 or “gemini-like”9 surfactants. Most previous reports on such noncovalent-type amphiphiles have mainly focused on their aqueous solution properties, and the number of papers reporting their stabilization properties in (macro)emulsion systems is very limited. Noori et al. reported that amphiphilic complexes of 1,6-bis(N,N-alkyldimethylammonium)adipate yielded toluene-in-water emulsions and that the dispersion stability increased with the increasing alkyl chain length.8 Recently, reversible control of emulsification/demulsification was achieved by Sun et al.10 using a CO2-switchable amphiphilic complex between oleic acid and a primary diamine compound with poly(oxypropylene) units. Systematic studies for the development of emulsion formulations with cost-effective environmentally friendly surfactant systems are necessary not only in academia but also in industry. In our previous work,11 we demonstrated that acylglutamic acids (CnGlu) form stoichiometric 1:1 gemini-like amphiphilic complexes with tertiary alkylamines (CnDMA). A typical chemical structure of the CnGlu-CnDMA complexes is shown in Figure 1. The complexes exhibit unique rheological behavior in aqueous solution. That is, a reduction of the pH from neutral to weakly acidic conditions leads to an increased viscosity of the solution as a result of the formation of wormlike micelles. A further reduction of the solution pH leads to a decreased viscosity, resulting from branching or interconnection of the wormlike micelles. These transitions are caused by changes in the acidity of the carboxylic acid headgroups with the pH. We have also reported that long-chain CnGlu-CnDMA complexes yield hydrogels at a certain pH range.12 Hereafter, we demonstrate the preparation of oil-in-water (O/W) type emulsions in the presence of amphiphilic CnGlu-CnDMA complexes as stabilizers. We also discuss the stabilization mechanism of the emulsion on the basis of analytical results, including data from differential


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scanning calorimetry (DSC), small and wide angle X-ray scattering (SWAXS), dynamic light scattering (DLS), and freeze-fracture transmission electron microscopy (FF-TEM) measurements. O

CH3(CH2)(n-2)

O

OH

N H

O OH

CH3(CH2)(n-1)

N

Figure 1. A typical chemical structure of CnGlu-CnDMA (n = 10, 12, 14, 16, and 18).

2. Experimental 2.1 Materials CnGlu-CnDMA complexes (n = 10, 12, 14, 16, and 18) were synthesized according to the procedure described in our previous papers.11,12 Hexadecane (Wako) was used as the oil phase. The pH was adjusted using 1 mol dm-3 HCl (Wako) and 1 mol dm-3 NaOH aqueous solutions (Wako). The water used in this study was deionized with a Barnstead NANO Pure DIamond UV system and filtered with a Millipore membrane filter (0.22 µm pore size). 2.2 Instruments DSC measurements were carried out using a Rigaku DSC8230 calorimeter. The measurement conditions were 1 °C min-1 for the scanning rate and 10–60 °C for the scanning range. We used an aluminum pan for these measurements and α-alumina particles as the standard material. SWAXS measurements were performed using an Anton Paar SAXSess camera equipped with a PANalytical PW3830 laboratory X-ray generator, a multilayer film Goebel mirror, a block collimator, a semi-transmissible beam stop, a TCS120 temperature controller, and an imaging plate detector. The apparatus was operated at 40 kV and 50 mA using Cu-Kα X-rays (wavelength of 0.154 nm). DLS measurements were carried out using an Anton-Paar Litesizer 500 particle analyzer. Emulsion samples were diluted with pure water when the sample transparency was not enough to measure the light scattering.


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A quick freezing specimen preparation device (Leica EM CPC) was used for the preparation of frozen samples for the FF-TEM measurements. A frozen replica prepared on a Hitachi High-Technologies FR-7000A apparatus was used for the freeze-fracture of specimens and vapor deposition for specimen preparation. Electron microscopy imaging was carried out using a Hitachi High-Technologies H-7650 TEM at an applied voltage of 120 kV.

