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Intrinsically Superhydrophobic Organosilica Sol-Gel Foams N. J. Shirtcliffe,* G. McHale, M. I. Newton, and C. C. Perry School of Science, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U.K. Received February 6, 2003. In Final Form: May 7, 2003 Intrinsically superhydrophobic foams with contact angles greater than 150° were prepared using a sol-gel phase-separation process. Hydrophobicity was built in by using organofunctionalized inorganic monomers and setting the conditions so that they were retained in the product. The materials were characterized by advancing and receding water contact angle measurements, scanning electron microscopy, and infrared spectroscopy. The sol-gel phase-separation preparation method used was simple and produced roughness and hydrophobicity in the same material, thus obviating the need for a hydrophobic coating to achieve superhydrophobicity. Superhydrophobicity was retained when the materials were cut or abraded. On heating, a rapid hydrophobic to hydrophilic transition was present at around 400 °C, generating a material that absorbed water rapidly. “Flat” sol-gel materials prepared without phase separation treated in the same manner showed a more gradual contact angle change, but the switch from hydrophobicity to hydrophilicity also occurred at around 400 °C.
Introduction A drop of water sitting on a surface will spontaneously spread or contract until the angle it makes with the surface (usually measured inside the drop) reaches a certain value. The equilibrium angle reached is determined by a balance between the interfacial forces for the solid-liquid (γSL), liquid-vapor (γLV), and solid-vapor (γSV) interfaces. This equilibrium can be ascribed to the balancing of the relative interfacial contact areas (ASL, ALV, and ASV), given the interfacial tensions for a particular solid-liquid-vapor system, so as to minimize the surface free energy.1-3 Wenzel4 showed that surface roughness produces a physical enhancement of hydrophobicity into superhydrophobicity. This occurs because the solid-liquid and solid-vapor area contributions to the surface free energy become much larger than the horizontal projections of the areas. Thus, the topography alters the relative contribution of the solid-liquid interface to the surface free energy. Wenzel’s equation predicts that the basic wetting behavior of a surface will be enhanced by roughness so that creating roughness on a flat surface with an equilibrium contact angle θe(flat) > 90° will increase the contact angle, while the same roughness on a surface with θe(flat) < 90° will decrease the contact angle. In practice, intimate contact is not usually maintained between liquid and solid on very rough surfaces with θe(flat) > 90° unless hydrostatic pressure is applied. The liquid drop, therefore, effectively sits upon a composite surface of the peaks of the topography and the air separating the surface features, and Cassie’s equation5 applies instead of Wenzel’s. Nonetheless, it is still possible to generate surfaces that are superhydrophobic (θr g 150°). One of the key differences to the predictions from Wenzel’s equation is that the effect of roughness on a surface further emphasizes superhydrophobicity, with the critical contact angle for * Corresponding author. E-mail:
[email protected]. Telephone: +44 (0)115 8486375. Fax: +44 (0)115 9486636. (1) De Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827-863. (2) Le´ger, L.; Joanny, J. F. Rep. Prog. Phys. 1992, 55, 431-486. (3) McHale, G.; Newton, M. I. Colloids Surf. 2002, A206, 193-201. (4) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (5) Que´re´, D. Physica A 2002, 313 (1-2), 32-46.
