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Morphology of Adsorbed Polymers and Solid Surface Wettability Gillian B. Kaggwa, Stefanie Froebe, Le Huynh, John Ralston,* and Kristen Bremmell Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia Received October 27, 2004. In Final Form: December 2, 2004 The adsorption of a polyacrylamide (MW 14600) and two polysaccharides (MW 9260 and 706 × 103) onto model silica surfaces of different hydrophobicities was investigated. In all cases, adsorption adhered to the Freundlich isotherm, reflecting the heterogeneous character of the solid substrates. The latter strongly influenced the character of the adsorbed polymer, with morphologies from chainlike structures to thin films and patches being observed. Surface roughness, polymer type, and molecular weight also play roles in controlling adsorbed polymer morphology. Surface wettability is strongly influenced by the thickness of the adsorbed layer.
Introduction Polymers are used in numerous industrial applications to modify the properties of solid surfaces to achieve a desired outcome. In mineral processing, for example, polymers are used as flocculants,1 dispersants,2,3 rheology modifiers,4 and depressants.5,6 Depressants are employed to reduce the hydrophobicity of targeted minerals. They act by adsorbing selectively onto hydrophobic gangue minerals, reducing the surface hydrophobicity and thus preventing bubble-particle attachment and subsequent capture.7-9 Depressants include functionalized polyacrylamides5,10,11 and polysaccharides.6,12-16 The morphology or topography of soft materials, such as an adsorbed polymer layer, can be characterized by using tapping mode atomic force microscopy (TMAFM). * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Clark, A. Q.; Herrington, T. M.; Petzold, J. C. Colloids Surf. 1990, 44, 247-261. (2) Pantchev, I.; Hunkeler, D. J. J. Appl. Polym. Sci. 2004, 92 (6), 3736-3743. (3) Das, K. K.; Somasunduran, P. Colloids Surf., A: Physicochem. Eng. Aspects 2003, 223 (1-3), 17-25. (4) Huynh, L.; Jenkins, P.; Ralston, J. Int. J. Miner. Process. 2000, 59 (4), 305-325. (5) Boulton, A.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2001, 61 (1), 13-22. (6) Pugh, R. J. Int. J. Miner. Process. 1989, 25 (1-2), 101-130. (7) Lin, K. F.; Burdick, L. In Polymeric Depressants, in Reagents in Mineral Technology; Somasundaran, P., Moudgil, B. M., Eds.; Marcel Dekker: New York, 1988; pp 471-483. (8) Morris, G.; D. Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2002, 67, 211-227 (9) Pugh, R. J. Int. J. Miner. Process. 1989, 25 (1-2), 131-146. (10) Gong, W.; Jenkins, P.; Ralston, J.; Schumann, R. Polymers in Mineral Processing, Proceedings of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 3rd, Quebec City, QC, Canada, Aug 22-26, 1999; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1999; pp 203216. (11) Chen, H. T.; Ravishankar, S. A.; Farinato, R. S. Int. J. Miner. Process. 2003, 72 (1-4), 75-86. (12) Morris, G. Ph.D. Thesis, Ian Wark Research Institute, University of South Australia, Adelaide, 1996. (13) Laskowski, J. S.; Liu, Q.; Bolin, N. J. Int. J. Miner. Process. 1991, 33 (1-4), 223-234. (14) Bulatovic, S. M. Miner. Eng. 1999, 12 (4), 341-354. (15) Jenkins, P. and Ralston, J. Colloids Surf., A: Physicochem. Eng. Aspects 1998, 139 (1), 27-40. (16) Rath, R. K.; Subramanian, S.; Laskowski, L. S. Langmuir 1997, 13 (23), 6260-6266.
The technique can image a surface without damage to the soft material, as it is based on the vertical oscillation of a tip at or near its resonant frequency. This basically allows the tip to gently tap over the surface, imaging soft or weakly bound materials in a nondestructive manner. The technique avoids the use of the standard contact-imaging mode, which involves scanning with the probe and sample in Born contact. This imaging mode is limited to hard solid surfaces. TMAFM can also be operated to image surfaces either in air17,18 or in solution,19,20 thus matching the environment in which adsorption processes take place. The technique has been used extensively since its development by Binnig et al.21 to image numerous structures, including biopolymers such as DNA.22,23 Surfactants, widely used in mineral processing as surface modifiers, have been imaged with TMAFM to understand the morphology of the adsorbed surfactant aggregates,24,25 building on the earlier concept of hemimicelles.26 Numerous images exist of surfactant structures adsorbed on defined surfaces, such as the work of Atkin et al.,27 Velegol et al.,28 and Davey et al.29 Ducker30 has reviewed adsorbed surfactant structures obtained through AFM imaging experiments, noting how the surface dictates the type of structure. Polymer-surfactant systems have (17) Pfau, A.; Schrepp, W.; Horn, D. Langmuir 1999, 15 (9), 32193225. (18) Akari, A.; Schrepp, W.; Horn, D. Langmuir 1996, 12 (4), 8571108. (19) Arita, T.; Kanda, Y.; Hamabe, H.; Ueno, T.; Watanabe, Y.; Higashitani, K. Langmuir 2003, 19 (17), 6723-6729. (20) Arita, T.; Kanda, Y.; Higashitani, K. J. Colloid Interface Sc. 2004, 273 (1), 102-105. (21) Binnig, G.; Quate, C. F.; Gerber, C. H. Phys. Rev. Lett. 1986, 56 (9), 930-933. (22) Medalia, O.; Englander, J.; Guckenberger, R.; Sperling, J. Ultramicroscopy 2001, 90 (2-3), 103-112. (23) Sanchez-Sevilla, A.; Thimonier, J,; Marilley, M.; Rocca-Serra, J.; Barbet, Jacques. Ultramicroscopy 2002, 92 (3-4), 151-158. (24) Tulpar, A.; Ducker, W. A. J. Phys. Chem. B 2004, 108 (5), 16671676. (25) Connell, S. D.; Collins, S.; Fundin, J.; Yang, Z.; Hamley, I. W. Langmuir 2003, 19 (24), 10449-10453. (26) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68 (12), 3562-3566. (27) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Phys. Chem. B 2003, 107 (13), 2978-2985. (28) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16 (6), 2548-2556. (29) Davey, T. W.; Warr, G.; Almgren, M.; Asakawa, Tsuyoshi. Langmuir 2001, 17 (17), 5283-5287.
