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Self-division of a mineral oil–fatty acid droplet
Chemical Physics Letters 640 (2015) 1–4
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Self-division of a mineral oil–fatty acid droplet István Lagzi Department of Physics, Budapest University of Technology and Economics, Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 18 August 2015 In final form 30 September 2015 Available online 9 October 2015
a b s t r a c t Self-division of a mineral oil–fatty acid droplet placed in an alkaline solution was investigated. The initially homogeneous mineral oil droplet containing various amounts of 2-hexyldecanoic fatty acid underwent a division process resulting in the formation of two droplets. One formed (‘daughter’) droplet contains middle-phase microemulsion (surfactant-rich phase), while the other contains mineral oil with 2-hexyldecanoic acid (surfactant-low organic phase). We found that the pH of the water phase has negligible effect on the ratio of the sizes of the ‘daughter’ droplets. However, the contact angle between two droplets highly depends on the pH of the alkaline solution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Investigation of division of micro- and macroscopic chemical objects such as micelles, vesicles and fatty acid droplets has gained much interest in the last few years due to the importance of creating a minimal artificial cell [1–12]. These chemical objects can mimic some aspects of a living cell and display ‘biological-like’ responses, i.e. they can react to the environmental stimuli. For instance they can move either randomly or directionally in a gradient of chemical substances (acid, salt, etc.), which can be considered as an artificial chemotaxis [13–24]. Moreover, some of these droplets can selfdivide driven by environmental factors such as pH and chemical composition [9,10]. Probably the most ingenious idea illustrating the self-replication of microscopic compartment is DNA replication in giant vesicles followed by vesicle growth and division triggered by addition of membrane precursor to DNA-amplified giant vesicles [9]. Significance of such investigations can be highlighted by the fact that simple models of protocells can be considered as simple bilayer vesicle structures or oil microdroplets placed in a water phase. These latter mentioned structures could represent even more primitive and simple examples of life on Earth than layer protected vesicles [25]. In this letter we show a new, simple example for self-division of a mixed mineral oil–fatty acid droplet in an aqueous environment. The droplet placed in an alkaline water phase can undergo self-division due to development from a two-phase system into a thermodynamically stable three-phase system.
A drop of mineral oil (MO, Sigma–Aldrich) (typically ∼10 L) that contains 5–90 v% of 2-hexyldecanoic acid (HDA, Sigma–Aldrich) and a small amount (ca. 5 mg mL−1 ) of oil red dye (Sigma–Aldrich) for better visualization was placed in a Petri dish (diameter of 12 cm) filled with an alkaline solution of potassium hydroxide (KOH, Sigma–Aldrich). The depth of the alkaline solution in the Petri dish was 1 cm. The drop was placed at the liquid–air interface and the division process was monitored by a digital camera (Canon EOS 20D). All experiments were carried out at room temperature (22 ± 0.5 ◦ C). MO and HDA are not soluble in water, however, they are miscible in all proportions. Deprotonated form of HDA acts as a surfactant in water and has critical micelle concentration (CMC) of 3.6 × 10−6 M at pH 10.5 [26].
E-mail address: [email protected] http://dx.doi.org/10.1016/j.cplett.2015.09.053 0009-2614/© 2015 Elsevier B.V. All rights reserved.
3. Results and discussion The process of droplet self-division is shown in Figure 1 (see also Movie 1 in Supplementary Content). After placing a homogeneous droplet containing the mixture of MO and HDA (yellow color) in the alkaline solution, at the perimeter of the droplet becomes blurry due to deprotonation of the HDA at the water–oil (w/o) interface (Figure 1). Several seconds later the appearance of a new phase (red color) can be sparsely observed in the oil phase due to microemulsion formation at the w/o interface of the droplet (Figure 1a). In parallel, there is deprotonation of HDA at the w/o interface of the droplet, and due to this process HDA becomes water soluble and acts as a surfactant, enriching the liquid–air interface and producing micelles in the water phase. However, the amount of HDA in the droplet is much higher than can be dissolved in the water
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I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
Figure 1. Self-division of a mineral oil–fatty acid droplet in an alkaline solution of KOH (2 M, pH 14.3). The mineral oil droplet contains initially 30 v% 2-hexyldecanoic acid. The scale bar represents 1 mm (a–d). Thermodynamically stable three-phase system (e); W: water phase with deprotonated 2-hexyldecanoic acid, M: microemulsion (surfactant-rich middle-phase), O: mineral oil with 2-hexyldecanoic acid (surfactant-low organic phase).
