Article pubs.acs.org/JPCC
Aggregation and Stabilization of Carboxylic Acid Functionalized Halloysite Nanotubes (HNT-COOH) Yongho Joo,† Yangjun Jeon,† Sang Uck Lee,‡ Jae Hyun Sim,¶ Jungju Ryu,† Sungyoung Lee,† Hoik Lee,† and Daewon Sohn*,† †
Department of Chemistry, Hanyang University, Seoul 133-791, Korea Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea ¶ Department of Chemistry, Virginia Tech., Blacksburg, Virginia 24061, United States ‡
S Supporting Information *
ABSTRACT: We modified the functional groups of holloysite nanotubes (HNT) from hydroxyl groups (HNT-OH) to carboxylic acids (HNT-COOH). Aggregation and dispersion properties of HNT-COOH under dry conditions were probed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, the degree of aggregation and dispersion of HNT-COOH in acidic, basic, and neutral solutions were measured by multiple angle polarized dynamic light scattering (MADLS). HNT-COOH formed aggregates in neutral solution; however, the material was dispersed in basic and acidic solutions. This occurrence is due to hydrogen bonds (HB) between the carboxyl groups of HNT-COOH in neutral solution, which decrease in acidic and basic solution due to charge dispersion.
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internal surface of HNT.8 Alteration of the HNT surface with various substances could be quite useful for various materials science applications. HNT-COOH can be easily collected and separated by adjusting the solution pH without the need for additional filtration, sonication, or centrifugation. By changing the solution pH, HNT-COOHs were easily dispersed, and the aggregation process was reversible. It is very important for halloysite to disperse without the use of mechanical methods such as ultrasonication, sonication, and centrifugation because such processes cause considerable fragmentation of HNT. The integrity of imogolite threads, long clay particles with HNT, were observed to greatly depend on exposure to ultrasonification. The size of HNT is not uniform in the solution state after ultrasonification for long periods of time.9 Here, the aggregation and dispersion behavior of modified halloysite in different pH solutions was studied in an attempt to identify stable conditions for halloysite solutions.
INTRODUCTION The nanotube structure of halloysite has led to increased interest in its applications as nanoreactors or nanotemplates,1 catalysts,2 and capillaries.3 Blending of halloysite with polymers or other additives can generate polymer/holloysite nanotube (HNT) nanocomposites and cement/HNT with increased mechanical strength and other properties.4 Furthermore, halloysite is a biocompatible material that has been used in drug delivery,5 cosmetics,6 and scaffold tissue engineering.6 In particular, HNT is very useful because it is obtained from natural, biocompatible sources with eco-friendly properties.7 The structure of HNT is shown in Figure 1. Halloysite consists of aluminosilicate nanotubes, similar to the structure of kaolinite, and has a molecular formula of Al2Si2O5(OH)4·nH2O. HNT are composed of a multiple layer structure. The space between the inner and outer layer of aluminum hydroxide is 10 Å in the hydrated form and 7 Å in the anhydrous form.8 Furthermore, halloysite has exclusive properties due to a small number of hydroxyl groups on its surface. The majority of the external morphology of HNT consists of siloxane groups (Si−O−Si); whereas, there are many functional groups (hydroxyl group, Al−OH) lining the © 2012 American Chemical Society
Received: April 23, 2012 Revised: July 11, 2012 Published: July 16, 2012 18230
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Figure 1. SEM and AFM images of HNT-COOH: (a) SEM image, (b) the length and diameter of HNT-COOH, and (c) schematic illustration of the morphology of HNT-COOH.
