pubs.acs.org/Langmuir © 2009 American Chemical Society
Ultrathin Nanofibrous Films Prepared from Cadmium Hydroxide Nanostrands and Anionic Surfactants† Xinsheng Peng, Santanu Karan, and Izumi Ichinose* Organic Nanomaterials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received December 10, 2008. Revised Manuscript Received February 6, 2009 We developed a simple fabrication method of ultrathin nanofibrous films from the dispersion of cadmium hydroxide nanostrands and anionic surfactants. The nanostrands were prepared in a dilute aqueous solution of cadmium chloride by using 2-aminoethanol. They were highly positively charged and gave bundlelike fibers upon mixing an aqueous solution of anionic surfactant. The nanostrand/surfactant composite fibers were filtered on an inorganic membrane filter. The resultant nanofibrous film was very uniform in the area of a few centimeters square when the thickness was not less than 60 nm. The films obtained with sodium tetradecyl sulfate (STS) had a composition close to the electroneutral complex, [Cd37(OH)68(H2O)n] 3 6(STS), as confirmed by energy dispersive X-ray analysis. They were water-repellent with a contact angle of 117°, and the value slightly decreased with the alkyl chain length of anionic surfactants. Ultrathin nanofibrous films were stable enough to be used for ultrafiltration at pressure difference of 90 kPa. We could effectively separate Au nanoparticles of 40 nm at an extremely high filtration rate of 14 000 L/(h m2 bar).
Introduction Thin films composed of randomly oriented nanofibers have attracted tremendous interest in recent years. They have extremely high surface area and porous structure, which are advantageous for many applications such as adsorption, filtration, heat retention, catalysis, sensor, and tissue engineering.1 Nanofibrous materials are generally produced in the form of thin films. A typical example is the electrospinning technique, which has been used for the preparation of various nanofibrous films of synthetic polymers, biomacromolecules, inorganic materials, and others.2-5 This technique provides nanofibers of a few tens to several hundreds of nanometers in diameter, and the resultant films have high porosity. Nanofibrous films of polymers have been prepared by interfacial polymerization or complexation of polymers at liquidliquid interfaces.6,7 Dispersions of as-prepared nanofibrous
materials were also possible to be converted to the films by simple casting.8 The better thickness control of such films has been achieved by layer-by-layer deposition using polyelectrolytes.9 Thin films of carbon nanotubes and other inorganic nanofibers have been prepared by filtration of their dispersions.10 This is a very simple technique widely applicable to the nanofibers of a few micrometers in length. Short nanofibers or nanorods have been assembled by the Langmuir-Blodgett technique.11 This method is able to control the thickness of the films but is limited to the preparation of very thin films of low porosity. We reported that cadmium hydroxide nanostrands with a diameter of 1.9 nm and the length of a few micrometers spontaneously formed in an aqueous solution of cadmium nitrate.12 Subsequently, similar nanostrands of copper and zinc hydroxides were found.13 These nanostrands were
*Corresponding author: fax +81-29-852-7449; e-mail
[email protected]. † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue.
(8) (a) Chiou, N.-R.; Epstein, A. J. Adv. Mater. 2005, 17, 1679–1683. (b) Sekitani, T.; Noguchi, Y.; Hata, K.; Fukushima, T.; Aida, T.; Someya, T. Science 2008, 321, 1468–1472. (c) Dong, W.; Cogbill, A.; Zhang, T.; Ghosh, S.; Tian, Z. R. J. Phys. Chem. B 2006, 110, 16819–16822. (d) Yuan, J.; Liu, X.; Akbulut, O.; Hu, J.; Suib, S. L.; Kong, J.; Stellacci, F. Nat. Nanotechnol. 2008, 3, 332–336. (9) (a) Everett, T. A.; Twite, A. A.; Xie, A.; Battina, S. K.; Hua, D. H.; Higgins, D. A. Chem. Mater. 2006, 18, 5937–5943. (b) Podsiadlo, P.; Sui, L.; Elkasabi, Y.; Burgardt, P.; Lee, J.; Miryala, A.; Kusumaatmaja, W. Carman, M. R.; Shtein, M.; Kieffer, J.; Lahann, J.; Kotov, N. A. Langmuir 2007, 23, 7901–7906. (10) (a) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273–1276. (b) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880–1886. (c) Zhang, X.; Du, A. J.; Lee, P.; Sun, D. D.; Leckie, J. O. J. Membr. Sci. 2008, 313, 44–51. (11) (a) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360–4361. (b) Yang, P. Nature (London) 2003, 425, 243–244. (c) Acharya, S.; Panda, A. B.; Belman, N.; Efrima, S.; Golan, Y. Adv. Mater. 2006, 18, 210–213. (12) Ichinose, I.; Kurashima, K.; Kunitake, T. J. Am. Chem. Soc. 2004, 126, 7162–7163. (13) (a) Luo, Y.-H.; Huang, J.; Jin, J.; Peng, X.; Schmitt, W.; Ichinose, I. Chem. Mater. 2006, 18, 1795–1802. (b) Peng, X.; Jin, J.; Kobayashi, N.; Schmitt, W.; Ichinose, I. Chem. Commun. 2008, 1904–1906.
