Langmuir 2008, 24, 6121-6127
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Adsorption of Modified Dextrins on Talc: Effect of Surface Coverage and Hydration Water on Hydrophobicity Reduction Agnieszka Mierczynska-Vasilev, John Ralston, and David A. Beattie* Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, UniVersity of South Australia, Mawson Lakes SA 5095, Australia ReceiVed January 30, 2008. ReVised Manuscript ReceiVed March 13, 2008 The adsorption of three modified dextrins on the basal plane of talc has been studied using in situ tapping mode atomic force microscopy (TMAFM). The images have been used to determine the layer thickness and coverage of the adsorbed polymers. Adsorption isotherms of the polymers on talc particles were also determined using the depletion technique. Values of the adsorbed amount at equilibrium were compared with the volume of adsorbed material as determined using in situ TMAFM, revealing the presence of significant amounts of hydration water in the adsorbed layer structure. This deduction was confirmed by comparing in and ex situ TMAFM images of the adsorbed dextrins. The effect of layer thickness, coverage, and hydration water content on the contact angle of talc particles treated with polymer was investigated using the Washburn method and the equilibrium capillary pressure (ECP) method. Distinct correlations were observed between adsorbed layer properties and the measured contact angles, with the ECP measurements especially highlighting the effect of the adsorbed polymer layer hydration water. The implications for the performance of the modified dextrins in flotation are discussed.
Introduction The adsorption of polymers on hydrophobic mineral surfaces is a well-studied topic in the physical chemistry of flotation.1,2 Water-soluble polymers are used to reduce the hydrophobicity of minerals of little commercial value (so-called gangue minerals) to prevent their recovery in flotation along with the valuable minerals (e.g., metal sulfides). This use of polymers is termed depression. One of the most prevalent gangue minerals in the flotation of metal sulfides is talc. Talc is a layered-silicate mineral that produces sheet-like particles with a dominant hydrophobic surface.3 A range of studies have been performed to select the polymers that are most effective in reducing talc flotation recovery,4–8 with a great deal of emphasis placed on maximizing the polymer adsorbed amount on the talc. As a result, adsorbed amount has been the primary indicator of polymer performance, with many studies being performed to empirically determine not only the best polymer but also those that employ rational polymer chemistry design to maximize adsorbed amount and hence performance.9 The major drawback of this approach is that it does not take into account the polymer characteristics that affect the process of bubble-particle attachment that lies at the heart of the flotation recovery process.10 The adsorbed polymer layer characteristics, * To whom correspondence should be addressed. E-mail: David.Beattie@ unisa.edu.au. (1) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 131–46. (2) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 101–130. (3) Deer, W. A.; Howie, R. A.; Zussman, J. An introduction to the rockforming minerals, 2nd ed.; Wiley: New York, 1992. (4) Beattie, D. A.; Huynh, L.; Kaggwa, G. B. N.; Ralston, J. Miner. Eng. 2006, 19, 598–608. (5) Morris, G. E.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2002, 67, 211–227. (6) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Langmuir 1997, 13, 6260– 6266. (7) Shortridge, P. G.; Harris, P. J.; Bradshaw, D. J.; Koopal, L. K. Int. J. Miner. Process. 2000, 59, 215–224. (8) Steenberg, E.; Harris, P. J. S. Afr. J. Chem. 1984, 37, 85–90. (9) Chen, H. T.; Ravishankar, S. A.; Farinato, R. S. Int. J. Miner. Process. 2003, 72, 75–86. (10) Nguyen, A. V.; Schulze, H. J.; Ralston, J. Int. J. Miner. Process. 1997, 51, 183–195.
