Drying Dissipative Structures of Nonionic Surfactants in Aqueous

Langmuir 2005, 21, 9889-9895

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Drying Dissipative Structures of Nonionic Surfactants in Aqueous Solution† Tsuneo Okubo,*,‡ Chika Shinoda,§ Keisuke Kimura,§ and Akira Tsuchida§ Institute for Colloidal Organization, Hatoyama 3-1-112, Uji, Kyoto 611-0012 Japan, Cooperative Research Center, Yamagata University, Yonezawa 992-8510 Japan, and Department of Applied Chemistry, Gifu University, Gifu 501-1193 Japan Received March 15, 2005. In Final Form: April 26, 2005 Macroscopic and microscopic dissipative structural patterns form in the course of drying a series of aqueous solutions of polyoxyethylenealkyl ethers. The shift from the single round hill with accumulated surfactant molecules to the broad ring patterns of the hill in a macroscopic scale occurs as the HLB (hydrophile-liophile balance) of the surfactant molecules increases. The patterns correlate intimately with the HLB values of the surfactants. Microscopic patterns of small blocks, starlike patterns, and branched strings are formed. The size and shape of the surfactant molecules themselves influence the drying patterns in part. The pattern area and the time to dryness have been discussed as a function of surfactant concentration and HLB of the surfactants. The convection flow of water accompanying the surfactant molecules, the change in the contact angles at the drying frontier between solution and substrate in the course of dryness, and interactions among the surfactants and substrate are important for the macroscopic pattern formation. Microscopic patterns are determined in part by the shape and size of the molecules, translational Brownian movement of the surfactant molecules, and the electrostatic and hydrophobic interactions between surfactants and/or between the surfactant and substrate in the course of solidification.

Introduction Most structural patterns in nature and experiments in the laboratory form via self-organization accompanied by the dissipation of free energy and in the nonequilibrium state. Among several factors in the free energy dissipation, evaporation and convection induced by the earth’s gravity are very important. Several papers1-16 on the pattern formation in the course of drying of monodispersed colloidal suspensions have been reported so far. Most of the papers have studied the particle distribution in the liquidlike suspensions. Electrostatic interparticle interac†

Part of the Bob Rowell Festschrift special issue. * To whom correspondence should be addressed. Fax: +81-77432-8270. E-mail: [email protected]. ‡ Institute for Colloidal Organization and Yamagata University. § Gifu University. (1) Vanderhoff, J. W.; Bradford, E. B.; Carrington, W. K. J. Polymer Sci. Symp. 1973, 41, 155. (2) Nicolis, G.; Prigogine, I. Self-organization in nonequilibrium systems; Wiley: New York, 1977. (3) Cross, M. C.; Hohenberg, P. C. Rev. Mod. Phys. 1993, 65, 851. (4) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1977, 36, 1078. (5) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Langmuir 1998, 14, 3418. (6) Uno, K.; Hayashi, K.; Hayashi, T.; Ito, K.; Kitano, H. Colloid Polym. Sci. 1998, 276, 810. (7) Gelbart, W. M.; Sear, R. P.; Heath, J. R.; Chang, S. Faraday Discuss. 1999, 112, 299. (8) van Duffel, B.; Schoonheydt, R. A.; Grim, C. P. M.; De Schryver, F. C. Langmuir 1999, 15, 7520. (9) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Langmuir 1999, 15, 957. (10) Brock, S. L.; Sanabria, M.; Suib, S. L.; Urban, V.; Thiyagarajan, P.; Potter, D. I. J. Phys. Chem. 1999, 103, 7416. (11) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. 2000, 104, 8635. (12) Ge, G.; Brus, L. J. Phys. Chem. 2000, 104, 9573. (13) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (14) Lin, X. M.; Jaenger, H. M.; Sorensen, C. M.; Klabunde, M. J. Phys. Chem. 2001, 105, 3353. (15) Kokkoli, E.; Zukoski, C. F. Langmuir 2001, 17, 369. (16) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441.

