Novel Method for Determining the Three-Phase Contact Angle of

A conceptually novel method has been developed for determining the three-phase contact angle of solid colloid particles adsorbed at the air−water or...
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Novel Method for Determining the Three-Phase Contact Angle of Colloid Particles Adsorbed at Air-Water and Oil-Water Interfaces Vesselin N. Paunov* Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom Received May 2, 2003. In Final Form: July 7, 2003 A conceptually novel method has been developed for determining the three-phase contact angle of solid colloid particles adsorbed at the air-water or oil-water interface. The method is applicable for particle diameters ranging from several hundred nanometers to several hundred micrometers. This new “gel trapping technique” (GTT) is based on spreading of the particles on an air-water or oil-water surface and subsequent gelling of the water phase with a nonadsorbing polysaccharide. The particle monolayer trapped on the surface of the gel is then replicated and lifted up with poly(dimethylsiloxane) (PDMS) elastomer, which allows the particles embedded within the PDMS surface to be imaged with high resolution by using a scanning electron microscope (SEM). The position of the particles with respect to the PDMS surface has been determined from the SEM images, which gives information on the particle contact angle at the air-water or the oil-water interface. Three samples of latex and silica particles of different size and surface chemistry have been examined by using the GTT. It has been found that the contact angles of sulfate polystyrene latex particles at the air-water and decane-water interface determined by the GTT are slightly lower than the contact angles of water drops on flat polystyrene substrates under air or decane.

1. Introduction Knowledge of the wettability of powderlike materials by different liquids is of paramount importance for a number of applications such as formulation of cosmetic and pharmaceutical products, paints and building materials, deposition of particulate coatings, antifoaming, flotation of minerals, and secondary oil recovery (see e.g. refs 1 and 2). Over the past two decades, there has been increasing interest in the behavior of small solid particles at liquid surfaces3-7 in relation to the properties of either foams or emulsions stabilized solely by particles. The effective particle diameter in various related applications can range from nanometers to millimeters, and a crucial property influencing their attachment to liquid interfaces is the particle hydrophobicity, usually described in terms of the three-phase contact angle θ of a particle at the liquid-fluid interface. For air-water and oil-water interfaces (θ is normally defined through the aqueous phase1), relatively hydrophilic particles have θ < 90° whereas more hydrophobic particles usually have θ > 90°. Particle wettability is an analogue to the hydrophilelipophile balance1 (HLB) number of surfactant molecules and depends on temperature and electrolyte concentration in the system.1,6,7 The development of a reliable method for determination of the three-phase contact angle of small particles attached * E-mail: [email protected]. (1) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963. (2) Adamson A. W. Physical Chemistry of Surfaces, 3rd ed.; John Wiley and Sons: New York, 1976. (3) Tadros, T. F.; Vincent, B. In Encyclopaedia of Emulsion Technology; Becher, P. Ed.; Marcel Dekker: New York, 1983; Vol. 1, Chapter 3. (4) Menon, V. B.; Wasan, D. T. Colloids Surf. 1986, 19, 89. (5) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38, 325. (6) Paunov, V. N.; Binks, B. P.; Ashby, N. P. Langmuir 2002, 18, 6946. (7) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.

to liquid surfaces has been a long-standing challenge for surface and colloidal science over the past few decades. The method of Washburn-Rideal8-11 relies on measuring the liquid penetration rate in a compressed powder bed and is normally used when enough particulate material is available for the bed preparation. However, particle polydispersity effects and loose packing can produce results that are difficult to interpret in terms of particle contact angle.11 Alternatively, the contact angles of powder particles are estimated by measuring the contact angle of a liquid drop on a powder tablet.12 The problems associated with this approach originate from the necessity of mechanical treatment of the particles (to form a tablet) which can compromise their surface properties and the porosity of the tablet surface. As a result, measured contact angles can vary with the size of the powder particles and the way of preparation of the powder tablet. Other methods13 rely on direct observation of particles attached to the oil-water (or air-water) interface by an optical microscope. The lower practical limit for the particle size where this method can be applied is 20-30 µm, and contact angles below ca. 30° or larger than ca. 150° are usually very difficult to measure for small particles. Clint et al.14 proposed another technique for determining the contact angle of colloid particles spread as a monolayer on the air-water or oil-water interface in a Langmuir trough. This method is based on measuring the “collapse” surface pressure at which, as the authors assumed, individual particles are expelled from the particle mono(8) Washburn, E. W. Phys. Rev. 1921, 17, 273. (9) Rideal, E. K. Philos. Mag. 1922, 44, 1152. (10) Chibovsky, E.; Holisz, L. Langmuir 1992, 8, 710. (11) Li, Z.; Giese, R. F.; van Oss, C. J.; Yvon, J.; Cases, J. J. Colloid Interface Sci. 1993, 156, 279. (12) Jouany, C.; Chassin, P. Colloids Surf. 1987, 27, 289. (13) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Colloids Surf., A 1993, 71, 207. Horvolgyi, Z.; Mate, M.; Daniel, A.; Szalma, J. Colloids Surf., A 1999, 156, 501. (14) Clint, J. H.; Taylor, S. Colloids Surf. 1992, 65, 61. Clint, J. H.; Quirke, N. Colloids Surf., A 1993, 78, 277.

