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Equilibrium Structure and Dynamics of (2R, 3R)-(+)-bis(decyloxy)succinic Acid at the Air-Water Interface B. Vijai Shankar and Archita Patnaik* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India ReceiVed July 5, 2007. In Final Form: October 4, 2007 A twin-tailed, twin-chiral fatty acid, (2R,3R)-(+)-bis(decyloxy)succinic acid was synthesized and its two dimensional behavior at the air-water interface was examined. The pH of the subphase had a profound effect on the monolayer formation. On acidic subphase, stable monolayers with increased area per molecule due to hydrogen bonding and bilayers at collapse pressures were observed. Highly compressible films were formed at 40 °C, while stable monolayers with increased area were observed at sub-room temperatures. Langmuir monolayers formed on subphases containing 1 mM ZnCl2 and CaCl2 revealed two dimensional metal complex formation with Zn2+ forming a chelate-type complex, while Ca2+ formed an ionic-type complex. Monolayers transferred from the condensed phase onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. Compression induced crystallites in 2D from monolayers and vesicle-like supramolecular structures from multilayers were the noted LB film characteristics, adopting optical imaging and electron microscopy. The interfacial monolayer structure studied through molecular dynamics simulation revealed the order and packing at a molecular level; monolayers adsorbed at various simulated specific areas of the molecule corroborated the (π-A) isotherm and the formation of a hexagonal lattice at the air-water interface.
Introduction Ever since Lewis Pasteur observed left- and right-handed sodium ammonium tartrate crystals, exploration of chirality and supramolecular chirality have deserved increased focus, spanning microscopic to mesoscopic and biological/biomimetic systems.1 Chiral transcription of individual chirality of a molecular system to the supramolecular assembly has recently invited increasing attention.2-4 In particular, supramolecular chirality in two dimensions has become the cynosure owing to its similarity as a simple model of highly organized biomembranes. Since chiral molecules have a complicated crystalline arrangement in bulk solids, it is simpler to study the chiral forces in two dimension, as the degrees of freedom in the other two planes are restricted. Surface pressure is a thermodynamic variable and permits systematic study of chiral forces in a membrane assembly. Monolayers at the air-water interface are an ideal example of physical systems where chirality has been studied in two dimensions.5 The asymmetry of intermolecular forces, the unique molecular structure, and the dynamic behavior make the interfacial region significantly different from a bulk liquid or a gas. Chiral monolayers at the air-water interface have been focused with greater interest owing to the direct consequence of many enzymatic processes at membrane surfaces.6 In real biological systems, the chirality of the biomembranes controls their selfassembly to execute specific bioreactions.7 * E-mail
[email protected]. Telephone: +91-44-2257-4217. Fax: +91-44-2257-4202. (1) (a) Pasteur, L. Ann. Chim. Phys. 1848, 24, 442. (b) Nandi, N.; Volhardt, D. J. Phys. Chem. B 2004, 108, 327. (2) Zhang, Y.; Chen, P.; Liu, M. Langmuir 2006, 22, 10246. (3) Crego-Calama, M.; Reinhoudt, D. N.; Ten Cate, M. G. Top. Curr. Chem. 2005, 249, 285. (4) Ajayaghosh, A.; Varghese, R.; George, S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141. (5) (a) Stewart, M. V.; Arnett, E. M. In Topics in Stereochemistry; Allinger, N. L., Eliel, E. L., Wilen, S. H., Eds.; Wiley: New York, 1984. (b) Alonso, C.; Artzner, F.; Lajzerowics, J.; Grubel, G.; Boudet, N.; Rieutord, F.; Petit, J. M.; Renault, A. Eur. Phys. J. E. 2000, 3, 63. (6) Zhang, Y. J.; Song, Y.; Zhao, Y.; Li, T. J.; Jiang, l.; Zhu, D. Langmuir 2001, 17, 1317.
