Film Formation of Nonionic Dendritic Amphiphiles at the Water Surface

Letter pubs.acs.org/Langmuir

Film Formation of Nonionic Dendritic Amphiphiles at the Water Surface Patrick Degen,*,†,§ Monika Wyszogrodzka,*,‡,§ and Christian Strötges† †

Institut für Physikalische Chemie II and ‡Organische Chemie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund, Germany S Supporting Information *

ABSTRACT: This study focuses on the modular synthesis of a new class of nonionic dendritic amphiphiles and their behavior at the water−air interface. Our approach is based on a modular architecture consisting of two different generations of hydrophilic polyol dendrons connected to a two-chain hydrophobic block. Caused by different polarities of polyol and aliphatic groups, the molecules are surface-active and, by analogy to phospholipids, can form well-organized Langmuir monolayers at the water surface. The self-association process and phase behavior of these molecules with two different headgroup sizes were investigated by means of surface pressure and surface potential area isotherms by surface shear rheology and Brewster angle microscopy. With these techniques, we were able to observe marked differences in the phase behavior of the two molecular generations.

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INTRODUCTION Nonionic amphiphiles have spawned a broad range of applications beginning with emulsion stabilization and detergency1 through catalysis2 to cosmetics and pharmaceuticals.3,4 Many of these amphiphilic molecules self-assemble to a range of different structures such as micelles, vesicles, and lamellar phases.5 In natural systems, the defined arrangement of lamellar phases enables the separation of different environments, which is the main reason for the formation of complex life. Cells contain several vesicular systems as part of the transport system or in order to gain higher reaction yields and therefore higher efficiency. Much research is done to imitate these biological structures and use them as efficient drug carriers or reaction chambers. In the 1960s, vesicle preparation from phosphatidylcholines using sonication was introduced as a method to generate model membranes for cell-like structures.6 Since then, other methods such as the use of electric fields7 and spontaneous formation from surfactant mixtures8 have been established. The formation of very stable vesicles, low production costs, and the ease of storage have led to the use of nonionic surfactants as alternatives to phospholipids.9 In addition, their self-organization process and the properties of formed aggregates make this class of molecules interesting as a model system for biological membranes.10 Therefore, a great amount of effort has been expended to design new, welldefined, biocompatible nonionic surfactants. Sugar-based amphiphiles are one important class of nonionic surfactants. For these molecules, interfacial and self-assembling behaviors have been explored in numerous studies.11−13 Related work was done by Cardullo et al., who investigated Langmuir and Langmuir−Blodgett films of sugar-coated fullerenes.14 © 2012 American Chemical Society

However, many commercially available nonionic amphiphiles possess poly(ethylene glycol) (PEG) as a headgroup (e.g., Pluronics, Brij, Triton). Even though PEG itself is very biocompatible,15,16 the production of the PEG-based surfactants requires the handling of highly toxic, flammable monomer ethylene oxide. In contrast to that, glycerol is readily available in bulk quantities as a byproduct from oleochemistry and is thereby becoming an attractive renewable raw material.10,17 Nevertheless, glycerol itself is not suitable for this purpose; therefore, oligomers are needed to increase the hydrophilic component and to adjust the hydrophilic−lipophilic balance (HLB) of the products.18,19 Because very pure products were developed (leading to FDA approval), their use either as a monomer or in the form of derivatives (mainly linear oligoglycerol esters) increased significantly. Thus, it is favorable to design new, well-defined, biocompatible nonionic surfactants based on glycerol and to investigate their self-assembly and selforganization process into nanostructures. In the literature, only a few examples of nonionic surfactants containing well-defined dendritic polyglycerol (PG) as a headgroup and long aliphatic chains as a hydrophobic part are known.20−22 For example, Trappmann et al. reported the micelle formation for this class of molecules containing a single aliphatic chain.21 Although Kressler et al. investigated the behavior of cholesterol-initiated polyglycerol derivatives at the air−water interface,23 to the best of our knowledge we report the first study of well-defined Janus dendrimers (represented in Figure Received: May 22, 2012 Revised: August 9, 2012 Published: August 9, 2012 12438