(bulk) phase, and thus it produces a thermodynamically stable new phase (Winsor type III microemulsion), the so-called surfactantrich middle-phase (M), which is insoluble both in organic (O) and water (W) phases (Figure 1e) [27–29]. In other words, the system transforms from a two-phase state into a three-phase state, in which the three phases, O, M and W, are thermodynamically stable. Middle-phase microemulsion contains both oil and water that form a bicontinuous structure. This middle-phase microemulsion has an intermediate density between the oil and water phases.
Oil red dye is yellow in the oil phase and red in the microemlusion phase, and thus the self-division process can be perfectly monitored. As the interfacial reaction between HDA and base progresses, more small droplets (size of ∼5–10 m) containing microemulsion form at the interface of MO droplet (‘mother’ droplet) (Figure 2). Later these small droplets can coalesce resulting in the formation of two distinct phases (O and M, Figure 1b), and finally the droplet divides into two ‘daughter’ droplets (Figure 1c and d).
I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
3
Figure 2. Formation of microdroplets containing middle-phase microemulsion at oil–water interface 15 s (a) and 20 s (b) after the beginning of the experiment. Small microdroplets can coalesce into a bigger droplet at the oil–water interface. Finally these bigger droplets merge into bulk phase containing middle-phase microemulsion.
Figure 3. ‘Daughter’ droplets formed due to self-division of mineral oil–fatty acid droplets in an alkaline solution of KOH (2 M, pH 14.3) containing initially 20 v% (a), 40 v% (b) and 60 v% (c) 2-hexyldecanoic acid. The scale bar represents 1 mm. Dependence of the ratio of the sizes (radii) of the two ‘daughter’ droplets formed in division process on the composition of mineral oil–fatty acid droplet (v% content of 2-hexyldecanoic acid) using various concentrations of KOH: red, 1 M (pH 14.0); green, 2 M (pH 14.3); blue, 3 M (pH 14.5) (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
Figure 4. ‘Daughter’ droplets formed due to self-division of mineral oil–fatty acid droplets containing initially 40 v% 2-hexyldecanoic acid in alkaline solutions of KOH: 1 M (pH 14.0) (a), 2 M (pH 14.3) (b) and 3 M (pH 14.5) (c). The scale bar represents 1 mm.
