VOLUME 6, NUMBER 2, FEBRUARY 2006 © Copyright 2006 by the American Chemical Society
Ru(II)-Based Metallosurfactant Forming Inverted Aggregates David Domı´nguez-Gutie´rrez, Marko Surtchev, Erika Eiser,* and Cornelis J. Elsevier* Van’t Hoff Institute for Molecular Chemistry, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received September 30, 2005; Revised Manuscript Received November 24, 2005
ABSTRACT Knowing the advantages of incorporating a transition metal into interfaces, we report on the first inverted aggregates formed using metallosurfactants. The metallosurfactant possesses four long linear tails that account for the shielding of the polar headgroup in apolar solvents. The nature of the so-formed aggregates changes dramatically from inverted vesicles (toluene) to inverted micelles (hexane). The size of the aggregates was determined using dynamic light scattering. Atomic force microscopy allowed us to study the dry structure of the vesicles on a glass surface.
Metallosurfactants, that is, surfactants containing transition metal ions, provide a way of incorporating properties to an interface, such as paramagnetism, color, or pH sensitivity, but most importantly redox and catalytic activity. Particularly, those that incorporate ruthenium(II) bipyridyl units stand out for their robustness, reactivity,1 and rich electro- and photochemical properties.2 Dialkylbipyridil ligands have been used successfully in the synthesis of both Ru- and Fe-based amphiphiles.3,4 Even though metallosurfactants are scarce compared to their nonmetallic counterparts, there has been a boost of interest because of their multiple applications in fields such as medicine,5,6 magnetic resonance imaging,7,8 drug delivery,9 and homogeneous catalysis.10-14 Initial studies focused on the lyotropic phase behavior of [Ru(terpy)2]2+ systems,15 but the first micellization was actually observed in solutions of nonlabile metallosurfactants containing a [Co(polyamine)]3+ center.16 Nevertheless, stud* Corresponding authors. E-mail:
[email protected]; elsevier@ science.uva.nl. 10.1021/nl051944v CCC: $33.50 Published on Web 12/31/2005
© 2006 American Chemical Society
ies on the relationship between structure and the physicochemical properties of these metalloaggregates remain limited17-19 and deal exclusively with spherical micelles. In addition, these aggregates have been studied mainly in aqueous solutions, while there are only few known metallosurfactants that aggregate in organic solvents, and if so, they are only polar organic solvents such as methanol.20 A simple way to design a metallosurfactant could be to replace an organic ion by a charged transition-metal complex, but this seldom renders a surface-active complex and rules for conventional surfactants can hardly apply to these systems. We have designed a metallosurfactant where the metal is an integral part of the headgroup, aimed at aggregation in apolar solvents, with an architecture that favors packing into inverted aggregates. This would result in the first metallosurfactant that aggregates into inverted micelles. We synthesized the new complex [Ru(bipy)(4,4′-diheptadecyl-2,2′-bipyridyl)2]Cl2 (1, Figure 1) by reacting 2 equiv of 4,4′-diheptadecyl-2,2′-bipyridine21 with 1 equiv of RuCl3‚
Figure 1. [Ru(bipy)(4,4′-diheptadecyl-2,2′-bipyridine)2]Cl2 (1). Ideal architecture of a surfactant (right) capable of aggregating into an inverted micelle. Hydrophobic moieties are depicted in blue and hydrophilic in red.
