Surface Chemistry of Flotation

1. Surface Chemistry of Flotation Dr. DMR Sekhar

2. Contents • Introduction • Surfaces • Physisorption • Chemisorption • Electrical double layer • Hydrophobic solutes/surfaces • Tetrahedral structures of water molecules • Water structure around non-polar solution • Galamba’s Model • SFVS studies by Wang • Summary • Interpretation: hydrophobic hydration 13. References

3. Introduction • Flotation is almost 115 years old technique used industrially to separate/recover minerals from the ores. However this technique is so universal that ions, fine oil droplets from aqueous phase, can also be recovered. • Formation of electrical double layer adjacent to charged mineral particle surfaces and adsorption of flotation reagents such as activators, depressants etcon the mineral surfaces is an established phenomenon that explains hydrophilic hydration of mineral. • Prof. JM Pratt established (1986) that floatability of minerals is associated with a change in the structure of water molecules near the mineral surface.

4. Surfaces • Unlike the atoms in the bulk of the mineral, atoms on the surface are bonded with other atoms below the surface and have broken bonds on the surface. The broken bonds result into surface charge. • Surfaces of mineral particles in aqueous suspension may gain (adsorb) some ions from aqueous phase or exchange ions from aqueous phase or lose (desorb) some ions into aqueous phase.

5. Physisorption • Low energy adsorption is called physical adsorption or physisorption where the energy of interaction is 1- 100 m eV. • Adsorption of gas molecules on activated carbon surface is physical adsorption caused by London Dispersion Forces a type of Van der Waals force. Van der Waals forces are weak and short ranged.

6. Chemisorption • If the adsorbate forms a surface chemical product on the adsorbent, then it is called chemisorption where the interaction energy is 1-10 eV. • Exchange of copper ion from aqueous phase with zinc ion from the surface of sphalerite is chemisorption. Copper sulfide formed on sphalerite surface is more insoluble (more stable) than zinc sulfide.

7. Electrical Double Layerhydrophilic hydration of mineral surfaces Stern layer Ψs Negatively charged diffuse layer + + + + + + + + + + + Counter/gegen ions - Bulk of the water positively charged mineral particle + - Ψd - - + - - - ζ + - + Electric potential - - - - - K-1 Debye length - Surface ions Slipping plane Stern plane • Zeta potential (ζ) is the potential difference between the slipping plane and the bulk of the aqueous phase. • System reaches isoelectric point if ζ is zero. ΨS& Ψdare surface and stern potentials.

8. Hydrophobic solutes/ surfaces • Water when cooled to 0oC solidifies into ice-1 (ordinary ice) that crystallizes in four coordinated, tetrahedral structure. Ice-1 is lighter than water. • Dissolution of nonpolar solutes in water is associated with large decrease of entropy. Nonpolar solutes being hydrophobic, do not interact with water molecules rather they occupy interstitial spaces in the loosely held molecular network of liquid water. • The large decrease of entropy upon the transfer of nonpolar solutes into liquid water is attributed by Frank and Evans (1945), to the ice berg like structures that might have formed around nonpolar/hydrophobic solutes.

9. Tetrahedral Structure of Water Molecules

10. Water Structure around Nonpolar Solutes • Maria, Fabio and Leonardo through their molecular dynamics simulation (2012) suggest, “liquid water can accommodate a small hydrophobic solute without altering its structural properties” however they note, “small fingerprints of the clathrate-like structure around a hydrophobic solute”. • More recent studies (Galamba, 2013) however suggest alteration of water structure around non polar solutes.

11. Galamba’s Model of Hydrophobic Hydration • Galamba’s results (2013) through molecular dynamic studies show, “a subset of water molecules in the first hydration shell of a nonpolar solute have a significantly enhanced tetrahedrality and a slightly larger number of hydrogen bonds, relative to the molecules in water at room temperature, consistent with the experimentally observed negative excess entropy and increased heat capacity of hydrophobic solutions at room temperature”. The tetrahedrality is comparable to that of liquid water at ∼10 °C without small contraction of the O−O distance observed in cold water.

12. Suggested Water Network Structure around Nonpolar Solutes - Galamba

13. Grunwald’s Model for Structure of Liquid Water • Grunwald proposed (1986) a two state model: State A is four coordinated with tetrahedral hydrogen bonds, has relatively low energy, low entropy, highly polar and large volume. State B is five coordinated, nearly half as polar as State A and has bifurcated hydrogen bonds. State B exists in two sub sates at equal concentration, one sub state has central molecule hydrogen bonded with two acceptors and three donors and the other has central molecule hydrogen bonded with three acceptors and two donors.

