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Diversified Pattern Formation in Self-Assembly of Bolaform Amphiphiles Bearing Mesogenic Groups at an Interface Dengli Qiu, Bo Song, Aolei Lin, Chunyu Wang, and Xi Zhang* Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received May 13, 2003. In Final Form: June 19, 2003
Introduction Although Gaudin et al. had proposed the existence of clusters of surfactant molecules at liquid/solid interfaces a long time before,1 Manne and Gaub have provided direct evidence that surfactants form micelle-like structures at the solution/solid interface for the first time using in situ atomic force microscopy (AFM).2 This method has been proven as a very useful technique for studying surfactant adlayer structure at the liquid/solid interface on a nanoscopic scale. Later, Ducker, Warr, and many others studied extensively the surface aggregation of various surfactants and revealed the roles of substrates, concentration, electrolyte type, pH value, alcohol, and temperature in determining the shapes of the surface aggregates.3 By introducing mesogenic groups into bolaform amphiphiles, we have obtained stable surface micelles, because the mesogenic groups and long spacers can enhance the intermolecular interactions among amphiphiles in the assemblies.4 We have shown that a bolaform amphiphile bearing azobenzene and long spacers can form stable nanometer-sized stripes with preferred orientation onto a mica sheet.4a,4b However, bolaform amphiphiles with short spacers have been considered a kind of surfactants that could not form high-ordered structures by themselves.4a,5 In this note, we have attempted to investigate the surface aggregation of a similar bolaform amphiphile but with short spacers. An interesting finding is that such amphiphile has shown diversified geometries at the solution/mica interface, including wormlike, spherical, and oriented rodlike micelles, which can be fine-tuned by change of concentration and addition of electrolyte. * To whom correspondence should be addressed. Fax: +86-4318980729. E-mail:
[email protected]. (1) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 958. (2) (a) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (b) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (3) (a) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (b) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558. (c) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (d) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (e) Liu, J. F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895. (f) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (g) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (h) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283. (4) (a) Gao, S.; Zou, B.; Chi, L. F.; Fuchs, H.; Sun, J. Q.; Zhang, X.; Shen, J. C. Chem. Commun. 2000, 1273. (b) Zou, B.; Wang, L. Y.; Wu, T.; Zhao, X. Y.; Wu, L. X.; Zhang, X.; Gao, S.; Gleiche, M.; Chi, L. F.; Fuchs, H.; Langmuir 2001, 17, 3682. (c) Nguyen, S. T.; Gin, D. L.; Hupp, J. T.; Zhang, X. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11849. (d) Zou, B.; Wang, M. F.; Qiu, D. L.; Zhang, X.; Chi, L. F.; Fuchs, H. Chem. Commun. 2002, 1008. (e) Wang, M. F.; Qiu, D. L.; Zou, B.; Wu, T.; Zhang, X. Chem.sEur. J. 2003, 9, 1876. (5) Yan, Y.; Huang, J. B.; Li, Z. C.; Han, F.; Ma, J. M. Langmuir 2003, 19, 972.
Figure 1. 1H NMR spectra of azo-6 in D2O. The concentration of azo-6 is 1.0 × 10-4 M (A) and 5.0 × 10-4 M (B).
Experimental Section The bolaform amphiphile used in this work is azobenzene4,4′-dicarboxylic acid bis(pyridiniohexyl ester) dibromide, denoted azo-6, as shown in Figure 1, which was kindly provided by H. Ringsdorf.6 In this study, freshly cleaved muscovite mica sheets (purchased from PLANO W. Plannet GmbH, Germany) were used as the substrate. Images were captured in situ and ex situ at the aqueous solution/mica interface using the commercial instrument Nanoscope IIIa AFM Multimode (Digital Instruments, CA) at room temperature. Si3N4 cantilevers (Park Scientific, CA) with nominal spring constants of 0.06 N/m for tapping mode in fluid (f ) 20-35 kHz) and silicon cantilevers (Digital Instruments) for tapping mode in air were used (f ) 200-300 kHz). An E scanner was selected for the Multimode. Phase and height images are presented. Scan rates varied from 1 to 4 Hz, and integral and proportional gains, from 0.2 to 1. To obtain surface micelles on mica, a freshly cleaved mica sheet was immersed into an aqueous solution of the amphiphile at the appropriate concentrations. For in situ measurements, amphiphile solutions of the appropriate concentrations were injected into the liquid cell and allowed to equilibrate for at least 1 h before the imaging. Between the different concentrations, the liquid cell was washed by the new solution 10 times (each time over 30 s). The solution was held within the liquid cell by an O-ring, which was rinsed with Milli-Q water and dried under a nitrogen flow before use. In situ AFM images of surfactant aggregates were obtained with tapping mode in fluid. For ex situ measurements, the substrate was taken out after adsorption for 30 min and dried for approximately 1-2 h in a desiccator (P2O5). The critical micelle concentration (cmc) of azo-6 is 2.5 × 10-4 M, which is deduced from the critical point appearing in the curve of solution conductivity versus concentration. These data are measured at room temperature with a digital conductivity instrument (DDS-11A, Shanghai, P. R. China).
