pubs.acs.org/Langmuir © 2009 American Chemical Society
Adsorption of Dextrin on Hydrophobic Minerals Audrey Beaussart, Agnieszka Mierczynska-Vasilev, and David A. Beattie* Ian Wark Research Institute, ARC Special Research Centre for Particles and Material Interfaces, University of South Australia, Mawson Lakes SA 5095, Australia Received March 27, 2009. Revised Manuscript Received May 21, 2009 The adsorption of dextrin on talc, molybdenite, and graphite (three naturally hydrophobic minerals) has been compared. Adsorption isotherms and in situ tapping mode atomic force microscope (TMAFM) imaging have enabled polymer adsorbed amount and morphology of the adsorbed layer (area coverage and polymer domain size) to be determined and also the amount of hydration water in the structure of the adsorbed layer. The effect of the polymer on the mineral contact angles, measured by the captive bubble method on cleaved mineral surfaces, indicates clear correlations between the hydrophobicity reduction of the minerals, the adsorbed amount, and the surface coverage of the adsorbed polymer. Predictions of the flotation recovery of the treated mineral phases have been confirmed by performing batch flotation experiments. The influence of the polymer surface coverage on flotation recovery has highlighted the importance of this key parameter in the predictions of depressant efficiency. The roles of the initial hydrophobicity and the surface structure of the mineral basal plane in determining adsorption parameters and flotation response of the polymer-treated minerals are also discussed.
Introduction In the separation of minerals by flotation, it is well established that the process takes advantage of the differences in wettability of the solid phases.1 However, in complex mineral systems, such as metal sulfide ores, similarities in the natural hydrophobicity of the valuable and unwanted (gangue) minerals do not allow a selective separation without recourse to addition of reagents into the mineral suspension. Water-soluble polymers, referred to as depressants, are added in the solution to alter the recovery of the naturally hydrophobic unwanted minerals.2,3 Polysaccharide polymers, such as starch or guar, have been shown to be particularly effective as depressants.2,4,5 The functional groups of these polymers are thought to impart strong hydrophilic character to the mineral phases on which they adsorb, thereby reducing the probability of successful bubble-particle attachment. One such polysaccharide polymer that has seen extensive use in flotation is dextrin. Dextrin is a low molecular weight derivative from starch, produced through acid hydrolysis. The basic composition unit for both dextrin and starch is R-D glucopyranose.6 Dextrin has been studied as a depressant of naturally hydrophobic minerals such as talc,7,8 carbonaceous gangue minerals,2 and coal.9 Hydrophobic interactions have been proposed as the main binding mechanism occurring between dextrin and naturally *Corresponding author. E-mail:
[email protected]. (1) Wills, B. A.; Napier-Munn, T. Chapter 12 - Froth Flotation. In Wills’ mineral processing technology: an introduction to the practical aspects of ore treatment and mineral recovery, 7th ed.; Elsevier: London, 2006. (2) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 101–130. (3) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 131–46. (4) Somasundaran, P.; Moudgil, B. M. Reagents in Mineral Technology; M. Dekker: New York, 1988; Vol. v, p 27. (5) Jenkins, P.; Ralston, J. Colloid Surf. A-Physicochem. Eng. Asp. 1998, 139, 27–40. (6) Coultate, T. P. Food: The Chemistry of its Components, 3rd ed.; Royal Society of Chemistry: Cambridge, 1996. (7) Beattie, D. A.; Huynh, L.; Kaggwa, G. B.; Ralston, J. Miner. Eng. 2006, 19, 598–608. (8) Beattie, D. A.; Huynh, L.; Kaggwa, G. B.; Ralston, J. Int. J. Miner. Process. 2006, 78, 238–249. (9) Miller, J. D.; Laskowski, J. S.; Chang, S. S. Colloids Surf. 1983, 8, 137–151.