rather than the bulk chemistry of the polymer, will ultimately control whether or not a polymer can reduce the contact angle of the surface sufficiently to prevent successful attachment and recovery. We have attempted to address this deficiency, combining measurements of layer thickness and hydrophobicity from acoustophoresis, ex situ tapping mode atomic force microscopy (TMAFM) imaging of polymers on the basal plane of talc, and particle contact angle measurements.11–13 The adsorbed layer characteristics of the adsorbed layer have been correlated successfully with the flotation recovery of talc in single and mixed mineral systems.11,12 In addition, adsorbed layer characteristics from in and ex situ TMAFM imaging of polymers adsorbed on methylated silica have been correlated with their ability to reduce the contact angle of this surface.14 In the current work we have extended our adsorbed layer characterization to include in situ (i.e., surfaces exposed to solution) TMAFM to image adsorbed polymers on the basal plane of talc. This methodology was chosen to ensure the images reflected the true nature of the polymer layer when adsorbed on a mineral surface in an aqueous suspension. In addition, we have coupled the data with talc particle contact angle measurements, acquired not only using the Washburn method15 but also using the equilibrium capillary pressure (ECP) technique.16,17 This technique has been recently extended to allow for the measurement of advancing and receding contact angles.18,19 Given that the receding contact angle is the most appropriate angle from a flotation perspective (bubble-particle attachment involves the (11) Beattie, D. A.; Huynh, L.; Kaggwa, G. B.; Ralston, J. Int. J. Miner. Process. 2006, 78, 238–249. (12) Beattie, D. A.; Huynh, L.; Mierczynska-Vasilev, A.; Myllynen, M.; Flatt, J. Can. Metall. Q. 2007, 46, 349–358. (13) Kaggwa, G. B.; Huynh, L.; Ralston, J.; Bremmell, K. Langmuir 2006, 22, 3221–3227. (14) Kaggwa, G. B.; Froebe, S.; Huynh, L.; Ralston, J.; Bremmell, K. Langmuir 2005, 21, 4695–4704. (15) Crawford, R.; Koopal, L. K.; Ralston, J. Colloids Surf. 1987, 27, 57–64. (16) Diggins, D.; Ralston, J. Coal Prep. 1993, 13, 1–19. (17) Diggins, D.; Fokkink, L. G. J.; Ralston, J. Colloids Surf. 1990, 44, 299– 313. (18) Stevens, N. Contact Angle Measurements in Particulate Systems; University of South Australia: Adelaide, 2005. (19) Sedev, R.; Stevens, N.; Ralston, J. J. Colloid Interface Sci. (Submitted).
10.1021/la8003382 CCC: $40.75 2008 American Chemical Society Published on Web 05/17/2008
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Figure 1. General structure for Dextrin-based polymers. CM (-OCH2COOH) and HP(-OCH2CH(OH)CH3) Dextrins contain substitutions primarily at position C6.
movement of a receding liquid front over the mineral surface), it is necessary to obtain this value if one is to get the true influence of the adsorbed polymer on surface hydrophobicity. A selection of modified dextrins has been chosen for this study. Dextrin is one of the most commonly used polymers for the depression of talc20 (along with polyacrylamides,4,21 carboxymethyl celluloses,5,7,8 and guars7,20,22). The three polymers are of low molar mass (a common characteristic of polymer depressants) and have been functionalized with different groups. The TMAFM images and contact angle data have been supplemented by adsorption isotherm determinations. Correlations are drawn between the morphology (roughness, thickness, coverage) and water content of the adsorbed polymer layers with the ability of the polymers to reduce the hydrophobicity and flotation recovery of talc.
Experimental Section Materials. Three different polysaccharide-based polymers were used as received from Penford Australia. These polymers are the following: Dextrin TY (regular maize Dextrin), carboxymethyl (CM) Dextrin, and hydroxypropyl (HP) Dextrin. These polymers have a degree of substitution less than 10%, (i.e., D.S. < 0.3). The general chemical structure of these polymers is presented in Figure 1. Molecular weight averages were calculated using size exclusion chromatography,11 and are as follows: 5000, 34 000, and 64 000 g mol-1 for Dextrin TY, CM, and HP, respectively. Cyclohexane (Analytical grade, 99%) was obtained from Chem-Supply, South Australia, and distilled before use. High-purity Milli-Q water was supplied by an Elga UHQ water system and had conductivity less than 1 × 10-6 S cm-1 and surface tension of 72.8 mN m-1 20 °C. The solution pH was adjusted with small quantities of analytical grade HCl and KOH solutions. All aqueous solutions were at pH 9 with a 10-2 M KCl background concentration, unless otherwise stated. All other chemicals were of analytical grade and were used without further purification. Dextrin TY, HP Dextrin, and CM Dextrin stock solutions of 2000 mg L-1 and 5000 mg L-1 were prepared by weighing the appropriate mass of solid polymer in 10-2 M KCl Milli-Q water, then stirring overnight to ensure complete hydration. CM Dextrin stock solutions were prepared using a gelling procedure developed by Penford, Australia. A small amount of Milli-Q water was added to the appropriate mass of polymer solid to form a paste. A 2 wt % HNO3 solution was added slowly, and the resultant mixture was left to stand and gel. Once gelled, Milli-Q water was added slowly with mixing to dissolve the gel and form a transparent solution. The resulting mixtures were diluted afterward to the desired concentration with background water (10-2 M KCl solution) and adjusted to pH 9. All polymer solutions were optically clear. The hydrodynamic (20) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Langmuir 1997, 13, 6260– 6266. (21) Nagaraj, D. R.; Wang, S. S.; Lee, J. S.; Magliocco, L. G. Acrylamidecontaining polymer and a polysaccharide as depressants for silicate gangue in flotation of sulfide ores; 95-475160, 5533626, 19950607; 1996. (22) Jenkins, P.; Ralston, J. Colloids Surf., A 1998, 139, 27–40.
