Langmuir 2006, 22, 3221-3227
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The Influence of Polymer Structure and Morphology on Talc Wettability Gillian B. Kaggwa, Le Huynh, John Ralston,* and Kristen Bremmell Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia ReceiVed August 24, 2005. In Final Form: January 16, 2006 Advancing water contact angles were measured on freshly cleaved talc faces as well as on talc particles. The intrinsic hydrophobicity of talc was shown to be due to the dominance of the apolar components of the work of adhesion. Polyacrylamides and polysaccharides adsorb onto the surface of talc, displaying strikingly different morphologies. Adsorbed amount, apparent layer thickness, and polymer structure control talc wettability.
1. Introduction Talc is a versatile material, encountered in a wide range of industrial processes. In the paint-making industry, the hydrophobic properties of talc are utilized to remove organic impurities.1-3 This influences the rheology and viscosity of water and oilbased formulations, reducing pigment settling and improving the appearance of the coating. As a filler, talc is used in composite materials,4,5 for example, to improve the strength, hardness, and abrasive resistance of various types of materials, typically polypropylene6-8 in the plastics industry. In mineral processing, talc is encountered as a hydrophobic gangue that contaminates concentrates, yet it may be depressed using appropriate reagents.9-11 In all of these applications, surface wettability is a critical parameter as it generally controls the interaction between talc particles and the various contacting media. Talc is a 2:1 layer phyllosilicate mineral with Mg3(Si4O10)(OH)2 as the unit structure.12 The layered structure is comprised of an octahedral brucite sheet sandwiched between two tetrahedral silica sheets. The two sheets are held together by ionic bonds, forming a layer with a thickness of 6.6 Å.13 The talc layers are linked by weak van der Waals forces and are approximately 2.8 Å apart.13 The layer charge is zero or very small, as there are no ions present between the layers. Upon breakage, talc forms two surfaces due to the facile cleavage of the layers (basal cleavage face) and the rupture of the ionic/covalent bonds within the layers * To whom correspondence should be addressed. E-mail: John.Ralston@ unisa.edu.au. (1) Paul, S. Surface Coatings: Science and Technology, 2nd ed.; John Wiley & Sons: New York, 1996. (2) Do¨ren, K.; Freitag, W.; Stoye, D. Water-Bourne Coatings: The EnVironmentally-Friendly AlternatiVe; Hanser Publishers: New York, 1994. (3) Abel, A. G. In Paint and Surface Coatings: Theory and Practice, 2nd ed.; Lambourne, R., Strivens, T. A., Eds.; Woodhead Publishing: Cambridge, U.K., 1999. (4) Unal, H.; Findik, F.; Mimaroglu, A. J. Appl. Polym. Sci. 2003, 88 (7), 1694-1697. (5) Ellis, T. S.; D’Angelo, J. S. J. Appl. Polym. Sci. 2003, 90 (6), 1639-1647. (6) Kim, K.-J.; White, J. L.; Shim, S. E.; Choe, S. J. Appl. Polym. Sci. 2004, 93 (5), 2105-2113. (7) Leong, Y. W.; Mohd Ishak, Z. A.; Ariffin, A. J. Appl. Polym. Sci. 2004, 91 (5), 3327-3336. (8) Liu, Z.; Gilbert, M. J. Appl. Polym. Sci. 1996, 59 (7), 1087-1098. (9) Morris, G.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2002, 67 (1-4), 211-227. (10) Jenkins, P.; Ralston, J. Colloids Surf., A 1998, 139 (1), 27-40. (11) Morris, G. Ph.D. Thesis, University of South Australia, 1996. (12) Roberts, W. L.; Campbell, T. J.; Rapp, G. R. J. Encyclopaedia of Minerals, 2nd ed.; Van Nostrand Reinhold: New York, 1990. (13) Bragg, L.; Claringbull, G. F. Crystal Structures of Minerals; G. Bell and Sons, Ltd.: London, 1965.