Scheme 1. Modification of Pure HNT to Carboxylic Acid Functionalized HNT-COOH
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AFM, SEM, and FTIR. An atomic force microscope (AFM) purchased from Park Science Corporation, was used to observe HNT-COOH on silicon wafers using a Si3N4 tip. The AFM images were obtained in noncontact mode, and the source of images was plotted in topographic mode. The scan rate and Zservogain were 0.7 Hz and 6 units, respectively. The range of the AFM experiment was from 1 to 5 μm. All samples were analyzed at room temperature. The self-assembled samples were prepared by dropping HNT-COOH solution on to precleaned silicon wafers and drying in a vacuum oven at a pressure of 1.1 Pa and 40 °C. HNT-COOH nanocomposites were examined using a CBSC-4000 M (SNE-4000 M) utilizing the samples used for AFM imaging. The surface of the silicon wafers including HNT-COOH was coated with gold, and scanning electron microscopy (SEM) was performed at a supply voltage of 15 kV. Fourier transform infrared (FTIR) spectra were obtained with a Bio-Rad FRS-6000 temperaturecontrolled accessory using KBr pellets mixed with HNTCOOH or pure HNT nanocomposite powder. FTIR scans were carried out from 0 to 4000 cm−1. Sample Preparation. HNT modified with amino groups (HNT-NH2) were synthesized using pure HNT via the following method. HNT were dispersed in 100 mL of toluene by sonication for 2 h and stirring (400 rpm) for 2 h. Next, 3 mL of triethylamine and APTES solution were added to the HNT solution, and the mixture was stirred for 1 day at 80 °C under a nitrogen atmosphere. The HNT-NH2 were washed several times with DI water and ethanol, then collected, and dried at 50 °C under vacuum at 1.1 Pa. HNT-COOH were synthesized from the HNT-NH2. A 2 mL portion of 0.1 M succinic anhydride and dry HNT-NH2 were mixed in DMF, and the mixture was stirred for 1 day. The collected HNT-COOH were
EXPERIMENTAL SECTION Materials. Sodium hydroxide (NaOH, >95%), hydrochloric acid (HCl, >90% solution), triethylamine, and 3-aminopropyltriethoxysilane (APTES) were purchased from the Sigma-Aldrich Company. Associated halloysites were provided by LG Chem, Ltd. The synthesis of carboxylic acid functionalized halloysite is detailed in the sample preparation section and has also been reported elsewhere.10 Reaction solutions were in toluene (Daejung Chemical & Metal Co. Ltd., Seoul, Korea) and DMF (Duksan Chemical Industry Co., Ltd., Seoul, Korea). A Milli-Q (Millipore, USA) system was utilized to prepare deionized water (DI water, resistivity of >18.2 Ω-cm).
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INSTRUMENTS DLS. Dynamic light scattering (DLS) experiments were performed with a UNIPHASE He−Ne laser operating at 632.8 nm. The electric energy of the laser was 30 mW, which was the maximum operating power. The detector equipment employed optical fibers coupled to an ALV/SO-SIPD/DUAL detection unit, which employed an EMI PM-28B power supply and ALV/ PM-PD preamplifier/discriminator. The correlation apparatus was an ALV-5000/E/WIN multiple tau correlator with 288 exponentially spaced channels. The laser beam passed through a cylinder scattering cell that included the sample located in a bath of index matching solvent (decaline) before exiting a 400 nm pinhole to reach the detector. One Glan-Thompson polarizer and two focal lenses were used for experiments. One of the focal lens and the polarizer were situated ahead of the laser, and the other focal lens was placed in front of the pinhole. Each correlation function was gathered at six different angles from θ = 30° to 105°. 18231
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cm−1, disappeared, and a vibration peak for carboxylic acids and secondary amides was observed from 3400 to 3600 cm−1. This peak was fairly weak and broad because of the small number of functional groups and hydrogen bonding interactions. Carbonyl vibrations appeared in the IR of HNT-COOH at 1580 and 1700 cm−1 for the amide and carboxylic acid groups, respectively. A peak at 1105 cm−1 indicates that a small distortion of the structure occurred after modification.14 Thus, modification of functional groups from hydroxyls to carboxylic acids was verified by the IR spectra.8 Figure 3 shows SEM images and schematic illustrations of HNT-COOH according to the pH of the solution. The morphology of HNT-COOH obtained from basic, acidic, and neutral solutions was observed by SEM and AFM. For the sample obtained from deionized water solution (pH 7), HNTCOOH were highly aggregated due to interactions of the carboxylic acid and amide groups of the halloysite tubes as seen in Figure 3b. In contrast, dry samples from acidic and basic solution were more dispersed than from neutral solution on the silicon wafers as seen in Figure 3a and c, which can be rationalized as resulting from repulsive forces. Dispersion was much greater in basic solution than in neutral solution because the carboxylic acids at high pH became carboxylate anions, which were repulsive and destabilizing to aggregation.11 Furthermore, the degree of dispersion was higher for dry HNT-COOH obtained from basic solution than from acidic solution, although an explanation of this observation was not readily available from images of the dry materials. There are many disadvantages of SEM. First, the images obtained from SEM and AFM do not naturally show the in situ morphology and state of aggregation of nanotubes in solution. The tendency of solution aggregation would only be observable using a direct imaging technique. Second, interactions between the silicon wafer and HNT-COOH can affect the sample disposition. To overcome these problems, multiple angle polarized dynamic light scattering (MA-DLS) experiments have been utilized for samples in the solution state.12 Dynamic light scattering is known as photon correlation spectroscopy. This technique is used to measure the size of
dried in a vacuum oven at 50 °C and 3.6 mPa.10 Scheme 1 outlines the synthesis of HNT-COOH. Acidic, basic, and neutral solutions (0.025 mg/mL) of HNTCOOH were prepared. Sodium hydroxide solution (pH 12) was used for the basic solution, and hydrochloric acid (pH 1) was used for the acidic solution. After HNT-COOH (0.5 mg) were dissolved in deionized water (1 mL), the solution was directly placed in the whistle light scattering cell (13 × 100 mm) using a 0.80 μm cellulose acetate membrane filter. A sonicator and ultrasonicator were used to disperse HNTCOOH in each solution for 2 h and 5 s, and each solution was kept at room temperature for 1 day.
