Fabrication of Superhydrophobic Cotton Fabrics with UV Protection

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Fabrication of Superhydrophobic Cotton Fabrics with UV Protection Based on CeO2 Particles Wei Duan, Anjian Xie,* Yuhua Shen,* Xiufang Wang, Fang Wang, Ye Zhang, and Jialin Li School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, People’s Republic of China

bS Supporting Information ABSTRACT: Water repellency and ultraviolet (UV) protection are desirable properties for textiles. In this paper, the cotton fabrics were first treated with CeO2 sol and then modified with a layer of dodecafluoroheptyl-propyl-trimethoxylsilane (DFTMS). The asobtained cotton sample was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transformation infrared spectroscopy (FTIR), water contact angle measurement, and UV
vis spectrophotometry. The modified cotton surface not only exhibited robust superhydrophobicity with a high contact angle of 158° and low roll-off angle of 14° for 5-μL water droplets, but also rendered excellent protection against UV radiation because of incorporation of CeO2 particles.

1. INTRODUCTION Recently, superhydrophobic surface with water contact angle (WCA) greater than 150° and low contact angle hysteresis (CAH) has been extensively investigated due to its importance in both fundamental research and practical applications.1
3 The superhydrophobic phenomenon can be commonly observed in many plants in nature such as the lotus leaf, which is known as the “Lotus effect” or self-cleaning effect.4,5 Inspired by the lotus leaf structure, numerous studies have been done to explore the physical and chemical mechanisms of superhydrophobicity in nature and it is further confirmed that the wettability of a solid surface is mainly dependent on its chemical composition and the geometrical structure of the surface. As markets in leisure and outdoor sporting textiles have been expanded, the needs for superhydrophobic fabrics have continuously increased. Superhydrophobic surfaces on textiles have been obtained by a number of different approaches.6
16 For instance, Gao and McCarthy8 prepared superhydrophobic polyester fabric by using a simple patented water-repellent silicone coating procedure. But the microfiber fabric with a single fiber as small as ∼2 μm needs to be tightly woven and this approach may not be suited to cotton textiles. Wang et al.13 successfully developed superhydrophobic surface on textiles by incorporating gold particles into cotton fabrics to induce a dual-size surface topology and subsequent modification with n-dodecanethiol. But this method is no doubt an expensive one. Michielsen and Lee4 fabricated artificial superhydrophobic surfaces using mechanical and chemical surface modification of nylon 6,6 woven fabric. Hoefnagels et al.14 created superhydrophobic cotton fabrics by in situ growing microsized silica particles on cotton fibers to generate a dual-size surface roughness, followed by a hydrophobization step. But large silica particles with a diameter about 1 μm might affect the softness and flexibility of natural cotton textiles. Daoud et al.15 deposited polytetrafluoroethylene thin films on cotton substrates by plused laser deposition to render the fabric superhydrophobicity with a CA of 151°. Zhang et al.16 deposited a nanoparticulate film of fluorocarbon chemical by plasma r 2011 American Chemical Society

treatment onto a cotton fabric surface and superhydrophobic cotton fabrics with a CA of 164° were achieved. Besides, multifunctional textiles with water- and oil-repellencies were achieved by combining fiber structure and polymer coatings more recently.17,18 Cotton has always been the principal clothing fabric due to its attractive characteristics such as softness, comfort, warmness, biodegradation, and low cost. However, the abundant waterabsorbing hydroxyl groups on cotton surfaces make the fabrics absorbent and easily stained by liquids. Therefore, additional finishes are required to impart superhydrophobicity and selfcleaning properties on cotton fabrics.12 It is known that surface wettability is controlled by the chemical composition and surface roughness of solids.19 Roughened surfaces have been commonly obtained by introducing nanosize particles onto the surface. There are several kinds of inorganic nanosize particles such as SiO2,11 ZnO,12 and TiO2,20 which have been introduced to cotton substrates to fabricate rough surface. In recent years, CeO2 as an important functional material has been receiving a great deal of attention.21,22 Nevertheless, it has been less investigated that CeO2 particles are introduced to cotton substrates to achieve water repellency. More importantly, compared to TiO2 (3.27 eV) and ZnO (3.37 eV), CeO2 has a relatively small band gap (3.1 eV), which is conducive to the transition of CeO2 valence electrons. It is proved that CeO2 is a better ultraviolet radiation absorbent, which have been widely used in sunscreen, textile fiber, paint, and so on. In this work, cotton fabrics with the bifunction of superhydrophobicity and UV-radiation protection were successfully fabricated by coating with CeO2 sol and subsequent modification with DFTMS. The as-obtained fabrics are greatly desired for industrial, medical, and military end-uses, as well as everyday life uses, such as beach umbrellas, protective clothing, and tents. Received: September 18, 2010 Accepted: March 2, 2011 Revised: February 23, 2011 Published: March 15, 2011 4441

