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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Controlling Bubble-Solid Surface Interactions with Environmentally Benign Interfacial Modifiers Marta Krasowska, Matin Kor, Piotr Pawliszak, Francesco L. Bernardis, Bronwyn H Bradshaw-Hajek, and David A Beattie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11770 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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The Journal of Physical Chemistry

Controlling Bubble-Solid Surface Interactions with Environmentally Benign Interfacial Modifiers

Marta Krasowska,1,2* Matin Kor,1 Piotr Pawliszak,1,2 Francesco L. Bernardis, 3 Bronwyn H. Bradshaw-Hajek,2 and David A. Beattie1,2*

1

Future Industries Institute, University of South Australia, Mawson Lakes SA 5095, Australia

2

School of Information Technology and Mathematical Sciences, University of South Australia, Mawson Lakes SA 5095, Australia 3

CP Kelco Oy, P.O. Box 500, 44101, Äänekoski, Finland

* [email protected], [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The influence of two biopolymers (xanthan gum and locust bean gum) on the interaction between bubbles and graphite has been elucidated using a combination of direct measurement techniques. Bubble-surface collisions (monitored using high speed video capture) reveal that when graphite is exposed to low concentration solutions of the two polymers, the timescale of bubble attachment is prolonged by 1 − 2 orders of magnitude, and the final receding water contact angle achieved on such surfaces is reduced by approximately 30 degrees. Single bubble flotation studies confirm the significant effect of such aspects of bubble-particle collisions on the collection efficiency of graphite particles, with marked reduction in flotation recovery across the particle size range, with greatest effect on the coarser particle sizes. The differences in performance of the two polymers in reducing bubbleparticle attachment is seen to be partly due to variation in adsorbed layer coverage of the two polymers on the graphite surface, as revealed by atomic force microscopy imaging. Both polymers can be expected to perform well in the prevention of flotation of graphitic/carbonaceous minerals.


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INTRODUCTION Bubble-solid surface collisions and interactions are topics of fundamental interest, as they encapsulate a range of phenomena and processes that can best be studied using physicochemical characterization of interfaces. Even without the presence of interfacial modifiers, the surface forces and hydrodynamics of the system present a significant degree of chemical and physical complexity that act as a barrier to fully understanding the process of a bubble coming into contact with a surface. Being able to alter these two aspects of bubblesurface encounters using chemical means is the focus of an intense amount of effort from both fundamental and applied chemists, as the use of interfacial modifiers provides fertile ground for the testing of hypotheses and provides chemical levers that can be pulled to control industrial processes in which bubble-surface interactions are important. Mineral flotation is one such industrial process that relies on control of bubble-surface encounters1-5. The recovery of minerals from ore is often achieved through the flotation process. In flotation, ore is crushed and ground to initially separate one mineral phase from another (termed liberation), and then the suspended minerals are conditioned with various chemicals to selectively alter their interfacial properties. At this stage, bubbles are passed through the slurry and collect minerals that are hydrophobic through an attachment process that relies on rapid rupture of wetting films between bubble and particle, and rapid spreading of the three phase contact line (TPCL) across the particle/solution interface. Mineral particles for which this process does not occur within ~10 ms are not collected4, 6. It is this contrast in bubble-solid surface interactions that controls which minerals are collected and which are not, and it is the role of interfacial modifiers to assist the process, either by preventing or encouraging bubbleparticle attachment. Encouraging successful bubble-particle attachment and collection of desired mineral particles most often relies on the adsorption of organic molecules (termed collectors) to 3 ACS Paragon Plus Environment


hydrophobise the surface of target minerals7-11 and the adsorption of frother molecules (surfactants) at the bubble surface4, 12-15. A high contact angle is usually enough to speed-up the process of bubble-particle attachment16-19, while a dynamic adsorption layer formed by the frother molecules at the bubble surface is responsible for longer lifetime of bubbles and formation of the froth layer, allowing the recovery of mineral particles. The reverse goal, preventing bubble-particle attachment, is a more complex process, due to the fact that the required type of solid surface modifier (termed a depressant) can act in one (or more) of three ways20-22: it can slow or prevent wetting film rupture; it can slow the speed of the TPCL movement; and it can reduce particle hydrophobicity so much that bubble-particle aggregates are unstable under conditions of turbulent flow (such as in a flotation cell). Without understanding how these three aspects of mineral particle rejection are connected to depressant molecule chemistry and interfacial characteristics, the design/selection of chemicals to prevent mineral recovery is essentially ‘shooting in the dark’. The results presented in this paper represent a significant effort toward connecting polymer chemistry with interfacial adsorbed layer characteristics, and with their efficacy in interrupting the three crucial stages of bubble-particle attachment. In recently published work from our group, we started the process of quantifying the influence of polymer modifiers on dynamic dewetting in a flotation-relevant system (carboxymethyl cellulose on molybdenite)21. Subsequent to that, we progressed our understanding of polymer modifier action with further single bubble flotation studies, this time for carboxymethyl cellulose on graphite (graphite is commonly recovered and purified via flotation; an essential step toward the production of graphene)22. The current work represents another step in our diagnostic approach to unpick the complex interplay of surface chemistry, physical and chemical heterogeneity and bubbleparticle attachment process (through model bubble-solid surface experiments as well as through bubble-particle collection efficiency).


