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Interfacial activities and aggregation behaviors of N-acyl amino acid surfactants derived from vegetable oils
Colloids and Surfaces A 559 (2018) 54–59
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Interfacial activities and aggregation behaviors of N-acyl amino acid surfactants derived from vegetable oils ⁎
T
⁎
Changyao Liu, Yuzhao Wang, Chenxing Chai, Sana Ullah, Guiju Zhang , Baocai Xu , Hongqin Liu, Li Zhao School of Food and Chemical Engineering, Beijing Key Laboratory of Flavor Chemistry, Beijing Higher Institution Engineering Research Center of Food Additives and Ingredients, Beijing Technology and Business University, No. 11 Fucheng Road, Beijing, 100048, People’s Republic of China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Amino acid surfactants Surface activities Vesicles Vegetable oils TEM Hydroxyl groups
The development of amino acid surfactants which are considered to be biodegradable and less toxic than traditional surfactants has been a subject of growing interests among chemists for the past 20 years. Within this category, N-acyl amino acid surfactants are popular due to their excellent interfacial properties and antimicrobial activities. In the present work, six new N-acyl amino acid surfactants were synthesized using vegetable oils (castor oil and cottonseed oil) and amino acids (glycine, alanine, and serine). Surface active properties of these surfactants were investigated. With the amide bonds acting as hydrogen bond donors and acceptors, globular and tubular vesicles were observed in the aqueous solutions of some prepared surfactants. The results indicated that hydroxyl groups on the hydrophobic tails for castor oil derivatives were associated with spherical vesicles formation, whereas serine residues bearing hydroxyl groups may be associated with the tubular vesicles.
1. Introduction Over the past few decades, the rising environmental awareness of both governments and consumers has had a strong impact on the choice of surfactants. Therefore, environmentally benign surfactants based on natural building blocks, as a potential substitute for traditional ⁎
petroleum-based surfactants, have received growing attentions [1]. Amino acids with carboxyl and amino groups are recently employed for the preparation of surfactants due to their excellent biodegradability and versatile properties [2,3]. Amino acid based surfactants usually have desirable properties, such as, low toxicity, non-irritation, low allergenic potential, good biodegradability, and harmless to marine
Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (B. Xu).
https://doi.org/10.1016/j.colsurfa.2018.09.042 Received 26 July 2018; Received in revised form 13 September 2018; Accepted 14 September 2018 Available online 15 September 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 559 (2018) 54–59
C. Liu et al.
organisms [4–6]. In particular, anionic N-acyl amino acid surfactants have been widely used in various areas [7,8], e.g., personal care products, cosmetics, household products, and detergents because of their good physicochemical properties and antimicrobial activities [9]. These surfactants have been reported to self-assemble into aggregates of various shapes induced by both hydrophobic effects and hydrogen bondings with the amide bonds acting as both donors and acceptors [2,10,11]. Therefore, the studies of the physical properties of N-acyl amino acid surfactants have both practical and theoretical significance. Traditionally, N-acyl amino acid surfactants are commercially produced by Schotten-Baumann reaction with fatty acyl chloride and amino acid reacting at high temperature [12]. However, the fatty acyl chlorides employed in this process are toxic; undesirable by-products and odor can’t be eliminated. Previously, our group has reported green synthesis of acyl glycine surfactants using oils (coconut, peanut and soybean oils) and sodium glycinate [13]. In the present work, we have synthesized six new N-acyl amino acid surfactants derived from castor and cottonseed oil with glycine, alanine, and serine residues as headgroups. Castor oil is a well-known source of ricinoleic acid bearing a hydroxyl group on the 12th carbon, and has been used as additives in food, medicine [14,15], surface coating [16], and lubricants [17]. On the other hand, cottonseed oil, commonly used as frying oil, consists lots of unsaturated fatty acids without hydroxyl groups. We study and compare the interfacial activities and assembly behaviors of surfactants synthesized from these two vegetable oils in hope of shedding light on the role of hydroxyl groups. Remarkably, vesicle formation was associated with the presence of hydroxyl groups on surfactant structures, and the morphologies of vesicles depended on the hydroxyl group position.
Scheme 2. Chemical structures of synthesized acyl amino acid surfactants (acidic forms and anionic forms).
were stirred at 160 °C for 5 h. After cooling, the mixtures were dissolved in water. White or yellow precipitates were formed upon the acidification by HCl aqueous solutions (Scheme 2). The acyl amino acid surfactants, i.e., the corresponding sodium salts of CAG, COG, CAA, COA, CAS, and COS, were prepared by neutralizing the acyl amino acids with NaOH aqueous solutions to pH 8–9 until completely clear solutions were obtained. These sodium salts were named as SCAG, SCOG, SCAA, SCOA, SCAS, and SCOS, respectively.
