Superhydrophobic Surface Fabricated from Fatty Acid-Modified

May 10, 2010 - The contact angle hysteresis of modified PCC film was investigated. .... sorbent powder for oil spill clean-ups from water and land sur...
0 downloads 3 Views 1MB Size
Download PDF
Ind. Eng. Chem. Res. 2010, 49, 5625–5630

5625

Superhydrophobic Surface Fabricated from Fatty Acid-Modified Precipitated Calcium Carbonate Zeshan Hu and Yulin Deng* School of Chemical and Biomolecular Engineering, IPST at GT, Georgia Institute of Technology, 500 10th Street N.W., Atlanta, Georgia, 30332-0620, U.S.A.

A very low-cost and simple method for fabricating a superhydrophobic surface with commercially available precipitated calcium carbonate (PCC) and fatty acid is reported in this research article. PCC particles were chemically modified from hydrophilic to hydrophobic in water with fatty acid. The thin film formed by these hydrophobic PCCs shows superhydrophobic and self-cleaning properties. The contact angle hysteresis of modified PCC film was investigated. A sliding angle of 1.75° could be obtained with oleic acid-modified PCC film at an oleic acid concentration of 2.52 wt %. Comparing with oleic acid, the PCC film modified by stearic acid has a higher sliding angle (∼10°). 1. Introduction One of the potential applications of the lotus effect is the self-cleaning property due to the rolling-off behavior of a water drop. The sliding angle of a water drop is a quantitative measurement of self-cleaning behavior. It is thought that both superhydrophobicity and low contact angle hysteresis are necessary for self-cleaning surface. Generally speaking, to fabricate a superhydrophobic surface (water contact angle, WCA >150°), nanoscaled roughness and hydrophobic surface (smooth surface contact angle is >90°) are two basic requirements.1 To achieve a high contact angle, many superhydrophobic surfaces are processed with fluorine-containing surfactants.2-5 Although very high contact angles can be obtained by surface modification using fluorine-containing surfactants, the cost of these chemicals is generally high. Reducing the cost, simplifying the fabrication process, increasing the durability of the final products, and using nontoxic materials are a few of the barriers that need to be solved for large-scale manufacturing of superhydrophobic surfaces. The theory of roughness-induced superhydrophobicity was discussed by Nosonovsky and Bhushan6 where Cassie-Baxter and Wenzel equations were used to analyze the contact angle with rough and heterogeneous surfaces. Bhushan and coworkers7 constructed hierarchical roughness and investigated the relation between roughness parameters and contact angle. Hsieh et al.8 reported a modified Cassie-Baxter model to investigate the influence of particle size on the superhydrophobic behavior of silica nanosphere arrays. There has been much attention paid to the formation mechanism of contact angle hysteresis that can be evaluated with the difference between the advancing water contact angle (AWCA) and the receding water contact angle (RWCA) or the difference of their cosine values.9-11 The latter was thought to be more sensitive to the variation of the sliding angle of the rolling-off water drop,12 especially at low contact angle range. The Cassie equation (eq 1) states that the apparent contact angle of a liquid on a heterogeneous surface where two components are present is the weighted average of the contact angles of each of them: cos θc ) f1 cos θ1 + f2 cos θ2

(1)

where θ1 and θ2 are Young’s contact angle over the flat surface of component 1 and 2, respectively, f1 and f2 are the respective * To whom correspondence should be addressed. E-mail: [email protected].