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hydrophobic mineral phases, e.g., talc,10-12 molybdenite,13 graphite,12,14 and sulfur.15 Although the adsorption of dextrin on these minerals individually has been reported previously,8,13,15,16 no experimental comparison of the adsorption and depressant action of the same dextrin across this series of similarly hydrophobic minerals has been reported to date.17 This distinction is critical as studies on different dextrins with varying molecular weight distributions do not enable direct comparisons to be made between data sets. In this paper, a detailed analysis of the adsorption of a specific dextrin depressant, Dextrin TYM, on talc, molybdenite, and graphite is reported to answer the fundamental question: do the chemical nature and structure of naturally hydrophobic mineral surfaces affect the adsorption and effectiveness of a polymeric depressant? The three minerals were chosen as they exhibit similarities in terms of particle shape and hydrophobicity. Due to the mineral crystal structure, the three pure minerals form platelike particles upon breakage, with two types of surfaces: nonpolar hydrophobic basal plane (particle face) and negatively charged hydrophilic edges. The hydrophobicity of the mineral particles is conferred by the basal plane as their surface area dominates the surface area of the edges. In addition, bulk (i.e., massive crystal) samples of all three minerals have a natural cleavage plane that makes it facile to produce near atomically flat substrates for adsorption studies. (10) Cuba-Chiem, L. Probing Polymer Adsorption At the Solid-liquid Interface with Particle Film ATR-FTIR Spectroscopy; University of South Australia: Mawson Lakes, 2007. (11) Du, H.; Miller, J. D. Int. J. Miner. Process. 2007, 84, 172–184. (12) Du, H.; Miller, J. D. Langmuir 2007, 23, 11587–11596. (13) Wie, J. M.; Fuerstenau, D. W. Int. J. Miner. Process. 1974, 1, 17–35. (14) Raju, G. B.; Holmgren, A.; Forsling, W. J. Colloid Interface Sci. 1997, 193, 215–222. (15) Brossard, K. S.; Du, H.; Miller, J. D. J. Colloid Interface Sci. 2008, 317, 18–25. (16) Subramanian, S.; Laskowski, J. S. Langmuir 1993, 9, 1330–1333. (17) Although refs 13 and 16 (on dextrin adsorption on molybdenite and graphite, respectively) use the same source of dextrin (JT Baker), no molecular weight characterization data are given in either work. In addition, given that the papers were published by different sets of authors almost 20 years apart, it is highly unlikely that the same batch of polymer (and thus molecular weight range) was used.
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The study follows from previous research on talc from this institute, using tapping mode AFM imaging of polymer adsorption, adsorption isotherms, and contact angle measurements to gain an insight into the role of adsorbed polymer morphology on hydrophobicity reduction.18,19 In addition to broadening the scope to include more than one hydrophobic mineral, the current study makes use of captive bubble contact angle measurements on cleaved mineral surfaces to directly probe the changes to the basal plane hydrophobicity while the surface is immersed in polymer solution. These measurements allow for more direct comparisons between the in situ AFM images and the effect of the polymer on the mineral hydrophobicity. Crucially, the predictions of the effectiveness of dextrin on the flotation of the three minerals are also tested using batch flotation studies.
Experimental Section Materials. Molybdenite particles used for adsorption isotherms and flotation were purchased from Fluka (purum powder). The Brunauer-Emmett-Tellet (BET) surface area was measured at 3.1 m2 3 g-1. The apparent particle size distribution, determined using a Malvern Instrument Mastersizer was 0.4-30.2 μm, with a D10 of 3.0 μm, a D50 of 7.4 μm, and a D90 of 15.5 μm. AFM images and contact angle measurements were taken on rock mineral samples from the Northern Territory, provided by the Mineralogy Department of the South Australia Museum. Freshly cleaved molybdenite surfaces were obtained by peeling the top layer of the mineral sample using a piece of sticking tape, revealing a flat freshly cleaved basal plane.18 Talc particles were purchased from Merck, Germany (>99 % pure). The BET surface area was measured at 2.9 m2 3 g-1. The particle size distribution was 0.5-100 μm, with a D10 of 3.5 μm, a D50 of 15 μm, and a D90 of 52 μm. A massive talc crystal from Delaware (USA) was obtained from the Mineralogy Department of the South Australian Museum, and cleaved flat surfaces produced for experiments in the same manner as for molybdenite. Graphite particles were obtained from Aldrich. The BET surface area was measured at 3.0 m2 3 g-1. The particle size distribution was 0.7-26.3 μm, with a D10 of 4.4 μm, a D50 of 9.7 μm, and a D90 of 19.3 μm. Graphite flat mineral samples were obtained by cleaving the top layer of a highly ordered pyrolytic graphite (HOPG) sample, purchased from SPI, Holgate Scientific (grade 1). All flat mineral surfaces and particles were free from significant impurities, as shown by X-ray photoelectron spectroscopy (XPS) analysis (see Supporting Information). Images of mineral flat surfaces and particles are shown in Figure 1. SEM micrographs were obtained using a high-resolution CamScan CS44FE with field emission electron source, equipped with a Robinson backscattered electron detector and an energy dispersive X-ray spectrometer. High-purity Milli-Q water was supplied by an Elga UHQ water system, with a conductivity less than 1 10-6 S 3 cm-1 and a surface tension of 72.8 mN 3 m-1 at 20 °C. The solution pH was adjusted by addition of small quantities of analytical grade HCl and KOH solutions. All experiments were conducted in 10-2 M KCl background electrolyte at pH 9, unless otherwise stated. A dextrin polymer supplied by Penford Australia was used in this study: Dextrin TYM (Regular Maize dextrin). The general structure of dextrin is depicted in Figure 2. The molecular weight average, determined by size exclusion chromatography (SEC),8 is 5600 g 3 mol-1 with a polydispersity index (Mw/Mn) of 5.6 (see Supporting Information). A solid polymer sample, as received from Penford, (18) Kaggwa, G. B.; Hyunh, L.; Ralston, J.; Bremmell, K. Langmuir 2006, 22, 3221–3227. (19) Mierczynska-Vasilev, A.; Ralston, J.; Beattie, D. A. Langmuir 2008, 24, 6121–6127.