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RESULTS AND DISCUSSION Figure 2 shows IR spectra of pure HNT and HNT-COOH. Peaks at 3703 and 3622 cm−1 were observed for hydroxyl
Figure 2. FTIR spectra of (a) pure HNT and (b) HNT-COOH at 298 K.
groups that exist on the internal surface of HNT. Hydroxyl groups on the surface of halloysite nanotubes (HNT) were modified to carboxylic acids (HNT-COOH). After substitution of the hydroxyl groups of HNT with amide and carboxylic acid groups, the broad surface alcohol IR peak, from 3200 to 3500
Figure 3. SEM images and a schematic illustration of HNT-COOH in dry form from (a) acidic (pH 1), (b) neutral (pH 7), and (c) basic solution (pH 12). 18232
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Figure 4. Correlation function plots of HNT-COOH (a) in acidic, (b) basic, and (c) neutral solution. The inset is the probability distribution graph according to decay time. (d) Γ = Dtq2 plot of HNT-COOH in acidic, basic, and neutral solution from polarization measurements. VV corresponds to the vertical−vertical polarized mode and A, N, and B correspond to acidic, neutral, and basic solutions, respectively. Numbers denote the angle of detection.
particles in the solution state. A monochromatic laser beam scattered from a specimen generates a time-dependent fluctuation because light that hits moving spherical particles in Brownian motion is Doppler shifted. The autocorrelation function, g(2)(τ), could be calculated from the intensity of light using the following eq 1: g(2)(τ ) =
I(0) ·I(t ) lim
t →∞
1 2T
∫T
Dt =
−T
I(t ′) ·I(t + t ′) dt ′
and (2)
Where A0 is the background signal, A is the instrument constant, Γ is the decay rate, τ is the decay time, and I(0) is the incident light intensity. The decay rate parameter can be obtained from the autocorrelation function and the scattered light intensity. In addition, the diffusion coefficient, Dt, can be calculated from the decay rate using the equation Γ = Dtq2 and eq 3:
q=
⎛ 4πn ⎞ ⎛ θ ⎞ ⎜ ⎟sin⎜ ⎟ ⎝ λ ⎠ ⎝2⎠
(4)
where q is the vector magnitude of scattering, λ is the incident light wavelength, n is the solution refractive index, θ is the scattering angle, kb is the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. Finally, using the Stokes−Einstein equation, eq 3, the hydrodynamic radius, Rh, of particles can be calculated from the diffusion coefficient, Dt, in dilute solution. Figure 4 shows the experimental results of correlation functions with vertical−vertical (incident beam is vertically polarized and the scattered beam is also vertically polarized) geometry; the inset is the probability distribution function of 0.5 mg·mL−1 HNT-COOH measured at scattering angles ⊖ = 30, 45, 60, 75, 90, and 105° in acidic (A), basic (B), and neutral (N) solution. From Figure 4a, according to the probability distribution graphs of HNT-COOH in basic solution (pH 12), the degree of distribution at each angle was well maintained, which indicates that HNT-COOHs were monodispersed at pH 12. From Figure 4b and c, although the degree of distribution of HNT-COOH increased in acidic and neutral solution, the degree of broadening can be ignored as the MA-DLS experiment progressed because the hydrodynamic radius was calculated from the mean decay time. Furthermore, aggregated
(1)
g(2)(τ ) = A 0 + A e−2Γτ
k bT 6πηR h
(3)
and 18233
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Table 1. Summary of Translational Diffusion Coefficient (Dt) and Hydrodynamic Radius (Rh) Values Obtained by MA-DLS Measurements for HNT-COOH in Acidic, Neutral, and Basic Solution Dt Rh
acidic solution (pH 1)
neutral solution (pH 7)
basic solution (pH 12)