3. Results and Discussion 3.1 Thermal behavior of CnGlu-CnDMA dispersed in aqueous media Before studying the emulsification procedure, the thermal behavior of aqueous dispersions of CnGlu-CnDMA complexes was studied. The aqueous samples were prepared as follows; CnGlu-CnDMA and water were added to a 30 cm3 glass vial and left to stand at 80 °C in a temperature-controlled water bath. Then the mixture was homogenized using a Nissei US-300T probe-type ultrasonic homogenizer (20 kHz) for 3 min. The DSC results are shown in Figure 2. These measurements were performed at a fixed concentration of CnGlu-CnDMA = 5 wt%. No peak was detected for the C10Glu-C10DMA system in the temperature range of 10–60 °C. In contrast, endothermic peaks were observed for the other CnGlu-CnDMA analogues, indicating the occurrence of gel–liquid crystal phase transitions in this temperature range, as reported in the literature.13

Figure 2. DSC thermograms obtained for aqueous dispersions of CnGlu-CnDMA. We further analyzed the gel–liquid crystal phase transition through SWAXS measurements in the aqueous CnGlu-CnDMA dispersion systems (n = 10, 12, 16 and 18). As shown in Figure 3, a sharp peak was observed at 25 °C in the wide angle region for the C16Glu-C16DMA and


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C18Glu-C18DMA systems, whereas the peak disappeared at 70 °C. Furthermore, such a peak was not detected for the C10Glu-C10DMA and C12Glu-C12DMA systems even at 25 °C. These X-ray scattering results are consistent with the DSC results shown in Figure 2. We also note that repeated peaks were observed in the small angle region, where the scattering vector q value is calculated as 1:2. This suggests the formation of lamellar assemblies in the aqueous dispersion systems, although the peaks are sometimes broad and weak.

Figure 3. SWAXS patterns measured for the aqueous dispersions of CnGlu-CnDMA (n = 10, 12, 16, and 18) at 25 and 70 °C. The CnGlu-CnDMA concentrations were set at 70 wt% (n = 10), 50 wt% (n = 12), 40 wt% (n = 16), and 40 wt% (n = 18). Clearly, the gel–liquid crystal phase transition temperature (Tc) increased with the increasing chain length of CnGlu-CnDMA (see Figure 2). Again, this is consistent with the behavior observed for phosphatidylcholine systems.13 It is important to note that, at room temperature (ca. 25 °C), C10Glu-C10DMA, C12Glu-C12DMA, and C14Glu-C14DMA were in the liquid crystal state (i.e., the alkyl chains were molten), whereas C16Glu-C16DMA and C18Glu-C18DMA were in the gel state (i.e., the alkyl chains were frozen). We noted that the Tc of C14Glu-C14DMA was very close to the room temperature, hampering the evaluation of the emulsion stability in the presence of this complex. The study of the C14Glu-C14DMA system was thus abandoned because of this difficulty. 3.2 Characterization of O/W emulsions stabilized by CnGlu-CnDMA On the basis of the DSC and SWAXS results, we examined the emulsification process as follows. In the first step, CnGlu-CnDMA (0.5 wt%) and water were added to a 30 cm3 glass vial and left to stand in a temperature-controlled water bath. The mixture was then homogenized using the probe-type ultrasonic homogenizer for 3 min. In the second step, hexadecane (20 wt%) was added to the glass vial, and the mixture was again shaken using the probe-type ultrasonic homogenizer for 3 min. Finally, the sample was left to stand at room temperature (ca. 25 °C).


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In order to see the temperature effect on the emulsion stability, we considered the following two conditions: the temperature was maintained at either (i) ca. 25 °C or (ii) ca. 80 °C in the first and second mixing steps. The visual observation results are shown in Figure 4. The emulsion samples were permitted to stand at the room temperature for 3 days in method (i) and for 1 week in method (ii), respectively. As shown in Figure 4a, the emulsion samples prepared by method (i) were not stable for all of the CnGlu-CnDMA systems. The emulsions with C10Glu-C10DMA and C12Glu-C12DMA experienced rapid creaming, whereas a separation of the oil phase was observed for the C16Glu-C16DMA and C18Glu-C18DMA systems. In addition, precipitation of the complex was also observed at the bottom of the vial in the C16Glu-C16DMA and C18Glu-C18DMA systems. On the other hand, the dispersion stability was significantly improved with C16Glu-C16DMA and C18Glu-C18DMA when method (ii) was applied for the emulsion preparation. This suggests that the temperature in the emulsification process is a key factor for controlling the dispersion stability of the emulsions. We thus characterized the emulsion samples prepared by method (ii) and investigated their stabilization mechanism.


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Figure 4. Visual appearance of the emulsion samples prepared by the two methods. The arrows indicate the oil phase separated from the emulsion phase. DLS measurements were performed in order to evaluate the size of the emulsion droplets. The results are shown in Figure 5, where only the data of two systems (i.e., C12Glu-C12DMA and C16Glu-C16DMA) are shown as typical examples. It can be seen that the droplet size for the C12Glu-C12DMA emulsion system is significantly larger than that for the C16Glu-C16DMA emulsion, even immediately after preparation. Larger droplet sizes cause rapid creaming, as observed in Figure 4b. More importantly, after one week, the droplet size significantly increased in the C12Glu-C12DMA system whereas it remained almost unchanged in the C16Glu-C16DMA system. This indicates that coalescence between emulsion droplets barely occurred in the C16Glu-C16DMA system.


