Multiple Structural Polymorphism - American Chemical Society

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Langmuir 2002, 18, 10163-10167

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Self-Assembly of Surface-Active Powder at the Interfaces of Selective Liquids. 1: Multiple Structural Polymorphism Yoshimune Nonomura,*,† Toru Sugawara,† Akio Kashimoto,† Keiichi Fukuda,† Hajime Hotta,† and Kaoru Tsujii‡ Tokyo Research Laboratories, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131-8501, Japan, and Frontier Research System for Extremophiles, Japan Marine Science and Technology Center, 2-15 Natsushima-Cho, Yokosuka-city, Kanagawa 237-0061, Japan Received June 3, 2002 A ternary system consisting of a surface-active powder (fluorinated silicone resin powder), fluorinated oil (perfluoropolymethylisopropyl ether, PFPE), and silicone oil (dimethylpolysiloxane, DMS) has been studied and found to exhibit multiple structural polymorphism in the mixed states. The mixed states were classified into seven different regions and ruled roughly by the composition ratio of the powder and the PFPE. For PFPE compositions which exceed the absorption limit of the powder, the continuous phase was PFPE and four mixed states were observed, that is, a network-structured state, an O/F emulsion state (O/F emulsion means a DMS-in-PFPE type emulsion), an O/F emulsion state with excess PFPE and DMS phases, and a powder dispersion state with excess PFPE and DMS phases. On the other hand, in the region in which the amount of PFPE was less than the absorption limit of the powder the continuous phase was DMS and three mixed states, powdery state, granular state, and dispersion state containing the PFPE/ powder mixture, were observed. Such self-assemblies of the resin powder and the multiple structural polymorphism were observed only when the degree of fluorination on the silicone resin powder was balanced. According to the results of X-ray photoelectron spectroscopy and sinking time measurements, the wettabilities of powder with PFPE and DMS are the governing factors to control the mixed state. The powder is dispersed in DMS or PFPE when it has too high an affinity to DMS or PFPE, and the O/F type emulsion is formed when it has balanced affinity to both oils. The multiple polymorphism of the mixed state is interesting not only from the scientific point of view but also for applications in the chemical and cosmetic industries.

Introduction Amphiphilic molecules and polymers can form supramolecular assemblies, such as micelles, microemulsions, and liquid crystals, to keep thermodynamically or kinetically stable conditions.1,2 However, there are few reports on surface-active powders controlling their dispersed state. Since Pickering reported an emulsion stabilized with powder particles in 1907,3 some additional solid-stabilized emulsions have been reported.4-20 The surface-active powders would be self-assembled in the emulsions mentioned above, because the powder particles † ‡

Kao Corp. Japan Marine Science and Technology Center.

(1) Solvent Properties of Surfactant Solutions; Shinoda, K., Ed.; Marcel Dekker: New York, 1967. (2) Surfactants and Polymers in Aqueous Solution; Lindman, J., Kronberg, H., Eds.; John Wiley & Sons Ltd.: West Sussex, U.K., 1998. (3) Pickering, S. U. J. Chem. Soc. 1907, 9, 2001. (4) Breen, P. J.; Wasan, D. T.; Kim, Y. H.; Nikolov, A. D.; Shetty. C. S. In Emulsion and Emulsion Stability; Sjoblom, J., Ed.; Marcel Dekker: New York, 1996; p 237. (5) Moore, W. C. J. Am. Chem. Soc. 1919, 41, 940. (6) Finkle, P.; Draper, H. D.; Hildebrand, J. H. J. Am. Chem. Soc. 1923, 45, 2780. (7) Kubo, S.; Hirano, I.; Yamada, J. IFSCC VIIIth International Congress, 1974; A8. (8) Menon, V. B.; Wasan, D. T. Colloids Surf. 1988, 29, 7. (9) Levine, S.; Bowen, B. D.; Paetridge, S. J. Colloids Surf. 1989, 38, 325. (10) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (11) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1994, 162, 1. (12) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (13) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748. (14) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (15) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17, 4540. (16) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708. (17) Binks, B. P.; Clint, J. H. Langmuir 2002, 18, 1270. (18) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374.