which roughness causes an increased apparent contact angle reduced to below 90°. Recent studies into superhydrophobic surfaces were started in 1996 by the spectacular example of a water drop with a contact angle of 174° on a surface obtained using a paper-sizing agent, alkylketene dimer (AKD), to provide a fractally rough surface.6,7 This is the best-known example of a superhydrophobic surface created using a chemical approach to take advantage of topographic effects. Since that time, superhydrophobic surfaces, defined as surfaces with contact angles greater than 150°, have been obtained using glass beads,8 the sol-gel process to generate a porous and hence rough surface,9,10 vacuum deposited PTFE thin films,11,12 anodic oxidation of aluminum surfaces,7 plasma polymerization,13,14 and, most recently, lithographic patterning of silicon wafers.15 In many of these cases the idea has been to create a high aspect ratio topographic surface and to then apply a thin (∼monolayer) hydrophobic coating. Typically the hydrophobic coating provides a contact angle of 115°-120° on a flat surface and the topography then enhances this to 150°-180°. Much of the current work is materials oriented with attempts to create hard, transparent, and selfcleaning surfaces.16,17 O ¨ ner and McCarthy15 suggested that a high contact angle in itself might not be a sufficient definition of (6) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125-2127. (7) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512-19517. (8) Fuji, M.; Fujimori, H.; Takei, T.; Watanabe, T.; Chikazawa, M. J. Phys. Chem. 1998, B102, 10498-10504. (9) Tadanaga, K.; Kitamuro, K.; Matsuda, A.; Minami, T. J. Sol-Gel Sci. Technol. 2003, 26 1-3, 705-708. (10) Tadanaga, K.; Morinaga, J.; Minami, T. J. Sol-Gel Sci. Technol. 2000, 19, 211-214. (11) Miller, J.; Veeramasuneni, J.; Drelich, J.; Yalamanchili, M. R. Polym. Eng. Sci. 1996, 36 (14), 1849-1855. (12) Palumbo, G.; D’Agostino, R.; Lamendola, R.; Corzani, I.; Favia, P. European Patents EP0985741 and EP0985740 with Procter and Gamble, 2000. (13) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. J. Phys. Chem. B 2000, 104 (37), 8836-8840. (14) Shirtcliffe, N.; Thiemann, P.; Stratmann, M.; Grundmeier, G. Surf. Coat. Technol. 2001, 142, 1121-1128. (15) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782.
10.1021/la034204f CCC: $25.00 © 2003 American Chemical Society Published on Web 06/06/2003
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superhydrophobicity; account should also be taken of the contact angle hysteresis, which determines how easy it is to set a droplet into motion. If the difference between the advancing and receding angles is low, a drop can easily slide on the surface. O ¨ ner and McCarthy have also shown that the length scale of the topography that can be used for superhydrophobicity can be from nanometers to tens of micrometers. Miwa et al.18 have shown that low tilt angles for sliding, less than 1° for a 7 mg drop, can be obtained on rough surfaces of superhydrophobic boehmite. Que´re´ et al. have studied the dynamics of interaction of droplets on superhydrophobic surfaces. Their work includes drops rolling down surfaces (due to the surface interactions, the drop rolls faster than a “marble” rolling under gravity),19 solids with a superhydrophobic coating bouncing from water,20 and drops rebounding from surfaces.21,22 It is of interest to note that in nature some plants, such as Nelumbo nucifera (L.) Druce, structure the surface of their leaves so that their chemical hydrophobicity is enhanced into superhydrophobicity.23 It has, however, proved difficult to utilize this superhydrophobic effect, as superhydrophobic surfaces become abraded and clogged easily. Plants can avoid this problem by continually renewing their surface wax layer, something that is not easy to reproduce with artificial surfaces. In this report, we concentrate on a very simple, phaseseparation method of producing controllably rough surfaces with intrinsic chemical hydrophobicity.24 Many of the previous studies of superhydrophobicity generate rough surfaces and treat them with surface modifiers. It is, however, possible to utilize the hydrophobicity of the final product to generate the roughness required for superhydrophobicity. The different methods of producing superhydrophobicity may also result in different levels of contact angle hysteresis. Our method utilizes the sol-gel approach, which is a relatively cheap method of manufacturing complex materials.25 In the sol-gel approach the final product is usually a metal oxide, but chemical modifications can be made to the starting materials to build in other functional groups, which can produce hydrophobic materials.26 These hydrophobic materials can be converted to hydrophilic materials by heating.24 Some work has been reported in the literature preparing rough sol-gels that are naturally hydrophilic and treating them to make them hydrophobic.10,17 Our method shows that it is possible to simply generate materials that are intrinsically superhydrophobic. In the phase separation method adopted in this report, some type of hardening process freezes a phase separation, which is arranged to occur concurrently with the hardening.27 This produces a cocontinuous material, consisting of a solid phase and another phase, which could be solid or liquid. If the second (16) Takeda, K.; Sasaki, M.; Kieda, N.; Katayama, K.; Kako, T.; Hashimoto, K.; Watanabe, T.; Nakajima, A. J. Mater. Sci. Lett. 2001, 20 (23), 2131-2133. (17) Nakajima, A.; Abe, K.; Hashimoto, K.; Watanabe, T. Thin Solid Films 2000, 376 (1-2), 140-143. (18) Miwa, M.; Nakajima, A.; Fujishima, A.; Hasimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754-5760. (19) Richard, D.; Que´re´, D. Europhys. Lett. 1999, 48 (3), 286-291. (20) Aussillous, P.; Que´re´, D. Nature 2001, 411 (6840), 924-927. (21) Richard, D.; Que´re´, D. Europhys. Lett. 2000, 50 (6), 769-775. (22) Richard, D.; Clanet, C.; Que´re´, D. Nature 2002, 417 (6891), 811811. (23) Barthlott, W.; Neinhuis, C. Planta 1997, 202 (1), 1-8. (24) Rao, A. V.; Kulkarni, M. M. Mater. Res. Bull. 2002, 37 (9), 16671677. (25) Sol-Gel Technology For Thin Films, Fibers, Preforms, Electronics and Speciality Shapes; Klein, L., Ed.; Noyes: Westwood, NJ, 1998. (26) Yoon, K. H.; Kim, M. W. Mol. Cryst. Liq. Cryst. 2001, 371, 365368. (27) Takahashi, R.; Nakanishi, K.; Soga, N. J. Ceram. Soc. Jpn. 1998, 106 (8), 772-777.
