Letter pubs.acs.org/JPCL
Morphological Transformations in Solid Domains of Alkanes on Surfactant Solutions Hiroki Matsubara,* Tetsumasa Takaichi, Takanori Takiue, and Makoto Aratono Department of Chemistry, Faculty of Sciences, Kyushu University, 812-8581 Fukuoka, Japan
Aya Toyoda and Kenichi Iimura Department of Advanced Interdisciplinary Sciences, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan
Philip A. Ash and Colin D. Bain Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom S Supporting Information *
ABSTRACT: Alkanes on surfactant solutions can form three distinct phases at the air− solution interface, a liquid phase (L), a solid monolayer phase (S1), and a hybrid bilayer phase (S2). Phase coexistence between any two, or all three, of these phases has been observed by Brewster angle microscopy of tetradecane, hexadecane, and their mixtures on solutions of tetradecyltrimethylammonium bromide. The morphologies of the domains depend on the competition between line tension and electrostatic interactions, which are essentially different depending on the pair of phases in contact. Domains of S1 in the L phase are long and thin; however, long, thin domains of L in an S1 phase are not stable but break up into a string of small circular domains. The bilayer S2 domains are always circular, owing to the dominance of line tension on the morphology. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
A
either to a frozen mixed monolayer or to an unusual bilayer structure in which the upper leaflet is a solid layer of pure alkane with hexagonal packing and upright chains while the lower leaflet remains a disordered liquid-like mixed monolayer.17−19 We will use the terminology L (liquid phase), S1 (solid monolayer phase), and S2 (hybrid bilayer phase) to identify these phases in this Letter. The discovery of three distinct surface phases motivated us to study the morphologies of alkane domains on a surfactant solution. For example, tetradecane (C14) and hexadecane (C16), respectively, exhibit S1 and S2 phases on the same surfactant solution (tetradecyltrimethylammonium bromide, C14TAB). Therefore, one can expect the S1−S2 transition as well as L− S 1 and L−S 2 transitions on the temperature (T)−oil composition (x2o) phase diagram. Considering that the structures of these phases are essentially different from each other (solid/liquid, monolayer/bilayer), a morphological transformation (a change in size and shape of domains) is likely to occur during the phase transition.
n interface between two bulk phases has an associated surface tension. Similarly, the two-dimensional analogue of surface tension, called line tension, arises at the boundaries between coexisting surface phases. The concept of line tension was originally proposed over 100 years ago by Gibbs1 and has re-emerged as a topical subject owing to its relevance to lipid rafts in cellular membranes.2−5 The size and shapes of domains are determined by the line tension, which favors a circular domain with minimum line energy, and electrostatic interactions between molecules within a domain, which favor smaller and elongated domains.6−8 Just as surfactants can stabilize dispersed phases in 3D systems, line active agents (or linactants) can stabilize 2D domains. For example, Schwartz and co-workers showed in a mixed Langmuir monolayer that molecules having both hydrocarbon and fluorocarbon chains partitioned at the phase boundaries between hydrocarbon-rich and fluorocarbon-rich phases, reducing the line tension and inducing peanut-shaped domains.9,10 We have shown previously that first-order wetting transitions can be induced in a wide range of oils by variation of the aqueous surfactant concentration, leading to the formation of mixed monolayers at the air−water interface.11−17 Depending on the combination of surfactant and oil, these mixed monolayers undergo a thermal phase transition upon cooling, © 2013 American Chemical Society
Received: January 24, 2013 Accepted: February 25, 2013 Published: February 26, 2013 844
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Figure 1. Ellipticity versus temperature at the air−surfactant solution interface. The composition of the coexisting oil phase was changed from xo2 = 0 to 1, left to right.