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The chosen system is graphite and two different biopolymers: xanthan gum and locust bean gum. Gums are polysaccharide polymers from a variety of plant/bacterial sources. Their use in flotation is favored over synthetic polymers as they are relatively cheap and environmentally benign. From a fundamental physical chemistry perspective, they represent an extra degree of complexity (broader molecular weight range, more diverse chemistry), but from a fundamental interfacial chemistry perspective, their adsorbed layer characteristics are as straightforward to measure and quantify as for a synthetic polymer (the adsorption of polyelectrolytes was recently thoroughly discussed in an article by Szilagyi et al.23). We present detailed characterization of their adsorbed layer properties, determined using atomic force microscopy. Their effect on bubble-surface interactions is determined both using high speed video capture of single bubble-flat solid surface collisions and with single bubble flotation in a Hallimond tube.

EXPERIMENTAL Solid Surfaces and Solutions Preparation. Highly oriented pyrolytic graphite, HOPG, flat surfaces with a surface area of 1 × 1 cm2 and of SPI-1 grade (mosaic spread angle of 0.4° +/- 0.1°), were purchased from SPI Supplies (U.S.A.), and used for Atomic Force Microscope (AFM) imaging and bubble collision experiments. Prior to each experiment, a fresh surface of HOPG was exposed by peeling the top layer of the mineral sample using clean tweezers and/or a scalpel blade (on rare occasions, when sticky tape needed to be used to peel the top layer of HOPG, AFM imaging was performed to ensure no adhesive residues were left on the surface). Graphite particles used for Hallimond tube flotation were purchased from Sigma Aldrich, Australia. The Brunauer-Emmett-Teller surface area was measured at 3.0 m2∙g-1. The particle size distribution was determined with AccuSizer 770, giving a particle distribution of 0.5 – 29 m, with a centre of the distribution at 11 m. 5 ACS Paragon Plus Environment


KCl (99%, AR) was purchased from Chem-Supply, Australia. In order to remove any surface active impurities KCl was further purified by calcination at 550 °C for 8 hours, recrystallization, and a second calcination (at 550°C for 8 hours). 10-2 M KCl solution was prepared using purified KCl and Milli-Q water (supplied by an Advantage A10 system (Millipore, U.S.A.), of a resistivity of 18.2 M∙cm and an interfacial tension of 72.4 mN∙m1 at 22 °C, and a total organic carbon component of less than 4 mg∙L-1). Prior to the solution preparation, Milli-Q water was boiled for 30 min, and once it cooled down, an ultrapure dried nitrogen stream (99.999%, BOC, Australia) was bubbled through a glass porous frit into it for 45 min. This step allows for CO2 removal so that any pH fluctuations due to CO2 dissolution are minimized. Once 10-2 M KCl solution was prepared its pH was adjusted to 9 using 10-1 M and 10-2 M KOH (volumetric grade, Scharlau, Spain). The choice of solution pH is determined by the application of mineral flotation, which is characteristically performed under mild alkaline conditions. Two polysaccharides: locust bean gum (type RL-200, trade name Genu gum, CP Kelco, U.S.A. – hereafter termed LBG), and xanthan gum (from Xanthomonas campestris, SigmaAldrich, Australia) were used for the experiments. Both polymers are sourced from or produced from biological material, and are expected to be of high molecular weight (in the MDa regime)24-25. LBG is a highly refined locust bean gum obtained from the seeds of the carob tree, while xanthan gum is an anionic polysaccharide composed of a β-(1→4)-D-glucopyranose glucan backbone with side chains of (1→3)-α-D-mannopyranose-(2→1)-β-D-glucuronic acid(4→1)-β-D-mannopyranose on alternating residues. About half of the terminal mannose residues are 4,6-pyruvated while most of the inner mannose residues are 6-acetylated. The general chemical structures of LBG and xanthan gum are given in Figure 1. 2000 mg∙L-1 stock solutions of gums were prepared following the procedure described by Stokes et al.26 in 10-2 M KCl at neutral pH and stirred for three hours. Afterward, to optimize the degree of solvation, 6 ACS Paragon Plus Environment

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the polymer solution was heated up to 80 °C for 45 minutes while continuously stirring. The solution was cooled down to room temperature and used within 24 hours. Polymer Adsorption. In order to ensure that the amount of polymer per surface area of graphite was the same in all types of experiments, the solid and particulate graphite samples were exposed to different amounts and concentrations of polymer solution. For AFM and bubble collision experiments, gums were adsorbed onto a flat and square (1 × 1 cm2) HOPG surface from 10 mL of either 2 mg·L-1 or 5 mg·L-1 polymers solution. For single bubble Hallimond tube flotation 0.003 g of graphite particles were stirred with 100 mL of either 18 or 45 mg·L-1 polymers solution (these solution concentrations allowed the amount of polymer per graphite unit surface area to be kept the same (either 0.02 mg or 0.05 mg of polymers per 1 cm2) for both flat HOPG surface and graphite particles). The adsorption time was 30 min, which is sufficient time for gums to adsorb onto hydrophobic surfaces20,27. After the adsorption, the flat HOPG surfaces were rinsed with 10-2 M KCl at pH 9 and used for the experiments. The graphite suspensions were transferred to the Hallimond tube after 30 min of adsorption time, and single bubble flotation experiment was carried out immediately. All experiments were conducted at 22 ± 1 °C.



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CH2OH OH OH

CH2OH OH

O

OH

OH

OH O

CH2OH

O

O OH HO

CH2OH

O

O

CH2

O OH HO

O

O

CH2OH

O OH HO

CH2OH

O OH HO

O

CH2OH

O OH HO

O

O

CH2

O OH HO

O

O OH HO

n

CH2OH

CH2OH

O O

O

CH2OR1 OH

O

O

OH

OH

O OH HO R2O

O

OH

R1 = H or

O

-CCH3 CO2-M+

R2, R3 = OH

n

O

O CO2-M+

O

M+ = Na+, K+, ½ Ca2+ or ½ Mg2+

HO CH2OR3

OH

C or

R2, R3 = H or

O

R2, = H, R3 = -CCH3

Figure 1. Chemical structures of locust bean gum (top) and xanthan gum (bottom).