(edge).14 Each face consists of siloxane groups that are electrically neutral, nonpolar in water, and, hence, hydrophobic. The edges consist of pH-dependent SiOH and MgOH groups, which are polar or hydrophilic in nature.15 Since the faces dominate the edges in terms of surface area, the hydrophobicity of talc is conferred by the faces. This is nicely illustrated in the work of Malandrini et al.16 who have shown that the surface wettability increases as the ratio between the hydrophobic faces/hydrophilic edges decreases. The individual hydrophobic and hydrophilic components of talc have been examined by Charnay et al.17 through adsorption experiments using surface molecular probes. The contact angle of talc is a topic of both considerable research and conjecture. Douillard et al.18 predicted a water contact angle of 70.6° for talc particles using immersion calorimetry experiments. On polished talc surfaces, they reported measured advancing and receding water contact angles of 60° and 50°, respectively. Schrader and Yariv19 reported an average advancing water contact angle of 83 ( 10° for talc particles deposited on tape backing. Through wetting enthalpy measurements, Michot et al.20 calculated a water contact angle of 80° for the talc face, by assuming that the edges of talc are perfectly wet by water. Streltsin21 measured an advancing water contact angle of 90° on a cleaved talc face, where the surface irregularities were removed through polishing. Bartell and Zuidema22 also directly measured the contact angle of various liquids on a freshly cleaved piece of talc. The classic textbook by Wark23 discusses the natural flotation of talc, while Chander et al.24 conducted contact angle measurements on talc and other hydrophobic solids. The origin and purity of the talc were not reported in these studies. (14) Fuerstenau, M. C.; Lopez-Valdivieso, A.; Fuerstenau, D. W. Int. J. Miner. Process. 1988, 23 (1), 161-170. (15) Malhammar, G. Colloids Surf. 1990, 44 (1), 61-69. (16) Malandrini, H.; Clauss, F.; Partyka, S.; Douillard, J. M. J. Colloid Interface Sci. 1997, 194 (1), 183-193. (17) Charnay, C.; Lagerge, S.; Partyka, S. J. Colloid Interface Sci. 2001, 233 (2), 250-258. (18) Douillard, J. M.; Zajac, J.; Malandrini, H.; Clauss, F. J. Colloid Interface Sci. 2002, 255 (2), 341-351. (19) Schrader, M.; Yariv, S. J. Colloid Interface Sci. 1990, 136 (1), 85-94. (20) Michot, L.; Villieras, F.; Franc¸ ois, M.; Yvon, J.; le Dred, R.; Cases, J. M. Langmuir 1994, 10 (1), 3765-3773. (21) Streltsin, G. S. Ph.D. Thesis. Mekhanbor Institute, Leningrad, Russia, 1953. In An Introduction to Theory of Flotation; Klassen, V. I., Maokrousov, V. A., Eds.; Butterworth: Guildford, England, 1963. (22) Bartell, F. E.; Zuidema, H. H. J. Am. Chem. Soc. 1936, 57 (1), 14491454. (23) Sutherland, K. L.; Wark, I. W. Principles of Flotation; AusIMM: Melbourne, Australia, 1955; Chapter 14. (24) Chander, S.; Wie, J. M.; Fuerstenau, D. W. AIChE Symp. Ser. 1975, 71, 183-188.
10.1021/la052303i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006
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Characterization of the solid surface is of fundamental importance with regards to surface wettability, as the contact angle can be influenced by both chemical and physical heterogeneity.25,26,30 Direct measurement of the water contact angle on the face of freshly cleaved talc of known purity has yet to be reported in the literature, while the influence of polymers on this wettability has received scant attention. In this study, we perform water contact angle measurements on the cleaved talc face, accompanied by tapping-mode atomic force microscopy (TMAFM) images of that face. The wettability of talc in the form of a packed particle bed was also measured using the Washburn technique.27 Polyacrylamides and polysaccharides are often used to reduce the wettability of talc.9-11,28,29 We recently demonstrated that the surface structure and hydrophobicity of a given mineral surface influence the morphology of the adsorbed polymer layer,30 and we now extend these findings to talc in a fundamental study. First, the adsorption of several polyacrylamides and polysaccharides onto talc has been assessed. Second, the form of the adsorbed polymer has been investigated using TMAFM, allowing the distribution and morphology of the polymer on the talc surface, surface roughness, and layer thickness to be characterized. Finally, the adsorbed amount and adsorbed polymer morphology were then correlated with the changes in the water contact angle before and after polymer adsorption. We concentrate on polysaccharides and polyacrylamides in this present study for, as we have demonstrated in our pioneering study of silica and modified silica surfaces, the morphology and structure of these ubiquitous adsorbing polymers exert a major influence on wettability.30 2. Materials 2.1. Talc Substrates. Talc particles were purchased from Merck, Germany (>99% pure) and used for adsorption isotherms and contact angle measurements. Analysis by X-ray photoelectron spectroscopy (XPS) showed the surface to be free from impurities. The particle size distribution determined using a Malvern Instruments Mastersizer was 0.5-100 µm, with a D10 of 3.5 µm, a D50 of 15 µm, and a D90 of 52 µm. The Brunauer-Emmett-Teller (BET) surface area was measured to be 2.9 m2 g-1. The talc sample used for contact angle and TMAFM imaging experiments was a rock mineral from Balmat (New York). The mineral sample was obtained from Ward’s Natural Science Establishment, Inc. XPS analysis of the cleaved talc crystal revealed that the surface