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phase can be removed, a porous material can be generated. The surfaces of this material are particularly interesting because they have the same high degree of semirandom roughness when cut on any plane. Increasing the time allowed for phase separation before the material becomes fully hardened increases the size of the domains of both phases. Thus, the materials approach adopted in this report has great flexibility for the study of superhydrophobicity because rough surfaces in the range of nanometers to micrometers can be created and these can either have intrinsic hydrophobicity or hydrophobicity added by surface treatment. The intrinsically hydrophobic materials have a hydrocarbon group on each silicon atom, so unlike the materials generated by Rao and Kulkarni,24 cut or abraded surfaces could be superhydrophobic without the need for any surface chemical treatment. In addition, with our intrinsically superhydrophobic sol-gel materials, transformations from superhydrophobic to hydrophilic can be studied by heating the materials. Experimental Section Sol-Gel Preparation. 0.1255 mol (25 mL) of methyltriethoxysilane (MTEOS) (98% Lancaster) was added to 0.0018 mol of HCl and 0.83 mol of water or 6.6 equiv (15 mL) of 0.12 M HCl (diluted from 37% HCl Analar Aldrich) and 0.327 mol (25 mL) of 2-propanol (Fisher p.a.) under rapid stirring using a magnetic stirrer. This mixture was sealed and allowed to react for 60 min at 22 °C; the container was not cooled and a small temperature rise was noted initially. The solution was separated into 4 mL aliquots, and 1 cm3 of ammonia solution, diluted from 35% stock (Fisher), was added to each. The concentration of the ammonia was varied, 1.1 and 2.2 M being used for the materials studied here. The reagents were stirred for 1 min before being decanted into 6 mL polystyrene containers and sealed. The materials were left sealed for 20 h to ensure that they gelled fully before the seal was pricked twice with a scalpel blade (5 mm slits) and the samples were allowed to dry at room temperature. The samples were not heated while wet to prevent excessive dissolution and redeposition.28 Compared to most sol-gels, these materials could be dried rapidly, drying over the course of a further 3 days and shrinking by 20-30%. Once dry, the samples were placed in a Pyrex beaker and heated to various temperatures to cross-link and eventually oxidize the materials. To avoid overheating, cracking, and inconsistencies, the samples were heated at a rate of 2.5 °C min-1 in a Carbolite AAF 11/3 furnace and held at the maximum temperature for 1 h before cooling at around 5 °C min-1. Propyl substituted silica foams could not be prepared in the manner described above using pure propyltriethoxysilane (PTEOS) (ABCR). Phase separation occurred more rapidly than with MTEOS, and gelation was slower. The reagents formed two separate liquid layers before the lower layer hardened to form a dense, transparent material. Substitution of methyltriethoxysilane (MTEOS) for two-thirds of the PTEOS (molar ratio) allowed mixed propyl-methyl sol-gel foams to be formed. Higher ammonia concentrations than those for the pure MTEOS gels were, however, necessary. About 20 times the concentration was required to produce an equivalent material. The sol-gel foams were sliced with a razor blade to reveal their internal structure. Measurements were carried out on samples cut from the centers of the monoliths. This allowed “flat” surfaces to be produced and used the bulk structure of the foam as opposed to the surface, where variation in the structure would be anticipated.29 Samples of one of the materials were abraded using 320 grit garnet paper (Wilkinson U.K.). Contact angles on these samples were measured with the contact line perpendicular to the sanding direction to reduce any influence of sanding grooves on the angle. (28) Nakanishi, K.; Takahashi, R.; Nakane, T.; Kitayama, K.; Koheya, N.; Shikata, H.; Soga, N. J. Sol-Gel Sci. Technol. 2000, 17, 191-210. (29) Kanamori, K.; Ishizuka, N.; Nakanishi, K.; Hirao, K. J. Sol-Gel Sci. Technol. 2003, 26 (1-3), 157-160.