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Figure 3. FTIR spectra of (a) the untreated cotton fabric and (b) the cotton fabric treated with CeO2 sol and hydrolyzed DFTMS.

Figure 1. SEM images of (a) original cotton fibers, and (b, c) cotton fibers treated with CeO2 sol at different magnifications; (d) EDS spectrum of the cotton fabric treated with CeO2 sol.

Figure 2. XRD patterns of (a) pristine cotton fabric, and (b) cotton fabric treated with CeO2 sol.

2. EXPERIMENTAL SECTION 2.1. Materials. Pure cotton fabrics purchased from a local fabric store were cleaned with deionized water and ethanol before drying for use. Chemicals including cerium nitrate hexahydrate, sodium hydroxide, acetic acid, and absolute ethanol were all of analytical reagent grade and purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Dodecafluoroheptyl-propyl-trimethoxylsilane (C13F12H18SiO3, DFTMS) used in this study was obtained from Xuegia Fluorine-Silicon Chemical Reagent Co. Ltd. (Harbin, China). For all experiments, deionized water was used. 2.2. Preparation of CeO2 sol. The CeO2 sol was prepared based on the method of Wu23 with some modifications. Cerium nitrate hexahydrate (0.06 M) was dissolved in ethanol at 60 °C under vigorous stirring. Then a solution of sodium hydroxide (0.10 M) in ethanol was slowly added into the above solution and further stirred for 4 h at 60 °C. The CeO2 sol was thus obtained. 2.3. Hydration of DFTMS. DFTMS (3%, w/w) was gradually added to ethanol to form a solution, and its pH was adjusted to 5.0 by acetic acid. The solution thus prepared was stirred for 60 min to form an alkylsilanol solution. 2.4. Treatment of Cotton Fabrics. The as-prepared CeO2 sol was coated onto the cotton fabric sample by dip
pad
cure

Figure 4. Shape of 5-μL water droplet on the surface of (a) the pure cotton sample, (b) the cotton sample treated with only hydrolyzed DFTMS, and (c) the cotton sample treated with CeO2 sol and hydrolyzed DFTMS. (d) Digital photograph image of the shape of 5-μL water droplets on the superhydrophobic cotton surface, and water droplets were dyed with methylene blue.

process. The cotton sample was first dipped into the CeO2 sol for 3 min and then padded with a two-roll laboratory padder giving a wet pick-up of 70
80%. Finally, the padded substrate was airdried for 20 min and cured at 170 °C for 3 min. This process was repeated three times to form a dense film of CeO2 xerogel on cotton fabrics. The treated fabrics were then immersed in the ethanol solution of hydrolyzed DFTMS for 24 h. Subsequently, the cotton fabrics were washed with anhydrous ethanol to remove any residual chemicals and allowed to dry in air at room temperature, then cured at 120 °C for 1 h in an oven. 2.5. Instruments and Characterization. The surface morphology of the treated fabrics was measured using scanning electron microscopy (SEM, Hitachi S4800, Japan). The average size of the fabric fibers was calculated based on SEM images. The cotton fabrics were also characterized by energy dispersive spectroscopy (EDS, attached to the SEM, operating at 20 keV). The crystallographic phase of the CeO2 was determined by powder XRD (shimadzu XRD-6000, CuKa radiation). The FTIR spectra were collected using a Nicolet Nexus-670 Fourier transform spectrophotometer with an attenuated total reflection (ATR) accessory. UV
vis characterization was conducted using a CARY 100 Bio UV
visible spectrophotometer with a CARY dual-cell Peltier accessory (UV-1800, Shimadzu, Japan). The CA measurements were performed using an optical video contact 4442