Results and Discussion 1H
NMR has been employed to study the micellization of azo-6 in aqueous solution. Figure 1 shows the 1H NMR spectra of azo-6 at the concentrations of 1.0 × 10-4 M (A) (below the cmc) and 5.0 × 10-4 M (B) (above the cmc). Comparing the 1H NMR spectra of (A) and (B), we can clearly see that the b and e protons superposed together, shown in Figure 1A, can be divided when the concentration is above the cmc, shown in Figure 1B. Moreover, the d and (6) (a) Hessel, V.; Ringsdorf, H.; Laversanne, R.; Nallet, F. Recl. Trav. Chim. Pays-Bas 1993, 112, 339. (b) Festag, R.; Hessel, V.; Lehmann, P.; Ringsdorf, H.; Wendorff, J. H. Recl. Trav. Chim. Pays-Bas 1994, 113, 222.
10.1021/la034814f CCC: $25.00 © 2003 American Chemical Society Published on Web 08/06/2003
Notes
Figure 2. In situ AFM image (600 nm × 600 nm) of azo-6 adsorbed at the solution/mica interface. The concentration of azo-6 is 1.0 × 10-4 M (A), 2.0 × 10-4 M (B), and 1.0 × 10-3 M (C).
e protons are shifted upfield, obviously shown in Figure 1B. These results suggest that azo-6 exhibits different aggregation behavior in solution before and after micellization. When micellization occurs, the different polar environment between the headgroups of azo-6 locating outside and the mesogenic groups locating inside the micelle is responsible for the changes in the 1H NMR spectra. We have investigated the surface aggregation of azo-6 with different concentrations by in situ and ex situ AFM. As shown in Figure 2A, in situ AFM reveals that azo-6 can form wormlike micelles spontaneously at the solution/ mica interface when the concentration of azo-6 is 1.0 ×
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Figure 3. AFM images (600 nm × 600 nm) of azo-6 adsorbed at the solution/mica interface with the addition of sodium salicylate. The concentration of azo-6 is kept at 1.0 × 10-4 M, but the concentration of sodium salicylate is 1.0 × 10-4 M (A, in situ), 5.0 × 10-4 M (B, in situ), and 5.0 × 10-4 M (C, ex situ).
10-4 M, below its bulk cmc. In this figure, the wormlike micelle is about 17 nm in width. Considering the formation of wormlike micelles at 1.0 × 10-4 M, this concentration must be above its surface critical micelle concentration (surface cmc). We have studied the surface aggregation of azo-6 at different concentrations systemically using in situ AFM and estimated that its surface cmc is about 5.0 × 10-5 M. This is understandable if we differentiate the surface from bulk concentrations. As pointed out by Xu7 and Ducker,3b for surfaces having a higher number (7) Xu, Z. H.; Ducker, W.; Israelachvili, J. Langmuir 1996, 12, 2263.