Mierczynska-VasileV et al. diameters of the polymers, measured by dynamic light scattering with the ZetasizerNano (Malvern Instruments, U.K.) were as follows: 3.6 ( 0.6 nm, 8.5 ( 0.4 nm, and 16.0 ( 0.7 nm for Dextrin TY, CM, and HP, respectively. Talc particles used for the Washburn and ECP contact angle experiments were purchased from Merck, Germany (>99% pure). X-ray photoelectron spectroscopy (XPS) analysis showed that the surface was free from impurities. The apparent particle size distribution (due to the plate-like structure of the talc particles) was 0.5-100 µm, with a D10 of 3.5 µm, a D50 of 15 µm, and a D90 of 52 µm, as determined using a Mastersizer X (Malvern Instruments, U.K.). The BET surface area was measured at 2.9 m2 g-1. The talc samples used for atomic force microscopy imaging experiments were rock minerals from Delaware (U.S.A.), provided by the Mineralogy Department of the South Australian Museum. Freshly cleaved talc for imaging was prepared by carefully adhering a small piece of sticking tape on a flat section of the mineral and gently peeling the tape to reveal a freshly cleaved mineral basal plane.13 Adsorption Isotherms. Adsorption studies were performed using the batch depletion method. A 5 wt % solid sample suspension at pH 9 was prepared, and a known volume of this suspension was added to individual capped vials. The required volume of 10-2 M KCl solution at pH 9 and stock polymer solution also at pH 9 were 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 UV-vis complexation method.23 It was assumed that the amount of polymer depleted from solution was adsorbed onto the solid surface. The adsorbed amount (Γ) was then calculated using the following equation:
Γ)
1 (c - ci)V mAS f
(1)
where m is mass of the solid substrate, As is the specific surface area of the solid substrate, ci and cf are the polymer concentrations before and after adsorption, and V is the volume of the suspension. The Langmuir model is a simple model that can be used to describe adsorption from solution onto solid surfaces:24
Ceq Ceq 1 ) m + m Γads Γ b Γ ads
(2)
ads
where Ceq is the equilibrium polymer concentration (mol L-1), Γads is the adsorbed amount (mol m-2), Γmads is the plateau adsorbed amount (mol m-2), and b is the Langmuir affinity constant which is equal to K/55.5, where K is the adsorption equilibrium constant and 55.5 mol L-1 is the concentration of the solvent (in this case water). In the case of polymer adsorption, many of the assumptions of the Langmuir model are invalid (adsorption reversibility, equal size of solvent and solute molecules, etc.). As such, no attempt was made to use the calculated parameters to determine thermodynamic m constants. However, relative comparisons of b and Γads values for a selection of polymers remain a useful means of gaining insight into the polymer adsorption process. Langmuir fitting was performed using the nonlinear curve fitting module of OriginPro 7.5. Contact Angle Measurements. Advancing water contact angle measurements were made on talc particles using the Washburn method.15,25 In our investigations, conditioned mineral samples were prepared for the contact angle investigations in exactly the same way as for the polymer adsorption studies. The samples were then filtered, dried overnight, and then analyzed. Measurements were made at room temperature (generally 22 °C) using the DCAT 21 Wilhelmy Balance. The value of the contact angle was calculated using the following equation: (23) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28, 350–361. (24) Hunter, R. Foundations of Colloid Science; OUP: Oxford, 2001. (25) Washburn, E. W. Phys. ReV. 1921, 17, 273–283.