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Table 1. Comparison of Predicted and Measured WCAs sample (R = 12.2 μm, D* = 1.2)

pure cotton

cotton treated with only hydrolyzed DFTMS

cotton treated with CeO2 sol and hydrolyzed DFTMS

predicted WCAs (deg)

θC
B = 0

99 < θC
B < 135

135 < θC
B < 180

measured WCAs (deg)

θC
B = 0

θC
B = 100

θC
B = 158

Figure 5. Static contact angle and roll-off angle of the cotton treated with CeO2 sol and hydrolyzed DFTMS as a function of the number of wash cycles.

angle instrument (model OCA 40, Dataphysics, Germany) at room temperature. The WCA was determined after a water droplet was placed on the fabric for 60 s. Each CA presented was the average value of those measured at five different locations of each fabric specimen. The roll-off angle was tested by inclining the sample to a certain degree. Evaluation of washing fastness or durability was carried out according to a standard test method for fabric coating (AATCC Test Method 61-2006 test no. 2A). This standard wash procedure is equivalent to five cycles of home machine launderings. For convenience, we use the equivalent number of home machine launderings in this paper.

3. RESULTS AND DISCUSSION SEM images were used to observe the morphology of original and treated cotton fibers. From Figure 1a, the woven cotton fabrics consist of conventional cotton fibers whose diameters are about 12.2 ( 0.7 μm (Figure S1, Supporting Information), and the pristine cotton fibers show very smooth surfaces. After sol
gel coating with CeO2 sol, the xerogel of CeO2 formed on fibers made the fiber surface rougher. It is clear to see that the cotton fibers were covered with a uniform and dense film of CeO2, as shown in Figure 1b and c. Higher magnification of the uniform area of CeO2 film is also shown in insertion in Figure 1c, and CeO2 particles show an average size of ∼15 nm. The aggregation of CeO2 nanoparticles could lead to irregular surface topography composed of “valleys” and “hills”. To determine the chemical elements of the treated cotton fabric, the cotton sample was characterized by EDS. Figure 1d gives a typical EDS spectrum recorded on the treated cotton fiber. Only O, Ce, Pt, and C elements could be detected from the EDS spectrum. Pt and C elements were from the coating layer used for the EDS measurement and the cotton substrate, respectively. No other elements were detected, confirming that CeO2 particles were effectively coated onto the cotton surface. Figure 2 shows the XRD patterns for the pristine and coated cotton fabric with CeO2 xerogel. Compared to curve a for the

pristine sample, three new weak peaks are observed at 2θ in the range of 25°
60° in the curve b, which are attributed to the diffraction peaks of the (111), (220), and (331) planes of CeO2 with cubic structure reported in the International Center for Diffraction Data (JCPDS data number 34-0394 card). No characteristic peaks were observed for the other impurities such as Ce2O3. Wettability is dependent on the geometrical structure and the chemical composition of the materials. To create a superhydrophobic surface, the cotton fabrics coated with CeO2 sol were further modified by self-assembly of DFTMS to reduce the surface energy. The superhydrophobic cotton fabric is studied by FTIR spectra (Figure 3). The spectrum of pure cotton (curve a) exhibits O
H stretching absorption around 3440 cm
1, C
H stretching absorption around 2800
3000 cm
1, and C
O
C stretching absorption around 1056 cm
1 and 1110 cm
1. These absorptions are consistent with those of the typical cellulose backbone.24,25 In the spectrum of the cotton fabric treated with CeO2 sol and DFTMS (curve b), a new peak appears at 791 cm
1 corresponding to Si
C stretching, which indicates the introduction of the siloxane. In addition, the peak at 550 cm
1 can be observed, which corresponds to Ce
O stretching due to the introduction of CeO2 particles. These results confirm chemically that the as-described treatments were successfully done. The wettability of the cotton fabrics was evaluated by static CAs and roll-off angles. Because the protruding fibers exhibit elasticity, and also force on the water droplet,13 it is difficult to yield accurate values for the advancing and receding water contact angles.14 Therefore, only static CAs are measured here, as shown in Figure 4. The WCAs on the pure and CeO2-treated cotton fabrics are almost zero, as shown in Figure 4a, indicating that the cotton fabrics untreated with DFTMS absorbed water drops without showing any hydrophobicity. This result stems from the hydrophilic hydroxyl groups on both surfaces of cotton fabric and CeO2 nanoparticles. The WCA on the cotton fabric treated with only the ethanol solution of hydrolyzed DFTMS is 100° (Figure 4b). However, for the DFTMS-modified cotton fiber in the presence of CeO2 particles, the static CA was found to be 158°, clearly revealing that the surface changed from superhydrophilic (Figure 4a) to superhydrophobic (Figure 4c, Video S1, Supporting Information). Figure 4d shows a digital photograph image of the shape of water droplets on the superhydrophobic surface, which gives a direct demonstration of the superhydrophobicity of the treated cotton fabrics. In addition, the roll-off angle on the superhydrophobic cotton surface was measured. For 5-μL water droplets the roll-off angle was as low as 14°. From these results, we conclude that our process could fabricate an excellent superhydrophobic cotton surface. Two classic theoretical models (Wenzel model26 and Cassie
Baxter model27) form the basic guidelines for the study of superhydrophobic surface. When a water droplet sits on a hydrophobic cotton fabric surface, its wetting behavior can be described by a modified Cassie
Baxter equation28 cos θC-B ¼ rf cos θe þ f
1 4443