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Figure 1. Digital pictures of a water droplet sitting on a flat mineral surface (left) and SEM images of mineral particles (right) for (a) talc, (b) molybdenite, and (c) graphite.
Figure 2. General structure for Dextrin based polymers (black circles, oxygen; gray circles, carbon; white circles, hydrogen). was used to prepare a stock solution of 2000 mg 3 L-1. The appropriate mass of solid polymer was dissolved in background electrolyte solution and stirred overnight to ensure complete hydration of the dextrin molecules. The hydrodynamic diameter and Zeta potential of Dextrin TYM, measured at pH 9 in 110-2 M KCl by dynamic light scattering with the ZetasizerNano (Malvern Instruments, U.K.), were found to be 4.0 ( 0.4 nm and -7.7 ( 0.2 mV, respectively. The measurement of a low negative Zeta potential for dextrin is not without precedent20 and most likely arises from the acid hydrolysis process (producing a small number of broken pyranose rings) or from a low degree of deprotonation of the hydroxyl groups on the glucopyranose rings. Irrespective of the exact reason for the negative charge, it is much smaller than that measured for polysaccharides considered to be true polyelectrolytes, such as carboxymethyl cellulose (an Aldrich CMC of Mw = 90 000 and DS = 0.7 produced a Zeta potential of approximately -40 mV at pH 9 in 110-2 M KCl). Adsorption Isotherms. Adsorption studies were performed using the batch depletion method. A 3.5 wt % solid sample suspension at pH 9 was prepared in 10-2 M KCl background solution, and the appropriate volume of this suspension was placed in individual vials. The required volumes of background solution and stock polymer solution at pH 9 were added to each vial to obtain samples of different initial concentrations. The vials (20) Goncalves, C.; Gama, F. M. Eur. Polym. J. 2008, 44, 3529–3534.
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Figure 3. Determination of the polymer surface coverage by image processing using Image J. (a) AFM height image, (b) AFM phase image, and (c) corresponding 8-bit black and white image after threshold adjustment by Image J.
mode Nanoscope III (Digital Instruments, Santa Barbara, California). Imaging was performed using a tapping mode fluid cell and tapping mode cantilevers (V-shaped cantilever configuration, Digital Instrument, Santa Barbara) with a typical spring constant of 0.2 N/m and a resonant frequency between 5 and 10 Hz. Fluid cell and cantilever were cleaned in ethanol, rinsed with high
quality Milli-Q water, and dried with nitrogen prior to imaging. All experiments were conducted in a class-100 clean room at 22 °C. Freshly cleaved mineral samples were mounted on a magnetic holder using double-sided adhesive tape and placed in a liquid cell where 10-2 M KCl solution was then gently flushed over the flat surface. After imaging the bare mineral, background electrolyte was exchanged with polymer solution by flowing unidirectionally the solution of desired concentration in the liquid cell using a syringe. Images were taken after exposing the mineral surface to the polymer solution for 30 min. Images were acquired in polymer solution to facilitate the imaging of the polymer layer with sequentially increasing concentration, without the potential complications of desorption due to electrolyte flushing. No degradation of the image quality was observed relative to similar images acquired in background electrolyte. For each experimental condition, images were repeated on three distinct spots of two different samples. As morphology and parameters of the adsorbed layer were found to be consistent, only one representative image is presented for each condition. Contrast between soft and hard material observed in the phase images enabled us to confirm the differences between the distinct polymer domains and the mineral substrate. The AFM parameters have been determined using 5 5 μm images, subjected to the second-ordered flattening. The Nanoscope software was used to assess the root-mean-squared (rms) roughness and average height of the polymer domains. Image J was used to determine the area fraction of polymer visually observed on the images.23 Height images obtained from the Nanoscope software were converted to 8-bit grayscale images. Images were then segmented into polymer features and background mineral by visually setting a threshold in accordance to the height and phase AFM images. The pixels were then converted to black or white if their brightness was, respectively, higher or lighter than the threshold brightness. An example of the image processing is shown in Figure 3. Threshold images were then processed in Image J to determine the coverage, the average perimeter, and consequently the average diameter of each domain. The domain circularity was also accessed using the formula 4Π* area/(perimeter)2, a value of 100% corresponding therefore to a perfect circle. Further image processing was performed using WXsM v3.0 (Nanotec24) to calculate the average height of the domains. This calculation determined the average height of every 5050 nm2 region within all domains on the surface, making the calculated average height more accurate than a simple average peak-to-valley distance (PTV, average value of the maximum height of each domain) and also making the calculation insensitive to the morphology of the domains. Contact Angle Measurements. Advancing contact angle (θa) and receding contact angle (θr) measurements on freshly cleaved mineral samples have been taken using the captive bubble
(21) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Biochem. 1956, 28, 350–356. (22) Hunter, R. J. Foundations of colloid science, 2nd ed.; Oxford University Press: Oxford, New York, 2001.