Thus, long-chain amino acid derivatives,8 hydroxy-substituted fatty acids,9 binol derivatives,10 and Gemini surfactants11 have been studied at the air-water interface. “Gemini” surfactants refer to dimeric surfactants with their head groups connected by either a rigid or a flexible spacer. They offer expanded structural diversity, as the length of hydrophobic chains, head groups, and the counterions can each be varied in a search for enhanced performance.12 While the properties of Gemini surfactants in aqueous solution have been the main source of investigation, recent studies have focused on monolayers of gemini surfactants at the air-water interface.13 Liu et al. reported the two dimensional crystallization of a Gemini surfactant with a phenyl methylene spacer.14 Tartaric acid based Gemini surfactants14-17 with long alkyl chains have been investigated under the class of natural surfactants. Acharya et al. reported the bulk-phase rheological and phase behavior of a dimeric Gemini-type surfactant with a rigid spacer.18 The self-assembling properties of enantiomerically pure sodium bis(decyloxy)succinates in aqueous solution have (7) Zarif, L.; Polidori, A.; Pucci, B.; Gulik-Krzywicki, T.; Pavia, A.; Reiss, J. Chem. Phys. Lipids 1996, 79, 165. (8) Du, X.; Shi, B.; Liang, Y. Langmuir 1998, 14, 3631. (9) Volhardt, d.; Siegel, S.; Cadenhead, D. A. J. Phys. Chem. B 2004, 108, 17448. (10) Kunitake, M.; Hattori, T.; Miyano, S.; Itaya, K. Langmuir 2005, 21, 9206. (11) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (b) Zana, R.; Talmon, Y. Nature (London) 1993, 362, 228. (c) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205. (12) Chen, X.; Wang, J.; Shen, N.; Luo, Y.; Li, Lin.; Liu, M.; Thomas, R. K. Langmuir 2002, 18, 6222. (13) (a) Jiang, M.; Zhai, X.; Liu, M. J. Mater. Chem. 2007, 17, 193. (b) Jiang, M.; Zhai, X.; Liu, M. Langmuir 2005, 21, 11128. (c) Talham, Chem. ReV. 2004, 104, 5479. (d) Liang, Z.; Wang, C.; Huang, J. Colloids Surf., A: Physicochem. Eng. Aspects 2003, 224, 213. (e) Menger, F. M.; Mbadugha, B. N. A. J. Am. Chem. Soc. 2001, 123, 875. (f) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (14) Zhou, M.; Liu, H. L.; Yang, H. F.; Liu, X. L.; Zhang, Z. R.; Hu, Y. Langmuir 2006, 22, 10877. (15) Buijnsters, P. J. J. A.; Rodriguez, C. L. G.; Willighagen, E. L.; Sommerdijk, N. A. J. M.; Kremer, A.; Camilleri, P.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Eur. J. Org. Chem. 2002, 1397. (16) Singh, A.; Lvov, Y.; Qadri, S. B. Chem. Mater. 1999, 11, 3196. (17) Fouquey, C.; Lehn, J. M.; Levelut, A. M. AdV. Mater. 1990, 2, 254. (18) Acharya, D. P.; Kunieda, H.; Shiba, Y.; Aratani, K.-i. J. Phys. Chem. B 2004, 108, 1790.
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been reported recently by us.19 The molecule was unique in its behavior with two vicinal carboxyl head groups attached directly to the chiral centers. However, there is no report so far on the 2D behavior of a chiral Gemini surfactant where the head groups are directly linked without any bridge between them. In the present work, we report the surface behavior of a chiral surfactant (2R,3R)-(+)-bis(decyloxy)succinic acid (BDSA) at the air-water interface and explore its phase behavior, pH and temperature effects on phase evolution, and the effect of metal ion complexation in two dimensions. Molecular dynamics (MD) simulation of the interfacial monolayer provided information on the interaction energy, molecular order, and packing at a molecular level, corroborating the experimentally observed monolayer properties.
