Spontaneous Formation of Fractal Structures on Triglyceride Surfaces

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J. Phys. Chem. B 2007, 111, 564-571

Spontaneous Formation of Fractal Structures on Triglyceride Surfaces with Reference to Their Super Water-Repellent Properties Wenjun Fang,*,†,‡ Hiroyuki Mayama,† and Kaoru Tsujii*,†,§ Nanotechnology Research Center, Research Institute for Electronic Sciences, Hokkaido UniVersity, N21, W10 Kita-ku, Sapporo 001-0021, Japan, Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, China, and CREST, JST, Kawaguchi, Saitama 338-8570, Japan ReceiVed: August 29, 2006; In Final Form: NoVember 2, 2006

Alkylketene dimer (AKD), a kind of wax, has been known to form fractal surfaces spontaneously and show super water-repellency. Such formation of water-repellent and fractal surfaces was also found in this work for triglycerides. Since the crystal phase transitions of these waxes were well studied, we studied the formation of their fractal surfaces through contact angle measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). From time-dependent contact angle measurements, it was found that the formation of super water-repellent surfaces with fractal structures occurred spontaneously also on the triglyceride surfaces at different temperatures. The freshly solidified triglyceride surfaces were almost transparent, and their initial contact angles of water were close to 110°. The surfaces then became rough and clouded after being incubated for a certain time at a specified temperature. The super water-repellent surfaces were quite rough and showed fractal structures with the dimension of ca. 2.2 calculated from the scanning electron microscopic (SEM) images by the box-counting method. The phase transformation from a metastable state to a stable cystalline one after the solidification from the melt of triglycerides was clearly observed by DSC and XRD measurements. The fractal crystalline structures and the super water-repellency resulted from this phase transformation and the crystal growth. Ensuring the initial sample solidified into the metastable state and curing the surface at an appropriate temperature are key factors for the successful preparation of fractal triglyceride surfaces by the solidification method.

1. Introduction Wettability of a liquid on a solid surface is governed by two factors, the chemical component and the geometric structure.1-3 Surface roughness, fractal structures in particular,4-9 can effectively enhance the wettability. Super water-repellency has attracted much attention, and many methods have been employed to produce such rough surfaces.5-26 In previous work, super water- and oil-repellent surfaces7,8 with fractal dimension of 2.19 were made of anodically oxidized aluminum surfaces treated with fluorinated compounds of low surface energy and super water-repellent poly(alkylpyrrole) films9 with a fractal dimension of 2.23 were electrochemically synthesized. Alkylketene dimer (AKD), a kind of wax, has been known to form fractal structures spontaneously on its surface made by the solidification from its melt and to give super water-repellency.5,6 The contact angle of water on the AKD surface was found to be as large as 174°, and the fractal dimension was 2.29. A preliminary research suggested that the mechanism of spontaneous formation of fractal structures on the AKD surface originated from a phase transformation from a metastable to a stable crystalline form.27 This work is an extension of the above research to elucidate further the mechanism of spontaneous formation of fractal structures on wax surfaces. * To whom correspondence should be addressed. E-mail: tsujik@ es.hokudai.ac.jp (K.T.); [email protected] (W.F.). Phone: (81)11-706-9356 (K.T.); (86)571-87952371 (W.F.). Fax: (81)11-706-9357 (K.T.); (86)57187951895 (W.F.). † Hokkaido University. ‡ Zhejiang University. § CREST, JST.

Triglycerides are chosen in this work as the wax samples, since the phase transition from a metastable to a stable crystalline form (“blooming phenomenon”)28 is well-known during their crystallization process from the melts. The complex polymorphic phenomenon of triglycerides has been extensively investigated.28-34 However, to the best of our knowledge, the fractal structures on their surfaces and the effects of the structures on the wettability have not been well studied. Three typical monoacid triglycerides, trimyristin (MMM), tripalmitin (PPP), and tristearin (SSS), were used in this work. The surfaces were prepared by the solidification from the melt of the triglycerides. Time-dependent contact angles were measured to investigate the transformation process of the surface from a fresh one to the fractal and super water-repellent one. Differential scanning calorimetry (DSC) and X-ray diffractometry (XRD) were also used to investigate the phase transition during the process of spontaneous formation of super waterrepellent surfaces with fractal structures. 2. Experimental Section 2.1. Surface Preparation. Trimyristin (Acros Organics, New Jersey) and tripalmitin and tristearin (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were purchased and used without further purification. A small amount of triglyceride sample was put on a plastic substrate (40 mm × 40 mm × 1 mm) and heated on an electric hot plate at a temperature slightly above the melting point (ca. 58, 63, and 73 °C for MMM, PPP, and SSS, respectively). Solidfication was then carried out by moving the sample quickly onto a cold plate cooled on an ice-water surface.