14. Four and Five Coordinated Water Networks( as modelled by E. Grunwald) D A A D D a • Familiar four coordinated water network. • & c) Model of nearest-neighbour network structure for complementary five-coordinated network states: (D) H-bond donors; (A) H-bond acceptors. D D A A c b A A A D D

15. Grunwald’s Model (continued) • According to the model of Grunwald the entry of hydrophobic solute into the water network displaces the fifth water molecule in State B there by converting to State A. • Solute perturbation of water network according to Grunwald is rearrangement of water network strands so as to create suitable cavities for the solute.

16. SFVS Studies on Hydrophobic Silica Surface: Wang et al • Wang et al (2012) report “The SFVS (sum frequency vibrational spectroscopy) spectra indicate that a very ordered water structure exists in water films at the hydrophilic silica surface during contact with a bubble and that the extent of hydrogen bonding increases with an increase in contact pressure”. • In contrast, the SFVS spectra of water at a hydrophobic silica surface show a lack of hydrogen bonding and are characterized by a distinct absorption at about 3700 cm-1 similar to the spectrum of the air/water interface. These results suggest the presence of a water void, or water exclusion zone, at the hydrophobic surface, ……..”.

17. Summary • Summing up, hydrophobic solutes perturb the water network strands to create suitable cavities and the tetrahedrality of water network, nearest to the hydrophobic solute increases. • The water exclusion zone near hydrophobic surface is the sum of cavities created by hydrophobic parts of the collector adsorbed on the mineral surface. • The increased tetrahedrality of water network near the hydrophobic surface may not be exactly similar to ice1. The resultant four coordinated State A upon displacement of fifth water molecule of State B by hydrophobic solute may still have to retain some properties of State B, for example low polarity and spatial positions/orientations. • Gershon Brill and JM Pratt reported first experimental evidence that a change in flotability is associated with a change in the structure of water at the interface.

18. Water exclusion zone adjacent to hydrophobic mineral Hydrophobic Hydration of Floatable Mineral Particle A) Hydrophobic hydration B) Bubble particle attachment Bulk water Water exclusion zone (merged cavities) Air Bubble Chemisorbed collector ions Water network with increased tetrahedrality Bulk Water Hydrophobic mineral particle Water network strands adjacent to hydrophobic surface with increased tetrahedrality Mineral Surface Attachment of a hydrophobic particle to an air bubble is a case of coalescence of water exclusion zone and air bubble

19. References • Adsorption, https://en.wikipedia.org/wiki/Adsorption • Electrical double layer, https://en.wikipedia.org/wiki/Double_layer_(interfacial) • Maria Montagna, Fabio Sterpone and Leonardo Guidoni, Structural and Spectroscopic Properties of Water around Small Hydrophobic Solutes, J. Phys. Chem. B 2012, 116, 11695−11700

20. References (continued ….) • N. Galamba, Water’s Structure around Hydrophobic Solutes and the Iceberg Model, J. Phys. Chem. B 2013, 117, 2153−2159 • Ernest Grunwald, Model for the structure of the Liquid Water Network, J.Am. Chem. Soc., 1986 & Thermodynamic Properties of Nonpolar Solutes in Water and the Structure of Hydrophobic Hydration Shells. Pages 5719 – 5731. • Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945, 13, 507. • Gershon Brill and John M. Pratt, The effect of doped zinc sulphide on ice nucleation. First experimental evidence that a change in flotability is associated with a change in the structure of water at the interface, S. Afr. J. Chem., 1982, 35(4) • Wang. X., Yin. X., Nalaskowski. J, Du. H., and Miller, J. D., Molecular features of water films created with bubbles at silica surfaces, Surface Innovations, Volume 3 Issue SI1, 2014.

21. Thanks • Thanks to Prof.ChVR Murthy, Principal, AU College of Engineering for making this Work Shop happen. • Thanks to Prof. V. Sujata, Prof. SV Naidu and Prof. P. King and to the Department of Chemical Engineering for whole hearted support. • Thanks to Prof. TC Rao and Er. DV Subba Rao Garu who are central to this work shop. • Thanks to Prof. JM Pratt, Prof. E. Grunwald, Dr. N. Galambaand Dr. X. Wang.