pubs.acs.org/Langmuir © 2010 American Chemical Society
Effect of Hydrophilic Groups of Ca Surfactants and Hydrophobic Chains of CnDMAO on Coordinated Vesicle Formation Hongshan Tian, Qi Ding, Juan Zhang, Aixin Song,* and Jingcheng Hao Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Received July 18, 2010. Revised Manuscript Received October 31, 2010 The effects of hydrophilic headgroups of Ca surfactants, calcium dodecylsulfate (Ca(DS)2), calcium dodecylsulfonate (Ca(DSA)2), and calcium laurate (CaL2) and hydrophobic chains of alkyldimethylamine oxide (CnDMAO, n=12, 14, 16) on the formation of Ca2þ-ligand coordinated vesicles was investigated in detail. On the basis of phase behavior studies, rheological properties and freeze-fracture transmission electron microscope (FF-TEM) images were measured. Quite different phase behaviors were observed in different surfactant systems. For a Ca surfactant with a highly polar group, Ca(DS)2, vesicles were observed in all Ca(DS)2/CnDMAO (n=12, 14, and 16) systems, whereas for Ca surfactant with lower polar group, Ca(DSA)2, vesicles can form in Ca(DSA)2/CnDMAO systems of n=14 and 16 but not for n=12. For CaL2, the surfactant with the least polar group, vesicles form only in the CaL2/C16DMAO system. The results demonstrate that in the systems formed by Ca surfactants and CnDMAO, the formation of vesicles is driven not only by interaction between Ca2þ and the N f O groups of CnDMAO but also by electrostatic and hydrophobic interactions. Vesicles prefer to form in Ca surfactants with highly polar headgroups and CnDMAO with long chain length.
Introduction Vesicle bilayers have served as models of cell membranes, microreactors, and templates for preparing a variety of functional materials. Since vesicles formed by phospholipids, such as lecithin, were first observed, great attention has been focused on constructing vesicles from different kinds of amphiphilic systems. Among those systems, cationic and anionic (catanionic) surfactants have attracted considerable interest since they were first reported by Kaler et al in 1989.1-7 However, precipitates are generally produced near equimolar ratios in catanionic surfactant aqueous solutions because the electrostatic repulsion between vesicle bilayers can be screened by excess salts formed by surfactant counterions.2,7-10 In contrast, vesicles obtained from saltfree catanionic surfactant mixtures provide very rich aggregation behaviors because the electrostatic interaction between the aggregates is not screened, which results in the stability of vesicles in
the salt-free catanionic surfactant systems at equimolar mixed ratios.7,11-26 The salt-free catanionic surfactant systems can be obtained through different routes. A very general route is to mix alkyltrimethylammonium hydroxide or alkyldimethylamine oxide (CnDMAO) with single-tailed acids in water.7,11-19 In these systems, salts can be avoided because OH- reacts with Hþ to form H2O, or N f O group of CnDMAO is charged by Hþ to form cationic surfactant (CnDMAOHþ). Another typical route is to form metal-ligand coordinated systems.20-28 Surfactants with multivalent metal counterions usually have high Krafft points and are hardly dissolved in water, which induces very simple phase behaviors in aqueous solutions at room temperature.28 However, when mixed with nonionic surfactants or cosurfactants, diverse aggregates are obtained.28-30 Among these systems, the formation of vesicles is found to be facile in mixtures of multivalent
*Corresponding author. E-mail:
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(15) Meister, A.; Dubois, M.; Belloni, L.; Zemb, Th. Langmuir 2003, 19, 7259– 7263. (16) Dubois, M.; Lizunov, V.; Meister, A.; Gulik-Krzywicki, Th.; Verbavatz, J. M.; Perez, E.; Zimmerberg, J.; Zemb, Th. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 15082–15087. (17) Li, H.; Hao, J. J. Phys. Chem. B 2008, 112, 10497–10508. (18) Silva, B. F. B.; Marques, E. F.; Olsson, U. Langmuir 2008, 24, 10746–10754. (19) Yamashita, Y.; Savchuk, K.; Beck, R.; Hoffmann, H. Curr. Opin. Colloid Interface Sci. 2004, 9, 173–177. (20) Waggoner, T. A.; Last, J. A.; Kotula, P. G.; Sasaki, D. Y. J. Am. Chem. Soc. 2001, 123, 496–497. (21) Wang, C.; Wang, S.; Huang, J.; Li, Z.; Gao, Q.; Zhu, B. Langmuir 2003, 19, 7676–7678. (22) Gazzaz, H. A.; Robinson, B. H. Langmuir 2000, 16, 8685–8691. (23) Owen, T.; Pynn, R.; Hammouda, B.; Butler, A. Langmuir 2007, 23, 9393– 9400. (24) Hoffmann, H.; Gr€abner, D.; Hornfeck, U.; Platz, G. J. Phys. Chem. B 1999, 103, 611–614. (25) Wang, Z.; Song, A.; Jia, X.; Hao, J.; Liu, W.; Hoffmann, H. J. Phys. Chem. B 2005, 109, 11126–11134. (26) Song, A.; Jia, X.; Teng, M.; Hao, J. Chem.;Eur. J. 2007, 13, 496–501. (27) Song, A.; Hao, J. Curr. Opin. Colloid Interface Sci. 2009, 14, 94–102. (28) Zapf, A.; Beck, R.; Platz, G.; Hoffmann, H. Adv. Colloid Interface Sci. 2003, 100 -102, 349–380. (29) Hornfeck, U.; Hammel, R.; Platz, G. Langmuir 1999, 15, 5232–5236. (30) Zapf, A.; Hammel, R.; Platz, G. Colloids Surf., A 2001, 183-185, 213–225.