surface fractions. All of these researches indicated that surface roughness, especially a roughness with hierarchical structure, is very important and high-energy sites of the surface are disadvantageous to the fabrication of the self-cleaning (rollingoff drops) surface. A superhydrophobic surface with a contact angle of higher than 150° does not guarantee the surface is a self-cleaning one.13,14 To achieve a self-cleaning surface, the surface not only has to be surperhydrophobic but also has to have a low contact angle hysteresis so the liquid droplets can easily roll off. Many researches have been focused on the fabrication of superhydrophobic surface with self-cleaning characteristic. Lee and Hwang15 prepared ultralow contact angle hysteresis over poly(tetrafluoroethylene) where template and replica technology was used to provide surface roughness using nanoporous alumina. Kusumaatmaja and Yeomans investigated the influence of roughness structure on hysteresis with numerical simulations method.16 Bhushan and co-workers17 investigated self-cleaning properties of a specimen, prepared with replication of lotus leaf. The influence of contact angle hysteresis on the rolling-off of water droplets was studied at different tilt angles. In Xiu and colleagues’ work,18 inorganic superhydrophobic, low surface energy silica coatings were prepared using sol-gel processing with tetraethoxysilane and trifluoropropyltrimethoxysilane as precursors. A contact angle of 172° and a contact angle hysteresis of 2° were obtained. Nimittrakoolchai and colleagues19 fabricated superhydrophobic surfaces through surface roughness enhancement by acid etching followed by surface modification with SiO2 nanoparticles and semifluorinated silane, where self-cleaning behavior of the coating layer was measured. Three types of superhydrophobic surfaces were fabricated by Reiner Furstner and colleagues.20 They found that water drops rolled off easily from silicon samples with a high spikes and an appropriate pitch. In the work of Nakajima and colleagues21 transparent superhydrophobic thin films with TiO2 photocatalyst were prepared by utilizing a sublimation material and subsequent coating of a (fluoroalkyl)silane. The water contact angle after 1800 h of outdoor exposure was investigated. Stain was thought to be the main reason for the water contact angle decrease. Plasma technique was widely used to construct roughness over polymer22 and cellulose materials.14 Mundo and colleagues23 found that contact angle hysteresis was sensitive to the power of the plasma. Wang and colleagues24 fabricated superhydrophobic calcium carbonate with self-cleaning characteristics with

10.1021/ie901944n  2010 American Chemical Society Published on Web 05/10/2010

5626

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010

Figure 1. Hierarchical structure comparison of a lotus leaf and PCC.

the use of nanoparticle calcium carbonate with a size of 40 nm and fluorine-containing compounds. Precipitated calcium carbonate (PCC) is a low-cost and widely used filler in papermaking, coating, composites, etc. A commercially available PCC possesses a hierarchical structure similar to that of lotus leaf. This research aims at fabricating a superhydrophobic surface using very low-cost PCC modified with fatty acids and investigating their self-cleaning characteristics. 2. Experimental Section 2.1. PCC Modification. In a typical modification process 20 g of dry powder of the PCC (Scalenohedral Albacar HO, Specialty Mineral Inc., U.S.A.) was well dispersed in 45 mL of DI water at 75 °C followed by adding 0.422 g of oleic acid (Alfa Aesar, reagent) under agitation. The suspension was continually stirred for 0.5 h to allow fatty acid to form waterinsoluble calcium salt on the PCC surface. The mixture was oven-dried at 120 °C. Both oleic acid and stearic acid as well as their calcium salts are insoluble in water Water solubility of oleic acid is higher than stearic acid. In order to investigate the influence of water solubility, the modification of PCC with stearic acid was also conducted in hexane rather than in water. The dry PCC powder was added to the stearic acid-hexane solution under agitation for 0.5 h at 50 °C. Finally, the solvent was removed by distillation, and the sample was dried at 120 °C for 2 h. In addition to commercial PCC, laboratorysynthesized PCC was also used. In a typical PCC synthesis process, 0.2 mol/L Na2CO3 and 0.4 mol/L CaCl2 were mixed at a molar ratio of 1:1 at room temperature with agitation. After aging for 1 h the precipitate was filtrated and washed with DI water and was dried at 120 °C for 2 h. The surface modification of laboratory-synthesized PCC with stearic or oleic acid is the same as that used for commercial PCC particles. A double pressure sensitive adhesive tape with one side sticking on a glass slice was used as the substrate. The modified PCC particles were spread on the surface of the adhesive tape and gently pressed with a glass cover. After pressing, the slide was flushed with air under pressure to remove free PCC particles. This process, including powder spreading, pressing, and air flushing were repeated several times until the adhesive tape was fully covered by PCC particles. 2.2. Measurement of Water Contact Angle. Water contact angle of the modified PCC on adhesive tape was measured with a FTÅ200 Dynamic contact angle analyzer (First Ten Ång-