2. Materials and methods 2.1. Materials Cottonseed oil and castor oil were analytical grade and purchased from Acros Organics. All amino acids, glycine (99%, Macklin), alanine (99%, Macklin), serine (99%, Macklin) were used as received. Other reagents were also used as received. All solutions were prepared with deionized water (ρ = 18.25 MΩ•cm). The acyl amino acids derived from vegetable oils, i.e., acyl glycine derived from castor oil (CAG), acyl glycine derived from cottonseed oil (COG), acyl alanine derived from castor oil (CAA), acyl alanine derived from cottonseed oil (COA), acyl serine derived from castor oil (CAS), and acyl serine derived from cottonseed oil (COS), were synthesized by a green method reported by our group [13]. Scheme 1 summarized the synthetic route of these surfactants. Amino acids were neutralized by NaOH aqueous solutions in three-necked flasks. The water was then evaporated, followed by the addition of corresponding vegetable oils and catalytic amount of CH3ONa while stirring. The reaction mixtures
2.2. Structure characterization The composition of cottonseed oil and castor oil were characterized by GC–MS on an Agilent Technologies 700GC/MS-7890 A GC system using a methylation method [18]. The chemical structures of the six acyl amino acids were analyzed by both infrared (IR) spectra on a Thermo Fisher Nicolet iS10 FTIR spectrometer and mass spectroscopy (MS) using an AB SCIEX API3200LC/MS/MS spectrometer. CAG: yield 90%. IR (vmax, cm−1): 3316 (NeH), 1645 (amide band I), 1549 (amide band II). ESI-MS: m/z = 312.1, 336.0, 338.0, 340.1, and 353.9 are assigned to palmitoyl glycine (C16:0), linoleoyl glycine (C18:2), oleoyl glycine (C18:1), stearoyl glycine (C18:0), and ricinoleoyl glycine (C18:1,containing −OH), respectively.
Scheme 1. Synthesis of acyl amino acids from vegetable oil and sodium salts of the corresponding amino acids. 55
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Fig. 1. Surface tension profiles of six acyl amino acid surfactants derived from caster oil (left) and cottonseed oil (right) at 25 °C. The lines are drawn to show the break points.
CAA: yield 83%. IR (vmax, cm−1): 3319 (NeH), 1650 (amide band I), 1540 (amide band II). ESI-MS: m/z = 326.3, 350.3, 352.2, 354.0, and 368.0 are assigned to palmitoyl alanine (C16:0), linoleoyl alanine (C18:2), oleoyl alanine (C18:1), stearoyl alanine (C18:0), and ricinoleoyl alanine (C18:1, containing −OH), respectively. CAS: yield 85%. IR (vmax, cm−1): 3362 (NeH), 1646 (amide band I), 1530 (amide band II). ESI-MS: m/z = 356.4, 380.4, 382.4, 384.4, and 398.2 are assigned to palmitoyl serine (C16:0), linoleoyl serine (C18:2), oleoyl serine (C18:1), stearoyl serine (C18:0), and ricinoleoyl serine (C18:1, containing −OH), respectively. COG: yield 88%. IR (vmax, cm−1): 3350 (NeH), 1623 (amide band I), 1536 (amide band II). ESI-MS: m/z = 284.3, 310.3, 312.3, 336.2, 338.2, and 340.2 are assigned to myristoyl glycine (C14:0), palmitoleoyl glycine (C16:1), palmitoyl glycine (C16:0), linoleoyl glycine (C18:2), oleoyl glycine (C18:1), and stearoyl glycine (C18:0), respectively. COA: yield 80%. IR (vmax, cm−1): 3326 (NeH), 1646 (amide band I), 1536 (amide band II). ESI-MS: m/z = 298.1, 324.1, 325.8, 350.0, 352.0, and 354.1 are assigned to myristoyl alanine (C14:0), palmitoleoyl alanine (C16:1), palmitoyl alanine (C16:0), linoleoyl alanine (C18:2), oleoyl alanine (C18:1), and stearoyl alanine (C18:0), respectively. COS: yield 86%. IR (vmax, cm−1): 3302 (NeH), 1643 (amide band I), 1545 (amide band II). ESI-MS: m/z = 314.2, 340.3, 342.2, 366.2, 368.2, and 370.2 are assigned to myristoyl serine (C14:0), palmitoleoyl serine (C16:1), palmitoyl serine (C16:0), linoleoyl serine (C18:2), oleoyl serine (C18:1), and stearoyl serine (C18:0), respectively.
2.5. Transmission electron microscope (TEM) measurements Mophology of surfactant aggregates were confirmed by using TEM measurements. One drop of each surfactant solution, prepared at a concentration of 5 x CMC of the respective surfactants, was placed onto a carbon-coated copper grid (300 mesh), and the excess liquid was absorbed with filter paper. One drop of staining agent (2% uranyl acetate aq. solution) was then added onto the same copper grid, followed by excess liquid again wiped away and natural drying. Negatively stained TEM micrographs were obtained by using an Oxford X-MAX JEM-2100 transmission electron microscope working at 120 kV. 3. Results and discussion 3.1. Castor and cottonseed oil compositions Vegetable oils are mixtures of various fatty acid triglycerides. The compositions of castor and cottonseed oil were determined by a routine methylation method in our group and recently published elsewhere [18]. The results showed that about 90% of castor oil was ricinoleic acid (C18:1, containing −OH) glyceride, whereas the main compositions of cottonseed oil was linoleic acid glyceride (C18:2, 47.4%), palmitic acid glyceride (C16:0, 31.6%) and oleic acid glyceride (C18:1, 13.9%). Therefore, there were hydroxyl groups on most of the surfactant monomers prepared by castor oil as they started with ricinoleic acid glyceride. 3.2. Surface activities
2.3. Surface tension measurements
The surface tension profiles of the six acyl amino acid surfactants derived from castor and cottonseed oil are illustrated in Fig. 1. Table 1 lists the CMC, minimum surface tensions, γCMC, maxiumum surface excess concentrations, Γmax, minimum surface area occupied by per surfactant molecule at the air/water interface, Amin, free energies of adsorption, ΔGads, and free energies of micellization, ΔGmic, of these surfactants compiled from Fig. 1. The CMC values were determined from the break points in these surface tension profiles, and γCMC values were the surface tensions at CMCs. Γmax and Amin were estimated using the approximate form of the Gibbs adsorption isotherm equations:
The critical micelle concentrations, CMCs, of the prepared acyl amino acid anionic surfactants were determined by surface tension measurements. The equilibrium surface tensions of these surfactants were obtained by Wilhelmy plate method using a Kibron Delta-8 at 25 °C. The results were obtained by averaging over three parallel measurements, and the standard deviations were within 0.2 mN m−1.