was free from detectable impurities. The bulk sample purity was estimated to be 97% using X-ray fluorescence (XRF) spectroscopy. The major impurity was CaO. The calcium oxide impurity is attributed to the presence of tremolite, (Ca2Mg5S18O22(OH))2. This mineral is a common association, as discussed by Huang and Fuerstenau.31 2.2. Chemicals. A 30 wt % polyacrylamide aqueous solution sample was supplied by Cytec Industries, U.S.A. This polyacrylamide was an unsubstituted polymer with a molecular weight of 16 200 g mol-1, determined through size exclusion chromatography. The polydispersity of this polymer, referred to as Polymer-N, was 3.9. The length of the fully extended Polymer-N, calculated from the dimensions of the monomer unit multiplied by the number of repeating units derived from the polymer molecular weight, is 54 nm. The average hydrodynamic diameter of the Polymer-N molecules was 4.9 nm, measured by dynamic light scattering. A 35 wt % polyacrylamide aqueous solution sample was also supplied by Cytec (25) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11-16. (26) Wenzel, R. N. Ind. Eng. Chem. 1949, 28, 988-994. (27) Washburn, E. W. Phys. ReV. 1921, 17, 273-283. (28) Chen, H. T.; Ravishankar, S. A.; Farinato, R. S. Int. J. Miner. Process. 2003, 72 (1-4), 75-86. (29) Pugh, R. J. Int. J. Miner. Process. 1989, 25 (1-2), 101-130. (30) Kaggwa, G.; Froebe, S.; Huynh, L.; Ralston, J.; Bremmell, K. Langmuir 2004, 21 (10), 4695-4704. (31) Huang, P.; Fuerstenau, D. W. Colloids Surf., A 2001, 177 (1), 147-156.
Kaggwa et al. Industries, U.S.A., with a 15% degree of random hydroxyl substitution and a molecular weight of 14 600 g mol-1. The polydispersity of this polyacrylamide, referred to for convenience as Polymer-H, is 3.4. The length of the fully extended Polymer-H is 63 nm. The Polymer-H molecules had an average hydrodynamic diameter of 5.8 nm. The general structure of the polyacrylamide repeating unit, along with the approximate dimensions, has been described in our previous study.30 The polysaccharides Dextrin-WY and hydroxypropylated starch (HP-Starch) were supplied by Penford Australia. Dextrin-WY has a molecular weight of 9260 g mol-1 and a polydispersity of 6.0. The molecular weight and polydispersity of HP-Starch are 706 000 g mol-1 and 11.4, respectively. HP-Starch has a 5.3% degree of hydroxypropyl substitution occurring at the C-2 and C-6 positions.30 Dextrin-WY is a derivative of starch and is made by the thermal degradation of starch under acidic conditions. The molecules of Dextrin-WY are smaller and more highly branched compared with those of HP-Starch, although they belong to the same family of polymers constructed from R-D-glucose. The average hydrodynamic diameter of the Dextrin-WY and HP-Starch molecules was 4.9 and 18 nm, respectively. Their general chemical structures, along with their approximate dimensions, have been described previously.30 High-purity Milli-Q water was supplied by a Millipore water system, with a conductivity less than 1 × 10-6 S cm-1 and surface tension of 72.8 mN m-1 at 25 °C. The solution pH was adjusted with small additions of analytical grade HNO3 and KOH solutions. All aqueous solutions were at pH 9, with a 10-3 M KNO3 background concentration, unless otherwise stated. All other chemicals were of analytical grade and were used without further purification.
3. Experimental Section 3.1. Polymer Preparation. The 30 and 35 wt % polyacrylamide aqueous solutions and Dextrin-WY powder were used to prepare stock solutions by dissolving the appropriate mass of polymer, either as a solid or from a concentrated aqueous solution, in 10-3 M KNO3 Milli-Q water. The polyacrylamide stock solutions were prepared daily, whereas Dextrin-WY stock solution was prepared and stirred overnight to ensure complete hydration of the Dextrin-WY molecules. A gelling procedure was used to prepare HP-Starch stock solutions.32 First, a small amount of water was added to the appropriate mass of powder to form a paste. 2 wt % KOH solution was then added slowly until the paste transformed into a gel (approximately 10-20 cm3 KOH is required). The resulting mixture was left to stand for 20 min before it was diluted to the desired concentration with Milli-Q water. All polymer solutions were optically clear. 3.2. Adsorption Isotherms. Adsorption studies were performed using the batch method. A 5 wt % solid sample suspension at pH 9 and 10-3 M KNO3 background concentration was first prepared and the appropriate volume of this suspension was added to individual sample vials. The required volume of 10-3 M KNO3 solution at pH 9 and stock polymer solution, also at pH 9, was added to each sample to yield samples of varying polymer concentrations. The resulting suspensions were then mixed on a rotary suspension mixer for 2 h, then centrifuged, and the supernatant was analyzed to determine the concentration of polymer left in solution via a complexation method.33,34 It was assumed that the amount of polymer depleted from solution was adsorbed onto the solid surface. The adsorbed amount, Γ, was then calculated from Γ)
1 (c - ci)V m‚As f
(1)
where m is mass of the solid substrate, As is the surface area of the solid substrate, ci and cf are the polymer concentration before and after adsorption, respectively, and V is the volume of the suspension. (32) Penford Australia. Lane Cove, Australia. Preparation of Starch Solution. Personal communication. (33) Dubois, M.; Gilles, K. A.; Hamiliton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28, 350-361. (34) Scoggins, M. W.; Miller, J. W. Anal. Chem. 1975, 47, 152-154.