Control experiments were carried out, in which a droplet of just one component (MO or HDA) was placed at the liquid–air interface. The MO droplet rests at the interface and neither physical nor chemical transformation occurs. In contrast, a self-propelled motion is observed when a droplet of HDA is placed at the interface, but no division occurs. This type of motion has been extensively studied in the past several years [13,15,21,22]. We also investigated the effect of the droplet composition on the characteristics of the divided droplets. Figure 3 shows the relative ratio of the ‘daughter’ droplets formed from different initial compositions of MO and HDA. It can be concluded that the greater the initial amount of HDA in MO, the larger the microemulsion phase is. Figure 3d illustrates this dependence in a quantitative way using 1 M (pH 14.0), 2 M (pH 14.3) and 3 M (pH 14.5) KOH solutions. It is evident that the variation of the pH of the alkaline solution in this range has negligible effect on the final size (ratio of the radii) of the ‘daughter’ droplets. This can be explained by the fact that in this pH range all carboxyl groups are deprotonated (the apparent pKa is ∼6–7). Therefore, the amount of the deprotonated form does not change in strong alkaline environment, and thus the pH does not affect the amount (the final size) of the microemulsion phase. However, it should be noted that the initial pH of the alkaline solution does have a pronounced effect on the contact angle (separation distance) of the two divided droplets (Figure 4), because the pH can directly affect the surface tension at water–oil and water–microemulsion interfaces [30]. Decreasing the pH of the water phase results in a decrease of the contact angle. Based on this observation, decreasing pH could lead to a full separation of the droplets, however, this has never been observed in experiments probably because of the surface tension difference between two different droplets (compositions), which sticks together the separated droplets [30]. 4. Conclusions The main aim of this study was to show a new concept of self-division of macroscopic compartments of size of several millimeters. A droplet of the mixture of an organic compound and fatty acid can undergo a self-division that is driven by the fatty acid chemistry at the w/o interface. Due to the nature of this process, these droplets cannot divide again, which is a limitation of this approach. However, it clearly shows the concept that a simple, initially homogeneous droplet containing two organic compounds can self-divide if the environmental factors change, for example in this study the pH of the surrounding water phase. Initially, the ‘mother’ droplet contains both chemicals (MO and HDA), and after division one ‘daughter’ droplet contains exclusively MO with low HDA content, while the other contains MO-HDA-water microemulsion, and these two droplets have different physical and chemical properties [31,32].
Acknowledgments This work was supported by the Hungarian Scientific Research Fund (OTKA K104666). We acknowledge helpful discussions with Profs Zoltán Hórvölgyi (Budapest University of Technology and Economics) and Rita Tóth (Swiss Federal Laboratories for Materials Science and Technology). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.09.053. References [1] P.A. Bachmann, P. Walde, P.L. Luisi, J. Lang, J. Am. Chem. Soc. 112 (1990) 8200. [2] P. Walde, R. Wick, M. Fresta, A. Mangone, P.L. Luisi, J. Am. Chem. Soc. 116 (1994) 11649. [3] J.W. Szostak, D.P. Bartel, P.L. Luisi, Nature 409 (2001) 387. [4] M.M. Hanczyc, S.M. Fujikawa, J.W. Szostak, Science 302 (2003) 618. [5] H. Takahashi, Y. Kageyama, K. Kurihara, K. Takakura, S. Murata, T. Sugawara, Chem. Commun. 