2.4. Dynamic light scattering (DLS) measurements
Γmax = −
Size distribution of surfactant aggregates was analyzed by DLS measurements using a Malvern Instrument Zetasizer Nano ZS equipped with a 22 mW solid state He-Ne laser (λ = 632.8 nm). The scanning scattering angle was 173°.
Amin =
1 ⎛⎜ ∂γ ⎞⎟ 2.303nRT ⎝ ∂ log C ⎠T
1 NA Γmax
(2)
(3)
where R and NA are the gas constant and Avogadro’s number, T is the 56
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Table 1 The CMC, γCMC, Γmax, Amin, ΔGads, and ΔGmic values of six acyl amino acid surfactants at 25 ℃. surfactants
CMC (mol·L−1)
γCMC (mN·m−1)
Γmax (μmol·m−2)
Amin (Å2)
ΔGmic (KJ·mol−1)
ΔGads (KJ·mol−1)
SCAG SCAA SCAS SCOG SCOA SCOS
1.09 × 10−3 1.49 × 10−3 7.61 × 10−4 8.56 × 10−4 1.86 × 10−3 5.68 × 10−4
39.13 38.79 41.47 35.00 38.25 35.70
1.68 × 10−10 1.17 × 10−10 1.69 × 10−10 1.57 × 10−10 1.20 × 10−10 1.96 × 10−10
99.13 141.74 98.15 105.51 138.06 84.94
−11.67 −11.33 −12.05 −11.93 −11.09 −12.37
−26.87 −26.10 −27.76 −27.47 −25.54 −28.48
Fig. 2. Apparent equivalent hydrodynamic diameter (DH) distribution of six acyl amino acid surfactants aqueous solutions at 5 x CMC.
range of 0.568–1.86 mM. Surfactants bearing the most hydrophobic alanine residues (SCAA and SCOA) have relative higher CMCs compared to the other surfactants. The strong hydrophobicity in the headgroup region of SCAA and SCOA may impede the water/headgroup interactions in the interfacial regions of micelles, and therefore, increase their CMCs. The values of γCMC were between 35.00 to 41.47 mN·m−1, and surfactants derived from cottonseed oil generally have lower γCMC values than those derived from castor oil with the same headgroups. Highest Γmax value and lowest Amin values were obtained for SCOS, indicating that SCOS monomers bearing −OH
absolute temperature, and n equals 2 for ionic surfactant solutions, although there were some debates on whether the Gibbs analysis should be revised [19,20]. ΔGads and ΔGmic were calculated by Rosen’s methodology:
CMC ) 55.5
(4)
ΔGads = ΔGmic − 6.022(γ0 − γCMC) Amin
(5)
ΔGmic = RT ln(
where γ0 is the surface tension of the bidistilled water. Table 1 shows the CMC values of synthesized surfactants were in the 57
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Fig. 3. TEM micrographs of aggregates in SCAG (a), SCAA (b), SCAS (c), and SCOS (d) aqueous solutions at 5 x CMC.
available anionic surfactant, linear alkylbenzene sulphonate (LAS), Figs. S1 and S2. The foamability was different for each surfactant, whereas the foam stability of cottonseed oil derivatives was generally better than that of castor oil derivatives, Fig. S3 and Table S1. Therefore, these amino acid derivatives can be potentially used for the formulation of personal care and household products.
groups on their headgroups packed more closely at the air/water interface. Given the existence of −OH groups on the hydrophobic tails for surfactants derived from castor oil, we conclude that the −OH groups on the headgroups behave differently from those on the tails. The negative values of ΔGmic and ΔGads for all surfactants imply that both micellization in water and adsorption at air/water interface are spontaneous processes at 298.15 K. The biggest absolute values of ΔGads and ΔGmic were also obtained for SCOS, suggesting the −OH groups on serine residues help with the micellization and adsorption. Properties, such as, detergency, foamability, and foam stability, of these surfactants were also studied (Supplementary information). They had comparable or even better detergency abilities than commercially
3.3. DLS measurements The sizes and distributions of the aggregates formed in six acyl amino acid surfactant aqueous solutions at 5 times of their CMC, 5 x CMC, were measured by DLS. Fig. 2 shows their apparent equivalent 58
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Natural Science Foundation of China (21676003), the Beijing Municipal Science and Technology Project (D17110500190000), and the Beijing Technology and Business University Youth Scholars Fund (PXM2018_014213_000033).