(23) Abramoff, M. D.; Magelhaes, P. J.; Ram, S. J. Biophotonics Int. 2004, 11, 36–42. (24) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78.
were placed on a rotary tumbler for 2 h and centrifuged. The supernatant was then analyzed to determine the concentration of polymer left in solution, via a complexation method using UV-vis spectroscopy.21 In this method, the amount of polymer depleted is assumed to be adsorbed onto the solid surface. The adsorbed amount (Γ) was then calculated using the following equation Γ ¼
1 ðcf -ci Þ 3 V m 3 As
ð1Þ
where m is mass of the solid substrate; As is the specific area of the solid substrate; ci and cf are the polymer concentration before and after adsorption; and V is the volume of the suspension. For polymers exhibiting high affinity isotherms, the data can be fitted using the Langmuir expression for adsorption at the solidwater interface22 θ ¼
Γads KCeq bCeq ¼ ¼ Γm 55:5 þ KC 1 þ bCeq eq ads
ð2Þ
where Γads is the adsorbed amount (mol 3 m-2); Γm ads is the plateau adsorbed amount (mol 3 m-2); Ceq is the polymer equilibrium solution concentration (mol 3 L-1); K is the adsorption equilibrium constant; and b is the Langmuir affinity constant (L 3 mol-1). This equation can be rearranged to Ceq 1 Ceq ¼ m þ m Γads b Γads Γads
ð3Þ
permitting the plateau adsorbed amount and the Langmuir affinity constant to be determined. The affinity constant can be used to determine the equilibrium constant, K (b = K/55.5, where 55.5 mol 3 L-1 is the concentration of the solvent, i.e., water, for dilute solutions). Polymer adsorption does not satisfy all of the assumptions to validate the Langmuir model (reversibility of the adsorption, equal size of the solvent, and solute molecules, etc.), therefore thermodynamic constants have not been calculated. However, determination of the maximum adsorbed amount and the affinity constant for the polymer on each mineral is worthwhile for comparison.
Tapping Mode Atomic Force Microscopy (TMAFM) Imaging. In situ TMAFM images were obtained using a Multi-
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Beaussart et al. Table 1. Langmuir Adsorption Parameters for Dextrin TYM Adsorption on the Three Minerals at pH 9
talc molybdenite graphite
Figure 4. Adsorption isotherms of Dextrin TYM on the three minerals at pH 9 (9, talc; b, molybdenite; 2, graphite). The lines represent the Langmuir fitting curves determined by the Langmuir model and calculated from the Langmuir analysis parameters given in Table 1. technique. A flat section of the mineral surface was adhered to a clean glass slide using double-sided tape. The freshly cleaved surface was then deposited on a holder and immersed for 30 min in a quartz cell containing the polymer solution at desired concentration. An air bubble was pushed below the exposed surface through a needle. The silhouette of the bubble was captured and imaged with a progressive scan CCD camera (JAI-CV-MOBX, Japan), and contact angles were determined by drawing the tangent close to the edge of the droplet. Measurements were taken in polymer solution to mirror the conditions of the in situ AFM imaging; dextrin does not adsorb to the liquid vapor interface and does not interfere with the contact angle measurement.25 Three measurements for three different bubbles were taken, and an average value was obtained. Experiments were conducted at 22 °C in a class-100 clean room. Flotation. Batch flotation tests were carried out in a 1 L Denver cell, Denver Equipment Company, Colorado, USA. The mineral suspension (2 wt % solids) was conditioned in 10-2 M KCl, pH 9, for 10 min prior to polymer addition and 30 min after the addition of polymer. The suspension was then transferred to the flotation cell and conditioned with methyl isobutyl carbinol (MIBC) at a concentration of 500 g/t for a further 2 min at 10 000 rpm. Air at a flow rate of 4 L/min was introduced, and concentrates were collected after 1, 3, 5, and 8 min of flotation. A further addition of MIBC at the concentration of 250 g/t was introduced after 3 min of flotation. The concentrates and the tailing were filtered, dried, and weighed to determine the recovery of the mineral. Flotation tests were carried out in duplicate, and standard deviations were calculated.
Results Adsorption Isotherms. The measured adsorption isotherms for Dextrin TYM adsorbed on the three minerals are depicted in Figure 4, with adsorbed amount plotted as a function of equilibrium polymer concentration. In all cases, Dextrin TYM adsorbs on the minerals, and the isotherms exhibit high affinity behavior: the adsorbed amount increases to quickly reach a plateau as the polymer concentration increases. The lack of a step in the isotherms suggests that either the polymer adsorbs on the mineral face only or has a similar affinity for the particle basal plane and the edge surface. Fitting curves determined for the Langmuir model using the nonlinear curve fitting tool in OriginPro7.5 are also shown in Figure 4. The value of the maximum adsorbed amount (Γm ads) and Langmuir affinity constant (b) are given in Table 1, together with (25) Kaggwa, G. B.; Froebe, S.; Huynh, L.; Ralston, J.; Bremmell, K. Langmuir 2005, 21, 4695–4704.
