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Comparison of Aggregation Behaviors between Ionic Liquid-Type Imidazolium Gemini Surfactant [C12-4-C12im]Br2 and Its Monomer [C12mim]Br on Silicon Wafer Mingqi Ao, Guiying Xu,* Jinyu Pang, and Taotao Zhao Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, P.R. China Received March 23, 2009. Revised Manuscript Received June 10, 2009 The aggregation of ionic liquid-type imidazolium gemini surfactant [C12-4-C12im]Br2 on silicon wafer, which is compared with its monomer [C12mim]Br, have been studied. AFM morphology images and contact angle measurements suggest that the aggregations of [C12-4-C12im]Br2 and [C12mim]Br on silicon wafer follow different mechanisms. Below the critical surface aggregation concentrations (CSAC), both surfactant molecules are adsorbed with their hydrophobic tails facing the air. But above the CSAC, [C12-4-C12im]Br2 molecules finally form a bilayer structure with hydrophilic head groups facing the air, whereas [C12mim]Br molecules form a multilayer structure, and with increasing its concentration, the layer numbers increase with the hydrophobic chains and hydrophilic head groups facing the air by turns. Besides, the watery wettability of [C12-4-C12im]Br2-treated silica surface is lower than that of [C12mim]Br at the concentration of 5.0 cmc, and the infrared spectroscopy suggests that the poorer watery wettability of [C12-4-C12im]Br2 may be relative to the less-ordered packing of methylene chains inside the aggregate. These different aggregation behaviors for the two surfactants ascribe to the different molecular structures and electrostatic interactions. This work would have certain theoretical guidance meaning on the modification of solid surface.
1. Introduction The aggregation of surfactants on a solid surface is a topic that has received steady attention in recent years.1-5 It is frequently used for surface property modulation, which is used in many fields, such as detergency, mineral flotation, lubrication, corrosion inhibition, tertiary oil recovery, and so on.6-10 Previously, a number of studies have been carried out to investigate the surfactant aggregation at the solid-aqueous solution interface, and valuable results have been obtained.11-15 Biggs et al.11 have investigated the adsorption kinetics and assembly of cetyltrimethyl ammonium bromide (CTAB) on silica surfaces by using optical reflectometry. They found that at concentrations above the critical micelle concentration (cmc), micelles participate in the adsorption process and the surfactant layers formed reflect the structure of component micelles. Clarence Charnay et al.12 have investigated the adsorption of gemini surfactants containing two *Corresponding author. E-mail:
[email protected]. Tel.: 86-53188365436. Fax: 86-531-88564750.
(1) Zheng, F.; Zhang, X.; Wang, W. Langmuir 2008, 24, 4661. (2) Han, X.; Hu, J.; Liu, H. L.; Hu, Y. Langmuir 2006, 22, 3428. (3) Reimer, U.; Wahab, M.; Schiller, P.; M€ogel, H. J. Langmuir 2005, 21, 1640. (4) Tulpar, A.; Ducker, W. A. J. Phys. Chem. B. 2004, 108, 1667. (5) Song, B.; Liu, G. Q.; Xu, R.; Yin, S. C.; Wang, Z. Q.; Zhang, X. Langmuir 2008, 24, 3734. (6) Weerawardena, A.; Drummond, C. J.; Caruso, F.; Mccormick, M. Langmuir 1998, 14, 575. (7) Drzymala, J.; Mielczarski, E.; Mielczarski, J. A. Colloids Surf. A. 2007, 308, 111. (8) Graca, M.; Bongaerts, J. H. H.; Stokes, J. R.; Granick, S. J. Colloid Interface Sci. 2007, 315, 662. (9) Telegdi, J.; Rigo, T.; Kalman, E. J. Electroanal. Chem. 2005, 582, 191. (10) Jing, G.; Wang, X.; Han, C. Desalination 2008, 220, 386. (11) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374. (12) Sedjerari, A. B.; Derrien, G.; Charnay, C.; Zajac, J.; Memprval, L. C. D.; Lindheimer, M. J. Colloid Interface Sci. 2009, 331, 281. (13) Esumi, K.; Matoba, M. Langmuir 1996, 12, 2130. (14) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (15) Zimin, D.; Craig, V. S. J.; Kunz, W. Langmuir 2004, 20, 8114.