dx.doi.org/10.1021/la302090k | Langmuir 2012, 28, 12438−12442

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The variation in the time interval (5 min−2 h) between the spreading process and the beginning of the experiments did not influence the data. Therefore, we started the different measurements 15 min after spreading. Vesicle formation was achieved by two different methods. First, we used a phase-transfer technique introduced by Pautot, Frisken, and Weitz25 to produce giant vesicles. A special advantage of this technique is the production of well-defined unilamellar vesicles, which can be filled with huge amounts of hydrophilic or hydrophobic compounds. The second approach was a sonication method according to Barenholz et al.26 Methods. To investigate the 2D phase behavior of these molecules at the water−air interface, we used several analytical methods such as surface pressure, surface potential, and surface rheology measurements during film compression. In addition, the Brewster angle microscopy (BAM) gave use information about the microscopic structure of the Langmuir layers. The size and stability of the vesicles were investigated optically (for the giant vesicles) and by dynamic light scattering (DLS). A detailed description of the methods that we used is provided in the Supporting Information.

1) that provides insight into the interfacial and colloidal properties at the air−water interface. We limited our study to

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RESULTS We observed clear differences in the pressure−area isotherms between both molecules (Figure 2). For generation [G2], the

Figure 1. Structure of amphiphiles with a glycerol dendron as the headgroup.

compounds having small headgroups, namely, generations [G1] and [G2] of glycerol dendrons. Amphiphiles consisting of higher generations of polyglycerol dendrons are water-soluble, and for that reason, they are not included in this study.

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EXPERIMENTAL SECTION

Synthesis. [G1] and [G2], well-defined polyol-polyether dendritic amphiphiles bearing two long aliphatic chains, were synthesized through the facile, easily scalable approach shown in Scheme 1.

Scheme 1. Synthesis of Compounds 1 and 2a

Figure 2. Surface pressure−area (π−A) isotherms for [G1]-(C18)2 and [G2]-(C18)2.

increase in the surface pressure is very smooth and starts at a very large area of approximately 4 nm2. In contrast, the isotherm of generation [G1] rises very fast but not before an area of 1.4 nm2/molecule was reached. Additionally, the slope of the isotherm in the compressed state is much higher for [G1]-(C18)2 than for [G2]-(C18)2, which is the first indication of the denser packing of the molecules in the monolayer. The required space for each molecule in the compressed state of the Langmuir layer is 0.45 nm2 for [G1]-(C18)2 and 1.6 nm2 for [G2]-(C18)2. This corresponds well to the dimensions of the dendritic headgroups, calculated by means of force field methods (MMFF94, Monte Carlo conformer search). A detailed description is presented in the Supporting Information. Additional information about the properties of the interfacial films were obtained by surface potential and surface rheology measurements presented for [G1]-(C18)2 in Figure 3. In contradiction to common oscillating drop or oscillating Langmuir trough techniques, the surface concentration of the Langmuir film stays constant during the torsion pendulum measurements. This allows us to investigate the interactions between soluble substances (e.g., proteins, dyes, and drugs) and the Langmuir layer.

a

(i) NaH, C18H36Br, THF/DMF; (ii) H2, Pd/C; (iii) NaH, C3H3Br, THF; (iv) [G1]-N3 or [G2]-N3, DIPEA, sodium ascorbate, CuSO4 (15 mol %), THF/H2O; (v) Dowex-50W resin, CH2Cl2/MeOH.

An alkyne-functionalized compound 5 obtained in a modular threestep reaction sequence serves as a hydrophobic block. The formation of the final compound 1 or 2 was achieved via a “click” reaction with an azide-functionalized [G1] or [G2] glycerol dendron, followed by the removal of the isopropylidene protecting group according to the previously described procedure by Wyszogrodzka and Haag.24 A detailed synthesis approach and all analytical data are presented in the Supporting Information. Sample Preparation. For Langmuir film preparation, dendritic amphiphiles [G1]-(C18)2 and [G2]-(C18)2 were spread on the pure water surface from a 10−3 M solution in chloroform (purchased from Fluka as p.a. grade). Water was obtained from a pure water system (Seralpur PRO 90 CN). 12439


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Figure 3. Surface rheology (elasticity and viscosity) and surface potential of the monolayer of [G1]-(C18)2 during compression.

