Ionic Liquids, Thermodynamics and Mechanisms of Cellulose Dissolution

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1 Doctoral Course on Cellulose Chemistry (PUU Aalto University) Ionic Liquids, Thermodynamics and Mechanisms of Cellulose Dissolution Alistair W. T. King (Docent in Organic Chemistry, University of Helsinki) Osasto / Henkilön nimi / Esityksen nimi

2 What are Ionic Liquids?

3 Definitions (IL, RTIL.etc.)

4 Why are Ionic Liquids Liquid? Unsolvated Molten Salts (m.p.) NaCl 801 o C [mmim]cl 125 o C [emim]cl 87 o C [emim][ntf 2 ] -3 o C Asymmetry, Diffusivity & Ion Size

5 Mathematical Formalism Z + & Z - : decrease when you have delocalised charges D = r + + r - : increase when you have larger ions A (Madelung Constant) : a measure of the packing efficiency (increases with irregularly sized or asymmetric ions). In a way it is a measure of the entropy change. S L (Gibbs Free Energy of Crystallisation) : apart from A, entropy also has some effect, in particular with long side-chains, although there is often conformational freedom even within crystalline compounds

6 Lattice Energies & MPs Averill & Eldredge, Principals of General Chemistry, v1.0

7 Low Vapour Pressure van der Waals (weak, short distance. steric interaction) H-bonding (strong, short distance. ~ 2 Å) Other dipolar interactions Columbic (strong, long distance up to 10 Å) Columbic Force Z + Z - / r 2 (inv. Sq. dist.) Long distance columbic interaction extends through the crystal or liquid and provides a strong cohesive force High charge density gives high cohesive energy density (CED) and high hence enthalpy of vapourisation ( H V ) Hildebrand Solubility Parameter

8 Cellulose Dissolution: Important Bonding Interactions [amim]cl Swatlowski et al., JACS, 2002, Zhang et al., Macromol., 2005, 8272.

9 Molten Salts for Cellulose Dissolution

10 Different Ionic Liquids for Cellulose Dissolution

11 Different Non-Derivatising Electrolytes for Cellulose Dissolution Show Lower-Critical Solution Temperature (LCST) Behaviour

12 Solubility in Sodium Hydroxide (Lower-Critical Solution Temperature) Soluble phase Sobue et al., Z. Phys. Chem., 1939, 43, 309.

13 What is the difference? Solubility in [P 4444 ][OH] (aq) (LCST or UCST behaviour is not observed)

14 Solvent Properties & Solvation Parameters

15 Bonding Interactions Covalent Bond: Electrons are shared between atoms over only a short distance. C- O bond energy is ~ 360 kj/mol Ionic Bond: Electrostatic interaction between static charges, e.g. NaCl (s) lattice energy is 756 kj/mol. Force decreases (1/r 2 ) over a longer distance (10Å) Hydrogen-Bond: Transfer of charge density to a Lewis acidic hydrogen atom to give a more persistent dipole-dipole interaction. They typically operate over ~ 2 Å and is are ~ 20 kj/mol for a O-H O that you typically find between cellulose AGUs. van der Waals: Very transient or induced electrostatic interactions between poorly polarisable species. Typically kj/mol at 3Å

16 Solvent Classification Polar vs Non-Polar - 2 Brønsted Acidic vs Basic Lewis Acidic vs Basic Protic / Aprotic / Ionic H-bond Acidic vs Basic

17 Common Parameter Scales Dielectric Constant: 1 parameter polarity scale, experimentally determined Hildebrand Parameter: 1 parameter polarity scale, applied to polymer solvation, like dissolves like, Square root of cohesive energy density (measure of cohesion between solvent molecules), experimentally determined from viscosity or enthalpy of vapourisation data Kamlet-Taft Parameters: 3 parameter polarity scale, experimentally determined and applied for all areas of chemistry (prediction of reactions and polymer solubility) Proton Affinity: 1 parameter scale, can be computed in a matter of hours to weeks for different species, a measure of Brønsted acidity, enthalpy of deprotonation in the gas phase.

18 Kamlet-Taft Parameterisation Solvatochromatic Probes Reichardt s Dye 4-Nitroaniline N,N-Diethyl-4-nitroaniline UV Solvatochromatic Shift π* = polarisability β = H-bond basicity α = H-bond acidity Kamlet, Taft et al. Linear Solvation Energy Relationships 1-40

19 Kamlet-Taft Parameterisation (H-bond accepting) (H-bond donating) (polarisable)

20 Application to LiCl/DMA* (Classical Cellulose Solvent) LiCl/DMA α = 0.57 β = 1.15 π* = 0.83 Interaction strength/energies can be related to the product of opposite parameters between solvent (LiCl/DMA) and solute (cellulose) *Spange et al. J. Polym. Sci. A: Polym. Chem. 1998, 36, 1945.