(1) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. Science 1989, 245, 1371–1374. (2) Raghavan, S. R.; Fritz, G.; Kaler, E. W. Langmuir 2002, 18, 3797–3803. (3) Andreozzi, P.; Funari, S. S.; Mesa, C.; Mariani, P.; Ortore, M.; Sinibaldi, R.; Spinozzi, F. J. Phys. Chem. B 2010, 114, 8056–8060. (4) Soussan, E.; Mille, C.; Blanzat, M.; Bordat, P.; Rico-Lattes, I. Langmuir 2008, 24, 2326–2330. (5) Wolf, C.; Bressel, K.; Drechsler, M.; Gradzielski, M. Langmuir 2009, 25, 11358–11366. (6) Lu, T.; Li, Z.; Huang, J.; Fu, H. Langmuir 2008, 24, 10723–10728. (7) Horbaschek, K.; Hoffmann, H.; Thunig, C. J. Colloid Interface Sci. 1998, 206, 439–456. (8) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792–13802. (9) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski, J. A. J. Phys. Chem. 1996, 100, 5874–5879. (10) Iampietro, D.; Kaler, E. W. Langmuir 1999, 15, 8590–8601. (11) Song, A.; Dong, S.; Jia, X.; Hao, J.; Liu, W.; Liu, T. Angew. Chem., Int. Ed. 2005, 44, 4018–4021. (12) Shen, Y.; Hao, J.; Hoffmann, H. Soft Matter 2007, 3, 1407–1412. (13) Zemb, Th.; Dubois, M.; Deme, B.; Gulik-Krzywicki, Th. Science 1999, 283, 816–819. (14) Dubois, M.; Deme, B.; Gulik-Krzywicki, Th.; Dedieu, J.-C.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, Th. Nature 2001, 411, 672–675.
18652 DOI: 10.1021/la102847f
Published on Web 11/18/2010
Langmuir 2010, 26(24), 18652–18658
Tian et al.
Article
metal surfactants and C14DMAO, driven by metal-ligand coordination between the divalent or multivalent metal ions and N f O groups of C14DMAO.24-28 In these systems, metal ions are tightly associated with the headgroups of surfactants, constructing vesicle membranes, which are not shielded by excess salts. The aggregates formed can be used as templates for preparing nano- or micromaterials because of the fixation of metal ions on the bilayer membranes, which can be precipitated through different reactions.25,27 A series of metal-ligand complexes formed between CnDMAO and anionic surfactants with multivalent metal ions (Zn2þ, Ca2þ, Ba2þ, Al3þ, etc.) as counterions was previously studied and reviewed by our group.27 However, the previous reports focused only on the metal-ligand interactions between metal ions and N f O groups of CnDMAO. The structures of surfactants that play very important roles in the formation of bilayer membranes have not yet been considered. To consider the aggregates in the surfactant systems, the packing parameter, p, has been widely used for many years.31
Phase Behavior Study. Phase behaviors were studied by visual inspection with the help of crossed polarizers. Samples were obtained by dissolving various amounts of Ca(DS)2, Ca(DSA)2, and CaL2 into CnDMAO micellar aqueous solutions with ultrasonication. Samples were allowed to equilibrate for at least 4 weeks at 25.0 ( 0.1 °C until they were unchanged over an extended period of time. The phase diagrams were obtained by observing the solutions in the calibrated test tubes at the same temperature. Rheological Measurements. Rheological measurements were carried out on a HAAKE RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti) for low viscosity samples and a cone-plate system (C35/1Ti L07116) for samples with high viscosity. In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep to ensure that the selected stress was in the linear viscoelastic region. Samples were measured at 25.0 ( 0.1 °C with the help of a cyclic water bath.