3H2O in DMF at 110 °C and subsequently with 1 equiv of 2,2′-bipyridine in THF. Although similar to Ru-complexes used for the preparation of LB films3, the choice of the counterion for this work is very important (ClO4- is more strongly condensing but huge compared to Cl-) to achieving a small headgroup area. The use of the [Ru(bipy)3]2+ unit as headgroup has proven effective in metallosurfactants18,19,22 and block-copolymers23-26 that can self-aggregate in aqueous solutions. Conversely, our metallosurfactant is not water-soluble, a feature attributed to the strongly hydrophobic character of the complex imposed by the alkyl chains of the modifiedbipyridine ligands. We estimate the headgroup size to be roughly 100 Å2. Following simple geometric arguments, the ratio of the overall surfactants’ volume, V, to that of the effective headgroup surface, a0, times the tail length, l, also known as the packing parameter, p, gives a good indication how surfactants will aggregate. Our aim is to form inverted micelles with our metal-containing headgroup forming the core of the micelles. To achieve this, we estimated a minimum of two modified dialkylbipyridines needed to create the desired effect, namely, giving our metallosurfactants a coneshaped geometry (Figure 1). Therefore, by altering one bi-
pyridine ligand, the aggregation behavior is radically changed and at the same time the surface-active capabilities are retained. Dynamic light scattering (DLS) measurements of 10-3 M solutions of 1 in toluene and 0.5 × 10-3 M solutions in hexane indeed indicated the presence of aggregates. Selfassembly of 1 in toluene results in large spherical aggregates of ∼500 nm in radius. This is too large to be due to the formation of inverted micelles. However, because p ≈ 1, we argue that our metallosurfactants form inverted bilayers that close on themselves, forming vesicles with toluene inside and outside of the bilayer. But how do the surfactants pack into these vesicle bilayers? The apolar nature of the solvent we used requires that the apolar chains face the solvent, hence toluene, while the cationic headgroups are situated in the core of the bilayer facing each other. The hypothesis is reinforced by the fact that the [Ru(bipy)3]Cl2 complex is not soluble in toluene. To further characterize our vesicles, we investigated them by means of AFM. Typical AFM images are shown in Figure 2. The vesicles appear with an ellipsoidal instead of a spherical shape because during the drying of the deposited droplet, the solvent does not evaporate immediately from the whole covered surface, but there is a moving drying front that orients the vesicles along its direction. In addition, the evaporation of the solvents causes the collapse of the vesicles as well. Indeed, our AFM measurements indicate that they have a flattened shape with rounded edges and a slightly convex top surface as seen in Figure 2b. The mean radius of the dry vesicles is 92 nm. Disagreement between AFM and DLS results for the determination of the radius has been found27 earlier, and it is attributed to the fact that DLS determines the hydrodynamic radius while solid AFM provides the radius of the collapsed dry aggregates. Knowing the radius of the condensed structures, the aggregation number Nagg can be then calculated as 1.1 × 105. With AFM, the thickness of the deposited particles can be measured by taking the cross section of the individual ones, obtaining
Figure 2. Typical AFM image (tapping mode, height) of individual vesicles adsorbed on fused silica from 10-3 M solutions of 1 in toluene (a) and corresponding cross-section analyses (b). The color scale in the image covers a range of 19 nm from dark to bright. The crosssections show the profiles along the corresponding lines indicated in the image. 146
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values between 12 and 14.5 nm. Because special care was taken to work under noninvasive imaging conditions, the measured thickness is definitely the true thickness of the deposited particles. The length of 1 assuming fully extended chains is 3 nm, which means that the thickness of the dry vesicles on the surface corresponds to those of two bilayers. Thus, this double bilayer confirms that the unilamellar vesicles consist of a bilayer closing onto itself. More unexpectedly, solutions of 1 in hexane present different aggregation behaviors. DLS experiments give a radius of about 5.5 nm per aggregate. This magnitude means unequivocally that the aggregates are inverted micelles. The corresponding Nagg is 78 ( 15, which lays between values found for other inverted micelles such as those of AOT (bis 2-ethylhexylsodium sulfosuccinate) in the absence of water28 (120 ( 20) and p-phenylenediamine boc-bis-glycamide29 (30 ( 10). Hexane creates a higher hydrophobic effect on the headgroup unit, which, despite its large area and rigidity, can pack into these tight inverted micelles (in fact, these inverted micelles are so small that they cannot be seen with AFM). After studying first the type of aggregates formed in toluene, we did not expect such a dramatic change in the aggregation behavior when using hexane as solvent because of geometric restrictions. One can envisage the need for more voluminous hydrophobic groups to ensure that the headgroup can be sufficiently sheltered in the aggregates. Changes in the effective headgroup of metallosurfactants have already been reported to take place in aqueous solutions18 as a result of different chain lengths. If this process of adapting the hydrophobic tails in a micellar interior requires changes in the effective headgroup area, analogous effects can be expected in the case of inverted micelles in order to fit the large headgroups and their corresponding counteranions in the limited volume available in the inverted micellar interior. In addition to architectural features, the influence of the solvent is determinant in the observed morphological changes, in agreement with simulations of block copolymers.30,31 Changing from toluene to hexane increases the solvophobicity of the metal-containing headgroup, forcing the monomers into the micellar pockets, where they can be accommodated because of the appropriated cone-shaped geometry of the surfactant. Acknowledgment. D.D.G. and C.J.E. thank NRSCCatalysis for financial support, project number 2001-10. Supporting Information Available: Detailed experimental procedures for DLS and AFM; synthesis and analytical data of 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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