10.1021/la0347509 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/21/2003

Determination of Three-Phase Contact Angle

layer into the adjacent fluid phases, which allows the collapse pressure to be related to the energy of particle adsorption at the liquid interface and hence to the particle contact angle. However, recently, Aveyard et al.15 showed by direct observation that for particle sizes of 0.2-2.6 µm the collapse mechanism of a particle monolayer at the air-water and the oil-water interface involves corrugation of the whole monolayer rather than expelling of individual particles from the interface. Hadjiiski et al.16 developed a film trapping technique (FTT) which is applicable for particle sizes of 1-10 µm. The FTT16 is based on capturing of individual particles in a liquid film on a flat solid substrate and analyzing the positions of the Newton interference fringes of the surrounding meniscus around the particles, which allows the shape of the liquid interface to be determined and the particle contact angle to be calculated. The FTT method assumes that the water film around the particles is ultimately stable and levels off away from the particle. Measurements of particle contact angles at the oil-water interface by the FTT have not been reported so far. Recently, another approach has been developed by Butt et al.17-19 where spherical particles have been attached to the cantilever of an atomic force microscope (AFM) and approach the air-water surface. By measurement of the equilibrium position of particles attached at the air-water surface or the “pull-off” force, the particle contact angle can be determined. This method allows both the receding and the advancing particle contact angle to be determined but requires sophisticated equipment (AFM) and good skills to fix the particles to the AFM cantilever without spreading glue all over the particle surface. Here I suggest a completely new approach for determining the contact angle of colloidal particles, which is based on the following gel trapping technique (GTT). The principle of the GTT is the following: Particles are spread on the air-water or the oil-water interface, and a nonadsorbing hydrocolloid polymer is used to gel the aqueous phase. Thus, the particle monolayer remains embedded on the gel surface and the particle positions with respect to the liquid interface remain fixed, which allows the top phase (air or oil) to be replaced with a poly(dimethylsiloxane) (PDMS) silicone elastomer. After curing the PDMS, the particle monolayer can be peeled off the aqueous gel and imaged on the PDMS surface with a scanning electron microscope (SEM) which provides enough spatial resolution to determine the particle position even for submicron particles. The particle contact angle measured through the PDMS phase is complementary (with respect to 180°) to the particle contact angle in the original system (air-water or oil-water interface). The methodology is illustrated schematically in Figure 1. I present results for the contact angles of a series of particles of different sizes and surface charge groups spread on the air-water and decane-water interfaces. This method can be applied to particles ranging from several hundred nanometers to several hundred micrometers in diameter. The lower limit of the particle size for the GTT depends on the spatial resolution of the SEM equipment and the particle material. The paper is organized as follows. Section 2 describes the materials and techniques used in (15) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969. (16) Hadjiiski, A.; Dimova, R.; Denkov, N. D.; Ivanov, I. B.; Borwankar, R. Langmuir 1996, 12, 6665. (17) Ecke, S.; Preuss, M.; Butt, H. J. J. Adhes. Sci. Technol. 1999, 13, 1181. (18) Yakubov, G. E.; Vinogradova, O. I.; Butt, H. J. J. Adhes. Sci. Technol. 2000, 14, 1783. (19) Preuss, M.; Butt, H. J. J. Colloid Interface Sci. 1998, 208, 468.

