Spontaneous Formation of Super Water-Repellent Fractal Surfaces in

14620

J. Phys. Chem. B 2008, 112, 14620–14627

Spontaneous Formation of Super Water-Repellent Fractal Surfaces in Mixed Wax Systems† Takayuki Minami,‡,§ Hiroyuki Mayama,‡ and Kaoru Tsujii*,‡,| Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido UniVersity (CRIS Building), N-21, W-10, Kita-ku, Sapporo 001-0021, Japan, Graduate School of Science, Hokkaido UniVersity, Japan, and CREST, JST, Japan ReceiVed: March 15, 2008; ReVised Manuscript ReceiVed: May 8, 2008

Alkylketene dimers (AKDs) and triglyceride waxes form fractal surfaces spontaneously and show super waterrepellent property. Spontaneous formation of fractal structures on their surfaces takes place when the metastable crystalline phase of the waxes transforms to the thermodynamically stable form of crystal. The triglyceride waxes form the meta-stable R-phase in whole specimen when solidified from their melt. In the case of AKD, on the other hand, only a small portion of the specimen solidifies in the meta-stable form of crystal. The surface of the AKD, however, becomes fractal in the whole part. We have, thus, examined the fractal structure formation in the mixed wax systems in which one wax forms fractal surfaces and the other one does not. In the stearic acid/tristearin mixed system as a typical one, the super water-repellent fractal surfaces form in the higher composition region of tristearin than that of the eutectic point in their mixture. 1. Introduction Super water- and/or oil-repellent surfaces have recently attracted much attention from both viewpoints of scientific interest and practical applications.1–26 Wettability of solid surfaces with liquids is governed by two factors, i.e., chemical and geometrical factors.27 The chemical factor determines the contact angle of a liquid on a flat surface, and the geometrical (roughness) factor enhances the wetting. Super water-repellent surfaces which show the contact angle greater than 150° must possess two factors of the above, since the contact angle cannot exceed 120° when the surface is flat.28 So, the roughness in solid surface is very important to realize any super liquidrepellent surfaces. Fractal is an ideal rough structure for the above purpose. The fractal surfaces possess very large real surface area compared with the projected one and show super water- and/or oil-repellent properties.1–10 Both waxes of alkylketene dimers (AKDs) and triglycerides interestingly form the fractal surfaces spontaneously.1,2,8,9 The AKD fractal surface was found to give the contact angle of 174° for water.1,2 The mechanism of spontaneous formation of fractal surfaces of the above two waxes has been studied and elucidated as follows: the wax must form a meta-stable crystalline phase when solidified from its melt, and then the fractal surfaces are formed spontaneously during the phase transition from a meta-stable to a stable crystalline form.8–10 We have found an empirical general rule of the above without any exceptions at least for the 15 wax samples tested.10 In the case of triglycerides, the whole wax specimen solidifies once to the meta-stable R-phase from its melt and then transforms to the thermodynamically stable β-crystal.8 In the AKD wax, on the other hand, the whole surface becomes a super water-repellent fractal one regardless of the transformation of only a small portion of the sample to the meta-stable crystal † Part of the “Janos H. Fendler Memorial Issue”. * To whom correspondence should be addressed. E-mail: tsujik@ es.hokudai.ac.jp. Phone: +81-11-706-9356. Fax: +81-11-706-9357. ‡ Research Institute for Electronic Science, Hokkaido University. § Graduate School of Science, Hokkaido University. | CREST, JST.

