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Langmuir 2008, 24, 10572-10574
Highly-Ordered Selective Self-Assembly of a Trimeric Cationic Surfactant on a Mica Surface Yanbo Hou, Meiwen Cao, Manli Deng, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ReceiVed June 27, 2008. ReVised Manuscript ReceiVed August 26, 2008 Novel trimeric cationic surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) has been synthesized, and its self-assembly morphology on a mineral surface has been studied. From its micelle solution, highly ordered bilayer patterns are obtained on a mica surface, whereas randomly distributed bilayer patches are formed on a silica substrate. The highly ordered bilayer patterns on mica are first caused by the matching of the special structure of DTAD headgroups with the negative charge sites on mica, which leads to the specific nucleation of DTAD on the mica surface via electrostatic interaction. Furthermore, hydrophobic interaction among the DTAD hydrocarbon chains results in the formation of the bilayer structure, and intermolecular hydrogen-bonding among the DTAD headgroups promotes the directional growth of such bilayer structures.
Defined and stable nanopatterns on solid surfaces over a macroscopic area are meaningful for the fabrication of molecular devices. Surfactants can self-assemble into micellar structures with defined shapes and sizes, which can be a simple, versatile, low-cost approach to fabricating nanopatterns on surfaces.1 Basically, two factors are important in the formation of ordered, stable surface patterns via molecular self-assembly. One is structure matching between surfactant molecular geometry and the crystalline structure of the substrate, which may regulate the oriented growth of self-assembled aggregates.2 The other is intermolecular interactions including hydrophobic interactions, hydrogen bonding, and π-π stacking, which may improve the stabilization and robustness of aggregate structures.3,4 Herein, we report the selective self-assembly of a novel trimeric cationic surfactant on a mica surface through electrostatic adsorption, hydrophobic interaction, and intermolecular hydrogen bonding. This work indicates that the structural features of an oligomeric surfactant can greatly affect its manner of adsorption, which may shed light on the design and development of self-assembly molecules for the fabrication of surface patterns. The molecular structure of trimeric cationic surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) is shown in Figure 1. It consists of a tris(chloroacetyl)diethyltriamine moiety as a spacer, 3 ammonium headgroups, and 3 hydrocarbon chains with 12 carbon atoms. The DTAD molecule has a trimeric starlike structure; moreover, three amide groups in a spacer group can generate a hydrogen-bonding network. The critical micelle concentration (cmc) of DTAD is 0.29 mM as determined by surface tension measurements (Supporting Information). * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. (1) Hameren, R. V.; Schon, P.; Buul, A. M. V.; Hoogboom, J.; Lazarenko, S. V.; Gerritsen, J. W.; Engelkamp, H.; Christianen, P. C. M.; Heus, H. A.; Maan, J. C.; Rasing, T.; Speller, S.; Rowan, A. E.; Elemans, J. A. A. W.; Nolte, R. J. M. Science 2006, 314, 1433–1436. (2) Yong, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703–705. (3) Song, B.; Wei, H.; Wang, Z.; Smet, M.; Dehaen, W.; Zhang, X. AdV. Mater. 2007, 19, 416–420. (4) Song, B.; Wang, Z.; Chen, S.; Fu, Y.; Smet, M.; Dehaen, W.; Zhang, X. Angew. Chem., Int. Ed. 2005, 44, 4731–4735.
Figure 1. Chemical structure of DTAD.
First, atomic force microscopy (AFM) is used to investigate the morphology of DTAD aggregates on a mica surface at concentrations of 0.08, 2, 10, and 20 mM, corresponding to 0.2, 5, 25, and 50 times its cmc, respectively (Figure 2). As seen, the DTAD aggregate morphology shows a concentration-dependent variation. At 0.08 mM (Figure 2A1-3), both continuous and semicontinuous islands are observed. Then, at a concentration of 2 mM (Figure 2B1-3), the aggregates change into ordered parallel stripes, although many circular islands can still be found. Most of the stripes are less than 1 µm in length and about 40 to 80 nm in width. At the higher concentration of 10 mM (Figure 2C1-3), highly stretched and parallel stripes come into existence, which are homogeneous over the whole image. The aggregate width remains unchanged while the length becomes much longer. The longest stripes are over 2 µm. At a much higher concentration of 20 mM (Figure 2D1-3), the magnitude of the aggregates becomes larger in both width and length. Many stripes are more than 100 nm wide appear on the surface, and each aggregate has a very smooth surface. From the section analysis, we can see also that with the increasing DTAD concentration the thickness of the aggregates
10.1021/la802021b CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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Langmuir, Vol. 24, No. 19, 2008 10573
Figure 3. Tapping-mode AFM images of the DTAD aggregate on a silica substrate prepared with 20 mM DTAD for 15 min: (A) height images, (B) 3D image, and (C) section analysis.
Figure 4. XPS core-level N 1s spectrum for DTAD adsorbed on mica.