10.1021/la047352u CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005
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also been studied to understand competitive adsorption processes.31,32 There are very few reports of the imaging of adsorbed polymers on mineral surfaces. Arita et al.19,20 have investigated changes in the morphology of adsorbed high molecular weight cationic polymeric flocculants with TMAFM. Their work showed that the morphology of an adsorbed single polymer changes as a function of time and that imaging in solution influences the apparent structure of the water-soluble polymers. Questions regarding the amount and disposition of polymer adsorbed on the solid surface, the thickness of the adsorbed layer, how the character of the solid surface influences the adsorption process, and the overall influence of these variables upon solid surface wettability remain unanswered. In this study, the adsorption of a polyacrylamide and two polysaccharides onto well-defined silica surfaces has been investigated through adsorption isotherms and TMAFM. These polymers are used to depress minerals of quite different hydrophobicities, such as talc, coal, carbonaceous pyrite, and lightly oxidized metal sulfides. We have chosen to use polymers that are used in practice. Inevitably these are polydisperse; however, the study of such systems is of paramount importance. Silica is the solid substrate of choice in this study. In the unmodified clean state it is hydrophilic and can be hydrophobized by heating-dehydroxylation and as well as by silanation.33 Through precise control of substrate chemistry, topology, and hydrophobicity, the link between substrate and adsorbed polymer structure can be deduced. The aims of this study are to quantify the adsorbed amount of polymer, characterize the adsorbed polymer layer in terms of the polymer distribution, surface roughness, and layer thickness, investigate the influence of polymer structure and type and solid surface hydrophobicity on both the adsorbed amount and adsorbed polymer morphology, and correlate the information gained from the diverse adsorption experiments with the resulting surface wettability. Experimental Section Materials. Chemicals. A 35 wt % polyacrylamide aqueous solution sample was supplied by Cytec Industries, with a 15% degree of random hydroxyl substitution and a molecular weight of 14600, determined through size exclusion chromatography. The polydispersity of this polyacrylamide, referred to for convenience as Polymer-H, is 3.4. Polymer-H has an average hydrodynamic diameter of 5 nm, determined through dynamic light scattering measurements. The length of the fully extended Polymer-H, calculated from the dimensions of the monomer unit multiplied by the number of repeating units based on the polymer molecular weight, is 63 nm. The general structure of the repeating unit along with the approximate dimensions is shown in Figure 1. The polysaccharides Dextrin-WY and hydroxypropylated starch (HP-Starch) were supplied by Penford Australia. DextrinWY has a molecular weight of 9260 and a polydispersity of 6.0. The molecular weight and polydispersity of HP-Starch are 706000 and 11.4, respectively. HP-Starch has a 5.3% degree of hydroxypropyl substitution occurring at the C-2 and C-6 positions (Figure (30) Ducker, W. A. Atomic force microscopy of adsorbed surfactant micelles. In Adsorption and Aggregation of Surfactants in Solution; Mittal, K. L., Shah, D., Eds.; Marcel Dekker: New York, 2003; Vol. 109, pp 219-242. (31) Fleming, B. D.; and Wanless, E. J. Microsc. Microanal. 2000, 6 (2), 104-112. (32) Liu, J.-F.; Min, G.; Ducker, W. A. Langmuir 2001, 17 (16), 48954899. (33) Yang, J.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139-6147.
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Figure 1. Molecular structure of the repeating unit of PolymerH.
Figure 2. Basic molecular structure of the polysaccharides dextrin and starch. For HP-Starch the hydroxyl sites (primarily C-2 and C-6) have been substituted with -OCH2(CHOH)CH3 (hydroxypropylated group). 2). 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 for Dextrin-WY and HP-Starch molecules is 5 and 16 nm. The general chemical structures of the polysaccharides are shown in Figure 2, along with their approximate dimensions. Trimethylchlorosilane (TMCS; redistilled, 99+%) was obtained from Aldrich and stored in a vacuum desiccator over silica gel (Chem-Supply, South Australia). Cyclohexane (analytical grade, 99%) was obtained from Chem-Supply, South Australia, and distilled before use. High-purity Milli-Q water was supplied by a Millipore water system and had a conductivity of less than 1 × 10-6 S cm-1 and surface tension of 72.8 mN m-1 (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. Solid Substrates. Oxidized silicon wafers (MEMC Electronic Materials Inc.) were used as the solid substrates. The root mean square (rms) roughness of the silicon wafers was less than 0.1 nm, and the peak to valley distance was approximately 0.3 nm. Clean hydrophilic silica surfaces were prepared by a method employed by Yang et al.33 This involves sonicating the silicon wafer in ethanol and cyclohexane for 30 s, to remove particulate contamination, followed by treatment in an air plasma (Harrick plasma cleaner/sterilizer PDC-32) for 30-40 s, as a first step in the removal of organic impurities. The wafers were then immersed in a mixture of H2SO4 (98%)/H2O2 (30%) (3:1 v/v) at 80 °C for 30 min and then rinsed with copious amounts of Milli-Q water, in a second stage of residual organic material removal. The wafers were then placed into hot Milli-Q water for 40 min, dried under high-purity nitrogen, and used immediately for surface modification. This procedure was carried out in a class100 clean room, ensuring minimal surface contamination. The wafers were hydrophilic, displaying wetting fringes and no finite contact angle. The surface chemistry and hydrophobicity of the substrates were altered through dehydroxylation and methylation. Clean hydrophilic silica was converted to a hydrophobic state through either dehydroxylation or methylation.33-35 Freshly cleaned silicon wafers were dehydroxylated in a furnace at 1050 °C (34) Hair, M. Silica Surfaces. In Silanes Surfaces and Interfaces; Leyden, D.; Ed.; Gordon and Breach Science Publishers: CO, 1986.