when solidified from the melt.9 This result would give us a quite interesting way to obtain the fractal surfaces and to control the fractal structures as well as the water repellency on wax surfaces. If we mix a small amount of a fractal-forming wax with an ordinary one which does not form the fractal structure, we may be able to have the fractal structures on the whole wax surfaces. This paper deals with the spontaneous formation of super waterrepellent fractal surfaces in mixed wax systems to confirm the above hypothesis. 2. Experimental Section 2.1. Materials. Tristearin was purchased from Tokyo Kasei Ltd. and used without further purification. The tristearin was employed as a wax sample which formed the fractal surfaces spontaneously. Fatty acid samples (lauric, 99.0% pure; myristic, 98.0%; palmitic, 95.0%; stearic, 95.0%; and behenic acid, 80.0%) were obtained from Wako Pure Chemical Co. and used as received. They were wax samples forming no fractal surface. Water used was ultrapure water treated with a membrane system (Milii-Q, Millipore Corporation). 2.2. Preparation of Sample Surfaces. A mixed sample of tristearin and a fatty acid (total amount of 200 mg) was put on a slide glass and heated to the melting point on a hot plate (EYELA RCX-1000H). The molten sample was fully mixed with a spatula, and then the sample on the slide glass was moved on a Petri dish which was precooled on an ice/water mixture. The sample wax was solidified quickly and then annealed at just below the melting temperature for about 1 week unless otherwise stated. The annealing temperatures were 35, 40, 40, 55, and 60 °C for the mixed samples of lauric, myristic, palmitic, stearic, and behenic acid with tristearin, respectively. The stable crystalline phase was formed during this annealing operation. 2.3. Measurements. Differential scanning calorimetry (DSC) was performed with a DSC apparatus (Rigaku Thermo Plus 2 DSC-8230). A mixed sample (5-10 mg) annealed as mentioned above was put in an aluminum cell and sealed tightly. The sample in the cell was heated and cooled at the rate of 2 °C/ min. The experimental runs were carried out twice. The second run was carried out just after the first run was finished, i.e., just

10.1021/jp802268j CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008

Formation of Fractal Surfaces in Mixed Wax Systems

J. Phys. Chem. B, Vol. 112, No. 46, 2008 14621

Figure 1. Some examples of DSC thermograms observed for the mixtures of stearic acid and tristearin. The sample was annealed at 55 °C for about 1 week for the first run, and the second run was carried out just after the first run was finished. The concentration put on each figure is that of tristearin in the wax mixture.

14622 J. Phys. Chem. B, Vol. 112, No. 46, 2008

Minami et al. Contact angle measurements were carried out with an optical contact angle meter (DropMaster 300, Kyowa Interface Science Co., Ltd.) at room temperature. A water droplet having a diameter of 1-2 mm was employed for the measurements. Five droplets were put on different places of a wax surface, and the five data points were averaged as a contact angle value. Surface structures of the wax samples were observed with a scanning electron microscope (Hitachi FE-SEM S-5200) after annealing for about 1 week. To set the sample on an aluminum SEM stage, a thin wax sample was pasted on the stage with a conductive carbon tape. Au-Pd alloy was sputtered for 90 s onto the sample surface with an ion-sputter (Hitachi type E-1030) before SEM observation. Fractal analysis for the cross-sections of the wax samples was made by the box-counting method. A wax sample was cut to be a square of about 5 mm × 5 mm and pasted perpendicularly on a SEM stage. The surface of a crosssectional image was traced at some different magnifications, and the trace curve was analyzed by the box-counting method.1,2 The fractal dimension of the cross-section, Dcross, was calculated from eq 1.

N(r) ∝ r-Dcross

(1)

where r, N(r), and D are the size of the box, the number of boxes occupied by the trace curve, and the fractal dimension, respectively. The fractal dimension of the surface, D, can be approximately written as D ) Dcross + 1.

Figure 2. Phase diagrams for the annealed mixtures of tristearin with stearic (a), lauric (b), and behenic acid (c). The diagrams of (a) and (c) are typical eutectic ones.

after solidified from the melt. The melting point was determined from the intersection between the straight line along the growing endothermic peak and the baseline. An X-ray diffraction (XRD) experiment was performed with an XRD apparatus of Rigaku RINT 2200 at room temperature (ca. 25 °C). Wax samples were ground with a mortar and put on a glass plate (5.0 cm × 3.5 cm × 0.15 cm). The glass plate was set on the sample holder of the XRD apparatus. Diffracted X-ray was detected with the scanning rate of 1°/min within the range of 2θ ) 15°∼30°. The measurements were performed under the conditions of 40 kV acceleration voltage, 40 mA current, and 4 accumulation time.