(1) (a) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223–2253. (b) Wang, X.; Chen, X.; Yoon, K.; Fang, D.; Hsiao, B. S.; Chu, B. Environ. Sci. Technol. 2005, 39, 7684–7691. (c) Shin, C.; Chase, G. G.; Reneker, D. H. AIChE J. 2005, 51, 3109–3113. (d) Gheith, M. K.; Pappas, T. C.; Liopo, A. V.; Sinani, V. A.; Shim, B. S.; Motamedi, M.; Wicksted, J. P.; Kotov, N. A. Adv. Mater. 2006, 18, 2975– 2979. (2) Onozuka, K.; Ding, B.; Tsuge, Y.; Naka, T.; Yamazaki, M.; Sugi, S.; Ohno, S.; Yoshikawa, M.; Shiratori, S. Nanotechnology 2006, 17, 1026–1031. (3) (a) Li, D.; Frey, M. W.; Baeumner, A. J. J. Membr. Sci. 2006, 279, 354– 363. (b) Sawicka, K.; Gouma, P.; Simon, S. Sens. Actuators, B 2005, 108, 585–588. (4) (a) Badami, A. S.; Kreke, M. R.; Thompson, M. S.; Riffle, J. S.; Goldstein, A. S. Biomaterials 2006, 27, 596–606. (b) Ma, Z.; Kotaki, M.; Inai, R.; Ramakrishna, S. Tissue Eng. 2005, 11, 101–109. (5) (a) Li, D.; Frey, M. W.; Joo, Y. L. J. Membr. Sci. 2006, 286, 104–114. (b) Ki, C. S.; Gang, E. H.; Um, I. C.; Park, Y. H. J. Membr. Sci. 2007, 302, 20–26. (c) Ogawa, T.; Ding, B.; Sone, Y.; Shiratori, S. Nanotechnology 2007, 18, 165607. (6) Ma, P. X. Mater. Today 2004, 7, 30–40. (7) Capito, R. M.; Azevedo, H. S.; Velichko, Y. S.; Mata, A.; Stupp, S. I. Science 2008, 319, 1812–1816.
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obtained as the highly dispersed solution since the surfaces had numerous positive charges. For example, about one-sixth of the cadmium atoms in cadmium hydroxide nanostrand were presumed to be positively charged. This value corresponds to one-third of the surface cadmium atoms. Such a feature was applied for capturing short DNA fragments from the extremely dilute solution.14 Aqueous dispersions of long and thin nanostrands and their composite fibers could be readily filtered on porous substrates. This gave a general method for the preparation of ultrathin nanofibrous films containing anionic dyes, proteins, conjugated polymers, etc.15-17 Metal hydroxide nanostrands formed stable electrostatic complexes with multiply charged molecular anions, which has been our strategy to make the nanofibrous films mechanically robust. The resultant films were highly hydrophilic and were often electrostatically charged. In the present study, we widely examined the electrostatic complexation of the nanostrands with various anionic surfactants. As a result, we found that nanofibrous films were obtainable from sodium alkyl sulfates of a certain length (SDeS, SDS, STS, SHS, and SOS). Surprisingly, they were mechanically robust enough to be used as ultrafiltration membranes. It was also unexpected that nanostrand/surfactant composite fibers converted to hydrophobic films. The water repellency could be controlled by choosing the length of the alkyl chain. Here we briefly report on a very simple preparation method of ultrathin nanofibrous films by mixing aqueous solutions of cadmium chloride, organic amine, and anionic surfactant and following by filtration.
