1. Surface Charge Surface Potential Surface Energy

2. Course 3130, Dr. Lokanathan Arcot 2 Origin of surface charge 1. Dissociation or ionization of a surface group (NH 3 ) + COOH COO - Carboxyl terminated Dissociated carboxyl Neutral Anionic pH > PI COOH COO - NH 2 (NH 3 ) + Amine terminated Ammonium terminated Neutral Cationic pH < PI Eample 1a: Cationic surfaceEample 1b: Anionic surface 2. Preferential adsorption of an ion from solution Ag + AgI Silver ion adsorption on Silver Iodide Eample 1a: Cationic surfaceEample 1b: Anionic surface Ag + Cl - Au Chloride ion adsorption on Gold Cl - Principles of Colloid and Surface Chemistry, Chapter 11, 3rd Edn, Paul and Raj Physical Chemsitry of Surfaces, Chapter 5, 6th Edn, Arthur and Alice

3. Course 3130, Dr. Lokanathan Arcot 3 Why is surface charge important Colloidal Stability Most Relevant Stabilization Strategies Repulsion between Like charges

4. Course 3130, Dr. Lokanathan Arcot 4 Relation between surface charge density And surface potential - Ve Surface Charge Density: Number of Charges per unit area Surface Potential: ( Electrical Potential) Charge produces Electric Field Thus a charged surface can influence the motion of other charged particles in surrounding + Potential due to surface at point ’A’ Point A Move the unit +ve charge From infinity to point ’A’ Unit +ve Electric Potential – Work done in moving a unit charge from infinity to a point - Ve potenital : -ve work done + Ve potenital : +ve work done -Ve charged Particle

5. Course 3130, Dr. Lokanathan Arcot 5 Effect of surface charge in Solution Electrical double layer Simplified representation Anionic ion -Ve Cationic ion +Ve Anionic colloidal particle -Ve

6. Course 3130, Dr. Lokanathan Arcot 6 Charges in different situations In water Charges in aqueous solution - solvated Charges giving rise to surface charge – not solvated

7. 7 Electrical double layer model : Why named ‘Double layer’ ? LAYER I Compact non-solvated Non-columbic Charge contributing layer LAYER II Solvated ions: Columbic Subdivided based on if they can move around Stern layer – No movement; Diffuse layer - mobile Course 3130, Dr. Lokanathan Arcot

8. 8 Electrical double layer model : Slipping Plane or Shear Plane – Zeta potential

9. Course 3130, Dr. Lokanathan Arcot 9 Electrical double layer model : Electrical Potential as a Func. Of Distance Ψ O - absolute value of surface potential (Very difficult to be experimentally measured) Ψ d - Surface potential at distance ‘d’ ζ – Potential at slipping plane (Easily experimentally measurable) Conclusion: Surface charge is responsible for Ψ O For a given solvent condition ζ is drectly related to Ψ O

10. Course 3130, Dr. Lokanathan Arcot 10 Electrokinetic Phenomena + Electrophoresis – Induced m otion of charged particles/ions in presence of applied electric field -

11. Course 3130, Dr. Lokanathan Arcot 11 Electrokinetic Phenomena Electroosmosis – Induced m otion of electrolyte solution located near a charged surface in presence of applied electric field The movement of counterions in diffuse layer of double layer under the influence of applied electric field drags the liquid and hence the osmotic flow +-

12. Course 3130, Dr. Lokanathan Arcot 12 Electrokinetic Phenomena Streaming Potential /Current – Electric potential/current induced by m otion of electrolyte solution over a charged surface. Forced movement of electrolyte solution displaces the counter ions from the double layer thus inducing a potential or current +-

13. Course 3130, Dr. Lokanathan Arcot 13 Zeta Potential of Nanoparticles and Large Surfaces 1 nm – few 1000s nm Macroscopic cm Nanoparticle Size Electrophoresis Surface Zeta Potential Streaming Potential

14. Course 3130, Dr. Lokanathan Arcot 14 Zeta Potential of Nanoparticles Using Laser Doppler Velocimetry Light wave couples for an instant + – Coupled light is randomly remitted in any direction Rayleigh Scattering by stationary particle λ λ λ λ Doppler Effect