3. Results 3.1. DSC Thermograms and the Phase Diagram. Figure 1 shows some examples of DSC thermograms observed for the mixtures of stearic acid and tristearin. Both DSC curves in the first and second runs are the same for the pure stearic acid (0 mol % sample). The sample for the first run was annealed at 55 °C for about 1 week and was a thermodynamically stable one. The second run was carried out just after the first run was finished, and the sample was the freshly solidified wax from the melt. The DSC curves for the pure tristearin (100 mol % sample), however, are completely different from those of stearic acid. The endothermic and the successive exothermic peaks at lower temperatures than the normal melting point were ascribed to the melting of the meta-stable R-form and the following transition to the stable β-crystal.8,10 So, one can see the existence of the meta-stable form of the crystal from these peaks below the normal melting temperature. We have measured the DSC curves for 14 mixed samples and have drawn a phase diagram (Figure 2a). The phase diagram obtained was a typical eutectic one. The eutectic point was around 25-28 mol % of tristearin. We have made similar phase diagrams also for the mixed systems of lauric acid/tristearin and behenic acid/tristearine as exhibited in Figures 2b and c, respectively. 3.2. X-ray Diffraction. Figure 3 shows some examples of XRD patterns observed for the mixtures of stearic acid and tristearin. The left side and the right side figure in each XRD pattern indicate the XRD result for the sample just after solidification and after annealing for about 1 week, respectively. One can see the completely different patterns in the left side and the right side figure for the tristearin wax (100 mol % sample). This result means that the meta-stable R-phase (left side figure) transforms to the stable β-form during the annealing process.8 For the stearic acid sample (0 mol %), on the other hand, both patterns are essentially the same indicating that the stable crystalline form is obtained even just after solidified from its melt.



Figure 3. Some examples of XRD patterns observed for the mixtures of stearic acid and tristearin. The left and the right side figure in each XRD pattern indicate the XRD result for the sample just after solidification and after annealing for about 1 week, respectively. The concentration put on each figure is that of tristearin in the wax mixture.


Minami et al.

Figure 4. Half-height width of the peak at 2θ ) 21.5° plotted against the composition of the mixed wax.

Figure 6. Contact angles of the water droplet on the mixed wax (stearic acid/tristearin) surfaces as a function of time after the solidification of the waxes from their melt. The compositions denoted in the insets of the figures are the mole percent of the tristearin in the wax mixtures.

Figure 5. Contact angles of the water droplet on the mixed wax samples annealed for about 1 week plotted against the composition of the wax samples.

We can recognize the meta-stable crystal of tristearin from the strong peak at 2θ ) 21.5° as mentioned previously.8 But unfortunately, the peak at 2θ ) 21.5° overlaps a peak of stearic acid. To distinguish these two peaks, the half-height width of the peak at 2θ ) 21.5° was plotted against the composition of the mixed wax in Figure 4. One can see the steep increase in the half-height width at about 24 mol %. This result means that the wax crystal just after solidification is mainly the metastable R-phase of tristearin when the composition is greater than 24 mol %. 3.3. Contact Angles. Figure 5 shows the contact angles of a water droplet on the mixed wax samples annealed for about 1 week plotted against the composition of the wax samples. Let us look at the mixture of stearic acid and tristearin as a typical case. The contact angle increases gradually with increasing composition up to 24 mol %. At the composition of 26 mol %, however, the contact angle suddenly jumps to a larger value than 150° and shows super water repellency. It is interesting to note that the composition of 26 mol % agrees well with the eutectic point in Figure 2a. Basically similar behaviors were observed also in other mixed systems, but some features were different in each mixture. In the mixtures of lauric acid and tristearin, the surfaces did not show super water repellency even at 86 mol % of tristearin. The contact angles increase with increasing alkyl chain length of the fatty acids in their mixtures with tristearin. In the mixed system of behenic acid and tristearin, the super water-repellent surfaces were obtained above the 40 mol % composition which agrees fairly well with the eutectic point again.