constant at around 2 μmol m−2 in the present case. BAM was performed at five selected molar ratios, with decreasing temperature. When the temperature was lowered at xo2 = 0, fingered domains were first observed at a temperature slightly below the L−S1 transition of the pure C14 system due to supercooling of the liquid phase (Figure 3A). As time evolved, these domains merged into the striped domains, and then, finally, the whole surface was covered with S1 phase. The shape of the 2D domains is determined by the competition between the line tension at the two-phase boundary and long-range electrostatic repulsions within the domains. In the simple expression,21,22 the line tension of the domain perimeter is given by
Surface freezing transitions were identified from discontinuities in the coefficient of ellipticity, ρ̅, as a function of temperature, T, at fixed mole fraction, xo2, of hexadecane in a droplet of hexadecane and/or tetradecane placed on the surfactant solution.20 According to the (T,xo2) diagram obtained, the shapes of the liquid and solid domains were observed by Brewster angle microscopy (BAM). Figure 1 shows the ellipsometric measurements performed at given molar ratio of C16 in the oil phase. The surfactant concentration, m, was fixed at 0.5 mmol kg −1 . The discontinuous change observed for xo2 = 0 corresponds to the L−S1 phase transition of pure C14.18,19 The L−S2 transition is characterized by a much larger change in ρ̅ at the transition temperature, as seen in the pure C16 system (xo2 = 1).18,19 At xo2 = 0.68, ρ̅ first changes discontinuously from the value for the L phase to the S2 phase and then increases to a value characteristic of an S1 phase when the temperature is further decreased. This sequence indicates that the S1 phase is thermodynamically preferred to the S2 phase at low T. The transition between the two solid phases has not been observed in either pure alkane but only in mixtures. Figure 2 shows the ellipsometry results in a quasi twocomponent surface phase diagram of alkanes. The surface density of the surfactant Γ can be changed depending on the surface phases; however, the change in Γ at the L−S1 transition was estimated to be 0.2 μmol m−2, and it was almost constant at the L−S2 transition.18,19 Therefore, the Γ is substantially
τ = γ Δl···
(1)
where γ denotes the surface tension of the domain walls in the air and Δl is the difference in the height between the domain and surroundings. When line tension dominates the phase behavior, domains are circular in order to minimize the energy penalty of the perimeter. On the other hand, when the electrostatic repulsion between surfactant ions in the same domain is dominant, the formation of elongated or striped domains may be observed.6−8 As Figure 4 illustrates, deformation of a domain reduces the number of ions in the interaction range of the center ion within the solid domain. For a solid−liquid phase transition in a pure surfactant monolayer, the density of the solid phase is higher than that of the liquid phase; therefore, the center ion experiences fewer repulsive interactions with ions in the liquid domain. Thermodynamic measurements on the L−S1 phase transition have shown, however, that the surface excess of the surfactant does not change much at the transition; the increase in density of the monolayer is driven by an increase in the surface excess of the alkane.19 Nevertheless, the S1 domains in Figure 3A have an elongated shape indicative of a domain shape dominated by electrostatic effects. We suggest that in the L phase, the head groups and counterions arrange themselves to minimize unfavorable electrostatic interactions. Such a rearrangement is difficult in the S1 phase in which the structure is driven by chain packing and the head groups are coplanar, with bound counterions lying in the Stern layer below the surfactant.23 All of the dipoles created between TMA+ and Br− ions are therefore lined up. Similar behavior is observed at xo2 = 0.06 (Figure 3B); however, the fingered domains first formed are more rounded,
Figure 2. The (T, xo2) phase diagram of the wetting films on the C14TAB solution surface. The surfactant concentration was fixed at m = 0.5 mmol kg−1. The arrows indicate the oil ratios where BAM was performed. 845
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Figure 3. BAM images of S1 and S2 domains taken at xo2 = 0 (A), 0.06 (B), 0.12 (C), 0.51 (D), and 1 (E). The scale bar indicates 50 μm.