Chart 1a
Experimental Section Synthesis. (2R,3R)-(+)-bis(decyloxy) succinic acid (BDSA) was synthesized according to the reported procedure.19,20 Pressure-Area Experiments. The pressure-area (π-A) isotherms were acquired from a computer controlled double barrier Langmuir trough (KSV 5000 Finland). A trough of total area 772.5 cm2 was fabricated from a single Teflon block with a dipping well at the center for transfer of films, while the barriers were made of hydrophilic Delrin. Ultrapure water was used as the subphase for all monolayer studies using a Millipore-Academic system. The temperature of the subphase was controlled with a Julabo F-36 temperature controller with an accuracy of (0.1 °C. Typically, the trough was cleaned with chloroform, followed by methanol (extrapure AR grade, SRL Fine chemicals, India) several times, and finally rinsed with ultrapure water. The monolayers were spread from 100 µL chloroform (Uvasol, Merck) solutions on the surface of ultrapure water (MilliporeAcademic). The pH of the subphase was altered by adding sulfuric acid (for acidic pH) or sodium bicarbonate (for basic pH). The surface pressure was measured by the Wilhelmy method with a platinum sensor of accuracy 0.1 mN/m. A delay of 30 min was allowed for the solvent to evaporate before acquiring the isotherms. The monolayers were compressed at an optimized speed of 10 mm/min. Langmuir-Blodgett films were prepared by transferring the monolayers at desired pressures by the vertical dipping method on hydrophilized Si(100) and quartz plates. Hydrophilization was done by etching the substrates with hot piranha solution (1:1 conc H2SO4/H2O2) at 70 °C followed by rinsing with Millipore water. The substrates were etched fresh prior to use and stored in Millipore water. Microscopy. Optical microscopy was performed with Euromex (Holland) microscope fitted with a color CCD camera (Samsung SD 310). Transmission electron microscopy was done with a JEOL 3010 electron microscope.
Results and Discussion Monolayer Behavior at the Air-Water Interface. The molecular structure of (2R,3R)-(+)-bis(decyloxy)succinic acid is shown in Chart 1a. The structure was geometry optimized using the Gaussian 03 package21 with the B3LYP/6-31G level (19) Shankar, B. V.; Patnaik, A. Langmuir 2007, 23, 3523. (20) Dulyea, L. M.; Fyles, T. M.; Dennis, W. M. Can. J. Chem. 1984, 62, 498. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford,
a (a) Molecular structure of (R,R) BDSA. (b)Geometry-optimized structure of BDSA (B3LYP/6-31G).
of theory and is depicted in Chart 1b. The pressure-area (π-A) isotherms of BDSA on Millipore water at 25 °C are shown in Figure 1. The isotherm showed an expanded behavior evidencing the sequence gas f liquid condensed f solid phase. The area per molecule by extrapolating the condensed region to zero pressure was found to be ∼70 Å2. Connolly surface calculations revealed the head group area of 71.86 Å2, suggesting an upright orientation at the air-water interface. The π-A-1 plots for the compression cycle shown in Figure 1 evinced the area per molecule to be 72.11 Å2, in line with the Connolly surface. Effect of Temperature on Phase Evolution. The π-A isotherms recorded at different subphase temperatures are shown in Figure 2. With increase in temperature of the subphase above room temperature (20 °C), the area per molecule decreased with a concomitant decrease in the collapse pressure. The phenomenon was attributed to increased translational motion of the molecules at the interface as well as enhanced vibrational degrees of freedom of the alkyl chains forming aggregated domains with decreased molecular area, as against a greater condensed phase area with restricted degrees of freedom. The latter manifested an efficient molecular packing at room temperature. At 10 °C, the isotherm had a maximum surface pressure of 50 mN/m with an additional phase formed around 42 mN/m. The area at which this new phase was formed corresponded to almost half of the liftoff area (82 Å2), suggesting the formation of bilayers beyond the collapse pressure. The AFM image of monolayer transferred at 25 mN/m on to HOPG (XYA type, NTMDT, Russia) as shown in Figure 2b revealed islands of bilayers formed through transfer-induced monolayer buckling. Effect of pH on Phase Evolution. Since the molecules possess carboxyl groups as the head groups, the pH of the subphase had a profound effect on the phases formed at the air-water interface. The π-A isotherms were recorded at subphase pH from 2 to 10, as shown in Figure 3. The isotherms differed distinctly by two S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
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Figure 1. The pressure-area (π-A) isotherm and the (π-A-1) profile of (R,R) BDSA on Millipore water at 25 °C. To the right, the geometry-optimized structure of (R,R) BDSA, constrained at the air-water interface is depicted with Connolly surface of the head group, showing an area per molecule of 71.86 Å2.