Adsorption of Modified Dextrins on Talc
cos θp )
(h2/t)nγlv,wηn (h2/t)wγlv,nηw
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(3)
where (h2/t)n and (h2/t)w are the wetting rates for nonwetting (10-2 M KCl solution) and wetting (cyclohexane) liquids with surface tensions γlv,n and γlv,w and viscosities ηn and ηw, respectively. The ECP technique is based on the measurement of the equilibrium capillary pressure in a packed bed of particles.16–19 Both the advancing and receding contact angles can be obtained using this methodology. For the advancing contact angle with respect to water, θw, the following equation is used:16,17
cos θw )
∆Pw γc × ∆Pc γw
(4)
∆P is the Laplace pressure for equilibrium measurement, γ is the liquid surface tension, and the “c” and “w” denote cyclohexane (wetting liquid) and water (nonwetting liquid). The determination of the receding contact angle requires that pressure be applied to the system using a micrometer syringe until the point at which liquid begins to recede from the bed. The following equation is then used to determine the receding contact angle:18,19 /w ∆Pw γc 1 - φS - φL cos θ ) × × ∆Pc γw 1 - φS - φL/c w
Figure 2. Adsorption isotherms for the three polymers on talc (2, HP Dextrin; 9, Dextrin TY; b, CM Dextrin). The lines represent Langmuir plots, based on the determined Langmuir constants in Table 1). Table 1. Langmuir Adsorption Parameters for Dextrin Adsorption on Talc m Mw [g mol-1] Γads [mg m-2]
(5)
where φL/ the retained liquid fraction and φS is the solid fraction of the particles in the particle bed. The retained liquid fraction is determined gravimetrically. Experiments were performed on bare and polymer-treated talc particles. Tapping Mode Atomic Force Microscopy Imaging. TMAFM imaging was performed in situ and ex situ, using a Multimode Nanoscope III (Digital Instruments, Santa Barbara, CA). A piezoelectric tube scanner E with the 10 × 10 µm scan size in the XY direction and a 2.5 µm vertical range was used. Tapping mode cantilevers (V-shape cantilever configuration) for imaging in liquids, with normal tip radii of curvature between 20 and 60 nm, cantilever lengths 100 and 200 µm, and spring constant 0.32 N/m, were used after cleaning (immersion in ethanol, rinsing with high quality Milli-Q water, and drying under high purity nitrogen). Tapping mode cantilevers for imaging in air (NT-MDT, Moscow, Russia), with a tip radius of curvature less than 10 nm, tip heights in the 10-20 µm range, and spring constants in the range 5.5 - 22.5 N/m, were also used after cleaning. The cantilevers were tuned within the specified resonant frequency range, typically around 270 Hz. All experiments were conducted in a class-100 clean room at 22 °C. For in situ experiments, freshly cleaved talc surfaces were conditioned in the AFM liquid cell for 30 min in a polymer solution of known concentration. After the designated conditioning time the polymer solution was exchanged for the background electrolyte solution. For the ex situ experiments, freshly cleaved talc surfaces were conditioned for 30 min in a polymer solution of known concentration. During immersion in polymer solution, the samples were orientated vertically to avoid any polymer deposition due to settling. After immersion in polymer solution, samples were rinsed with high quality Milli-Q water in order to remove excess polymer then dried in air for 30 min and imaged. The root-mean-squared (rms) roughness and the peak-to valley (PTV) distance for the imaged surfaces were determined. The apparent layer thickness (∆PTV) was calculated as the difference between the PTV distances for the bare talc surface and the polymer covered surfaces.13,14 The sizes of the images for the rms roughness calculations and PTV determinations were 3 × 3 µm. For each polymer, rms roughness and PTV were calculated for three separate images and average values reported. The surface coverage of the imaged surfaces was assessed using the Bearing function available in the AFM Nanoscope software.13 Bearing analysis determines what percentage of the image surface lies above an arbitrarily chosen height, in our case above the height of the bare talc mineral surface.