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METHODS Bubble-Surface Collisions and Receding Contact Angle Measurements. Bubble-surface collisions and the dewetting kinetics experiments on bare and polymer-modified HOPG surfaces were carried out using a rising microbubble apparatus21-22. This approach takes advantage of small (~ 400 m – 450 m bubbles – the size range for microfluidic chip used in this study), single bubbles being released by a microfluidic chip and rising in a borosilicate column of a square (30 mm × 30 mm) cross-section containing background electrolyte solution. A sample of HOPG is mounted on the top of the column (~ 8 cm above the point of bubble formation), just beneath the solution surface and the bubble collisions with the HOPG surface are recorded at a frequency of 1000 Hz with a high speed CCD camera (SA3, Photron) connected to a stereomicroscope (SZ-CTV, Olympus). In addition to observing the film rupture and the dewetting process, this type of approach allows for the determination of the bubble terminal velocity as well as film drainage time. The time interval between subsequent bubbles was long enough to ensure the bubble rise was isolated from any effects coming from the previous and/or the next rising bubble. A schematic diagram of the bubble-surface collision experimental setup is given elsewhere (see Supporting Information of Kor et al.21). 30 – 50 collisions were recorded and analysed for each solution condition. A Matlab code was used to determine and extract data of bubble size, bubble position vs time, three phase contact line (TPCL) parameters (i.e. dewetting radius and, receding contact angle vs time). Full details of this process are given in the Supporting Information for Kor et al.21.



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Single Bubble Flotation. The experiments were conducted in a modified Hallimond tube28 shown in Figure 2. The flotation tube was made of two borosilicate columns: a bottom column of square cross-section of 20 × 20 mm2, and an upper circular one of a diameter of 12 mm. Both columns were joined via a three-way valve. The particle suspension was fed to the bottom column while particle-free background electrolyte solution filled the top one. This design helps to reduce entrainment (collection of particles without wetting film rupture and bubble-particle attachment – i.e. carried along with a stream of bubbles by liquid flow). Prior to the experiment, a dilute particle suspension (0.003 wt%) in 10-2 M KCl at pH 9, with or without polymer, was gently poured into the lower section of the flotation column (the three-way valve was turned to position ‘A’ – as indicated in Figure 2). The top part of the column was then filled with particle-free 10-2 M KCl at pH 9. The position of the three-way valve was then changed, allowing flow between both parts of the Hallimond tube (position ‘B’ in Figure 2), so that the graphite particle suspension came into contact with the background electrolyte above it. At this point the bubbles were generated by the same microfluidic chip as used in the rising microbubble apparatus. This ensured the same bubble size in both experiments and allowed precise control over the frequency of bubble generation. 200 bubbles were generated and allowed to rise to the top of the Hallimond tube. The solution in the Hallimond tube concentrate receiver was collected and analysed in terms of number and size of floated particles. The flotation recovery is expressed in terms of the bubble-particle collection efficiency, Ecoll, and is defined as29: 𝐸𝑐𝑜𝑙𝑙 = 1

𝑁𝑝𝑓

(1)

(𝑑𝑝 + 𝑑𝑏)2𝑃𝑁𝐶ℎ𝑠

4𝜋


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where Npf is the number of particles collected per bubble, PNC is the number of particles in that size fraction per cm3, hs is the height of suspension, dp and db are particle and bubble diameters,

respectively.

Figure 2. Schematic representation of the modified Hallimond tube for single bubble flotation. (A) the position of three-way valve during feeding and after flotation; (B) the position of threeway valve during flotation.

Particle Number and Size Determination. An Accusizer C770 Optical Particle Sizer (Particle Sizing Systems, Inc., U.S.A.) was used for the particle number determination and sizing. The Accusizer combines light scattering and light obscuration to allow measurement in the range of particle sizes from 0.5 μm to 500 μm. Light scattering, in which a single particle 11 ACS Paragon Plus Environment


scatters light with an angular dependence dependent on its diameter and relative refractive index, is used for particles up to several microns. For larger particles a light obscuration, or blockage methodology, is used. In the latter case, a particle passing through a narrow area of uniform illumination causes a fraction of the illuminating beam to be blocked or deflected away by an amount approximately equal to the cross-sectional area of the particle. Peak Force Mode Atomic Force Microscope. Peak Force Mode Atomic Force Microscope, PFM AFM, operates in a non-resonant mode and the cantilever oscillation is performed at frequencies well below the cantilever resonance (usually 2 kHz), thus avoiding lateral forces (and potential surface damage) by intermittently contacting the sample. In addition, PFM AFM allows imaging at a controlled (and low, in the order of pN) feedback force. Such low feedback force is of pivotal importance when imaging soft molecular samples such as adsorbed (and hydrated) polymer layers that could be damaged/deformed under high feedback forces30. In situ imaging was performed using a commercially available quartz fluid cell in open configuration, i.e. without an o-ring and tubing. This minimizes introduction of any impurities into the system. Bare mineral surfaces were conditioned for 30 minutes in a desired polymer solution. After conditioning the polymer solution was exchanged for 10-2 M KCl at pH 9 solution. In order to minimize the effect of solution evaporation (and therefore an increase in the ionic strength of the solution) during scanning, a small amount of solution (~50 L) was added every 30 min, using a glass airtight Hamilton syringe. A piezoelectric scanner E, with a maximum 10 μm × 10 μm scan size in the XY direction and nominal 2.5 μm in the Z direction was used to collect several 2 μm × 2 m images of the sample. High resolution (1024 × 1024 pixels) topographic images were acquired using silicon nitride cantilevers with a resonance frequency between 40 and 75 kHz, a spring constant between 0.12 and 0.48 N·m-1, and a sharp silicon (nominal tip radius 2 nm) tip (SCANASYST-FLUID+, Bruker, U.S.A.). This results in 12 ACS Paragon Plus Environment