S2 domain is composed of an island of frozen hexadecane molecules floating on the 2D liquid, and therefore, the electrostatic effects are probably negligible. Furthermore, because the alkane chains in the upper layer of the S2 domain are exposed directly to the air phase at the edges of the domain, the line tension is high, and circular domains are favored. This situation is in contrast to the S1 domains where the hydrocarbon chains at the edges of the solid domains are in contact with the surrounding L phase, leading to a much lower line tension (see cartoons in Figure 2). Akimov et al.3 and some other researchers5,26 introduced an elastic deformation term into eq 1 that stores mechanical energy accompanied with stretching or shrinking of hydrocarbon chains at the domain boundary but simultaneously reduces the exposure of hydrocarbon chains to the water phase. The theory gave much smaller τ than that expected from eq 1; however, they also claimed that the calculated τ was still sufficient to maintain circular domain when lipid rafts in a bilayer are 1−2 nm thicker than the surroundings. The difference in thickness between S2 and L phases estimated by X-ray reflectometry (17.74 Å)27 was within this range, and therefore, our theory about the relation between the appearance of the circular domain and the equilibrium structure of the S2 phase was valid at least qualitatively. At xo2 = 0.51, one can see the sequence of transitions from L to S2 and then to S1 phases. Here, only the circular domains were observable at the first transition (Figure 3D). S2 domains increased in size until they mostly covered the surface. The line tension made the remaining L phase circular. If the temperature was kept constant at 9.6 °C for a while, the S1 phase gradually replaced the S2 phase, as confirmed by the coexistence of three distinct regions with different brightness on the BAM image.
Figure 4. Schematic diagrams for the relation between electrostatic repulsions and domain morphology. The number of interacting surfactant ions (black dots) with the center ion (white) is decreased by the deformation of the circular domain.
and the following stripe domain is wider than that at xo2 = 0. These differences point to an increase in the line tension with increasing xo2, probably because an increase in the hexadecane content in the S1 domain leads to larger Δl, that is, a larger difference in thickness between the coexisting phases. We note a surprising feature of the BAM image in Figure 3A and B; at low temperatures, a large number of small circular domains of the L phase persist. While elongated shapes are stabilized for narrow solid domains in a liquid monolayer, this appears not to be the case for narrow liquid domains in a solid monolayer. Once the liquid domains reach a threshold width, they break up into a series of droplets in order to reduce the perimeter length in a manner reminiscent of the Rayleigh instability of liquid jets.24 The middle image in Figure 3A captures liquid domains during this breakup process. BAM images for the pure C16 system (Figure 3E) are essentially different, with all of the domains being circular. The 846
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The brightest one is the S2 phase, and the next brightest one is the S1 phase. The circular shapes of the S2 domains in the S1 phase are again explained by the high line tension of the S2 domains. As in Figure 3A and B, L domains in an S1 phase adopt a rounded shape. These liquid domains become increasingly circular as the hexadecane content increases, consistent with an increase in the L−S1 line tension. Finally, we examine the phase behavior at xo2 = 0.12, which passes the triple point at T = 7.8 °C (Figure 3C). At a temperature very close to the triple point (5.9 °C), three-phase coexistence was realized. The first point to notice is that the S2 domain is convex inward with some spikes toward