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overnight and allowed to cool in a sealed desiccator.34,35 For methylation, clean wafers were exposed to the vapor of pure TMCS for 30 min, then rinsed with cyclohexane and copious amounts of Milli-Q water, cleaned with a CO2 gun, and dried under a stream of high-purity nitrogen. Cleaned silicon wafers were also methylated in freshly prepared 1 wt % TMCS solution in cyclohexane for 30 min, under a dry nitrogen atmosphere in a glovebag. They were sonicated in cyclohexane for 30 min to remove residual TMCS and any byproducts. They were then washed with cyclohexane, followed by rinsing with Milli-Q water, cleaning with a CO2 gun, and finally drying under a stream of high-purity nitrogen. The dehydroxylation and methylation of the silicon wafers were also carried out in a clean room at room temperature. Silica particles used for adsorption isotherms were supplied by Sigma-Aldrich (99%). Prior to experimentation the silica particles were cleaned with a series of hot aqua regia washes, followed by rinsing with Milli-Q water until the pH was neutral. The silica particles were then treated with caustic KOH to remove any residual organic impurities and again rinsed thoroughly with Milli-Q water until the pH was again neutral. Analysis by X-ray photoelectron spectroscopy (Perkin-Elmer Physical Electronics Division (PHI) 5100 XPS system) showed that the sample was free from impurities. The particle size distribution was 0.5-10 µm, determined using a Malvern Instrument Mastersizer, with a D10 of 1.41 µm, a D50 of 3.68 µm, and a D90 of 7.89 µm. The BET specific surface area of the sample was 5.2 m2 g-1. The surface chemistry and hydrophobicity of the silica particles were altered through dehydroxylation and solution methylation in a class100 clean room, following procedures similar to those used for the silicon wafers. There was no detectable change in the BET specific surface area following surface modification. Methods. Polymer Preparation. The 35 wt % polyacrylamide aqueous solution and Dextrin-WY sample 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. The Dextrin-WY stock solution was prepared and stirred overnight to ensure complete hydration of the DextrinWY molecules. HP-Starch stock solutions were prepared daily, by adding a small amount of Milli-Q water to the appropriate mass of polymer solid, forming a paste.36 A small volume of 2 wt % KOH solution was then added slowly, and the resulting mixture was left to stand and gel for 20 min, after which Milli-Q water was added slowly with mixing, to dissolve the gel. All the prepared solutions were optically clear. Adsorption Isotherms. Adsorption studies were performed using the batch method. A 5 wt % solid sample suspension at pH 9 was prepared, and a known volume of this suspension was added to individual 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 vial, yielding samples of different polymer concentrations. The resulting suspensions were then mixed on a rotator for 2 h, centrifuged, and analyzed to determine the concentration of polymer left in solution, using a complexation method.37,38 It was assumed that the amount of polymer depleted from solution was adsorbed onto the solid surface. The adsorbed amount (Γ) can then be calculated using the following equation:
achieved (it is acknowledged that such a state is difficult to define for polydisperse polymers), a period of 2 h was allowed for equilibration. Experiments were performed at 22 °C. Adsorption studies were preformed in duplicate, and the results shown are the average from the experiments. Contact Angle Measurements. Contact angle measurements were made on flat modified silica surfaces using the captivebubble technique. The captive-bubble technique involves advancing and receding a small air bubble (2-4 mm in diameter) using a motorized syringe. The silhouette of the bubble was captured and imaged with a progressive scan CCD camera (JAI CV-M10BX, Japan), and the contact angle was then determined by drawing a tangent close to the edge of the bubble. The advancing and receding water contact angles for each surface were measured using this technique. For an experiment conducted in the presence of polymers, 15 min was allowed for equilibrium to occur, after which there was no detectable contact angle change with time. Experiments were conducted at 22 °C. TMAFM Imaging. Surface imaging through TMAFM 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, thin silicon nitride Si3N4 cantilever (Digital Instruments, Santa Barbara, CA) 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, rinsing with copious amounts of highquality Milli-Q water, and drying under high-purity nitrogen before use. All experiments were conducted in a class-100 clean room at 22 °C. Silicon wafers were cleaned, then either dehydroxylated or methylated, and finally conditioned in a polymer solution (without background electrolyte to avoid the influence of salt recrystallization effects during imaging) of known concentration for 30 min. When conditioned in the presence of polymer, the wafers were placed upright (rather than horizontal) to ensure any polymer that is present on the surface when imaged was due to adsorption. Samples were then rinsed with high-quality Milli-Q water to remove excess polymer, gently dried under a stream of high-purity nitrogen, and imaged immediately in air or solution. The rms roughness and the peak to valley (PTV) distance for the imaged surface were determined using standard AFM procedures. 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, which correspond to an adsorbed amount of 0.9 mg/m2 on the adsorption isotherm. The surface coverage of the imaged surface was assessed by using the bearing function available in the AFM software. The surface coverage represents the area fraction of adsorbed polymer on a solid surface with an area of 1 µm2.
