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Influence of Particle Hydrophobicity on Particle-Assisted Wetting Ailin Ding,†,‡ Bernard P. Binks,§ and Werner A. Goedel*,†,‡,|,⊥ Organic & Macromolecular Chemistry and Material Science & Catalysis, University of Ulm, 89081 Ulm, Germany, Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K., Department of Polymer Physics, BASF Aktiengestellschaft, 67056 Ludwigshafen, Germany, and Physical Chemistry, Chemnitz University of Technology, 09111 Chemnitz, Germany Received August 27, 2004. In Final Form: November 17, 2004 The surface properties of silica particles significantly influence their efficiency in particle-assisted wetting. A series of small particles of controlled surface hydrophobicity was mixed with a nonvolatile oil. These mixtures were applied onto a water surface; the structures formed were subsequently solidified by photopolymerization and observed using scanning electron microscopy. For the most hydrophilic particles, only lenses of pure oil formed, with the particles being submerged into the aqueous phase. The most hydrophobic particles help to form patches of stable homogeneous mixed layers composed of oil and particles. In these cases the particles adhere to the air-oil as well as to the oil-water interfaces. For particles with intermediate hydrophobicity, lenses and patches of mixed layers were observed. These three different observations verify that the hydrophobicity of the particle surface determines the wetting behavior of the oil at the water surface.
Introduction The wetting of a surface by an organic liquid (oil) has attracted much attention since it is of great importance both in scientific terms and in technological processes. Wetting in general is understood as an interplay of shortrange forces like hydrogen bonding and long-range forces like van der Waals or dispersion forces. If both are favorable, a liquid can spread out on a surface to form layers of any thickness. In the case of unfavorable longrange forces, only partial wetting is observed and the liquid forms lenses thatsdepending on the short-range forcess may or may not coexist with a layer of limited thickness. Intense research has been conducted to explore ways of enhancing wetting with the aid of low molecular weight surfactants.1 On the other hand, it is known that particles can have surfactant-like properties. They have a strong tendency to adhere to fluid interfaces, mainly due to the reduction of the total interfacial energy upon replacing part of the liquid-liquid or liquid-vapor interface by a liquidparticle interface. One consequence of this is the ability of particles to stabilize emulsions, the so-called Ramsden or Pickering emulsions.2-8 Since particles may act as surfactants, one might think of using them as wetting * To whom correspondence should be addressed. Telephone: 0049-371-5311731. Fax: 0049-371-5311371. E-mail:
[email protected]. † Organic and Macromolecular Chemistry, University of Ulm. ‡ Technical University of Chemnitz. § University of Hull. | Materials Science and Catalysis, University of Ulm. ⊥ BASF Aktiengestellschaft. (1) Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (3) Finkle, P. Draper, H. D.; Hildebrand, J. H. J. Am. Chem. Soc. 1923, 45, 2780. (4) Schulman, J. H.; Leja, J. Trans. Faraday. Soc. 1954, 50, 598. (5) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (6) Binks, B. P. Curr. Opin. Colloid Interf. Sci. 2002, 7, 21. (7) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloid Interface Sci. 2003, 100-102, 503. (8) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748.
agents similar to low molecular weight surfactants. However, particles at surfaces usually are quoted as preventing wetting of a surface; e.g. small wax particles on the cuticula of plant leaves are responsible for the socalled Lotus effect9 (superhydrophobicity combined with self-cleaning if exposed to water droplets), which in the meantime has been successfully translated into technical applications.10 The main reason for this behavior is the fact that in those cases the particles are attached to solid surfaces and increase the contact area of the liquid instead of decreasing it as they do in Pickering emulsions. Recently it was discovered that particles can be used as a wetting aid if a liquid interface is involved. Mixtures of oil and suitable particles form wetting layers on a water surface although the oil alone is nonwetting.11,12 The principle of this effect is outlined schematically in Figure 1. Particles easily form monolayers on a water surface.13,14 An oil, that usually forms lenses, can be dragged into the free space of that monolayer via capillary forces. If an excess of oil is used, in principle one can form a thick layer of oil, the particles adhering either to the upper or the lower interface or partitioning between both (Figure 1c-e). Depending on the wettability of the particles, it may well happen that lenses and a monolayer of particles coexist (Figure 1a) or that the particles are completely enclosed within the oil lenses (Figure 1b). However, the formation of mixed wetting layers composed of particles and a liquid is not completely understood at the moment. In the first experiments described in ref 11, it seemed to depend markedly on the surface properties of the particles. Particles coated with methacrylate groups that had a chemical structure very similar to the used oil were efficient in assisting wetting, while particles with a (9) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (10) Dambacher, G. T. Kunstst. Plast. Eur. 2002, 92, A18. (11) Xu, H.; Goedel, W. Langmuir 2003, 19, 4950. (12) Goedel, W. Europhys. Lett. 2003, 42, 607. (13) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (14) Fulda, K. U.; Tieke, B. Adv. Mater. 1994, 6, 288.