stroms, U.S.A.). A DI water drop (4 µL) was first hanged on the syringe needle above the test sample. The PCC sample on the adhesive tape surface, which was supported on a movable stage, was raised until it contacted the water drop, and then another 1 µL of DI water was pulled out from the syringe. The photo of the water drop was taken by a camera of the instrument. The image was analyzed by software, and the water contact angle was obtained. Such obtained WCA was taken as AWCA. For the measurement of receding contact angle, 5 µL of DI water drop was first contacted with the sample. After pulling out another 5 µL of DI water the water drop was then slowly sucked by the syringe until 5 µL of a DI water drop was formed. A picture was taken, the image was analyzed again, and the WCA obtained in this way was used as RWCA. All of WCA was a result of an average of six measurements, and an error bar represents the standard deviation from six repeated measurements. In order to measure the sliding angle, a slide of the sample was supported on two stages at two ends. One of them was fixed, and the other one could be raised. A 15 µL drop of DI water was placed on the specimen. One end of the slide was raised manually and slowly (average 2°/min in the range of 0°-15° at 5°/min in the range of 15°-90°). Once the drop began sliding, the stage movement was stopped, and the tilted angle was measured. Six sliding angle measurements were taken for each sample, and the average of the repeated measurements was reported. 2.3. Characterization. Morphology of the PCC particle was visualized with a Hitachi S-800 SEM using an acceleration voltage of 12 kV. FTIR spectra of the samples were characterized with a Magna-IR spectrometer 550. The spectra were recorded with a transmission mode. A specimen slice was prepared by pressing a mixture of the sample and KBr powder in a mold using a piston. 3. Results and Discussion 3.1. Hierarchical Structure of Commercial PCC. Scalenohedral PCC has been widely used as filler in papermaking where a hierarchical structure can increase brightness and opacity of paper because of its higher reflection surface area than general rhombohedral calcite. The morphology of the PCC and a lotus leaf are compared in Figure 1. The particle on the surface of a lotus leaf possesses a size of ∼10 µm with some convexes of ∼1 µm. Similarly, the PCC particle shows an average size of 2 µm with some branches of ∼150 nm.

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010

5627

Figure 2. Dependency of superhydrophobicity of PCC on oleic acid content.

Figure 3. Image comparisons of AWCA and RWCA.

Therefore, theoretically it is possible that the submicrometer hierarchical structure of Albacar HO PCC can be used for surface roughness modification in superhydrophobic surface fabrication. 3.2. Superhydrophobicity of Modified PCC. AWCAs and RWCAs of the modified PCC are given in Figure 2 a. With the increase of oleic acid content in modified PCC both AWCA and RWCA show a maximum value. The maximum AWCA is 164°, corresponding to the optimal content of oleic acid of 2.15 wt %. The contact angle hysteresis, which is the difference between AWCAs and RWCAs is given in Figure 2b. A hysteresis of nearly zero was obtained at the optimal content. The water drop images for the measurement of AWCA and RWCA are given in Figure 3. A slight difference of contact area between AWCA and RWCA drops can be found at the content of 1.09 wt %, which corresponds to a contact angle hysteresis of 2.5°. No evident difference between the images of AWAC and RWAC can be found for the content of 2.51 wt %. It has been known that calcium oleate is a weak hydrophobic precipitate with a WCA of 92°.25 However, for a fully covered surface with uniform -CH3 groups the WCA is 108°.26 Therefore, a uniform monolayer adsorption of oleate molecules on a PCC surface with a good orientation should have a higher hydrophobicity than the aggregated calcium oleate on the PCC surface. The rolling-off sliding angle of the 15-µL DI water drop is also given in Figure 2b. A minimum value of the sliding angle can be found. Although both contact angle hysteresis and the sliding angle show a minimum value, the change in the sliding angle is more marked than that of the contact angle hysteresis as the oleic acid content increases. The sliding angle for oleic acid content of 0.375 wt % could not be measured because the drop stuck completely to the surface. It seems that the sliding angle is more sensitive to oleic acid content than is contact angle hysteresis.

Figure 4. Calculated Young’s contact angle.

The Cassie-Baxter equation has been widely used to study the relation between apparent WCA and surface roughness: cos θa ) f(cos θy + 1) - 1

(2)

where f is the solid surface fraction, (1 - f) is the air surface fraction with water, θais the apparent contact angle, and θyis the Young’s contact angle on a flat/smooth surface. Therefore, the solid surface fraction or surface roughness f only depends on the surface structure. For a given structure, f is a constant. If we assume the specimen with the maximum AWCA has a monolayer adsorption of fatty acid, its θycan be taken as 108°.26 The solid surface fraction f, calculated form eq 2 is 0.0556. The Young’s contact angle θy for all samples can be calculated from eq 2 if the solid surface fraction of 0.0556 is a constant for all samples. The results are given in Figure 4 where the value of 92° shown by the dashed line is the WCA of solid calcium oleate,25 which was measured using a saturated aqueous

5628

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010

Figure 5. SEM image of self-synthesized PCC.