p ¼ v=ðla0 Þ
characterized by FF-TEM observations. A small amount of sample was placed on a 0.1-mm-thick copper disk covered with a second copper disk. Then, the copper sandwich with the sample was plunged into liquid propane that had been cooled by liquid nitrogen to freeze. Fracturing and replication were carried out on a Balzers BAF-400D equipment at -150 °C. Pt/C was deposited at an angle of 45°. The replicas were examined in a JEOL JEM1400 electron microscope operated at 120 kV. Conductivity Measurements. Conductivity measurements were performed on a DDS-307 conductivity meter (China) at 25.0 ( 0.1 °C. The two-phase solutions were detected under stirring.
ð1Þ
Here v and l are the volume and length of hydrophobic chain, and a0 is the area of the hydrophilic groups of amphiphilic molecules. The value of p is usually a guide to predicting the kind of aggregates forming in aqueous solutions of surfactant systems. When p is small (p e 1/3), global micelles are usually obtained; then, rod-like or wormlike micelles (1/3 < p e 1/2), bilayers (1/2 < p e 1), and reverse structures (p > 1) can form with the increase in p. Therefore, the aggregation behaviors of amphiphilic systems should be affected by many factors through changing the value of p, such as the polar groups, the chain length, and the structure of the hydrophobic chains. To detect whether and how the metal-ligand complexes can be affected by surfactant structures, in the present paper, three anionic surfactants with the same chain length (C12) and counterion (Ca2þ) but different hydrophilic groups, calcium dodecylsulfate (Ca(DS)2), calcium dodecylsulfonate (Ca(DSA)2), and calcium laurate (CaL2) were selected to be mixed with CnDMAO with different chain lengths (n=12, 14, 16). The phase behaviors and rheological properties of these systems were studied, demonstrating that they are very different because of the different hydrophilic groups of the Ca surfactants and different chain lengths of CnDMAO. The results reveal that the formation of vesicle bilayers is not only driven by the metal-ligand coordination between the metal ions and N f O groups but also strongly affected by the structure of surfactants. In this respect, our results provide new understanding for metal-ligand coordinated vesicles.
Experimental Methods Chemicals and Materials. Alkyldimethylamine oxide (CnDMAO, n=12, 14, 16) aqueous solutions with the concentration of 30 wt % were received as a gift from the affiliate of Clariant Company (Germany) in China. The solutions were freeze-dried and then crystallized three times in acetone. Calcium dodecylsulfate (Ca(DS)2), calcium dodecylsulfonate (Ca(DSA)2), and calcium laurate (CaL2) were obtained by dripping excess CaCl2 solutions into sodium dodecylsulfate, sodium dodecylsulfonate, and sodium laurate solutions, respectively. The preparations were stirred and the precipitates formed were filtered until Ca2þ and Cl- ions were not detected. They were then dried at 50 °C for 24 h. Water was distilled three times. (31) Israelachvili, J.; Mitchell, D. J.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525–1568.
Langmuir 2010, 26(24), 18652–18658
Freeze-Fracture Transmission Electron Microscope (FFTEM) Observations. The microstructure of the gel-phase was
Results and Discussion Ca-Surfactant (Ca(DS)2 , Ca(DSA)2 , and CaL2)/ C12DMAO/H2O Systems. (i). Phase Behaviors. CaL2, Ca(DSA)2, and Ca(DS)2 all have poor solubility in water at room temperature because of their high Krafft points induced by the divalent metal counterion (Ca2þ). The Krafft point of Ca(DS)2 is ∼50 °C.32 For Ca(DSA)2 and CaL2, both of their Krafft points were not observed up to 100 °C. The 100.0 mmol 3 L-1 C12DMAO aqueous solution is a transparent L1 phase with very low viscosity, consisting of spherical micelles. When different Ca surfactants were dissolved in C12DMAO micellar solutions, different phase behaviors were observed. The phase behaviors of CaL2, Ca(DSA)2, and Ca(DS)2 in C12DMAO aqueous solutions are shown in Figure 1a-c, respectively. For CaL2 (Figure 1a), at low CaL2 concentrations (