the surrounding S1 phase. Moreover, the boundary between S1 domains within the L phase is rounded, which is different from the observations in Figure 3A and B. It is possible that the S1 domains transiently retain the shape of the S2 domains from which they were formed. At xo2 = 0.12, the S2 domain is encroached upon by the S1 domain upon cooling because the low-temperature phase is S1. The inward spikes in the top panel of Figure 3C suggest that the S1 phase nucleates at the boundary between the L and S2 phases. Nucleation at this boundary seems reasonable because this is the place where there is the least need for oil transport. The equilibrium is almost attained at 5.2 °C, where the surface is covered with the S1 phase except for some small dots of the S2 phase. Because the alkanes used have negligible vapor pressure and water solubility, the excess oil remains as lenses in equilibrium with the surface phases. The transport of oil through the L phase is required both for S1 and S2 phase formation. The diffusion coefficient in the L phase would be similar to that of bulk oil, while that in the crystalline S1 phase is expected to be much lower.25 This may explain why the isolated regions of the L phases remain in a continuous S1 phase for a long period of time, as seen in Figure 3A and B. In summary, we have mapped out the surface phase diagram for pure and mixed alkanes with chain lengths of 14 and 16 on a 0.5 mM solution of C14TAB. While pure tetradecane and pure hexadecane show only L−S1 and L−S2 phase transitions, respectively, mixtures of alkanes may show all three phases, and there is a triple point at a mole fraction of hexadecane of 0.1. All phase transitions are first order. The coexistence of three phases with distinctive structures allowed us to explore the effect on domain morphology of competition between line tension and electrostatic repulsions within domains. The L−S2 and S1−S2 phase boundaries are dominated by line tension, owing to the height mismatch between the monolayer and bilayer phases. For the L−S1 phase boundary (both monolayer phases), the line tension is expected to be small, and consequently, the S1 domains form as narrow fingers. The morphology is consistent with an increasing line tension as the mole fraction of hexadecane in the oil increases. Surprisingly, long, narrow domains of L in an S1 matrix do not appear to be stabilized in the same way as the inverse system and break up into a line of small spherical islands. Domain morphologies are affected by pre-existing domain structures; therefore, when S2 domains in an L phase convert to S1, they maintain the circular shape of the S2 domains, at least transiently. Transport kinetics of oil through the surface phases appears to play a significant role in the nucleation of new phases and in long-lived liquid islands in the solid S1 phase.
Letter
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, including the materials used, ellipsometric measurements, and Brewster angle microscopy. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Cosmetology research grant.
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REFERENCES
(1) Gibbs, J. W. The Scientific Papers of J. Willard Gibbs; Dover: New York; 1961. (2) Baumgart, T.; Hess, S. T.; Webb, W. W. Imaging Coexisting Fluid Domains in Biomembrane Models Coupling Curvature and Line Tension. Nature 2003, 425, 821−824. (3) Akimov, S. A.; Kuzmin, P. I.; Zimmerberg, J.; Cohen, F. S.; Chizmadzhev, Y. A. An Elastic Theory for Line Tension at a Boundary Separating Two Lipid Monolayer Regions of Different Thickness. J. Electroanal. Chem. 2004, 564, 13−18. (4) Umeda, T.; Suezaki, Y.; Takiguchi, K.; Hotani, H. Theoretical Analysis of Opening-Up Vesicles with Single and Two Holes. Phys. Rev. E 2005, 71, 011913. (5) Towles, K. B.; Dan, N. Line Tension and Coalescence in Heterogeneous Membranes. Langmuir 2007, 23, 13053−13058. (6) McConnell, H. M. Structures