10.1021/jp065589o CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006

Fractal Structures on Triglyceride Surfaces

J. Phys. Chem. B, Vol. 111, No. 3, 2007 565

Figure 1. Time-dependent contact angles and some photographs of water droplets on the trimyristin (MMM) surfaces cured at different temperatures. The inset shows the expansion of the changes at the three higher temperatures.

Figure 2. Time-dependent contact angles and some photographs of water droplets on the tripalmitin (PPP) surfaces cured at different temperatures. The inset shows the changes at the three higher temperatures.

The sample was solidified within 10-15 s, and the fresh solid surface was obtained. After the initial contact angle was measured, the sample was put into an incubator (FF-30N, Tokyo Garasu Kikai Co. Ltd.) for heat treatment at a specified temperature. 2.2. Contact Angle Measurements. Time-dependent contact angles were measured to monitor the growth of the fractal surfaces with an optical contact angle meter (DropMaster 300, Kyowa Interface Science Co. Ltd.) at room temperature. Two surface samples were prepared for every measurement. Droplets of ultrapure water (Milli-Q, Millipore Corp.) of 3 µL were placed on 10 different positions of each surface, and the averaged value was adopted as the measured contact angle. The contact angles of water/acetic acid and water/1,4-dioxane mixtures on the flat and the fractal surfaces were measured to

test the validity of the theory of wetting on the fractal surfaces. For the preparation of mixed solvents, 99.9% acetic acid (Wako) and 99.5% 1,4-dioxane (Sigma-Aldrich Inc., St. Louis) were used. 2.3. Scanning Electron Microscopic (SEM) Observations. Images of the triglyceride surfaces and their cross sections were taken using a field-emission scanning electron microscope (FESEM, S-5200, Hitachi) after coating the samples with a thin layer of sputtered alloy of gold and palladium by using an ion sputter (E-1030, Hitachi). The SEM images of the cross sections at several magnifications were used to evaluate the dimention of the fractal surfaces. 2.4. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (Thermo Plus 2 DSC8230, Rigaku) was used to observe the phase transition of triglycerides. The

566 J. Phys. Chem. B, Vol. 111, No. 3, 2007

Fang et al. samples (3-4 mg) were sealed in an aluminum pan, and an empty pan was used as a reference. The heating rate employed was 3 °C/min. 2.5. X-ray Diffractometry (XRD). The transformation from a metastable to a stable crystalline state was recorded by powder X-ray diffractometer (RINT 2200, Rigaku) with Ni-filtered Cu KR (0.154 18 nm) radiation, voltage 40 kV, and current 40 mA. The 2θ angle was calibrated with copper. The sample holder (25 mm × 17 mm × 2 mm) was made of copper to ensure good heat conductivity. The temperature stability was of (0.5 °C. The scanning was performed with a counting time of 1 s/0.02° between 1 and 40° of 2θ. 3. Results

Figure 3. Time-dependent contact angles and some photographs of water droplets on the tristearin (SSS) surfaces cured at different temperatures.

3.1. Time-Dependent Contact Angles of Water on the Triglyceride Surfaces. Spontaneous formation of super waterrepellent triglyceride surfaces at different temperatures was monitored by the time-dependent contact angles of water, as shown in Figures 1-3. The results in these figures clearly show

Figure 4. Typical SEM images with different magnifications for the fractal triglyceride surfaces. The surfaces of (a,b) MMM (contact angle of water (CA) ) 156.1°), (c, d) PPP (CA ) 157.7°), and (e, f) SSS (CA ) 156.6°) were obtained after incubation at 40 °C for 3.5 h, at 50 °C for 1 h, and at 55 °C for 29 h, respectively.



Figure 5. Cross-sectional SEM images of the fractal triglyceride surfaces: (a) MMM incubated at 40 °C for 3.5 h; (b) PPP incubated at 50 °C for 1 h; (c) SSS incubated at 55 °C for 29 h.

Figure 6. Plots of log N(r) versus log r for the cross-sectional trace curves of the fractal triglyceride surfaces: (a) MMM; (b) PPP; (c) SSS. r is the box size, and N(r) is the corresponding box number obtained by the box-counting program. L is the upper limit scale of the fractal (self-similar) behavior.