The contact angles of water on the mixed wax (stearic acid/ tristearin) surfaces were plotted as a function of time after the solidification of the waxes from their melt in Figure 6. The temperature was kept constant at 55 °C during the above process. It can be seen from the figure that it takes longer to obtain the super water-repellent surface (contact angle >150°) for the higher composition of tristearin. This may be due to the higher melting point of the mixed wax of higher composition of tristearin. 3.4. SEM Observations of the Mixed Wax Surfaces and Fractal Analysis of the Surfaces. Figure 7 shows some examples of the SEM images of the mixed wax (stearic acid/ tristearin) surfaces annealed. The rough part in the surfaces increases with increasing composition of tristearin and then shows complete roughness on the whole part of the surface above 26 mol % composition. It is interesting to note that the morphology of the sample surface at 24 mol % is much different

Figure 7. Some examples of the SEM images of the mixed wax (stearic acid/tristearin) surfaces annealed. The rough part in the surfaces increases with increasing composition of tristearin and then becomes rough completely on the whole part of the surface above 26 mol %. The concentration put on each figure is that of tristearin in the wax mixture.


Figure 8. Cross-sectional views and their trace curves of a rough surface at several magnifications for the sample of 43 mol % of tristearin in the mixture of tristearin/stearic acid.

from others. The eutectic mixture may result in this unique morphology. Cross-sectional views and their trace curves of a rough surface at several magnifications are shown in Figure 8 for the sample of 43 mol % as an example. The box-counting method was applied to these trace curves, and the fractal dimension was determined. The results were shown in Figure 9. The fractal dimensions of the trace curves are around 1.2; i.e., the fractal dimensions of the surfaces are ∼2.2 for the samples above 26 mol %. The sample surface at 7 mol % is essentially twodimensional. 4. Discussion 4.1. Formation of Super Water-Repellent Fractal Surfaces in Eutectic Systems. As shown previously in Figures 2a and 5, the contact angle on the surface of the stearic acid/tristearin


Figure 10. Contact angle vs mixed composition plot for the stearic acid/tristearin system drawn on the phase diagram of them.

Figure 11. SEM image of the surface of the lauric acid/tristearin mixture (74 mol % of tristearin). No rough structure was observed on the surface.

system increases abruptly at about the eutectic composition. We have plotted the contact angle values on the phase diagram in Figure 10 for the stearic acid/tristearin mixed system. One can clearly see that the super water-repellent fractal surfaces form spontaneously in the right-hand side (higher composition side of tristearin) from the eutectic point.

Figure 9. Plot of log N(r) vs log r (the box-counting) for the cross-sectional trace curves of rough surfaces of wax mixtures (tristearin/stearic acid). The concentration put on each figure is that of tristearin in the wax mixture.


Minami et al. Crystallization of tristearin in such a situation is like a recrystallization from a solvent. The tristearin crystal could be the stable β-form when solidified from a solvent. To substantiate this assumption, we have crystallized the tristearin from a hexane solution and checked the phase behavior. Figure 12 shows the DSC curve of tristearin just after crystallization from the hexane solution. We do not observe any endothermic and/or exothermic peak(s) before normal melting and see that the tristearin crystal is a stable one. A similar crystallization process could take place also in the mixture of myristic acid and tristearin since no abrupt jump in the contact angle was observed in Figure 5. 5. Conclusions

Figure 12. DSC curve of tristearin wax just after crystallization from the hexane solution.