Results and Discussion Γ)
1 (c - ci)V mAs f
(1)
where m is the mass of the solid substrate, As is the surface area of the solid substrate, ci and cf are the polymer concentrations before and after adsorption, respectively, and V is the volume of the suspension. Preliminary adsorption studies indicated that, after 15 min of equilibrium, no further change in solution concentration was detected. To ensure that equilibrium was (35) Burneau, A. Hydroxyl Groups on Silica. In The Surface Properties of Silicas; Legrand, A. P., Ed.; John Wiley & Sons: Chichester, U.K., 1998; p 165. (36) Personal communication with Penford, Australia. (37) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28 (3), 350-361. (38) Scoggins, M. W. and Miller, J. W. Anal. Chem. 1975, 47 (1), 152-154.
Adsorption Isotherms. The adsorption behavior of all three polymers is shown in Figure 3. In no case is an adsorption plateau evident; rather there is a steady increase in the adsorption density with equilibrium concentration. For convenience we report adsorption densities at specific polymer concentrations. Polymer-H does not adsorb onto hydrophilic silica but does adsorb onto methylated and dehydroxylated silica to give an adsorbed amount of 2.7 and 0.7 mg/m2, respectively, at 80 ppm (equilibrium concentration) in Figure 3a. The adsorption of Dextrin-WY onto dehydroxylated silica and methylated silica as a function of equilibrium concentration is shown in Figure 3b. Similarly, Dextrin-WY does not adsorb onto hydrophilic silica but does adsorb weakly onto dehydroxylated silica to an adsorbed amount
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Figure 3. Adsorption behavior of (a) Polymer-H, (b) DextrinWY, and (c) HP-Starch onto methylated, dehydroxylated, and hydrophilic (HP-Starch only) silica surfaces.
of 0.2 mg/m2 at an equilibrium concentration of 80 ppm, which only increases to 0.4 mg/m2 at an equilibrium concentration of 200 ppm. The adsorption of Dextrin-WY onto methylated silica shows an enhanced adsorption density, compared with that of dehydroxylated silica, yielding adsorption densities of 0.5 mg/m2 at 80 ppm (equilibrium concentration) and 0.8 mg/m2 at 200 ppm (equilibrium concentration). The adsorption behavior of HP-Starch is shown in Figure 3c. HP-Starch adsorbs weakly onto hydrophilic silica, followed by methylated silica and then dehydroxylated silica. The polymer adsorbs onto dehydroxylated silica to give an adsorption density of 1.9 and 1.7 mg/m2 on methylated silica and 1.4 mg/m2 on hydrophilic silica, all at 80 ppm equilibrium concentration. The adsorption of Polymer-H onto the model surfaces obeys the Freundlich isotherm, as shown in Figure 4a. The Freundlich isotherm is given by the following equation:
Γabs ) kFCeq1/nF
(2)
where kF and nF are empirical constants. The constant kF is described as the adsorbent capacity, and 1/nF is the adsorption affinity constant. This isotherm is an empirical isotherm that describes the adsorption of species onto
Figure 4. Freundlich isotherm adsorption behavior of (a) Polymer-H, (b) Dextrin-WY, and (c) HP-Starch onto the hydrophilic (HP-Starch only), dehydroxylated, and methylated silica surfaces.
heterogeneous surfaces39-41 and is derived by assuming a certain distribution function of sites with different free energies (∆G°), supposing that Langmuir adsorption takes place at each type of site.42 Freundlich isotherms do not approach monolayer coverage; instead the adsorbed amount increases with polymer concentration, a feature exhibited by all polymers in this study. A plot of the logarithm of the adsorbed amount against the logarithm of the equilibrium concentration gives a straight line defined by the following equation:
log Γ ) log kF + (1/nF) log Ceq
(3)
The adsorption data for Polymer-H on the model surfaces showed a good fit with the Freundlich isotherm (Figure 4a), evident from the high correlation factor (R2) in Table 1. This is convincing, for the Freundlich isotherm is reported to adequately describe adsorption onto heterogeneous surfaces such as dehydroxylated and methylated silica.43 The experimental data for the adsorption (39) Hunter, R. Foundations of Colloid Science, 2nd ed.; OUP: Oxford, 2001. (40) Yang, C.-H. J. Colloid Interface Sci. 1998, 208 (2), 379-387. (41) Li, H.; Xu, M,; Shi, Z.; He, B. J. Colloid Interface Sci. 2004, 271 (1), 47-54. (42) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (43) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: Canada, 1997.
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Table 1. Freundlich Isotherm Parameters for Polymer Adsorptiona polymer
solid surface
adsorption density at Ceq ) 80 ppm (mg/m2)
1/nF adsorption affinity constant
kF (L/gsolid)
correlation factor (R2)
Polymer-H
dehydroxylated silica methylated silica dehydroxylated silica methylated silica hydrophilic silica dehydroxylated silica methylated silica
0.80 2.65 0.30 1.25 1.35 1.92 1.68
0.91 0.74 0.48 1.04 1.33 0.40 0.56
0.07 0.52 0.12 0.02 0.01 2.10 0.88
0.99 0.92 0.69 0.94 0.97 0.99 0.99
Dextrin-WY HP-Starch
a
Adsorption densities at an equilibrium polymer concentration of 80 ppm shown for comparative purposes.