Scheme 1. Structures of Anionic Surfactants
scanning electron microscope (SEM, Hitachi S-4800), a transmission electron microscope (TEM, JEOL 1010), and a highresolution transmission electron microscope (HR-TEM, JEM 2100F) equipped with an energy-dispersive X-ray analysis system (EDX). The specimens for TEM and HR-TEM observations were prepared by dropping the dispersion on a carboncoated TEM grid, wiping the solution from the edge of the grid, and then drying in vacuum. SEM was used for the observation of the films on membrane filters. To prevent electric charging, the specimens were coated with 2 nm thick platinum layer by using a Hitachi e-1030 ion sputter at the pressure of 10 Pa and the current density of 10 mA. Water contact angle measurements were carried out by using a Drop Master 300 (Kyowa). The drop volume was 1.5 μL. The value was averaged from five measurement points. UV-vis absorption spectra were obtained by using a Shimazu UV-3150 spectrophotometer.
Experimental Section
Results and Discussion
Materials. Sodium decyl sulfate (SDeS), sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), sodium hexadecyl sulfate (SHS), and sodium octadecyl sulfate (SOS) were purchased from Fluka. CdCl2 3 2.5H2O and 2-aminoethanol were purchased from Kanto Chemical. Au nanoparticles (concentration: 1011-13 units/mL) were obtained from British Biocell International and used for filtration experiments after diluting 10 times with ethanol.
In the present study, cadmium hydroxide nanostrands were prepared in an aqueous solution containing 2.0 mM cadmium chloride and 0.4 mM 2-aminoethanol. In this condition, about 10% of cadmium ions are converted to the nanostrands, and the rest exist as the hydrated ions. The conversion rate could be increased up to 25% or so. However, with the increase of the rate, hexagonal crystals of cadmium hydroxide were prone to precipitate as byproduct.12 The selection of anionic surfactant was also significant. Cadmium ions readily precipitate at pH 9 or higher. Therefore, alkaline surfactants such as sodium salts of fatty acids cannot be used. When dilute aqueous solutions of alkylphosphonic acids (C12H25PO(OH)2, C14H29PO(OH)2, C18H37PO(OH)2) were adjusted to near pH 7 with NaOH and mixed with the above nanostrand solution, long and narrow nanostrands disappeared, and instead nanoparticles of 20-40 nm were produced. Probably, strong coordination bonds of the phosphonate group to the cadmium ion result in the decomposition of the nanostrands. In sharp contrast, sodium alkyl sulfates shown in Scheme 1 did not change the morphology of the nanostrands. When 333 μL of 1 mM sodium tetradecyl sulfate (STS) was mixed with the above nanostrand solution (10 mL), weakly gelled composite fibers appeared, and the quasi-transparent gel could be dispersed in the solution by mildly stirring. In this condition, the concentration of STS was 0.032 mM, and about one-sixth of the cadmium ions had been converted to the nanostrands. Figure 1 shows SEM images of the nanofibrous film prepared by filtering the dispersion on a porous aluminum membrane. The film was composed of flat bundlelike fibers with a width of 30-60 nm. As shown in the inset of Figure 1a, the bundlelike structure was made of nanofibers of about 5 nm in width. The apparently flexible composite fibers had the length of several micrometers and uniformly covered
Preparation of Nanostrand/Surfactant Composite Fibers and the Films. Cadmium hydroxide nanostrands were prepared by mixing equal volume of aqueous solutions of 4 mM CdCl2 and 0.8 mM 2-aminoethanol under vigorous stirring and aging for 10 min. Then a certain volume of 1 mM surfactant solution at pH of about 7 was added into the nanostrand solution (usually 10 mL), and the mixture was slowly stirred for 30 min. In the typical experiment, the molar ratio of surfactant to cadmium chloride was 1/60 in the final mixture. The resultant weakly gelled quasi-transparent dispersion was suction filtered on a porous alumina membrane (Whatman, Anodisc 25, pore diameter 0.2 μm) or a track-etched polycarbonate membrane (Whatman, Nucleopore) at decreasing pressure from atmosphere to 10 kPa. The filtration area was 1.9 cm in diameter. The pore size of the latter membrane was selected from 0.2, 0.4, 0.8, or 1.0 μm depending on the length of nanostrand/surfactant composite fibers. The nanofibrous film was obtained on the membrane filter after rinsing with pure water of 10 mL several times on the filtration cell and drying in air. Characterization. The structure and composition of the composite fibers and the films were characterized by using a (14) Ichinose, I.; Huang, J.; Luo, Y.-H. Nano Lett. 2005, 5, 97–100. (15) Luo, Y.-H.; Huang, J.; Ichinose, I. J. Am. Chem. Soc. 2005, 127, 8296– 8297. (16) Peng, X.; Jin, J.; Ericsson, E. M.; Ichinose, I. J. Am. Chem. Soc. 2007, 129, 8625–8633. (17) Peng, X.; Jin, J.; Ichinose, I. Adv. Funct. Mater. 2007, 17, 1849–1855.