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Figure 1. Schematic diagram of the gel trapping technique for determining contact angles of microparticles at an oil-water (A-F) and air-water (C-F) interface.

this method. The experimental details are explained in section 3. The results for the particle contact angles are discussed in section 4, and section 5 summarizes the main conclusions. 2. Materials and Methods The method presented in this paper became possible after the identification of a suitable hydrocolloid that forms a strong aqueous gel but has a very low surface activity at the air-water or the oil-water surface. Gellan gum20 is a gel-forming polysaccharide secreted by the microbe Sphinogomonas elodea and consists of monosaccharides β-D-glucose, β-D-glucuronic acid, and R-L-rhaminose in molar ratios of 2:1:1. At high temperatures, gellan polymers in aqueous solutions are in a disordered random coil state. When cooled to gelling temperature, this hydrocolloid forms double helices which aggregate to form junction zones. The presence of small amounts of cations (e.g., Na+, Ca2+) stabilizes the double helices and forms a 3D gel network.21,22 The gellan gum (Kelcogel) was a gift from CPKelco (U.K.). Gellan gum was initially dispersed as 0.5 wt % in miliQ water and heated to 95 °C in a water bath for 15 min to dissolve and hydrate. To purify gellan from surface-active contaminations, the hot solution was passed twice through a C18-silica chromatographic column (Phenomenex) preactivated with an acetonitrile-water (80:20) mixture and flushed several times with hot miliQ water. The column was heated during the filtration to avoid undesirable retention of gellan on the adsorbent due to gelling. The gellan solution eluted from the column was collected, dried up, and redissolved by the same procedure as 2.0 wt % aqueous solution. Bubbles were removed from the hot solution by centrifugation. The gelling temperature20-22 of 2.0 wt % gellan solution is in the range of 40-45 °C, but the setting of the gel does not happen instantaneously even if the solution is quickly cooled to room temperature. The surface activity of gellan in aqueous solutions is discussed in the next section. n-Decane (Sigma) was used as an oil phase after additional purification by passing through a chromatographic alumina column (three times). PDMS Sylgard 184 elastomer (Dow Corning) was used in a ratio of 10:1 with (20) Sanderson, G. R. Gellan gum. In Food Gels; Harris, P., Ed.; Elsevier: New York, 1990. (21) Millas, M.; Rinaudo, M. Carbohydr. Polym. 1996, 30, 177. (22) Nakajiama, K.; Ikehara, T.; Nishi, T. Carbohydr. Polym. 1996, 30, 77.

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Figure 2. Surface tension of the air-water interface (b) and interfacial tension at the n-decane-water interface (9) as functions of the concentration of gellan in the water phase.

Figure 3. Three-phase contact angles of water drops on a flat PS slide in air (b) and three-phase contact angles of water drops on a flat PS slide under n-decane (9) as functions of the concentration of gellan in the water phase.

respect to the curing agent. Two samples of monodisperse surfactant-free latex particles (IDC) and one sample of octadecyl (C18)-modified hydrophobic silica particles (microParticles GmbH, Germany) of sizes ranging from 2.15 to 9.6 µm were used without further purification. 2-Propanol (BDH, U.K.) was used as a spreading solvent to spread particles at the air-water and the decane-water interface. A LEO Electron Microscope S360 SEM with a secondary electron detector was used to image the particle monolayers on PDMS. Before imaging, samples were coated with ∼10 nm carbon layer (spectrally pure graphite) by using an Edwards High Vacuum evaporator. Contact angles of aqueous drops on flat polystyrene substrates under air or decane were measured by using a Kru¨ss Drop Shape Analysis System-10 (DSA-10) with a thermostated chamber.