The increase in the surface potential during film compression indicates the appearances of effective dipoles perpendicular to the water surface. In the first part of the compression, the surface potential value is typical of organic amphiphilic molecules such as fatty acids and phospholipids.27 Over an area of approximately 1.2 nm2, the surface potential rises a second time. Interestingly, this increase correlates with an increase in the surface elasticity and even more for the surface viscosity. The increase in surface viscosity during film compression and the absolute value agree with data obtained for typical Langmuir monolayers.28 As far as we know, this is the first time that the rheological behavior of the film could correlate directly with the change in molecular orientation in the film. Obviously, some reorientation in the monolayer (e.g., slipping away of the hydrate sheet, interaction of the headgroups, and hydrophobic reorientation) results in an increase in the surface potential. In the same way, this reorientation enables a better interaction of the monolayer molecules, resulting in higher elasticity. However, the surface potential is almost constant during complete compression, and no significant increase in surface elasticity or viscosity could be observed for generation [G2] (Supporting Information). Therefore, we can conclude that for [G2]-(C18)2 no densely ordered packing of the molecules could be reached. This was confirmed by an optical investigation of the interface during compression using BAM (Figure 3). The BAM images indicate the formation of condensed aggregates (bright structures) enclosed with an expanded phase (dark surrounding) for both generations. For [G1]-(C18)2, the first occurrence of aggregates at very low surface pressure (approximately 2 mN/m) corresponds to the plateau in the isotherm (details in Supporting Information). After a small regime (from 2 to 7 mN/m) where the formation of aggregates occurs, the size of the aggregates increases until a closed layer could be observed. In contradiction, for [G2]-(C18)2 the first occurrence of aggregates, at 10 mN/m did not correspond to a plateau regime but this can be explained by the smooth progression of the isotherm. At a surface pressure of between 25 and 35 mN/m, the shape of the aggregates changes from an angular to a snowflake structure and this process correlates with the plateau regime observed in the isotherm (Figure 2). In contrast to [G1]-(C18)2, a closed layer could not be observed for [G2](C18)2, even at the maximum surface pressure. Instead, further compression results in film rupture and the formation of nonspecific aggregates. This behavior indicates, in good agreement with other results, the soft properties of the layer of [G2]-(C18)2.

Figure 4. BAM images of the monolayers of generations [G1] (left) and [G2] (right) during compression.

Because of the results concerning the planar surface, the formation of vesicles is based on these molecules. In the initial experiments, we could prove the formation of giant vesicles prepared by phase-transfer processes (Figure 5).

Figure 5. (a) Pictures from floating giant unilamellar vesicles of [G1](C18)2 in the water phase. (b) Oil lenses (toluene) at the top of vesicles occur after phase transfer. Vesicles are stable for hours and can be destabilized by the addition of Triton X-100.

These vesicles were easily produced, unilamellar, and filled with huge amounts of sucrose and aniline blue. The size of the vesicles was approximately 3 mm, and despite the significant osmotic pressure between the inner and outer phases, they were stable for several hours. The oil lenses (toluene) at the top of the vesicles occur after phase transfer and indicate the ability of the vesicles to incorporate a huge quantity of lipophilic compounds. In addition to the phase-transfer technique, we used a sonication method for vesicle formation.26 The resulting liquid shows pale turbidity and birefringence observed with two 12440


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drug-delivery potential. Additionally, we intend to study the interfacial characterization of dendron−lipid mixtures on the planar water−air surface.

crossed polarizers. This is a strong indication of Lα phases in the solution. Using this procedure, we found vesicles in the nanometer range. Thus, the aggregates are too small for optical microscopy but are measurable by DLS. The results of DLS measurements are shown in Figure 6.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and synthesis and characterization of compounds 1 and 2. Description of the methods mentioned in the text and additional results (rheology curve, isotherm, and vesicle image). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and monika. [email protected]. Author Contributions §

These authors contributed equally.

Notes

Figure 6. Size and PDI of vesicles of G1 produced by swelling (run numbers 1−5). The addition of Triton X-100 leads to the rapid destruction of the vesicles (run numbers 6−10).

The authors declare no competing financial interest.

The prepared vesicles have a diameter of approximately 250 nm and a low polydispersity (PDI). This indicates the formation of uniform vesicles. The lipid bilayer can be destroyed by the surfactant Triton X-100.29 Therefore, the rapid destruction of the aggregates after the addition of Triton X-100 is a strong indication of the formation of vesicles. The destruction of the vesicles is reflected in the increasing size and PDI shown in Figure 6 and also in the right image of Figure 5.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Prof. R. Haag (FU Berlin, Germany) and Prof. H. Rehage (TU Dortmund, Germany) for the opportunity to use their lab-space during synthetic work and characterization. We thank K. Biskup and Ch. Kördel (FU Berlin, Germany) for help with synthesis and W. Münch (FU Berlin, Germany) for his help with HPLC purification.