Figure 5. A comparison of the adsorbed amount at 80 ppm equilibrium concentration for each polymer on a given solid surface.
of Dextrin-WY on methylated silica and dehydroxylated silica fit the Freundlich isotherm albeit not overly well for dehydroxylated silica (evident by the low R2 value in Table 1) as shown in Figure 4b. Note that the adsorption onto dehydroxylated silica is rather weak in any case. The adsorption of HP-Starch adheres to the Freundlich isotherm in all cases (Figure 4c and Table 1). Table 1 summarizes the parameters calculated from fitting the adsorption data to the Freundlich isotherm. Recall that the constant kF gives an indication of the adsorbent capacity, whereas 1/nF gives a measure of the adsorption affinity: the greater the value of nF, the greater the adsorption affinity.40-43 Table 1 shows that the values of kF and nF increase with increasing surface hydrophobicity for Polymer-H, correlating with the adsorption data. For Dextrin-WY, the nF and kF values do not correlate with the adsorption data as the Freundlich isotherm does not fit the data well, particularly the adsorption onto dehydroxylated silica, as shown by the low correlation factor in Table 1. For HP-Starch the nF and kF values both correlate with the trend seen for the adsorption data. The largest nF and kF values correspond to the strongest adsorption onto dehydroxylated silica, followed by methylated silica and hydrophilic silica. Surface hydrophobicity dominates the adsorption affinity and the adsorption density for the low molecular weight polymers Polymer-H and Dextrin-WY as shown in Figure 5. HP-Starch adsorbs onto silica and dehydroxylated silica to give a greater adsorbed amount at 80 ppm equilibrium concentration than the other polymers (Figure 5). In the case of methylated silica, Polymer-H gives the greater adsorbed amount, followed by HP-Starch and then Dextrin-WY (Figure 5). It is important to note that the adsorption isotherms were all carried out at pH 9 with a 10-3 M KNO3 background concentration. In these conditions the ζ potential or the surface charge of silica, dehydroxylated silica, and methylated silica remains
Figure 6. Height and phase images, captured in air, for (a) hydrophilic silica, (b) dehydroxylated silica, (c) vapor methylated silica, and (d) solution methylated silica surfaces. Lateral scale: 1 × 1 µm. Vertical scale: 0-5 µm.
constant.44 The driving force for adsorption is therefore nonelectrostatic in origin, for the polymers are uncharged (shown by the absence of any H+ and OH- consumption (44) Snoswell, D. R. E.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107 (13), 2986-2994.
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Table 2. Roughness, Peak to Valley Distance, and Contact Angles (Advancing and Receding) for Solid Surfaces
solid surface clean hydrophilic silica dehydroxylated silica (1050 °C) vapor methylated silica (30 min) solution methylated silica (30 min)
water contact rms PTV angle ((2°) roughness distance ((0.05 nm) ((0.05 nm) advancing receding 0.11
0.43
spreading
0.12
0.45
45
40
0.21
0.9
76
70
0.32
1.3
86
71
in potentiometric titrations). Broseta et al.45 have also found that the adsorption of a nonionic polyacrylamide increases with surface hydrophobicity, which is in good agreement with this study. Nonelectrostatic interactions, primarily the hydrophobic interactions, have also been proposed to be the major driving force for the adsorption of polyacrylamides10 and polysaccharides8,12,15 onto a hydrophobic mineral. Hydrogen bonding also may also play a role, evidenced by the different adsorption behavior of HP-Starch onto silica. This will be explored in a future, related spectroscopic study.46 TMAFM and Surface Wettability. Bare Solid Surfaces. The surface features of the solid substrates in the presence and absence of polymer were characterized by TMAFM. The height, phase, and amplitude images of the solid substrates in the absence of polymer are shown in Figure 6. The images were obtained in air, and height images were subjected to a second-order flattening process. Table 2 shows the rms roughness and PTV distance of the bare solid substrates. The advancing and receding water contact angles for the bare substrates are also shown in Table 2. The rms roughness and PTV distance of clean hydrophilic silica (Figure 6a) are 0.11 and 0.43 nm over a 1 µm × 1 µm area, respectively. The low rms roughness and PTV distance indicate that the surface is highly smooth and clean. These values are similar to those reported by Yang et al.33 The contact angle measurements showed instant and spontaneous spreading of the water droplet over the entire surface. Figure 6b shows the TMAFM image of the dehydroxylated silica surface. The rms roughness and PTV distance for this surface are 0.12 and 0.45 nm, respectively. These values are similar to those for clean hydrophilic silica; therefore, the heat treatment of silica does not result in a distinct change of surface morphology. The water contact angle is, however, altered by the dehydroxylation process due to the change in surface chemistry. The increase in the water contact angle (Table 2) through dehydroxylation is due to mutual condensation of the surface silanol groups to form siloxane bridges.33,35,47 The TMAFM image of a TMCS vapor methylated silica for 30 min is shown in Figure 6c. The image shows a distinct change in the surface morphology compared with that of the clean hydrophilic and dehydroxylated silica surfaces. The substrates that have been methylated with TMCS vapor have an rms roughness and PTV distance that are slightly greater than those of the hydrophilic and dehydroxylated silica. The measured advancing and receding water contact angles are 76° and 70°, respectively. (45) Broseta, D.; Medjahed, F. J. Colloid Interface Sci. 1995, 170 (1), 457-465. (46) Chiem, L.; Huynh, L.; Ralston, J.; Beattie, D. J. Colloid Interface Sci., manuscript in preparation. (47) Laskowski, J. S.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 29 (4), 670-679.
Figure 7. Height and phase images for (a) Polymer-H (100 ppm) adsorbed onto dehydroxylated silica, (b-d) Polymer-H (25, 75, and 150 ppm) adsorbed onto vapor methylated silica, captured in air, and (e) Polymer-H (75 ppm) adsorbed onto methylated silica captured in solution. Lateral scale: 1 × 1 µm. Vertical scale: 0-5 and 0-10 µm (c only).