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-2 Γm ads [mg 3 m ] ((0.06)
b [L 3 mol-1]
R2
0.99 1.04 3.24
(2.26 ( 0.01) 105 (4.15 ( 0.01) 105 (1.35 ( 0.01) 106
0.97 0.97 0.97
the correlation coefficient for the Langmuir fits. From Figure 4 and Table 1, it can be seen that Dextrin TYM adsorbed to a greater extent and with higher affinity on graphite particles. The maximum adsorbed amount was roughly similar in the case of adsorption on talc and molybdenite, although Dextrin TYM presents a slightly higher uptake and affinity when adsorbed on molybdenite. TMAFM Imaging. The tapping mode atomic force microscopy (TMAFM) images of freshly cleaved talc, molybdenite, and HOPG taken in situ are depicted in Figure 5 a, b, and c, respectively. The untreated surfaces are characterized by an rms roughness value of 0.34 nm for talc, 1.31 nm for molybdenite, and 1.13 nm in the case of HOPG. These low values of roughness indicate that the freshly cleaved mineral surfaces appear very smooth at the scale at which the images have been taken. However, more heterogeneities due to the imperfect cleavage of the mineral surfaces may be present at millimeter scale.26 Images of the flat basal planes of these minerals exhibiting low roughness have been reported previously.18,27,28 Figure 6 contains in situ height images of Dextrin TYM after 30 min adsorption on the cleaved mineral surfaces as the polymer concentration increases from 25 to 50 mg 3 L-1. These experiments were performed using the same area of each mineral exposed to increasing concentration of polymer solution (images were acquired in polymer solution). Corresponding rms roughness, average height, and surface coverage of the polymer domains are reported in Table 2. Dextrin TYM adsorbed on the talc surface as hemispheres randomly dispersed all over the surface as can be seen in Figure 6 a and b. Phase images (not shown) confirm the differences between the polymer domains and the talc substrate by revealing the contrast between soft and hard materials. To support this conclusion, the rms roughness of an area considered as bare substrate has been measured on both images, giving values (0.35 nm in Figure 6 a and 0.46 nm in Figure 6 b) similar to the roughness of the freshly cleaved surface of talc (0.34 nm) shown in Figure 5 a. When the Dextrin TYM concentration increased from 25 to 50 mg 3 L-1, the size of the domains clearly increased, which is reflected in the values of the average height and polymer coverage reported in Table 2. In situ height images of Dextrin TYM adsorbed on molybdenite for polymer concentrations of 25 mg 3 L-1 and 50 mg 3 L-1 are depicted on Figure 6 c and d, respectively. Dextrin TYM adsorbed on the molybdenite surface as randomly shaped hemispheres of inhomogeneous sizes. The rms roughness of small areas measured between hemispheres on both images 6c and d give values (1.12 nm at 25 mg 3 L-1 and 1.16 nm at 50 mg 3 L-1) similar to the value measured for the freshly cleaved molybdenite surface (1.31 nm) depicted in Figure 5 b. This supports the conclusion that polymer domains are distributed over the bare molybdenite surface. Contrary to adsorption on talc, the Dextrin TYM morphology remains roughly similar when the polymer concentration (26) Tiwari, R. K.; Yang, J.; Saeys, M.; Joachim, C. Surf. Sci. 2008, 602, 2628– 2633. (27) Komiyama, M.; Kiyohara, K.; Li, Y.; Fujikawa, T.; Ebihara, T.; Kubota, T.; Okamoto, Y. J. Mol. Catal. A: Chem. 2004, 215, 143–147. (28) Zhang, X. H.; Zhang, X.; Sun, J.; Zhang, Z.; Li, G.; Fang, H.; Xiao, X.; Zeng, X.; Hu, J. Langmuir 2007, 23, 1778–1783.
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Figure 5. AFM height images (5 5 μm) of freshly cleaved bare mineral substrates: (a) talc, (b) molybdenite, and (c) graphite.