10.1021/la047858c CCC: $30.25 © 2005 American Chemical Society Published on Web 01/20/2005
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Figure 1. Schematic illustration of particle-assisted wetting.
dissimilar surface coating (polyisobutene chains) were inefficient. At that time, it was assumed that the particles do not help the organic liquid to wet the water if the particle surface is completely incompatible with the organic liquid. Only, if the liquid partially wets the particles can a homogeneous hybrid layer be formed. The influence of the surface thermodynamics was investigated in a simple theoretical treatment.12 It was predicted that particles with a contact angle of 90° (either on the oil-water interface or on the oil-air interface) should be most suitable to assist wetting. Thus the most suitable particles should be those having intermediate hydrophobicity. In previous investigations, a series of particles of varying hydrophobicity was used to study the formation and stability of Pickering emulsions.5,15 Here we use the same series to investigate in a more systematic way the influence of particle hydrophobicity on particle-assisted wetting. Experimental Section The organic liquid, trimethylolpropane trimethacrylate (TMPTMA), and the photoinitiator, benzoinisobutyl ether, were purchased from Aldrich Chemicals. TMPTMA was purified by pressing it through silica gel before usage and then mixed with 5 wt % of the photoinitiator. Water (18 mΩ/cm; total organic carbon < 5 ppm) was passed through a Milli-Q water system. A series of fumed silica particles was prepared and provided by Wacker-Chemie (Burghausen). They were coated via treatment with dicholorodimethylsilane. According to the supplier, there are no indications of inhomogeneous coating of the particles. They are characterized as described in ref 5 by the amount of silanol groups on the surface. About 100 mg of mixtures of the organic liquid (1.5 wt % with respect to chloroform), silica particles (0.3 wt % with respect to chloroform), and chloroform were applied onto the surface of water-filled Petri dishes (22.9 cm2 surface area) at 20 °C. After evaporation of the chloroform, the structures were solidified by photopolymerization via 8 h of irradiation with UV light of 360 nm wavelength (intensity of 1.05 mW/cm2; Umex GmbH, Dresden) and then transferred to poly(methyl methacrylate) (PMMA) substrates or electron microscopy grids using horizontal transfer (placing the substrate horizontally below the water surface and then lowering the water level16). The UV lamp was equipped with a filter plate retaining infrared radiation. No significant temperature changes occurred on the water surface. The copper or gold electron microscopy grids were purchased from Plano Co. The images were obtained using a Zeiss DSM 962 scanning electron microscope (SEM) and a Hitachi S-5200 field emission scanning electron microscope (FESEM). Balzers Union MED010 (15) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727. (16) Araki, T.; Oinuma, S.; Iriyama, K. Langmuir 1991, 7, 738.
Figure 2. High-resolution field emission scanning electron microscopy (FESEM) images of silica particles used in this study (14% SiOH). and EVM052 deposition systems were used to sputter ∼20 nm of Pd/Au alloy on the sample surfaces to increase their conductivity. The surface tension of water in the presence of TMPTMA was measured with a KSV-Sigma 70 tensiometer using a du Nou¨y ring and Huh-Mason correction. Water and 5 wt % TMPTMA were vigorously stirred overnight, and then the aqueous phase was separated from the excess of TMPTMA prior to the experiment. Interfacial tensions of TMPTMA against air and water were measured from the shape analysis of pendant drops with a G10 Kru¨ss interfacial tension analyzer. The system measurement error is lower than 2.8%.
Results and Discussion Mixtures of a nonvolatile polymerizable oil, trimethylolpropane trimethacrylate (TMPTMA), with silica particles dissolved/dispersed in chloroform, were applied to a water surface. After evaporation of the chloroform, the structures formed on the water surface were solidified by photopolymerization, then transferred to solid substrates, and imaged with the help of SEM and high-resolution FESEM. The core of these investigations is to use particles of different surface hydrophobicity, compare their influence on the wetting of the water surface by the oil, and identify the location of the particles in the interfaces via electron microscopy. A series of commercial small silica particles was utilized in which the hydrophobicity was modified by reacting the originally hydrophilic surface to different extents with dichlorodimethylsilane. The reactive hydrophilic silanol groups (SiOH) on the surface of the particles were thus converted into hydrophobic dimethylsiloxane groups.17 We assume that the most hydrophilic particle surface has 100% silanol groups, and the hydrophobicity of the other particles is described in terms of the percentage of silanol groups remaining on their surfaces. The most hydrophobic (17) Binks, B. P.; Clint, J. H. Langmuir 2002, 18, 1270.