hydrodynamic diameters (DH). The DH distributions of surfactants derived from castor oil were generally bigger than those of surfactants derived from cottonseed oil except for serine. The DH distributions of SCAG and SCAA were centered at 900 and 150 nm, respectively, whereas the diameters were around 10 nm for SCOG and SCOA aggregates. For serine, the mean DH values are around 100 nm and 250 nm for SCAS and SCOS, respectively. These results may be explained by the existence of −OH group on both the ricinoleyl groups for surfactants derived from castor oil and serine headgroups facilitated big aggregates, e.g., vesicles, formation (Fig. 2).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2018.09.042. References
3.4. TEM measurements [1] R. Marchant, I.M. Banat, Biosurfactants: a sustainable replacement for chemical surfactants? Biotechnol. Lett 34 (2012) 1597–1605. [2] R. Bordes, K. Holmberg, Amino acid-based surfactants – do they deserve more attention? Adv. Colloid Interface Sci. 222 (2015) 79–91. [3] L. Pérez, A. Pinazo, R. Pons, M. Infante, Gemini surfactants from natural amino acids, Adv. Colloid Interface Sci. 205 (2014) 134–155. [4] J. Xia, Protein-Based Surfactants: Synthesis: Physicochemical Properties, and Applications, CRC Press, 2001. [5] M. Husmann, K. Menting, H. Rieckert, H. Ring, J. Weise, W. Zinser, Secondary fatty acid amide derivatives: amino-acid based surfactants for household, industrial and personal care applications, SOFW J. 130 (2004) 22–29. [6] D. Yea, S. Lee, S. Jo, H. Yu, J. Lim, Preparation of environmentally friendly amino acid‐based anionic surfactants and characterization of their interfacial properties for detergent products formulation, J. Surfact. Deterg. 21 (2018) 541–552. [7] M. Takehara, Properties and applications of amino acid based surfactants, Colloids Surf. 38 (1989) 149–167. [8] G.O. Reznik, P. Vishwanath, M.A. Pynn, J.M. Sitnik, J.J. Todd, J. Wu, Y. Jiang, B.G. Keenan, A.B. Castle, R.F. Haskell, Use of sustainable chemistry to produce an acyl amino acid surfactant, Appl. Microbiol. Biotechnol. 86 (2010) 1387–1397. [9] J. Xia, Y. Xia, I.A. Nnanna, Structure-function relationship of acyl amino acid surfactants: surface activity and antimicrobial properties, J. Agric. Food Chem. 43 (1995) 867–871. [10] N. Joondan, S. Jhaumeer-Laulloo, P. Caumul, M. Akerman, Synthesis, physicochemical, and biological activities of novel N‐acyl tyrosine monomeric and Gemini surfactants in single and SDS/CTAB–mixed micellar system, J. Phys. Org. Chem. (2017) 30. [11] S. Roy, J. Dey, Effect of hydrogen-bonding interactions on the self-assembly formation of sodium N-(11-acrylamidoundecanoyl)-L-serinate, L-asparaginate, and Lglutaminate in aqueous solution, J. Colloid Interface Sci. 307 (2007) 229–234. [12] E. Jungermann, J. Gerecht, I. Krems, The preparation of long chain N-acylamino acids, J. Am. Chem. Soc. 78 (1956) 172–174. [13] G.J. Zhang, C.X. Chai, T.T. Tan, B.C. Xu, Y.W. Zhou, H.Q. Liu, L. Zhao, N. Wang, Green synthesis and surface properties of acyl glycine surfactants derived from vegetable oils, Tenside Surfact. Deterg. 53 (2016) 284–290. [14] R. Wilson, B. Van Schie, D. Howes, Overview of the preparation, use and biological studies on polyglycerol polyricinoleate (PGPR), Food Chem. Toxicol. 36 (1998) 711–718. [15] A.J. Kelly, J. Kavanagh, J. Thomas, Castor oil, bath and/or enema for cervical priming and induction of labour, Cochrane Database System. Rev. (2001) CD003099-CD003099. [16] A. Thomas, Fats and fatty oils, Ullmann’s Encyclopedia of Industrial Chemistry, (2000). [17] B.K. Sharma, A. Adhvaryu, Z. Liu, S.Z. Erhan, Chemical modification of vegetable oils for lubricant applications, J. Am. Oil Chem. Soc. 83 (2006) 129–136. [18] Y. Xu, H. Liu, B. Xu, G. Zhang, Synthesis, characterization, and surface properties of amide amine oxides based on natural vegetable oil, J. Dispers. Sci. Technol. 39 (2018) 585–593. [19] F.M. Menger, L. Shi, S.A. Rizvi, Re-evaluating the Gibbs analysis of surface tension at the air/water interface, J. Am. Chem. Soc. 131 (2009) 10380–10381. [20] F.M. Menger, L. Shi, S.A. Rizvi, Additional support for a revised Gibbs analysis, Langmuir 26 (2009) 1588–1589. [21] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers, J. Chem. Soc. Faraday Trans. 2: Mol. Chem. Phys. 72 (1976) 1525–1568. [22] J.N. Israelachvili, Intermolecular and Surface Forces (With Applications to Colloidal and Biological Systems), Academic Press, London, Orlando, San Diego, New York, Toronto, Montreal, Sydney, Tokyo, 1985. [23] A. Mohanty, J. Dey, 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 Acs J. Surf. Colloids 23 (2007) 1033–1040. [24] A. Mohanty, J. Dey, Spontaneous formation of vesicles and chiral self-assemblies of sodium N-(4-dodecyloxybenzoyl)-L-valinate in water, Langmuir Acs J. Surf. Colloids 20 (2004) 8452–8459. [25] A. Ghosh, J. Dey, Effect of hydrogen bonding on the physicochemical properties and bilayer self-assembly formation of N-(2-hydroxydodecyl)-L-alanine in aqueous solution, Langmuir Acs J. Surf. Colloids 24 (2008) 6018–6026.
