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Gemini-like Molecular Clips and Tweezers: The Influence of Structure and Guest Binding on Interfacial Tension :: P. Degen,*,† S. Leick,† H. Rehage,† J. Polkowska,‡ and F.-G. Klarner‡ †
:: :: Lehrstuhl fur Physikalische Chemie II, Universitat Dortmund, Otto Hahn Strasse 6, 44227 Dortmund, :: :: ‡ Germany and Institut fur Organische Chemie, Universitat Duisburg-Essen, 45117 Essen, Germany Received January 7, 2009. Revised Manuscript Received February 6, 2009
In a series of experiments, we studied the interfacial activity of aromatic aliphatic molecules with rigid geminilike structures at the interface between toluene and water. These molecules, called clips and tweezers, have rigid central benzene or naphthalene spacer-units, each substituted with two polar groups as well as two rigid aromatic side walls. They can serve as host molecules and selectively bind a variety of electron-deficient aromatic and aliphatic guest molecules. In different experiments, we compared the interfacial tensions with the calculated hydrophilic-lipid-balance (HLB) values of these molecules. The measured interfacial tensions depend as much on the HLB values as on the geometric structure of the water insoluble molecules. The concentration dependence of the surface tension gave evidence for the formation of inverse micellar aggregates, which were formed in the oil phase above a well-defined value of the bulk concentration. The presence of aggregates in the organic liquid could also be investigated by dynamic light scattering measurements. We observed typical diameters of the inverse micellar aggregates in the order of 5.6 nm, and the critical micelle concentrations (cmc’s) coincided well with the results of interfacial tension measurements. From the surface excess in the vicinity of the cmc, we calculated the space occupied by a single clip molecule on the self-assembled monolayer. The observed molecular surface area was in agreement with the effective molecular diameters of the molecules. In additional experiments, we could also show that complexes with aromatic guest molecules such as 1,2-4,5-tetracyanobenzene (TCNB) led to a reduction of the amphiphilic clip properties.
1.
Introduction
Molecular recognition is important in many areas of biological and supramolecular chemistry, e.g., in antigen-antibody recognition or enzyme-substrate binding.1 Most of these processes take place at interfaces, and therefore surface active substances are ideal model systems for biological systems. In addition, the amphiphilic character of such receptor molecules simplifies the preparation of ordered structures, which is important for chemical sensors. A relatively new class of Gemini-like receptor molecules, called molecular :: tweezers or clips, was described by Klarner et al.2 Gemini or dimeric surfactants generally contain two hydrophilic and two hydrophobic groups per molecule. The linkage (“spacer”) between the two hydrophilic groups can be either a flexible hydrophilic,3,4 flexible hydrophobic,5 or rigid hydrophobic molecule,6,7 whereas the two hydrophobic chains are flexible in most cases. The special molecules presented here have a rigid spacer-unit substituted with two polar groups as well as two rigid aromatic side walls. They are called clips if the aromatic side walls are stretched straightly and are called tweezers if the side walls are angular (see Figure 1). These compounds can selectively bind a variety of electron-deficient *Corresponding author. E-mail:
[email protected].
:: (1) Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vogtle, F.; Suslick, K. S. Comprehensive Supramolecular Chemistry; Elsevier: Oxford, 1996. (2) Kamieth, M.; Burkert, U.; Corbin, P. S.; Dell, S. J.; Zimmermann, S. :: C.; Klarner, F.-G. Eur. J. Org. Chem. 1999, 11, 2741. (3) Okahara, M.; Masuyama, A.; Sumida, Y.; Zhu, Y.-P. J. Jpn. Oil Chem. Soc. (Yukagaku) 1988, 37, 716. (4) Rosen, M. J.; Zhu, Z. H.; Hua, X. Y. J. Am. Oil Chem. Soc. 1992, 69, 30. (5) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (6) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (7) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083.
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DOI: 10.1021/la900072w
aromatic and aliphatic guest molecules.8 The specific hostguest relationships have been extensively studied in dilute :: organic or aqueous solutions by Klarner et al. and Schrader 9-12,28-32 et al. Through the combination of a preorganized rigid molecular aromatic structure and strategically placed, flanking polar groups, the water-insoluble molecules obtain striking amphiphilic properties. The behavior of the molecular clips and tweezers at the water/air interface as Langmuir monolayers has been reported in previous publications.13,14 The focus of this study was the investigation of the adsorption process of these molecules from the organic liquid (toluene) at the interface between toluene and water. The dynamic interfacial tension is a favored indicator to investigate the adsorption process of amphiphilic molecules at a liquid/liquid interface. The formation of a Gibbs adsorption layer leads to a reduction of the interfacial tension, and the dynamic of this reduction as well as the equilibrium interfacial tension value are characteristic for the amphiphilic molecules. Additionally, the determination of the interfacial tension versus bulk concentration can be use to determine the :: :: (8) Klarner, F.-G.; Panitzky, J.; Blaser, D.; Boese, R. Tetrahedron 2001, 57, 3673. :: (9) Klarner, F.-G.; Panitzky, J.; Preda, D.; Scott, L. T. J. Mol. Model. 2000, 6, 318. :: (10) Klarner, F.-G.; Burkert, U.; Kamieth, M.; Boese, R.; Benet-Buchholtz, J. Chem.;Eur. J. 1999, 5, 1700. :: (11) Klarner, F.-G.; Burkert, U.; Kamieth, M.; Boese, R. J. Phys. Org. Chem. 2000, 13, 604. :: (12) Klarner, F.-G.; Benkhoff, J.; Boese, R.; Burkert, U.; Kamieth, M.; Naatz, U. Angew. Chem., Int. Ed. 1996, 35, 1130. :: (13) Degen, P.; Rehage, H.; Klarner, F.-G.; Polkowska, J. J. Colloid Polym. Sci. 2005, 284, 44. (14) Degen, P.; Optenhostert, T.; Rehage, H.; Verhaelen, C.; Lange, M.; :: Polkowska, J.; Klarner, F.-G. Langmuir 2007, 23, 11611.
Published on Web 3/4/2009
Langmuir 2009, 25(11), 6094–6099
Degen et al.
Article
Figure 1. Structures of the investigated molecular clips (I and II) and tweezers (III and IV) with a central benzene spacer-unit (n = 0) and a naphthalene spacer-unit (n = 1), respectively. Clip I is substituted by different polar groups (a-c). Clip II and tweezers III and IV are only substituted by acetoxy groups (a). In addition, the schematic structure of the complex TCNB@Ia is shown.
formation of micelles of the amphiphilic molecules. The concentration at which the formation of these aggregates occurs for the first time is the critical micelle concentration (cmc).15-17 In our case, the amphiphilic molecules are only soluble in organic liquids and can form inverse micellar systems. Inverse micelles, also called water-in-oil microemulsions (w/o-MEs), were microdroplets of nanometer size, stabilized in the nonaqueous solution by a surfactant. The existence of inverse micelles of oil-soluble surfactants in nonpolar solvents has been reported in many studies,18 but, up to now, it had not been described for the molecular clips we investigated here. In addition to interfacial tension measurements, a possible micelle formation could be observe by light scattering experiments, UV-vis spectroscopy of solute dye molecules, or conductivity measurements. Because of the complicated synthesis of the molecular clips and tweezers, we did not have enough material to investigate the phenomenon of aggregate formation in more details by all of these methods. The light scattering technique has the advantage that no contamination of the probe by dye molecules was necessary, and a very small amount of the solution (