Figure 1. A mixed state diagram of the PF-5 silicone resin powder/PFPE/DMS mixtures: region I, network-structured state; region II, O/F emulsion 1 state; region III, O/F emulsion 2 state; region IV, powdery state; region V, granular state; region VI, O + FS + F; region VII, OF,S.

are adsorbed and aligned at the interface between water and oil. Such an adsorption is observed only when the

10.1021/la020511u CCC: $22.00 © 2002 American Chemical Society Published on Web 11/21/2002

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Figure 2. Optical microscopic photographs of the PF-5 silicone resin powder/PFPE/DMS mixtures: (a) 30:68:2 (v/v/v), networkstructured state (FS,O); (b) 28:65:17 (v/v/v), O/F emulsion 1 state (O/FS + F); (c) 13:27:60 (v/v/v), O/F emulsion 2 state (O + O/FS + F); (d) 31:8:61 (v/v/v), granular state (O + OF,S).

powder has equal affinity for both water and oil and the adsorption energy, E, is positive.9 In recent years, Binks et al. have reported the effect of wettability on the dispersed state of the powder particles.12-17 These authors performed a thermodynamic analysis of the systems. Nagayama et al. have developed an “emulsion template” method to prepare supraparticle assemblies and found not only the solid-stabilized emulsion but also ball-like aggregates.18-20 The result suggests to us that surfaceactive powders could form many self-assembled structures such as molecules or polymers of a surface-active nature. We have focused on fluorinated silicone resin powder, because it has a surface-active character and disperses in both fluorinated oils and silicone oils. In the present paper, we characterize the mixed states of the ternary system which consists of fluorinated silicone resin powder, fluorinated oil (perfluoropolymethylisopropyl ether, PFPE), and silicone oil (dimethylpolysiloxane, DMS) to find a novel self-assembled system. Effects of the composition and the wettability of powders with both oils were also investigated. Experimental Section Materials. The silicone resin powder, PFPE, and DMS used in this study were commercially available Tospearl 145A (GE Toshiba Silicone Co. Ltd.; specific gravity, 1.00 g cm-3; spherical, average diameter of 4.5 µm), FOMBLIN HC/04 (Ausimont K.K.; CF3-[(OCF(CF3)CF2)n-(OCF2)m]-OCF3, n/m ) 20/40; MW ) 1500; specific gravity ) 1.79 g cm-3), and KF96A(6CS) (Shinestu Chemical Co., Ltd.; (CH3)3SiO[Si(CH3)2O]nSi(CH3)3; MW ) 900; specific gravity ) 0.93 g cm-3), respectively. PFPE and DMS were used as received. Fluorinated powders which were treated with a fluorinated agent, diethanolamine salt of perfluoroalkyl phosphate (PF-x silicone resin powder, x ) 0.5-10, where the initial letters “PF-x” mean that the powder is modified with x wt % of the fluorinated agent), were purchased from Daito Kasei Co.21 The degree of surface modification was analyzed by means of contact angle with water, PFPE, and DMS for powder tablets; (19) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2385. (20) Velev, O. D.; Nagayama, K. Langmuir 1997, 13, 1856. (21) JP, 4-330007, JP, 63-250311.