Figure 6. AFM height images (5 5 μm) of adsorbed Dextrin TYM onto minerals in situ at different concentrations: (a) 25 mg 3 L-1 on talc;
(b) 50 mg 3 L-1 on talc; (c) 25 mg 3 L-1 on molybdenite; (d) 50 mg 3 L-1 on molybdenite; (e) 25 mg 3 L-1 on graphite; and (f) 50 mg 3 L-1 on graphite (images acquired at pH 9). The boxes section in the images have been analyzed for rms roughness, giving a value of (a) 0.35 nm, (b) 0.46 nm, (c) 1.12 nm, (d) 1.16 nm, (e) 0.86 nm, and (f) 0.87 nm.
increases. This is also reflected in the parameters presented in Table 2, where all parameters (rms roughness, average height of the domains, and polymer surface coverage) do not significantly increase between 25 and 50 mg 3 L-1. The close values of maximum adsorbed amount obtained from adsorption isotherms of talc and molybdenite are reflected in a similar morphology and size of the polymer domains on the two surfaces. In situ height images of Dextrin TYM at 25 and 50 mg 3 L-1 adsorbed on graphite are presented in Figure 6 e and f, respectively. Dextrin TYM adsorbed on HOPG with the same morphology as Langmuir 2009, 25(17), 9913–9921
on talc and molybdenite, i.e., randomly distributed hemispherical domains. Similar to molybdenite, the domain morphology and size do not significantly change when increasing the polymer concentration. The rms roughness of the mineral surface measured in the area between patches on Figure 6 e and f gives values (0.86 nm at 25 mg 3 L-1 and 0.87 nm at 50 mg 3 L-1) slightly lower than the roughness of the freshly cleaved HOPG surface (1.13 nm). This is most likely due to the reduction in the number of steps on the HOPG surface when the roughness is measured on smaller areas. Again, the low roughness values for the regions of the image DOI: 10.1021/la9010778
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Table 2. RMS Roughness, Average Height of Polymer Domains, Area Fraction of Polymer Coverage, Number of Domains, Average Diameter, and Average Circularity of the Domains Calculated from Images Depicted on Figure 6 polymer concentration [mg 3 L-1]
rms roughness [nm] ((0.05)
average domain height [nm] ((0.05)
area fraction of polymer coverage [%] ((1)
number of domains
average diameter of the domains [nm] ((a)
average circularity of the domains [%] ((a)
bare talc 25 50 bare molybdenite 25 50 bare HOPG 25 50 a Standard error.
0.34 1.87 12.14 1.31 8.45 9.13 1.13 8.91 10.80
8.12 23.41 24.77 22.74 21.03 27.80
9 21 14 17 36 38
236 209 180 188 107 101
108 ( 4 189 ( 7 177 ( 6 191 ( 7 371 ( 44 304 ( 31
79 ( 1 75 ( 1 82 ( 1 79 ( 1 51 ( 2 71 ( 2
Table 3. Adsorbed Amount (Γ), Adsorbed Volume (V), Volume of Water in the Adsorbed Layer (Vw), and Proportion of Hydration Water for Dextrin TYM onto the Three Mineralsa (pH 9), Calculated at 50 mg 3 L-1 Γ from adsorption isotherms [mg m-2] ((0.06)
V 10-9 from adsorption isotherms [m-3] ((0.04)
V 10-9 from AFM parameters [m-3] ((0.01)
Vw 10-9 [m-3] ((0.05)
wt % of water in layer ((2)
talc 0.99 0.68 4.92 4.24 81 molybdenite 1.04 0.72 3.87 3.15 75 graphite 3.24 2.23 10.60 8.37 72 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).
Table 4. Effect of Dextrin TYM on Mineral Wettability Measured with a Captive Bubble Technique after 30 Minutes of Polymer Adsorption at 50 mg 3 L-1 (pH 9) TYM 50 mg L-1
bare mineral
talc molybdenite graphite
Δθ
θa
θr
θa
θr
Δθa
Δθr
[deg] ((2)
[deg] ((2)
[deg] ((2)
[deg] ((2)
[deg] ((4)
[deg] ((4)
93 91 90
74 67 70
79 77 72
53 45 33
14 14 18
21 22 37
between the features support the conclusion that the polymer adsorbs in discrete domains. The rms roughness, height, and coverage of the polymer domains remain almost constant when the polymer concentration increases from 25 to 50 mg 3 L-1. The higher adsorbed amount of Dextrin TYM on graphite particles is mirrored by the higher coverage (around double the coverage values of talc and molybdenite) of the polymer domains on the cleaved HOPG surface. Water Content. Differences in the adsorbed amount obtained from adsorption isotherms and AFM images allow us to calculate the volume of hydration water attached to the polymer chains. From the AFM images, the volume of adsorbed material over the area of the AFM image can be estimated by multiplying the average height (see Experimental section) by the area coverage of the domains. This volume per unit area can then be compared with the amount/volume of polymer adsorbed as calculated from the adsorption isotherms.19 The amount of hydration water for Dextrin TYM, shown in Table 3, is found to be similar when absorbed on the three surfaces, approximately 75% of the total adsorbed mass. This value is in line with QCM-D determinations of the adsorbed layer water content for polysaccharides (between 80 and 95%)29,30 including polysaccharides that adsorb discretely on a hydrophobic surface.29 Although the values of water content are similar for Dextrin TYM, small differences are observed. The amount of hydration (29) Hedin, J.; Loefroth, J.-E.; Nyden, M. Langmuir 2007, 23, 6148–6155. (30) Malmstroem, J.; Agheli, H.; Kingshott, P.; Sutherland, D. S. Langmuir 2007, 23, 9760–9768.