TABLE 1: Fractal Parameters for the Super Water-Repellent Triglyceride Surfaces triglyceride

D

L/µm

l/µm

(L/l)D-2

MMM PPP SSS

2.21 2.24 2.23

50 21 16

0.1 0.1 0.1

3.7 3.6 3.2

the progressive formation of the super water-repellent surfaces. The freshly solidified triglyceride surfaces are almost transparent, and the initial contact angles of water on them are close to 110°. The surfaces then become rough and clouded after incubated for a certain time at a specified temperature, which is well-known as “blooming phenomenon”28 in the chocolate industries. The contact angle on the rough and clouded surface increases with time at each temperature, and the initial increase is quite rapid. The final stage of this change gives the contact angles greater than 150°. It is also obvious that the temperature plays a dominant role in the spontaneous changing process of the surfaces. It takes more than 5 days for the trimyristin (MMM, mp ca. 58 °C) surface to become super water-repellent at room temperature, but it takes only 1.5 h when the surface is treated at 40 °C. Because the tripalmitin (PPP, mp ca. 63 °C) molecule has longer hydrocarbon chains than the MMM molecule, the PPP surface takes 6.4 h to become super water-repellent at 40 °C. It is interesting to note that the PPP surface exhibits its super water-repellency after only a 10-min heat treatment at 50 °C but the tristearin (SSS, mp ca. 73 °C) surface needs 65 h. It can be expected that both of the PPP and SSS surfaces take quite a long time to become super water-repellent at room temperature. The rate of spontaneous formation of super waterrepellent surfaces strongly depends on the temperature and the size of the molecule. The shorter chain triglyceride molecules may move more easily even in the crystalline phase than the longer chain ones do. This may be the reason why the MMM

surface shows super water-repellency in a shorter time and at lower temperatures. 3.2. Fractal Structures on the Triglyceride Surfaces. SEM images of different magnifications for the super water-repellent triglyceride surfaces are shown in Figure 4. The beautiful fractal and flowerlike structures are clearly seen. The surfaces exhibit extreme roughness and then enhance the water-repellency. SEM images of the cross-sectional view of the fractal triglyceride surfaces are shown in Figure 5. The thicknesses of the rough fractal structures on the three kinds of super waterrepellent triglyceride surfaces are about 3-4 µm, and those of the flakes on the surfaces are about 0.1-0.2 µm. The SEM images with several magnifications of the cross sections of the fractal triglyceride surfaces were used to determine the fractal dimension. Trace curves of the cross section were obtained with the tool of Adobe Photoshop 7.0.1. A box-counting program was then used to count the number, N(r), of boxes with the size, r, to cover the trace. The real scale of the box was obtained from the box-counting size, the scale bar, and the pixel of the images. The relationship of the fractal dimension, D, with the box size and box number is expressed as

N(r) ∝ r -D

(1)

The dimension Dcross of the trace curves can be calculated from the slope of the plot of log N(r) versus log r with the boxcounting data. The fractal dimension D of the surface was then evaluated as D ) Dcross + 1.5,6 The box-counting results for cross sections of the fractal triglyceride surfaces are shown in Figure 6. For the investigated triglyceride surfaces, the fractal dimension D and the upper limit scale L of fractal behavior can be obtained from the plots in Figure 6. The lower limit scale l, unfortunately, cannot be detected. It might be reasonable,


Fang et al.

Figure 7. DSC thermograms at the heating/cooling rate of 3 °C/min for different triglyceride samples: (a-c) MMM; (d-f) PPP; (g-i) SSS. Key: (a, d, g) freshly solidfied triglycerides; (b, e, h) super water-repellent surfaces; (c, f, i) stable samples stored for more than 1 year. Arrows: f, heating; r, cooling; V, endothermic peak; v, exothermic peak.