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Figure 1. Electron micrographs of gold-coated foams taken at a magnification of ×2000 and at 6 kV: (a) MTEOS with 1.1 M ammonia, heated to 300 °C (foam 1); (b) MTEOS with 2.2 M ammonia, heated to 300 °C (foam 2); (c) PTEOS/MTEOS with 22 M ammonia, heated to 300 °C (foam 3). Flat surfaces with similar methylsilyl- chemistry were prepared using a method derived from that of Nakajima et al.17 0.00218 mol (0.435 mL) of MTEOS was added to a stirred mixture of 0.065 mol (5 mL) of 2-propanol and 0.00036 mol of HCl and 0.0165 mol of water, which is about 7.6 molar equiv (0.3 mL of 1.2 M hydrochloric acid). The mixture was stirred for 2 h and then dip-coated onto cleaned slides and allowed to dry. This produced layers of material that could be stabilized by heating to 225 °C and could be further heated to 550 °C, above which the glass substrate became too soft. This material was used as a reference flat surface for contact angle testing, as the surface was flat when observed under a JEOL JSM-840A scanning electron microscope, showing no observable features (300 nm or greater). Characterization. The materials were characterized using infrared spectroscopy, electron microscopy, and contact angle measurements. Infrared measurements were taken using a Nicolet Magna FTIR with a Spectra-Tech diffuse reflection accessory. Samples were loaded into the sample holder and heated to 100 °C for 20 min to remove any moisture. The spectrometer was flushed with dry air continuously before and during measurement. For electron microscopy, the samples were fixed to aluminum stubs with clear nail varnish before being coated with about 20 nm of gold in an Edwards sputter coater. The samples were then imaged using a JEOL JSM-840A scanning electron microscope with an acceleration voltage of 6 kV; the images presented here were all measured at a magnification of ×2000. The contact angle of water on the different substrates was measured using a G1 device by Kru¨ss. A Razel model A syringe pump was attached to a small needle in an XYZ manipulator to enable a drop to be slowly increased and decreased in size; the flow rate of water was (0.661 mL h-1. This equipment allowed measurement of advancing and receding angles, as the flow rate was low enough that the angles measured were quasistationary. The shape of the drop was recorded onto videotape, and the contact angles were measured from the images. The drop size was increased from zero to around 0.01 mL and decreased again until the drop detached from the surface. Angles were measured at half the maximum size and just below maximum advancing and receding. Two drops were measured on each of two different samples in every case, resulting in eight measurements for each angle. Pore Volumes. Water could not penetrate the foams prepared at low temperatures, but nonpolar solvents could. Their external volumes were estimated by their displacement of water,
and their internal volumes by their displacement of toluene. Samples were wedged into a volumetric flask so that they were held below the measurement line, and the flask was filled with water and weighed. This was compared with the same sample dried and filled with toluene by evacuating the foam covered with toluene (Fischer p.a.) to about 150 mbar for 20 min; the flask was filled to the line with toluene and weighed once more. The organofunctional silica foams swelled in organic solvents, particularly in 1-butanol. To investigate this, samples were measured using Vernier callipers, soaked for 12 h in 1-butanol (Aldrich G.C.) after evacuation; their dimensions were then remeasured in a pool of butanol. Upon drying, the samples returned to their original size. Thermogravimetry. Small samples (about 50 mg) of solgel foams were placed in a thermogravimeter (Stanton Redcroft TG 760), and mass loss was measured while heating the sample at a rate of 3 °C min-1 from room temperature up to 750 °C. Surface Treatment. After contact angle measurement, samples of some of the heat-treated gels found to be hydrophilic were hydrophobized by grafting on trimethylsilyl- or propyldimethylsilyl- groups. Samples (5 × 5 × 2 mm3 foam or 3 cm2 flat) were immersed in 2 mL of a 5% volume solution of trimethylsilyl chloride (TMSCl) (Aldrich, 98%) or propyldimethylsilyl chloride (PDMSCl) (ABCR) in toluene (Fisher p.a.), lightly agitated, and allowed to stand at room temperature for 2 days before being removed and dried at 100 °C for 30 min in a vacuum oven.