46 (2010) 8791. [6] K.P. Browne, D.A. Walker, K.J.M. Bishop, B.A. Grzybowski, Angew. Chem. Int. Ed 49 (49) (2010) 6756. [7] F. Caschera, S. Rasmussen, M.M. Hanczyc, ChemPlusChem 78 (2013) 52. [8] S. Koga, D.S. Williams, A.W. Perriman, S. Mann, Nat. Chem. 3 (2011) 720. [9] K. Kurihara, M. Tamura, K. Shohda, T. Toyota, K. Suzuki, T. Sugawara, Nat. Chem. 3 (2011) 775. [10] I. Derényi, I. Lagzi, Phys. Chem. Chem. Phys. 16 (2014) 4639. [11] T. Banno, T. Toyota, Langmuir 31 (2015) 6943. [12] T. Banno, R. Kuroha, S. Miura, T. Toyota, Soft Matter 11 (2015) 1459. [13] M.M. Hanczyc, T. Toyota, T. Ikegami, N. Packard, T. Sugawara, J. Am. Chem. Soc. 129 (2007) 9386. [14] Y. Sumino, K. Yoshikawa, Chaos 18 (2008) 026106. [15] T. Toyota, N. Maru, M.M. Hanczyc, T. Ikegami, T. Sugawara, J. Am. Chem. Soc. 131 (2009) 5012. [16] I. Lagzi, S. Soh, P.J. Wesson, K.P. Browne, B.A. Grzybowski, J. Am. Chem. Soc. 132 (2010) 1198. [17] T. Miura, H. Oosawa, M. Sakai, Y. Syundou, T. Ban, A. Shio, Langmuir 26 (2010) 1610. [18] S.J. Ebbens, J.R. Howse, Langmuir 27 (2011) 12293. [19] G. Zhao, M. Pumera, J. Phys. Chem. B 116 (2012) 10960. [20] S. Sengupta, M.E. Ibele, A. Sen, Angew. Chem. Int. Edit. 51 (2012) 8434. [21] I. Lagzi, Cent. Eur. J. Med. 8 (2013) 377. [22] J. Cejkova, M. Novak, F. Stepanek, M. Hanczyc, Langmuir 30 (2014) 11937. [23] G. Zhao, M. Pumera, Lab Chip 14 (2014) 2818. [24] S. Nakata, M. Nagayama, H. Kitahata, N.J. Suematsu, T. Hasegawa, Phys. Chem. Chem. Phys. 17 (2015) 10326. [25] M.M. Hanczyc, Philos. Trans. R. Soc. Lond. B Biol. Sci. 366 (2011) 2885. [26] M.H. Mohamed, L.D. Wilson, K.M. Peru, J.V. Headley, J. Colloid Interface Sci. 395 (2013) 104. [27] Cosgrove Terence (Ed.), Colloid Science: Principles, Methods Applications, Wiley, 2010. [28] T. Nishimi, Macromol. Symp. 270 (2008) 48. [29] T. Doan, E. Acosta, J.F. Scamehorn, D.A. Sabatini, J. Surfactants Deterg. 6 (2003) 215. [30] J.S. Huang, M.W. Kim, Phys. Rev. Lett. 47 (1981) 1462. [31] V.G. Rao, C. Banerjee, S. Ghosh, S. Mandal, J. Kuchlyan, N. Sarkar, J. Phys. Chem. B 117 (2013) 7472. [32] V.G. Rao, S. Mandal, S. Ghosh, C. Banerjee, N. Sarkar, J. Phys. Chem. B 117 (2013) 1480.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Self-division of a mineral oil–fatty acid droplet István Lagzi Department of Physics, Budapest University of Technology and Economics, Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 18 August 2015 In final form 30 September 2015 Available online 9 October 2015
a b s t r a c t Self-division of a mineral oil–fatty acid droplet placed in an alkaline solution was investigated. The initially homogeneous mineral oil droplet containing various amounts of 2-hexyldecanoic fatty acid underwent a division process resulting in the formation of two droplets. One formed (‘daughter’) droplet contains middle-phase microemulsion (surfactant-rich phase), while the other contains mineral oil with 2-hexyldecanoic acid (surfactant-low organic phase). We found that the pH of the water phase has negligible effect on the ratio of the sizes of the ‘daughter’ droplets. However, the contact angle between two droplets highly depends on the pH of the alkaline solution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Investigation of division of micro- and macroscopic chemical objects such as micelles, vesicles and fatty acid droplets has gained much interest in the last few years due to the importance of creating a minimal artificial cell [1–12]. These chemical objects can mimic some aspects of a living cell and display ‘biological-like’ responses, i.e. they can react to the environmental stimuli. For instance they can move either randomly or directionally in a gradient of chemical substances (acid, salt, etc.), which can be considered as an artificial chemotaxis [13–24]. Moreover, some of these droplets can selfdivide driven by environmental factors such as pH and chemical composition [9,10]. Probably the most ingenious idea illustrating the self-replication of microscopic compartment is DNA replication in giant vesicles followed by vesicle growth and division triggered by addition of membrane precursor to DNA-amplified giant vesicles [9]. Significance of such investigations can be highlighted by the fact that simple models of protocells can be considered as simple bilayer vesicle structures or oil microdroplets placed in a water phase. These latter mentioned structures could represent even more primitive and simple examples of life on Earth than layer protected vesicles [25]. In this letter we show a new, simple example for self-division of a mixed mineral oil–fatty acid droplet in an aqueous environment. The droplet placed in an alkaline water phase can undergo self-division due to development from a two-phase system into a thermodynamically stable three-phase system.