electron probe microanalyzer (EPMA); and X-ray photoelectron spectroscopy (XPS) methods. Preparation and Characterization of the Ternary Mixtures. The mixtures of the powders and the PFPE were prepared with a conditioning mixer (Thinky MX-201) at 2000 rpm for 3 min. After adding DMS into the mixture above, the mixture was stirred again under the same conditions. The ternary mixtures thus obtained were transferred into screw-capped test tubes and kept in a thermostated water bath at 25 ( 1 °C. The mixed states of the samples were observed with a Nikon OPTIPHOTO-2 optical microscope. Determination of continuous and separated phases of the mixtures was made by observing the dispersibility of a drop of them in either PFPE or DMS. PFPE continuous (DMS continuous) mixtures are easily dispersed in PFPE (DMS) and remained as drops in DMS (PFPE). Separations of PFPE and/or DMS were observed within several tens of minutes after preparation of the mixtures. Then, the mixed states were checked 3 h after preparation to make a diagram of the fluorinated silicone resin powder/PFPE/DMS ternary system. Analysis and Measurements. Viscosity measurements were carried out by an A & D vibro viscometer CVJ5000 at 25 ( 1 °C. The separation rate of PFPE or DMS from mixtures was measured by a Turbiscan MA2000. EPMA was performed with a JEOL FE-SEM JSM-6330F scanning electron microscope equipped with an Oxford Link ISIS EDS. XPS spectra were measured with a JEOL JPS-9000MX X-ray photoelectron spectrometer using Al XR X-ray radiation whose X-ray voltage was 10 kV. Atomic composition ratios of fluorine and silicon atoms on the surface of PF-x silicone resin powders, P(F or Si), were calculated from the results of XPS as follows:22

∑n(F

P(F or Si) ) n(F1s or Si2p)/

1s

or Si2p) × 100

where n(F1s or Si2p) is a correction value of an XPS peak area assigned as F1s or Si2p peaks, and ∑n(F1s or Si2p) is the sum of correction values of XPS peak areas assigned as F1s and Si2p peaks. Wettability of powders was evaluated from the sinking time into water, PFPE, or DMS.15 Fifty milligrams of powder was placed carefully and evenly on the surface of 20 cm3 of PFPE or (22) Ikeo, N.; Iijima, Y.; Niimura, N.; Shigematsu, M.; Tazawa, T.; Matsumoto, S.; Kojima, K.; Nagasawa, Y. Handbook of X-ray Photoelectron Spectroscopy; JEOL: Tokyo, 1991; p 12.

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Figure 3. Fraction of the separated PFPE or DMS oil with time: (9) separated PFPE oil in PF-5 silicone resin powder/ PFPE/DMS (35:50:15, v/v/v) (O/F emulsion 1 state); (0) supernatant DMS oil in PF-5 silicone resin powder/PFPE/DMS (35:5:60, v/v/v) (granular state). DMS contained in a tube of diameter 32 mm at room temperature. The time taken for all of the powder to disappear from the liquid surface was measured and is adopted as the immersion time. Interpretation of Symbolism. In the diagram, S, F, and O denote the powder, PFPE, and DMS, respectively. FS (OS) indicates a state in which the powder particles disperse in PFPE (DMS). SO,F means that the powder component includes PFPE and DMS oils in its inside space among particles. O/F means a DMS-in-PFPE type emulsion. O/FS + F (O + OF,S) means an emulsion (a dispersion) with a separated phase of PFPE (a supernatant of DMS). We denote the volume fraction of DMS in the system as R (R ) DMS/(DMS + PFPE + the powder)) and the volume fraction of PFPE in the PFPE/ powder mixture as β (β ) PFPE/(PFPE + powder)).

Results and Discussion (a) Effects of the Composition on the Mixed States. Figure 1 shows a mixed state diagram of the PF-5 silicone resin powder/PFPE/DMS ternary system. The mixed states were classified into seven different regions and were ruled roughly by a composition ratio of the powder and PFPE. When the amount of PFPE exceeded the limiting amount of the oil absorption to the powder (β > 0.25), the continuous phase was PFPE (Figure 1, I-III and VI), while otherwise (β < 0.25), the continuous phase was DMS (Figure 1, V and VII). The details of the seven mixed states are described as follows: (i) Network-Structured State. When the PFPE amount exceeds the limit of the oil absorption to the powder (β > 0.25) and the DMS content, R, is 0-0.05, the mixture is in the “network-structured state” (FS,O) (Figure 1, I; Figure 2a). In this state, powder particles form a network structure in the PFPE medium. (ii) O/F Emulsion 1 State. When R is 0.05-0.30, the mixture is in the “O/F emulsion 1 state” (O/FS + F), which is a milky DMS-in-PFPE type emulsion (Figure 1, II; Figure 2b). The mixture is fluid, while the grain size of DMS droplets on which powder particles are adsorbed is several tens of micrometers. Flocculation and creaming of DMS droplets and the separation of excess PFPE phase at the bottom are observed in the emulsion system. The emulsion droplets are easily flocculated mainly because of small electrostatic repulsion between droplets in the nonaqueous emulsion system. The volume of the excess PFPE phase was measured at intervals for 2 weeks. PFPE