Figure 6. Dependency of the hydrophobicity of synthesized PCC on oleic acid content. Table 1. Fraction of Real Contact Area of Water Drop to Solid Projected Areaa PCC

oleic acid content (wt %)

advancing WCA (deg)

solid surface fraction, f

Albaca HO self-synthesized

2.16 2.11

164 150

0.0556 0.193

a

Calculated from Cassie’s Law.

solution of calcium oleate. The last two points obtained from the PCC modified with high fatty acid content, however, shows a low WCA which is even lower than that of pure calcium oleate. In terms of eq 1, a smooth surface consisting of two surface components will give an apparent WCA between the values of two pure components. Therefore, these results suggest that the excessive calcium oleate may possess an orientation with the hydrophilic calcium carboxylate group toward the surface. In this case the θy will be lower than 92°. The above results suggest that excessive oleate should be avoided in the process of superhydrophobic PCC.

Figure 8. FTIR spectrum of modified PCC.

3.3. Importance of Hierarchical Structure on Superhydrophobicity. In order to study the effect of surface roughness and morphology of PCC on the superhydrophobicity, calcite calcium carbonate particles were synthesized in the laboratory using a homogeneous precipitation reaction of calcium chloride and sodium carbonate aqueous solution. As shown in Figure 5, the PCC particle shows flat morphology, and no evident hierarchical structure can be found. The WCA of the PCC modified with the same procedure and chemical is given in Figure 6. Similarly a maximum WCA can be found. However, comparing with the results of Figure 2, both AWCA and RWCA are smaller, and the contact angle hysteresis is much higher than that of modified scalenohedral Albacar HO PCC. The sliding angle of the water drop on the surface could not be measured for all of these samples due to the drop sticking onto the surface. These results suggest a poor self-cleaning behavior of the synthesized PCC compared to that of the modified scalenohedral Albacar HO PCC. The fraction

Figure 7. Dependency of superhydrophobicity on stearic acid content and the effect of the modification method.

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010

5629

Figure 9. Comparison of self-cleaning behavior of modified PCC.

of real contact area of the water drop to solid projected area was calculated with Cassie’s Law. The results are given in Table 1. It can be found that the solid surface fraction f of Albacar HO PCC is much smaller than that of synthesized PCC in our laboratory. These results demonstrate the hierarchical structure is very important for the fabrication of a superhydrophobic surface with self-cleaning function. 3.4. Influence of Fatty Acid Chemistry on Superhydrophobicity. Stearic acid was used to replace oleic acid in the modification of Albacar HO PCC with the same modification procedure and conditions. The results of WCA and hysteresis are given in Figure 7. Both AWCA and RWCA show tendencies to similar to that of PCC modified with oleic acid. However, both advancing and receding contact angles are evidently smaller, and the hysteresis and sliding angle are slightly higher than those of oleic acid-modified PCC, especially in low fatty acid content range. The fatty acid content for the maximum WCA value is evidently higher than that of the PCC modified with oleic acid. The above results cannot be explained simply by the hydrophobicity of fatty acid molecules because oleic acid should not be more hydrophobic than stearic acid. A possible explanation for the above results might be that a lower solubility of stearic acid in water than that of oleic acid resulted in the formation of calcium salt aggregates of stearic acid, which is disadvantageous to the formation of a uniform adsorption layer of calcium fatty acid salt on PCC surface. To reduce the fatty acid aggregate formation, a good solvent for stearic acid, hexane, was also used in the PCC modification treatment. The results are given Figure 7b where almost zero hysteresis and sliding angle were obtained. FTIR spectra of related specimen are given in Figure 8. No characteristic absorption peaks of stearic acid can be found in all modified PCCs, which suggests that stearic acid has either chemically or physically bonded to the PCC surface. In the modification of aqueous solution, peak 1580 cm-1 corresponds to the C-O stretching vibrations of the carboxylate group.27 The adsorption near 1100 cm-1 is assigned to the O-Ca band of calcium stearate,28 which suggests the formation of calcium stearate. For the sample prepared in a nonaqueous solvent, characteristic absorption of both stearic acid and stearate cannot be found, which probably is overlapped by the absorption of PCC. Osman and colleagues29 modified calcite at 80 °C in toluene, which is similar to our modification in hexane. 3.5. Comparison of Self-Cleaning Properties of Modified PCC. The force needed to start a water drop moving over a solid surface was described by the following equation:30 F ) γLV(cos θR - cos θA)

