Polymorphism in the Long-chain n -Alkylammonium Halides and Related Compounds

1. Polymorphism in the Long-chain n-Alkylammonium Halides and Related Compounds Studied by a Combination of X-Ray Diffraction and Thermal Analysis Methods Gert Kruger, Dave Billing, Melanie Rademeyer

2. My Polymorphism Credentials (From Ancient Times)

3. Outline • Introduction to what is of interest to us • Alkylammonium halides • Some crystal structures • The use of powder diffraction and thermal analysis • Further examples

4. The Light Source of Africa • The Candle

5. SASOL – South Africa’s Producer of Synthetic Fuels and Waxes Synthetic waxes are produced by Fischer-Tropsch technology. Output from the Sasolburg plant: 730 Kt per year including hard and medium waxes and liquid paraffins in the C5-C20 range.

6. SASOL – Synthetic Fuels and Waxes from Coal Liquid fuels are produced at two huge plants in Mpumalanga. At Sasolburg industrial chemicals and waxes are produced in the new 10.5 meter diameter Sasol Advanced Synthol (SAS) reactor shown in front of the Circulating Fluidized Bed (CFB) reactor it replaces.

7. The Commercial Importance of the Wax Industry • Candles • Polishes • Cosmetics • Fruit coatings

8. Waxes and their Components • Natural Waxes • This group includes plant, animal, mineral waxes • They contain alkanes but also esters, alcohols, acids • Synthetic Waxes • From Fischer-Tropsch and other synthetic routes • Contain normal alkanes, isoalkanes, cycloalkanes • Petroleum waxes • A similar blend of paraffins from crude oil

9. Our aims • To understand the factors involved in the crystal packing of synthetic and natural waxes • To mimic the desirable properties of expensive natural waxes by suitably modifying synthetic waxes • To achieve this we model natural waxes by a range of long-chain substances showing extreme inter-molecular interactions

10. The polymethylene chain in: Decane, C10H22 Octadecanol, C18H37OH D-12-Hydroxyoctadecanoic acid methyl ester, C18H36OHCO2CH3 Dioctadecyl tetrasulfide, C36H74S4 Examples of Alkanes and Substituted Alkanes

11. What do we know about their crystal packing? • Fundamental work on general packing considerations by many authors • Experimental work over the past fifty years using diffraction and spectroscopy

12. Kitaiigorodskii – Closest Packing - Bumps and Hollows Plane Groups: p1, p2, pm

13. Kitaiigorodskii – Structure of Normal Paraffins Configuration of an aliphatic chain Minimum energy - the flat zig-zag carbon chain

14. Kitaiigorodskii – Close Packing of Chain Molecules Three possible types of packing: Hexagonal, oblique, rectangular cell

15. Kitaiigorodskii – Sideways Packing of Normal Paraffins Types of close-packed arrays of aliphatic chains

16. Kitaiigorodskii – End Packing of Normal Paraffins • Adjacent layers never stack through mirror plane • Single-layer structures give skew unit cell • Double-layer structures give orthorombic cells

17. Alkane Packing Example • n-Decane - packing like the stacking of pencils or cigarettes in a box

18. Styles of Packing in the Polymorphs of n-alkanes Triclinic, n even (CnH2n+2 6<n<26) Orthorhombic, n odd (11<n<39) Monoclinic, n even (28<n<36)

19. n-Alkane Subcell Orthorhombic O

20. Polymorphism in long-chain compounds • Exhibited by most long-chain compounds • Types: • Stacking differences • Conformational polymorphism • Solvates • Polymorph-dependent physical properties include: • hardness • solubility • changes in melting point • density • compressibility

21. n-Alkyl Ammonium Salts In a recent project we tried to prepare, crystallize and characterize as many crystal forms as possible of the series of compounds: • with extended long chain or cyclic alkane (n>10) • introduce H-bonded layer with X = Cl-, Br-, I-, phosphate, sulphate, etc. • also organic/inorganic hybrids with PbI2, etc.

22. Why Study n-Alkyl Ammonium Halides if we are really interested in Waxes? • Long-chain alkyl ammonium halides are good model compounds for the study of wax components and their intermolecular interactions • The ionic end groups form extended planar H-bonded networks that anchor the paraffinic chains, much like slanted columns on a flat platform • These compounds are much easier to crystallize than the alkanes, giving us a crystallographic grip on the problem

23. n-Alkyl Ammonium Halides – Typical Crystal Packing • They crystallize with ammonium and halide layers; hydrocarbon layers

24. Crystallization Strategies • Two-fold aim: • to obtain good quality single crystals • and as many polymorphic forms as possible. • Crystallize at different temperatures • e.g. room temperature, refrigerator (3ºC), freezer (-10ºC), hot solvent, from the melt • Use solvents with different polarities • Vary solvent evaporation rate • Employ solvent and vapour diffusion techniques

25. Experimental Methods Employed or Considered • X-ray diffraction - single crystal & powder techniques • Thermal analysis - DSC and TGA • “Hot-stage” thermal microscopy • Electron microscopy & diffraction • AFM - “Atomic Force Microscopy” • Solid state NMR • Molecular modelling • Energy calculations