15. Course 3130, Dr. Lokanathan Arcot 15 Incident Light Frequency v (nu) V = 0 V > 0 Scattered Light ν (Same as incident) Scattered ν + Δν or Increase in Freq Basis of Laser Doppler Velocimetry Observer/ Detector Observer/ Detector What about V < 0 ? Particle moving away from observer ν - Δν Incident Light Frequency v (nu) V is Velocity of Nanoparticle

16. Course 3130, Dr. Lokanathan Arcot 16 Basis of Laser Doppler Velocimetry Interferometry Scattered ν + Δν Reference ν Interference Resultant Beat Frequency

17. Course 3130, Dr. Lokanathan Arcot 17 Basis of Laser Doppler Velocimetry Interferometry Interference Resultant Beat Frequency The Frequency of the Scattred light is directly linked to velocity of Particles How do we get the velocity of Particle from Doppler shift?

18. Course 3130, Dr. Lokanathan Arcot 18 From Doppler Shift to Electrophoretic Mobility How to measure Electrophoretic Mobility U E ? We know Field Strength (V/cm) because we apply it Need to find Velocity of Particle Doppler Effect: Frequency increase or decrease Δfreq

19. Course 3130, Dr. Lokanathan Arcot 19 From Electrophoretic Mobility to Zeta Potential Helmholtz Smoluchowski equation

20. Course 3130, Dr. Lokanathan Arcot 20 Setup for Zeta Potential Nanoparticle Measurement Sample Cell Provision for applying voltage while measuring Velocity of particles From Velocity and Voltage we can calculate U E From U E we can calculate Zeta potential

21. Course 3130, Dr. Lokanathan Arcot 21 Nanoparticle Zeta Potential Measurement Sample Cell Observer/ Detector

22. Course 3130, Dr. Lokanathan Arcot 22 Stern plane Ψ0Ψ0 Debye Length (Low) ΨdΨd ζ Distance from charged surface Electrical potential High Electrolyte Low Electrolyte Diffuse layer High ionic strength Or High electrolyte Debye Length (High) ζ Electrolyte Shear Plane Principles of Colloid and Surface Chemistry, Chapter 12, 3rd Edn, Paul and Raj Double Layer Model – Effect of Ionic Strength

23. Course 3130, Dr. Lokanathan Arcot 23 Double Layer Model – Ionic Strength Principles of Colloid and Surface Chemistry, Chapter 12, 3rd Edn, Paul and Raj Debye Length (1/κ) – The distance over which the potential 1/e of surface potential Ψ 0 ε 0 – Permittivity of space ε r – Permittivity of medium Κ – Boltzmann Constant T – Temperature N A – Avagadro Number I – Ionic Strength The Zeta Potential Measured will decline with increasing ionic strength So? If you are comparing surface charge of two surfaces, make sure the I are same.

24. Course 3130, Dr. Lokanathan Arcot 24 Effect of pH on Zeta potential Dissociation or ionization of a surface group depending on Isoelectric pH (NH 3 ) + COOH COO - Neutral Anionic pH > PI COOH COO - NH 2 (NH 3 ) + Neutral Cationic pH < PI Cationic surface Anionic surface Example PI 9.0 Example PI 4.0 pH = 0 - 8pH = 9 - 14 pH = 0 - 4pH = 5 - 14 NH 2 COOH Discussion: Predict the sign of Zeta potential 2.0 COOH, NH 3 + + Ve 6.0COO –, NH 3 + + or – Ve depending on relative conc. 11.0COO –, NH 2 - Ve pH Zeta signGroups

25. Course 3130, Dr. Lokanathan Arcot 25 Zeta Potential of Nanoparticles and Large Surfaces 1 nm – few 1000s nm Macroscopic cm Nanoparticle Size Electrophoresis Surface Zeta Potential Streaming Potential

26. Course 3130, Dr. Lokanathan Arcot 26 Streaming Potential Negatively Charged Surface Shear Plane Solvent Flow Potential Develops A flow of fluid over charged surface removes the counter-ions at and above the shear plane Imbalance of charge in double layer causes a potential

27. Course 3130, Dr. Lokanathan Arcot 27 Streaming Potential: Parallel Plate Capillary System Streaming Current I S Conduction Current I C ΔPΔP V / I C Apply Pressure (ΔP) to make water flow Counterions from ’diffuse layer’ above shear plane are removed (Streaming Current I S ) Streaming potential causes a flow of ions in Stern Layer in direction opposite to flow of liquid constituting the Conduction Current I C Creates a potential (Streaming Potential V)

