Langmuir 2007, 23, 5255-5258
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Contact Angles of Submillimeter Particles: Connecting Wettability to Nanoscale Surface Topography Keith M. Forward,† Amanda L. Moster,† Daniel K. Schwartz,‡ and Daniel J. Lacks*,† Department of Chemical Engineering, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed February 16, 2007. In Final Form: March 12, 2007
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A method is presented for the measurement of contact angles of particles in the size range 100-1000 µm, using an optical microscope. This method is used to characterize the wettability of polyethylene particles produced in a gas-phase polymerization process. The as-polymerized particles are shown to be significantly more hydrophobic than melt-processed polyethylene materials. The surface structure of the particles is examined with scanning electron microscopy, and the increased hydrophobicity of the as-polymerized particles is attributed to the nanoscale texture of the particle surfaces.
I. Introduction The wettability of particulate materials, which is influenced by the chemical composition and physical structure of the particle surfaces, has a range of consequences. For example, transport properties such as fluidization and granular flow are strongly affected by particle aggregation, which is in turn controlled by particle surface properties including wettability. Particle wettability is also of critical importance in flotation processes and when used as emulsion stabilizers (Pickering emulsions). Furthermore, particles with highly hydrophobic surfaces could serve as the basis of novel applications, such as those that mimic the self-cleaning functions of the Lotus leaf. The wettability of a material can be characterized by the contact angle describing the three-phase (liquid-particle-air) interface. Although the contact angles of macroscopically flat surfaces are easily determined by sessile drop methods,1 determining the contact angles of particles presents an experimental challenge. A range of methods have been used for this purpose, but a generally ideal method does not exist. Wicking methods such as the Washburn approach determine the average wettability of a packing of particles based on the liquid uptake of the packing,2 but obtaining reliable results with these methods is difficult;3 further drawbacks occur because the contact angle determination is based on a pore model with an arbitrary geometric parameter that must be determined for each sample, and because the particle packing may not give the required pore size range when the sample includes coarser particles. Tensiometry methods move a particle through an interface,4,5 but the contact angle determination proceeds from an analysis based on spherical particles, so that the accuracy is diminished for particles that are not perfectly spherical. Other methods image particles sitting on a surface,6-8 but the imaging is carried out from above the particle, which necessitates an analysis based on spherical particles to determine the contact angle. * To whom correspondence should
[email protected]. † Case Western Reserve University. ‡ University of Colorado.
be
addressed.
E-mail:
(1) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827. (2) Chibowski, E.; Gonzalez-Caballero, F. Langmuir 1993, 9, 330. (3) Chibowski, E.; Perea-Carpio, R. AdV. Colloid Interface Sci. 2002, 98, 245. (4) Zhang, L.; Ren, L.; Hartland, S. J. Colloid Interface Sci. 1997, 192, 306. (5) Preuss, M.; Butt, H.-J. J. Colloid Interface Sci. 1998, 208, 468. (6) Paunov, V. N. Langmuir 2003, 19, 7970. (7) Mohammadi, R.; Amirfazli, A. J. Disper. Sci. Technol. 2004, 25, 567. (8) Hadjiiski, A.; Dimova, R.; Denkov, N. D.; Ivanov, I. B.; Borwankar, R. Langmuir 1996, 12, 6665.
In this Letter, we describe a general method for the direct measurement of the contact angles of particles in the size range 100-1000 µm. This method uses a conventional optical microscope and has advantages over previous methods based on optical microscopy,9,10 as discussed below. This method is used to characterize the wettability of polyethylene particles produced in a gas-phase polymerization process, for which we find contact angles that are significantly larger than those for melt-processed polyethylene materials, with these differences being directly related to the nanoscale texture of the particle surfaces. II. Methodology for Contact Angle Measurement of Particles The method is depicted in Figure 1. A drop of ultrapure (18 MΩ) water is placed between two microscope slides separated by a spacer, and this sandwich is held together by clamps. Because the water drop is contained within an enclosed space between the microscope slides and the spacer, evaporation of the water is prevented (also in regard to evaporation, the slides are placed on 1/4 in. aluminum blocks to minimize heating from the microscope lamp). Microscope slides constructed from several different materials are used, as described below. The spacer is constructed from polyethylene and has a thickness of 750 µm. Prior to use, the slides are cleaned by lightly wiping with isopropanol (70%). The measurements are made on particles with diameters ranging from 200 to 400 µm. A particle is placed at the water-air interface; the particle and water positions adjust to attain the equilibrium water-air-particle geometry (this equilibrium geometery forms within seconds, and does not change within the time frame of the experiments). Digital pictures are taken of the water-particle-air interface under a microscope. The ImageJ image processing software11 is used to enlarge the pictures and digitally find the edges that denote the interfaces, and to measure the contact angle (the contact angle measurement requires a user to mark the angle on a screen within the ImageJ program). The method does not require a particular particle shape and can be applied to irregularly shaped particles. The precision of the measurement is limited by the fact that the projection of the water-air interface, when viewed from above, is not sharp due to the meniscus associated with the wetting of the microscope slide. The apparent width of this interface, which hinders the measurement of the contact angle, depends on the hydrophobicity (9) Velev, O. D.; Denkov, N. D.; Paunov, V. N.; Kralchevsky, P. A.; Nagayama, K. J. Colloid Interface Sci. 1994, 167, 66. (10) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Langmuir 1996, 12, 997. (11) Rasband, W. S. ImageJ, U. S. National Institutes of Health, Bethesda, MD, http://rsb.info.nih.gov/ij/, 1997-2005.