Figure 2. (a) Pressure-area (π-A) isotherms of (R,R) BDSA at different temperatures of the subphase. (b) AFM image of (R,R) BDSA monolayer transferred at 25 mN/m at 25 °C on HOPG revealing islands of transfer induced bilayers.
aspects: (1) increased area per molecule from ∼70 to 81.32 Å2 upon decreasing the pH from 7 to 4 to 2; (2) formation of a plateau-like constant collapse pressure region at pH 2-4. At acidic pH, either the head groups were protonated, or the two dimensional assembly was susceptible to extensive hydrogen
bonding. Intramolecular hydrogen bonding in the present case could not account for the observed increase in the specific area, as it would lie within the total area occupied by the molecule at the interface. Since close packing of the molecule is highly restricted when the hydrogen bonding is intermolecular, we
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Figure 3. Pressure-area (π-A) isotherms of (R,R) BDSA at different pH of the subphase. Inset illustrates the intermolecular H-bonding accounting for the increased area per molecule. The image to the right shows the isotherm on pure water subphase at pH ) 10.
Figure 4. Optical micrographs of 25 layers of LB films of BDSA transferred at 25 mN/m (a) on water subphase (b) on subphase containing 1 mM ZnCl2 (c) on subphase containing 1 mM CaCl2.
attribute the latter for the observed increased area per molecule at acidic pH. An opposite trend was expected to be true for an increase in the subphase pH. Upon increasing the pH to 7.0, the area per molecule decreased and once again increased to 93.87 Å2 at pH 10. The π-A isotherm at pH 10 on pure water subphase, showed a very little surface pressure of 0.2 mN/m with an area per molecule of ∼25 Å2, suggesting the dissolution of the molecule in the subphase (Figure 3, right panel). Therefore, the isotherm at pH 10 in Figure 3 was acquired with 1 M NaCl in the subphase. In all the cases, the isotherms exhibited a flat collapse region, suggesting the formation of bilayers and multi-bilayers as the collapse structures. The self-assembling mode could be predicted by calculating the critical packing parameter p given by p ) V/(as. lc) where V is the volume of the hydrocarbon chain, lc is the critical chain length assumed to be equal to the fully extended chain length, and as is the head group area.22 The value of p ) 1 suggested the formation of planar bilayers at the respective surface molecular densities achieved through compression of the monolayer film. Domain Shape Dependent 2D Lattice Model of the LB Monolayers. The films formed at the air-water interface were transferred onto hydrophilic quartz substrates at 25 mN/m and (22) Pashley, M. R.; Karaman, M. E. Applied Colloid and Surface Chemistry; John Wiley: New York, 2004.
were examined through optical and electron microscopic techniques. The optical images of the multilayer LB films are shown in Figure 4. The images revealed varied shapes such as rings, fractal-like domains, and rods resembling vesicular structures. The TEM images of the monolayers transferred at 25 mN/m onto carbon-coated copper grids are shown in Figure 5. The micrographs revealed 2D cuboids and polyhedra. HRTEM images of the crystals formed at the air-water interface showed an antipodal arrangement of molecules in the crystal lattice, characteristic of an enantiomerically pure crystal.23 An enantiomerically pure monolayer will have the molecules packed with zero tilt angle with respect to its nearest neighbor,24 and such enantiomerically pure membranes exhibit enhanced stability by chiral bilayer effect.25 Molecular Dynamics Simulation of the Interfacial Monolayers. Molecular dynamics (MD) simulation has been attempted in recent times for amphiphiles at the air-water interface.26 MD simulations of long-chain carboxylic acids have revealed their (23) Eckhardt, C. J.; Peachey, N. M.; Swanson, D. R.; Takacs, J. M.; Khan, M. A.; Gong, X.; Kim, J.-H.; Wang, J.; Uphaus, R. A. Nature (London) 1993, 362, 614. (24) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (25) Furhop, J. H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (26) Okamura, E.; Fukushima, N.; Hayashi, S. Langmuir 1999, 15, 3589.