9918 DOI: 10.1021/la9010778
water in Dextrin TYM when adsorbed on talc is slightly higher than for the two other surfaces. The lowest amount of water is obtained for graphite, followed by molybdenite. It is likely that there is some influence of the mineral surface structure on hydration water content of the polymer layer, perhaps through some alteration of the adsorbed polymer layer conformation when adsorbed on the different minerals. However, it would appear that this is only a small effect. Contact Angle Measurements. Contact angle measurements were performed using the captive bubble method. Measurements of the wettability of freshly cleaved flat surfaces have been taken in background electrolyte and compared with contact angle values of the same surface after 30 min immersion in polymer solution at 50 mg 3 L-1. Advancing contact angles of the three bare surfaces are higher than 90°, which reflects the natural hydrophobicity of the three minerals. The hysteresis (defined as the difference between advancing and receding contact angles31-33) is relatively high for the three minerals (approximately 20°) due to the imperfect cleavage of the basal plan over a large area. Indeed, at millimeter scale, hydrophilic steps on the freshly cleaved surfaces might be present which were not detected at the micrometer scale on the AFM images. As can been seen from Table 4, Dextrin TYM at 50 mg 3 L-1 decreases both advancing and receding contact angles of the three minerals. For the three (31) Lam, C. N. C.; Wu, R.; Li, D.; Hair, M. L.; Neumann, A. W. Adv. Colloid Interface Sci. 2002, 96, 169–191. (32) Everett, D. H. Pure Appl. Chem. 1980, 52, 1279–1293. (33) Good, R. J. J. Adhes. Sci. Technol. 1992, 6, 1269–1302.
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Article Table 5. Calculated Values of Rmax and k from Flotation Recovery of the Three Minerals after Adsorption of Dextrin TYM at Different Concentrations (pH 9) polymer concentration [mg L-1]
talc
molybdenite
graphite
Rmax [%] 0 25 50
91 ( 3 89 ( 4 91 ( 3
96 ( 2 94 ( 8 97 ( 4
0.91 ( 0.14 0.49 ( 0.07 0.30 ( 0.02
0.78 ( 0.06 0.23 ( 0.04 0.16 ( 0.01
98 ( 1 100 ( 4 100 ( 2
k [min-1]
Figure 7. Effect of Dextrin TYM on the flotation recovery (at pH 9) of minerals for polymer concentration of 25 mg 3 L-1 (-9-, bare talc; -b-, bare molybdenite; -2-, bare graphite; -0-, TYM on talc; O-, TYM on molybdenite; -4-, TYM on graphite). The lines are curves generated from the fitting of eq 4 to each data set.
0 25 50
0.70 ( 0.04 0.10 ( 0.01 0.07 ( 0.01
the untreated particles for the three minerals is very high due to their natural hydrophobicity. Dextrin TYM decreases the recovery of the three minerals when the polymer concentration is 25 mg 3 L-1. The lowest effect of the dextrin is obtained for talc, followed by molybdenite, and the largest effect is obtained for graphite particles. It can be seen that for the three minerals the decrease in recovery after 8 min of flotation for Dextrin TYM adsorbed at 50 mg 3 L-1 is higher than 25 mg 3 L-1. However, the tendency observed at 25 mg 3 L-1 remains the same, with the smallest decrease in recovery being obtained for talc particles, then molybdenite, and the largest decrease occurring for dextrin adsorption on graphite. The flotation recovery data as a function of time have been fitted to the standard flotation rate equation.35 R ¼ Rmax ð1 -expð -ktÞÞ
Figure 8. Effect of Dextrin TYM on the flotation recovery (at pH 9) of minerals for polymer concentration of 50 mg 3 L-1 (-9-, bare talc; -b-, bare molybdenite; -2-, bare graphite; -0-, TYM on talc; O-, TYM on molybdenite; -Δ-, TYM on graphite). The lines are curves generated from the fitting of eq 4 to each data set.
surfaces, the decrease of the receding contact angle is higher than the effect on the advancing one. From an industrial perspective, one should take into consideration the receding contact angle due to the similarity between the measurement and the mechanism of bubble-particle attachment (involving the dewetting of a mineral surface, with a receding liquid front passing over the mineral).34 The effect of the polymer on talc and molybdenite hydrophobicity is approximately the same, whereas a larger increase in wettability is observed for the polymer adsorbed on graphite. The changes to the mineral wettability correlate with the parameters calculated from the adsorption isotherms as well as the surface coverage of the polymer domains from the TMAFM images. Considering these parameters, a clear increased effect of the polymer is expected on the flotation recovery of graphite; the effect of Dextrin TYM on the flotation recovery of talc and molybdenite could be predicted to be similar. Flotation. Single mineral flotation experiments in the absence and presence of Dextrin TYM at 25 mg 3 L-1 (Figure 7) and 50 mg 3 L-1 (Figure 8) have been performed to correlate the adsorption behavior and the effect of the polymer layer on the mineral wettability with the ability of the polymer to depress the mineral particles. As depicted in Figures 7 and 8, the recovery of (34) Koca, S.; Savas, M. Appl. Surf. Sci. 2004, 225, 347–355.