Results and Discussion On addition of base to hydrolyzed MTEOS, phase separation and gelation occurred simultaneously, producing gels with pores ranging from a few hundred nanometers up to tens of micrometers in diameter. The materials gelled and phase separated at rates dependent upon the amount and concentration of the ammonia added. When MTEOS materials were prepared using 2.2 and 1.1 M ammonia solution, both types of samples became visibly cloudy after about 150 min at 22 °C. The samples prepared with 2.2 M ammonia became solid 20 min later, while those with 1.1 M ammonia required an additional 45 min. The mixed PTEOS/MTEOS system showed phase separation after approximately 1 min of reaction. Figure 1 shows a set of micrographs of different samples heated to the same temperature to dry and cross-link the
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Table 1. Properties of Different Sol-Gels conditions
properties
composition
conc. of NH4OH/NH3
heating temp/°C
volume loss on heating/%
density of solid/(g cm-3)
pore volume/%
volume change in butanol/%
MTEOS MTEOS MTEOS MTEOS MTEOS MTEOS/PTEOS
1.1 M 1.1 M 1.1 M 1.1 M 2.2 M 22 M
none 300 400 500 225 225
3 31 48 b b
1.2 1.2 1.3 1.7 1.3 0.7
75 75 65 70a 80 59
84 12 1 1 b b
a
Result calculated from shrinkage and solids volume. b Not measured.
Figure 3. Diffuse infrared reflection absorbance spectra of MEOS sol gels heated to different temperatures; all gels were prepared with 1.1 M ammonia solution.
organosilica; the pictures show materials consisting of strings of globules around a network of pores. In this type of system the pores are interlinked, as is the solid. The sizes of the primary particles and pores were, as observed in other systems,28,30 smaller in the sample that set more rapidly (Figure 1a and b). Use of PTEOS required a considerably higher ammonia concentration to produce the same size features as those of the pure MTEOS materials (Figure 1a and c). Figure 2 shows a set of small samples cut from a single monolith that were heated to different temperatures. The particles in the unheated sample had an average diameter of 3.1 µm; after heating to 300 °C, these shrank slightly to 2.8 µm, and further heating to 400 °C caused them to shrink further to 2.4 µm. The piece heated to 750 °C showed further shrinkage of the primary particles, down to 1.8 µm. During heating, only a small amount of macroscopic shrinkage of the materials could be measured up to just below 400 °C; around 400 °C, the samples began to shrink more noticeably (Table 1). At around 600 °C the materials began to darken; they began to lighten again at temperatures slightly above this, but the shrinkage continued. At 750 °C, the materials were white once again. The percentage pore volume also began to change more rapidly at around 400 °C; at this point, the solid material also began to densify. The material derived from a mixture of
PTEOS and MTEOS had a lower solid density and considerably lower macropore volume than that derived from MTEOS alone. This suggests the presence of smaller or closed pores in the material. As can be seen in Table 1, heating the samples reduced their tendency to swell in 1-butanol, and hydrophilic foams (those heated to 400 °C or above) showed insignificant swelling. This suggests that the solid material was not fully cross-linked initially and that the bulk material was hydrophobic. It could also point to the presence of nanoporosity in the material. Cross-linking of the silanol groups appeared to take place up to 400 °C, reducing the tendency of the material to swell, and may be implicated in the hydrophobic to hydrophilic transition that occurred at the same point that swelling stopped. Figure 3 is a set of diffuse infrared reflection spectra showing how the chemical nature of the surface of the material changed on heating. Interpretation can be undertaken using information from previous studies.31 During heating, the siloxane network grew, as indicated by the Si-O-Si bands at around 1050 and 1150 cm-1 that broadened and shifted apart. The O-H band at 3200-3900 cm-1 shifted continually to higher wavenumber with increasing annealing temperature as surface water was lost and hydrogen bonded groups cross-linked, leaving lone hydroxide moieties. Above 400 °C the intensities of bands relating to methyl groups at 13801470 cm-1 and 2900-3000 cm-1, which increased up to this point, began to decline. This continued up to about 750 °C, where the CH3 regions became featureless. From 400 °C the Si-CH3 band region at around 750 cm-1 changed shape, as it decreased in intensity and an SiO-Si band at around 800 cm-1 increased in intensity.