Influence of Polymer Structure on Talc Wettability Preliminary adsorption studies showed that, after 15 min of equilibrium, no further change in solution concentration was detected. To ensure that equilibrium was achieved (it is acknowledged that such a state is difficult to define for polydisperse polymers), a period of 2 h was allowed for equilibrium adsorption to be achieved. Experiments were performed at 22 °C. 3.3. Contact Angle Measurements. Static contact-angle measurements on freshly cleaved talc faces were performed by adhering a small piece of adhesion tape to a flat section of the mineral and gently peeling the tape, exposing the talc face. The sample was then mounted onto a section of double-sided adhesive tape on a clean glass slide, keeping the exposed face on the top side. A small drop (approximately 100 µL) of aqueous solution was deposited on the surface using a micropipet. The silhouette of the droplet was captured and imaged with a progressive scan CCD camera (JAI CV-M10BX, Japan), and the advancing contact angle was determined by drawing a tangent close to the edge of the droplet. Five measurements were taken, and an average value was reported. Experiments were conducted at 22 °C in a class-100 clean room. Contact angle measurements were also made on talc particle beds using the Washburn technique.27,35 For measurements in the presence of polymer, samples were prepared from talc particles that had been treated in the following manner: Talc suspensions were prepared and conditioned in polymer solutions of the desired concentration for 20 min. The suspensions were left to settle, the supernatant was decanted, and the wet particles were then placed in a sealed desiccator and dried overnight under vacuum. Contact angle measurements were performed on dried samples. Measurements were made at room temperature, generally 22 °C. 3.4. TMAFM Imaging. TMAFM imaging was performed with a Nanoscope III (Digital Instruments). Ultrasharp, noncontact silicon cantilevers (NT-MDT, Moscow) with resonant frequencies varying between 200 and 400 Hz were used for imaging in air. Imaging in solution was performed using a tapping-mode fluid cell and a narrow silicon nitride (Si3N4) cantilever (Digital Instruments, Santa Barbara) with a typical spring constant of 0.2 N/m and a resonant frequency between 5 and 10 Hz. The cantilever and tip were cleaned by immersion in ethanol, rinsed with copious amounts of high-quality Milli-Q water, and dried under high-purity nitrogen before use. All experiments were conducted in a class-100 clean room at 22 °C. Freshly cleaved talc was conditioned with a small droplet of polymer solution (without background electrolyte to avoid the influence of salt recrystallization effects during imaging) of known concentration for 30 min. After conditioning, the sample was gently rinsed with high-quality MilliQ-water to remove excess polymer, then dried in air and immediately imaged. The root-mean-squared (rms) roughness and the peak-to-valley (PTV) distance for the imaged surface were determined using the standard AFM procedures. The apparent layer thickness (∆PTV) was calculated as the difference between for the PTV distances for the bare talc surface and the polymer-covered surfaces. The polymer concentration used in the imaging experiments was determined using information drawn from the adsorption isotherms, so that the desired polymer concentration corresponded to an adsorbed amount of 0.9 mg/m2. It is important to note that the ratio between the solid surface and the polymer concentration is different between the adsorption and imaging experiments. The polymer concentrations used in the imaging experiments are close estimates that corresponded to an adsorbed amount of 0.9 mg/m2 on the adsorption isotherm at pH 9.30 The surface coverage of the imaged surface was assessed using the bearing function available on the AFM software. The measured surface coverage represents the area fraction of adsorbed polymer on a solid surface with an area of 1 µm2.
4. Results 4.1. Contact Angle of Talc. Figure 1 shows an image of a water droplet lying on a flat and smooth section of the cleaved (35) Crawford, R.; Koopal, L. K.; Ralston, J. Colloids Surf. 1987, 27 (1), 57-64.