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Figure 3. Scanning electron microscopy (SEM) images of (a) TMPTMA lenses on an air-water interface; (b) TMPTMA lenses formed from the mixture of TMPTMA and particles (87% SiOH); (c-g) patches formed from mixtures of TMPTMA and particles of decreasing hydrophilicity: (c) 80%, (d) 51%, (e) 36%, (f) 24%, and (g) 14% SiOH.
particles possess 14% of silanol groups on their surface. Since the particles were too small to be observed under normal SEM, high-resolution FESEM was utilized. The primary particles of average diameter of 20 nm can aggregate to form larger clusters. As an example, the images of the particles bearing 14% silanol groups are shown in Figure 2. The images of the other types of particles do not differ significantly from these.
If the organic liquid TMPTMA is applied to a water surface, either as pure substance or dissolved in a volatile solvent, it does not form a uniform wetting layer, but rather retracts into lenses of various sizes. The interfacial tensions at 25 °C of the air-TMPTMA interface and the TMPTMA-water interface were measured to be γao ) 32.86 mN/m and γwo ) 18.99 mN/m, respectively. They are similar to the corresponding interfacial tensions of
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Figure 4. High-resolution FESEM images of the air-oil (TMPTMA) surface (a and b) and the oil (TMPTMA)-water interface (c and d) of photo-cross-linked TMPTMA lenses.
other water-insoluble esters (e.g. in the case of n-butyl acetate, γao ) 23.60 mN/m and γwo ) 13.40 mN/m 18). From these values and the tension of the interface between air and water that was saturated with TMPTMA (γaw ) 51.74 mN/m), one can calculate an equilibrium spreading coefficient of Se ) γaw - γao - γwo ) -0.11 mN/m. The negative value of Se is in accordance with the fact that TMPTMA alone forms lenses on a water surface. The fact that the presence of TMPTMA significantly lowers the surface tension of water indicates that the lenses coexist with a very thin layer, presumably a monomolecular layer on the water surface. Figure 3 shows, at comparably low magnification, images of structures formed by the oil TMPTMA and its mixtures with particles on a water surface, transferred to a solid substrate (Figure 3a) and electron microscopy grids (Figures 3b-g). The oil alone forms lenses as shown in Figure 3a. For the two most hydrophilic types of silica particles (silanol contents of 100% and 87%), very similar lenses were observed (see for example Figure 3b). On the contrary, the most hydrophobic silica particles with silanol contents equal to and lower than 51% help to form patches of stable homogeneous layers on the water surface (Figure 3d-g). In the experiments, the area per particle was deliberately chosen to be two times that of a close-packed monolayer. Thus the formation of patches is in agreement with a partial coverage of the surface by densely packed particles. It is worth noting that the size of the patches and the area covered by the layer depend on the particle hydrophobicity. Upon increasing the silanol content from 14% to 36% (Figure 3e-g), the patch size is increased from a few hundred µm2 to several cm2. The total area covered by these patches increased from 10% to 40% of the Petri dish. When the silanol content increases to 51% (Figure (18) Santos, B. M. S.; Ferreira, A. G. M.; Fonseca, I. M. A. Fluid Phase Equilib. 2003, 208, 1.
3d), the patch size begins to diminish again. This might imply that particles with a silanol content of 36% have an optimum compatibility with liquid TMPTMA and thus facilitate wetting more efficiently than the other particles. For particles with intermediate hydrophobicity, possessing silanol contents of 67% and 80%, both TMPTMA lenses and patches of mixed layers were observed for each sample (Figure 3c). In addition, the area covered by the patches (approximately 20%) is much smaller than those formed from more hydrophobic particles. The different observations described above indicate that the hydrophobicity of the particle surfaces ultimately determines the wetting behavior of TMPTMA at an airwater interface. For hydrophobic particles, the driving force of particle-assisted wetting is attributed to the reduction of the total interfacial energy of the system that occurs if the particles partially replace the bare airTMPTMA and TMPTMA-water interfaces. That is, adsorption of particles to one or both interface(s) gives rise to a gain in energy that facilitates the wetting of water by the organic liquid. Therefore, one can expect the particles to be visible at at least one of the interfaces of the oil layer or at both. The more hydrophilic the particles are, the more one can expect them to adhere to the TMPTMA-water interface only or to completely transfer into the bulk water phase.12 This deduction was partially proven by high-resolution FESEM images of the various interfaces involved (see below). To identify whether the particles are adsorbed to any of the interfaces or even to both, the surfaces of the droplets and patches were imaged with high-resolution electron microscopy. In Pickering emulsions, particles adsorbed to oil-water interfaces have been observed via SEM using a freeze fracture technique15,19 or immobilizing them (19) Simovic, S.; Prestidge, C. A. Langmuir 2003, 19, 3785.