tions have been pointed out as one of the important factors in the dissipative structures. Hydrophobic and hydrophilic interactions are also demonstrated to be important in the drying process.14-16 Gelbart et al.4,5,7 examined the mechanism of solvent dewetting in annular ring structures formed by drying a diluted metal colloid on a substrate. Shimomura et al.17 and other researchers have studied intensively the dissipative patterns in the processes of film formation by drying polymer solutions. In previous papers from our laboratory,18,19 drying dissipative patterns on a cover glass have been observed for colloidal crystal suspensions of colloidal silica and monodispersed polystyrene spheres, which are hydrophilic and hydrophobic at their surfaces. The colloidal crystal is undoubtedly one of the most simplest and most convenient systems for the study of dissipative structures on a laboratory scale. For example, accurate structural information on the processes of dissipative pattern formation is available for colloidal crystal suspensions using reflection spectroscopy in real time. Quite similar macroscopic and microscopic dissipative structural patterns were observed between the colloidal samples of silica and polystyrene spheres. The broad ring patterns of hills with accumulated spheres and spoke-like and ring-like cracks formed on a macroscopic scale were observed. From these observations, the existence of the small circular convection cells proposed by Terada20-22 was supported. The primitive patterns of valleys were formed already in the concentrated suspensions before dryness, and they grew with fine cracks (17) Shimomura, M.; Sawadaishi, T. Curr. Opin. Colloid Interface Sci. 2001, 6, 11. (18) Okubo, T., Okuda, S.; Kimura, H. Colloid Polym. Sci. 2002, 280, 454 (19) Okubo, T.; Kimura, K.; Kimura, H. Colloid Polym. Sci 2002, 280, 1001. (20) Terada, T.; Yamamoto, R.; Watanabe, T. Sci. Paper Inst. Phys. Chem. Res. Jpn. 1934, 27, 173; Proc. Imper. Acad. Tokyo 1934, 10, 10. (21) Terada, T.; Yamamoto, R.; Watanabe, T. Sci. Paper Inst. Phys. Chem. Res. Jpn. 1934, 27, 75. (22) Terada, T.; Yamamoto, R. Proc. Imper. Acad. Tokyo 1935, 11, 214.

10.1021/la050692a CCC: $30.25 © 2005 American Chemical Society Published on Web 07/29/2005