28. Course 3130, Dr. Lokanathan Arcot 28 Streaming Potential: Rotating Disk System

29. Course 3130, Dr. Lokanathan Arcot 29 Zeta Potential and Streaming Current/Potential Electrophoresis 2004, 25, 187–202 Zeta Potential from Streaming Current – Zeta Potential V = streaming potential (V) ΔP = pressure difference across the channel = viscosity of the solution (kg/m/s) ε r = relative permittivity of the solution (-) ε 0 = electric permittivity of vacuum (F/m) σ = conductivity of polyelectrolyte solution (S/m) – Zeta Potential I S = streaming Current R = Resistance of channel ΔP = pressure difference = viscosity of the solution ε r = relative permittivity of the solution ε 0 = electric permittivity of vacuum σ = conductivity of polyelectrolyte solution Zeta Potential from Streaming Potential These equations hold only for thin double layers

30. Course 3130, Dr. Lokanathan Arcot 30 Ways to Find Surface Charge Not to be confused with Surface Potential Titrations: Anionic – Cationic Titration Estimating anionic surface by titrating against cationic polymer Acid Base Titration Estimating Carboxyl surface surface by titrating against Base NaOH

31. Course 3130, Dr. Lokanathan Arcot 31 Example of Zeta Potential and Surface Charge Soft Matter, 2008, 4, 2238–2244 | 2239 Cationization of Cellulose NanoCrystals (CNC) Negatively Charged due to Sulfate groups e – Sulfate groups e – Chloride The number of anionic groups is higher than the cationic groups introduced

32. Course 3130, Dr. Lokanathan Arcot 32 SUMMARY of Part I  Origin of Surface Charge  Surface Charge and Colloidal Stability  Surface Charge and Surface Potential  Electrical Double Layer Model Shear/Slipping Plane  Electrokinetic Phenomena Phoresis, Osmosis, Streaming  Nanoparticle Zeta potential (Laser Doppler Velocimetry)  Surface Potential - Streaming Potential/Current  Surface Charge – Titrations  Example of Surface Charge and Potential

33. Course 3130, Dr. Lokanathan Arcot 33 Short Break

34. Surface energy Atoms in bulk Atoms at surface Unsatisfied Bonding interactions What are bonding interactions? 34

35. Intermolecular interactions  Lifschitz-van der Waals-forces (LW). These occur between all atoms and molecules. They are due to correlation between the electromagnetic fields created around molecules due to the distribution and motions of electrons within them (permanent and fluctuating dipole moments)  Acid-Base interactions, AB) Interactions between electrophilic and nucleophilic groups *Only the LW forces are of importance at distances larger than a few Å  Electrostatic interactions and electrical double layer forces are also long range interactions 35

36. Lifshitz- van der Waals interaction between neutral molecules 1 and 2  The interaction is electromagnetic, it is always attractive and it occurs between all molecules r 12 A, B = coefficients that depend on molecular dipole moments, polarisabilities and temperature r 12 = intermolecular distance U = interaction energy  A general equation for the interaction energy is  When r is larger than a few molecular diameters, the interaction becomes negligible 36

37. Acid-base interactions Lewis’ acids and bases Acid: a group that tends to receive electron Base: a group that tends to donate electron  Acid-base interactions are of importance only at very short distances (0,1 - 0,5 nm) but are of essential importance for the adhesion between surfaces  Example: plasma treatment of cellulose Base Acid Acid Base 37

38. Surface energy Origin: Inter-molecular/atomic interactions Quantification: Liquid surface – Suface tension Solid surface – Surface energy 38

39. Surface energy of materials For solids, actual surface energy often depends on the history of the surface Surface energy = the work of the cleavage, it can depend on how material is cleaved Most common units of surface tension: r 39

40. SURFACE ENERGY MEASURING METHODS 40

41. Methods to measure surface tension of liquid F = force, mN L = wetted length, mm Liquid air Pt plate  http://www.kruss.de/ Wilhelmy plate method Du Noüy ring method Pt/Ir ring air F (force)  41

42. Contact angle measurement Methods to measure surface energy of Solid http://www.kruss.de/ Video microscope Light source Syringe Drop Lenses 42