10.1021/la700471y CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
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Letters Table 2. Contact Angle Results for As-Polymerized PE Particles slide material PETGa Acrylica PSa PCb All materialsc
low MW PE angle (deg)
high MW PE angle (deg)
109 ( 5 111 ( 4 109 ( 3 104 ( 4 106 ( 3
130 ( 4 128 ( 5 132 ( 4 135 ( 2 133 ( 2
a Average for 3 particles. b Average for 17 particles for low MW PE, 15 particles for high MW PE. c Average for 26 particles for low MW PE, 24 particles for high MW PE.
Figure 1. Experimental methodology for measuring contact angles of particles: (a) view from the top; (b) view from the side (note that the portion of the spacer in front is omitted); (c) view from the top, zoomed in. The microscope slides are shown in yellow, the spacer is shown in gray, the water drop is shown in blue, and the particle is shown in black. Table 1. Liquid-Air Interface Thickness slide material
thickness (µm)
glass PETG acrylic PC PS
173 ( 4 47 ( 3 67 ( 2 29 ( 2 38 ( 3
of the microscope slides. For this reason, the investigation is carried out with microscope slides constructed from hydrophobic materials rather than glass; microscope slides are constructed from glycolmodified polyethylene terephthalate (PETG), acrylic, polycarbonate (PC), and polystyrene (PS). The importance of the hydrophobic microscope slides is evident in Figure 2: the water-air interface is much narrower on the PC slide than on a glass slide, which allows a more precise identification of the contact angle. The measured apparent width of the water-air interface on each type of microscope slide is given in Table 1. Results for the contact angles of polyethylene particles (described below) with water are shown in Table 2. The contact angle results obtained with the various microscope slides are within the error estimates determined from the distributions of measured contact angles. The present method has several advantages over previously reported approaches based on optical microscopy. The method
reported here requires only a conventional benchtop optical microscope and is thus more easily implemented than the approach described in ref 9 where the particle sits at a horizontal water-air interface, which necessitates a special experimental setup to image the three-phase interface from the side. In addition, the method reported here is applicable to particles with a full range of contact angles, both hydrophilic and hydrophobic. A related method was also described in ref 10. In that work, glass slides were used to sandwich particles without any sort of spacer. In fact, we initially used a similar approach; however, as shown above, we observed broad meniscus lines using glass slides. We also found that a spacer was necessary to maintain an air-water interface with reproducible dimensions and to isolate particles from the slide surface; without a spacer, many particles were pinned between the slides, causing large errors. Additionally, the spacer acts to seal the system and prevent evaporation of the water drop; in the absence of the spacer, the heat from the optical microscope causes evaporation at a rate that significantly alters the interface over period of a few minutes. Thus, the method described in this letter is broadly applicable for a large range of particle sizes, shapes, and compositions.
III. Application to Polyethylene Particles The methodology described above is used to determine the wettability of polyethylene (PE) particles produced in a commercial reactor based on the widely used gas-phase polymerization process.12 In this process the polymer is produced in particulate form, with ∼90% (by mass) of the particles in the size range 100-1000 µm. The reactor conditions (e.g., feed composition and temperature) are varied to produce polymers with different characteristics such density and molecular weight. Contact angles are measured for particles of two types of PE produced by this gas-phase polymerization process: a low molecular weight (MW) PE, and a high MW PE. The results, shown in Table 2, indicate that particles of high MW PE are significantly more hydrophobic than particles of low MW PE. Figure 3 shows typical pictures for the water-particle-air interface: it is evident that the high MW PE particle sits further outside the water drop than the low MW PE particle, due to its more hydrophobic character. Contact angles are also obtained for water on macroscopically flat surfaces produced by melting and then cooling samples of
Figure 2. Pictures of the water-particle-air interface for PE particles: (a) using glass microscope slides; (b) using PC microscope slides.