(30) Takahashi, R.; Sato, S.; Sodeawa, T.; Suzuki, K. J. Am. Ceram. Soc. 2001, 84, 1968-76.
(31) Perry, C. C.; Li, X. J. Chem. Soc., Faraday Trans. 1991, 87, 761-766.
Figure 2. Electron micrographs of gold-coated foams taken at a magnification of ×2000 and at 6 kV. Pieces of MTEOS foam with 1.1 M ammonia heated to different temperatures: top left, unheated; top right, 300 °C; bottom left, 400 °C; bottom right, 750 °C.
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Shirtcliffe et al. Table 2. Advancing and Receding Contact Angles of Drops of Water on Organo-silica Flat Surfaces and Foams. Foam 1- MTEOS Sol-Gel Produced Using 1.1 M Ammonia, See Figure 1A: Foam 2- Sol-Gel Produced Using MTEOS and 2.2 M Ammonia, See Figure 1b: Foam 3- Sol-Gel Produced Using PTEOS/MTEOS and 22 M Ammonia, See Figure 1c materials adv. angle, rec. angle, hyst., sample temp/°C posttreatment Θ/deg Θ/deg ∆Θ/deg
Figure 4. Representative thermogravimetric trace during heating of MTEOS, 1.1 M ammonia sol-gel in air at 3 °C min-1.
Figure 4 shows the typical loss of mass during heating of a small sample of a MTEOS sol-gel foam. There are three main regions where changes occurred: from 140 to 155 °C, from 225 to 505 °C, and from 595 to 715 °C. The lowest temperature change was probably due to loss of chemisorbed water, as only changes in the hydroxide stretch region were noted in the infrared spectra. The next, and far more extended change, was probably due to silanol groups binding together; it began around the temperature where this would be expected to and continued over a large temperature range, which was also expected.31 The final mass loss was probably due to the loss of carbon from the material, although dehydroylation of silica materials usually also continues to a small degree.31 The various measurements show that the inorganicorganic sol-gel foams could be heated to about 225 °C without changing much; they then cross-linked and densified slowly upon further heating to about 500 °C. At just below 400 °C, organic groups started to disappear from the surface, perhaps during rearrangement of the gel as it formed Si-O-Si linkages. At temperatures above about 550 °C, the organic material was gradually lost, some of it being initially converted to carbon. Contact Angles. Contact angle data are presented in Table 2. Values of advancing and receding angles measured at the different drop sizes agreed quite closely, as did repeat measurements, although some contact line stick-slip was observed, giving an error in the averages of (2°. It is important to note that the angles at very small drop size varied considerably, but these were not considered in the averages shown. Flat films of methyl silicate materials were hydrophobic (contact angle > 90°) but became more hydrophilic when heated, reaching approximately 90° (neither hydrophobic nor hydrophilic) at 400 °C; the decrease in contact angle continued above this temperature. As infrared spectra indicated that loss of methyl groups from the surface began at around this temperature, this is probably the reason for the change in contact angle. When the materials were made into porous foams, strong contact angle amplification was evident. Foams heated to temperatures below 400 °C were superhydrophobic, defined as advancing contact angles in excess of 150° (sometimes 130°, as there is some disagreement in the literature), and those heated to above 400 °C became hydrophilic and imbibed water until their pores were filled. It is interesting to note that the contact
flat flat flat flat flat flat foam 1 foam 1 foam 2 foam 3 foam 1 foam 1 foam 1
300 400 500 550 500 500 300 300 300 300 400 400 400
none none none none TMSCl PDMSCl none abraded none none none TMSCl PDMSCl
107 90 81 54 97 93 153 156 155 152 absorbed 156 155
87 69 67 31 60 64 137 152 149 126 absorbed 149 148
20 21 14 12 35 29 16 4 6 26 7 7
angle on a flat gel heated to 400 °C is about 90°, and this temperature also appeared to be the critical temperature for foams, as a few samples heated to this temperature remained superhydrophobic but most imbibed water rapidly. Hydrophilic samples treated with TMSCl or PDMSCl showed an increase in their advancing contact angle to values close to those observed on the intrinsically hydrophobic samples. The contact angles measured on the rough samples were more similar to those of their intrinsically hydrophobic counterparts than those of the smooth ones, suggesting that the contact angles on the foams were unaffected by small changes in θ(flat). The receding contact angles on flat surfaces were all lower than 90°, suggesting that the surfaces consisted of polar and nonpolar regions. The flat hydrophilic samples treated with TMSCl or PDMSCl