The contact angle hysteresis is low for the vapor methylated surface, correlating with earlier observations.35,48,49 (48) Crawford, R.; Koopal, L. K.; Ralston, J. Colloids Surf. 1987, 27 (1), 57-64.
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Table 3. Roughness, Peak to Valley Distance, and Apparent Layer Thickness (∆PTV) of Adsorbed Polymer-H solid substrate 100 ppm/dehydroxylated silica 25 ppm/30 min vapor methylated silica 75 ppm/30 min vapor methylated silica 150 ppm/30 min vapor methylated silica 75 ppm/30 min vapor methylated silica, imaged in solution
rms roughness ((0.05 nm)
PTV distance ((0.05 nm)
∆PTV ((0.05 nm)
area fraction of polymer coverage
adsorbed polymer morphology
0.16 0.43 0.43 0.52 0.50
0.68 1.75 1.70 1.85 2.15
0.25 0.85 0.80 0.95 1.25
0.77 0.35 0.53 0.64 0.48
thin film linear chainlike structures dense chainlike structures dense chainlike structures swollen chainlike film
The image of the solution TMCS methylated silica surface is shown in Figure 6d. The image shows that the surface is rougher than the other solid surfaces, demonstrated by an increase in the rms roughness and PTV distance, being approximately 0.32 and 1.3 nm, respectively. The surface is suitable for determining the influence of surface roughness on the distribution and appearance of adsorbed polymer. The advancing water contact angle for this surface was 86°, greater than that for the vapor methylated surface and in agreement with past studies.35,47,50,51 The receding water contact angle was 71°. The solution methylated silica surface has a greater contact angle hysteresis than that of the vapor methylated silica surface, reflecting its greater surface heterogeneity. Polymer-H. Polymer-H adsorbs onto both dehydroxylated and methylated silica, but reaches a greater adsorbed amount on the methylated silica surfaces, as shown through adsorption isotherms (Figure 3a). Figure 7a shows the height and phase images of Polymer-H adsorbed onto dehydroxylated silica taken in air. There is a slight increase in the rms roughness and PTV distance (Table 3) for this surface compared with those for the underlying dehydroxylated silica surface. Polymer-H appears to adsorb onto dehydroxylated silica as a very thin film, supported by the low apparent layer thickness of 0.25 nm. The area fraction of polymer covered was found to be 0.77 as shown in Table 3. Images of adsorbed Polymer-H on vapor methylated silica in air were carried out at three different initial polymer concentrations. Figure 7b shows the height and phase images of 25 ppm Polymer-H adsorbed onto the vapor methylated surface. Polymer-H has a distinct and different morphology on this surface compared with that on the dehydroxylated surface. The polymer is adsorbed as structured polymer “chains”. The average length and width of the chainlike structures are 180 and 20 nm, respectively. When this is correlated with the length of an individual polymer chain, the structures appear to be 3-4 chains end on end. A change in the surface roughness occurs between the polymer-covered and bare surfaces, as shown in Table 3. The thickness of the polymer film, estimated from the PTV distance with reference to the bare methylated surface, was found to be 0.85 nm. Parts c and d of Figure 7 show the adsorbed structure of Polymer-H at 75 and 150 ppm, respectively, on the vapor methylated silica surface. The rms roughness and PTV distance for these images are shown in Table 3. The calculated apparent layer thickness for all three images is roughly 0.9 nm. This correlates well with the width of the polymer structure (0.99 nm) in Figure 1, which suggests that the polymer is adsorbing in a flat conformation. A comparison of the three images clearly shows that the area fraction of surface covered with adsorbed polymer (49) Mahnke, J.; Stearnes, J.; Hayes, R. A.; Fornasiero, D.; Ralston, J. Phys. Chem. Chem. Phys. 1999, 1, 2793-2798. (50) Lamb, R. N.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1 1982, 78 (1), 61-73. (51) Pashley, R.; Kitchener, J. A. J. Colloid Interface Sci. 1979, 71 (3), 491-500.
increases as the polymer concentration increases, as shown in Table 3. This correlates very well with the results obtained from the adsorption experiments (Figure 3a). The adsorbed polymer layer was also imaged in solution. The images taken in solution in situ were compared with those taken in air (ex situ). The influence of the solution on the appearance and distribution of the adsorbed polymer layer could then be investigated. An image of 75 ppm Polymer-H adsorbed on vapor methylated silica taken in solution is shown in Figure 7e. This image was compared with the image taken in air (Figure 7d). There is an increase in the rms roughness and PTV distance (shown in Table 3) values for this image compared with the ex situ image. The polymer appears to be more swollen in situ. The apparent layer thickness of the adsorbed polymer imaged in solution is slightly larger than that in air. The
Figure 8. Height and phase images for (a) Dextrin-WY (400 ppm) adsorbed onto dehydroxylated silica, (b) Dextrin-WY (150 ppm) adsorbed onto vapor methylated silica, captured in air, and (c) Dextrin-WY (150 ppm) adsorbed onto vapor methylated silica captured in solution. Lateral scale: 1 × 1 µm. Vertical scale: 0-5 and 0-10 µm (c only).