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in the course of solidification. The branch-like fractal patterns of sphere association were observed on a microscopic scale. The capillary forces between spheres at the air-liquid interface and the different rates of the convection flows of water and spheres at the drying front were important for these pattern formations. Drying dissipative structures have also been studied for a series of colloidal silica spheres ranging from 29 nm to 1 µm in diameter.23 Macroscopic and microscopic structural patterns were studied in the course of drying a suspension of Chinese black ink on a cover glass and in a dish.24 Clear broad ring and spoke-like patterns of the rims accumulating with particles formed especially in the central region of the pattern. The convection of water and colloidal particles at different rates under gravity and the translational and rotational Brownian movement of the particles were important for the macroscopic pattern formation. Microscopic patterns were influenced strongly by the translational Brownian diffusion of the particles and the electrostatic and/or between the particles and the substrate in the course of solidification of the particles. For the Chinese ink, direct observation of the convection flow was done mainly in a dish. The drying patterns of aqueous suspensions of monodispersed bentonite particles were investigated in detail.25 The drying dissipative structures have been studied for linear-type macrocations, poly(allylamine hydrochloride).26 Macroscopic broad ring patterns, where the polymers accumulate densely at outside edge, were formed. Furthermore, beautiful string-like fractal patterns were observed on a microscopic scale. Drying experiments were done for n-dodecyltrimethylammonium chloride, which is one of the typical cationic surfactant molecules.27 Broad ring patterns of hills from accumulated surfactant molecules formed around the outside edges of the film on a macroscopic scale. Starlike, branchlike, arclike, and small blocklike microstructures were also observed. Furthermore, drying structural patterns were studied for a series of anionic surfactant molecules, sodium n-alkyl sulfates (n-alkyl ) n-hexyl, n-octyl, n-decyl, n-dodecyl, n-hexadecyl, and n-octadecyl).28 From these studies on drying dissipative structures, the macroscopic broad ring patterns for various solutions and suspensions were surprisingly similar. Interestingly, microscopic patterns such as branchlike, stringlike, arclike, and small blocklike ones were reflected based on the shape, size and flexibility of the solute molecules. In this paper, the dissipative structures of a series of nonionic detergents, polyoxyethylenealkyl ethers, are studied in detail. Experimental Section Materials. The nonionic surfactants studied in this paper were polyoxyethylene-n-cetyl ethers (POEmC16), CH3(CH2)n-1O(CH2CH2O)mH, n ) 16, m ) 2, 5, 10, 20, and polyoxyethylenen-stearyl ethers (POEmC18), n ) 18, m ) 5, 10, 20. All of the samples were purchased from Sigma-Aldrich Co. and used without further purification. (23) Okubo, T.; Yamada, T.; Kimura, K.; Tsuchida, A. Colloid Polym. Sci. 2005, 283, 1007. (24) Okubo, T.; Kimura, H.; Kimura, T.; Hayakawa, F.; Shibata, T.; Kimura, K. Colloid Polym. Sci. 2004, 283, 1. (25) Yamaguchi, T.; Kimura, K.; Tsuchida, A.; Okubo, T.; Matsumoto, M. Colloid Polym. Sci. In press. (26) Okubo, T.; Kanayama, S.; Ogawa, H.; Hibino, M.; Kimura, K. Colloid Polym. Sci. 2004, 282, 230. (27) Okubo, T.; Kanayama, S.; Kimura, K. Colloid Polym. Sci. 2004, 282, 486. (28) Kimura, K.; Kanayama, S.; Tsuchida, A.; Okubo, T. Colloid Polym. Sci. 2005, 283, 898.

Okubo et al.

Figure 1. Drying patterns of the aqueous solutions (0.05 mL) of POE2C16 on a cover glass at 25 °C. (a) w ) 1 × 10-9 M, (b) 1 × 10-7 M, (c) 1 × 10-5 M, (d) 0.01 M, (e) 0.06 M, (f) 0.1 M. The length of the bar is 2.0 mm. The water used for the sample preparation was purified by a Milli-Q reagent grade system (Milli-RO5 plus and Milli-Q plus, Millipore, Bedford, Mass.). Observation of the Dissipative Structures. A total of 0.05 mL of the aqueous solution was dropped carefully and gently on a micro cover glass (30 mm × 30 mm, thickness No.1, 0.12 to 0.17 mm, Matsunami Glass Co., Kishiwada, Osaka) in a dish (60 mm in diameter, 15 mm in depth, Petri Co., Tokyo). The cover glass was used without further rinsing in most cases. The extrapolated value of the contact angle for pure water was 31 ( 0.2° from the drop profile of a small amount of water (0.2, 0.4, 0.6 and 0.8 µL) on the cover glass. A pipet (1 mL, disposable serological pipet, Corning Lab. Sci. Co.) was used for the dropping. Macroscopic and microscopic observations were done on the film formed after the solution was dried completely on a cover glass in a room air-conditioned at 25 °C and 65% in humidity. Macroscopic dissipative structures were observed with a digital HD microscope (type VH-7000, Keyence Co., Osaka) and a Canon EOS 10 camera with a macro-lens (EF 50 mm, f ) 2.5) and a life-size converter EF. Microscopic structures were observed with a laser 3D profile microscope (type VK-8500, Keyence) and a metallurgical microscope (Axiovert 25CA, Carl-Zeiss, Jena GmbH). Observation of the microscopic patterns was also done with an atomic force microscope (type SPA400, Seiko Instruments) and with a transmission electron microscope (Hitachi, H8100).