shortly after producing the pendant drop, but this may induce a small reduction of the tension due to incomplete thermostating of the drop at room temperature. Nevertheless, an interfacial tension reduction of 1.5 mN/m for 2.0 wt % gellan has no practical importance for the GTT application since the introduced correction in the corresponding contact angle (through the Young equation1,2) is within the experimental error. As shown below, it has no measurable effect on the contact angles of water drops on flat polystyrene substrates under air and decane. 3.2. Contact Angles of Gellan Solutions on Polystyrene (PS) Substrates. To check how the presence of gelling agent affects the contact angle of PS latex particles, the three-phase contact angles of drops of an aqueous solution of (purified) gellan on flat PS slides (Agar Scientific, Ltd.) were measured under air and n-decane, respectively, by using the same equipment (Kru¨ss DSA10). Both the receding and the advancing contact angles were measured as functions of the gellan concentration. The results for the average contact angles for water drops on PS slides in air and those for water drops on PS slides under n-decane are shown in Figure 3. The average contact angle measured with water drops on PS substrates (in air) was measured to be θaw ) 82 ( 6°. The corresponding advancing and receding contact angles were determined to be θr ) 76° and θa ) 88°, respectively. One sees from Figure 3A that the average contact angle is practically insensitive to the gellan concentration up to 2.0 wt %. From this finding, combined with the fact that gellan barely perturbs the air-water surface tension (see Figure 2), one can conclude from the Young equation,1,2 cos θaw ) (γPS/a - γPS/w)/γaw, that the surface activity of gellan at the PS-water interface is also very low, thus not influencing the three-phase contact angle measured through the water phase. Figure 3 shows a similar trend for the contact angle of water drops on flat PS substrates under decane. The average contact angle for pure water in this case was found to be θdw ) 128 ( 4° with advancing contact angle θa ) 132° and receding contact angle θr ) 124°. The average contact angle also appears to be insensitive to the gellan concentration in the water phase even at a concentration of 2.0 wt %, similarly to the case of PS/air/water drops (Figure 3). This is consistent with the result from Figure 2 which indicates very low surface activity of gellan at the decane-water interface and that in Figure 3 that indicates low surface activity of gellan at the PS-water interface (cf. the Young equation, cos θdw ) (γPS/d - γPS/w)/γdw, for PS/decane/water). These results

3. Experimental Section 3.1. Surface Tension of Gellan Solutions. To check the surface activity of gellan at liquid surfaces, the surface tension of gellan solutions at the air-water and decanewater interfaces was measured at different gellan concentrations. Surface tension was measured by the pendant drop technique with the Kru¨ss DSA-10. Gellan was purified by the procedure described in section 2. Hot gellan solution was loaded into the microsyringe, and pendant drops were produced inside the thermostated chamber at 25 °C. The value of the surface tension was monitored until the drop gelled, and the lowest value was recorded, normally just before setting of the gel. Interfacial tension at the decane-water interface was measured by producing pendant drops in a thermostated rectangular cuvette at 25 °C with the Kru¨ss DSA-10. No measurable change in the pendant drop shape was encountered upon gelling. The results for the surface tension of aqueous gellan solutions are summarized in Figure 2 for air-water and decane-water interfaces. One sees from Figure 2 that the (air-water) surface tension of gellan solutions practically does not depend on the gellan concentration up to 2.0 wt %. For the decane-water interface, it was found that there is a very low surface activity corresponding to a reduction of the interfacial tension from 52.6 mN/m for pure water to 51.1 mN/m for 2.0 wt % gellan, possibly due to traces of surface-active contamination. This apparent very slight reduction of γdw for 2.0 wt % gellan solution at the decane-water interface could also be due to a temperature effect since the pendant drop often “skins” when produced in the thermostated decane phase due to subsurface gelling. To avoid this difficulty, the reading for the decane-water interfacial tension was taken very