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Figure 2. TEM (a) and high-magnification TEM (b) images of nanostrand/STS composite fibers. The specimen was prepared from the same solution as in Figure 1. The inset in (a) is a TEM image of a cadmium hydroxide nanostrand. The inset in (b) is a HR-TEM image of a part of the composite fiber. Scheme 2. Formation of Nanostrand/Surfactant Composite Fiber
Figure 1. SEM images of a nanofibrous composite film covering a porous alumina membrane. The specimen was prepared from the mixed solution of cadmium chloride, 2-aminoethanol, and STS in the molar ratio of 60:12:1. The top view (a) and cross-sectional images (b and c) were obtained after coating with a 2 nm thick platinum layer. The inset in (a) is the corresponding high-magnification image. the pores of the alumina membrane of 0.2 μm in diameter. The film on the membrane had a thickness of about 60 nm, as confirmed by cross-sectional SEM observations (Figure 1b,c), and the thickness could be controlled by choosing the volume of the solution to be suction filtered. Judging from the highmagnification image, the individual composite fibers appeared to have a width of 20 nm, which is thinner than the width in Figure 1a. However, this is probably due to the deformation of the composite fibers by being torn. TEM images of nanostrand/STS composite fibers are shown in Figure 2. It is clear that flat bundlelike fibers are composed of parallel assembly of cadmium hydroxide nanostrands that have a width of 1.9 nm.12 As a reference, we showed a TEM image of a single nanostrand in the inset of Figure 2a. The interval length of the parallel assembly was 3.7 nm in average. The length appeared to be shorter when the flat bundlelike fiber was tilted from the horizontal plane. As shown in the high-magnification image, the interval was slightly changed even in the single fiber in the range 3.6-4.0 nm (Figure 2b). The width of the nanostrands was not easy to observe in the HR-TEM image. However, relatively dark lines of about 2 nm were occasionally observed. The representative image is shown in the inset of Figure 2b. As described before, about one-sixth of the cadmium atoms in cadmium hydroxide nanostrand are positively charged. The surfaces should be covered by anionic surfactant. However, the resultant surfactant-coated nanostrands are energetically disfavored in water. Thus, the outermost surfaces must be partially adsorbed by the surfactant, which gives dispersibility to the coated nanostrands (Scheme 2). We believe that the extent of this adsorption is not very high. Because in this experiment, STS concentration (0.032 mM) is about 2 orders of magnitude lower than the CMC (2.4 mM),18 and almost all the surfactant should be used up for the electrostatic neutralization of the nanostrands. Then, the surfactant-coated nanostrands further (18) Li, X.; Zhang, G.; Dong, J.; Zhou, X.; Yan, X.; Luo, M. J. Mol. Struct. 2004, 710, 119–126.
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assemble into bundlelike fibers due to the hydrophobic interaction, as generally seen in other self-assembly systems in water.19-21 Judging from the high dispersibility, the bundlelike fibers should be partially adsorbed with the surfactant, too. This is further confirmed by the following experiments. When we attempted to extract the bundlelike fibers in hexane, no any nanofibrous structures were found in the hexane phase. Instead, some white aggregates were formed at the interface between water and hexane phases. The fibers dispersed in water cannot be very hydrophobic. After subtracting the width of the nanostrand (1.9 nm), the observed interval (3.6-4.0 nm) indicates that the thickness occupied by the surfactant is 1.7-2.1 nm. This value is considerably short, as compared to the molecular length of STS (2.4 nm) in its extended conformation. Therefore, the alkyl chains between neighboring nanostrands should have interdigitated structures. The thickness of flat bundlelike fibers is still unclear, but it seems to be in the range of 4-6 nm, summing up the results of SEM and TEM observations. To study the effect of alkyl chain on morphology, we examined the composite fibers of SDeS, SDS, SHS, and SOS with cadmium hydroxide nanostrands by using TEM. SDeS and SDS gave bundlelike fibers with an average width of about 30 nm. On the other hand, anionic surfactants with a long alkyl chain (SHS and SOS) gave the fibers of 55 nm on average, which was slightly wider than that of STS (50 nm). :: (19) Meijer, J. T.; Roeters, M.; Viola, V.; Lowik, D. W. P. M.; Vriend, G.; van Hest, J. C. M. Langmuir 2007, 23, 2058–2063. (20) Clark, T. D.; Tien, J.; Duffy, D. C.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 7677–7682. (21) Zangi, R.; Berne, B. J. J. Phys. Chem. B 2006, 110, 22736–22741.