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a pixel size of 1.95 nm, i.e. comparable to the cantilever nominal tip radius. Scan rates employed in imaging were 0.99 Hz or lower. The images for analysis were selected based on the reproducible structural details of adsorbed polymers and by comparing the height profiles collected in the trace and retrace scanning direction. The images were analysed using WSxM 4.0 SPMAGE 09 Edition (Nanotec)31 and NanoScope Analysis v1.5 (Bruker, USA) software packages. In order to remove the image tilt the AFM images were fitted with a first-order plane fit. At least three independent samples (i.e. different HOPG surface and independently prepared polymer solution) were used for each of the solution conditions and multiple spots per sample were imaged.

RESULTS AND DISCUSSION Wetting Film Drainage and Stability at Bare HOPG. To determine the effect of the polymers on bubble-surface collisions, it is first necessary to characterize the collisions of bubbles against a bare HOPG surface. This has been done in the past for polyelectrolytes32, dextrins20, cellulose21,22. The representative sequence of images presenting the bubble collision and attachment to freshly cleaved HOPG surface in 10-2 M KCl at pH 9 is shown in the top panel of Figure 3. The air bubble rises with its terminal velocity when it’s far from the HOPG surface. When the bubble approaches close to the HOPG surface (~ 0.5 – 1 bubble diameter away) it starts to slow down until it stays motionless beneath the HOPG surface. This is the moment when the wetting film formed between the solid surface and the air bubble starts to drain. The film drains and, depending on the surface forces (at smaller film thicknesses) or the occurrence of film thickness fluctuations (due to for example presence of surface roughness or nanobubbles; at larger film thicknesses) it either stays intact (i.e. no bubble attachment) or it ruptures (resulting in the bubble attachment to the solid surface). Once the film drains (a very rapid process − in the case of a 400 m bubble colliding with 13 ACS Paragon Plus Environment


a bare HOPG surface the drainage time equals 5.2 ± 2.8 ms22), and ruptures (frame at t = 0 ms in the top panels of Figure 3) the three phase contact line (TPCL) is formed (t = 1 ms in the top panel of Figure 3). The diameter of the TPCL expands until it reaches its static (final) value. The contact angle measured at the three phase contact point, i.e. the dynamic receding contact angle, changes as the TPCL progresses and is a function of the HOPG hydrophobicity as well as TPCL velocity. The evolution of the dynamic receding contact angle and the bubble dewetting parameter is presented in the bottom panel of Figure 3. As one can see it takes only a few (5 – 7) ms for the bubble to reach a static receding contact angle of 65° – 69 °. The wetting film rupture between bare HOPG and the air bubble indicates that attractive forces within such a film dominate. Since there are no adsorbing species present in the system, and the solution is Newtonian, the interaction forces are due to DLVO (named after Derjaguin, Landau, Verwey and Overbeek33) forces, and they are a sum of van der Waals and electrostatic interactions34,35. These forces can be used to understand the process of bubble-surface attachment. The Hamaker coefficient for a carbon-water-air system is negative (-5.68 × 10-20 J)36 resulting in repulsive van der Waals interactions (see dotted black line in Figure 4). Also, the electrostatic component of the force between negatively charged HOPG (-31 mV in 10-2 M KCl at pH 9, as determined from streaming potential experiment using ZetaSpin (Zetametrix, U.S.A.) and a negatively charged air bubble (-38 mV in 10-2 M KCl37) is repulsive (see dashed line in Figure 4). The sum of both components results in a total repulsive force acting across the wetting film between bare HOPG and the air bubble (see solid red line in Figure 4). Such a repulsive force, which is non-zero at separations (film thicknesses) as large as tens of nm, will prevent the thin film rupture and bubble attachment to the HOPG surface. However the wetting film ruptures, indicating that there must be some attractive forces responsible for the process.


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Figure 3. A representative sequence of images showing the film rupture and dewetting at a freshly cleaved HOPG surface, pH 9, 10–2 M KCl (top panel). Diameter of the dewetting perimeter (black squares and left y-axis) and dynamic receding contact angle (grey triangles and right y-axis) as a function of time for the same condition. Bubble diameter, db = 432 m.



Figure 4. The normalized (in respect to bubble radius, Rb) interaction forces between an air bubble and HOPG surface in 10-2 M KCl at pH 9 vs thin film thickness, h. Non-retarded van der Waals interactions (dotted black line) were calculated for a sphere-flat surface geometry38. The Hamaker coefficient used for HOPG-water-air system was -5.68 × 10-20 J36. The electrostatic interactions for the system were computed using constant potential boundary conditions and the Hogg-Healy-Fuerstenau (HHF) approximation39. The zeta potential value for bare surface HOPG (-31 mV) and -38 mV37 were used in place of surface potentials, Rb = 200 m. Wu et al.22 estimated from the drainage time that the wetting film between bare HOPG and an air bubble ruptures at thicknesses of 90 – 130 nm. It is also well-known and documented that air in the form of nano-40-44 or submicron-45-46 bubbles/features is present at HOPG and hydrophobic surfaces immersed in aqueous solution. Wrobel was the first to propose that the gas can exist in aqueous solution in two forms: (i) dissolved gas molecules, and (ii) gas nuclei47. These nuclei formed at the defect (with the lowest local surface energy) are precursor to growing (nano-/micro-) bubbles. The importance of nano-/micro- bubbles, as well as the type 16 ACS Paragon Plus Environment