Determination of Three-Phase Contact Angle

show that gellan has very low surface activity both at the air-water surface and at hydrophobic interfaces such as the decane-water and the PS-water interface. The latter justifies the use of gellan as the gelling agent in our method for determining the contact angle of hydrophobic microparticles as described in the next section. 3.3. Particle Contact Angles. Hot gellan solution at 2.0 wt % was prepared as described in section 2 and dispensed in a thermostated Petri dish at 50 °C. The latex particle suspensions were used as supplied from the manufacturer. The following procedure was used to spread both latex and silica particles at the liquid surface with the help of a spreading solvent, 2-propanol (IPA). The methodology is summarized in Figure 1A-F for particles at an oil-water interface and in Figure 1C-F for the case of the air-water interface. Here we describe both procedures in more details. (a) Particles at the Air-Water Interface. The latex particle suspension was mixed with IPA (50:50), and 10 µL of the mixture was injected with a microsyringe on the surface of the gellan solution at 50 °C. Initially, IPA spreads and distributes the particles over the liquid surface, and then the particle monolayer quickly contracts (as IPA evaporates and/or dissolves in the bulk phase) and forms a white-off spot on the liquid surface of relatively high surface concentration of particles. The hydrophobic silica particles (dry powder) were spread on the water surface by the same procedure as 1 wt % suspension in IPA (prepared by redispersing with ultrasound to break up particle aggregates). After spreading of the particles, the system was quickly cooled to room temperature in a covered Petri dish and left for 30 min to allow the gel to set. Gellan solution starts gelling after cooling to room temperature and forms a strong gel which shows no visible sign of syneresis even after 2 days of storage. Sylgard 184 PDMS elastomer was prepared by mixing PDMS with curing agent in a ratio of 10:1 and degassing in a centrifuge at 2000 rpm to produce a clear bubble-free liquid. The liquid PDMS elastomer was poured over the gelled water phase with the particle monolayer, and the system was left to cure at room temperature for 48 h. Then, the solidified elastomer was peeled off the gel surface, thus trapping the particles in a position complementary to that at the air-water interface. The elastomer sample was immersed in a water bath at 95 °C for 2 min to dissolve residual amounts of gellan on the surface of the cured PDMS and the partially embedded particles and then was washed thoroughly with miliQ water.23 The sample was further prepared for SEM imaging by deposition of about 10 nm of a thin carbon layer over the particle monolayer on the PDMS surface. Samples were imaged with SEM at different angular positions of the secondary electron detector, and images were processed with image analysis software to take measurements of the particle position with respect to the PDMS surface. The angular position of the particles at the PDMS surface is complementary (with respect to 180°) to the original contact angle of the particle when adsorbed at the air-water surface (see Figure 1C-F). A notable swelling of the spot under focus on the PDMS surface was observed after several minutes of exposure to the electron beam. The effect was more pronounced for the smallest particles where higher resolution was used. However, despite this local temperature induced expansion of the elastomer, the position of the particle contact line did not seem to show a notable (23) An alternative procedure is also applicable (not used here) by soaking the samples in 10 mM EDTA solution at room temperature to dissolve the gellan residue on the PDMS surface.

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change. The PDMS surface and the particles away from the area of focus were also unaffected. In all cases, SEM images were taken immediately after focusing on an individual particle to avoid effects from the temperature expansion of the PDMS and the particles. (b) Particles at the Decane-Water Interface. In this case, the procedure was very similar to that for the air-water interface (see Figure 1A-F). Hot 2.0 wt % gellan solution was dispensed in a thermostated Petri dish at 50 °C, and n-decane prewarmed to 50 °C was layered on the top of the gellan solution. A sample of 10 µL of a (50:50) mixture of IPA and latex particle suspension or 10 µL of an IPA dispersion of 1 wt % C18-modified silica particles was injected very close to the decane-water interface. After spreading of the particles, the system (Petri dish) was cooled to room temperature and left for 30 min to allow the gel to set. The oil phase (decane) was then gently removed and replaced with the liquid PDMS elastomer. The rest of the sample preparation procedure (e.g., curing of the PDMS) is identical to that for the air-water interface (see above). 4. Results and Discussion The value of the particle contact angle was determined from the SEM images: (i) When the spherical particles were immersed in the PDMS below their equatorial line, the measured quantities were the diameter of the particle contact line dc and the particle equatorial diameter d, from which the contact angle θ was calculated from the equation

sin θ )

dc d

(1)

(ii) When the particles were immersed in the PDMS above their equator line so that the particle equatorial diameter was not measurable from the images, we measured the diameter of the particle contact line dc and the height of the visible part of the particle hc (determined from the image). The latter are connected by the equation

hc π β-δ sin β ) cos2 dc 4 2

(

)

β)π-θ

(2)

where δ is the angle of observation of the particle with the electron detector (δ ) 0 when observing the particle from above). Note that the real height H ) d(1 - cos β)/2 of the particle above the PDMS surface is different from the “visible height” hc which depends on the angle of observation δ. Equation 2 is valid only for observation angles smaller than β, that is, for δ < π - θ. The latter equation can be resolved for θ at fixed values of δ, dc, and hc. Alternatively, for highly monodisperse particles, the average equatorial diameter can be determined from the SEM images of particles deposited on a solid surface and then eq 1 can be used (instead of eq 2) without inflicting significant error in θ due to polydispersity. Figure 4 shows typical SEM images of the three particle samples (A, B, and C, see Table 1 for specifications) prepared as described in the previous section for the airwater surface. The images in the first and the second column are taken at different spatial resolutions and angles of observation, δ ) 60° and δ ) 85°, respectively. The results are summarized in Table 2. Note that both samples of PS latex particles (of average particle diameters 9.6 and 3.9 µm) showed reproducibly very similar positions with respect to the PDMS surface, corresponding to an average contact angle of 74° and 73°, respectively. This