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density of charged sites, as occurs on the mica surface, the adsorbed molecules of higher density are now sufficiently close to each other that hydrophobic chain association can occur even at a low bulk solution concentration of surfactant and can form aggregates. With an increase in concentration, the interfacial surfactant aggregates of azo-6 have shown micelles with short-range order at 2.0 × 10-4 M (approximate to its cmc, as shown in Figure 2B; the width of the micelles is about 14 nm) and spherical micelles at 1.0 × 10-3 M (above its cmc, as shown in Figure 2C; the diameter of the spherical micelle is about 17 nm). This is an interesting phenomenon that differs from the traditional micelle aggregation behavior. Azo-6 only forms loosely packed wormlike surface micelles at lower concentration. With the concentration increasing, the surface micelles seem less meandering and more densely packed than those in Figure 2A. We can see clearly aggregates that are in shortrange order locally at the interface, as shown in Figure 2B. However, for a further increase of the concentration, the basal layer is fully occupied at the solution/mica interface and the bromide counterions could be adsorbed on the basal layer through the electrostatic interaction. This may allow the spherical aggregates of azo-6 to adsorb as the second layer at the upper layer.8 In other words, such higher concentration of anion could induce the amphiphiles to adopt a shape closer to their “free” solution structure to adsorb at the outlayer.4c This indicates that azo-6 forms spherical micelles in the bulk solution, and then the formation of wormlike micelles is probably induced by the mica substrate. To confirm our assumption, we have used different substrates as adsorptive matrixes, such as hydrophilic and hydrophobic silica and polyelectrolyte multilayer modified mica sheets, to study the formation of surface micelles in the same conditions except substrates. In situ AFM observation has confirmed that azo-6 only forms some irregular spherical aggregates on these substrates. However, comparative studies by in situ and ex situ AFM observation have shown that the surface micelle of azo-6 cannot be stable against drying because of the short spacer. Keeping the concentration of azo-6 at 1.0 × 10-4 M, we have studied the effect of sodium salicylate (NaSal) on the formation of surface micelles at the solution/mica interface. It is clearly seen that the wormlike structure in Figure 2A has been induced to be less meandering and more densely packed with addition of NaSal (1.0 × 10-4 M), as shown in Figure 3A. Comparing Figure 3A with Figure 2B, we find that the addition of electrolyte has shown a similar effect with increase of concentration. When the NaSal concentration is increased to 5.0 × 10-4 M, the micellar structures turn to interesting oriented (8) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 224.
Notes
rodlike micelles of about 11 nm in width, as shown in Figure 3B. This phenomenon is rationalized by the possibility that salicylate ions could embed in the micelles of azo-6 to decrease the repulsive interaction among headgroups in favoring the formation of the long-range order. This result is consistent with Warr’s proposition that a strongly binding counterion, such as salicylate ion, could induce a sphere-forming surfactant to exist as cylinder aggregates.9 Such interesting stripes would disappear when the concentration of NaSal is up to about 1.0 × 10-3 M. These results indicate that the addition of electrolyte can induce the formation of surface micelles with diversified geometries. In addition, we have studied whether the rodlike micellar structures in Figure 3B can be stable against the drying process. For this purpose, a mica sheet was immersed into azo-6 solution under the addition of NaSal and adsorbed for about 30 min at the solution/solid interface, the same conditions as in Figure 3B. Then we took the sheet out, put it into a desiccator (P2O5) for drying, and then imaged it through ex situ AFM. The ex situ AFM image (Figure 3C), when compared with the image in Figure 3B, shows quite similar surface structures of oriented stripes, which are separated by about 10 nm in width. This result demonstrates that the surface micelles of azo-6 can only be stable against drying when sodium salicylate is added. We would think that the salicylate ion could act as a bis-counterion, linking the individual micelles and forming a stable structure with long-range order. Conclusion In summary, we have successfully obtained a series of 2-D aggregates with various geometries at an interface through in situ and ex situ AFM. We have previously reported that a bolaform amphiphile bearing azobenzene with long spacers (azo-11) can only form well-defined nanostripes,4b while azo-6 with short spacers forms even diversified aggregates, including wormlike, spherical, and rodlike surface micellar structures in different conditions. This suggests that, besides the influence of the introduction of mesogenic groups, there is a strong spacer effect on self-organization of amphiphiles, leading to the formation of stable molecular assemblies confined to the interface. Acknowledgment. The research was funded by the Major State Basic Research Development Program (Grant No. G2000078102), Key Project of the Ministry of Education, and the Natural Science Foundation of China. The authors thank Dr. Lifeng Chi for helpful discussion. LA034814F (9) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5, 88.