Figure 2. Series of tapping-mode AFM height images (2.5 µm × 2.5 µm), 3D images (625 nm × 625 nm), and corresponding section analysis on mica substrates prepared with different DTAD concentrations for 15 min: (A1-3) 0.08 mM, (B1-3) 2 mM, (C1-3) 10 mM, and (D1-3) 20 mM.
becomes more and more regular. However, no matter what the surfactant concentration, the thickness of the aggregates maintains a constant value of about 3.1 ( 0.2 nm. The length of one DTAD chain plus its headgroup is about 1.7 nm as derived from the CPK model. The observed thickness of 3.1 nm from the AFM data is about 2 times this value, suggesting a bilayer structure of the adsorbate. Different from that on mica, only randomly distributed bilayer patches with a thickness of 3.4 ( 0.4 nm are observed for DTAD absorbed on silica (Figure 3). The bilayer structure is approved by the layer thickness, 2.69 nm, of the cast film, which is deposited on a silicon wafer and determined by X-ray diffraction (XRD). Although both silica and mica are hydrophilic surfaces, there is an obvious difference between the aggregate morphologies on
mica and silica. Therefore, the mica structure should be one of the key factors in inducing the ordered and selective morphology of DTAD. It is well known that the mica surface has alumino-silicate six-ring sites compensated by K+.2 The ammonium ions of DTAD can exchange K+ via electrostatic interaction. Moreover, the spatial arrangement of the three ammonium headgroups of DTAD matches the alumino-silicate six-ring sites on the crystalline mica surface and thus can nucleate on the surface with selectivity in the orientation of the DTAD adsorption.5 Besides, intermolecular hydrogen bonding between the amide groups of DTAD molecules has been proven to play an important role in promoting the orientation of the surface patterns. As shown in the result of X-ray photoelectron spectroscopy (XPS) (Figure 4), the XPS N 1s signal is distinctive of adsorbed DTAD. The DTAD molecule includes nitrogen atoms in three different functional groups (i.e., a secondary amide bond, a tertiary amide bond, and an ammonium group). Deconvolution of the raw data of DTAD suggests that N 1s signal is composed of three peaks occurring at 402.2, 400.9, and 400.4 eV with a relative intensity ratio of about 3:1:2, consistent with the proportion of three types of nitrogen in DTAD. Normally, the N 1s signal in ammonium groups shows a high binding energy of about 402-403 eV6,7 whereas it appears as a peak at 399.5-399.9 eV for the secondary amide group6 and at 400.9-401.1 eV for the tertiary amide group. (5) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160–168. (6) Rojas, O. J.; Ernstsson, M.; Neuman, R.; Claesson, P. M J. Phys. Chem. B 2000, 104, 10032–10042. (7) Rojas, O. J.; Ernstsson, M.; Neuman, R; Claesson, P. M Langmuir 2002, 18, 1604–1612.
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Figure 5. (A) Possible model of DTAD molecular arrangement on mica as a zigzag shape. Orange hexagonal grids represent mica lattices, blue spheres represent ammonium headgroups, and pink rods represent starlike spacers. (B) Possible bilayer model of DTAD molecules. Light-yellow stripes represent hydrocarbon chains.
However, the amide N 1s shows about 1 eV higher binding energy when forming H bonds with each other.8 For DTAD, the N 1s signals at 402.2 and 400.9 eV are assigned to ammonium and tertiary amide groups, respectively. However, the signal of the secondary amide group (400.4 eV) is about 0.7-0.8 eV higher than that of the reported neutral amide N 1s. This indicates the existence of hydrogen bonds between the secondary amide groups in DTAD aggregates on mica. On the basis of the above discussion, we propose a model of DTAD adsorption on the mica surface (Figure 5). First, the DTAD adsorption should be impelled by electrostatic binding between cationic ammonium headgroups and negative mica lattices. (8) Whelan, C. M.; Cecchet, F.; Baxter, R.; Zerbetto, F; Clarkson, G. J.; Leigh, D. A.; Rudolf, P. J. Phys. Chem. B 2002, 106, 8739–8746.
Letters
Compared with single-chained or gemini surfactants, the DTAD molecule has more positive charges and ought to suffer a stronger inducing effect of the mica surface. From the calculation of the PC model, three parts of the DTAD spacer group are about 0.75, 0.75, and 0.38 nm long from the nitrogen atom of the central tertiary amide to each of the other nitrogen atoms of the headgroups, respectively (Figure 5A). Because the distance between each mica lattice site is 0.5 nm,9 three ammonium headgroups of DTAD are supposed to occupy three mica lattice sites as a triangle shape. This is reasonable with respect to spacial distance matching. Duval et al.10 reported that for gemini surfactants only those with a spacer length longer than six atoms could span two mica sites. Here, DTAD owns at least eight atoms between any two charge groups, basically matching the distance of mica lattice sites. After nucleation, neighboring sites are favored for the following adsorption because of the hydrophobic effect between the side chains of DTAD. The strong hydrophobic interaction between the DTAD molecules results in the compactly packed aggregate. Simultaneously, hydrogen bonding between the secondary amide groups leads DTAD molecules to array as a zigzag shape. With more and more DTAD molecules involved, hydrophobic interaction among DTAD hydrocarbon chains generates the formation of the bilayer structure as well as the lateral expansion of the bilayer patches, and the hydrogen bonding between the molecules mentioned above directs the orientated growth of the DTAD bilayer stripes. In summary, the adsorption of trimeric cationic surfactant DTAD on a mica substrate has been studied, and a highly ordered and oriented bilayer is obtained. Such a unique aggregation on mica is induced by the match of the three charges of DTAD headgroup with the negative charge sites of mica and is assisted by intermolecular hydrogen bonding and hydrophobic interaction. This work sheds light on the modulation of oligomeric surfactant self-assembly on a solid surface by controlling the surfactant structure and selecting the surface structure. Acknowledgment. This work is supported by the National Natural Science Foundation of China and the National Basic Research Program of China (20633010 and 2005cb221300). Supporting Information Available: Synthesis procedures, compound characterization data, surface tension measurement, and AFM, XRD, and XPS experiments with DTAD. This material is available free of charge via the Internet at http://pubs.acs.org. LA802021B (9) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602–7607. (10) Duval, F. P.; Zana, R.; Warr, G. G. Langmuir 2006, 22, 1143–1149.
