Langmuir 2002, 18, 6821-6829
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Wetting Behavior of Silicone Oils on Solid Substrates Immersed in Aqueous Electrolyte Solutions T. Svitova,†,‡ O. Theodoly,† S. Christiano,§ R. M. Hill,§ and C. J. Radke*,† Chemical Engineering Department, University of California, Berkeley, California 94720-1462, Institute of Physical Chemistry RAS, Moscow, Leninsky Pr. 31, 117915 Russia, and Dow Corning Corporation, P.O. Box 994, Midland, Michigan 48686-0994 Received January 2, 2002. In Final Form: June 12, 2002 Equilibrium contact angles are reported for silicone oils (poly(dimethylsiloxane)s) on polymer-coated and uncoated solid-silicon substrates immersed in aqueous electrolyte solutions. Solid-substrate wettability to water ranges from highly hydrophilic to highly hydrophobic based on water/air contact angles. Although silicone oils in air spread completely on all of the studied substrates, these same surfaces when immersed in aqueous media exhibit finite contact angles against silicone oils that depend strongly on the substrate surface energy. A detailed investigation of the pH influence on the wetting behavior of silicone oil on the solid substrates is pursued where a clear correlation emerges between the changes of substrate-surface zeta potential (ζ) and the oil-wetting behavior on substrates immersed in aqueous solution. Also, the influence of inorganic KCl and CaCl2 electrolytes on the wetting behavior of silicone oils on solid substrates is studied. KCl does not produce a noticeable effect on the wetting behavior of silicone oils. CaCl2, in general, increases surface hydrophilicity, with the exception of the chitin-coated silicon-wafer substrate. For this polymer surface, the oil/water contact angle decreases with increasing CaCl2 concentration, indicating stronger oil wetting. Specific interactions between chitin surface functional groups and calcium ions are confirmed by ζ-potential measurements. Finally, we find that the classic Bartell-Osterhof equation, based on the thermodynamics of ideal wetting, well describes the wetting behavior of silicone oil in water on apolar and moderately polar solid substrates of different functionality, provided that accurate measurements are available for the corresponding air-in-water contact angles and for the equilibrium surface and interfacial tensions. The same Bartell-Osterhof equation yields invalid predictions for contact angles of oils in water on substrates wettable by both liquid phases and for contact angles of water on the substrates studied when they are immersed in silicone oils.
Introduction Wetting and spreading phenomena are intensely investigated due to their widespread applications and fundamental scientific importance.1,2 These investigations provide information on the nature of surfaces and the interactions between spreading fluids and substrates. Almost universally, wettability studies are focused on contact-angle measurements of liquids on solid substrates in air. Interfacial wettability (i.e., the wetting of solids in three-phase systems containing two immiscible liquids3) has attracted only limited attention.3-7 Nevertheless, this case is important in a number of practical applications. Some studies of the wetting behavior by an aqueous phase against low-energy surfaces immersed in nonpolar media are available.3-5 The reverse case, however, of a solid substrate immersed in an aqueous medium and contacted * To whom correspondence should be addressed. † University of California. ‡ Institute of Physical Chemistry RAS. § Dow Corning Corp. (1) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (2) Contact Angle, Wettability, and Adhesion; Advances in Chemistry Series, Vol. 43; American Chemical Society: Washington, DC, 1964. (3) Johnson, R. E.; Dettre, R. H. Wetting of Low-Energy Surfaces. In Wettability, Berg, C. J., Ed.; Surfactant Science Series, Vol. 49; Marcel Dekker: New York, 1993; pp 1-73. (4) Tamai, Y.; Makunchi, J.; Suzuki, M. J. Phys. Chem. 1967, 71, 4176. (5) Smolders, C. A. In Wetting; SCI Monograph No. 25; Society of Chemicals Industry: London, 1967; p 318. (6) El-Shimi, A.; Goddard, E. D. J. Colloid Interface Sci. 1974, 48 (2), 249. (7) Ershov, A. P.; Esipova, N. E.; Zakharova, M. A.; Zorin, Z. M.; Iskandaryan, G. A.; Madzharova, E. A.; Sergeeva, I. P.; Sobolev, V. D.; Svitova, T. F.; Churaev, N. V. Colloid J. 1994, 56 (1), 36-41.