Dextrin TY CM Dextrin HP Dextrin
5000 34000 64000
b [L mol-1]
R2
0.62 ( 0.02 (3.91 ( 0.01) × 105 0.99 0.80 ( 0.02 (3.50 ( 0.01) × 106 0.99 1.05 ( 0.05 (3.00 ( 0.01) × 106 0.99
Bearing analysis was performed on three separate 3 × 3 µm images for each polymer. The measured surface coverage represents the area fraction of adsorbed polymer on the talc surface. The polymer concentration used in the imaging experiments was 100 ppm.
Results Adsorption Isotherms. The measured adsorption isotherms for the three polymers on talc are shown in Figure 2, with adsorbed amount plotted as a function of equilibrium polymer concentration. In all three cases, the polymers adsorb on talc and the isotherms exhibit high affinity behavior. The isotherm data were fitted using the Langmuir model. The values of the maximum m adsorbed amount (Γads ) and Langmuir affinity constant (b) are given in Table 1, together with the correlation coefficient for the Langmuir fits. Upon comparing the molecular weights and Langmuir parameters, it can be seen that the plateau adsorbed amount increases with the molecular weight of the polymers, with Dextrin TY having the smallest adsorbed amount and molar mass, and HP Dextrin having the largest adsorbed amount and molar mass.26 The Langmuir affinity constants for the polymers did not follow this general trend, with Dextrin TY exhibiting the smallest affinity constant and CM Dextrin the highest affinity constant for the talc surface. This disconnect between the affinity constant and the variation in molar mass across the three dextrins indicates that the type of functional group substitution determines to some extent the binding interaction between the polymers and the talc surface. Specifically, it would appear that the substitution of the carboxymethyl group has had a positive effect on the binding of the polymer to the talc. This is consistent with in situ ATR FTIR analysis of carboxymethyl substituted cellulose polymers on talc, where the carboxyl group asymmetric and symmetric stretching peaks indicate a chemical interaction with the talc surface.27–29 (26) Fleer, M. A.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (27) Cuba-Chiem, L. Probing polymer adsorption at the solid-liquid interface with particle film ATR-FTIR spectroscopy; University of South Australia: Adelaide, 2007.
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Figure 3. Images (1 × 1 µm) of bare, freshly cleaved talc surface.
TMAFM Imaging. The tapping mode atomic force microscopy (TMAFM) image of a freshly cleaved talc surface is shown in Figure 3. The untreated talc surface is characterized by a rms roughness of 0.045 nm and peak-to-valley (PTV) distance of 0.20 nm. The extremely low rms roughness and PTV distance indicate that the surface is very smooth. Similar data have been obtained previously.13 The cleaved surface is expected to be very hydrophobic, with sessile drop contact angle measurements of the same sample giving a value of 90 ( 3°.13 Figure 4 shows the in situ TMAFM height and phase images of 100 ppm Dextrin TY, CM, and HP adsorbed onto a talc surface. Phase images are important in this study as they allow one to determine which surface features are soft and which are hard, enabling features due to the adsorbate layer to be identified. In the case of Dextrin TY and HP Dextrin, the images consist of hemispherical features with some degree of variation in the size and distribution of the features between the two polymers. The phase images enable us to determine that the areas between the hemispherical features are likely to be bare talc. The correlation between the features in the image with changes in the phase response of the cantilever is clearly seen in the 2D height and phase cross sections included in Figure 4, indicating that HP Dextrin and Dextrin TY form islands of adsorbed polymer separated by areas of bare talc. Also supporting this conclusion is the determination of the rms roughness of a section of the exposed bare talc in the image for Dextrin TY, giving a value (0.06 nm) almost identical to that of the freshly cleaved talc surface shown in Figure 4 (0.05 nm). In contrast to HP Dextrin and Dextrin TY, CM Dextrin forms an almost complete layer, with some small pits. It is difficult to determine whether these pits extend to the bare talc surface. The morphology of the adsorbed layers can be described quantitatively in terms of area fraction of coverage (determined using the bearing function) and layer thickness (∆PTV). These data are given in Table 2. The individual PTV distances are averages taken over a 2-D cross section of the AFM image. Also given in Table 2 are the rms roughness values for the images. The AFM images of the adsorbed layers allow one to make an estimate of the polymer adsorbed amount. The average height multiplied by the area coverage allows one to determine the volume of adsorbed material over the area of the AFM image. This volume per unit area can then be compared with the adsorbed amount of each polymer on talc determined from the batch depletion method. The data comparing adsorbed amounts per unit area, expressed as adsorbed volume per unit area for both AFM and batch depletion results, are given in Table 3. The data indicate quite clearly that the adsorbed amount of material per