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Figure 3. SEM images of nanostrand/STS composite fibers, their surface properties, and elemental composition. The composite fibers were prepared at the molar ratios of 60:0.015 (a), 60:0.06 (b), 60:0.9 (c), and 60:3.3 (d) for cadmium atom and STS. The changes of contact angle with the molar ratio (STS/Cd) and EDX spectra of the nanofibrous films (a-d) are shown in (e) and (f). The insets in (a-d) are photo images of a water droplet on the corresponding films. The corresponding SEM images revealed that the width of composite fibers increased for three surfactants with a long alkyl chain (see Supporting Information). The clean surface of the film of cadmium hydroxide nanostrands gave a contact angle of 26°. Those of SDeS and SDS films were 89° and 92°, respectively. Surprising is the stability of nanostrand/SDeS (or SDS) composite fibers to water. In spite of the short alkyl chain, these surfactants strongly adsorb to the surfaces of the nanostrands and keep the composite fibers water-repellent (see Supporting Information). The water contact angle increased to 115°-117° when STS, SHS, and SOS were used. In general, the contact angle is strongly affected by the morphology as well as the chemical properties of the material surface.22 However, both factors must be very close for the latter three surfactants with a long alkyl chain. That is to say, the outermost surfaces of these composite fibers are coated with hydrophobic chains in the same fashion. We also examined the effect of STS concentration on the morphology of the composite fibers. As shown in Figure 3a, bundlelike assemblies were obtained even at the molar ratio of 60:0.015 for cadmium atom and STS. At this condition, about (22) Wu, X.; Zheng, L.; Wu, D. Langmuir 2005, 21, 2665–2667.
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1.5% of the positive charges on cadmium hydroxide nanostrand can be neutralized by the surfactant. The resultant fibers had an average width of about 8 nm. The film showed a water contact angle of 27°, which is almost the same as the value on the film of cadmium hydroxide nanostrands alone (26°) (see Supporting Information). When the molar ratio was 60:0.06, the average width and the contact angle increased to 15 nm and 56°, respectively (Figure 3b). These values increased to 50 nm and 111° at the molar ratio of 60:0.9 (Figure 3c). However, we did not observe significant changes in the average width when the ratio was not less than 60:1 (Figure 3d). The changes of contact angle with STS/Cd ratio are plotted in Figure 3e. The value quickly increased with small amount of STS and exceeded 90° when the ratio was 0.003 (or at the concentration of 6 10-5 M for STS). The contact angle gradually increased until the STS/Cd ratio became 0.0167 (or 60:1 for cadmium atom and STS), and after that, the value was within the range 115°-117°. The composition of nanofibrous films (Figure 3a-d) was examined by energy dispersive X-ray analysis (EDX). The spectra are shown in Figure 3f. The atomic ratio of sulfur to cadmium increased from trace to about 1% when the molar ratio (Cd/STS) increased from 60:0.015 to 60:0.06 and then gave 14% at the molar ratio of 60:0.9. The atomic ratio was DOI: 10.1021/la8040693
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Figure 4. SEM images of nanostrand/STS composite fibers on track-etched polycarbonate membrane (a) and the nanofibrous film after filtering 20 and 40 nm Au nanoparticles (b). The composite fibers were prepared from the same solution as in Figure 1. The former image was taken at the early stage of the film growth on a 1 μm pore. The film thickness in (b) is 100 nm. 17% when the film was prepared at the molar ratio of 60:3.3. Judging from the saturated contact angle, the sulfur content should have the upper limit of 17% in the ratio to cadmium. In our preparation process, the films on membrane filters were rinsed with 10 mL of pure water several times and dried in air. Therefore, it is reasonable that excess surfactant is removed from the nanofibrous films. In our previous studies, the composition of cadmium hydroxide nanostrand in water was proposed as [Cd37(OH)68(H2O)n]6+, which has been confirmed by titration experiments using anionic dyes.12-14 When the positive charges are completely electrically neutralized by STS, the composite ratio, [Cd37(OH)68(H2O)n] 3 6 (STS), gives a sulfur content of 16.2% in the ratio to cadmium. From the sulfur content (17%) and the change in contact angle, SEM, and