43. Contact Angle  The contact angle can be measured relatively easily, but both absolute values of angles and reproducibility of measurements are influenced by porosity and surface heterogeneity  The most common way to measure the surface energy of solids  A liquid that does not completely wet a surface forms a finite contact angle on it  At equilibrium  sg =  ls +  lg cos  (Young’s equation)   lg  sg  = contact angle 43

44. θ – contact angle.  total surface tension,  d  Lifshitz van der Waals component,    Electron acceptor component,   Electron donor component. The subscript ‘S’ and ‘L’ stand for solid surface and liquid respectively. C. J. Van Oss et al., Chemical Reviews 1988, 88, 6, 927 Liquid  s  d mJ/m 2  s  mJ/m 2  s  mJ/m 2 Water21.825.5 Formamide392.2839.6  -Bromonapthalene4400 Surface energy componenets from CA  It is often useful to formally divide surface tension into contributions from LW and AB interactions  =  LW +  AB additionally,  AB  + &   44

45. Thermodynamics of adhesion interaction due to Lifschitz-van der Waals’ forces (dispersion, dipole/dipole) interaction due to acid/base interactions Acid: acceptor of electrons (e.g. -NH 3, -OH) Base: donor of electrons (-C=O.  -electron systems) -ve adhesion +ve no adhesion 45

46. Solid Surface chemical interactions Quantification: Components can be determined experimentally by Contact angle measurements (different liquids) Measurements of adsorption from liquid Inverse gas chromatography  LW  + &   Cationic, anionic charge Surface charge can be experimentally determined by Conductometric titrations Zeta potential measurement (Potential) Other properties: Magnetic, Gravity 46

47. Course 3130, Dr. Lokanathan Arcot 47 Lotus Flower Mechanism of Water Repellance What about physical properties of surface? Topography

48. How do we make use of surface energy information? Example: Bacterial Adhesion to Surface Zhang et. al. RSC Adv., 2013, 3, 12003-12020 Designing a surface that will resist bacterial adhesion! 48

49. Designing a non-sticky surface http://www.wpi.edu/Pubs/ETD/Available/etd082206162049/unre stricted/Arzu_Atabek_MSthesis.pdf 49

50. Designing a non-sticky surface WDS'10 Proceedings of Contributed Papers, Part III, 25–30, 2010. -ve adhesion +ve no adhesion 50

51. Examples of surface energy manipulation of renewable materials Lignin 51

52. How to change surface chemistry and surface topography simultaneously? 52

53. 53 Coating paper with hydrophobic nanoparticles Chemistry & topography PAH – Poly(allylamine hydrochloride) Polycation + - - - - - - SiO 2 PAH Anionic surface Cationic surface + + + + AKD – Alkyl ketene dimer Silica SiO 2 AKD Hydrophobic Cationic Paper -ve charge

54. Contact angle of water on fine paper surface coated with hydrophobically modified silica nanoparticles The rough paper surface becomes superhydrophobic !!! 54

55. Contact Angle 65 0 ±2 0 Contact Angle 150 0 ±2 0 Contact Angle 110 0 ±2 0 Lignin surface Lignin surface coated with 1g/100ml Aerosil R972 Lignin surface coated with 2g/100ml Aerosil R972 Effect of hydrophobic silica on the surface structure and contact angle (lignin model surface) 55

56. Course 3130, Dr. Lokanathan Arcot 56 Advancing Contact Angle Receding Contact Angle Contact Angle Hysterisis: Dynamic Vs Static Contact Angle Add/Remove Volume Method Tilted Plane Method

57. Course 3130, Dr. Lokanathan Arcot 57 Advancing Contact Angle Receding Contact Angle Contact Angle Hysterisis Hysterisis is the difference between Advancing and Receding Contact Angle H = θ a - θ r Mechanism: Pinning of Liquid Front Causes of Pinning: High rougness Chemical Inhomogeneity Low HHigh H Example of Chemical Homogeneity

58. Summary of Part II Origin of Surface Energy Definition of Surface Energy and Surface Tension Methods to Measure Liquid Surface Tension Method to Measure Surface Energy of Solid Surface Surface Energy Components Thermodynamics of Adhesion Physcial property of Surface and Surface Energy Examples – Bacterial Adhesion - Lignin Hydrophobization Dynamic Contact Angle - Hysterisis Course 3130, Dr. Lokanathan Arcot 58