In the right-hand side from the eutectic point, the solid phase that first appeared is pure tristearin when the molten mixed wax is cooled down. This pure tristearin crystal is the metastable R-phase as mentioned before in Figures 3 and 4. Then, the meta-stable R-phase transforms to the stable β-form crystal during the annealing process, and the fractal surfaces form spontaneously. On the left-hand side from the eutectic point, the crystal of stearic acid precipitates first in the molten wax on cooling. This crystal of stearic acid is a thermodynamically stable one since the stearic acid does not form fractal surfaces. When the temperature attains the eutectic one, the remained liquid wax containing both stearic acid and tristearin solidifies at once. The crystalline phase of tristearin formed in this process may be the stable β-form, since the crystal structure of stearic acid is similar to the stable phase of tristearin (stearic acid, monoclinic; tristearin, triclinic). One can see the similar relationship between the contact angles and the phase diagram also in the behenic acid/tristearin mixtures (see Figures 2c and 5). But in this case, the accordance between the steep contact angle increase and the eutectic point is not so clear. The rough surface of behenic acid may contribute to the enhancement of wetting even though it is not in fractal nature. We have no phase diagram for the mixed systems of myristic and palmitic acid with tristearin and cannot discuss the relationship between the fractal surface formation and the eutectic point. 4.2. Effect of Alkyl Chain Length of Fatty Acid on the Formation of Fractal Surfaces. As shown in Figure 5, the contact angle on the surface of the wax mixture increases with increasing alkyl chain length of the fatty acid mixed. The contact angles in the mixture of palmitic acid and tristearin do not exceed 150°. The rough surface structure in this wax system could be in nonfractal nature, since the fractal surface always shows the larger contact angle than 150°.1–10 An abrupt jump in the contact angle at about eutectic composition was not observed in the systems of lauric and myristic acid with tristearin. Figure 11 shows a SEM image of the surface of the lauric acid/tristearin mixture. One can see that no rough structure forms on the surface even after annealing. The contact angle is also quite small in the mixture of lauric acid and tristearin (Figure 5). In this case, no clear eutectic point was observed as shown in Figure 2b. Lauric acid is always in the liquid state until the whole tristearin sample precipitates out.

Spontaneous formation of super water-repellent fractal surfaces has been studied in mixed wax systems in which one wax forms a fractal structure and the other one does not. In the stearic acid/tristearin mixed system as a typical one, the super water-repellent fractal surfaces form in the higher composition region of tristearin than that of their eutectic point. A similar result was obtained also in the system of behenic acid/tristearin mixture, although the accordance between the steep contact angle increases and the eutectic point is not so clear. No super water-repellent fractal surface is obtained when solidified from the mixture of lauric acid and tristearin. In this case, lauric acid behaves like a solvent, and the stable tristearin crystal forms unlikely solidified from the melt. Acknowledgment. We thank the Ministry of Education, Culture, Sports, Science and Technology, Japan, for the financial support, a Grant-in-Aid for Scientific research (B) (No. 16310077). References and Notes (1) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (2) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (3) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011. (4) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid Interface Sci. 1998, 208, 287. (5) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453. (6) Kurogi, K.; Yan, H.; Mayama, H.; Tsujii, K. J. Colloid Interface Sci. 2007, 312, 156. (7) Yan, H.; Kurogi, K.; Tsujii, K. Colloids Surf. A 2007, 292, 27. (8) Fang, W.; Mayama, H.; Tsujii, K. J. Phys. Chem. B 2007, 111, 564. (9) Fang, W.; Mayama, H.; Tsujii, K. Colloids Surf. A 2008, 316, 258. (10) Minami, T.; Mayama, H.; Nakamura, S.; Yokojima, S.; Shen, J.W.; Tsujii, K. Soft Matter 2008, 4, 140. (11) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (12) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213. (13) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365. (14) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220. ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (15) O (16) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (17) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (18) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (19) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 2012. (20) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357.

Formation of Fractal Surfaces in Mixed Wax Systems (21) Mohammadi, R.; Wassink, J.; Amirfazli, A. Langmuir 2004, 20, 9657. (22) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (23) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299. (24) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458. (25) Callies, M.; Que´re´, D. Soft Matter 2005, 1, 55.

J. Phys. Chem. B, Vol. 112, No. 46, 2008 14627 (26) Feng, X.; Jiang, L. AdV. Mater. 2006, 18, 3063. (27) For example: Tsujii, K. In Surface ActiVity - Principles, Phenomena, and Applications; Tanaka, T., Ed.; Academic press: New York, 1998; pp 52-54. (28) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321.

JP802268J

Spontaneous Formation of Super Water-Repellent Fractal Surfaces in

Recommend Documents