and Transitions in Lipid Monolayers at the Air−Water Interface. Annu. Rev. Phys. Chem. 1991, 42, 171−195. (7) Nandi, N.; Vollhardt, J. Anomalous Temperature Dependence of Domain Shape in Langmuir Monolayers: Role of Dipolar Interaction. J. Phys. Chem. B 2004, 108, 18793−18795. (8) Hossain, M. M.; Iimura, K.; Kato, T. Comparison of Phase Behavior between Water Soluble and Insoluble Surfactants at the Air− Water Interface. Appl. Surf. Sci. 2010, 257, 1129−1133. (9) Trabelsi, S.; Zhang, S.; Lee, T. R.; Schwartz, D. K. Linactants: Surfactant Analogues in Two Dimensions. Phys. Rev. Lett. 2008, 100, 037802. (10) Trabelsi, S.; Zhang, Z.; Zhang, S.; Lee, T. R.; Schwartz, D. K. Correlating Linactant Efficiency and Self-Assembly: Structural Basis of Line Activity in Molecular Monolayers. Langmuir 2009, 25, 8056− 8061. (11) Aratono, M.; Kawagoe, H.; Toyomasu, T.; Ikeda, N.; Takiue, T.; Matsubara, H. Interfacial Films and Wetting Behavior of the Air/ Hexadecane/Aqueous Solution of a Surfactant System. Langmuir 2001, 17, 7344−7349. (12) Matsubara, H.; Ikeda, N.; Takiue, T.; Aratono, M.; Bain, C. D. Interfacial Films and Wetting Behavior of Hexadecane on Aqueous Solutions of Dodecyltrimethylammonium Bromide. Langmuir 2003, 19, 2249−2253. (13) Wilkinson, K. M.; Bain, C. D.; Matsubara, H.; Aratono, M. Wetting of Surfactant Solutions by Alkanes. Chem. Phys. Chem. 2005, 6, 547−555. (14) Matsubara, H.; Aratono, M.; Wilkinson, K. M.; Bain, C. D. Lattice Model for the Wetting Transition of Alkanes on Aqueous Surfactant Solutions. Langmuir 2006, 22, 982−988. (15) Matsubara, H.; Shigeta, T.; Takata, Y.; Ikeda, N.; Sakamoto, H.; Takiue, T.; Aratono, M. Effect of Molecular Structure of Oil on Wetting Transition on Surfactant Solutions. Colloids Surf., A 2007, 301, 141−146. 847
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(16) Wilkinson, K. M.; Lei, Q.; Bain, C. D. Freezing Transitions in Mixed Surfactant/Alkane Monolayers at the Air−Solution Interface. Soft Matter 2006, 2, 66−76. (17) Sloutskin, E.; Sapir, Z.; Bain, C. D.; Lei, Q.; Wilkinson, K. M.; Tamam, L.; Deutsch, M.; Ocko, B. M. Wetting, Mixing, and Phase Transitions in Langmuir−Gibbs Films. Phys. Rev. Lett. 2007, 99, 136102. (18) Matsubara, H.; Ohtomi, E.; Bain, C. D.; Aratono, M. Wetting and Freezing of Hexadecane on an Aqueous Surfactant Solution: Triple Point in a 2-D film. J. Phys. Chem. B 2008, 112, 11664−11668. (19) Ohtomi, E.; Takiue, T.; Aratono, M.; Matsubara, H. Freezing Transition of Wetting Film of Tetradecane on Tetradecyltrimethylammonium Bromide Solutions. Colloid Polym. Sci. 2010, 288, 1333− 1339. (20) Drude, P. The Theory of Optics; Dover: New York; 1959. (21) McConnell, H. M.; Moy, V. T. Shapes of Finite TwoDimensional Lipid Domains. J. Phys. Chem. 1988, 92, 4520−4525. (22) McConnell, H. M.; Koker, R. D. Note on the Theory of the Sizes and Shapes of Lipid Domains in Monolayers. J. Phys. Chem. 1992, 96, 7101−710. (23) Ohtomi, E.; Ikeda, N.; Tokiwa, Y.; Watanabe, I.; Tanida, H.; Takiue, T.; Aratono, M.; Matsubara, H. Thin−Thick Transition of Foam Film Driven by Phase Transition of Surfactant−Alkane Mixed Adsorbed Film. Chem. Lett. 2012, 41, 1300−1302. (24) Eggers, J. Nonlinear Dynamics and Breakup of Free-Surface Flows. Rev. Mod. Phys. 1997, 69, 865−930. (25) Yamakawa, H.; Matsukawa, S.; Kurosu, H.; Kuroki, S.; Ando, I. A Study of the Dynamics of n-Alkane in the Rotator Phase by Using the Pulse-Field-Gradient Spin−Echo 1H NMR Method. Chem. Phys. Lett. 1998, 283, 333−336. (26) May, S. Trans-Monolayer Coupling of Fluid Domains in Lipid Bilayers. Soft Matter 2009, 5, 3148−3156. (27) Matsubara, H.; Takaichi, T.; Takiue, T.; Tanida, H.; Uruga, T.; Yano, Y. F.; Aratono, M. X-ray Reflectivity Measurements for Freezing Transitions of Alkane Wetting Film on Surfactant Solution Surface. . Bull. Chem. Soc. Jpn. 2013, DOI: 10.1246/bcsj.20120263.
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