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of gas, on the flotation recovery, was recently discussed by Vaziri Hassas et al.48. Their molecular dynamic simulations matched the AFM images, revealing that while the attached N2 nanobubbles were stable and exhibit a high water contact angle (90 °), the CO2 nanobubbles had very different behavior and were spreading after attachment to a pyrite surface to form something that appeared to be a layer of CO2 molecules, corresponding to even higher water contact angles (almost 180°)48. Ozun et al. further proved that the flotation recovery of oxidized pyrite in CO2 saturated water gradually increased49. Also, in our recent studies, we have observed that the stability of the wetting films on HOPG decreased with increasing amount of dissolved air and we have attributed it to an increasing number of nano- and submicronbubbles nucleated at the graphite surface50. Mishchuk elaborated a model that can explain such ‘hydrophobic attraction’ in the framework of classical DLVO forces51. This model takes into account water depletion near hydrophobic particles and nanobubbles formed on their surface. In such a case, the macroscopic air bubble is in contact with (nano-/micro-) bubble, and not a HOPG surface. For a system characterized as air (macroscopic bubble)-water-air (nano-/microbubble), the Hamaker coefficient is positive (3.7 × 10-20 J)33, resulting in attractive van der Waals interactions (see dotted black line in Figure 5). The electrostatic interaction between two bubbles is repulsive (see dashed blue line in Figure 5), and the total force of interactions between such surfaces predicts two minima: (i) a deep primary minimum for h < 3 nm, and a shallow secondary minimum for h values between 16 and 43 nm (see solid red line in Figure 5). The presence of a secondary minimum is due to the fact that the electrostatic repulsions decrease exponentially with h, while the van der Waals interactions decrease more slowly. As a result, the foam film destabilization in this attractive region is possible. The aggregation in a secondary minimum was observed for several types of solid particles52-57. Weise and Healy also pointed out that the secondary minima becomes more important for the systems where the



interaction approaches the sphere-wall configuration55 (and a nanobubble interacting with a 400 mm bubble can be approximated to such a case). The combination of a non-zero attractive force being non-zero at separations as large as tens of nm, and the fact that the height of the nanobubbles at HOPG surfaces is known to be between 10 and 80 nm42, means that the film would rupture at separations (film thicknesses) as large as 26 – 123 nm (with respect to the HOPG surface). This is in a relatively good agreement with the estimation given by Wu et al.22.

Figure 5. The normalized (in respect to nanobubble radius, Rnb) interaction forces between an air bubble and nanobubble in 10-2 M KCl at pH 9 vs surface separation, h. Non-retarded van der Waals interactions (dotted black line) were calculated for the sphere-sphere geometry38. The Hamaker coefficient used for air-water-air system was: 3.7 × 10-20 J33. The electrostatic interactions for the system were computed using constant potential boundary conditions and the Hogg-Healy-Fuerstenau (HHF) approximation39. Zeta potentials of -38 mV37 were used in place of surface potentials, Rb = 200 m, Rnb = 40 nm.


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As a means to further investigate the feasibility of our explanation for the observed bubble attachment data (i.e. the presence of a layer of nanobubbles at the solid surface), an experiment on foam film drainage kinetics (as well as its stability) was performed using a modified bubblesurface collisions apparatus58 which is schematically presented in Figure S1. Bubbles are allowed to rise and collide with a free air-water interface. The physical system parallels between the two cases (in terms of measured and experienced surface forces) are clear. In both cases, a bubble interacts with a layer of air across an intervening aqueous medium. This approach for our work was necessary, as there is no available experimental approach to study a drainage and stability of thin liquid films formed between a macroscopic bubble and a nanobubble, but the same forces act between two macroscopic bubbles or between a macroscopic bubble and an air-solution interface. Study of interactions across a foam film has been most often facilitated by the use of the thin film balance where the critical thickness of rupture of such foam film can be determined using monochromatic light interferometry59-62. In our approach we studied the interaction of a rising bubble colliding with the air-solution interface. An example of the sequence of images that can be obtained with this technique is given in the top panel in Figure S2, and the determined film thickness between the bubble and the free air-solution interface is provided in the lower panel of Figure S2. The measured critical thickness of rupture overlaps with the position of a secondary minimum (16 – 43 nm) in Figure 5, indicating that it is likely that a film between a nanobubble and a macroscopic bubble would also rupture in that secondary minimum. Wetting Film Drainage and Stability at Polymer-Modified HOPG. Representative sequences of images presenting the bubble collision and attachment to polymer-modified HOPG surfaces are shown in the top panels of Figures 6 and 7. Also given in these Figures are data for the diameter of the wetting perimeter (the distance across the bubble region that is in contact with the surface) and the determined receding contact angle. Once the film ruptures,



the three phase contact is formed. This takes a few ms for bare HOPG vs a few s for polymermodified HOPG surface, t = 0 ms is the moment when the bubble is closest to the solid surface with no detectable motion in the direction normal to the solid surface.

Figure 6. Sequence of images showing the film rupture and dewetting after 30 min of immersion in LBG solution of concentration 2 mg∙L–1, pH 9, 10–2 M KCl (top panel). Diameter of the dewetting perimeter (black circles and left vertical-axis) and dynamic receding contact angle (grey triangles and right vertical-axis) as a function of time for same condition. Bubble diameter, db = 429 m.