21 Some Values for Molecular Solvents and Ionic Liquids

22 First Time Used on Ionic Liquids

23 Cellulose Dissolution and Regeneration Chloride, phosphate, phosphonate acetate and N-Oxides are all basic species in the absence of a protic environment

24 Effect of Water on Parameters Water, as a protic solvent, decreases basicity of the media initiating regeneration

25 Parameter Space for Dissolution

26 Proton Affinity Calculations (enthalpy of deprotonation in the gas-phase) [HB] + (g) à B: (g) + H + (g) (conjugate acid) (unconjugated base) (proton) ZPE = zero-point energy, E ELEC = electronic energy (main contribution)

27 Anions Proton Affinities with [emim] + Cation Thermal Stability vs Proton Affinity Dissolution vs Proton Affinity *King et al. RSC Adv. 2012, 8020.

28 Cation Proton Affinities with Propionate Anions Parviainen et al. ChemSusChem, 2013, 6, 2161

29 Effect of Increasing Cation Acidity

30 Thermodynamics

31 Thermodynamic Considerations The enthalpy gain from breaking H-bonds is important for cellulose dissolution for the wide range of basic ionic solvents : ionic liquids, electrolytes & NMMO H 2 O Therefore, we need to start thinking about the Gibbs equation G diss = H diss - T S diss G diss = Gibbs free energy of dissolution H diss = Enthalpy of dissolution S diss = Entropy of dissolution T = Temperature G < 0 for dissolution to occur! Or in other words H < T S

32 Enthalpy of Reaction ( H) Enthalpy is a measure of the net energy from transitioning from one state to another, e,g: Enthalpy of mixing Enthalpy of fusion/sublimation/vapourisation Enthalpy of reaction In an organic reaction this is the net energy that is produced during bond breakage and formation on going from reactants to products (chemical energy) A reaction can either release or store energy in a spontaneous reaction, i.e. H alone not predict if a reaction goes forward or backwards! Exothermic ( H is negative) vs Endothermic ( H is positive)

33 Entropy of Reaction ( S) (Boltzmann s Entropy Equation) S = Entropy of an ideal gas (particles with a distribution of kinetic energy levels) W = number of accessible states (i.e. accessible energy levels) S = change in accessible states to distribute energy over T S = energy spread over the accessible states

34 Entropy of Reaction ( S) Kinetic Theory of Gasses Matter is a collection of atoms with different kinetic energy levels Origns of thermodynamics à 2 nd Law Boltzmann partition function Entropy equation

35 Boltzmann s Grave Brahms Beethoven Schubert Strauss (Jr) Mozart Strauss (Sr)

36 Entropy in Chemistry Boltzmann s Entropy Equation: Entropy is a measure of the freedom of motion in a system or more precisely a measure of how the energy can be partitioned over the available energy levels For a particular substance (starting material or product) under certain equilibrium conditions the entropy (energy per mol*kelvin) can be sub-divided into 5 main components: Electronic, Translational, Rotational, Vibrational & Configurational Entropy Energy levels G diss = H diss - T S diss

37 Entropy Contributions Translational Entropy Rotational Entropy Vibrational Entropy Configurational (Conformational) Entropy 6 rotatable bonds, 3 favoured torsional angles à 3 6 = 729 Conformers S = k b ln ~730

38 Entropy of Mixing Flory Huggins Theory (Ideal Mixing Situation) (Total number of units) N 1 = number of free units N 2 = number of linked units x = polymer length (Mol fractions)

39 Entropy of Mixing (2 Liquids) S = k b ln W (Boltzmann s Equation) k b = Boltzmann constant W= number of accessible states S 1 <<< S 2 ( S mix )

40 Entropy of Mixing (1 Liquid + 1 Flexible Polymer) S = k b ln W (Boltzmann s Equation) k b = Boltzmann constant W= number of accessible states S 1 << S 2 ( S mix )

41 Entropy of Mixing (1 Liquid + 1 Rigid Polymer) S = k b ln W (Boltzmann s Equation) k b = Boltzmann constant W= number of accessible states S 1 < S 2 ( S mix )

42 Entropy of Mixing Flory Huggins Theory (Ideal Mixing Situation) 400 (Total number of units) N 1 = number of free units N 2 = number of linked units x = polymer length k N x N1 N2 S Entropy of Mixing, S mix Degree of Polymerisation, DP

43 Entropy of Mixing vs Chain-Length DP cellulose is soluble in water DP < 6!* Cellobiose has restricted bond rotation so S from crystalline state to solution is rapidly diminishing as DP increases. S = k b ln W (Boltzmann s Equation) k b = Boltzmann constant W= number of possible states Taylor, Trans. Faraday, Soc., 1957, 53, 1198

44 Entropy Contributions Translational Entropy Rotational Entropy Vibrational Entropy Configurational (Conformational) Entropy 6 rotatable bonds, 3 favoured torsional angles à 3 6 = 729 Conformers S = k b ln ~730