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12 Intensity distribution (%)

(a)

1 week after preparation

10 8 6 4

Immediately after preparation

2 0 10

100

Diameter (nm)

1000

10000

12 Intensity distribution (%)

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(b)

10

1 week after preparation

8 6 4

Immediately after preparation

2 0 10

100

Diameter (nm)

1000

10000

Figure 5. Particle size distribution obtained from DLS measurements for the (a) C12Glu-C12DMA and (b) C16Glu-C16DMA emulsion systems. These emulsion samples were prepared by method (ii); the samples were cooled to room temperature after the homogenization at 80 °C and then stored at the room temperature for the standing period. We also performed FF-TEM measurements in order to see the surface morphology of the emulsion droplets. The results are shown in Figure 6. Again, only the results obtained for the C12Glu-C12DMA (Figure 6a) and C16Glu-C16DMA (Figure 6b) systems are shown here. In Figure 6b, a stripe-structure is observed on the emulsion droplet, while no significant structure is seen in the continuous phase. Since the FF-TEM technique provides morphological information about fractured surface, the observed stripes suggest the presence of a lamellar arrangement on the droplet. In this case, the lamellar structure is expected to be formed parallel to the oil/water interface under an assumption that the droplet is fractured like “terraced fields” on the curved surface. We note that a similar morphology was reported for a cosmetic emulsion system stabilized by an amphiphilic random copolymer.14 In contrast, such a stripe-structure is not observed in the C12Glu-C12DMA emulsion system (Figure 6a). This suggests that C12Glu-C12DMA is adsorbed at the oil/water interface but the adsorbed layer morphology is less ordered than that of the


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C16Glu-C16DMA emulsion.

Figure 6. FF-TEM images of emulsion droplets stabilized by (a) C12Glu-C12DMA and (b) C16Glu-C16DMA. The emulsion samples employed for these measurements were prepared by method (ii); the samples were cooled to room temperature after the homogenization at 80 °C, and then a sample replica for FF-TEM measurements was prepared within 1 day from the cooling. On the basis of the results shown in Figures 2–6, we propose a possible mechanism regarding the stabilization of the CnGlu-CnDMA emulsion systems. Since the Tc of C16Glu-C16DMA dispersed in water is ca. 39 °C, the alkyl chains of C16Glu-C16DMA are molten in the first and second homogenization processes at 80 °C. During the second homogenization with hexadecane, C16Glu-C16DMA forms an adsorbed layer at the oil/water interface. Then, subsequent cooling to 25 °C leads to gelation of the adsorbed layer, preventing the oil droplets from coalescing. We observed a separation of the oil phase even for the C16Glu-C16DMA system when the first and second homogenizations were performed at 25 °C (Figure 4a). This results from a limited adsorption capability of the complex below its Tc. On the other hand, the Tc of C12Glu-C12DMA dispersed in water is lower than 25 °C. This implies that the alkyl chains of C12Glu-C12DMA are molten even during storage of the emulsion system. As a result, the adsorbed layer is not able to prevent the coalescence of oil droplets, and hence the size of the emulsion droplets increases with time. This further leads to relatively fast creaming. A similar stabilization mechanism has been reported in our previous paper, where a phosphorylcholine-type zwitterionic gemini amphiphile was employed as an emulsion stabilizer.15 However, in this previous work, such a stripe structure was