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Figure 5. TEM images of LB films of BDSA transferred at 25 mN/m showing (a) cubes; (b) cuboids and polyhedra; (c) HRTEM image of the cuboid marked (i) showing the lattice with repeated spacing of 0.24 nm. Inset shows the FFT pattern of the lattice evidencing antipodal arrangement of BDSA in a two-molecule rectangular unit cell. (d) Ray trace replica of the polyhedra (i) and (ii) of image (b).
organizational behavior and surface molecular density at the air-water interface.27 NVT MD simulation was carried out with octadecyl ammoniumchloride at the interface as a function of concentration, inorganic salt concentration, and temperature.28 The latter two parameters did not affect the volume of a single molecule of the surfactant, but influenced the morphology of the aggregate. Recently, Khurana et al. reported the fully atomistic
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simulation of a gemini surfactant at the air-water interface29 and observed bending of the spacer chain with increasing spacer length. The present investigation involves molecular dynamics simulation of Langmuir monolayers of BDSA at different surface molecular densities using Hyperchem version 7.5 (Hypercube Inc., USA). Subsequently, the experimentally observed surface pressure-area (π-A) isotherms of BDSA monolayers were simulated. The MM+ force field as an extension of MM2 designed for small organic molecules like the BDSA was applied. The MM+ force field used the potential energy and the torsional and bond angles under periodic boundary conditions. The intermolecular potential functions comprised (i) short-range van der Waals interaction of non-Lennard-Jones type that combined an exponential repulsion with an attractive 1/r6 dispersion interaction term; (ii) long-range Coulombic potentials without the usual electrostatic charge-charge interaction and atomic charges. Instead, electrostatic contributions were accounted for by defining a set of bond dipole moments along with the bond stretching parameters. The total energy could be written as U ) Ubonds + Uangles + Udihedrals + UVDW + Ucoul. The initial configuration for simulation is shown in Figure 6(c) as a 9-molecule hexagonal lattice. The lattice was conceived from the SAED pattern of the BDSA LB films transferred at 25 mN/m (area/molecule ) 70 Å2) of the condensed phase (vide Figure 6a). Figure 6b illustrates simulations performed within a periodic boundary box of 9 BDSA molecules in a hexagonal lattice with a specific orientation on a water slab of 56 molecules. The water slab was obtained from an equilibrated periodic system with a vertical (Z) height of 4 Å (head group region of the BDSA) and the X, Y dimensions identical to that of the 9-molecule monolayer, confined in a periodic box at different specific areas of the molecule. The cutoff radii30 of the water molecules were fixed at 2.4 Å. The water slab was geometry-optimized using the Newton-Raphson method for the first two 500 cycles followed by steepest descent to reduce the potential energy of the system before starting the simulation. The BDSA molecules were
Figure 6. (a) HRTEM image of BDSA monolayers showing a hexagonal lattice at the air-water interface. (b) Snapshot of a 9-molecule hexagonal lattice on a pre-equilibrated water slab of 56 water molecules for a BDSA area per molecule 70 Å2. (c) Schematic representation of 9-molecule hexagonal arrangement of BDSA adopted for simulation for an area per molecule of 70 Å2.