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Figure 4. Scanning electron microscope images of monolayers of monodisperse particles on the surface of PDMS obtained by the gel trapping technique for the air-water interface: (A) sulfate PS latex particles of an average diameter of 9.6 µm; (B) sulfate PS latex particles of an average diameter of 3.9 µm; (C) C18-modified silica particles of an average diameter of 2.15 µm. (For A1, B1, C1, and C2, the observation angle δ ) 60°; for A2 and B2, δ ) 85°). Table 1. Physical Properties of the Particle Samples Used particle sample diameter/µm A B C

9.6 ( 0.71 3.9 ( 0.31 2.15 ( 0.19

surface groups sulfate latex sulfate latex silica-C18

area per surface group/nm2 manufacturer N/Aa 2.14 N/Ab

IDC IDC microParticles GmbH

Table 2. Three-Phase Contact Angles of Particle Samples Determined by Our Methoda particle contact particle contact angle particle angle at air-water at decane-water sample diameter/µm interface/deg interface/deg A B C a

9.6 ( 0.71 3.9 ( 0.31 2.15 ( 0.19

74 ( 4 73 ( 4 97 ( 5

111 ( 4 101 ( 3 136 ( 3

Particle specification is according to Table 1.

a

According to IDC, the latex surface area is not large enough for precise surface charge determination by conductometric titration but the area per surface sulfate group is expected to be between 2.5 and 5.0 nm2. b According to microParticles GmbH, the silica surface has been saturated by C18-alkyl chains.

value is slightly lower than the PS-air-water contact angle of 82° measured on a flat PS substrate, but this is consistent with the fact that the PS latex particles have sulfate charged groups on their surface and are expected to be more hydrophilic than the flat PS substrate. The average contact angle of the C18-silica particles was found

to be 97°, which value is again slightly lower than the contact angle of water drops on C18-functionalized glass slides.24 The corresponding SEM images for the same particles captured at the n-decane-water interface are presented in Figure 5 for different magnifications and observation angles. In this case, the two samples of PS particles showed significantly different average contact (24) The contact angle of water drops on C18 alkyl-silanized microscope glass slides was measured in an independent experiment to be θ ) 104 ( 2.

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Figure 5. Scanning electron micrographs of monolayers of monodisperse particles on the surface of PDMS obtained by the gel trapping technique for the n-decane-water interface: (A) sulfate PS latex particles of an average diameter of 9.6 µm; (B) sulfate PS latex particles of an average diameter of 3.9 µm; (C) C18-modified silica particles of an average diameter of 2.15 µm. (For A1, B1, and C1, the observation angle δ ) 60°; for A2, δ ) 80°; for B2, δ ) 85°; for C2, δ ) 0°).

angles, 111° and 101°, respectively. Both are lower than the average contact angle of water drops on flat PS slides under decane, θ ≈ 128°, which again can be attributed to the expected lower hydrophobicity of the charged latex particles compared to that of the PS slides. The average contact angle of the C18-modified silica particles at the decane-water interface was determined from the SEM images to be very high, θ ) 136°, indicating that the silica surface is possibly densely coated with octadecyl groups, as noted by the manufacturer. Despite the robustness of the suggested technique for determining contact angles of microparticles which works for both air-water and oil-water interfaces, several issues and discussion points regarding this method still remain open and require additional research: (i) What is the smallest particle size for which this method can be reliably applied for contact angle determination? The answer to this question depends on the particular spatial resolution of the SEM equipment, the type of the

electron detector, and the contrast between the PDMS and the particle material. For example, we have been able to obtain SEM images of 250 nm PS latex particles on a PDMS surface (not presented in this paper) at the very limit of our scanning electron microscope. However, for metal particles on PDMS the contrast is expected to be much higher than that for PS on PDMS; hence metal particles of sizes even lower than that could be imaged and characterized with the GTT. A remaining problem, however, is the thermal expansion of the PDMS upon focusing on small areas with particles on the PDMS surface. We found that the latter problem can be partially resolved by using a higher concentration (ratio of 9:1) of curing agent which corresponds to a higher degree of crosslinking in the PDMS network. For submicron particles, line tension effects may also contribute to size dependence of the contact angles.25,26 (ii) What is the lowest value of the contact angle that can be determined by this gel trapping technique? This limit