however, that l has the same value as the thickness of the flakelike crystal on the triglyceride surfaces observed by SEM, i.e., l ≈ 0.1 µm. Hence, the surface area magnification factor (L/l)D-2 can be estimated.5,6 The fractal parameters for the triglyceride surfaces are listed in Table 1. 3.3. DSC and XRD Measurements. Triglycerides have three main polymorphic states,28-34 R, β′, and β, which have different melting points. The R and β′ forms are in metastable state, and the β form is in the stable one at room temperature and atmospheric pressure. The β′ form is hardly observed. Figure 7 shows the DSC curves for the different samples, the freshly solidified triglycerides (Figure 7a,d,g), the super water-repellent surfaces (Figure 7b,e,h), and the stable triglycerides (Figure 7c,f,i) which have been stored for long time (more than 1 year). It is clear that there exist three peaks for the fresh samples, especially for PPP and SSS. The two endothermic peaks correspond to the melting of the R (lower temperature) and β (higher temperature) forms, respectively. The exothermic peak

corresponds to the crystallization from the R form in metastable state to the stable β form. When the R form transforms directly to the β form without noticeable melting, the endothermic peak does not appear. This may be the case in the transition of MMM (Figure 7a). In the DSC curves for the samples of the super water-repellent surfaces and the aged triglycerides, the melting and crystallization peaks of the R form disappear. The metastable R form might undergo a solid-melt-solid or a solid-solid transformation into the stable β form under different thermal conditions, resulting in the increase of the roughness of the surfaces. These results indicate that the growth of the rough and water-repellent triglyceride surfaces takes place during the phase transformation from the metastable state to the stable one. Furthermore, XRD patterns were recorded for PPP and SSS at some time and temperature intervals, using temperaturecontrolled X-ray diffractometry. The results are presented in Figure 8. The triglyceride samples were made by the same procedure as that of the surface preparation from melt solidifica-



Figure 8. XRD patterns of PPP and SSS at different temperatures and time intervals: (a) PPP; (b) SSS. The initial record was performed at 28 °C. The sample was then heated to the next temperature at the heating rate of 2 °C/min and was held at that temperature for 10 min. Two records were carried out with the time interval of 2 h at each temperature.

Figure 9. Relative intensities of XRD peaks of R (2θ ) 21.5°) and β (2θ ) 19.3°) forms as a function of time. Dashed lines show the temperature changes in the heating program. Key: (a) PPP; (b) SSS.

tion. The initial record of XRD pattern was performed at 28 °C. The sample was then heated to the next temperature at the heating rate of 2 °C/min and was kept at that temperature for 10 min before measurement. At each temperature, two separate records were carried out with the time interval of 2 h. In the wide-angle region shown in Figure 8, the samples of the fresh triglyceride crystal have a single strong refelection at 2θ ) 21.5° (d ) 0.415 nm) indicating the R state.30 The stable β state is typically characterized by the very strong refelection at 2θ ) 19.3° (d ) 0.460 nm). The peaks in the XRD parttern of fresh samples are broader than those of aged samples. It means that the R state is a metastable one and it is, at least partly, disordered, which is similar to the liquid state mainly with difference in the mobility of the molecules.31 With the increase of temperature, triglycerides undergo a thermally activated transformation from the metastable state to the stable crystalline one. The peaks of the β state become sharper and

TABLE 2: t150-Values of Triglyceride Sufaces Showing Super Water-Repellency at Different Incubation Temperatures MMM

PPP

SSS

T/°C

t150/h

T/°C

t150/h

T/°C

t150/h

20.0 30.0 35.0 40.0 42.5

125.2 24.6 5.3 1.5 0.7

40.0 42.5 45.0 47.5 50.0

6.4 1.6 0.5 0.3 0.2

50.0 52.5 55.0 57.5

64.7 24.6 7.4 5.5

sharper, and their intensities become stronger and stronger. This shows the growth of the stable crystal and the increase of the degree of crystallization. The relative intensities of R and β states for PPP and SSS along with the heating program are shown in Figure 9. It is found that the transformation from R to β state depends strongly


Figure 10. Plots of ln(1/t150) versus 1/T for the triglycerides. t150 (h) is the time requirement with the water contact angle of 150° on the surface. T (K) is the temperature of heat treatment.

upon the temperature and predominantly begins at about 35 °C for PPP and 45 °C for SSS. 4. Discussion 4.1. Mechanism of Spontaneous Formation of Fractal Surfaces of Triglycerides. From the results of DSC and XRD measurements shown in Figures 7 and 8, one can understand clearly that the fresh triglyceride crystals made by the rapid solidification from the melts are in the metastable R state. Under heat treatment of the samples at a specified temperature, the metastable phase spontaneously transforms into the thermodynamically favorable crystalline form because it has higher Gibbs energy than the stable phase.32 A lot of microcrystals of stable β form grow from the metastable phase. With the formation of the stable microcrystals on the surface, the fractal structures gradually grow and the surface becomes rough as observed by SEM in Figures 4 and 5 and finally becomes super waterrepellent. As a result, the contact angle increases with time at each temperature as shown in Figures 1-3. Because of the low mobility of the three long hydrcarbon chains in triglyceride molecules, the formation of the stable β form is a thermally