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Table 4. Roughness, Peak to Valley Distance, and Apparent Layer Thickness (∆PTV) of Adsorbed Dextrin-WY rms roughness ((0.05 nm)
PTV distance ((0.05 nm)
∆PTV ((0.05 nm)
area fraction of polymer coverage
adsorbed polymer morphology
400 ppm/dehydroxylated silica
0.28
1.07
0.62
0.75
150 ppm/30 min vapor methylated silica 150 ppm/30 min vapor methylated silica, imaged in solution
0.70 0.51
1.85 1.75
0.85 0.85
0.53 0.42
inhomogeneous film with protruding features dense inhomogeneous film dense inhomogeneous film
solid substrate
Table 5. Roughness, Peak to Valley Distance, and Apparent Layer Thickness (∆PTV) of Adsorbed HP-Starch solid surface
rms roughness PTV distance ∆PTV area fraction of ((0.05 nm) ((0.05 nm) ((0.05 nm) polymer coverage
200 ppm/hydrophilic silica 125 ppm/dehydroxylated silica
0.13 0.55
0.60 2.75
0.20 2.30
0.38 0.77
150 ppm/30 min vapor methylated silica 150 ppm/solution methylated silica
0.70 1.30
2.20 5.30
1.30 4.00
0.52 0.51
150 ppm/30 min vapor methylated silica, imaged in solution
1.46
4.25
3.35
0.42
important outcome, of course, is that the essential features of the adsorbed polymer remain the same irrespective of whether the images were obtained in or ex situ. Dextrin-WY. Dextrin-WY adsorbs onto dehydroxylated and methylated silica, but to a greater degree on methylated silica (Figure 3b). Figure 8a shows an image of Dextrin-WY adsorbed onto dehydroxylated silica, revealing an inhomogeneous film consisting of small protruding features separated by smoother zones. The rms roughness and PTV distance (Table 4) of this surface are slightly greater than those of bare dehydroxylated silica, and the calculated apparent layer thickness indicates that the polymer is adsorbed as a thin film with a thickness of 0.62 nm and a fractional surface coverage of 0.75. Figure 8b shows the height and phase images of 150 ppm Dextrin-WY adsorbed onto the surface of vapor methylated silica. The polymer adsorbs as a dense inhomogeneous film. The rms roughness and PTV distance (Table 4) increase compared with those of the bare vapor methylated surface, and the resulting apparent layer thickness of 0.85 nm indicates that the adsorbed polymer film is thicker than that on the dehydroxylated surface. The fractional surface coverage is 0.53. An image of 150 ppm Dextrin-WY adsorbed onto the 30 min vapor methylated silica captured in solution is shown in Figure 8c. The corresponding rms roughness, PTV distance, apparent layer thickness of 0.85 nm, and fractional coverage are, in general, slightly smaller than those for the ex situ case (Table 4). HP-Starch. The adsorption of the HP-Starch, a high molecular weight polymer, was also imaged using TMAFM both in air and in solution. HP-Starch adsorbs weakly onto clean hydrophilic silica as shown in Figure 3c. Figure 9a shows the image, taken in air, of HP-Starch adsorbed onto clean hydrophilic silica. It appears as a thin patchy film. The rms roughness and PTV values for this surface (Table 5) are slightly greater than those of the original bare silica surface. A comparison of the PTV distance for this surface with that of the bare surface shows that a thin adsorbed polymer layer is 0.2 nm thick. The image correlates well with the adsorption isotherm (Figure 3c), which shows that HP-Starch adsorbs onto hydrophilic silica only weakly, giving an area fraction of adsorbed polymer of 0.38. HP-Starch adsorbs onto dehydroxylated silica to give the greatest adsorbed amount compared with the other model substrates, as shown in the adsorption isotherms (Figure 3c). The polymer appears as distinct, small polymer
adsorbed layer morphology thin patchy film inconsistent film with distinct polymer patches thin inconsistent film thick inconsistent film constricted by underlying surface swollen film
patches with a diameter of 41 nm and a height of 3.1 nm, as shown in Figure 9b. The corresponding rms roughness and PTV distance for this surface are greater than those of the bare surface, and the apparent layer thickness is calculated to be 2.3 nm and the area fraction 0.77. The influence of surface roughness on the adsorption of HP-starch was investigated by comparing the images of HP-Starch, adsorbed onto two methylated substrates of approximately the same contact angle, but different roughnesses (Figure 9c,d). The first substrate was methylated with the vapor of TMCS, while the second was methylated in a solution of TMCS in cyclohexane. The roughness of the substrate methylated in solution is greater than that of the vapor methylated substrate, as shown in Table 2. Figure 9c shows HP-Starch adsorbed onto the vapor methylated silica. The rms roughness and the PTV value increase in the presence of adsorbed HPStarch. The image of HP-Starch adsorbed onto the solution methylated (Figure 9d) silica clearly shows a denser, less coherent filmlike structure compared with that of the vapor methylated case (Figure 9c). The observed surface structure is largely influenced by the underlying substrate roughness. In Figure 9d, the high surface roughness inhibits the formation of a continuous film. Further examination of this image reveals that the average length of the filmlike structures was 48.0-62.5 nm, which compares very well with the peak to peak distances of the bare substrate, i.e., 47-62.5 nm. The roughness and structure information lead us to propose that the polymer lies on the hydrophobic zones of the methylated surface. A comparison of the two images (Figure 9c,d) reveals that the area fraction of the adsorbed polymer is the same on both surfaces (shown in Table 5), even though the resulting surface roughness and layer thickness are distinctly different. An image of HP-Starch adsorbed onto vapor methylated silica was taken in solution, as shown in Figure 9e. While the image is slightly blurred due to the difficulty in imaging polymers in solution, the rms roughness, PTV distance, and apparent layer thickness are all increased compared with those of the ex situ images (Table 5), although the area fraction is slightly less. This strongly suggests that the HP-Starch is swollen in solution. Surface Wettability. The wettability of the model solid surfaces was assessed through contact angle measurements. The contact angle of clean hydrophilic silica in the presence of all polymers remained zero, as no bubblesurface adhesion occurred. Figure 10a shows the contact
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Figure 10. Contact angle of (a) dehydroxylated silica, (b) vapor methylated silica, and (c) solution methylated silica as a function of initial polymer concentration for all polymers.