unit area in the AFM images is higher than that determined from the isotherms. The difference is very large in the case of CM Dextrin. The most likely explanation for the mismatch is that the features observed in the TMAFM images includes hydration water attached to the polymer chains, making the imaged volume much larger than the volume of actual adsorbed polymer material. The observation of hydration water in the adsorption of macromolecules is commonplace in QCM and SPR studies of protein and polymer adsorption30–32 (QCM measures mass of adsorbed material which includes hydration water; SPR senses refractive index differences and thus does not see hydration water). Water percentages determined from these studies are often higher than 80%, and in some cases adsorbed biological macromolecule layers can have water contents above 95%. In the latter case, the carbohydrate content of the adsorbing biological macromolecules has been implicated in the ability of the layer to retain/couple such high amounts of water.30 Confirmation of the extent of hydration water in the adsorbed polymer layers can be obtained by performing ex situ TMAFM imaging (after immersion and drying) of the talc surface. The height and phase images for CM Dextrin acquired in this manner can be seen in Figure 5 (reproduced in part, with permission, from ref 12). Clearly, in the absence of the adsorption medium, and allowing time for evaporation, the layer imaged by ex situ AFM is much less extensive in its coverage and thickness. Taking into account the volume of the adsorbed polymer on the talc surface, as well as density of dextrin (1450 kg/m3), the adsorbed CM Dextrin amount was calculated to be 0.74 mg m-2, which is very close to the calculated amount from adsorption isotherm experiments of 0.8 mg m-2. Similar reductions in adsorbed volume are seen for ex situ images of both HP Dextrin and Dextrin TY (not shown). Knowing the actual adsorbed mass from batch depletion experiments makes it possible to estimate the percentage of hydration water in the adsorbed layers of the different polymers. CM Dextrin has the highest percentage, followed by Dextrin TY. HP Dextrin has the lowest amount of hydration water, which may initially seem counter-intuitive given the larger adsorbed amount of this polymer relative to the other two dextrins. The differences in hydration water content may be more related to the functional groups attached to the dextrins. Carboxymethyl groups will be fully deprotonated at pH 9 and thus very polar and likely to attract a significant amount of hydration water. The hydroxypropyl substitution is much less polar and contains a hydrophobic moiety that may alter the ability of the polymer to hydrogen bond with surrounding water.
(28) Cuba-Chiem, L. T.; Huynh, L.; Ralston, J.; Beattie, D. A. Langmuir, in press. (29) Cuba-Chiem, L. T.; Huynh, L.; Ralston, J.; Beattie, D. A. Miner. Eng., in press.
(30) Malmstroem, J.; Agheli, H.; Kingshott, P.; Sutherland, D. S. Langmuir 2007, 23, 9760–9768. (31) Hedin, J.; Loefroth, J.-E.; Nyden, M. Langmuir 2007, 23, 6148–6155. (32) Larsson, C.; Rodahl, M.; Hoeoek, F. Anal. Chem. 2003, 75, 5080–5087.
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Figure 4. AFM height (left) and phase (right) images (3 × 3 µm) of adsorbed polymers onto talc taken in situ: (a) Dextrin TY, (b) CM Dextrin, and (c) HP Dextrin (100 ppm polymer concentration). Adjacent to each set of images are 2D cross section profiles of height (upper) and phase (lower), taken along the black lines in each image. The boxed section in the height image of (a) has been analyzed for rms roughness, giving a value of 0.06 nm. Table 2. Root Mean Square (rms) Roughness, Peak to Valley Distance (PTV), Apparent Layer Thickness (∆PTV), and Area Fraction of Polymer Coveragea rms [nm] ((0.05)
PTV [nm] ((0.05)
∆PTV [nm] ((0.05)
area fraction
0.05 9.02 2.33 2.77
0.20 11.00 13.00 9.53
10.80 12.80 9.33
16 93 10.5
no polymer Dextrin TY CM Dextrin HP Dextrin a
Adsorbed Dextrin: 100 ppm on talc, images taken in situ.