Fang et al. activated process and it depends strongly upon the temperature. Hence, quite different values of the contact angles were observed on the triglyceride surfaces with the same time of heat treatment at different temperatures. Then, one can understand that ensuring the fresh sample solidified into the polymorphic R form and curing the surface at an appropriate temperature are key factors for the successful preparation of fractal super water-repellent triglyceride surfaces. 4.2. Activation Energy of Spontaneous Formation of Super Water-Repellent Surfaces. It is well-known that a solid surface with water contact angles larger than 150° is called a super water-repellent one. Here, we make an attempt to estimate quantitatively the activation energies of spontaneous formation of super water-repellent surfaces from the contact angle measurements. The time values (t150) required for the triglyceride sufaces cured at various temperatures to become super-water repellent are listed in Table 2. On the basis of chemical kinetic theory, the rate constant k of a transformation depends on the temperature T according to the Arrhenius equation

k ) Ae-Ea/RT

(2)

where A is the preexponential factor, Ea is the activation energy, and R is the gas constant. Generally, the activation energy Ea can be considered as a constant, and then

ln k ) ln A -

()

Ea 1 R T

(3)

The plots of ln(1/t150) versus 1/T for the three triglycerides investigated are shown in Figure 10. The activation energies are obtained from the slopes of the straight lines to be 178, 290, and 306 kJ/mol for MMM, PPP, and SSS, respectively. 4.3. Wettability of the Triglyceride Surfaces. The relationship between the equilibrium contact angle θf on a fractal solid surface and θ on a flat one is given by5,6

cos θf ) (L/l)D-2 cos θ

(4)

Figure 11 shows the plots of cos θf versus cos θ from the contact angles of water/acetic acid mixtures (Figure 11a) and water/ 1,4-dioxane mixtures (Figure 11b) on the fractal and the flat surfaces of the triglycerides. The solid straight lines show the

Figure 11. Plots of cos θf versus cos θ for the triglyceride surfaces. The measurements were done using (a) water/acetic acid mixtures and (b) water/1,4-dioxane mixtures. The solid straight line of cos θf versus cos θ is the theoretical prediction with the surface area magnification factor (L/l)D-2 of 3.6. θf and θ are the contact angles on the fractal and flat surfaces, respectively.

Fractal Structures on Triglyceride Surfaces theoretical prediction from eq 4 assuming the surface area magnification factor (L/l)D-2 of 3.6. One can see that the slope predicted from the fractal theory is in good agreement with that observed from the contact angle measurements. 5. Conclusions Spontaneous formation of fractal structures with the fractal dimension of 2.2 has been observed on triglyceride surfaces. The fractal surfaces show the super water-repellent property. The spontaneous formation of fractal structures results from the phase transition from the metastable R to the stable β cystalline phase and the growth of a lot of microcrystals. The rates of the phase transition and the super water-repellent surface formation depend strongly upon the temperature and the molecular size. The activation energies of the formation of super water-repellent triglyceride surfaces are 178, 290, and 306 kJ/mol for trimyristin, tripalmitin, and tristearin, respectively. Ensuring the initial sample solidified into the metastable state and curing the surface at an appropriate temperature are key factors for fractal surface preparation. Acknowledgment. We are grateful for the financial support from the 21st century COE program “Center of Excellence for Advanced Life Science on the Base of Bioscience and Nanotechnology” of Hokkaido University, Sapporo, Japan, and a Grant-in-Aid for Scientific Research (B) (No. 16310077) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References and Notes (1) Tsujii, K. In Surface ActiVity Principles, Phenomena, and Applications; Tanaka, T., Ed.; Academic Press: New York, 1998; pp 52-54. (2) Callies, M.; Que´re´, D. Soft Matter 2005, 1, 55. (3) McHale, G.; Newton, M. I.; Shirtcliffe, N. J. Eur. J. Soil Sci. 2005, 56, 445. (4) Hazlett, R. D. J. Colloid Interface Sci. 1990, 137, 527. (5) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (6) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (7) Tsujii, K; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011.

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Spontaneous Formation of Fractal Structures on Triglyceride Surfaces

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