Langmuir 2009, 25(17), 9913–9921
ð4Þ
The fits allow one to determine values for the maximum recovery at infinite time (Rmax) and the flotation rate constant (k). These values are given in Table 5 for Dextrin TYM adsorbed on the three mineral particles at 25 and 50 mg 3 L-1. For all three minerals, the value of Rmax remains close to the baseline mineral flotation value. This indicates that, given enough time, all the mineral particles would be recovered irrespective of the addition of a depressant. However, Dextrin TYM was found to have a large effect on the rate of flotation for all minerals. Indeed after polymer adsorption at 50 mg 3 L-1, the flotation rate constant is seen to decrease by a factor of 3 in the case of talc, of 5 for molybdenite, and by a factor of 10 for graphite particles. Such significant reductions in the values of k but not in the values of Rmax have been reported previously in the case of adsorption of low molecular weight polysaccharides on talc particles.8
Discussion The data described in this work indicate clearly the role of the polymer surface coverage on the effectiveness of a depressant. However, a number of features of the data were unexpected and raise some questions. First, why does the polymer adopt the morphology seen with AFM? Second, why does the polymer adsorb to a much greater extent on graphite? Third, why does the polymer have a greater effect on molybdenite recovery when the adsorbed amount and the induced change in the mineral hydrophobicity are similar to that seen for polymer-treated talc? We will address each of these questions in turn. The morphology of the adsorbed layer is found to be similar on the three minerals, with Dextrin TYM forming hemispheres randomly dispersed on the substrates. The polymer domains are (35) Lynch, A. J.; Johnson, N. W.; Manlapig, E. V.; Thorne, C. G. Mathematical models of flotation; Elsevier Scientific: Amsterdam, 1981; pp 57-96.
DOI: 10.1021/la9010778
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significantly larger than the size of Dextrin TYM in solution (around 4 nm), suggesting multiple polymer chains coupled with substantial amounts of hydration water within each domain. The reason why this morphology is adopted is not immediately apparent, but there are two plausible explanations. The first explanation is that there may be some polymer domain-domain repulsion caused by the small negative charge on the polymer chain (see Materials section). To test the domain-domain repulsion hypothesis, imaging of the adsorbed Dextrin TYM was performed on talc at pH 9. The solution was then changed for one at pH 3 (without polymer) and the polymer layer reimaged. pH change does not affect the adsorption density of dextrin on talc,36 nor will it alter the talc basal plane-dextrin interaction. At this new pH, the Zeta potential of Dextrin TYM is less than half that at pH 9 (see Supporting Information), and any polymer domain-domain repulsion should be reduced. The TMAFM images (see Supporting Information) revealed only small changes in the adsorbed layer characteristics (increase in domain height but no significant change in separation of domains or number of domains), indicating that electrostatic repulsion between the domains is not likely to be responsible for the observed morphology. The second explanation has its basis in the ability of Dextrin TYM to retain a large amount of hydration water when adsorbed, combined with the high hydrophobicity and low roughness of the substrate. The polymer/hydration water domain may be behaving almost like a liquid droplet, trying to minimize the interaction with the very hydrophobic surface. The extreme hydrophobicity of the surface would prevent the droplet from spreading over the surface. From another perspective, the extreme smoothness of the surfaces provides no significant defects/pinning points to prevent the polymer/hydration water from “rolling up” like a liquid droplet. The above explanation is speculative and needs to be considered while bearing in mind that the polymer must have some degree of hydrophobic attraction to the surface. It is difficult to confirm or refute the hypothesis, but it remains a potential explanation for the observed morphology and merits further study. The second question raised by the data presented in this work relates to the increased extent of adsorption of Dextrin TYM on graphite compared to talc and molybdenite. The adsorption data indicate that there must be some difference between graphite and the other two surfaces that encourages greater Dextrin TYM adsorption. The hydrophobicity of the substrates cannot be used as an explanation for this observation as the three substrates studied all have very similar hydrophobicities (increased substrate hydrophobicity does increase the adsorption of polymers that bind hydrophobically, as shown in recent work from this group on the adsorption of a substituted polyacrylamide37). Enhanced adsorption of dextrin on graphite has been attributed to the presence of impurities in graphite samples by Subramanian and Laskowski.16 However, as indicated in the Supporting Information, no metal ion impurities other than