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Figure 7. Sequence of images showing the film rupture and dewetting after 30 min of immersion in xanthan gum solution of concentration 2 mg∙L–1, pH 9, 10–2 M KCl (top panel). Diameter of the dewetting perimeter (black circles and left y-axis) and dynamic receding contact angle (grey triangles and right y-axis) as a function of time for same condition. Bubble diameter, db = 432 m.



Figure 8. Average TPVL velocity, VTPCL, of bubble dewetting at LBG-modified HOPG surface (black bars), and xanthan gum-modified HOPG surface (grey bars) for periods of dewetting. The average values taken from experiments for bubble diameters, db = 430 ± 9 m.

The initial receding contact angle for polymer-modified HOPG surface is significantly lower than the initial receding contact angle for bare HOPG surface. In addition, the receding contact angle changes at similar rates for short times (i.e. t ≤ 20 ms) for both polymers, while for longer times the dewetting is significantly slower for LBG. For t = 1 ms, the receding contact angles for LBG and xanthan gum are 19° and 16°, respectively, with the contact angle increasing with time. After 10 ms the receding contact angle values increase to 31° and 25° for LBG and xanthan gum, respectively. This is the largest rate of increase in the values of the contact angle in the data, as the TPCL expansion velocity is the largest at the beginning of the dewetting process and the kinetic energy of the moving TPCL line is the driving force for the 22 ACS Paragon Plus Environment

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process (during the first 10 ms the average velocity of TPCL, VTPCL, during the dewetting process on LBG-modified HOPG is 8031 ± 127 nm·ms-1 and VTPCL for xanthan gum-modified HOPG is 5987 ± 101 nm·ms-1 – compare TPCL expansion velocity data for both polymers in Figure 8). At longer times (and lower velocity of the TPCL movement), the rate of increase of contact angle becomes smaller for both polymers, with the TPCL expansion velocity being slower for LBG-modified HOPG surface. In the second 10 ms time block, VTPCL decreases to 1407 ± 51 nm·ms-1 for LBG-modified HOPG and to 1481 ± 62 nm·ms-1 for xanthan gummodified HOPG. In the same time period, the receding contact angle increases to 33° and 26° for LBG and xanthan gum, respectively. Between 20 and 40 ms of the dewetting process the VTPCL decreases further by an order of magnitude (this is the most significant decrease during entire dewetting process) to 149 ± 15 nm·ms-1 for LBG-modified HOPG and to 152 ± 11 nm·ms-1 for xanthan gum-modified HOPG. During this time, the receding contact angles are 34° and 27° for LBG and xanthan gum, respectively. For the time interval 41 − 110 ms, the VTPCL is 35 ± 2 nm·ms-1 for LBG-modified HOPG and 45 ± 2 nm·ms-1 for xanthan gummodified HOPG. During the next 120 ms there is still a significant decrease in the average velocity of the TPCL (VTPCL decreases to 23 ± 2 nm·ms-1 for LBG-modified HOPG and to 27 ± 2 nm·ms-1 for xanthan gum-modified HOPG), assisted by a small increase in the receding contact angle values (36° for LBG-modified HOPG and 29° for xanthan gum-modified HOPG). The values of VTPCL for longer dewetting times continuously decrease. For the LBGmodified HOPG surface the VTPCL values are: 8 ± 0.5 nm·ms-1 (for time interval 231 − 730 ms), 4 ± 0.5 nm·ms-1 (for time interval 731 − 1930 ms), 2 ± 0.5 nm·ms-1 (for time interval 1931 − 3000 ms), and 1 ± 0.5 nm·ms-1 (for t > 3000 ms). For the xanthan gum-modified HOPG surface the VTPCL values are: 14 ± 2 nm·ms-1 (for time interval 231 − 730 ms), 6 ± 2 nm·ms-1 (for time interval 731 − 1930 ms), 3 ± 1 nm·ms-1 (for time interval 1931 − 3000 ms), and 1.5 ± 0.5 nm·ms-1 (for t > 3000 ms). At t > 3500 ms, the bubble attached to LBG-modified HOPG surface



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reaches its final static contact angle (and the average TPCL velocity is below 0.5 nm·ms-1) of 38 °, while the contact angle evolution proceeds in the case of the bubble attached to xanthan gum-modified HOPG surface. At t = 5000 ms the receding contact angle reaches 35 °, and at t = 7000 ms 36° and the average TPCL velocity is below 0.5 nm·ms-1. Since the bubble collision with a solid surface is a dynamic process there is always some scatter in the data and multiple experiments were necessary. Table 1 lists the average values of static (final) receding contact angles and the average values for the film drainage time for each polymer-modified HOPG surface. Given the variation in these parameters, one would expect there to be a varying effectiveness of the two polymers to reduce flotation recovery of graphite. Table 1. Comparison of Average Values of Static Receding Contact Angle and Drainage Time for Polymer-Modified HOPG Surfaces Static receding contact Time of film drainage, tdr [ms] angle, rec [degrees] LBG, 2 mg·L-1

41 ± 13

49 ± 25

Xanthan gum, 2 mg·L-1

36 ± 6

103 ± 39

Single Bubble Hallimond Tube Flotation. The experimental collection efficiency data, Ecoll, for bare and polymer-modified graphite particles in 10-2 M KCl at pH 9 are presented in Figures 9A and 9B, respectively. Collection efficiency is very high for bare graphite particles (Figure 9A). This is not unexpected considering the high receding contact angle and increased probability of nano-/microbubble nucleation at the rough hydrophobic surface46, 63-64. The Ecoll also depends on the particle size: the lowest Ecoll corresponds to particles 3 m and smaller, and the highest Ecoll to the largest particles. Ecoll increases by the factor of 4 within the studied particle size range. Wu et al.22 observed a similar trend in their experiments, however these were conducted at a lower salt concentration (10-3 M KCl) and the Ecoll values for the same particle size were lower. With lower salt concentration, the repulsive electrostatic interaction between an air bubble and a graphite particle is stronger and of longer range (-1 ~ 9.6 nm). At 24 ACS Paragon Plus Environment