To further confirm the morphologies of these big aggregates, TEM micrographs of SCAG, SCAA, SCAS, and SCOS aqueous solutions were shown in Fig. 3. Globular vesicles were formed in the aqueous solutions of SCAG and SCAA, whereas SCAS and SCOS self-aggregated into tubular vesicles. The diameters of the observed spherical vesicles were around 1 μm and 100–200 nm for SCAG and SCAA, respectively, consistent with the DLS measurements. The tubules have length L ≤ 4 μm, and diameters d ≤ 40 nm and 100 nm for SCAS and SCOS, respectively. These are consistent with the DLS results showing the equivalent hydrodynamic diameters of aggregates formed from SCOS were larger than those from SCAS. The TEM results indicated that the aggregates’ morphologies depended on the structures of both amino acid residues and hydrophobic tails. The −OH groups on the hydrophobic tails, for surfactants derived from castor oil, facilitated the formation of spherical vesicles; whereas tubules were more stable for surfactants bearing −OH groups on their serine headgroups. According to the famous geometry based packing parameter theory proposed by Israelachvili et al. [21,22], double-tailed surfactant tends to form vesicles and other bilayer structures, while surfactant with single-chain forms spherical micelles. Our results show that these Nacyl amino acid surfactants are exceptions. The current results are consistent with many reported literature results showing that various highly organized nanometer-scale structures were formed by N-acyl amino acid derivatives [2,11,23,24]. Previous FTIR and NMR studies have shown that intermolecular hydrogen bonding between the NeH moiety and C]O group of the amide bonds plays an important role during the self-assembly for these surfactants [11,25]. Here, our results further illustrate that the morphologies of the aggregates depend on the side chain type of amino acid and functional groups on the hydrophobic tails. Further analysis may be done by synthesizing pure surfactants using methyl esters of the corresponding fatty acids instead of oils. 4. Conclusion N-acyl amino acid surfactants are a class of “green” surfactants that can be used as replacements for traditional surfactants. Six N-acyl amino acid surfactants were synthesized directly using vegetable oils and corresponding amino acids in this paper. Their structures were confirmed by IR and MS. Chracterizing parameters of surfactants, including, CMC, γCMC, Γmax, Amin, ΔGads, and ΔGmic were obtained. Interestingly, the aggregation behaviors depended largely on the structures of both amino acid residues and hydrophobic chains. The presence of hydroxyl groups on the tails of castor oil derivatives tend to self-assemble into globular vesicles, while acyl serine derivatives bearing hydroxyl groups on the headgroups formed tubular vesicles. Acknowledgements The authors gratefully acknowledge supports for this work by the National Key R&D Program of China (2017YFB0308701), the National
59
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Interfacial activities and aggregation behaviors of N-acyl amino acid surfactants derived from vegetable oils ⁎
T
⁎
Changyao Liu, Yuzhao Wang, Chenxing Chai, Sana Ullah, Guiju Zhang , Baocai Xu , Hongqin Liu, Li Zhao School of Food and Chemical Engineering, Beijing Key Laboratory of Flavor Chemistry, Beijing Higher Institution Engineering Research Center of Food Additives and Ingredients, Beijing Technology and Business University, No. 11 Fucheng Road, Beijing, 100048, People’s Republic of China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Amino acid surfactants Surface activities Vesicles Vegetable oils TEM Hydroxyl groups
The development of amino acid surfactants which are considered to be biodegradable and less toxic than traditional surfactants has been a subject of growing interests among chemists for the past 20 years. Within this category, N-acyl amino acid surfactants are popular due to their excellent interfacial properties and antimicrobial activities. In the present work, six new N-acyl amino acid surfactants were synthesized using vegetable oils (castor oil and cottonseed oil) and amino acids (glycine, alanine, and serine). Surface active properties of these surfactants were investigated. With the amide bonds acting as hydrogen bond donors and acceptors, globular and tubular vesicles were observed in the aqueous solutions of some prepared surfactants. The results indicated that hydroxyl groups on the hydrophobic tails for castor oil derivatives were associated with spherical vesicles formation, whereas serine residues bearing hydroxyl groups may be associated with the tubular vesicles.
1. Introduction Over the past few decades, the rising environmental awareness of both governments and consumers has had a strong impact on the choice of surfactants. Therefore, environmentally benign surfactants based on natural building blocks, as a potential substitute for traditional ⁎
petroleum-based surfactants, have received growing attentions [1]. Amino acids with carboxyl and amino groups are recently employed for the preparation of surfactants due to their excellent biodegradability and versatile properties [2,3]. Amino acid based surfactants usually have desirable properties, such as, low toxicity, non-irritation, low allergenic potential, good biodegradability, and harmless to marine
Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (B. Xu).