by a nonpolar liquid is much less studied.6-9 A pertinent example is oil-in-water emulsion deposition on solid surfaces. In their extensive review, Johnson and Dettre collected detailed information about the wetting of low-energy surfaces.3 These authors state that the starting point for the study of liquid/liquid/solid wettability is the BartellOsterhof equation, which predicts the oil/water contact angle on a substrate from the corresponding oil/gas and water/gas contact angles:10
γo/w cos Θo/w ) γo cos Θo/a - γw cos Θw/a
(1)
where γo/w is the water/oil interfacial tension; Θo/w is the interfacial contact angle of an aqueous-immersed oil droplet contacting the solid, as measured through the oil phase; γw is the air/aqueous phase surface tension; Θw/a is the contact angle of the aqueous phase on the solid against air; γo is the oil/air surface tension; and Θo/a is the contact angle of oil on the solid against air. Note that we gauge the oil-in-water contact angle through the oil phase rather than that commonly reported through the water phase.10 Equation 1 is based on Young’s relation and the assumptions that the contact angles for both liquids in air are greater than zero and the air/solid and liquid/solid interfacial energies are unchanged by the presence of the second fluid. Thus, if either fluid spreads on the solid when (8) Grosse, I.; Mueller, H. J. Tenside, Surfactants, Deterg. 1998, 35 (1), 65-70. (9) Perwuelz, A.; Novais da Olivera, T.; Caze, C. Colloids Surf., A 1999, 147, 317-329. (10) Bartell, F. E.; Osterhof, H. J. Colloid Symp. Monogr. 1927, 5, 113.
10.1021/la020006x CCC: $22.00 © 2002 American Chemical Society Published on Web 08/09/2002
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Svitova et al.
Table 1. Buffer Solutions and Surface and Interfacial Tensions
buffer
pH
surface tension, mN/m
acetate, 0.1 M CH3COONa + 0.1 M CH3COOH citrate, 0.1 M citric acid + 0.2 M NaOH acetate, 0.17 M CH3COONa + 0.1 M CH3COOH citrate, 0.06 M citric acid + 0.16 M NaOH 0.1 M sodium phosphate 0.01 M CH3COONa 0.01 M NaHCO3
4.67 5.15 5.45 6.05 7.0 7.3 8.3
69.85 71.05 70.16 71.5 72.4 69.5 72.4
immersed in air, the Bartell-Osterhof relation is suspect. Nevertheless, Bartell and Zuidema11 performed experimental measurements for various organic liquid droplets resting on a talc sheet immersed in water. They show that even when the organic-fluid contact angle in air is zero (i.e., complete spreading in air), agreement between the calculated interfacial contact angle from eq 1 and the corresponding measured angle is acceptable for some organic liquids. For the surface-active liquid amyl alcohol, however, a substantial difference (80°) is observed between the measured contact angle and that theoretically predicted. El-Shimi and Goddard6 measured advancing and receding contact angles of different oils (linear hydrocarbons, cyclohexane, n-hexanol, and mineral oil) on several lowenergy surfaces (Teflon, Nylon 11, poly(methyl methacrylate), and bovine hoof keratin) submerged in water. Only for heptane and n-hexanol on Teflon did they find fair agreement between measured contact angles and those calculated according to eq 1. Silicone oils (poly(dimethylsiloxane)s, PDMSs) are nowadays widely used in cosmetics and personal-care products. They have a low surface tension of 21 mN/m, and, accordingly, they spread in air on most low-energy surfaces except Teflon. Some contact angles of silicone oil drops on solid substrates immersed in aqueous media have been measured.7,12 Surprisingly, PDMS oils do not spread completely even on a PDMS elastomer surface when that elastomer is either immersed in water or in aqueous surfactant solutions.12 For a fused quartz surface, the cationic surfactant cetyltrimethylammonium bromide (CTAB), just below the critical micelle concentration (cmc) at 10-5 M, slightly improves the wetting ability of silicone oil. Nevertheless, even at optimum concentration, the CTAB surfactant solution wets quartz better than does the silicone oil.7 The purpose of this paper is to investigate the wettability of silicone oils to aqueous-submerged solid substrates of widely differing chemical nature, polarity, and nascent water wetting in air. The influence of pH and salt on oil/ water contact angles is studied on smooth silicon wafers coated with various polymers. ζ-Potential measurements are performed for chitin and chitosan coatings immersed in aqueous electrolyte solutions, to clarify the pH and calcium-ion influence on oil-wetting behavior of these substrates. Since silicone oils spread in air on all substrates studied here formally invalidating the Bartell-Osterhof expression, we also test measured Θo/w values against those predicted by eq 1.