A drop of mineral oil (MO, Sigma–Aldrich) (typically ∼10 L) that contains 5–90 v% of 2-hexyldecanoic acid (HDA, Sigma–Aldrich) and a small amount (ca. 5 mg mL−1 ) of oil red dye (Sigma–Aldrich) for better visualization was placed in a Petri dish (diameter of 12 cm) filled with an alkaline solution of potassium hydroxide (KOH, Sigma–Aldrich). The depth of the alkaline solution in the Petri dish was 1 cm. The drop was placed at the liquid–air interface and the division process was monitored by a digital camera (Canon EOS 20D). All experiments were carried out at room temperature (22 ± 0.5 ◦ C). MO and HDA are not soluble in water, however, they are miscible in all proportions. Deprotonated form of HDA acts as a surfactant in water and has critical micelle concentration (CMC) of 3.6 × 10−6 M at pH 10.5 [26].
E-mail address: [email protected] http://dx.doi.org/10.1016/j.cplett.2015.09.053 0009-2614/© 2015 Elsevier B.V. All rights reserved.
3. Results and discussion The process of droplet self-division is shown in Figure 1 (see also Movie 1 in Supplementary Content). After placing a homogeneous droplet containing the mixture of MO and HDA (yellow color) in the alkaline solution, at the perimeter of the droplet becomes blurry due to deprotonation of the HDA at the water–oil (w/o) interface (Figure 1). Several seconds later the appearance of a new phase (red color) can be sparsely observed in the oil phase due to microemulsion formation at the w/o interface of the droplet (Figure 1a). In parallel, there is deprotonation of HDA at the w/o interface of the droplet, and due to this process HDA becomes water soluble and acts as a surfactant, enriching the liquid–air interface and producing micelles in the water phase. However, the amount of HDA in the droplet is much higher than can be dissolved in the water
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I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
Figure 1. Self-division of a mineral oil–fatty acid droplet in an alkaline solution of KOH (2 M, pH 14.3). The mineral oil droplet contains initially 30 v% 2-hexyldecanoic acid. The scale bar represents 1 mm (a–d). Thermodynamically stable three-phase system (e); W: water phase with deprotonated 2-hexyldecanoic acid, M: microemulsion (surfactant-rich middle-phase), O: mineral oil with 2-hexyldecanoic acid (surfactant-low organic phase).
(bulk) phase, and thus it produces a thermodynamically stable new phase (Winsor type III microemulsion), the so-called surfactantrich middle-phase (M), which is insoluble both in organic (O) and water (W) phases (Figure 1e) [27–29]. In other words, the system transforms from a two-phase state into a three-phase state, in which the three phases, O, M and W, are thermodynamically stable. Middle-phase microemulsion contains both oil and water that form a bicontinuous structure. This middle-phase microemulsion has an intermediate density between the oil and water phases.
Oil red dye is yellow in the oil phase and red in the microemlusion phase, and thus the self-division process can be perfectly monitored. As the interfacial reaction between HDA and base progresses, more small droplets (size of ∼5–10 m) containing microemulsion form at the interface of MO droplet (‘mother’ droplet) (Figure 2). Later these small droplets can coalesce resulting in the formation of two distinct phases (O and M, Figure 1b), and finally the droplet divides into two ‘daughter’ droplets (Figure 1c and d).
I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
3
Figure 2. Formation of microdroplets containing middle-phase microemulsion at oil–water interface 15 s (a) and 20 s (b) after the beginning of the experiment. Small microdroplets can coalesce into a bigger droplet at the oil–water interface. Finally these bigger droplets merge into bulk phase containing middle-phase microemulsion.