Figure 4. Mixed state diagrams of (a) PF-0 silicone resin powder/PFPE/DMS and (b) PF-10 silicone resin powder/PFPE/ DMS mixtures.

separation started after 30 min from emulsion preparation and had attained a stationary state after 10 days (Figure 3). The DMS-in-PFPE type emulsion was kept at the stationary state. The DMS-in-PFPE type emulsion is easily obtained and is stable without any surfactants in the system. (iii) O/F Emulsion 2 State. When the DMS content is more than that of the O/F emulsion 1 state (R > ca. 0.3), the mixture is in the “O/F emulsion 2 state” (O + O/FS + F) (Figure 1, III; Figure 2c). The DMS oil phase separates out by coalescence, and the PFPE phase by creaming. The size of DMS droplets is larger and in wider distribution than that of the O/F emulsion 1 state. (iv) Powdery State. When the PFPE content is less than the limiting amount of the oil absorption to the powder (β < 0.25) and the DMS content is also less than a certain volume, the mixture is in the “powdery state” (SO,F). In this state, the powder component contains both PFPE and DMS oils in its inside space among particles (Figure 1, IV).

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Figure 5. Viscosity of the PF-5 silicone resin powder/PFPE/ DMS mixture plotted against DMS composition: (a) β ) 0.7; (b) β ) 0.2.

(v) Granular State. When more DMS is added to the powdery state, the mixture becomes in the “granular state” (O + OF,S) (Figure 1, V; Figure 2d). The powder particles are granulated in the DMS medium. PFPE is absorbed among particles and acts as a binding material. The slurry separates a clear DMS supernatant after standing for a couple of hours and attains a stationary state after about a week (Figure 3). (vi) Other States. When either powder or PFPE content is small ( 0.3, the viscosity decreased with increasing amount of DMS, probably because of the separated DMS oil and/or the

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Figure 6. Effects of the degree of fluorination on (a) atomic composition ratios of fluorine (9) and silicon (0) atoms on the surface of PF-x silicone resin powders and (b) sinking time of PF-x silicone resin powders into PFPE (9), DMS (0), and water (O). The arrows mean that the sinking time should be longer since the powder still remains on the liquid surface after 48 h. Above 1 wt %, arrows apply to both the DMS data and the water data.

larger emulsion drop size than that in the O/F type emulsion 1 state. The effect of DMS composition on the viscosity at β ) 0.2 is shown in Figure 5b. When the mixture was in the powdery state (R < 0.15), it was impossible to measure the viscosity. When liquid, the viscosity decreased with increasing DMS composition, because the volume of the continuous phase increased. When R is 0.4-0.45, the viscosity could not be measured by the method employed in this work, because the vibro viscometer indicated a sign of “out of range” irrespective of fair reproducibility of the observed values. We do not understand the reason for the above. As one can see from the above results, the dispersed state of the powders in the oils is reflected in the viscosity. (c) Effects of the Powder Wettability on the Mixed States. Powder particles should be situated in a liquidliquid interface to stabilize the emulsions. Powder particles adsorb on the liquid-liquid interface only when adsorption energy, E, is positive.9,13

E ) πR2γAB(1 + cos θ)2 where E is the energy required to remove a particle from an interface into a liquid phase, R is the radius of the powder, γAB is the interfacial tension between A and B phases, and θ is the contact angle which a powder particle makes with the liquid-liquid interface. Binks suggested that the type and the stability of emulsions were controlled with θ. We have also investigated the effects of the wettability of PF-x silicone resin powder on the structure of the mixed state.

