An Introduction of Salt-Free Systems 曾琇瑱 淡大數學系 徐治平 臺大化工系

1. An Introduction of Salt-Free Systems 曾琇瑱 淡大數學系 徐治平 臺大化工系

2. Charged entity (surface) in an electrolyte solution

3. + - + - + - + - + - + - Salt-free Dispersion • The dispersion medium contains no or negligible amount of ionic species except those dissociated from the dispersed entities.

4. H2O NaCl Na+ + Cl- Ex. Polyelectrolytes • Polyelectrolytes are polymers bearing dissociable functional groups, which, in polar solvents (water), can dissociate into charged polymer chains (macroions) and small counterions. • Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. PAA (polyacrylic acid) + H2O + H3O+ -

5. Synthetic Polyelectrolytes • PSS (polystyrene sulfonate ) A flexible polyelectrolyte Counterion Microion (charged backbone) Constitution formula Simulation model

6. PPP (polyparaphenylene) A stiff polyelectrolyte Counterion Microion (charged backbone) Constitution formula Analytical model

7. Natural Polyelectrolytes • Proteins Primary protein structure is sequence of a chain of amino acids Amino acids NH2 Amino acid COOH R 1/4 basic units may be ionized : basic : lysine, arginine, histidine (-NH2+) acidic : aspartic, glutamic (-COO-) 0.4 nm Linear charge density = 1 /( 4  4 Å) ≒ 0.6 e / nm

8. 2 nm • Deoxyribonucleic acid (DNA) 0.34 nm Linear charge density = 2 e / 3.4 Å ≒ 6 e / nm  One of the most highly charged systems

9. Applications • Polyelectrolyte gel for artificial muscles, cartilage, organs, etc. • Hydrophobically modified polyelectrolytes and polyelectrolyte block-copolymers for biomedical applications • Ion exchange resins for separation, purification, and decontamination processes • Controlled drug delivery • Composite polyelectrolyte self-assembled films for sensorapplications • Layer-by-layer polyelectrolyte-based thin films for electronic and photonic applications • Polyelectrolyte multilayer membranes for materials separation • Nanostructures of Polyelectrolyte–Surfactant Complexes

10. SDS (sodium dodecyl sulfate) Oil loving tail Water loving head Counterion Hydrophobic group Hydrophilic group 2 nm Ex. Surfactants (like a tadpole) • An amphiphilicmolecule! A surfactant consists of a hydrophobic (non-polar) hydrocarbon "tail" and a hydrophilic (polar) "head" group.

11. Conc. Monomers 14 12 10 8 CMC 6 Micelles 4 2 0 0 1 Surfactant conc. Monolayer CMC Micelle Self-Assembly • In order to minimize interactions between solvent and the insoluble portion of an amphiphilic molecule, the monomers aggregate into ordered structures. Below CMC only monomers are present • Above CMC there are micelles in equilibrium with monomers • After that, they can act as emulsifiers

12. Surfactant Aggregates Surfactant Reverse cubic phase Cubic phase Lamellar phase Hexagonal phase Cylindrical/Rod-like micelles Irregular bi-continuous phase Sphericalmicelles Reversemicelles Monolayer Water Oil Monolayer Phase diagram of a surfactant-water-oil ternary solution

13. Oil (Non-polar) = 50 % S Surfactant= 25 % Water = 25 % 75 25 50 50 25 75 W O 75 50 25 Only certain region creates reverse micelles

14. Applications • As wash and cleaning reagents

15. Oil-in-water emulsion (micelle) Water-in-oil emulsion (reverse micelle) • As emulsion stabilizers

16. Emulsion polymerization • As micro/nano-reactors for material synthesis (organic or inorganic particles)

17. Microemulsion 1 Aq. phase of metal salt Mix 1. Fusion 2. Exchange 3. Reduction 4. Nucleation 5. Growth Metal or metal oxide nanoparticle Aq. phase of reductant Microemulsion 2 silver colloids (yellow), gold colloids (red) and silver-core, gold-shell particles (violett)