Figure 3. ‘Daughter’ droplets formed due to self-division of mineral oil–fatty acid droplets in an alkaline solution of KOH (2 M, pH 14.3) containing initially 20 v% (a), 40 v% (b) and 60 v% (c) 2-hexyldecanoic acid. The scale bar represents 1 mm. Dependence of the ratio of the sizes (radii) of the two ‘daughter’ droplets formed in division process on the composition of mineral oil–fatty acid droplet (v% content of 2-hexyldecanoic acid) using various concentrations of KOH: red, 1 M (pH 14.0); green, 2 M (pH 14.3); blue, 3 M (pH 14.5) (d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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I. Lagzi / Chemical Physics Letters 640 (2015) 1–4
Figure 4. ‘Daughter’ droplets formed due to self-division of mineral oil–fatty acid droplets containing initially 40 v% 2-hexyldecanoic acid in alkaline solutions of KOH: 1 M (pH 14.0) (a), 2 M (pH 14.3) (b) and 3 M (pH 14.5) (c). The scale bar represents 1 mm.
Control experiments were carried out, in which a droplet of just one component (MO or HDA) was placed at the liquid–air interface. The MO droplet rests at the interface and neither physical nor chemical transformation occurs. In contrast, a self-propelled motion is observed when a droplet of HDA is placed at the interface, but no division occurs. This type of motion has been extensively studied in the past several years [13,15,21,22]. We also investigated the effect of the droplet composition on the characteristics of the divided droplets. Figure 3 shows the relative ratio of the ‘daughter’ droplets formed from different initial compositions of MO and HDA. It can be concluded that the greater the initial amount of HDA in MO, the larger the microemulsion phase is. Figure 3d illustrates this dependence in a quantitative way using 1 M (pH 14.0), 2 M (pH 14.3) and 3 M (pH 14.5) KOH solutions. It is evident that the variation of the pH of the alkaline solution in this range has negligible effect on the final size (ratio of the radii) of the ‘daughter’ droplets. This can be explained by the fact that in this pH range all carboxyl groups are deprotonated (the apparent pKa is ∼6–7). Therefore, the amount of the deprotonated form does not change in strong alkaline environment, and thus the pH does not affect the amount (the final size) of the microemulsion phase. However, it should be noted that the initial pH of the alkaline solution does have a pronounced effect on the contact angle (separation distance) of the two divided droplets (Figure 4), because the pH can directly affect the surface tension at water–oil and water–microemulsion interfaces [30]. Decreasing the pH of the water phase results in a decrease of the contact angle. Based on this observation, decreasing pH could lead to a full separation of the droplets, however, this has never been observed in experiments probably because of the surface tension difference between two different droplets (compositions), which sticks together the separated droplets [30]. 4. Conclusions The main aim of this study was to show a new concept of self-division of macroscopic compartments of size of several millimeters. A droplet of the mixture of an organic compound and fatty acid can undergo a self-division that is driven by the fatty acid chemistry at the w/o interface. Due to the nature of this process, these droplets cannot divide again, which is a limitation of this approach. However, it clearly shows the concept that a simple, initially homogeneous droplet containing two organic compounds can self-divide if the environmental factors change, for example in this study the pH of the surrounding water phase. Initially, the ‘mother’ droplet contains both chemicals (MO and HDA), and after division one ‘daughter’ droplet contains exclusively MO with low HDA content, while the other contains MO-HDA-water microemulsion, and these two droplets have different physical and chemical properties [31,32].