Table 3. Adsorbed Amount (Γ) and Adsorbed Volume (V) for Three Polymers and Water on Talca
Dextrin TY CM Dextrin HP Dextrin
Γ [mg m-2] (adsorption isotherms)
V × 10-9 [m-3] (adsorption isotherms)
V × 10-9 [m-3] (AFM)
V × 10-9 [m-3] volume of water (AFM)
% of water in layer
0.62 0.80 1.05
0.43 0.55 0.73
1.73 11.90 0.98
1.30 11.35 0.25
75 95 26
a Adsorbed volume per unit area data from the batch depletion experiments are calculated using the adsorbed mass per unit area (Table 1) and the density of dextrin (1450 kg/m3).
Contact Angle Measurements. Contact angle measurements have been performed using two methods: the Washburn method,
and the equilibrium capillary pressure method. The Washburn technique requires that the samples be dried prior to analysis,
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Figure 5. AFM height (left) and phase (right) images (1 × 1 µm) of CM Dextrin on talc taken ex situ. Reproduced in part, with permission, from ref 12. Copyright 2007 Elsevier. Table 4. Washburn and ECP Contact Angle Results for Treated and Untreated Talc Powder ECP contact angle [deg] ((3)
no polymer TY CM HP
Washburn contact angle [deg] ((1)
advancing
receding
80 70 59 74
80 71 25 67
60 58 13 57
and the contact angles measured are advancing water contact angles. ECP measures the advancing and receding contact angles, and it does so when the packed bed is wet. The difference between advancing and receding contact angles is well-acknowledged in the field of wetting33–36 but not so in the area of polymer depressant studies. Contact angle data for talc conditioned the absence and presence of 100 ppm polymer are given in Table 4. The advancing contact angle of untreated talc determined by the Washburn method is 80°, in agreement with earlier studies.11 The data for Dextrin TY (70°) and HP Dextrin (74°) are similar, especially when one considers the accuracy of the measurement ((1°). Both polymers are able to reduce the advancing contact angle of talc. The advancing Washburn value for CM Dextrin is markedly lower than the other two polymers, at 59°. The observed trend (CM Dextrin > Dextrin TY > HP Dextrin) does not correlate with the adsorbed amounts of the polymers, as determined from the batch depletion studies (all polymers are likely to be approaching plateau coverage for the initial polymer concentration used). The adsorbed amount data alone would indicate that HP Dextrin would have the greatest effect, followed by CM Dextrin, then Dextrin TY. The contact angle values must also be influenced by chemical differences between the three polymers. Although the contact angle data does not correlate with adsorbed amount from the depletion isotherms, the observed contact angle trend does correlate with the surface coverage of the three polymers determined by TMAFM. The ECP data (also given in Table 4) indicate that untreated talc particles, conditioned simply with electrolyte solution, give values of 80° and 60° for the advancing and receding contact angles, respectively. This represents a fairly large contact angle hysteresis, which would be expected for particles with chemically heterogeneous surfaces (90% hydrophobic surface area from basal (33) Ralston, J.; Newcombe, G. Static and Dynamic Contact Angles. In Colloid Chemistry in Mineral Processing; Laskowski, J. S., Ralston, J., Eds.; Elsevier Science Publishing Company: Amsterdam, 1992; pp 173-200. (34) Petrov, J. G.; Ralston, J.; Schneemilch, M.; Hayes, R. A. Langmuir 2003, 19, 2795–2801. (35) Sedev, R.; Fabretto, M.; Ralston, J. J. Adhes. 2004, 80, 497–520. (36) Priest, C.; Sedev, R.; Ralston, J. Phys. ReV. Lett. 2007, 99.
plane and 10% hydrophilic surface area). The data for the three polymers using the ECP technique are also given in Table 4. For the polymer-treated talc, the advancing contact angle values obtained for Dextrin TY (71°) and HP Dextrin (67°) are the same within the accuracy of the measurement and are very similar to the Washburn values. The receding contact angles for talc treated with these two polymers (Dextrin TY, 58°; HP Dextrin, 57°) are also the same within the accuracy of the measurement, and they represent only a very small change to the receding contact angle value for untreated talc. The ECP data for CM Dextrin (advancing, 25°; receding, 13°) represents an amplification of the differences seen between this polymer and Dextrin TY and HP Dextrin when measured using the Washburn method. As with the Washburn data, the ECP data overall correlates more with the surface coverage determined by the TMAFM images than with the adsorbed amounts determined from batch depletion studies.