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not observed on the oil droplet surface. Control of the liquid/liquid interfacial properties is key in the field of emulsion technology. In particular, the following classic emulsions exhibit an independent “third phase” between two liquid phases, improving the stability against droplet coalescence: Pickering emulsions,16 three-phase emulsions,17 and liquid crystal emulsions.18 We have also proposed a new amphiphilic material concept called “active interfacial modifier (AIM)”.19,20 AIMs cotain moieties attracted to each of the immiscible liquid phases, but are intrinsically insoluble in each phase. As a result of their nature, AIMs form an independent interfacial layer or phase around the emulsion droplets. In our current example, the gel phase formed on the oil droplets acts as a third phase preventing the oil droplets from coalescing, similar to the classic emulsion systems mentioned above and our AIM emulsion systems. 3.3 Dispersion stability as a function of the pH CnGlu-CnDMA is a pH-sensitive material, and hence the stability of the emulsion samples is necessarily affected by the pH. Figure 7 shows the visual appearance of emulsions stabilized by C16Glu-C16DMA at different pH values. These samples were prepared by the preparation method (ii), and then the pH of the aqueous phase was changed by adding small amounts of aqueous HCl or NaOH solutions. The samples were manually shaken for 1 min at this stage. The pictures shown in Figure 7 were taken after 1 h from this additional shaking step. The dispersion stability of the emulsion samples was relatively high at pH 6.2 and 6.9. In contrast, the emulsion sample at pH 5.2 experienced aggregation of the oil droplets and subsequent rapid creaming. The pH change to the acidic condition results in the protonation of the carboxylate headgroups. The dissociation constant of the C16Glu-C16DMA complex to the monomers (i.e., C16Glu and D16DMA) is not known in the emulsion system. However, in our previous work focusing on the pH-sensitive wormlike micellar solution of C12Glu-C12DMA, we suggested that the charge density (or acidity) of the carboxylate headgroups controls the overall curvature of the micelles, even if the pH shift also leads to a change in the protonation degree of C12DMA.11 From this point, we hypothesize that, at the acidic pH, the electrostatic repulsion between oil droplets is significantly reduced as a result of the protonation of the carboxylate headgroups, and hence aggregation between droplets occurs. In addition, the desorption of the C16Glu-C16DMA complex from the oil/water interface may also occur at this pH as a result of the aggregation or phase separation (solidification) of the complex itself. Again this behavior is consistent with our previous finding that the C12Glu-C12DMA complex precipitates in relatively strong acidic solution.


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Clearly, the desorption of the C16Glu-C16DMA complex from the interface will lead to coalescence of the droplets, prompting a subsequent rapid creaming event. At pH 11.3, the dispersion stability was also low and an oil phase was separated at the top of the sample. The carboxylate headgroups are fully deprotonated at this pH. Although the dissociation constant of the complex to the monomers is again not known under the current conditions, it seems likely that the pH shift causes dissociation of C16Glu into the aqueous phase. In other words, the adsorption layer formed on the oil droplets collapses in the alkaline solution, and thus coalescence of the oil droplets is observed. In conclusion, the results obtained here demonstrate that the dispersion stability of this emulsion system can be controlled by changing the aqueous pH, as expected.

Figure 7. Visual appearance of the emulsion samples stabilized by C16Glu-C16DMA at different pH values.

4. Conclusions In this study, we have prepared O/W emulsions in the presence of amphiphilic 1:1 stoichiometric complexes of CnGlu-CnDMA. Relatively stable emulsions were obtained when C16Glu-C16DMA (or C18Glu-C18DMA), hexadecane, and water were emulsified at 80 °C and then stored at room temperature. The Tc of C16Glu-C16DMA dispersed in water was determined to be ca. 39 °C. Thus, this complex forms an adsorbed layer at the oil/water interface during the emulsification process above the Tc, and then changes into a gel during storage at room temperature. The gel phase formed at the oil/water interface prevents the oil droplets from coalescing. We have also demonstrated that the dispersion stability of the emulsions can be controlled by changing the aqueous pH. This study provides a smart preparation method for emulsion systems with noncovalent-type amphiphiles as stabilizers.


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Acknowledgements This work was supported by the Cosmetology Research Foundation. We also thank Dr. Koji Tsuchiya (Tokyo University of Science) for his helpful comment. References 1. Sakai, K.; Abe, M. In Encyclopedia of Biocolloid and Biointerface Science (Volume 1, First Edition); Ohshima, H., Ed.; John Wiley & Sons: New Jersey, 2016; Chapter 42. 2. Zhu, L.; Tang, Y.; Wang, Y. Constructing Surfactant Systems with the Characteristics of Gemini and Oligomeric Surfactants through Noncovalent Interaction. J. Surfactant Detergent 2016, 19, 237–247. 3. Zhang, Y.; Feng, Y.; Wang, Y.; Li, X. CO2-Switchable Viscoelastic Fluids Based on a Pseudogemini Surfactant. Langmuir 2013, 29, 4187–4192. 4. Sun, N.; Shi, L. J.; Lu, F.; Xie, S. T.; Zheng, L. Q. Spontaneous Vesicle Phase Formation by Pseudogemini Surfactants in Aqueous Solutions. Soft Matter 2014, 10, 5463–5471. 5. Li, Y.; Li, H.; Chai, J.; Chen, M.; Yang, Q.; Hao, J. Self-assembly and Rheological Properties of a Pseudogemini Surfactant Formed in a Salt-free Catanionic Surfactant Mixture in Water. Langmuir 2015, 31, 11209–11219. 6. Lu, H.; Shi, Q.; Wang, B.; Huang, Z. Spherical-to-Wormlike Micelle Transition in a Pseudogemini Surfactant System with Two Types of Effective pH-responsive Groups. Colloids Surf. A 2016, 494, 74–80. 7. Páhi, A. B.; Király, Z.; Mastalir, Á.; Dudás, J.; Puskás, S.; Vágó, Á. Thermodynamics of Micelle