Acknowledgments This work was supported by the Hungarian Scientific Research Fund (OTKA K104666). We acknowledge helpful discussions with Profs Zoltán Hórvölgyi (Budapest University of Technology and Economics) and Rita Tóth (Swiss Federal Laboratories for Materials Science and Technology). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.09.053. References [1] P.A. Bachmann, P. Walde, P.L. Luisi, J. Lang, J. Am. Chem. Soc. 112 (1990) 8200. [2] P. Walde, R. Wick, M. Fresta, A. Mangone, P.L. Luisi, J. Am. Chem. Soc. 116 (1994) 11649. [3] J.W. Szostak, D.P. Bartel, P.L. Luisi, Nature 409 (2001) 387. [4] M.M. Hanczyc, S.M. Fujikawa, J.W. Szostak, Science 302 (2003) 618. [5] H. Takahashi, Y. Kageyama, K. Kurihara, K. Takakura, S. Murata, T. Sugawara, Chem. Commun. 46 (2010) 8791. [6] K.P. Browne, D.A. Walker, K.J.M. Bishop, B.A. Grzybowski, Angew. Chem. Int. Ed 49 (49) (2010) 6756. [7] F. Caschera, S. Rasmussen, M.M. Hanczyc, ChemPlusChem 78 (2013) 52. [8] S. Koga, D.S. Williams, A.W. Perriman, S. Mann, Nat. Chem. 3 (2011) 720. [9] K. Kurihara, M. Tamura, K. Shohda, T. Toyota, K. Suzuki, T. Sugawara, Nat. Chem. 3 (2011) 775. [10] I. Derényi, I. Lagzi, Phys. Chem. Chem. Phys. 16 (2014) 4639. [11] T. Banno, T. Toyota, Langmuir 31 (2015) 6943. [12] T. Banno, R. Kuroha, S. Miura, T. Toyota, Soft Matter 11 (2015) 1459. [13] M.M. Hanczyc, T. Toyota, T. Ikegami, N. Packard, T. Sugawara, J. Am. Chem. Soc. 129 (2007) 9386. [14] Y. Sumino, K. Yoshikawa, Chaos 18 (2008) 026106. [15] T. Toyota, N. Maru, M.M. Hanczyc, T. Ikegami, T. Sugawara, J. Am. Chem. Soc. 131 (2009) 5012. [16] I. Lagzi, S. Soh, P.J. Wesson, K.P. Browne, B.A. Grzybowski, J. Am. Chem. Soc. 132 (2010) 1198. [17] T. Miura, H. Oosawa, M. Sakai, Y. Syundou, T. Ban, A. Shio, Langmuir 26 (2010) 1610. [18] S.J. Ebbens, J.R. Howse, Langmuir 27 (2011) 12293. [19] G. Zhao, M. Pumera, J. Phys. Chem. B 116 (2012) 10960. [20] S. Sengupta, M.E. Ibele, A. Sen, Angew. Chem. Int. Edit. 51 (2012) 8434. [21] I. Lagzi, Cent. Eur. J. Med. 8 (2013) 377. [22] J. Cejkova, M. Novak, F. Stepanek, M. Hanczyc, Langmuir 30 (2014) 11937. [23] G. Zhao, M. Pumera, Lab Chip 14 (2014) 2818. [24] S. Nakata, M. Nagayama, H. Kitahata, N.J. Suematsu, T. Hasegawa, Phys. Chem. Chem. Phys. 17 (2015) 10326. [25] M.M. Hanczyc, Philos. Trans. R. Soc. Lond. B Biol. Sci. 366 (2011) 2885. [26] M.H. Mohamed, L.D. Wilson, K.M. Peru, J.V. Headley, J. Colloid Interface Sci. 395 (2013) 104. [27] Cosgrove Terence (Ed.), Colloid Science: Principles, Methods Applications, Wiley, 2010. [28] T. Nishimi, Macromol. Symp. 270 (2008) 48. [29] T. Doan, E. Acosta, J.F. Scamehorn, D.A. Sabatini, J. Surfactants Deterg. 6 (2003) 215. [30] J.S. Huang, M.W. Kim, Phys. Rev. Lett. 47 (1981) 1462. [31] V.G. Rao, C. Banerjee, S. Ghosh, S. Mandal, J. Kuchlyan, N. Sarkar, J. Phys. Chem. B 117 (2013) 7472. [32] V.G. Rao, S. Mandal, S. Ghosh, C. Banerjee, N. Sarkar, J. Phys. Chem. B 117 (2013) 1480.