https://doi.org/10.1016/j.colsurfa.2018.09.042 Received 26 July 2018; Received in revised form 13 September 2018; Accepted 14 September 2018 Available online 15 September 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 559 (2018) 54–59
C. Liu et al.
organisms [4–6]. In particular, anionic N-acyl amino acid surfactants have been widely used in various areas [7,8], e.g., personal care products, cosmetics, household products, and detergents because of their good physicochemical properties and antimicrobial activities [9]. These surfactants have been reported to self-assemble into aggregates of various shapes induced by both hydrophobic effects and hydrogen bondings with the amide bonds acting as both donors and acceptors [2,10,11]. Therefore, the studies of the physical properties of N-acyl amino acid surfactants have both practical and theoretical significance. Traditionally, N-acyl amino acid surfactants are commercially produced by Schotten-Baumann reaction with fatty acyl chloride and amino acid reacting at high temperature [12]. However, the fatty acyl chlorides employed in this process are toxic; undesirable by-products and odor can’t be eliminated. Previously, our group has reported green synthesis of acyl glycine surfactants using oils (coconut, peanut and soybean oils) and sodium glycinate [13]. In the present work, we have synthesized six new N-acyl amino acid surfactants derived from castor and cottonseed oil with glycine, alanine, and serine residues as headgroups. Castor oil is a well-known source of ricinoleic acid bearing a hydroxyl group on the 12th carbon, and has been used as additives in food, medicine [14,15], surface coating [16], and lubricants [17]. On the other hand, cottonseed oil, commonly used as frying oil, consists lots of unsaturated fatty acids without hydroxyl groups. We study and compare the interfacial activities and assembly behaviors of surfactants synthesized from these two vegetable oils in hope of shedding light on the role of hydroxyl groups. Remarkably, vesicle formation was associated with the presence of hydroxyl groups on surfactant structures, and the morphologies of vesicles depended on the hydroxyl group position.
Scheme 2. Chemical structures of synthesized acyl amino acid surfactants (acidic forms and anionic forms).
were stirred at 160 °C for 5 h. After cooling, the mixtures were dissolved in water. White or yellow precipitates were formed upon the acidification by HCl aqueous solutions (Scheme 2). The acyl amino acid surfactants, i.e., the corresponding sodium salts of CAG, COG, CAA, COA, CAS, and COS, were prepared by neutralizing the acyl amino acids with NaOH aqueous solutions to pH 8–9 until completely clear solutions were obtained. These sodium salts were named as SCAG, SCOG, SCAA, SCOA, SCAS, and SCOS, respectively.
2. Materials and methods 2.1. Materials Cottonseed oil and castor oil were analytical grade and purchased from Acros Organics. All amino acids, glycine (99%, Macklin), alanine (99%, Macklin), serine (99%, Macklin) were used as received. Other reagents were also used as received. All solutions were prepared with deionized water (ρ = 18.25 MΩ•cm). The acyl amino acids derived from vegetable oils, i.e., acyl glycine derived from castor oil (CAG), acyl glycine derived from cottonseed oil (COG), acyl alanine derived from castor oil (CAA), acyl alanine derived from cottonseed oil (COA), acyl serine derived from castor oil (CAS), and acyl serine derived from cottonseed oil (COS), were synthesized by a green method reported by our group [13]. Scheme 1 summarized the synthetic route of these surfactants. Amino acids were neutralized by NaOH aqueous solutions in three-necked flasks. The water was then evaporated, followed by the addition of corresponding vegetable oils and catalytic amount of CH3ONa while stirring. The reaction mixtures
2.2. Structure characterization The composition of cottonseed oil and castor oil were characterized by GC–MS on an Agilent Technologies 700GC/MS-7890 A GC system using a methylation method [18]. The chemical structures of the six acyl amino acids were analyzed by both infrared (IR) spectra on a Thermo Fisher Nicolet iS10 FTIR spectrometer and mass spectroscopy (MS) using an AB SCIEX API3200LC/MS/MS spectrometer. CAG: yield 90%. IR (vmax, cm−1): 3316 (NeH), 1645 (amide band I), 1549 (amide band II). ESI-MS: m/z = 312.1, 336.0, 338.0, 340.1, and 353.9 are assigned to palmitoyl glycine (C16:0), linoleoyl glycine (C18:2), oleoyl glycine (C18:1), stearoyl glycine (C18:0), and ricinoleoyl glycine (C18:1,containing −OH), respectively.
Scheme 1. Synthesis of acyl amino acids from vegetable oil and sodium salts of the corresponding amino acids. 55
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Fig. 1. Surface tension profiles of six acyl amino acid surfactants derived from caster oil (left) and cottonseed oil (right) at 25 °C. The lines are drawn to show the break points.
CAA: yield 83%. IR (vmax, cm−1): 3319 (NeH), 1650 (amide band I), 1540 (amide band II). ESI-MS: m/z = 326.3, 350.3, 352.2, 354.0, and 368.0 are assigned to palmitoyl alanine (C16:0), linoleoyl alanine (C18:2), oleoyl alanine (C18:1), stearoyl alanine (C18:0), and ricinoleoyl alanine (C18:1, containing −OH), respectively. CAS: yield 85%. IR (vmax, cm−1): 3362 (NeH), 1646 (amide band I), 1530 (amide band II). ESI-MS: m/z = 356.4, 380.4, 382.4, 384.4, and 398.2 are assigned to palmitoyl serine (C16:0), linoleoyl serine (C18:2), oleoyl serine (C18:1), stearoyl serine (C18:0), and ricinoleoyl serine (C18:1, containing −OH), respectively. COG: yield 88%. IR (vmax, cm−1): 3350 (NeH), 1623 (amide band I), 1536 (amide band II). ESI-MS: m/z = 284.3, 310.3, 312.3, 336.2, 338.2, and 340.2 are assigned to myristoyl glycine (C14:0), palmitoleoyl glycine (C16:1), palmitoyl glycine (C16:0), linoleoyl glycine (C18:2), oleoyl glycine (C18:1), and stearoyl glycine (C18:0), respectively. COA: yield 80%. IR (vmax, cm−1): 3326 (NeH), 1646 (amide band I), 1536 (amide band II). ESI-MS: m/z = 298.1, 324.1, 325.8, 350.0, 352.0, and 354.1 are assigned to myristoyl alanine (C14:0), palmitoleoyl alanine (C16:1), palmitoyl alanine (C16:0), linoleoyl alanine (C18:2), oleoyl alanine (C18:1), and stearoyl alanine (C18:0), respectively. COS: yield 86%. IR (vmax, cm−1): 3302 (NeH), 1643 (amide band I), 1545 (amide band II). ESI-MS: m/z = 314.2, 340.3, 342.2, 366.2, 368.2, and 370.2 are assigned to myristoyl serine (C14:0), palmitoleoyl serine (C16:1), palmitoyl serine (C16:0), linoleoyl serine (C18:2), oleoyl serine (C18:1), and stearoyl serine (C18:0), respectively.