ð1Þ

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Figure 6. Scheme of fabrication process of superhydrophobic surface on cotton substrate including CeO2 coating and subsequent DFTMS modification.

where θC
B is the apparent CA on a rough and porous surface, θe is the equilibrium CA on the corresponding smooth surface, f is the fraction of the projected area of the solid surface wetted by water, and r is the surface roughness of the wetted area. For a texture with uniform cylindrical features characterized by radius R and intercylinder spacing 2D, r and f in Cassie
Baxter relationship (eq 1) become r = (π
θe)/sin θe and f = R sin θe/ (R þ D). Substituting these expressions into eq 1 and defining a dimensionless spacing ratio D* = (R þ D)/R enables us to rearrange the Cassie
Baxter equation into a more convenient form4,18,29 cos θC-B ¼

1 ½ðπ
θe Þcos θe þ sin θe 
1 D

ð2Þ

Using this formula we can report a comparison of the theoretical predicted CAs and measured CAs when θe is varied. From the SEM image of the pure cotton fibers (Figure 1a), the average spacing ratio (D*) calculated was 1.2 (Table 1). For the pure cotton sample, the equilibrium CA on smooth and hydrophilic cotton surface should be less than 30° (θe < 30°), and substituting these values into eq 2 gives θC
B = 0°; for the cotton sample treated with only hydrolyzed DFTMS, 90° < θe < 120° (WCA on smooth surfaces cannot exceed 120° through tailoring surface chemistry28), and 99° < θC
B < 135°. These predicted CAs are in good agreement with the measured CAs, as shown in Table 1. But for the DFTMS-modified cotton fiber in the presence of CeO2 particles, the surface roughness of the cotton fiber increases due to the incorporation of CeO2 particles, r . 1. So compared to the DFTMS-modified cotton without CeO2 particles θC
B should be larger than 135° and goes toward 180°, which can be estimated by eq 1. Again, the theoretical value is in agreement with the measured CAs (Table 1). For practical applications, the durability of the superhydrophobic surface is important. In our study, the affinity between the CeO2 layer and the cotton fiber was tested by immersing the fabric into a sonication bath containing ethanol. After continuous sonication for 20 min, the CA values remained essentially unchanged. In addition, the wash durability test revealed that the static WCAs of a 5-μL droplet still remained above 150° even after 30 cycles, and roll-off angle of a 5-μL droplet remained lower than 20° after 15 cycles of home laundering (Figure 5).

Figure 7. UV
vis transmission spectra of the original cotton fabric and the cotton fabric treated with CeO2 sol and hydrolyzed DFTMS.