Results and Discussion Macroscopic and Microscopic Drying Patterns of Nonionic Surfactants. Figure 1 shows the typical patterns in the drying POE2C16 solutions, their concentrations ranging from 10-9 M to 0.1 M. At low concentrations of the surfactant, the pattern area shrank in the center from the initial area of the liquid drop. This shrinking phenomenon has been clarified to correlate intimately with the critical micelle concentration (cmc) of the surfactant solution.27,28 When the solute concentration is lower than the cmc, the initial solution area should

Dissipative Structure of Nonionic Surfactants

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Figure 2. Drying patterns of the aqueous solutions (w ) 0.06 M, 0.05 mL) of POE2C16 on a cover glass at 25 °C. Photographs b and c show enlargements of window b in photograph a, and photographs d and e are those of window d. (a) The length of the bar is 2.0 mm, (b) 200 µm, (c) 40 µm, (d) 200µm, (e) 40 µm.

shrink without solidification until the solution is concentrated in the course of drying and its concentration reaches the cmc. For concentrations of the surfactants higher than the cmc, the shrinking of the solution area stops. In other words, the solution area shrinks when the contact angle between solution and substrate is large, and the shrinking stops when the angle is small enough at the concentrations of surfactant higher than the cmc. However, quantitative agreement between the predicted values of cmc and the reference values of cmc was not always excellent for the POEmCn samples including POE2C16. It should be mentioned further that sodium poly-R-L-glutamate and the hydrochloride of poly-L-lysine shrank when their concentrations were lower than the critical concentration, m*, where these polymers form the structured conformation at the air-water interface, and their surface tensions start to decrease sharply as the polymer concentration increases.29,30 At concentrations of POE2C16 higher than 1 × 10-5 M, the surface of the dried film was very rough and blocklike. Quite extended surface patterns are shown in Figure 2. Small granules of around 50 µm in size were observed. The thickness of the film observed directly using the 3D profile microscope demonstrates a single round hill pattern in the central region for POE2C16 as is shown below in Figure 3a. The HLB (hydrophile-lyophile balance) value of POE2C16 is around 3,31,32 which means that the surfactant is highly hydrophobic and has low surface activity. Association of the molecules is highly plausible. A substantial decrease in the translational movement of (29) Okubo, T.; Onoshima, D.; Kimura, K.; Tsuchida, A., publication in preparation. (30) Okubo, T. Kobayashi, K. J. Colloid Interface Sci. 1998, 205, 433. (31) Becher, P. Nonionic Surfactants; Shick, M. J., Ed.; Marcel Dekker: New York, 1967; p 608. (32) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 249.

Figure 3. Thickness (d) of the dried film as a function of radius (r) at 25 °C. In water, 0.06 M, 0.05 mL, (a) POE2C16, (b) POE10C16, (c) POE 20C16.

Figure 4. Drying patterns of the aqueous solutions (0.05 mL) of POE5C16 on a cover glass at 25 °C. (a and d) w ) 1 × 10-9 M, (b and e) 1 × 10-2 M, (c and f) 0.06 M. The length of the bar is (a-c) 2.0 mm and (d-f) 0.5 mm.

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Figure 5. Drying patterns of the aqueous solutions (0.06 M, 0.05 mL) of POE5C16 on a cover glass at 25 °C. Photographs b and c show enlargements of window b in photograph a, and photographs d and e are those of window d. (a) The length of the bar is 2.0 mm, (b) 200 µm, (c) 40 µm, (d) 200 µm, (e) 40 µm.




the associated molecules is one of the main reasons for the single hill pattern in the center. It should be recalled that broad ring patterns formed very often for spherical colloidal suspensions. The main

cause of the broad ring formation is the convection flow of the solvent and the colloidal spheres. Especially important is the flow of the colloidal spheres from the center area toward the outside edges in the lower layer


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Figure 9. Drying patterns of the aqueous solutions (0.06 M, 0.05 mL) of POE5C18 on a cover glass at 25 °C. Photographs b and c show enlargements of window b in photograph a, and photographs d and e are those of window d. (a) The length of the bar is 2.0 mm, (b) 200 µm, (c) 40 µm, (d) 200µm, (e) 40 µm.