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quaternary ammonium groups bound by an ethylene oxide spacer chain at the silica-aqueous solution interface using 1H NMR. They found that the adsorbed mode of gemini surfactants on silica was largely relative to the spacer length. Kunio Esumi et al.13 have studied the adsorption of a series of quaternary ammonium cationic surfactants by measuring adsorption density, ζ potential, and dispersion stability, and found that the double-chain or triplechain surfactants adsorbed strongly on the silica surface in comparison with their single-chain counterparts. Ionic liquid-type imidazolium surfactants are a new generation of surfactants. Compared with conventional ammonium surfactants, the ionic liquid-type surfactants show several advantages due to the existence of imidazolium head groups. For example, as the cationic micelle systems, they would display significantly stronger tendency toward self-aggregation, and thus they could be used in the preparation of functional materials,16,17 or may be applied to modify various types of chemical reactions.18,19 As the cationic reverse-micelle systems, they show higher capacity for solutes than those of quaternary ammonium cationic surfactants;20,21 the stronger attraction between the imidazolium ring and cosurfactants could facilitate the immobilization of the latter on the (W/O) interface, and thus they can form compact membrane.22 Furthermore, because of the existence of imidazolium head groups, they should have novel properties on solid surface, and may have potential application in many areas, such as the adsorption of these amphiphiles on solid surface producing (16) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Yong, Z. Angew. Chem., Int. Ed. 2004, 43, 4988. (17) Kuang, D.; Brezesinki, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534. (18) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36, 2346. (19) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G. Langmuir 1999, 15, 2631. (20) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (21) Paul, B. K.; Motra, R. J. Colloid Interface Sci. 2005, 288, 261. (22) Leodidis, E. B.; Hatton, A. T. J. Phys. Chem. 1990, 94, 6411.
Published on Web 06/25/2009
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Figure 1. Chemical structures of ionic liquid-type gemini imidazolium surfactant and its monomeric analogue.
two-dimensional supramolecular nanostructures,23-28 which have the potential to be used as lateral nanomaterials or templates of biomimetic or biomineralization processes.29-32 In addition, the imidazolium surfactants have good antimicrobial activities, and the modified materials may be used as antimicrobial materials.33,34 Therefore, the study of aggregation behavior of imidazolium surfactants adsorbed on solid surface has great significance. However, most of the studies of the cationic surfactant aggregation on solid surfaces have been focused on the quaternary ammonium cationic surfactant system,11-13,35-38 the aggregation of ionic liquid-type gemini surfactants on solid surface is so far rare. Previously, we have investigated the surface activity of the ionic liquid-type gemini imidazolium surfactants ([Cn-4-Cnim]Br2, n = 10,12,14) compared with their corresponding monomers ([Cnmim]Br, n =10,12,14) in aqueous solution,39 and found that the surface activity of [Cn-4-Cnim]Br2 is higher than that of [Cnmim]Br. Herein, we report for the aggregation of [C12-4C12im]Br2 and [C12mim]Br on a silica surface. Significantly different aggregation behaviors have been observed. For [C12mim]Br, a multilayer aggregate is formed on silica surface, whereas [C12-4-C12im]Br2 forms only a bilayer structure. These different aggregation behaviors for the two surfactants may ascribe to the different molecular structures and electrostatic interactions. This work would have certain theoretical guidance meaning on the modification of solid surface.
2. Experimental Section 2.1. Materials. The ionic liquid-type gemini imidazolium surfactant [C12-4-C12im]Br2 was synthesized and purified as described previously and the critical micelle concentration (cmc) is 0.72 mmol/L.39 Its corresponding monomer [C12mim]Br was synthesized according to ref.40 and the cmc is 9.8 mmol/L which is in accordance with that reported in ref 41. Their structures are (23) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. 1988, 27, 113. (24) Ahlers, M.; Mueller, W.; Reichert, A.; Ringsdorf, H.; Venzmer, J. Angew. Chem., Int. Ed. 1990, 29, 1269. (25) Kunitake, T. Angew. Chem., Int. Ed. 1992, 31, 709. (26) Oshovsky, G. V.; Reinhoudt, D. N.; Verboom, W. Angew. Chem., Int. Ed. 2007, 46, 2366. (27) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthauser, J.; Vanos, N. M.; Zana, R. Science 1994, 266, 254. (28) Mao, G. Z.; Tsao, Y. H.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461. (29) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (30) Luk, Y. Y.; Abbott, N. L. Curr. Opin. Colloid Interface Sci. 2002, 7, 267. (31) Berti, D. Curr. Opin. Colloid Interface Sci. 2006, 11, 74. (32) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (33) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1907. (34) Jiang, X.; Zhou, L. Chin. J. Org. Chem. 2004, 24, 563. (35) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (36) Cao, M. W.; Song, X. Y.; Wang, J. B.; Wang, Y. L. J. Colloid Interface Sci. 2006, 300, 519. (37) Schulz, J. C.; Warr, G. G. Langmuir 2000, 16, 2995. (38) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1998, 199, 169. (39) Ao, M. Q.; Xu, G. Y.; Zhu, Y. Y.; Bai, Y. J. Colloid Interface Sci. 2008, 326, 490. (40) Baltazar, Q. Q.; Chandawalla, J.; Anderson, J. L. Colloids Surf., A 2007, 302, 150. (41) Vanyur, R.; Biczok, L.; Miskolczy, Z. Colloids Surf., A 2007, 299, 256.