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Figure 3. Pictures of the water-particle-air interface and SEMs for PE samples: (a) water-particle-air interface for low MW PE; (b) SEM of low MW PE; (c) water-particle-air interface for high MW PE; (d) SEM of high MW PE; (e) SEM of melt-cooled surface of high MW PE (the SEM of the melt-cooled surface of low MW PE looks very similar, and thus is not shown).
these PE particles. The contact angles for drops of water on these melt-cooled surfaces are measured with a sessile drop method. The advancing contact angles are 100 ( 1° for the low MW PE and 101 ( 1° for the high MW PE (the receding contact angles were 78 and 81°, respectively); these results are consistent with literature reports for the contact angle of water on polyethylene surfaces of approximately 100°.13-15 In comparing the results for the as-polymerized particles and the melt cooled surfaces, we clearly see that the as-polymerized particles are significantly more hydrophobic than the melt-cooled surfaces. It is well-known that surface roughness can increase the hydrophobicity of surfaces and cause θapp, the apparent contact angle, to exceed θ, the intrinsic contact angle for a smooth surface. In the complete wetting limit, the Wenzel equation cos θapp ) (12) Xie, T.; McAuley, K. B.; Hsu, J. C. C.; Bacon, D. W. Ind. Eng. Chem. Res. 1994, 33, 449. (13) Edge, S.; Walker, S.; Feast, W. J.; Pacynko, W. F. J. Appl. Polym. Sci. 1993, 47, 1075. (14) Banik, I.; Kim, K. S.; Yun, Y. I.; Kim, D. H.; Ryu, C. M.; Park, C. E. J. Adhesion Sci. Technol. 2002, 16, 1155. (15) Puukilainen, E.; Koponen, H.-K.; Xiao, Z.; Suvanto, S.; Pakkanen, T. A. Colloid Surf. A: Physicochem. Eng. Aspects 2006, 287, 175.
r cos θ applies,16,17 where r is a measure of the surface roughness (r g 1, r ) 1 for a smooth surface). In the incomplete wetting limit, the Cassie-Baxter equation cos θapp) f cos θ - (1 - f) applies,17,18 where f is the fraction of the surface wetted. Surface roughness causes the apparent contact angle to exceed the intrinsic contact angle in both of these limits (for θ > 90°). To address the role of surface roughness on the wettability of these materials, scanning electron micrographs (SEMs) were obtained of the as-polymerized particles and the melt-cooled surfaces. Before SEM imaging, the surfaces were prepared by sputtering to form 5 nm thick Pd layers. Representative SEMs are shown in Figure 3. The SEMs show that the surface roughness is much greater on the as-polymerized particles than on the meltcooled surface. This result makes sense physically: the melt surface is smooth, and so the solid surface obtained by cooling a melt is smooth. In contrast, the as-polymerized particles are formed as polymer chains grow in the solid state (below the melting temperature), and so surface roughness develops because (16) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (17) Marmur, A. Langmuir 2003, 19, 8343. (18) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
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the polymers grow heterogeneously on the particle. Thus the greater contact angles obtained for the as-polymerized particles in comparison to the melt-cooled surfaces can be attributed to the greater surface roughness on the particles. The SEMs also show the surface roughness is significantly greater on particles of high MW PE than on particles of low MW PE. Thus the greater contact angles obtained for the high MW particles in comparison to the low MW particles can be attributed to the greater surface roughness on the high MW particles. This difference in surface roughness is a consequence of different reactor conditions. Although both polymers were produced with the same catalyst type, the reactor was operated at different feed concentrations and temperatures. The temperature is likely to be the most relevant difference in regard to the surface morphology: the low MW PE is produced at a higher temperature, which would allow more annealing to occur that smooths the surface. Recently, solvent19,20 and plasma21 processing techniques, and direct solution-phase polymerization techniques,22 have been developed to enhance the hydrophobicity of polyethylene and
Letters
polypropylene films. We show that commercial gas-phase polymerization processes produce polyethylene with enhanced hydrophobicity, due to the surface texture that arises naturally during the polymer growth. The enhanced hydrophobicity has not been noticed previously due to the difficulty of characterizing the wettability of particles in this size range, and because the as-polymerized materials are normally melted before further use, which removes the enhanced hydrophobicity. By varying reactor conditions, we can tailor the surface roughness and thus the hydrophobicity of the as-polymerized PE. The PE samples examined here are produced in current commercial reactorssit is possible that even greater hydrophobicity can be produced in as-polymerized PE by adjusting the reactor conditions. Acknowledgment. We thank Karl Jacob, Bob Jorgensen, Tom McNeil, Tom Spriggs, and Mike Turner for valuable discussions and insight, and for providing us with the polyethylene samples. This project was funded by the Dow Chemical Co. LA700471Y
(19) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (20) Lu, X.; Zhang, C.; Han, Y. Macromol. Rapid Commun. 2004, 25, 1606. (21) Fresnais, J.; Chapel, J. P.; Poncin-Epaillard, F. Surf. Coat. Technol. 2006, 200, 5296.
(22) Han, W.; Wu, D.; Ming, W.; Niemantsverdriet, H. J. W.; Thune, P. C. Langmuir 2006, 22, 7956.