of the liquid drop, which was observed directly from the movement of the aggregates of the spheres.18 Aggregation, however, took place very rarely. The flow is enhanced by the evaporation of water at the liquid surface, resulting in lowering of the suspension temperature in the upper region. When the spheres reach the edges of the drying frontier at the outside region of the liquid, some of the spheres will turn upward and go back to the center region. However, the movement of most spheres may stop at the frontier region because of the disappearance of water. This process must be followed by the broad ring-like accumulation of the spheres near the round edges. It should be noted here that the importance of the convection flow of colloidal spheres in the ring formation has often been reported in the process of film formation.9,33 We should note further here that a very rough blocklike hill appeared in the center region in addition to a broad ring for the platelike bentonite suspensions especially at high particle concentrations and with the coexistence of sodium chloride.25 These hills in the central area have not been observed for suspensions of any kind of spherical particles hitherto. The translational movement of the associated platelike particles accompanied by the convectional flow of water will be highly restricted. The rotational movement of the highly associated molecules must also be difficult considering their anisotropic shape, and the sliding movement will be major especially in the area close to the substrate plane. This restricted Brownian movement must be correlated deeply with the appearance of the hill in the center. Thus, it is concluded that POE2C16 is highly hydrophobic and that association of the molecules should occur. The associated molecules are large in colloidal size and anisotropic in shape, and the single round (33) Latterini, L.; Blossey, R.; Hofkens, J.; Vanoppen, P. Langmuir 1999, 15, 3582.

Figure 10. Thickness (d) of the dried film as a function of radius (r) at 25 °C. In water, 0.06 M, 0.05 mL, (a) POE5C18, (b) POE10C18, (c) POE20C18.

hill then appears due to the highly restricted Brownian movement of the associated molecules. Figure 4 demonstrates typical examples of the drying patterns formed with POE5C16 studied over the wide range of concentrations of 1 × 10-9, 0.01, and 0.06 M, though the experiments were actually done at six different concentrations ranging from 1 × 10-9 to 0.1 M. The pattern area increased as the surfactant concentration increased from 1 × 10-9 to 1 × 10-5 M. However, the patterns are quite similar over a wide range of surfactant concentrations. The single round hills were also observed at medium and high concentrations. However, with the addition of a rough hill, thin broad ring regions appeared. From the extended pictures shown in Figure 5, it is clear that some fibril-like ordered structures formed. The HLB of POE5C16 is estimated to be ca. 7, and the hydrophilic and liophilic forces of the surfactant are balanced. It is highly plausible that the hydrophobic affinitive interactions between POE5C16 molecules are not as strong compared with POE2C16 and that the microscopic fibril structures favorably formed. Measurements of the thickness distribution of the dried film supports the belief that a thin broad ring forms in addition to the central hill in the dried film, though the figure showing this is omitted in Figure 3. The typical drying patterns of POE10C16 at 0.06 M are shown in Figure 6. Again, the patterns at concentrations between 1 × 10-9 M and 1 × 10-5 M were quite similar to those at 1 × 10-9 M, though the pictures showing this