TEM images, it would be safe to say that nanostrand/STS composite films prepared at the molar ratio of 60:1 or more have a composition close to the above electroneutral complex. Formation of the electroneutral complex is also supported by the stability of the composite fibers for hydration. In the present study, the nanostrands were prepared at the condition of 60:12 for cadmium chloride and 2-aminoethanol. If 10% of the cadmium ions are converted to the nanostrands, the positive charges on the nanostrands can be neutralized by STS just at the molar ratio of 60:1. The thing is that one-sixth of cadmium atoms in the nanostrand are positively charged.14 The electrostatic interaction between nanostrand and STS must be strong since the composite fibers give water-repellent films after rinsing with pure water. Figure 4a shows nanostrand/STS composite fibers at the early stage of covering on a pore of 1 μm diameter. It is obvious that the fibers form a uniform film on a porous substrate from the beginning. The nanofibrous film grows by stacking the quasi-two-dimensional network of flat bundlelike fibers. As a result, the film has a constant thickness, as seen in Figure 1. Such nanofibrous network was mechanically robust. In fact, we confirmed that a 100 nm thick film was useful as an ultrafiltration membrane under pressure difference of 90 kPa. Figure 4b shows the film after filtering a mixed solution of 20 and 40 nm Au nanoparticles. The particles remained on the surface have a diameter of 40 nm or more. Apparently, the
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film has large pores of 100 nm or so on its surface. However, the inside is more densely packed. The rejection of 40 nm nanoparticles was 98%, as confirmed by UV-vis absorption spectra of the permeate. In sharp contrast, 20 nm nanoparticles freely passed through the film. One may think it strange that water readily goes through hydrophobic films. As expected by the Young-Laplace equation, an extremely high pressure difference is required to make water permeate in a nanometer-sized hydrophobic pore. However, it was not the case with our nanofibrous films. Although the films have apparently hydrophobic surfaces for water droplets, the inner surfaces seem to be not very hydrophobic. A very small amount of adsorbed STS may remain in the inner surfaces. Filtration rate of the nanoparticle solution (40 nm) was 14 000 L/(h m2 bar), which was 5-10 times faster than commercial filtration membranes with similar rejection properties. The films on porous substrates were not very stable for washing by vibration or back-flushing. There is room for improvement on the repeated use or the filtration of large volumes of aqueous solutions. However, the high filtration rate (or low pressure operation) must be a significant advantage in practical use.
Conclusions In the present study, we demonstrated that cadmium hydroxide nanostrands gave bundlelike fibers after forming the electrostatic complexes with anionic surfactants in water. They were readily converted into ultrathin nanofibrous films by filtration. The films made of the nanostrand/surfactant composite fibers were water-repellent. They showed a contact angle of 115°-117° when the surfactant concentration was enough to compensate the positive charges on the nanostrand. The nanofibrous film uniformly covering the pores of 1 μm was mechanically robust enough to be used for ultrafiltration. To be emphasized is the simplicity in preparation. The substrates with micro- or submicron pores are converted to ultrathin filtration membranes with pores of a few tens of nanometers readily and at low cost. It was unexpected for us that simple alkyl sulfates gave bundlelike fibers stable against hydration. This finding opens a wide range of functionalization of nanofibers through chemically designing the structure of alkyl sulfate. For example, hydrophobicity, molecular recognition properties, and bioaffinity of nanofibrous films will be widely controlled by choosing the surfactant structure. Furthermore, the method described here has good compatibility with the present membrane technology. Acknowledgment. This research was partially supported by Japan Science and Technology Agency, CREST. Supporting Information Available: TEM and SEM images of nanostrand/surfactant composite fibers and photo images of a water droplet on the corresponding films. This material is available free of charge via the Internet at http://pubs. acs.org.
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