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should depend on the adhesion of the PDMS to the particles, which is specific to the particle material. For the PS latex particles used in this work, the adhesion to PDMS appears to be so strong that some of the particles break in half when scraped from the surface of the PDMS. (iii) Given that gelation starts below the gelation temperature as the gellan solution is cooled to room temperature, is the topography observed with the SEM really representative of that at room temperature? The method works for room temperature since there is a kinetic effect involved. Gellan does not form gel instantaneously, as it is cooled off below its gelling point and the solution maintains its fluidity for a while before the gel sets at room temperature. Hence the particles adsorbed at the air-water or the oil-water interface can respond to the interfacial forces and acquire the corresponding contact angle at the maintained temperature. This is also supported by the results from the contact angle measurements on flat substrates (Figure 3), where the same question is relevant. However, the measured contact angles of gellan solutions are similar to the ones without gellan at room temperature. The same conclusion follows from the surface tension and interfacial tensions in the presence of gellan (Figure 2), as well. It may also be possible to adapt the GTT method to measure contact angles at other temperatures (below the gelling point of gellan), but this will be a subject of additional research. (iv) What is the stability of the gel with respect to (a) a slow evaporation of water from the gel as the PDMS is curing and (b) possible gel syneresis, and how would these affect the measured contact angles? (a) Water evaporation was eliminated since the Petri dish, containing the gel and the particles, was completely sealed off by the top layer of PDMS. Water loss was not detected over 2 days at room temperature (as PDMS is curing). However, water loss was observed for a sample that was attempted to be cured for 2 h at 50 °C. (b) Several samples were checked for gel syneresis before adding the top layer of PDMS in a covered Petri dish with a long-focus microscope objective. No visible signs of syneresis or cracking of the gel surface were observed. The samples containing particles at the gel-PDMS interface were not checked for syneresis since the gel adhered to the cured PDMS and it was difficult to separate them without breaking of the gel layer. This is an indication that syneresis is unlikely to have occurred as it would facilitate the separation of the two layers. On the other hand, if the gel releases water (syneresis) during (25) Aveyard, R.; Clint, J. H.; Paunov, V. N.; Nees, D. Phys. Chem. Chem. Phys. 1999, 1, 155. (26) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Colloids Surf., A 1999, 146, 95.

Paunov

the curing of the PDMS, then in both cases of particles at the air-water and the oil-water interface one would measure one and the same contact angle (particle-waterPDMS). The fact that the contact angles of latex and silica particles at air-water and oil-water are different and close to the ones measured on flat substrates reassures that the method works without a significant gel syneresis at least at room temperature. (v) What would be the effect of interaction of different surface groups on the particles with the gellan hydrocolloid, and how would this affect measured particle contact angle with the gel trapping technique? Negatively charged microparticles are not expected to be influenced by gellan in their adsorption at the liquid surface; however, additional research is to be done in order to reveal whether positively charged particles can be tackled in the same way as negatively charged particles. Such work is currently under way. 5. Conclusions In summary, a novel gel trapping technique (GTT) has been developed for determining the three-phase contact angle of colloid particles adsorbed at an air-water or an oil-water interface. The method is based on gelling of the water phase with non-surface-active hydrocolloid polymer (gellan) after the particle adsorption at the liquid surface and subsequent replication of the particle monolayer with PDMS elastomer. SEM imaging of the particle position on the PDMS surface provides information on the particle contact angles at the air-water or the oil-water interface, respectively, and gives much higher resolution than optical microscopy which makes the method applicable even for submicron size particles. Surface and interfacial tension studies as well as contact angle measurements for water drops on flat PS substrates indicate that the presence of the hydrocolloid gellan in the water phase barely perturbs the interfacial free energy at the air-water, decanewater, and PS-water interfaces and has no effect on the measured three-phase contact angle. Results for the contact angles of several samples of particles of different size and surface chemistry at the air-water and decanewater interfaces obtained by using the GTT have been presented. Acknowledgment. The author appreciates the provision of the gellan sample by CPKelco, U.K., and the technical support from Mr. Tony Sinclair for the SEM images. LA0347509