2.5. Transmission electron microscope (TEM) measurements Mophology of surfactant aggregates were confirmed by using TEM measurements. One drop of each surfactant solution, prepared at a concentration of 5 x CMC of the respective surfactants, was placed onto a carbon-coated copper grid (300 mesh), and the excess liquid was absorbed with filter paper. One drop of staining agent (2% uranyl acetate aq. solution) was then added onto the same copper grid, followed by excess liquid again wiped away and natural drying. Negatively stained TEM micrographs were obtained by using an Oxford X-MAX JEM-2100 transmission electron microscope working at 120 kV. 3. Results and discussion 3.1. Castor and cottonseed oil compositions Vegetable oils are mixtures of various fatty acid triglycerides. The compositions of castor and cottonseed oil were determined by a routine methylation method in our group and recently published elsewhere [18]. The results showed that about 90% of castor oil was ricinoleic acid (C18:1, containing −OH) glyceride, whereas the main compositions of cottonseed oil was linoleic acid glyceride (C18:2, 47.4%), palmitic acid glyceride (C16:0, 31.6%) and oleic acid glyceride (C18:1, 13.9%). Therefore, there were hydroxyl groups on most of the surfactant monomers prepared by castor oil as they started with ricinoleic acid glyceride. 3.2. Surface activities
2.3. Surface tension measurements
The surface tension profiles of the six acyl amino acid surfactants derived from castor and cottonseed oil are illustrated in Fig. 1. Table 1 lists the CMC, minimum surface tensions, γCMC, maxiumum surface excess concentrations, Γmax, minimum surface area occupied by per surfactant molecule at the air/water interface, Amin, free energies of adsorption, ΔGads, and free energies of micellization, ΔGmic, of these surfactants compiled from Fig. 1. The CMC values were determined from the break points in these surface tension profiles, and γCMC values were the surface tensions at CMCs. Γmax and Amin were estimated using the approximate form of the Gibbs adsorption isotherm equations:
The critical micelle concentrations, CMCs, of the prepared acyl amino acid anionic surfactants were determined by surface tension measurements. The equilibrium surface tensions of these surfactants were obtained by Wilhelmy plate method using a Kibron Delta-8 at 25 °C. The results were obtained by averaging over three parallel measurements, and the standard deviations were within 0.2 mN m−1.
2.4. Dynamic light scattering (DLS) measurements
Γmax = −
Size distribution of surfactant aggregates was analyzed by DLS measurements using a Malvern Instrument Zetasizer Nano ZS equipped with a 22 mW solid state He-Ne laser (λ = 632.8 nm). The scanning scattering angle was 173°.
Amin =
1 ⎛⎜ ∂γ ⎞⎟ 2.303nRT ⎝ ∂ log C ⎠T
1 NA Γmax
(2)
(3)
where R and NA are the gas constant and Avogadro’s number, T is the 56
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Table 1 The CMC, γCMC, Γmax, Amin, ΔGads, and ΔGmic values of six acyl amino acid surfactants at 25 ℃. surfactants
CMC (mol·L−1)
γCMC (mN·m−1)
Γmax (μmol·m−2)
Amin (Å2)
ΔGmic (KJ·mol−1)
ΔGads (KJ·mol−1)
SCAG SCAA SCAS SCOG SCOA SCOS
1.09 × 10−3 1.49 × 10−3 7.61 × 10−4 8.56 × 10−4 1.86 × 10−3 5.68 × 10−4
39.13 38.79 41.47 35.00 38.25 35.70
1.68 × 10−10 1.17 × 10−10 1.69 × 10−10 1.57 × 10−10 1.20 × 10−10 1.96 × 10−10
99.13 141.74 98.15 105.51 138.06 84.94
−11.67 −11.33 −12.05 −11.93 −11.09 −12.37
−26.87 −26.10 −27.76 −27.47 −25.54 −28.48
Fig. 2. Apparent equivalent hydrodynamic diameter (DH) distribution of six acyl amino acid surfactants aqueous solutions at 5 x CMC.
