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Langmuir 2001, 17, 3526-3531
Rearrangements of Metastable Micelles to Different Molecular Bilayers on Planar Graphite, Mica, Silicon, and Hydrocarbon Surfaces Christian Messerschmidt,† Christian Draeger,† Andrea Schulz,† Ju¨rgen P. Rabe,‡ and Ju¨rgen-Hinrich Fuhrhop*,† Freie Universita¨ t Berlin, FB Biologie, Chemie, Pharmazie, Institut fu¨ r Chemie/Organische Chemie, Takustr. 3, D-14195 Berlin, Germany, and Humboldt-Universita¨ t zu Berlin, Institut fu¨ r Physik, Invalidenstr. 110, D-10115 Berlin, Germany Received June 10, 2000. In Final Form: March 16, 2001 Bis(2,2′-bipyridyl)(dihexadecyl-2-[2,2′-dipyridylmethylene] malonate) ruthenium(II) dihexafluorophosphate, 1, formed multilayered micelles (denoted C16-micelles) upon sonication of aqueous suspensions. The C16-micelles collapsed upon transfer to gold, mica, and silicon surfaces and rearranged to planar bilayers. These bilayers appeared in different arrangements under the atomic force microscope. On graphite, the ruthenium headgroups and the alkyl chains lay flat and were both in direct contact with the substrate. On mica and silicon wafers, upright-standing interdigitated bilayers were found exclusively. Self-assembly of a dodecylsilane layer containing cracks on the silicon surface induced the formation of irregular double and triple layers of 1. Bulk polyethylene or octadecylthiol layers on gold with smooth surfaces did not disrupt the micelles. C18- and C22-micelles made of the corresponding homologues of 1 were much more stable on most surfaces, C14-micelles were destroyed on all surfaces. The variability of micelle-substrate interactions is discussed qualitatively.
Introduction
Experimental Section
Adsorbed supramolecular structures remain stable on surfaces if the intermolecular forces are stronger than the forces between the molecules and the surface, that is, if the spreading coefficient is positive.1 We recently reported on multilayered micelles made of bis(2,2′bipyridyl) (dioctadecyl-2-[2,2′-dipyridylmethylene] malonate) ruthenium(II) dihexafluorophosphate, which could be isolated in the dry state and did not change their spherical shape on solid silicon or mica. They did not contain an entrapped water volume but were made of interdigitated bilayers only. Therefore, they are not vesicles. On graphite surfaces, these micelles have been destroyed and rearranged to planar bilayers. On mica or silicon surfaces, however, the micelles remained stable. The reason for this stability was traced back to the multilayer arrangement with intercalated alkyl chains and unique back-to-back headgroup interactions.2 In the present paper, we examine somewhat less stable micelles of the C16- and C14-homologues as well as equally stable C22-micelles in order to investigate the effect of surface forces on sensitive spherical bilayer structures. The classical planar surfaces used in atomic force microscopy (AFM) experiments (gold, mica, silicon, graphite) and hydrocarbons were chosen as materials. The C16micelle was found to differentiate between them, and the results with this particular lipid sphere are reported. We also compare the micelle’s collapse or fusion to the known phenomena of vesicle fusion, which produces bilayers on solid surfaces.1,3-6
Bis(2,2′-bipyridyl) (dihexadecyl-2-[2,2′-dipyridylmethylene] malonate) ruthenium(II) dihexafluorophosphate, 1, as well as the C22- and C14-homologues were synthesized analogously to its octadecyl compound described earlier. Multilayered micelles of 1 have been formed according to a procedure previously reported.2 Mica (Plano, Wetzlar/Germany) and highly oriented pyrolitic graphite (HOPG, Plano) have been freshly cleaved prior to use. Silicon wafers (Aurel) were cleaned with a mixture of hydrogen peroxide/ammonia/water 1:1:10 at 80 °C for 30 min and rinsed thoroughly with Millipore water. The same wafer pieces were hydrophobized by self-assembly using a 10-3 M solution of dodecyltrichlorosilane (Aldrich, used as received) in tetrachloromethane for 24 h (contact angle ∼110°). For the polyethylene experiments, a commercial foil was taken. Gold surfaces were prepared according to Wagner et al.7 and immersed in a 10-3 M solution of octadecyl thiol (Aldrich, used as received) in tetrachloromethane for 24 h (contact angle ∼110°). AFM measurements were performed using a Digital Instruments Nanoscope IIIa (Santa Barbara, CA) in tapping mode. All images were taken under ambient conditions. Silicon cantilevers (Digital Instruments) with a spring constant of 45-60 N/m and a resonance frequency in the range of 270-350 kHz were used. The scanning rate was usually 1.5 Hz. Micelles were adsorbed by placing a 10 µL droplet of a sonicated 5 × 10-4 M solution of 1 on the different surfaces. Excess fluid was blotted off after 30 s. Transmission electron microscopy (TEM) was conducted as follows: A droplet (5 µL) of the freshly sonicated aqueous solution of 1 was placed on hydrophilized carbon films on copper wire grids (60 s plasma treatment at 8 W using a BALTEC MED 020 device). Excess fluid was blotted off and air-dried on the
* To whom correspondence should be addressed. † Freie Universita ¨ t Berlin. ‡ Humboldt-Universita ¨ t zu Berlin. (1) Nissen, J.; Gritsch, S.; Wiegand, G.; Ra¨dler, J. O. Eur. Phys. J. B 1999, 10, 335. (2) Draeger, C.; Bo¨ttcher, C.; Messerschmidt, C.; Schulz, A.; Ruhlmann, L.; Siggel, U.; Hammarstro¨m, L.; Berglund-Baudin, H.; Fuhrhop, J.-H. Langmuir 2000, 16, 2068. (3) Sackmann, E. Science 1996, 271, 43.
(4) Ra¨dler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 4539. (5) Salay, L. C.; Carmona-Ribeiro, A. M. J. Phys. Chem. B 1998, 102, 4011. (6) Rapuano, R.; Carmona-Ribeiro, A. M. J. Colloid Interface Sci. 1997, 193, 104. (7) Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867.
10.1021/la0008131 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/16/2001
Rearrangements of Metastable Micelles
Figure 1. (A) Cryo-transmission electron micrograph of a micellar solution of bis(2,2′-bipyridyl)(dihexadecyl-2-[2,2′-dipyridylmethylene] malonate) ruthenium(II) dihexafluorophosphate, 1. The white and black striations are each 2 nm wide. (B) Model of a cross section of the micelle. grids. Microscopy was carried out using a Philips CM12 TEM operated at 100 kV accelerating voltage at a low electron dose (