interfacial tension against PDMS (10 cSt), mN/m 37.3 32.7 34.4 30.7 42.3 40.5 41.5
substrate roughness on contact angles and contact-angle hysteresis is well-known.13-17 Therefore, we chose silicon wafers as the primary solid substrate because this surface is one of the smoothest available. Rectangular silicon wafers, approximately 20 × 10 mm in size, were cleaned by a saturated solution of KOH in ethanol and then rinsed with distilled and deionized water. Just prior to use as a substrate for contact-angle measurements, these plates were treated by plasma cleaning (PDC-32 G, Harrick Scientific Corp., Ossining, NY) for 2 min at medium power level (60 W) in air. Thus, the clean silicon surface is actually covered by a thin hydrophilic layer of SiO2. Advancing contact angles of water in air on the cleaned silicon plates are always less than 20°. Thin polymer films were deposited on the surface of plasmacleaned silicon-wafer square plates, 25 × 25 mm, using spin coating. The silicon plates were affixed to the top of a centrifuge rotor. When the speed of rotation reached 2500-3000 rpm, 3-5 drops of polymer solution, 20-30 µL each in volume, were deposited onto the rotating plate. Rotation continued for 1-2 min. Substrates were then stored in clean beakers at room temperature. The set of polymer-coated substrates includes two rather hydrophilic polymers from a class of polysaccharides, chitin (poly{N-acetyl}-D-glucosamine, Aldrich), from crab shells, and chitosan (poly-D-glucosamine, Aldrich), which is the deacetylated derivative of chitin, and a hydrophobic polystyrene coating (PS, Aldrich). Chitin and chitosan are used as model substrates because, to some extent, they imitate the surface chemistry and functionality of skin or hair surfaces. For the coatings, a saturated solution of chitin in glacial acetic acid, a 3 wt % solution of chitosan in 10 wt % acetic acid, and a 3 wt % solution of polystyrene in toluene were used. The middle portion of the silicon plates having the most uniform coating was cut and used for further investigation. Coating-film thickness is measured by ellipsometry (Sentech Instruments GmbH, Germany) and is found to vary slightly from sample to sample. For PS coating, the film thickness is 250 ( 30 nm, for chitin films it is 200 ( 60 nm, and for chitosan it is 150 ( 50 nm. Thus, all polymer films are thick enough to represent bulk material. KCl or CaCl2 (both from Fisher Scientific, analytical grade) in distilled water, deionized by a MilliQ system (Millipore Co., Bedford, WA) serve as the electrolyte solutions. These solutions exhibit a pH of about 6.5. We use two different methods for changing pH of the aqueous phase. On the acid side, we employ HCl, whereas on the basic side we use KOH, sodium bicarbonate, and sodium acetate, as listed in Table 1. Several other buffers were also used to study the effect of pH on interfacial contact angles. These are also listed in Table 1. Sodium phosphate, HCl, and KOH are from Fisher Scientific (analytic grade). All remaining chemicals in Table 1 are from Aldrich-Sigma (>99% reagent grade purity). A number of different PDMS samples are used as the oil phase, all from Gelest, Inc., at greater than 95% purity. Physicochemical characteristics and suppliers of these silicone oils are listed in Table 2. Methods. Surface and interfacial tensions are measured at ambient temperature by the pendant-drop method and a Kruss
Experiment Materials. The materials employed as solid substrates are clean silicon wafers (Montco Silicon Technologies, Inc., PA) and polymer-coated silicon wafers. The strong influence of solid(11) Bartell, F. E.; Zuidema, H. H. J. Am. Chem. Soc. 1936, 58, 1449. (12) Bergeron, V.; Cooper, P.; Fisher, C.; Giermanska-Kahn, J.; Langevin, D.; Pouchelon, A. Colloids Surf., A 1997, 122, 103-120.