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hand, the F/O type emulsion (PFPE-in-DMS type emulsion) was not observed for all ternary systems in the present work. PF-5 silicone resin powder has a suitable balanced affinity to form a stable O/F type emulsion. Namely, the affinity of the powder with PFPE is somewhat higher than that with DMS as shown in the sinking time. Then, the F/O emulsion is not obtained even when the system contains much DMS oil. This is a quite similar situation to that in which the high hydrophilic-lipophilic balance (HLB) surfactants make only O/W emulsions and not W/O emulsions. If a PF-x silicone resin powder having a higher affinity with DMS is used, we may be able to obtain F/O type emulsions. The wettability is thus presumed to be the most important factor to control the mixed state in the nonaqueous systems. Figure 7. Effects of the degree of fluorination on the mixed states and the viscosity of the PF-x silicone resin powder/PFPE/ DMS mixtures (35:50:15, v/v/v).

(c-1) Fluorination of the Silicone Resin Powder. The atomic composition ratios and the wettability of the PF-x silicone resin powders (x ) 0, 0.5, 1, 3, 5, 10) are shown in Figure 6. The results of XPS measurements indicated that the atomic composition ratio of fluorine atoms increased with increasing amount of fluorinated agent, x (Figure 6a). On the other hand, the atomic composition ratio of silicon atoms of the silicone resin decreased with increasing x. It is consequently seen that the fluorinated sites and the naked silicone resin substrate are intermingled on the powder surface. The sinking time of the powder into PFPE became shorter and that into DMS became longer as x increased (Figure 6b). The wettability of the powder with PFPE is enhanced with the degree of fluorination, while that with DMS is reduced. (c-2) Mixed States of the Fluorinated Powders and the Oils. The effects of x on the mixed states and the viscosities of the PF-x silicone resin powder/PFPE/DMS mixtures (35:50:15, v/v/v) are shown in Figure 7. The O/F type emulsions were obtained when x ) 1, 3, 5 wt % (O/F emulsion 1 state (O/FS + F)). The powder was dispersed in DMS when x ) 0, 0.5 wt % (OS + F) and was dispersed in PFPE when x ) 10 wt % (O + SF + FS). The powder is dispersed in DMS or PFPE when it has too high an affinity to DMS or PFPE, and the O/F type emulsion is formed when it has a balanced affinity to both oils. On the other

Conclusions We have studied a ternary system consisting of fluorinated silicone resin powder, PFPE, and DMS and observed multiple structural polymorphism, that is, a networkstructured state, an O/F emulsion state, a powdery state, a granular state, an O/F emulsion state with excess PFPE and DMS phases, a powder dispersion state with excess PFPE and DMS phases, and a dispersion state including PFPE and the powder. According to the results of XPS and sinking time measurements, the wettability of the powder with PFPE and DMS is the governing factor to control the mixed state. The O/F emulsion state and the granular state were stable due to the adsorption of powder particles on the interface between two oil phases, PFPE and DMS, although no surfactant was added to the ternary system. The emulsion is the first solid-stabilized oil-in-oil type emulsion which is theoretically predicted by Binks et al.17 These mixed states are interesting not only from the point of view of colloid science but also for their applications in the chemical and cosmetic industries. A surfactantfree homogenizing technique is useful to improve the properties of chemical and consumer products.23,24 Studies on the mechanism and the application of the powder selfassembly are in progress and will be published soon. LA020511U (23) Kamogawa, K.; Sakai, T.; Momozawa, N.; Shimazaki, M.; Enomura, M.; Sakai, H.; Abe, M. J. Jpn. Oil Chem. Soc. 1998, 47, 159. (24) Sakai, T.; Sakai, H.; Abe, M. Langmuir 2002, 18, 3763.