range of 0.568–1.86 mM. Surfactants bearing the most hydrophobic alanine residues (SCAA and SCOA) have relative higher CMCs compared to the other surfactants. The strong hydrophobicity in the headgroup region of SCAA and SCOA may impede the water/headgroup interactions in the interfacial regions of micelles, and therefore, increase their CMCs. The values of γCMC were between 35.00 to 41.47 mN·m−1, and surfactants derived from cottonseed oil generally have lower γCMC values than those derived from castor oil with the same headgroups. Highest Γmax value and lowest Amin values were obtained for SCOS, indicating that SCOS monomers bearing −OH
absolute temperature, and n equals 2 for ionic surfactant solutions, although there were some debates on whether the Gibbs analysis should be revised [19,20]. ΔGads and ΔGmic were calculated by Rosen’s methodology:
CMC ) 55.5
(4)
ΔGads = ΔGmic − 6.022(γ0 − γCMC) Amin
(5)
ΔGmic = RT ln(
where γ0 is the surface tension of the bidistilled water. Table 1 shows the CMC values of synthesized surfactants were in the 57
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Fig. 3. TEM micrographs of aggregates in SCAG (a), SCAA (b), SCAS (c), and SCOS (d) aqueous solutions at 5 x CMC.
available anionic surfactant, linear alkylbenzene sulphonate (LAS), Figs. S1 and S2. The foamability was different for each surfactant, whereas the foam stability of cottonseed oil derivatives was generally better than that of castor oil derivatives, Fig. S3 and Table S1. Therefore, these amino acid derivatives can be potentially used for the formulation of personal care and household products.
groups on their headgroups packed more closely at the air/water interface. Given the existence of −OH groups on the hydrophobic tails for surfactants derived from castor oil, we conclude that the −OH groups on the headgroups behave differently from those on the tails. The negative values of ΔGmic and ΔGads for all surfactants imply that both micellization in water and adsorption at air/water interface are spontaneous processes at 298.15 K. The biggest absolute values of ΔGads and ΔGmic were also obtained for SCOS, suggesting the −OH groups on serine residues help with the micellization and adsorption. Properties, such as, detergency, foamability, and foam stability, of these surfactants were also studied (Supplementary information). They had comparable or even better detergency abilities than commercially
3.3. DLS measurements The sizes and distributions of the aggregates formed in six acyl amino acid surfactant aqueous solutions at 5 times of their CMC, 5 x CMC, were measured by DLS. Fig. 2 shows their apparent equivalent 58
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Natural Science Foundation of China (21676003), the Beijing Municipal Science and Technology Project (D17110500190000), and the Beijing Technology and Business University Youth Scholars Fund (PXM2018_014213_000033).
hydrodynamic diameters (DH). The DH distributions of surfactants derived from castor oil were generally bigger than those of surfactants derived from cottonseed oil except for serine. The DH distributions of SCAG and SCAA were centered at 900 and 150 nm, respectively, whereas the diameters were around 10 nm for SCOG and SCOA aggregates. For serine, the mean DH values are around 100 nm and 250 nm for SCAS and SCOS, respectively. These results may be explained by the existence of −OH group on both the ricinoleyl groups for surfactants derived from castor oil and serine headgroups facilitated big aggregates, e.g., vesicles, formation (Fig. 2).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2018.09.042. References
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To further confirm the morphologies of these big aggregates, TEM micrographs of SCAG, SCAA, SCAS, and SCOS aqueous solutions were shown in Fig. 3. Globular vesicles were formed in the aqueous solutions of SCAG and SCAA, whereas SCAS and SCOS self-aggregated into tubular vesicles. The diameters of the observed spherical vesicles were around 1 μm and 100–200 nm for SCAG and SCAA, respectively, consistent with the DLS measurements. The tubules have length L ≤ 4 μm, and diameters d ≤ 40 nm and 100 nm for SCAS and SCOS, respectively. These are consistent with the DLS results showing the equivalent hydrodynamic diameters of aggregates formed from SCOS were larger than those from SCAS. The TEM results indicated that the aggregates’ morphologies depended on the structures of both amino acid residues and hydrophobic tails. The −OH groups on the hydrophobic tails, for surfactants derived from castor oil, facilitated the formation of spherical vesicles; whereas tubules were more stable for surfactants bearing −OH groups on their serine headgroups. According to the famous geometry based packing parameter theory proposed by Israelachvili et al. [21,22], double-tailed surfactant tends to form vesicles and other bilayer structures, while surfactant with single-chain forms spherical micelles. Our results show that these Nacyl amino acid surfactants are exceptions. The current results are consistent with many reported literature results showing that various highly organized nanometer-scale structures were formed by N-acyl amino acid derivatives [2,11,23,24]. Previous FTIR and NMR studies have shown that intermolecular hydrogen bonding between the NeH moiety and C]O group of the amide bonds plays an important role during the self-assembly for these surfactants [11,25]. Here, our results further illustrate that the morphologies of the aggregates depend on the side chain type of amino acid and functional groups on the hydrophobic tails. Further analysis may be done by synthesizing pure surfactants using methyl esters of the corresponding fatty acids instead of oils. 4. Conclusion N-acyl amino acid surfactants are a class of “green” surfactants that can be used as replacements for traditional surfactants. Six N-acyl amino acid surfactants were synthesized directly using vegetable oils and corresponding amino acids in this paper. Their structures were confirmed by IR and MS. Chracterizing parameters of surfactants, including, CMC, γCMC, Γmax, Amin, ΔGads, and ΔGmic were obtained. Interestingly, the aggregation behaviors depended largely on the structures of both amino acid residues and hydrophobic chains. The presence of hydroxyl groups on the tails of castor oil derivatives tend to self-assemble into globular vesicles, while acyl serine derivatives bearing hydroxyl groups on the headgroups formed tubular vesicles. Acknowledgements The authors gratefully acknowledge supports for this work by the National Key R&D Program of China (2017YFB0308701), the National
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