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Figure 1. Advancing water contact angle on a freshly cleaved talc surface.
talc surface. The static advancing water contact angle on the freshly cleaved talc face was 90 ( 3°. The advancing water contact angle for talc particle beds was found to be 80 ( 2°, measured by the Washburn technique. This value is lower than that of the cleaved talc face because of the influence of the hydrophilic edges of the talc particles. The 90° contact angle measured for the talc face is in good agreement with that measured by Streltsin21 and Bartell and Zuidema22, and in fair agreement with the estimates of Michot et al.20 Recall that talc comprises a hydrophobic face that accounts for 90% of the surface area while the hydrophilic edge contributes the remaining 10%.12,13 The Cassie25 equation may be used to estimate the contact angle of the hydrophobic talc face:
cos θC ) LA cos θA + LB cos θB
(2)
where θC is the composite contact angle of 80° for talc, measured from the Washburn experiments, and LA and LB ) 1 - LA are the area fractions of the siloxane groups of the talc face and the hydrophilic groups of the talc edge, respectively. If θA and θB are the contact angles on a surface composed of the siloxane groups of the talc face and the hydrophilic groups of the talc edge, respectively, then the calculated contact angle for the hydrophobic talc face is 85°. The hydrophobicity of talc can also be assessed through the work of adhesion, WA, of water to talc. The relationship between the contact angle, θ, and the work of adhesion is given by
WA WC
cos θ ) -1 + 2
(3)
where WC ) 2γL is the work of cohesion for water, and γL ) 72.8 mJ/m2. If the work of cohesion is greater than the work of adhesion, then the surface is regarded as nonwetting or hydrophobic. The work of adhesion, WA, described by Oss36 and Good37 is divided into two components: AB WA ) WLW A + WA
(4)
AB where, WLW A and WA are the Lifshitz-van der Waals (LW) and the acid-base (AB) components, respectively. The LW component takes into account the van der Waals interactions, which include orientation, induction, and dispersion contributions. The polar interactions that originate from electron-donor/electronacceptor interactions, including hydrogen bonding, are accounted for by the acid-base (AB) component of the work of adhesion. In the case of talc, only the hydroxyl groups of the edges are available for hydrogen bonding, for the hydrophobic face consists of nonpolar siloxane groups. The LW component of the work
(36) van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994. (37) Good, R. J.; van Oss, C. J. In Modern Approaches to Wettability; Schrader, M. E., Loeb, G. I., Eds.; Plenum: New York, 1992.
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Kaggwa et al.
Table 1. Calculated and Experimental Parameters Used to Determine the Work of Adhesion, WA, of Water onto Talc talc parameters
A × 10-20 J
WLW A (mJ/m2)
γ+ S (mJ/m2)
γS (mJ/m2)
WAB A (mJ/m2)
WA (mJ/m2)
49.0 31.5
65.3 52.4
2.0 2.4
2.0 2.7
20.1 32.2
85.4 84.6
9.1a
calculated Giese et al.b a
γLW S (mJ/m2)
From ref 38. b Ref 39.
of adhesion, WLW can be calculated using the following A equation:36,37 LW 1/2 WLW ) 2(γLW S γL )
(5)
where γLW S is the LW component of the solid surface energy, and LW 2 γLW L ) 21.8 mJ/m for water. γS can be calculated from
γLW S )
A 24πl02
(6)
where A is the Hamaker constant of the material, and l0 ) 1.57 ( 0.09 Å is the average equilibrium distance between two objects.36 The Hamaker constant for talc is 9.1 × 10-20 J, reported by Lins et al.38 The AB component, which is usually asymmetric, is described by two numerical parameters: one is the electron donor, γi (or Lewis base), and the other is the electron acceptor, γ+ i (Lewis acid). The AB component of the work of adhesion, WAB A for a single material is given by36,37 + - 1/2 + 1/2 + 2(γWAB A ) 2(γS γL ) S γL )
(7)
+ 2 For water, γL ) γL ) 25.5 mJ/m by definition. According 39 to Giese et al., the strengths of the acid and base sites are relatively small and are approximately equal for talc and other phyllosilicate minerals; therefore, we will assume that γ+ S ) γS and calculate these two parameters such that the contact angle, θ, calculated through eq 3, is equal to 80° (the measured contact angle for high-purity talc). Table 1 shows the calculated values used to determine the work of adhesion of water to talc. The apolar component of the work of adhesion (WLW A ) is larger than the polar component ( WAB ). This is reasonable because the basal oxygen atoms do not A function as Lewis bases to any significant degree, for they are tetrahedrally coordinated silicon atoms. In addition to this, the small surface area of edges containing OH groups, available for hydrogen bonding with water molecules, also reduces the WAB A component to the work of adhesion. The calculated results are in good agreement with the experimental values determined by Giese et al.39 on a talc sample of high purity, as shown in Table 1. 4.2. Adsorption Isotherms. The adsorption behavior of the four polymers onto talc is depicted in Figure 2. All the polymers show a high affinity for the talc surface, with HP-Starch adsorbing to the greatest extent, Polymer-N the least, and Dextrin-WY and Polymer-H falling between these two limits. In each case, a plateau is reached and the isotherm may be conveniently described
(38) Lins, F. F.; Middea, A.; Adamian, R. Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the 1st UBC-McGill Bi-annual International Symposium on Fundamentals of Mineral Processing, Vancouver, B.C., August 20-24, 1995; Laskowski, J. S., Poling, G. W., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1995; p 61-75. (39) Giese, R. F.; Constanzo, P. M.; Van Oss, C. J. Phys. Chem. Miner. 1991, 17, 611-616.