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Ao et al. illustrated in Figure 1. Pure water (18 MΩ cm-1) was obtained from the Milli-Q system and used in all experiments. The solid substrates used were one-side polished silicon wafers (111) purchased from Department of Microelectronics, Peking University, China. Before use, the silicon wafers were ultrasonicated in ethanol and pure water for 5 min, respectively. After rinsed with water, the substrates were treated in a freshly prepared mixture of H2SO4 (98%) and H2O2 (30%) (v/v = 7/3) at 130 °C for 45 min followed by rinsed thoroughly with water and etched in a 30% hydrofluoric acid for 10 s. Then, the substrates were rinsed with water again and dried with a nitrogen stream. Finally, the substrates were baked at 1000 °C for 120 min. After such treatments, a thin oxide layer (∼100 nm) formed on the silica surfaces.42
2.2. Methods.
2.2.1. AFM Morphology Measurement.
AFM images were obtained using a SPA-400 AFM (Seiko Instrument, Japan) operating in the tapping mode with silicon nitride cantilever tips. For the morphology measurements, the substrates were immersed in the [C12-4-C12im]Br2 or [C12mim]Br solutions and equilibrated for 6 h. They were then taken out and dried under a nitrogen flow. More than three regions per sample were imaged.5,36 2.2.2. Contact Angle Measurement. An optical contact angle meter (Tracker, Teclis-IT Concept, France) was used to determine the static contact angle with the drop sessile down mode. Two microliters of water was dropped on the silica surfaces and the contact angle was then calculated by tracker software. In the experiments, at least three silicon wafers were used to repeat the contact angle measurements for each concentration, and at least three different areas of each silicon wafer were used to measure the contact angles. The reported values are averages of these measurements. 2.2.3. Infrared Spectroscopy (IR). Transmission infrared spectra were obtained using a VECTOR22 FT-IR spectrophotometer in transmission mode, after the silica substrates were left to equilibrate in [C12-4-C12im]Br2 or [C12mim]Br solutions for 6 h and dried with nitrogen flow. The spectral resolution was 4 cm-1, and 1000 scans were performed.
3. Results and Discussion 3.1. Aggregation Morphology of [C12-4-C12im]Br2 and [C12mim]Br Surfactants on Silicon Wafers. The AFM height images of the silica substrates treated with different concentration solutions of [C12-4-C12im]Br2 are shown in Figure 2A-F, respectively. It is observed that the morphology changes significantly with increasing the concentration. At the concentration of 0.5 cmc (Figure 2A), few aggregates are observed on the silica surface and the surface exhibits subnanometer roughness. At the concentration of 0.75 cmc (Figure 2B), many small circular islands appear on the surface. When the concentration increases to 1.0 cmc (Figure 2C), the bigger and irregular islands are observed. When the concentration is further increased to 2.0 cmc (Figure 2D), a lot of semicontinuous islands are observed. Then, these islands increase in size, at the concentration of 3.0 cmc (Figure 2E), the whole silica surface is nearly covered by the surfactant aggregates. Finally, at the concentration of 5.0 cmc (Figure 2F), a film with small holes is formed on silica surface. To make an intensive study, cross-sectional analysis of the image is performed. Figure 2G is the height profile along the line in Figure 2E, which shows the height of the aggregates is about 3.0 nm. No matter what kind of aggregates (Figure 2B-F) are in existence, the island heights all keep at this value. Geometry optimization is performed using Density-functional method by employing the generalized gradient approximation (GGA) with (42) Ding, L.; Zhou, W. W.; Chu, H. B.; Jin, Z.; Zhang, Y.; Li, Y. Chem. Mater. 2006, 18, 4109.