45 Rigidity of Cellobiose Glycosidic Linkage (MD) ψ & φ are dihedral angles between adjacent glucose units ω & υ are conformers of the C6 OH Shen et al., J. Am. Chem. Soc., 2009, 14786

46 Cellobiose Configurational Entropy (MD Study) beta(1à4) linkage gives lowest configurational entropy! Pereira et al., Biophys. J., 2006, 4337

47 Cellulose Rigidity Strong C 3 OH O 5 & C 2 OH O 6 Hydrogen-Bonds

48 Effect of H-bond Accepting (Basic) Ionic Liquids on Configurational Entropy of Cellulose (MD Study) [bmim]cl Kamlet-Taft beta value = 0.95 H 2 O Kamlet-Taft beta value = 0.14 Mostofian et al., J. Phys. Chem. B., 2014, 11037

49 Hard Anions Weaken H-bonds

50 Bonding Interactions (Ab initio MD) [emim][oac]:cellobiose Acetate Emim-cation Payal et al., Phys. Chem. Chem. Phys., 2014, 17458

51 Mechanism of Chain Liberation in [bmim]cl (MD study) Anions bind strongly to the hydroxyl groups of the exterior strands of the bundle, forming negatively charged complexes. Chloride Binding also weakens the intrastrand hydrogen bonds present in the cellulose strands, providing greater strand flexibility. Cations then intercalate between the individual strands, likely due to charge imbalances, providing the bulk to push the individual moieties apart and initiating the separation. [bmim]-cation Rabideau et al., J. Phys. Chem. B, 2013, 3469

52 Cellulose Crystallinity (Cellulose Iα) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

53 Cellulose Crystallinity (H-bonding contacts) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

54 Cellulose Crystallinity (Expanded HB Contacts) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

55 Cellulose Crystallinity (space filling - van der Waals radii) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

56 Cellulose Crystallinity (space filling - van der Waals radii) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

57 Cellulose Crystallinity (expanded short vdv contacts) Nishiyama et al., J. Am. Chem. Soc., 2003, 14300

58 Cellulose Crystallinity (Planes of interaction) y (H-bonding) z (van der Waals) z (Covalent) H-bonding in one plane, van der Waals in another plan and the polymer (covalent interactions) in the 3rd plane

59 [TMGH][OAc] Crystal Structure (Cellulose-dissolving ionic liquid) King et al. Angew. Chem., 2011, 50, 6301

60 Hydrophobicity

61 The Hydrophobic Effect* Solvation of hydrophobic materials by water restricts the conformational freedom (decreasing entropy) of water due to cage formation Therefore minimisation of surface area to volume ratio of hydrophobic cavities allows for higher entropy! i.e. phase-separation or aggregation LCST behaviour in aqueous solutions is due to the hydrophobic effect! *Chandler, Nature. 2005, 640.

62 Hydrophobic Effect? G diss = H diss - T S diss T S G > 0 G = 0 ( H = T S) G < 0 Soluble phase Sobue et al., Z. Phys. Chem., 1939, 43, 309.

63 What is the difference? Solubility in [P 4444 ][OH] (aq) (LCST or UCST behaviour is not observed)

64 Cellulose Dissolution in NMMO.xH 2 O (MD Study) Calculation of enthalpy and entropy changes upon increasing water content Rabideau et al., J. Phys. Chem. B. 2015,

65 Pairwise Energy DifferenceBetween Crystalline & Dissociated States Pure Water NMMO:H 2 O NMMO ( N ) Cellulose ( C ) Water ( W ) All Constituents ( Tot ) NMMO affords hydrophobic interactions that are not available in pure water. Rabideau et al., J. Phys. Chem. B. 2015,

66 Hydrophobic Effect? Soluble phase? Quellen swollen Swollen phase where hydrophobic surfaces interact. Sobue et al., Z. Phys. Chem., 1939, 43, 309.

67 Summary Different contributions to the thermodynamics of solvation of cellulose from the crystalline state to solution state are apparent, G = H T S: Enthalpy: basic and van der Waals interactions provide enthalpy gains to the Gibbs free energy of mixing too acidic mixtures decrease the enthalpy gain from H-bonding Entropy: cellulose rigidity means increases in molecular weight incur a severe configurational entropy penalty (aqueous) basicity and hydrophobicity both ease this issue by stabilising intramolecular H-bonds and van der Waals surfaces.

68 Physical-Kinetic Limitations when raw cotton fibres are placed in certain swelling agent, the radial expansion of the cellulose in the secondary wall causes the primary wall to burst. As the expanding swollen cellulose pushes its way through these tears in the primary wall, the latter rolls up in such a way as to form collars, rings or spirals which restrict the uniform expansion of the fibre and forms baloons Balooning & Helicies