Figure 2. Adsorption behavior of (a) Polymer-N ([), Polymer-H (2), Dextrin-WY (9), and (b) HP-Starch (b) onto talc at pH 9, 10-3 M KNO3. Table 2. Langmuir Isotherm Parameters for Polymer Adsorption (pH 9, 10-3 M KNO3) polymer Polymer-N Polymer-H Dextrin-WY HP-Starch
Γm ads (mg/m2)
b (M-1)
K (M-1)
∆Gads (kJ/mol)
R2
0.77 1.05 0.97 5.07
4.9 × 3.3 × 105 6.2 × 105 5.04 × 108
2.74 × 1.84 × 107 3.46 × 107 2.80 × 1010
-42 -41 -43 -60
0.99 0.99 0.99 0.98
105
107
by the Langmuir model,40,41 with due acknowledgment of its limitations when applied to high molecular weight polymer adsorption.42 The values of Γm ads (the maximum adsorbed amount), b (Langmuir affinity constant), K (equilibrium constant), and ∆Gads (the free energy of adsorption) are given in Table 2, together with the correlation coefficient for the Langmuir fits. The adsorption data for all four polymers onto talc show a good fit with the Langmuir model, evident from the high correlation factors (R2) in Table 2. In the case of the two low (40) Hunter, R. Foundations of Colloid Science, 2nd ed.; OUP: Oxford, 2001. (41) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (42) Fleer, G. J. Adsorption of Polymers; Marcel Decker: New York, 1988.
Influence of Polymer Structure on Talc Wettability
Langmuir, Vol. 22, No. 7, 2006 3225
Table 3. Roughness, PTV Distance, Apparent Layer Thickness (∆PTV), Area Fraction of Polymer Coverage, and Adsorbed Polymer Morphology on Talc adsorbed polymer on talc
rms roughness (nm) (( 0.05)
PTV distance (nm) (( 0.05)
∆PTV (nm) (( 0.05)
area fraction of polymer coverage
adsorbed polymer morphology
no polymer Polymer-N (300 ppm) Polymer-H (300 ppm) Dextrin-WY (200 ppm) HP-Starch (50 ppm)
0.075 2.08 2.58 2.18 3.16
0.23 4.54 5.57 5.87 6.78
4.30 5.34 5.64 6.55
0.20 0.55 0.44 0.35
beadlike structures spherical domains randomly shaped patches thick-branched polymer film
molecular weight polyacrylamides, Polymer-H adsorbs to give a greater maximum adsorbed amount compared to that of Polymer-N (Table 2). Table 2 shows that the polyacrylamides have similar affinity for the talc surface, reflected by the similar affinity constants. For the polysaccharides, the high molecular weight HP-Starch has a higher affinity and adsorbs onto talc to a much greater extent than the low molecular weight DextrinWY. However, Dextrin-WY and Polymer-H have similar maximum adsorbed amounts. In comparing the polymers, it is evident that HP-Starch has the highest affinity and adsorbs to the greatest extent on the talc surface, probably due to its high molecular weight dominating the adsorption free energy.43 In all cases, ∆G°ads is between -40 and -60 kJ mol-1. We note that a value of approximately -50 kJ mol-1 is expected for hydrophobic bonding,53 while a value of half this magnitude is consistent with the free energy of hydrogen bond formation.54 For the three lower molecular weight polymers, the affinity constants and adsorption free energies are very similar, suggesting that the adsorption mechanism is also similar. Certainly, in the case of the polyacrylamide, the absence of a spectral shift in the spectra of the adsorbed polymer is indicative of a hydrophobic interaction.51 In our previous study, we showed that the adsorption of these particular polymers onto model silica surfaces obeyed the Freundlich isotherm, which describes adsorption onto heterogeneous surfaces.30 In this study, adsorption followed the Langmuir isotherm, indicating a homogeneous surface. This means that adsorption is dominated by the homogeneous basal cleavage face of talc, and that the hydrophilic talc edges play a rather minor role. This work agrees with previous studies of the adsorption of both polyacrylamides44 and polysaccharides9,10 onto talc, where the adsorption was apparently dominated by the talc face. However, in other studies, it is proposed that the talc edge is involved in the adsorption of polysaccharides.45,46 (43) Fleer, G. J.; Stuart, M. A. C.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (44) Gong, W.; Jenkins, P.; Ralston, J.; Schumann, R. In Polymers in Minerals Processing; Laskowski, J. S., Ed.; Canadian Institute of Mining, Metallurgy and Petroleum: Quebec, Canada, 1999; pp 203-216. (45) Wang, J.; Somasundaran, P.; Nagaraj, D. R. Miner. Eng. 2004, 18 (1), 77-81. (46) Rath, R. K.; Subramanian, S.; Laskowski, L. S. Langmuir 1997, 13 (23), 6260-6266. (47) Yang, J.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139-6147. (48) Okusa, H.; Kurihara, K.; Kunitake, T. Langmuir 1994, 10 (10), 35773581. (49) Israelachvili, J. N.; Alcantar, N. A.; Maeda, N.; Mates, T. E.; Ruths, M. Langmuir 2004, 20 (9), 3616-3622. (50) Pugh, R. J. Int. J. Miner. Process. 1989, 25 (1-2), 131-146. (51) Chiem, L. T.; Huynh, L.; Ralston, J.; Beattie, D. B. J. Colloid Interface Sci., in press. (52) Hubard, S. R.; Hendrickson, W. A.; Lambright, D. G.; Boxer, S. G. J. Mol. Biol. 1990, 213, 215-226. (53) Huang, H. H.; Calara, J. V.; Bauer, D. L.; Miller, J. D. Recent DeV. Sep. Sci. 1978, 4, 115-133. (54) Wang, J.; Somasundaran, P. J. Colloid Interface Sci. 2006, 293, 322332.