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Figure 2. AFM height images (10 10 μm) of silica substrates treated with different [C12-4-C12im]Br2 concentrations: (A) 0.50, (B) 0.75, (C) 1.0, (D) 2.0, (E) 3.0, and (F) 5.0 cmc. (G) Corresponding section analyses for E.
the PW91 functional. Double numerical plus polarization (DNP) basis set was chosen for computing the length of [C12-4-C12im]Br2 and [C12mim]Br. The result indicates that for the monomer counterpart of [C12-4-C12im]Br2, namely, [C12mim]Br, the fully extended hydrophobic chain plus the headgroup molecule is about 1.8 nm. The island height values of [C12-4-C12im]Br2 are higher than 1.8 nm but lower than 3.6 nm, indicating that the surfactant aggregates may form the bilayer structure, and that either the hydrocarbon chain of the bilayer structure may have a tilt angle with respect to the surface or that the chain is significantly disordered. Different from [C12-4-C12im]Br2, its corresponding monomer [C12mim]Br shows another morphology on silica surface with increasing its concentration. The AFM height images of silica substrates treated with different concentration solutions are shown in Figure 3A-E, respectively. As can be seen, for the concentration lower than 1.0 cmc (images A and B in Figure 3), no or little aggregates are observed on the silica surfaces. At the concentration of 2.0 cmc (Figure 3C), many semicontinuous Langmuir 2009, 25(17), 9721–9727
islands appear on the surface and the height is about 3.4 nm (Figure 3F), which indicates the formation of the bilayer structure. With increasing the concentration to 3.0 cmc (Figure 3D) and then to 5.0 cmc (Figure 3E), the islands merge with their neighbors. The height of islands increases from about 3.4 nm (Figure 3F) to 12.4 nm (Figure 3G), and then to 76.2 nm (Figure 3H), which indicates the formation of multilayer structure on the silica surface and that the numbers of layers increases with the increase in surfactant concentration. The surface coverage on the silica surface is estimated by using the height histogram of each image for both [C12-4-C12im]Br2 and [C12mim]Br. The results are revealed in Figure 4. It may be seen that the surface coverage of [C12-4-C12im]Br2 exhibits a linear dependence on its concentration, and finally, it reaches about 81% at 5.0 cmc. Different from [C12-4-C12im]Br2, the surface coverage of [C12mim]Br rises rapidly from 0 to about 46% in the range from 1.0 to 2.0 cmc, and then it remains nearly constant, although the concentration increases. This may be attributed to the formation of multilayer structure and will be explained below. DOI: 10.1021/la901005v
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Figure 3. AFM height images (10 10 μm) of silica substrates treated with different [C12mim]Br concentrations: (A) 0.75, (B) 1.0, (C) 2.0, (D) 3.0, and (E) 5.0 cmc. For images C-E, their corresponding section analyses are provided, which are F-H, respectively.
Figure 4. The surfactant coverage on the silica surface as a function of [C12-4-C12im]Br2 and [C12mim]Br concentrations, respectively.