(3)

where γLVis the surface tension of the liquid. Therefore, selfcleaning properties can be evaluated with cos θR - cos θA. The

hystereses of the contact angles for different PCCs are given in Figure 9. It can be seen that the oleic acid-modified Albacar HO PCC gives much lower hysteresis due to its hierarchical structure than does the PCC synthesized in the laboratory. The hysteresis of the latter is very high so that a sticky of water drop was formed. Furmidge31 derived an equation, which gives the conditions that a drop will remain stuck on the surface: πbγLV(cos θR - cos θA) g FgVsin R

(4)

where b is the radius of the solid-liquid contact, R is the angle of inclination, F the liquid density, g is the gravity acceleration, and V the drop volume. Although many self-cleaning surfaces have been reported in literature, generally speaking, the processes and materials adopted in these approaches are relatively complicated with high cost. For our case, eq 4 can be simplified as: 15.38b(cos θR - cos θA) g sin R

(5)

where b is the radius of the solid-water contact with a unit of cm, which will dramatically decrease with the increase of WCA over a hydrophobic surface. The sliding angle tendency shown in Figure 9b fits well to the hysteresis results shown in Figure 9a within the experimental error. However, a sliding angle calculated from eq 5 is much smaller than the experimental data. It can be found that Albaca HO PCC modified with stearic acid can give a sliding angle less than 10° in a wide range of stearic acid content. 4. Conclusion Commercial Albaca HO PCC that possesses a hierarchical rough structure can be used for fabricating a superhydrophobic surface. Surface treatment with fatty acid is important and can convert a PCC surface from hydrophilic to hydrophobic. Oleic acid-modified scalenohedral Albacar HO PCC can achieve a sliding angle of 1.75°, and stearic acid-modified scalenohedral Albacar HO PCC has a sliding angle ∼10° for a wide range of stearic acid content. Literature Cited (1) Zhang, X.; Jarn, M.; Peltonen, J.; Pore, V.; Vuorinen, T.; Levanen, E.; Mantyla, T. Analysis of Roughness Parameters to Specify Superhydrophobic Antireflective Boehmite Films made by the Sol-Gel Process. J. Eur. Ceram. Soc. 2008, 28, 2177. (2) Xu, W. G.; Liu, H. Q.; Lu, S. X.; Xi, J. M.; Wang, Y. B. Fabrication of Superhydrophobic Surfaces with Hierarchical Structure through a Solution-Immersion Process on Copper and Galvanized Iron Substrates. Langmuir 2008, 24, 10895.

5630

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010

(3) Kim, D.; Hwang, W.; Park, H. C.; Lee, K. H. Superhydrophobic Nanostructures Based on Porous Alumina. Curr. Appl. Phys. 2008, 8, 770. (4) Goncalves, G.; Marques, P. A. A. P.; Trindade, T.; Pascoal, C.; Gandini, A. Superhydrophobic Cellulose Nanocomposites. J. Colloid Interface Sci. 2008, 324, 42. (5) Li, S. H.; Zhang, S. B.; Wang, X. H. Fabrication of Superhydrophobic Cellulose-based Materials through a Solution-Immersion Process. Langmuir 2008, 24, 5585. (6) Nosonovsky, M.; Bhushan, B. Roughness-induced Superhydrophobicity: a way to Design Non-adhesive Surfaces. J. Phys.: Condens. Matter 2008, 20, 225009. (7) Bhushan, B.; Koch, K.; Jung, Y. C. Biomimetic Hierarchical Structure for Self-cleaning. Appl. Phys. Lett. 2008, 93, 093101. (8) Hsieh, C. T.; Chen, W. Y.; Wu, F. L.; Shen, Y. S. Fabrication and Superhydrophobic Behavior of Fluorinated Silica Nanosphere Arrays. J. Adhes. Sci. Technol. 2008, 22, 265. (9) Johnson, R. E.; Dettre, R. H. Contact Angle Hysteresis. 3. Study of an Idealized Heterogeneous Surface. J. Phys. Chem. 1964, 68, 1744. (10) Dettre, R. H.; Johnson, R. E. Contact Angle Hysteresis. 4. Contact Angle Measurements on Heterogeneous Surfaces. J. Phys. Chem. 1965, 69, 1507. (11) Morra, M.; Occhiello, E.; Garbassi, F. Contact Angle Hysteresis on Oxygen Plasma Treated Polypropylene Surfaces. J. Colloid Interface Sci. 1989, 132, 504. (12) Xiu, Y. H.; Zhu, L. B.; Hess, D. W.; Wong, C. P. Relationship between Work of Adhesion and Contact Angle Hysteresis on Superhydrophobic Surfaces. J. Phys. Chem. C. 2008, 112, 11403. (13) Balu, B.; Kim, J. S.; Breedveld, V.; Hess, D. W. Tunability of the Adhesion of Water Drops on a Superhydrophobic Paper Surface via Selective Plasma Etching. J. Adhes. Sci. Technol. 2009, 23, 361. (14) Balu, B.; Breedveld, V.; Hess, D. W. Fabrication of “Roll-off” and “Sticky” Superhydrophobic Cellulose Surfaces-via Plasma Processing. Langmuir 2008, 24, 4785. (15) Lee, S.; Hwang, W. Ultralow Contact Angle Hysteresis and NoAging Effects in Superhydrophobic Tangled Nanofiber Structures Generated by Controlling the Pore Size of a 99.5% Aluminum Foil. J. Micromech. Microeng. 2009, 19, 035019. (16) Kusumaatmaja, H.; Yeomans, J. M. Modeling Contact Angle Hysteresis on Chemically Patterned and Superhydrophobic Surfaces. Langmuir 2007, 23, 6019. (17) Bhushan, B.; Jung, Y. C.; Koch, K. Self-Cleaning Efficiency of Artificial Superhydrophobic Surfaces. Langmuir 2009, 25, 3240. (18) Xiu, Y. H.; Hess, D. W.; Wong, C. P. UV-Resistant and Superhydrophobic Self-Cleaning Surfaces Using Sol-Gel Processes. J. Adhes. Sci. Technol. 2008, 22, 1907.