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Figure 7. Structural properties of the interface: (a) Molecular skeleton of BDSA. (b) Simulated plot of total energy of the BDSA monolayer at the interface. (c) Variation of O-C*-C*-O torsion angle as a function of increased surface molecular density (d-g) Changes in various torsion and bond angles of the respective bonds of BDSA. The bond angles are an average of 18 data points for the corresponding area per molecule.
configured at the interface with their carboxyl head groups immersed in the water slab and the alkyl chains in the air. The surface molecular density was varied from gas to the condensed phase of BDSA (vide Figure 1 showing the π-A isotherm) by restraining BDSA molecules in the hexagonal lattice, thus yielding the corresponding specific area per molecule. The equations of motion were integrated in the canonical constant NVT ensemble for a simulation time of 1 ps in 0.1 fs steps during the simulation. The molecular skeleton of BDSA is shown in Figure 7a. Figure 7b depicts the minimum-energy configuration for a calculated
ideal monolayer area of 67.84 Å2, in good correlation with the experimental area of 72.86 Å2. The varying conformations of the molecules at different phases of the monolayer were analyzed by calculating the torsion angles, O-C*-C*-O, C*-O-C1C2, C10-C9-C8-C7, and the bond angle C*-O-C1, as illustrated in Figure 7 against their specific area per molecule. A C10C9-C8-C7 torsion angle of ∼180° is observed with complete relaxation of the alkyl tails, away from the water subphase, as compared to the contrasting lowest torsion angle C*-O-C1C2 in Figure 7e. Similarly, an opposite trend between the torsion
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Figure 8. Snapshots of the simulated interface of the nine-molecule BDSA hexagonal lattice at the end of 1 ps simulation time with area per molecule: (a) 105 Å2 (b) 92.51 Å2 (c) 74.06 Å2 (d) 67.84 Å2 (e) 45.03 Å2. Snapshots in the left panel are viewed through the Z axis, while on the right panes are viewed through the XZ plane. The water molecules are not shown for clarity. The atom colors are as follows: red, oxygen; blue, carbon; gray, hydrogen.
angle C*-(CdO)-OH and bond angle C*-O-C1 in Figure 7f and 7g was attributed to the impact of water subphase H-bonding in the former. Equilibrium snapshots of systems at various areas per molecule from 105 to 45.03 Å2 are shown in Figure 8; significant anisotropy is seen corresponding to the ideal monolayer area of ∼67.8 Å2, where the BDSA molecules tend to orient perpendicular to the interface, as against at other areas with extended tails. 2D Metal Complex Formation. It is well established that the presence of bivalent cations in the subphase assist cationdependent condensation of monolayers at the air-water inter(27) Rouvillard, S.; Perez, E.; Ionoc, R.; Voue, M.; De Coninck, J. Langmuir 1999, 15, 2749.
face.31 The isotherms were measured on subphases containing 1 mM ZnCl2 and 1 mM CaCl2. The isotherms, as shown in Figure 9a,b evinced distinct differences in their features, characteristic of the metal ions. The presence of metal ions formed more condensed isotherms, and the (π-A-1) plots revealed the liftoff values of 60.10 and 62.31 Å2 for Zn2+ and Ca2+, respectively. Huehnerfuss et al. reported the effect of cations on the ordering of saturated fatty acid monolayers at the air-water (28) Yuan, S.; Chen, Y.; Xu, G. Colloids Surf., A 2006, 280, 108. (29) Khurana, E.; Nielsen, S. O.; Klein, M. L. J. Phys. Chem. B 2006, 110, 22136. (30) For cut-off radius value, refer to Hyperchem Version 7.5, manual, Hypercube Inc. (31) Neumann, V.; Gericke, A.; Huehnerfuss, H. Langmuir 1995, 11, 2206.