576.16 nm2/ms 423.75 nm
155.30 nm2/ms 1572.11 nm
1620.89 nm2/ms 150.54 nm
HNT-COOH particles were sufficiently monodispersed in solution because the degree of distribution was suitable to process the DLS experiment, and there was not an error in calculating the inference result of the hydrodynamic radius using the distribution data. From Figure 4d, the decay rate was calculated from the equation Γ = 1/τ; τ, the mean value of decay time, was calculated by the equation according to scattering angle. From these two parameters, Figure 4 shows the translation diffusion coefficient (Dt), the slope of the linear fitting of each dot (Γ = Dtq2) shown in Figure 4d. The hydrodynamic radius (Rh) was derived from the Stokes− Einstein equation, eq 4. Table 1 shows the Dt and Rh values of HNT-COOH in the three solutions. The Dt values were 1620.9 nm2/ms at pH 12, 576.2 nm2/ms at pH 1, and 155.3 nm2/ms at pH 7. Furthermore, the degree of aggregation/dispersion in solution according to pH was confirmed from the Rh values. Rh values were 150 nm at pH 12, 423 nm at pH 1, and 1572 nm at pH 7. These results correlated to the SEM images, which were generated under dry conditions. Figure 5 schematically shows the interaction types for HNTCOOH aggregation. The flexible side chains can generate reliable conformation for hydrogen bond (HB) networks through interactions among amide and carboxyl groups of side chain and HNT surface oxygen. There are three side chain related HB interactions as show in Figure 5a. If HNT-COOH are closed each other, side chains become interacting through HB-1 (chain−chain and chain−surface interaction) and HB-2 (chain−surface interaction) to HB-3 (chain−chain interaction). Such three types of HB interactions can be maximized at neutral conditions, because acidic and basic conditions include undesirable effects of extra protons at deprotonation disturbing effective HB interactions (RF 1). The reason that HNTCOOH displayed greater aggregation in acidic solution than in basic solution can be explained as follows. In the case of acidic conditions, extra protons surround the oxygen of carboxyl and amide groups of the side chain and HNT surface oxygen which reduces HB interactions. However, extra protons can mediate interaction between HNT-COOH via the fourth HB interaction, HB-4 in Figure 5b. Protons in solution tend to form positively charged oxonium complexes with oxygen atoms connected to two Si atoms on the surface of HNT-COOH. The proton-oxygen bond length was found to be 1.01 Å, which is in good agreement with previous theoretical and experimental studies.13 Two nanotube molecules connect with each other through oxonium ions, which are the most stable sites for the protons. This theory is in agreement with the work of Zhang et al.13 Although the bond between the proton and an oxygen atom is not very strong, such proton bonding with the oxygen of Si−O−Si on the external surface of HNT can act as a bridge connecting the HNT. This interaction is possible because most of the HNT surface structure consists of Si−O−Si morphology,8 which is similar to the structure of silica particles or silicon oxide materials,13 and the whole process can occur without having to overcome any energy barrier. Thus, dispersion is reduced due to interactions of protons in acidic solution via HB-4. In contrast, basic conditions cause
Figure 5. Computer simulation of interactive and repulsive properties of HNT-COOH in (a) neutral, (b) acidic, and (c) basic solution. Colored spheres are white for hydrogen, red for oxygen, purple for aluminum, yellow for silicon, blue for nitrogen, and black for carbon atoms.