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CONCLUSIONS We have shown that this new class of “two-armed” glyceroldendron-based amphiphiles is able to form well-defined Langmuir monolayers. In addition, we could prove that the headgroup size and therefore the packing parameter have a strong influence on the interfacial properties at the air−water interface. Molecule [G1]-(C18)2 is able to form a very dense planar layer (BAM images). In such a layer, the molecules are closely packed and the hydrophobic interaction of the aliphatic chains dominates the properties of the layer. This dense packing results in a high surface viscosity and elasticity that is generally also found for phospholipids (e.g., DPPC). Compared to [G1]-(C18)2, the larger headgroups of [G2]-(C18)2 lead to a packing parameter that is not ideal for dense packing on a planar surface. Accordingly, the space between the molecules is higher than for [G1]-(C18)2, and the interaction of the hydrophobic chains is much weaker. Furthermore, the partial solubility of G2-C18 has to be taken into account because this could explain the different behavior in comparison to that of [G1]-(C18)2. However, we did not observe significant hysteresis in the isotherms, which speaks against the partial solubility of [G2]-(C18)2. In initial experiments, we could prove the formation of giant unilamellar vesicles (with diameters of up to 3 mm). These vesicles were easily formed and appeared to be stable for up to hours. In the second fabrication process, we were able to generate much smaller vesicles. The vesicles are stable for up to days and have a narrow size distribution and a mean diameter of approximately 250 nm. Further studies will focus on a detailed investigation of the vesicle properties, stability, and

(1) Schick, M. J. Nonionic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1987. (2) Lipshutz, B. H.; Ghorai, S. Transition-Metal-Catalyzed CrossCouplings Going Green: In Water at Room Temperature. Aldrichim. Acta 2008, 41, 59−72. (3) Uchegbu, I. F.; Florence, A. T. Nonionic Surfactant Vesicles (Niosomes) - Physical and Pharmaceutical Chemistry. Adv. Colloid Interface Sci. 1995, 58, 1−55. (4) Stamatov, S. D.; Stawinski, J. Regioselective and Stereospecific Acylation across Oxirane- and Silyloxy Systems as a Novel Strategy to the Synthesis of Enantiomerically Pure Mono-, Di- and Triglycerides. Org. Biomol. Chem. 2007, 5, 3787−3800. (5) Rosen, B. M.; Percec, V. Single-Electron Transfer and SingleElectron Transfer Degenerative Chain Transfer Living Radical Polymerization. Chem. Rev. 2009, 109, 5069−5119. (6) Saunders, L.; Perrin, J.; Gammack, D. B. Ultrasonic Radiation of Some Phospholipid sols. J. Pharm. Pharmacol. 1962, 14, 567−572. (7) Angelova, M. I.; Saleau, S.; Meleard, P. H.; Faucon, J. F.; Bothorel, P. Preparation of Giant Vesicles by External AC Electric Fields. Kinetics and Applications. Prog. Colloid Polym. Sci. 1992, 89, 127−131. (8) Kaler, W. K.; Murthy, A. K.; Rodriguez, B. E.; Jaradzinski, J. A. N. Spontaneous Vesicle Formation in Aqueous Mixtures of Single-Tailed Surfactants. Science 1989, 245, 1371−1374. (9) Mozafari, M. R. Nanomaterials and Nanosystems for Biomedical Applications; Springer: Dordrecht, The Netherlands, 2007. (10) Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina, C. From Glycerol to Value-Added Products. Angew. Chem., Int. Ed. 2007, 46, 4434−4440. (11) Bhattacharya, S.; Acharya, S. N. G. Vesicle and Tubular Microstructure Formation from Synthetic Sugar-Linked Amphiphiles. Evidence of Vesicle Formation from Single-Chain Amphiphiles Bearing a Disaccharide Headgroup. Langmuir 2000, 16, 87−97.