(13) Dettre, R. H.; Johnson, R. E. Contact Angle Hysteresis. In Contact Angle, Wettability, and Adhesion; Advances in Chemistry Series, Vol. 43; American Chemical Society: Washington, DC, 1964; pp 112-144. (14) Cazabat, A. M.; Cohen Stuart, M. A. J. Phys. Chem. 1986, 90, 5845. (15) Rye, R. R.; Mann, J. A.; Yost, F. G. Langmuir 1996, 12, 555. (16) Gerdes, S.; Cazabat, A. M.; Strom, G. Langmuir 1997, 13, 7258. (17) Marmur, A. Colloids Surf., A 1998, 136, 209.
Wetting Behavior of Silicone Oils
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Figure 1. Schematic of the home-built experimental apparatus for oil/water/solid wetting-behavior studies. Table 2. Physicochemical Properties of Silicone Oils
name and source
molecular mass, Da
density, g/cm3
kinematic viscosity, cSt
surface tension against air, mN/m
interfacial tension against water, mN/m
PDMS, Gelest Inc. (>95% pure) PDMS, Gelest Inc. (>95% pure) PDMS, Gelest Inc. (>95% pure) PDMS, Gelest Inc. (>95% pure)
700-800 1000-1500 5000-7000 9000-10000
0.913 0.930 0.96 0.968
5.0 10 100 200
21.1 ( 0.3 21.3 ( 0.2 21.4 ( 0.3 21.3 ( 0.2
41.5 ( 0.3 41.3 ( 0.3 41.4 ( 0.2 41.4 ( 0.2
DSA-10 tensiometer. A home-built device is adopted for the dynamic and equilibrium contact-angle measurements, as illustrated in Figure 1. This apparatus is mounted on a pressurized vibration-isolation table from Newport (model VH-3036-OPT). The video system includes a Cole-Parmer fiber-optic light source, two polarizers (Cole-Parmer) that eliminate stray light reflections and also fine-tune the light intensity, and a Pulnix video camera. Positions of the camera, sample holder, and drop dispenser (Gilmont microsyringe, GS 1201, 2 mL with a 0.5 mm diameter stainless steel needle) are adjustable in three directions by means of multimovement optical stages. Glass rods (90°-angle bent) serve as solid-substrate holders. Solid samples are glued to the flattened end of the glass rod by melted paraffin. The drop of oil, usually less than 1 mm3 in volume, is formed in the aqueous phase beneath the solid sample, its diameter is measured, and then it is slowly brought into a contact with the solid substrate and the needle is retracted. The image of the drop is captured by an IMAQ frame-grabber that interrogates the image via a specially designed program (virtual instrument, or VI) in LabView (National Instruments). The VI determines the drop edge coordinates, drop height, diameter of the drop-solid contact, and the left, right, and average contact angles with a maximum speed of 8 measurements per second. A sub-VI provided by National Instruments (IMAQ Get Angles VI) permits angle determinations, with a precision of (0.5°. Interfacial contact angles so measured are designated as static oil-in-water angles, Θo/w. Likewise, static water-in-oil contact angles, Θw/o, are obtained by measuring through the oil phase the contact angles of small (