deprotonation of the carboxyl group. Therefore, the negatively charged side chains electrostatically repulse each other (RF1) and HNT surface oxygen (RF2) as shown in Figure 5c. The HB-4 interaction also cannot be expected due to lack of protons. Consequently, it is worth mentioning that the 18234
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(6) (a) Price, R. R.; Gaber, B. P.; Lvov, Y. M. J. Microencapsulation 2001, 18, 713. (b) Veerabadran, N.; Price, R.; Lvov, Y. M. Nano J. 2007, 2, 215. (c) Chang, C.-W.; Spreeuwel, A.; Zhang, C.; Varghese, S. Soft Matter 2005, 6, 5157. (7) Hassan-Nejad, M.; Ganster, J.; Bohn, A.; Pinnow, M.; Volkert, B. Macromol. Symp. 2009, 280, 123. (8) (a) Cavallaro, G.; Donato, D.; Lazzara, G.; Milioto, S. J. Phys. Chem. C. 2011, 115, 20491. (b) Cavallaro, G.; Lazzara, G.; Milioto, S. Langmuir 2011, 27 (3), 1158. (c) Guimaraes, L.; Enyashin, A. N.; Seifert, G.; Duarte, H. A. J. Phys. Chem. C 2010, 114, 11358. (d) Du, M.; Guo, B.; Jia, D. Polym. Int. 2010, 59, 574. (e) Wei, W.; Abdullayev, E.; Hollister, A.; Mills, D.; Lvov, Y. Macromol. Mater. Eng. 2012, 302, 342. (f) Churchman, G. J.; Carr, R. M. Clays Clay Miner. 1975, 23, 382. (g) Alexander, L. T.; Faust, G. T.; Hendrick, S. B.; Insley, H.; McMurdie, H. F. Am. Mineral. 1943, 28, 1. (h) Joussein, E.; Petit, S.; Churchman, J.; Theng, B.; Righi, D.; Delvaux, B. Clay Miner. 2005, 40, 383. (i) Kautz, C. Q.; Ryan, P. C. Clays Clay Miner. 2003, 51, 252. (j) Hillier, S.; Ryan, P. C. Clays Clay Miner. 2002, 37, 4879. (9) Tani1, M.; Liu, C.; Huang, P. M. Geoderma 2004, 118, 209. (10) (a) Pan, J.; Yao, H.; Xu, L.; Ou, H.; Huo, P.; Li, X.; Yan, Y. J. Phys. Chem. C. 2011, 115, 5440. (b) An, Y.; Chen, M.; Xue, Q.; Liu, W. J. Colloid Interface Sci. 2007, 311, 507. (11) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (12) Shetty, A. M.; Georgina, M. H.; Wilkins.; Nanda, J.; Solomon, M. J. J. Phys. Chem. C 2009, 113, 7129. (13) (a) Zhang, Q.; Tang, S.; Wallacec, R. M. Appl. Surf. Sci. 2001, 172, 41. (b) Jing, Z.; Lucovsky, G. J. Vac. Sci. Technol., B 1995, 13, 1613. (c) Yokozawa, A.; Miyamoto, Y. Phys. Rev. B 1997, 55, 13783. (d) Karna, S. P.; Pugh, R. D.; Chavez, J. R.; Shedd, W.; Brohers, C. P.; Singaraju, B. K.; Vitiello, M.; Pacchioni, G.; Devine, R. A. B. IEEE Trans. Nucl. Sci. 1998, 45, 2408. (14) Yariv, S.; Shoval, S. Clays Clay Miner. 1976, 24, 253.
aggregation mechanism of HNT-COOH is based on the HB interactions, which can be controlled by pH conditions. Figure S1 (in the Supporting Information) shows the zeta potential graph of the HNT-COOH nanotube. HNT-COOH has a positive charge (40.1 mV) at acidic solution (pH 1), little negative charge (−30.5 mV) at neutral solution (pH 7), and negative charge (−45 mV) at base solution (pH 12).5a Charge repulsion of HNT according to the solution state also affects the aggregation and dispersion properties.
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CONCLUSION We have demonstrated a method to modify the hydroxyl functional groups of halloysite nanotubes to give carboxylic acid functionalized HNT. Furthermore, the degree of aggregation and dispersion of HNT-COOH was observed by SEM and MA-DLS in acidic, neutral, and basic solution. Among the techniques, multiple angle polarized dynamic light scattering (MA-DLS) measurements were useful for studying the degree of aggregation and dispersion of nanoparticles and associating species. Aggregation and dispersion properties are very important for the utilization of HNT in applications such as nanocomposite materials,1 catalysts,2 capillaries,3 drug delivery systems,5 and cosmetics.6 By adjusting the solution pH, the aggregation/dispersion properties of HNT-COOH could be controlled.
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ASSOCIATED CONTENT
* Supporting Information S
Figure S1 as mentioned in the text. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the research fund from Korea Research Fund (KRF) for the basic research program. REFERENCES
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