4.3. TMAFM Imaging of Talc and Adsorbed Polymer. Image of the Bare Talc Surface. The surface features of the cleaved talc face in the absence and presence of polymer were characterized by TMAFM. The images were performed in air, and height and phase images were subjected to a second-order flattening process. Table 3 shows the rms roughness and PTV distance of the bare talc face. It is evident from the height image that the surface of freshly cleaved talc is clean and smooth with a surface roughness and PTV distance of 0.08 and 0.23 nm, respectively. The very low surface roughness for talc is comparable to that of clean hydrophilic silica30,47 and freshly cleaved mica.48,49 In this study, TMAFM images have been performed in air. We have shown in a previous study30 that images, taken in air or in aqueous solution, of these polymers adsorbed onto modified silica surfaces, are essentially the same, with little perturbation except for some slight swelling in the solution case. In the current study, all the images have therefore been accumulated in air. Images of Talc in the Presence of Adsorbing Polymers. Polymer-N adsorbs onto the talc surface as distinct beadlike structures (Figure 3a,b). The rms roughness of this surface is 2.08 nm, and the PTV distance is 4.54 nm. The calculated apparent layer thickness is 4.30 nm. The morphology of adsorbed Polymer-H onto freshly cleaved talc is shown in Figure 3(c). The polymer adsorbs as spherical patches with a size range of 6090 nm. The spherical domains consist of aggregated polymer molecules that have an average hydrodynamic diameter of 5.8 nm in solution. These structures are distributed randomly over the surface. The rms roughness of this surface is 2.58 nm, and the PTV distance is 5.57 nm, determined from the height image. The calculated apparent layer thickness is 5.34 nm. The images of adsorbed Polymer-N (Figure 3b) and Polymer-H (Figure 3c) correlate well with the adsorption isotherms, as the measured area fraction of polymer coverage shows a greater value for Polymer-H compared with Polymer-N (Table 3). In addition to this, the layer thickness value of the adsorbed polymer layer correlates rather well with the average hydrodynamic diameter of the Polymer-N and Polymer-H molecules in solution, indicating that the polymers adsorb as a single layer and that their adsorbed structure is not significantly different from that in solution. Similar behavior has been observed when certain proteins, for example, myoglobin, adsorb at the solid-water interface.52 The image of Dextrin-WY adsorbed onto freshly cleaved talc is shown as Figure 3d. The image distinctly shows that the morphology of this polymer, when adsorbed onto the talc surface, is very different to that of the polyacrylamides. The polymer adsorbs as randomly shaped patches and significantly increases the roughness of the talc surface. The resulting apparent layer thickness is 5.64 nm, correlating quite well with the average hydrodynamic diameter of the Dextrin-WY molecules in solution (4.98 nm). HP-Starch adsorbs onto talc as a branched chain structure as shown in Figure 3e. In the presence of HP-Starch, the roughness
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Figure 4. Advancing water contact angle of a talc particle bed as a function of ([) Polymer-N, (2) Polymer-H, (9) Dextrin-WY, and (b) HP-Starch concentration.