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(12) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Headgroup and Hydrocarbon Tail Effects on the Surface Tension of Sugar-Based Surfactant Solutions. Langmuir 2004, 20, 10935−10942. (13) Jung, J. H.; Do, Y.; Lee, Y.-A.; Shimizu, T. Self-Assembling Structures of Long-Chain Sugar-Based Amphiphiles Influenced by the Introduction of Double Bonds. Chem.Eur. J. 2005, 11, 5538−5544. (14) Jonas, U.; Cardullo, F.; Belik, P.; Diederich, F.; Giigel, A.; Harth, E.; Herrmann, A.; Isaacs, L.; Müllen, K.; Ringsdorf, H.; Thilgen, C.; Uhlmann, P.; Vasella, A.; Waldraff, C. A. A.; Walter, M. Synthesis of a Fullerene[60] Cryptate and Systematic Langmuir - Blodgett and ThinFilm Investigations of Amphiphilic Fullerene Derivatives. Chem.Eur. J. 1995, 4, 243−251. (15) Harris, J. M.; Chess, R. B. Effect of Pegylation on Pharmaceuticals. Nat. Rev. Drug Discovery 2003, 2, 214−221. (16) Veronese, F. M.; Pasut, G. PEGylation, Successful Approach to Drug Delivery. Drug Discovery Today 2005, 10, 1451−1458. (17) Márquez-Alvarez, C.; Sastre, E.; Pérez-Pariente, J. Solid Catalysts for the Synthesis of Fatty Esters of Glycerol, Polyglycerols and Sorbitol from Renewable Resources. Top. Catal. 2004, 27, 105− 117. (18) Schütz, C.; Dwars, T.; Kragl, U. Investigation on the Biocatalytic Oligomerization of Glycidol. Lett. Org. Chem. 2006, 3, 679−684. (19) Shrestha, L. K.; Dulle, M.; Glatter, O.; Aramaki, K. Structure of Polyglycerol Oleic Acid Ester Nonionic Surfactant Reverse Micelles in Decane: Growth Control by Headgroup Size. Langmuir 2010, 26, 7015−7024. (20) Noiret, N.; Rivaux, Y.; Brochette, P.; Algarra, L.; Pa-tin, H. Polyglyceryl Amines As Surfactants I: Symmetrical Polyglyceryl Amines and Aqueous Solution Properties. J Surfact. Deterg. 1999, 2, 349−355. (21) Trappmann, B.; Ludwig, K.; Radowski, M. R.; Shukla, A.; Mohr, A.; Rehage, H.; Bottcher, C.; Haag, R. A New Family of Nonionic Dendritic Amphiphiles Displaying Unexpected Packing Parameters in Micellar Assemblies. J. Am. Chem. Soc. 2010, 132, 11119−11124. (22) Wyszogrodzka, M.; Haag, R. Synthesis and Characterization of Glycerol Dendrons, Self-Assembled Monolayers on Gold: A Detailed Study of Their Protein Resistance. Biomacromolecules 2009, 10, 1043− 1054. (23) Reuter, S.; Hofmann, A. M.; Busse, K.; Frey, H.; Kressler, J. Langmuir and Langmuir-Blodgett Films of Multifunctional, Amphiphilic Polyethers with Cholesterol Moieties. Langmuir 2011, 27, 1978−1989. (24) Wyszogrodzka, M.; Haag, R. A Convergent Approach to Biocompatible Polyglycerol ″Click″ Dendrons for the Synthesis of Modular Core-Shell Architectures and Their Transport Behavior. Chem.−Eur. J. 2008, 14, 9202. (25) Pautot, S.; Frisken, B. J.; Weitz, D. A. Production of Unilamellar Vesicles Using an Inverted Emulsion. Langmuir 2003, 19, 2870−2879. (26) Barenholz, Y.; Gibbes, D.; Litman, B. J.; Goll, J.; Thompson, T. E.; Carlson, F. D. A Simple Method for the Preparation of Homogenous Phospholipid Vesicles. Biochemistry 1977, 16, 2806− 2810. (27) Oliveira, O. N.; Bonardi, C. The Surface Potential of Langmuir Monolayers Revisited. Langmuir 1997, 13, 5920−5924. (28) Krägel, J.; Li, J. B.; Miller, R.; Bree, M.; Kretzschmar, G.; Mohwald, H. Surface Viscoelasticity of Phospholipid Monolayers at the Air/Water Interface. Colloid Polym. Sci. 1996, 274, 1183−1187. (29) Edwards, K.; Almgren, M.; Bellare, J.; Brown, W. Effects of Triton X-100 on Sonicated Lecithin Vesicles. Langmuir 1989, 5, 473− 478.

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