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Figure 9. Pressure-area (π-A) isotherms and (π-A-1) plots of BDSA at 25 °C on subphases containing (a) 1 mM ZnCl2 and (b) 1 mM CaCl2.
interface.32 Sakai et al. studied the modes of binding of Zn2+ to 12-hydroxystearic acid through external reflection IRRAS, evincing the chelate-type binding of Zn2+ to the carboxyl groups.33 The area per molecule of fatty acid salt with metal ions at the air-water interface has been correlated with Pauling’s electronegativity of the corresponding counterion, and a determination of the degree of the covalent or ionic nature of the bond has been made.34 In the present case, as the two carboxyl head groups are connected by a rigid spacer, the formation of chelate-type complex could be more favorably expected. The monolayers formed with Zn2+ showed enhanced surface pressure when compared to that of Ca2+. We attribute these characteristics to the formation of a “chelate-type” complex, where the two dimensional metal complex formation requires a “preferred bite-site” for the carboxyl oxygens toward the Zn2+. The isothermal compressibility of a Langmuir monolayer enables us to identify the phase transitions occurring at the airwater interface.35 Also, it provides insights into the in-plane bilayer transitions such as the liquid expanded (LE) T liquid condensed (LC) and LC T solid (S), which could not directly be deciphered from the π-A isotherms. Compressibility in two dimensions is defined as
K ) [-1/A(dA/dπ)]
(1)
where dA and dπ are changes in the specific area and surface pressure respectively of a Langmuir monolayer at the air-water interface. More conveniently, the reciprocal of compressibility (K-1), termed the elastic modulus, is plotted. The higher the K-1 value, the lower is the interfacial elasticity.36 The isothermal elastic moduli of BDSA on the subphases containing Zn2+ and Ca2+ are shown in Figure 10. The curves were smoothed by adjacent averaging with 10 points twice, using Origin 7.0 plotting software. (32) Gericke, A.; Huehnerfuss, H. Thin Solid Films 1994, 245, 74. (33) Sakai, H.; Umemura, J. Colloid Polym. Sci. 2002, 280, 316. (34) Datta, A.; Kmetko, J.; Yu, C.-J.; Richter, A. G.; Chung, K.-S.; Bai, J.-M.; Dutta, P. J. Phys. Chem. B 2000, 104, 5797. (35) Liu, G.; Yang, S.; Zhang, G. J. Phys. Chem. B 2007, 111, 3633. (36) Joncheray, T. J.; Denoncourt, K. M.; Meier, M. A. R.; Schubert, U. S.; Duran, R. S. Langmuir 2007, 23, 2423.
Figure 10. Elasticity modulus plots of BDSA at 25 °C on subphases containing 1 mM ZnCl2 and 1 mM CaCl2.
The K-1-A plot could be analyzed by dividing into four segments: in the case of Zn2+, the gaseous phase (G) 100 to 70 Å2, an LE phase from 67 Å2 to a small region of 68 Å2, followed by a coexistent region of LE-LC until 62 Å2. Further compression of the film formed the condensed phase or the solid phase, followed by collapse into bilayers and multi-bilayers. The presence of Ca2+ in the subphase showed similar behavior, but with an extended LE-LC coexistence region up to 58 Å2. A high elastic modulus of 101.2 mN/m is associated with the condensed phase. Zn2+, therefore, formed a more condensed and stable film at the air-water interface because of its “intramolecular”-type chelate complex.
Conclusions The monolayer behavior of a new Gemini-type, twin chiral, twin-tailed molecule (2R,3R)-(+)bis (decyloxy)succinic acid at the air-water interface has been explored. The pH effect on the monolayer phases studied revealed the formation of intermolecular hydrogen bonding in acidic pH with increased area per molecule
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and formation of water-soluble self-assembles in basic pH. A decrease in temperature of the subphase manifested in the close packing of the molecules in the condensed phase with differing degrees of freedom, attributed to the alkyl chains of BDSA. Molecular dynamics simulations performed on the monolayers of BDSA on the water surface corroborated the experimentally observed area per molecule. The presence of metal ions in the subphase resulted in 2D metal complex formation with Zn2+ forming a chelate-type complex and Ca2+ an ionic-type complex.
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Acknowledgment. The present research work is supported by Department of Science and Technology (DST), Govt. of India under the grant SP/SI/H-37/2001. B.V.S. acknowledges research fellowship from IIT Madras. The authors thank Mr. Bharat Kulkarni and Prof. K.A. Suresh, Raman Research Institute, Bangalore, India, for AFM analysis. LA701998C