Figure 3. Height and phase images are given for (a,b) adsorbed Polymer-N (300 ppm), (c) adsorbed Polymer-H (300 ppm), (d) adsorbed Dextrin-WY (300 ppm), and (e) adsorbed HP-Starch (50 ppm) adsorbed onto talc. Lateral scale: 5 × 5 µm (for panel a only) and 1 × 1 µm. Vertical scale: 0-20 nm.
of talc increases significantly, as is the case with the PTV distance. The rms roughness and PTV distance, for this polymer covered surface, is 3.16 and 6.78 nm, respectively. The layer thickness for the adsorbed HP-Starch was calculated to be 6.55 nm. This
value is lower than that of the measured average hydrodynamic diameter of the HP-Starch molecules in solution, suggesting that the polymer ‘flattens out’ upon adsorption. 4.4. Contact Angle of Talc in the Presence of Adsorbing Polymers. Figure 4 is a plot of the advancing water contact angle of talc particles, determined by the Washburn technique, as a function of the initial polymer concentration. There is a slight decrease in contact angle for Polymer-N as a function of concentration. This decrease is more pronounced for DextrinWY, Polymer-H, and HP-Starch, in increasing order. For example, at an initial polymer concentration of 100 ppm, the contact angle decrease of HP-Starch is 15°, whereas it is only 6° for PolymerN. A comparison between the two polysaccharides of contrasting molecular weight shows that HP-Starch, with a higher molecular weight, decreases the contact angle of talc to a greater extent than Dextrin-WY and the other low molecular weight polyacrylamides. In addition to this, HP-Starch was found to adsorb onto talc with the greatest affinity compared with Dextrin-WY. The adsorbed morphology of the two polysaccharides on the talc face demonstrates that HP-Starch adsorbs onto talc as a dense branched network, whereas the lower molecular weight DextrinWY adsorbs as smaller, randomly shaped patches. The resulting layer thickness also follows the same trend with the higher molecular weight polymer adsorbing to give a greater thickness. The fact that HP-Starch adsorbs with a higher affinity for the talc surface and a larger layer thickness explains why this polymer increases the wettability of talc to a greater extent than DextrinWY and the low molecular weight polyacrylamides. We propose that the large adsorbed amount and polymer layer thickness provides a greater number of hydrophilic -OH groups in an optimum orientation, with respect to the solution phase, to increase the wettability of the underlying surface, as suggested by Pugh.50 For the two low molecular weight polyacrylamides, Polymer-H adsorbs onto talc to a greater maximum adsorbed amount and increases the wettability of this surface to a greater degree than does Polymer-N. The two polymers also adopt a similar morphology on the talc surface; that is, they adsorb as spherical domains with similar apparent layer thicknesses. The increased adsorption explains why Polymer-H decreases the contact angle of talc, with the polymer structure also playing a role. PolymerH, unlike Polymer-N, is substituted with hydrophilic hydroxyl groups that aid in increasing the talc wettability.
Influence of Polymer Structure on Talc Wettability
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and, in the case of HP-Starch, there is no correlation at all. In the case of the latter polymer, the determination of L is exceedingly difficult, for it is likely that polymer is also adsorbed between the dominant domains observed in Figure 3e.
Conclusion
Figure 5. The experimental contact angle of talc versus the fraction of adsorbed polymer for ([) Polymer-N, (2) Polymer-H, (9) DextrinWY, and (b) HP-Starch. The solid lines are the linear Cassie equation prediction.
The Cassie equation, used effectively in section 4.1, may be used to describe the surface wettability of the talc in the presence of adsorbed polymer. Advancing water contact angles for talc as a function of the area fraction of adsorbed polymer are given in Figure 5, derived from contact angle measurements made on particles and polymer TMAFM imaging for area fractions. For Polymer-N and Dextrin-WY, the experimental values were in good agreement with the Cassie predictions, reflecting the discrete nature of the adsorption process. For Polymer-H, deviations from Cassie become pronounced at polymer L greater than about 0.4,
The wettability of talc was investigated by directly measuring the advancing water contact angle on freshly cleaved talc as well as in a talc particle bed. The intrinsic hydrophobicity of talc was assessed by considering the apolar and polar components to the work of adhesion. It was found that the apolar contributions exceeded the polar components, reflecting the talc crystal structure and bonding and explaining the mineral’s intrinsic hydrophobicity. The polyacrylamide and polysaccharide polymers adsorbed onto the talc surface in a Langmuirian manner, indicating that the homogeneous talc face dominated the adsorption process in terms of adsorption sites. The wettability of talc in the presence of polymers was correlated to both adsorption data and adsorbed polymer morphology. Both polymer structure and molecular weight were found to influence the results. A strong correlation exists between the adsorbed amount, the adsorbed morphology, layer thickness, and the subsequent talc wettability. This was strikingly evident in the case of the high molecular weight polymer, HP-Starch, which decreased the wettability of talc to the greatest extent. This was attributed to a greater adsorbed amount and apparent layer thickness in comparison with the low molecular polymers. This investigation of talc surfaces, in conjunction with our previous study of silica surfaces30 demonstrates that adsorbed amount, adsorbed polymer morphology, apparent layer thickness, and polymer structure influence solid surface wettability. Acknowledgment. Support from the Australian Research Council and AMIRA International is gratefully acknowledged. Fruitful discussions with Daniel Fornaserio, David Beattie, and Rossen Sedev are warmly acknowledged. LA052303I