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are omitted in this paper. Now, the broad ring appeared clearly (see also Figure 3b), which supports the convection flow of the surfactant molecules being active. Figure 6 shows the extended structures in the broad ring and central regions at w ) 0.06 M. The surfaces are still very rough, but netted granulelike and stringlike microstructures are observed. The HLB of POE10C16 is estimated to be around 11, which means that the molecules are rather hydrophilic and no association occurs. Ordering of the molecules is highly plausible for POE10C16. The macroscopic and microscopic patterns of POE20C16 (HLB ) ca. 15) at w ) 0.06 M are shown in Figure 7. Broad rings formed for the solutions examined, their concentrations ranging from 1 × 10-9 to 0.02 M. Interestingly, the microscopic surfaces were rather smooth, and their structures were fine and rodlike in many cases as is clear in Figure 9. Now, let us discuss a series of POEmC18 samples. Figures 8 and 9 demonstrate the macroscopic and microscopic patterns of the POE5C18 solution. Surprisingly, especially the macroscopic patterns of POE5C18 are quite similar to those of POE5C16. A single round hill and a thin broad ring appeared for both samples as is clear in Figure 10b. The HLB of the former is ca. 6 and quite close to that of the latter, ca. 7, which supports the similarity in patterns. Figure 11 displays the patterns of POE10C18 (HLB ) ca. 10). A broad ring appeared, and the fine string-like structures formed. Again, their macroscopic patterns appeared to be between those of POE5C16 (HLB ) ca. 7 and POE10C16 (HLB ) ca. 11). These systematic changes in the drying patterns are explained quite clearly by the HLB values. Figures 12 and 13 display the macroscopic and microscopic patterns of POE20C18, the HLB being ca. 14. A broad ring clearly formed in the picture, and the surfaces were rather smooth. Again the patterns were quite similar to those of POE20C16 (HLB ) ca. 15).

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Now, Figure 14 demonstrates the systematic change in the macroscopic patterns as a function of the HLB at the highest surfactant concentration of 0.1 M. At low HLB values, the patterns are the single round hill type, and


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Figure 14. Drying patterns of the aqueous solutions (0.1 M, 0.05 mL) of surfactants on a cover glass at 25 °C. (a) POE2C16, (b) POE5C16, (c) POE10C16, (d) POE20C16, (e) POE5C18, (f) POE10C18, (g) POE20C18. The length of the bar is 2.0 mm. Table 1. S and T Values in the Drying Patterns of Non-ionic Surfactants at 25 °C surfactant

HLB

[surfactant] (M)

S (mm2)

T (min)

POE5C18 POE5C18 POE5C18 POE5C16 POE5C16 POE5C16 POE10C18 POE10C18 POE10C18 POE10C16 POE10C16 POE10C16 POE20C18 POE20C18 POE20C18 POE20C16 POE20C16 POE20C16

6 6 6 7 7 7 10 10 10 11 11 11 14 14 14 15 15 15

0.01 0.04 0.1 0.01 0.04 0.1 0.01 0.04 0.1 0.01 0.04 0.1 0.01 0.04 0.1 0.01 0.04 0.1

46.3 54.8 65.8 41.0 50.9 69.5 55.9 61.0 72.6 55.1 62.9 79.5 62.9 72.6 95.5 60.5 78.6 102.8

152 101 166 109 146 110 228 159 155 122 225 185

the film surface is very rough. On the other hand, the patterns are the broad ring type and the surface is smooth at high HLB values. Drying Film Area and Drying Time. The drying film area, S, and the drying time, T, are compiled in Table 1. The S values increased smoothly as the HLB of the surfactant increased and/or the surfactant concentration increased. These results are quite understandable because the surface tension decreases as the HLB and/or

surfactant concentration increases. On the other hand, the T values increased as the HLB increased or the surfactant concentration decreased. An increase in T with an increase in HLB is peculiar at a first glance, because decrease in the surface tension should result in an increase in S and then a decrease in T. It is highly plausible that the water activity of an aqueous solution of a high HLB surfactant is lowered sharply compared with that of a solution of a low HLB surfactant. In other words, the nature of the deliquescence is significant for a solution of high HLB surfactants. In conclusion, the drying dissipative patterns correlate intimately with the HLB values of the nonionic surfactants used in this work. The shrinking of the dried patterns takes place when the surfactant concentrations are lower than the critical micelle concentration and the contact angles between solution and substrate are high. For the rod-shaped molecules including the nonionic surfactant molecules studied in this work, brock-like, starlike and string-like microscopic patterns were often observed, which supports the belief that the shape and size of the individual surfactant molecules also influence the drying dissipative structures in part. Acknowledgment. The Ministry of Education, Science, Sports and Culture is thanked for grants-in-aid for Scientific Research on Priority Area (A) (11167241) and for Scientific Research (B) (11450367). LA050692A

Drying Dissipative Structures of Nonionic Surfactants in Aqueous

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