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Figure 5. High-resolution FESEM image of the air-oil surface (a and b) and the oil-water interface (c and d) of a patch formed from the mixture of TMPTMA and particles (36% SiOH).
Figure 6. High-resolution FESEM image of the air-oil surface (a and b) and the oil-water interface (c and d) of a patch formed from the mixture of TMPTMA and particles (80% SiOH).
through particle fusion.20 Here we neither can freeze the interface as fast as needed for freeze fracture SEM nor can we sinter the particles through gentle heating. We thus resorted to solidifying the oil via room-temperature photopolymerization. It is known that the photopolym-
erization of TMPTMA yields a gel at a conversion degree of only 3%-4%.21,22 This gel behaves already as a solid. The surface structure and contact angles are thus not substantially changed during the subsequent completion of the polymerization.23 Since a conversion of only a few
(20) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006.
(21) Rosenberg, J. E.; Flodin, P. Macromolecules 1988, 21, 2041. (22) Rosenberg, J. E.; Flodin, P. Macromolecules 1989, 22, 155.
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percentdoes not substantially change the polarity of the organic liquid, we can assume that the structures observed at the solidified layer closely reflect the structures formed in the liquid state. As a reference, close up images of the upper and lower surfaces of the photo-cross-linked lenses of the pure oil are shown in Figure 4. Besides some indication of wrinkling, the images are essentially featureless, especially if compared to the images shown further below. In the case of mixtures with particles of 100 and 87% silanol group content, the top and bottom surfaces of the lenses formed have essentially the same appearance as the lenses of the pure oil (images not shown here). It is thus very likely that these particles do not adhere to any of the interfaces but completely submerge into the bulk water phase. The interfaces of the patches formed in the most advantageous case (36% silanol content) have a structure significantly deviating from the structure of the pure TMPTMA (Figure 5). By comparison with the images of the pure oil and of the particles, we can conclude that the particles adhere to both interfaces of the layer. (In this experiment the mixture ratio between TMPTMA and particles, 5:1 by weight and 9.4:1 by volume, was chosen in such a way that the thickness of an oil layer formed on top of a closepacked layer of particles is thick enough to prevent the penetration of individual aggregates from the top to the bottom.) By comparing the images of the upper surface (Figure 5a,b) and the bottom surface (Figure 5c,d), it seems that the density of particles is higher on the bottom surface, implying that the particles tend to adhere to the TMPTMA-water interface more than to the airTMPTMA surface. For particles with intermediate surface hydrophobicity, both lenses and patches of mixed layers were observed (Figure 6). The lenses again are essentially free of particles. The images of the patches indicate that the particles are also adsorbed at both the top (Figure 6a,b) and the bottom interfaces (Figure 6c,d). (23) A similar situation arises in the determination of contact angles via the “gel trapping technique”: Paunov, V. N. Langmuir 2003, 19, 7970.
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It was assumed that, with increasing hydrophilicity of the particle surfaces, the location of particles will shift from being attached to both interfaces (air-oil and oilwater) to being attached to only the oil-water interface, and finally to being completely immersed into the bulk water phase. In this work, we did not observe such a clear transition. In the case of intermediate hydrophobicity, one observes coexistence between lenses and layers with particles at both interfaces. The reasons for this behavior are not completely understood at the moment. One possible cause might be the heterogeneity of the particle shapes, which may cause a preferential adsorption/desorption of subsets of the particles to the various interfaces involved. Conclusions In summary, the hydrophobicity of silica particle surfaces significantly influences the wetting behavior of oil/particle mixtures on a water interface. Only particles with surface silanol content lower than 50% can assist the wetting of water by TMPTMA and thus lead to a homogeneous layer, the most efficient type of particle having 36% of silanol groups. Thus, we conclude that particle-assisted wetting is predominately a function of the hydrophobicity of the particle surface and might be achieved with particles made of various chemical substances, provided the surface properties are tuned accordingly, e.g. via chemical reaction, adsorption of amphiphiles, or adjusting the pH more or less toward the isoelectric point. Especially the latter aspects might allow the switching between the wetting and the nonwetting condition by external stimuli and might thus be of significant technological relevance. Acknowledgment. We thank K. Landfester, B. Rieger, H. Auweter, and R. Iden for supporting these investigations, P. Walther for guidance in FESEM measurements, and Wacker-Chemie, Burghausen, for providing the silica particles. This work was financially supported by the Deutsche Forschungs Gemeinschaft through the research training group 328 of the University of Ulm and the priority program 1052. LA047858C