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higher salt concentration, as in this case, the repulsive electrical double layer is compressed (-1 ~ 3.03 nm) and therefore the bubble-particle attachment is more probable, resulting in higher collection efficiency. Figure 9B presents collection efficiencies for LBG- (blue open circles for 18 mg·L-1, and blue closed circles for 45 mg·L-1) and xanthan gum- (red open circles for 18 mg·L-1, and red closed circles for 45 mg·L-1) modified graphite particles. Even for the lower (18 mg·L-1) polymer concentration the Ecoll is significantly smaller than for bare graphite particles. Also the trend in Ecoll as a function of particle size is different from the case of bare graphite particles: the highest Ecoll is recorded for the smallest (< 5 m) particles. The Ecoll decreases by a factor of ~ 3 for 18 mg·L-1 LBG-modified graphite particles of the largest size, and by a factor of ~ 2 for 18 mg·L-1 xanthan gum-modified largest graphite particles. There is a small difference in Ecoll between 18 mg·L-1 LBG-modified and 18 mg·L-1 xanthan gum-modified graphite particles smaller than 10 m, with slightly higher Ecoll for 18 mg·L-1 LBG-modified particles (as predicted based on the bubble-surface collisions – see Table 1). For larger particles the Ecoll is the same within the experimental error. Increasing the polymer concentration (see Figure 9B - blue closed circles for 45 mg·L-1 LBG-and red closed circles for 45 mg·L-1 xanthan gummodified graphite particles) results in even lower Ecoll, with no noticeable dependence on particle size. This is more likely particle entrainment than a real flotation recovery. The decrease in the Ecoll for polymer-modified graphite particles is due to: (i) smaller receding contact angles, (ii) longer drainage time and (iii) longer time required to establish the TPCL sufficient for stable bubble-particle aggregate – this is in good agreement with bubblepolymer-modified HOPG collision experiments. When the wetting film ruptures and the TPCL is formed, the speed of the dewetting process is a function of the contact angle. It is also well established experimentally that the TPCL motion is slower on non-rigid surfaces20-22. Upon polymer adsorption the contact angle decreases and the surface becomes softer. Therefore, the 25 ACS Paragon Plus Environment


rate of TPCL spreading is slower and the ‘final’ dewetting perimeter established between a polymer-modified graphite particle and an air bubble is not large enough for the formation of a stable bubble-particle aggregate. As a result, due to gravity, larger particles will detach from the bubble surface during the flotation process.


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Figure 9. (A) – Bubble-particle collection efficiencies for untreated graphite, as a function of particle diameter (B) – Bubble-particle collection efficiencies for polymer-modified graphite, as a function of particle diameter. In all cases db(average) ~ 430 m, 10-2 M KCl at pH 9. Topography of Polymer-Modified HOPG Surfaces. The bubble-surface collision data and the single bubble flotation data point to some variation in the effectiveness of the two biopolymers in reducing graphite hydrophobicity and altering bubble-surface interactions. Some insight into the reasons for the difference can be gained from imaging of the adsorbed layers on the graphite surface, using atomic force microscopy. 2 m × 2 m topographic AFM images presenting adsorbed layers of both polymers on the HOPG surface are shown in Figure 10. Figures 10A and 10B present LBG adsorbed from 2 mg·L-1 and 5 mg·L-1 solutions, respectively, while Figures 10C and 10D present xanthan gum, adsorbed from 2 mg·L-1 and 5 mg·L-1 solutions, respectively. LBG at 2 mg·L-1 forms a thin layer almost uniformly covering the HOPG surface, both basal plane and edges. The thickness of the polymer layer (measured with respect to the holes in the polymer layer) is 0.7 ± 0.1 nm, while the root-mean-square (RMS) roughness is 0.58 nm. Upon increasing the concentration of the solution from which adsorption takes place (to 5 mg·L-1), the surface topography changes: (i) the area not covered with the polymer is significantly smaller, (9 – 12% of not covered area for 5 mg·L-1 solution vs 18 – 22% of not covered area for 2 mg·L-1 solution), as the size of the holes decreases for LBG adsorption from 5 mg·L-1 solution, (ii) there are single strands of LBG polymer adsorbing on top of the existing polymer layer. The height of such strands is 1.0 ± 0.2 nm, while their length falls between 100 nm and ~ 1 m, the RMS roughness of the surface not covered with single strands of LBG also increases and equals 0.69 nm. The appearance of an altered morphology of the adsorbed polymer would indicate that the initial polymer layer does not preclude additional adsorption, but that this adsorption occurs through an altered mechanism. 27 ACS Paragon Plus Environment


Figure 10. 2 × 2 m topographic AFM images presenting adsorbed layers of the adsorbed polymers on HOPG surface. The images were acquired in situ, after 30 min immersion in: (A) 2 mg·L-1 LBG, (B) 5 mg·L-1 LBG, (C) 2 mg·L-1 xanthan gum, and (D) 5 mg·L-1 xanthan gum solutions. Scale bar = 400 nm.