The AFM morphology images and surface coverage of [C12-4C12im]Br2 suggest that the surfactant adsorption follows a nucleation and growth mechanism continued by coalescence of the 9724 DOI: 10.1021/la901005v
layered islands. The molecules cannot form the aggregates and the surface coverage is low at low surfactant concentrations. Upon the concentration increasing, the head groups of surfactant ions adsorb on the silica surface because of electrostatic interactions. When the charged sites at the surface are almost occupied by surfactants ions, the previously adsorbed molecules act as nuclei for the formation of small circular aggregates, which are bilayer by interactions between the hydrophobic chains. With the further increase in the concentration, the aggregates occupy more surface area and lead to the increase in surface coverage, and these bilayer aggregates would then start to merge with each other to form semicontinuous islands, inducing the surface coverage to increase further. Finally, a highly continuous flat bilayer forms at a high surfactant concentration, where the surface coverage reaches its highest value. But for [C12mim]Br, the growth mechanism is different. After the formation of nuclei and the bilayer structure, the additional surfactants, instead of increasing the surface coverage, continue to accumulate on the surface layer by layer because of the electrostatic attraction with counterions Br- adsorbed on Langmuir 2009, 25(17), 9721–9727
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the aggregate surface and hydrophobic interactions with hydrophobic chains, namely, counterions Br-, play the role of a bridge. Eventually, a multilayer structure is formed and the surface coverage almost remains at about 46%. For [C12-4-C12im]Br2, it forms a continuous flat bilayer rather than the multilayer. The probable causes are as follows. First, the spacer between the hydrophilic head groups limits the freedom of the monomer parts. For them, it is energetic unfavorable to form the multilayer film. Second, according to Blomberg’s research result proved by surface force measurements, when gemini surfactants with the short spacer adsorbed on solid surface, only one counterion is coadsorbed with the dimeric surfactant, whereas no counterion coadsorption occurs for that with longer spacer at low surfactant concentration.43 Besides, the electrical conductivity experiments show that the counterion (Br-) binding degree of [C12-4-C12im]Br2 is 0.70,39 which is smaller than that of [C12mim]Br (0.76).44 These both reflect the weaker degree of counterion (Br-) of [C12-4-C12im]Br2 binding on the aggregate surface, leading to the lower surface negative charge density for the additional [C12-4-C12im] cations adsorbed on the surface. To probe the difference between [C12-4-C12im]Br2 and [C12mim]Br further, the ab initio calculation was employed. Geometry optimization was performed on the ionic liquids without imposing any symmetry constraints using density functional theroy method (DFT). DFT was employed with the three-parameter hybrid exchange functional of Becke and the Lee, Yang, and Parr correlation functional (BLYP)45,46 elements modeled using the double-Zeta plus polarization basis sets. Mullikan population analysis was carried out on the optimized geometries to derive the charges of the ions. The result indicates that for [C124-C12im]Br2, the [C12-4-C12im] cation and one bromide anion have the electric charges of 1.476 and -0.738, respectively. They have more charges than those of [C12mim] cation (0.701) and bromide anion (-0.701) in [C12mim]Br, respectively. This induces the larger electrostatic interactions between the ions in [C12-4C12im]Br2. The assembly behavior of the ionic liquid molecules may be dependent on the delicate balance between the electrostatic attractive interaction and the electrostatic repulsive interaction. The overall electrostatic interaction is determined by three terms: (i) the repulsion among the cations, (ii) the repulsion among the anions, and (iii) the attraction between the cations and the anions. An increase in the net charge on the cation and anion induces a rapid energy increase when forming the multilayer, because of the greater repulsive interaction contribution than the attractive interaction. Thus, it is harder for [C12-4-C12im]Br2 to form the multilayer structure. 3.2. Contact Angles. Normally, the hydrophobic and hydrophilic property of solid surface can be measured by the contact angles.36 The static contact angles of the silica surface treated with [C12-4-C12im]Br2 and [C12mim]Br solutions at different concentrations are shown in Figure 5. As can be seen, the contact angles depend on the surfactant concentrations. For [C12-4-C12im]Br2, the variation of contact angles against [C12-4-C12im]Br2 concentration exhibits two segments: an upward segment shifts from ∼51° at 0.05 cmc to ∼64° at 0.5 cmc and then a downward segment shifts from the maximum to ∼41° at 5.0 cmc. For [C12mim]Br, the variation in contact angles against [C12mim]Br concentration has the similar tendency when its concentration is lower than 2.0 cmc. The contact angles also increase first and pass (43) Blomberg, E.; Verrall, R.; Claesson, P. M. Langmuir 2008, 24, 1133. (44) Inoue, T.; Ebina, H.; Dong, B.; Zheng, L. Q. J. Colloid Interface Sci. 2007, 314, 236. (45) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (46) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1993, 37, 785.
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Figure 5. Static contact angles of silica substrates treated with [C12-4-C12im]Br2 and [C12mim]Br solutions at different concentrations.
through a maximum value of ∼62° at 1.0 cmc, and then decrease to ∼43° at 2.0 cmc. But at 3.0 cmc, the contact angle increases again to ∼52° and decreases to ∼37° at 5.0 cmc. Obviously, even at the quite low concentrations of [C12-4-C12im]Br2 (0.05 cmc) or [C12mim]Br (0.5 cmc), the initial contact angles, which are ∼51 and ∼52° for [C12-4-C12im]Br2 and [C12mim]Br, respectively, are larger than that of the bare silica surface (