26. Previous Work • Many authors contributed to the rich literature on the subject, mostly work on the short-chain chlorides • Solid-solid phase transitions on heating: • Chlorides: Tsau and Gilson (1968); Busico et al, (1983); Terreros et al, (2000) • Bromides: Tsau and Gilson (1968) • Structural information: • PXRD and TA: Tsau and Gilson (1974) • Chlorides: Schenk and Chapuis, 1986; Pinto et al, 1987;Silver et al (1996) • Bromides: Lunden (1974) • Di-alkyl Bromides: Nyburg (1996) • Thermal Analysis, NMR, etc. – many more

27. Common Structural Forms in the Alkyl Ammonium Halides • Phase transitions similar to those of n-paraffins • Chain kinks give additional low-temperature conformational polymorphs Temp

28. Polymorphic Forms of n-Alkylammonium Halides at Room Temp lamellar thickness – long spacing i – tilted, interdigitated k – kinked, non-interdigitated • - tilted, non-interdigitated

29. Polymorphic Forms of n-Alkylammonium Halides at High Temp Temperature • - perpendicular, non-interdigitated, rotating Liquid crystal, hydrocarbon chains melted • - perpendicular, non-interdigitated

30. Our Single Crystal Structure Determinations • n-Undecylammonium bromide monohydrate (C11Br.H2O) • n-Tridecylammonium bromide monohydrate (C13Br.H2O) • n-Tetradecylammonium bromide monohydrate (C14Br.H2O) • n-Pentadecylammonium bromide monohydrate (C15Br.H2O) • n-Hexadecylammonium bromide monohydrate (C16Br.H2O) • n-Octadecylammonium bromide monohydrate (C18Br.H2O) • n-Hexadecylammonium chloride (C16Cl) • n-Octadecylammonium chloride (C18Cl) • n-Octadecylammonium iodide (C18I) • Platy habit of the crystals formed made it very difficult to obtain single crystals big and perfect enough for single crystal X-ray studies.

31. Focus on the C18 Polymorphs:First the C18 Chlorides (C18Cl)

32. n-Octadecylammonium Chloride Kinkedk Form C18Cl-k single crystals grown from methanol at room tempSMART CCD data, structure refined to an R-factor of 0.083crystal system: orthorhombic, space group: Pna21cell: 70.90 x 5.45 x 5.36 Å, Z=4

33. n-OctadecylAmmonium Chloride Fully Extended i Form • Crystallized from methanol, determined from powder diffraction data (lab diffractometer data) followed by Rietveld refinement • Space group: P21 • Cell: 5.655, 7.214, 24.573 Å, 93.07 degrees • R (weighted profile) 8.15 % • R (Bragg)/ 3.14 %

34. n-Octadecyl Ammonium Bromide Hydrate • single crystals grown from hexane at room temperature • structure determined at room and low temperatures • refined to an R-factor of 4.5% • crystal system: monoclinic, space group: Cc • cell: 4.803 x 58.192 x 7.909 Å, β = 105.86 deg, Z=4

35. n-OctadecylAmmonium Iodide Triclinic, P1bar a = 6.4799, b = 7.1515, c = 22.941,  = 98.610,  = 90.763,  = 91.466

36. Molecular Conformations: Deviations from the Ideal C18Cl k-form – gauche bond between C2 and C3 C18I i-form –bond between C3 and C4 rotated 10 deg

37. Packing in the Polymorphs Observed crystal forms: • i • m • k • a

38. Non-interdigitated C18Cl-k packing

39. Interdigitated C18Cl-i packing

40. Interdigitated C18Br hydrate packing

41. Interdigitated C18I packing

42. Typical Chain Tilting C18Cl-k

43. Packing Examples

44. N-H…Cl interactions Average N-H-Cl bond values: • H-Cl = 2.3 Å • N-Cl = 3.2 Å • Bond Angles: N-H-Cl = 170°

45. Hydrogen Bonding Network in the Bromides Interaction distances in C18Br.H2O Two N-H...Br interactions, one N-H...O interaction and two O-H...Br interactions

46. N-H…I interactions • Average N-H-I bond values: • H-I = 2.7 Å • N-I = 3.5 Å • N-H-I = 169° (136 °)

47. C18I Ionic Layer

48. Remarks on the use of Powder XRD and Thermal Analysis • Determination of crystal structures • Identification of polymorphs • Identification of compounds in a series • Determination of phase transition temperatures and enthalpies • Visual confirmation of phase changes

49. PXRD Structure of thei form of n-OctadecylAmmonium Chloride • Starting model: extrapolation, rotation, translation of the published C10Cl structure • Lab data, capillary, Cu K alpha1, indexed with Treor, Rietveld refinement with X’Pert Plus, no restraints • Molecular deficiencies are obvious • Space group: P21 , Cell: 5.655, 7.214, 24.573 Å, 93.07 deg • R (expected) 3.213 % • R (profile) 6.351 % • R (weighted profile) 8.150 % • R (Bragg) 3.149 %

50. Typical powder pattern - C18Br.H2O Capillary sample – Cu radiation Lamellar reflections