18. Preparation of nanotubes via surfactant micelle-template

19. As containers for targeted drug delivery

20. Problem I – Stability of a micelle system oil oil Water phase contains counterions • Electrical potential outside a micelle (ionic surfactant) • Total interaction energy between two micelles • Critical coagulation (coalescence) concentration • Various shapes: planar, cylindrical, spherical

21. Problem II – Ionic distribution inside a reverse micelle water phase contains counterions water • Electrical potential inside a reverse micelle • Ionic distribution • Presence of other entity • Influence of ionic size

22. stable unstable Stability of a Colloidal Dispersion DLVO Theory: Total interaction energy VT = Electrical (repulsive) energy VR + van der Waals (attractive) energy VA Stable system VT > 0; Unstable system VT < 0

23. Increase in electrolyte concentration Double Layer Compression Mechanism

24. concentration gradient electrical gradient Electrical Double Layer • Electrical potential • Ionic distribution

25. Only the electrostatic stabilization is considered Steric stabilization is neglected oil oil Problem I

26. a0: radius of particle b :valence of counterions r: distance from particle center - + + + - - + a0 + O r + + + - + - - Analysis Analytical model =charged backbone+dissociated counterions oil

27. - + + Let + - Form factor ω=0, θ=0: planar ω=1, θ=1: cylindrical ω=2, θ=1: spherical - + a0 + O r + + + - + - - Poisson-Boltzmann Equation

28. boundary conditions - + + + - - + a0 + O r + + + - + - -

29. Multiplying both sides by 2(dy/dx) gives Planer micelle (ω=0)

30. Let If , Therefore, Spherical micelle (ω =2)

31. Let K0,K1=zeroth and first-order modified Bessel function of the second kind Cylindrical micelle (ω =1)

32. If x/a<<1 and , then

33. x/a << 1 x/a << 1 Potential distributionnear a single micelle

34. boundary conditions If then Potential distributionbetween two identical micelles using analogy yspherical = ycylindrial =

35. Electrostatic Interaction Energy • Osmotic pressure • Electrical energy (repulsive force) • Derjarguin approximation (planar spherical)

36. DLVO theory • Total energy: At CCC: D=Dc is a critical distance

37. Present result Correction factor Fa,c=Fa,c(b,ys,ym,c) Comparison with Schulze-Hardy rule Schulze-Hardy rule

38. 0.839

39. Problem II water Distribution of ions in a submicron-sized reverse micelle

40. cylindrical or spherical cavity planar slit Analysis

41. Results

43. planar slit outside inside outside inside

44. Effect of Ionic Size Tsao, Sheng, and Lu, J. Chem. Phys. 113, 10304 (2000) – Ionic size becomes unimportant when (R/a)>40

45. Distributions of Electrical Potential and Ionic Concentration* * Borukhov, Andelman, & Orland, Electrochimica Acta. 46, 221 (2000)

46. Main Results

47. a=0 nm Xs=0.519 R=15 nm a=0.5 nm Xs=0.641 a=1 nm Xs=0.775 =1 nm-2, Kd=5 nm-3 1.Neglecting size effect will underestimate the charge density on surfactant shell 2. Size effect is inappreciable if (R/a) exceeds about 40

48. a=0 nm 0.5 nm 1 nm =1 nm-2, Kd=5 nm-3 The larger the XS, or the larger the size of counterions, the greater the deviation in CS if the size of counterions is neglected

49. =1 nm-2 Kd=5 nm-3 a=0.5 nm a=0.7 nm Kd=1 nm-3 Increase in the size of a reverse micelle has the effect of raising the degree of dissociation of surfactants; XS reaches the equilibrium value when R/a exceeds a certain value

50. Kd=1 nm-3 a=0 nm a=0.3 nm Kd=10 nm-3 =free energy per surfactant molecule due to dissociation The larger the counterions, the smaller the chemical potential, i.e., the steric effect of counterions is positive to the decrease of chemical potential