showed higher hysteresis than the untreated samples, suggesting that the spread of surface groups was not as even on these substrates or that the coverage of the organic groups was lower. Wei et al.32 made a study of various silane hydrophobizing agents on glass and conclude that they are gradually hydrolyzed and removed when in contact with water and that incomplete coverage reduces the receding angle but barely affects the advancing angle. The initial coverage of the agents that they used was dependent on the surface treatment method used. Foams generally exhibited considerably reduced contact angle hysteresis compared to the equivalent flat surfaces. In other words, the receding angles were enhanced more than the advancing angles by the roughness. Exceptions to this were foams of types 1 and 3 heated to 300 °C and cut with a razor blade. The contact angle hysteresis could have been caused by chemical heterogeneity of the surface, which could have arisen from a difference in chemical structure between the pore surface and the broken surface of the sample. As foam 1 abraded with sandpaper and foams treated with TMSCl (with high hysteresis when flat) showed low contact angle hysteresis, this is unlikely to have been the case. The PTEOS containing foam (foam 3) showed the greatest contact angle hysteresis, probably due to the lower pore volume of this material. O ¨ ner and McCarthy15 have shown that the receding angle on a superhydrophobic surface is strongly dependent on the pattern on the surface. (32) Wei, M.; Bowman, R.; Wilson, J.; Morrow, N. J. Colloid Interface Sci. 1993, 157, 154-159.
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Patterns where the contact line of a drop cannot sit on a row of peaks or a ridge show higher receding contact angles than those where it can. The extreme case would be a flat surface with pillars compared to the same surface with holes of the same depth as that of the pillars. Both surfaces could be superhydrophobic to advancing drops, but only that with pillars would show a high receding angle. Patterns with lines of pillars are also more likely to cause contact line pinning than more complex patterns. In the case of the foams produced here, foam 3 and foam 1 have similarly sized primary particles but there are fewer pores in foam 3 (Table 1). This could have produced a pattern on foam 3 where the contact line of the receding drop can become pinned on a row of high areas close to one another. The receding angle on the MTEOS foam materials produced using 1.1 molar ammonia (foam 1) was lower on a sample cut with a razor blade than on the same material abraded with sandpaper, on an almost identical material treated with PDMSCl, or on the chemically identical foam 2 cut in the same way. When measuring the electron micrographs of the materials, foam 1 cut with a blade was difficult to image, as parts of the surface moved around as they charged up. We suggest that surface damage during cutting reduced the effective roughness of these samples, but we have been unable to measure this directly. Conclusions Phase-separation foams prepared from MTEOS (methyltriethoxysilane) are porous materials that show intrinsic superhydrophobicity. They can be cut or abraded and
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maintain their high water contact angles, unlike hydrophilic materials coated with hydrophobizing agents or phase-separation sol-gel foams made from mixtures of unsubstituted and substituted reagents. Some of the materials were more robust than others in this respect. This characteristic would allow regeneration of superhydrophobicity on dirty samples by abrading their surfaces. High advancing and receding angles of water on the materials showed that superhydrophobic materials with low contact angle hysteresis could be produced. The water contact angles on the materials could be reduced by heat treatment. Heating the intrinsically hydrophobic materials to 400 °C or above produced hydrophilic materials with similar structure. These materials could be made superhydrophobic once more by treating them with TMSCl or PDMSCl. The water contact angles and contact angle hysteresis on the sol-gel foams appeared to be more strongly determined by the pattern morphology than by the contact angle on an equivalent flat surface if it was greater than 90°. Around θa(flat) ) 90°, or 400 °C, a sudden transition from superhydrophobic to absorption of water occurred. Acknowledgment. The financial support of the U.K. Engineering and Physical Sciences Research Council (EPSRC) and the MOD Joint Grant Scheme under Grant GR/02184/01 and the support of Dr. E. O’Keefe of Qinetiq as coordinator are gratefully acknowledged. LA034204F