These results suggest that the CeO2 particles and DFTMS molecules were strongly attached to the cotton fibers. Figure 6 shows the schematic illustration of the fabricating process of superhydrophobic surface. There are two steps including the preparation of CeO2 coating on cotton substrate and the subsequent self-assembly of DFTMS on the CeO2 surfaces. As shown in Figure 6, after coating CeO2 sol on the cotton fiber, hydrogen bonds were formed between OH groups on the CeO2 layer and OH groups on cotton fiber surfaces.30 When the cotton fabric was immersed into DFTMS ethanol solution for 24 h at room temperature, DFTMS was hydrolyzed at first (Figure 6), most of the Si
OCH2CH3 groups in DFTMS were converted into Si
OH groups. Then, Si
OH of hydrolyzed DFTMS could react with OH groups on both the CeO2 layer and cotton fiber surfaces. These factors may contribute to the enhancement of the attachment between the cotton fiber substrate and the CeO2 particles. Figure 7 shows the UV-shielding ability of the treated cotton fabric. From Figure 7 we observed a marked transmittance decrease in the UV region, and transmission was reduced to nearly zero in the wavelength below 350 nm, which indicates that the cotton fabric treated with CeO2 sol and hydrolyzed DFTMS may offer better protection from UV radiation than the pristine cotton fabric. There are two possible reasons for this. One is that CeO2 causes a good UV absorption as a result of the excitation of 4444

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Industrial & Engineering Chemistry Research CeO2 valence electrons. The other possible reason is that the formation of CeO2 nanoparticles on the fabric surface imparts a very efficient UV scattering because of the large refractive index of CeO2 nanoparticles.

4. CONCLUSION In summary, the cotton fabrics treated with CeO2 sol and DFTMS showed excellent hydrophobicity and the WCA could reach 158°. The incorporation of CeO2 particles can not only cause a surface roughness for enhancing the hydrophobicity but also results in good UV-shielding property. In addition, the fabrication process of such bifunctional fabrics can easily be applied without expensive equipment and chemicals, which is desirable for use in multiple fields such as functional material devices in the future. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 giving size distribution of the cotton fiber; video S1 showing superhydrophobicity of the modified cotton fabric measured by CA instrument. This information is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-551-5108090. Fax: þ86-551-5107342. E-mail: [email protected] (A.X.), [email protected] (Y.S.).

’ ACKNOWLEDGMENT This work is supported by the National Science Foundation of China (20871001, 31070730, and 50973001), the Research Foundation for the Doctoral Program of Higher Education of China (20070357002), the Important Project of Anhui Provincial Education Department (ZD2007004-1), Key Laboratory of Functional Material of Inorganic Chemistry and Key Laboratory of Environment-friendly Polymer Materials and Inorganic Materials of Anhui Province. ’ REFERENCES (1) Tsai, H. J.; Lee, Y. L. Facile Method to Fabricate Raspberry-Like Particulate Films for Superhydrophobic Surfaces. Langmuir 2007, 23, 12687. (2) Tuteja, A.; Choi, W. J.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F. Design Parameters for Superhydrophobicity and Superoleophobicity. MRS Bull. 2008, 33, 752. (3) Kulkami, S. A.; Ogale, S. B.; Vijayamohanan, K. P. Tuning the Hydrophobic Properties of Silica Particles by Surface Silanization Using Mixed Self-Assembled Monolayers. J. Colloid Interface Sci. 2008, 318, 372. (4) Michielsen, S.; Lee, H. J. Design of a Superhydrophobic Surface Using Woven Structures. Langmuir 2007, 23, 6004. (5) Tavana, H.; Amirfazli, A.; Neumann, A. W. Fabrication of Superhydrophobic Surfaces of n-Hexatriacontane. Langmuir 2006, 22, 5556. (6) Xue, C. H.; Jia, S. T.; Zhang, J.; Tian, L. Q. Superhydrophobic Surfaces on Cotton Textiles by Complex Coating of Silica Nanoparticles and Hydrophobization. Thin Solid Films 2009, 517, 4593. (7) Nystrom, D.; Lindqvist, J.; Ostmark, E.; Hult, A.; Malmstrom, E. Superhydrophobic Bio-Fibre Surfaces via Tailored Grafting Architecture. Chem. Commun. 2006, 34, 3594.

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