(19) Nimittrakoolchai, O. U.; Supothina, S. Deposition of organic-based superhydrophobic films for anti-adhesion and self-cleaning applications. J. Eur. Ceram. Soc. 2008, 28, 947. (20) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and Self-cleaning Properties of Artificial Superhydrophobic Surfaces. Langmuir 2005, 21, 956. (21) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent Superhydrophobic thin Films with Selfcleaning Properties. Langmuir 2000, 16, 7044. (22) Morra, M.; Occhiello, E.; Garbassi, F. Contact Angle Hysteresis in Oxygen Plasma Treated Poly(tetrafluoroethylene). Langmuir 1989, 5, 872. (23) Di Mundo, R.; Palumbo, F.; d’Agostino, R. Nanotexturing of Polystyrene Surface in Fluorocarbon Plasmas: From Sticky to Slippery Superhydrophobicity. Langmuir 2008, 24, 5044. (24) Wang, H.; Tang, L. M.; Wu, X. M.; Dai, W. T.; Qiu, Y. P. Fabrication and Anti-frosting Performance of Super Hydrophobic Coating Based on Modified Nano-Sized Calcium Carbonate and Ordinary Polyacrylate. Appl. Surf. Sci. 2008, 253, 8818. (25) Luangpirom, N.; Dechabumphen, N.; Saiwan, C.; Scamehorn, J. F. Contact Angle of Surfactant Solutions on Precipitated Surfactant Surfaces. J. Surfactants Deterg. 2001, 4, 367. (26) Johnson, R. E.; Dettre, R. H.; Brandreth, D. A. Dynamic Contact Angles and Contact Angle Hysteresis. J. Colloid Interface Sci. 1977, 62, 205. (27) Dhanabalan, A.; Mello, S. V.; Oliveira, O. N. Preparation of Langmuir-Blodgett Films of Soluble Polypyrrole. Macromolecules 1998, 31, 1827. (28) Wang, C. Y.; Xu, Y.; Liu, Y. L.; Li, J. Synthesis and Characterization of Lamellar Aragonite with Hydrophobic Property. Mater. Sci. Eng., C 2009, 29, 843. (29) Osman, M. A.; Suter, U. W. Surface Treatment of Calcite with Fatty Acids: Structure and Properties of the Organic Monolayer. Chem. Mater. 2002, 14, 4408. (30) Wolfram, E.; Faust, R. In Wetting, Spreading and Adhesion; Padday, J. F., Ed.; Academic Press: London, 1978; pp 213-215. (31) Furmidge, C. G. Studies at Phase Interface. 1. Sliding of Liquid Drops on Solid Surfaces and a Theory for Spray Retention. J. Colloid. Sci. 1962, 17, 309.

ReceiVed for reView December 8, 2009 ReVised manuscript receiVed March 26, 2010 Accepted April 14, 2010 IE901944N