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Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine Salt and Its Dynamic Adsorption on Sandstone. J. Phys. Chem. B 2008, 112, 15320–15326. 8. Noori, S.; Naqvi, A. Z.; Ansari, W. H.; Akram, M.; Kabir-ud-Din. Synthesis and Investigation of Surface Active Properties of Counterion Coupled Gemini Surfactants. J. Surfactant Detergent 2013, 17, 409–417. 9. Sakai, H.; Okabe, Y.; Tsuchiya, K.; Sakai, K.; Abe, M. Catanionic Mixtures Forming Gemini-like Amphiphiles. J. Oleo. Sci. 2011, 60, 549–555. 10. Xu, P.; Wang, Z.; Xu, Z.; Hao, J.; Sun, D. Highly Effective Emulsification/Demulsification with a CO2-switchable Superamphiphile. J. Colloid Interface Sci. 2016, 480, 198–204. 11. Sakai, K.; Nomura, K.; Shrestha, R. G.; Endo, T.; Sakamoto, K.; Sakai, H.; Abe, M. Wormlike Micelle Formation by Acylglutamic Acid with Alkylamines. Langmuir 2012, 28, 17617–17622. 12. Sakai, K.; Sawa, M.; Nomura, K.; Endo, T.; Tsuchiya, K.; Sakamoto, K.; Abe, M.; Sakai, H. pH-sensitive Wormlike Micelle and Hydrogel Formation by Acylglutamic Acid-Alkylamine Complex. Chem. Lett. 2016, 45, 655–657.


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13. Koynova, R.; Caffrey, M. Phases and Phase Transitions of the Phosphatidylcholines. Biochim. Biophys. Acta 1998, 1376, 91–145. 14. Oka, T.; Miyahara, R.; Teshigawara, T.; Watanabe, K. Development of Novel Cosmetic Base Using Sterol Surfactant. I. Preparation of Novel Emulsified Particles with Sterol Surfactant. J. Oleo Sci. 2008, 57, 567–575. 15. Sakai, K.; Fukushima, A.; Misono, T.; Endo, T.; Sakamoto, K.; Sakai, H.; Abe, M. Emulsification by Phosphorylcholine-type Gemini Amphiphile as Active Interfacial Modifier. Chem. Lett. 2015, 44, 247–249. 16. Pickering, S. U. Emulsions. J. Chem. Soc. Trans. 1907, 91, 2001–2021. 17. Tajima, K.; Imai, Y.; Tsutsui, T. Structure of Three-Phase Emulsion Stabilized with Phospholipid Bilayer-Assembly and Its Stability. J. Oleo. Sci. 2002, 51, 285–296. 18. Suzuki, T.; Takei, H.; Yamazaki, S. Formation of Fine Three-Phase Emulsions by the Liquid Crystal Emulsification Method with Arginine β-Branched Monoalkyl Phosphate. J. Colloid Interface Sci. 1989, 129, 491–500. 19. Sakai, K.; Ikeda, R.; Sharma, S. C.; Shrestha, R. G.; Ohtani, N.; Yoshioka, M.; Sakai, H.; Abe, M.; Sakamoto, K. Active Interfacial Modifier: Stabilization Mechanism of Water in Silicone Oil Emulsions by Peptide-Silicone Hybrid Polymers. Langmuir 2010, 26, 5349–5354. 20. Sakai, K.; Iijima, S.; Ikeda, R.; Endo, T.; Yamazaki, T.; Yamashita, Y.; Natsuisaka, M.; Sakai, H.; Abe, M.; Sakamoto, K. Water-in-Oil Emulsions Prepared by Peptide-Silicone Hybrid Polymers as Active Interfacial Modifier: Effects of Silicone Oil Species on Dispersion Stability of Emulsions. J. Oleo Sci. 2013, 62, 505–511.


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Oil-in-Water Emulsions Stabilized by Acylglutamic AcidâAlkylamine

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Oil-in-Water Emulsions Stabilized by Acylglutamic AcidâAlkylamine