Discussion The data obtained from the AFM images and the contact angle measurements allow us to gain insights into the way in which polymers can affect the wettability of hydrophobic minerals. The use of polymer adsorbed amount alone is a blunt instrument for predicting whether a polymer will be better or worse at reducing the contact angle of talc. It will be effective for some variations in polymer structure, such as comparing a range of polymers with similar chemistry but varying molecular weight. However, when there is a significant chemical difference between the polymers, adsorbed amount does not translate through to the effect of the polymers on the contact angle. The AFM images of the adsorbed polymers illustrate that there are far greater differences between the adsorbed morphology of the polymer layer (coverage, roughness, thickness) than there is between the adsorbed mass of the different polymers. The contact angle data indicate that these differences translate directly to the effect of the polymer on the surface wettability. Dextrin TY and HP Dextrin have similar area fraction coverages and layer thicknesses, albeit with a different distribution of polymer features across the surface. The contact angle reduction of talc treated with these polymers is similar. CM Dextrin, which has by far the highest area fraction coverage, has a much larger effect on the contact angle. The reason behind the larger coverage for a polymer which does not have a larger adsorbed mass is the presence of the hydration water. The charged functional group (at pH 9, the carboxyl group on the substitution will be deprotonated) clearly has a major impact on the ability of the polymer to retain a large amount of hydration water, in spite of the polymer being adsorbed on a very hydrophobic surface. The hydroxypropyl moiety on HP Dextrin is unlikely to increase the degree of hydration over and above that of the straightforward
Adsorption of Modified Dextrins on Talc
Dextrin TY, as it contains an even mix of additional hydrophobic (-CH3) and hydrophilic (-OH) sections. The observation of large amounts of hydration water and large contact angle reductions not only highlights the need to measure contact angles using the ECP method but indicates that there are more ways in which a polymer depressant may prevent bubbleparticle attachment than simply reducing the underlying mineral hydrophobicity. Pugh1 was one of the first to address the possibility of an adsorbed depressant layer inducing hydrationrepulsive forces between minerals and air-bubble. In situ TMAFM, combined with more traditional methods of determining adsorbed amount, gives us an experimental tool to determine the extent of hydration water in adsorbed depressant layers. Measuring this quantity will allow us to investigate whether a tightly bound hydration layer on a polymer-coated hydrophobic mineral particle will be effective in preventing thin film rupture when a bubble encounters that particle in a flotation cell.10
Conclusions The adsorption of three modified dextrins onto talc has been studied with adsorption isotherms, in and ex situ tapping mode atomic force microscopy, and particle contact angle measurements using the Washburn and equilibrium capillary pressure techniques. AFM images have been used to characterize the adsorbed layer
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properties in terms of coverage and layer thickness. A mismatch between the volume of adsorbed material on the basal plane of talc deduced from the AFM images and the amount of material adsorbed as determined from the depletion isotherms on talc particles has highlighted the important role played by hydration water in the morphology of the adsorbed polymer layer. The coverage and degree of water content of the adsorbed polymer layers is linked with the ability of the polymers to reduce the contact angle of talc particles. The ECP contact angle measurements especially highlight the need to measure the contact angle of solid particles (talc in this case) with adsorbed polymers in a hydrated state. The presence of significant amounts of hydration water suggests that polymer depressants may affect bubbleparticle attachment in more ways than simply the reduction of particle hydrophobicity. Acknowledgment. Financial support for this study was received from the Australian Research Council and AMIRA International sponsors of Project P498B (Polymers at Mineral Interfaces), which include Penford Australia, Cytec, CP Kelco, Xstrata, Rio Tinto, and Anglo Platinum. Russell Schumann of Levay and Co. Environmental Services is acknowledged for his for assistance with SEC measurements. LA8003382