Figure 9. Height and phase images for (a) HP-Starch (200 ppm) adsorbed onto clean hydrophilic silica, (b) HP-Starch (125 ppm) adsorbed onto dehydroxylated silica, (c) HP-Starch (150 ppm) adsorbed onto vapor methylated silica, (d) HP-Starch (150 ppm) adsorbed onto solution methylated silica, captured in air, and (e) HP-Starch (150 ppm) adsorbed onto vapor methylated silica captured in solution. Lateral scale: 1 × 1 µm. Vertical scale: 0-5 and 0-10 µm (c and d only).
angle of dehydroxylated silica as a function of initial polymer concentration for all polymers. In the presence of all polymers both the advancing and receding contact
angles of dehydroxylated silica decreased with increasing polymer concentration. Dextrin-WY and Polymer-H changed the advancing water contact angle of dehydroxylated silica by 20 ( 2° and 24 ( 2°, respectively, over the polymer concentration range investigated. Polymer-H adsorbs onto dehydroxylated silica with a greater adsorption density than Dextrin-WY at approximately the same area fraction but with a smaller apparent layer thickness. This slightly greater decrease in contact angle in the presence of Polymer-H compared with Dextrin-WY may be due to a different orientation of the hydrophilic hydroxyl groups of the adsorbed polymer. Polymer-H is a linear polymer, while Dextrin-WY is highly branched. The hydroxyl groups of Polymer-H, unlike those of DextrinWY, may be oriented in such a way that they form a hydrophilic layer that reduces the contact angle. The hydroxyl groups of Dextrin-WY may be hidden with the branched structure of the polymer and not as well oriented to cause a greater reduction in the contact angle. From both dynamic and static surface tension experiments, as reported elsewhere,52,53 we know that the three polymers
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used in this investigation do not adsorb at the liquidvapor interface. Thus, the effect on wettability can confidently be ascribed to the changes at the solid-liquid interface. In the presence of HP-Starch the contact angle of dehydroxylated silica decreases by 43° over the concentration range investigated. HP-Starch has a greater adsorption density on dehydroxylated silica compared with the other two polymers, although the area fraction is very nearly the same. The apparent layer thickness is considerably greater for HP-Starch. It appears that the thicker layer is the primary cause of the lack of bubble-surface adhesion, screening the bubble from the underlying substrate. The specific orientation of hydrophilic OH groups may also play a role. The variation of the contact angle for vapor and solution methylated silica as a function of polymer concentration is shown in parts b and c of Figure 10, respectively. In the presence of Polymer-H and Dextrin-WY the advancing water contact angle remains constant while the receding contact angle decreases slightly with polymer concentration. Both the advancing and receding water contact angles of the methylated silica surface decrease significantly with increasing concentration of HP-Starch. HP-Starch changes the advancing water contact angle of methylated silica by 27 ( 2° over a concentration range of 0-400 ppm. Certainly the adsorbed layer thickness and possibly the conformation explain why HP-Starch is the only polymer to influence the wettability of the methylated surface. For the two low molecular weight polymers the hydrophobicity of the underlying surface is dominant. Conclusion In this study, the adsorption behavior of three nonionic polymers, a polyacrylamide and two polysaccharides, on model silica surfaces was determined through adsorption isotherms. The experimental data showed acceptable fits to the Freundlich isotherm. Surface hydrophobicity influenced the maximum adsorbed amount for the low molecular weight polymers, whereas HP-Starch adsorbs onto both hydrophilic and hydrophobic surfaces. The surface morphology in the absence and presence of polymer was assessed through TMAFM experiments. The (52) Kaggwa, G. Ph.D. Thesis, University of South Australia, in preparation. (53) Beattie, D.; Huynh, L.; Kaggwa, G.; Bremmell, K.; Ralston, J. Int. J. Miner. Process., submitted for publication.
Kaggwa et al.
appearance and distribution of the adsorbed polymer layer as well as the apparent layer thickness were determined through imaging experiments. Both polymer structure and type influence the morphology of the adsorbed molecular layer. For example, Polymer-H, a linear polymer, adsorbed onto methylated silica as chainlike structures, whereas Dextrin-WY, which is highly branched, adsorbed as an inhomogeneous film. The polymer molecular weight influences the adsorbed amount, the appearance and distribution of the adsorbed layer, and the apparent layer thickness. For the case of the two polysaccharides, HP-Starch adsorbed to a greater extent on the model surfaces compared with Dextrin-WY. It also adsorbs onto the model surfaces to give a greater surface roughness and polymer layer thickness than Dextrin-WY. The underlying surface roughness also influences the morphology of the adsorbed polymer layer. Images of HPStarch adsorbed onto vapor and solution methylated surfaces of different roughnesses, but similar hydrophobicities, showed that increased surface roughness hindered the formation of a continuous film. The wettability of dehydroxylated silica increased strongly in the presence of HP-Starch, whereas Polymer-H and Dextrin-WY were far less effective. This behavior was attributed to the greater adsorbed layer thickness in the case of HP-Starch. For the methylated silica surface only HP-Starch altered the surface wettability. The greater adsorbed layer thickness compared with that of the lower molecular weight polymers is largely responsible for the change in wettability. For Polymer-H and Dextrin-WY, the substrate hydrophobicity is dominant. It is significant that the role of surface heterogeneity is so clearly identified in the study. The adherence to the Freundlich isotherm in the adsorption behavior is entirely consistent with the substrate heterogeneity and a strong influence upon adsorbed polymer disposition. This has strong ramifications for future substrate design and modeling investigations. Acknowledgment. Financial support from the Australian Research Council and AMIRA International is gratefully acknowledged. Valuable discussions with David Beattie, Daniel Fornasiero, and Jonas Addai-Mensah are warmly acknowledged. LA047352U