For xanthan gum (Figures 10C and 10D), a different polymer adsorption pattern is observed. Upon initial inspection, it would appear that xanthan gum has much lower coverage at 2 mg·L-1, and that the coverage increases when the concentration is raised to 5 mg·L-1. However, this does not align with what has been observed in the bubble rise and the flotation experiments. If the adsorbed layer is indeed the scattered features at 2 mg·L-1, then the coverage of the polymer film is only 4-8%. It is unlikely that such a low coverage could result in such a large effect on 28 ACS Paragon Plus Environment

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HOPG hydrophobicity and graphite flotation recovery. It is more likely that the xanthan gum forms a complete layer, and that the observable features in the images (10C and 10D) are xanthan gum adsorbed on top of the initial layer (similar to the features seen for LBG at 5 mg·L-1). The additional features on top of the (presumed) underlying film appear to be in the form of single strands (or their aggregates Figure 10C). Additional support for this interpretation can be gained by closer inspection of an AFM image of such a xanthan-coated HOPG sample, and interrogation of the roughness of areas that appear to be uncoated basal plane – as shown in Figure S3 in the supporting information. The roughness of this region of the image is greater than has been determined for bare HOPG in our previous work (which is atomically smooth), supporting the interpretation that there is a thin near-continuous layer of xanthan gum underneath more visible strands/features of xanthan gum. Each such strand is of the height of 0.8 nm ± 0.1 nm and length of 80 – 500 nm. The strand’s height indicates that renaturation of xanthan has occurred under relatively high salt concentration, and the adsorbed xanthan molecules are in a double helical state65. The strands on top of the initial polymer layer preferentially adsorb at the terrace edges. RMS roughness is higher than in the case of LBG and it equals 0.88 nm. In the case of adsorption from 5 mg·L-1 xanthan gum solution, the strands (or rather their aggregates) are of the same height (0.8 nm ± 0.1 nm) but much shorter. The RMS roughness decreases in comparison to adsorption from 2 mg·L-1 xanthan solution and equals 0.59 nm. Upon comparing the two sets of images, one other important difference becomes apparent. The layer of LBG adsorbed from 2 mg·L-1 is likely to be less rigid than the other adsorbed layers. This information is inferred from the sharpness of the AFM images around the step edges on the terraces of the HOPG surface. For the other layers, which are likely softer and less compact, the edges of terraces on HOPG are less sharp/blurred (compare Figure 10A with Figures 10B − D), pointing toward the possibility that the LBG layer formed from lower



polymer concentration is less rigid than in three other cases, where sharp edges of HOPG are clearly seen. Provided that we can proceed with the assumption that xanthan has an underlying film with additional features on top, the AFM images correlate well with the observed bubble-surface collision and single bubble flotation data. Considering the images for the lower concentration, the complete surface coverage by xanthan gum would be expected to reduce the hydrophobicity to a greater extent relative to that obtained with LBG. This follows through to a lower collection efficiently in flotation. The much longer time for thin film drainage can be directly related to the solid surface hydrophobicity. The receding final (static) water contact angle for xanthan gum is lower. This means there will be more interactions between water molecules and xanthan gum-modified HOPG surface, resulting in less hydrodynamic slip at solid-solution interface and hence slower wetting film drainage. The slower dewetting speed in the lower (less than 100 nm·s-1) TPCL velocity regime in case of LBG-modified HOPG is most likely due to two factors: (i) adsorbed layer of LBG being less rigid than the layer of xanthan gum, and (ii) LBG layer having holes (a few nm to ~ 100 nm in lateral dimension) in the adsorbed layer, hence significant chemical and physical heterogeneity on the length-scale compared with the TPCL speed (tens of nm·s-1 and lower). These two factors will have an effect on how quickly the surface is dewetted, as well as the extent of contact line pinning. In the case of adsorption from LBG and xanthan gum solutions of higher concentration (5 mg·L-1), the bubble never attaches to the polymer-modified HOPG surfaces indicating that both polymers at 5 mg·L-1 act as a very efficient flotation depressants for HOPG/graphite. There are expected to be differences in the ability of the polymer layers to impede the movement of a three phase contact line, as the two polymers are known to be hydrated to different degrees when adsorbed onto a hydrophobic surface (see Stokes et al26, for a study of the two polymer


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types on hydrophobic PDMS). This difference may be responsible for the altered velocity of three phase contact line movement on the two different adsorbed layers.

CONCLUSION Locust bean gum and xanthan gum are very effective polymer reagents for reducing the flotation of graphite. Even small doses of the polymer result in complete suppression of flotation due to a reduction in graphite hydrophobicity, resulting in significant prolonging of the time for drainage and rupture of a wetting film between a bubble and a polymer-modified HOPG/graphite surface. The small differences in the performance of the two polymers can be attributed to differences in coverage and rigidity of the two polymers on the graphite surface. Of almost equal interest and significance to the performance of the two polymers in preventing bubble-particle attachment for graphite is the determination that in the absence of polymer, the recovery of graphite is attributable to the attractive surface forces between air bubbles and a layer on nanobubbles at the surface of the bare graphite surface. It is highly likely that the polymers in the study act not solely as a means to reduce the hydrophobicity of the graphite surface, but they will also act to prevent the formation of nanobubbles at the graphite surface, or to displace existing nanobubbles.

SUPPORTING INFORMATION The schematic of the apparatus used for foam film thickness determination, the foam film drainage in 10-2 M KCl at pH 9, and an expanded AFM image presenting adsorbed layer of xanthan gum adsorbed from 2 mg·L-1 solution are given in the Supporting Information file. ACKNOWLEDGEMENT



This work was performed in part at the South Australian node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers. DAB acknowledges the financial support from the Australian Research Council (Linkage Project LP0990646, and Discovery Project DP110104179).

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