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

My Polymorphism Credentials (From Ancient Times)

Outline Introduction to what is of interest to us Alkylammonium halides Some crystal structures The use of powder diffraction and thermal

analysis Further examples

The Light Source of Africa

The Candle

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.

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.

The Commercial Importance of the Wax Industry

Candles Polishes Cosmetics Fruit coatings

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

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

Examples of Alkanes and Substituted Alkanes

The polymethylene chain in: Decane, C10H22 Octadecanol, C18H37OH D-12-Hydroxyoctadecanoic acid

methyl ester, C18H36OHCO2CH3 Dioctadecyl tetrasulfide, C36H74S4

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

Kitaiigorodskii – Closest Packing - Bumps and Hollows

Plane Groups: p1, p2, pm

Kitaiigorodskii – Structure of Normal Paraffins

Configuration of an aliphatic chainMinimum energy - the flat zig-zag carbon chain

Kitaiigorodskii – Close Packing of Chain Molecules

Three possible types of packing:Hexagonal, oblique, rectangular cell

Kitaiigorodskii – Sideways Packing of Normal Paraffins

Types of close-packed arrays of aliphatic chains

Kitaiigorodskii – End Packing of Normal Paraffins

a) Adjacent layers never stack through mirror planeb) Single-layer structures give skew unit cellc) Double-layer structures give orthorombic cells

Alkane Packing Example

n-Decane - packing like the stacking of pencils or cigarettes in a box

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)

n-Alkane Subcell

Orthorhombic O

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

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.

N+H

HH

X -

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

n-Alkyl Ammonium Halides – Typical Crystal Packing

They crystallize with ammonium and halide layers; hydrocarbon layers

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

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

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

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

i – tilted, interdigitated

k – kinked, non-interdigitated

- tilted,non-interdigitated

lamellar thickness – long spacing

Polymorphic Forms of n-Alkylammonium Halides at Room Temp

- perpendicular,non-interdigitated, rotating

- perpendicular,non-interdigitated

Liquid crystal, hydrocarbon chainsmelted

Temperature

Polymorphic Forms of n-Alkylammonium Halides at High Temp

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.

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

Polymorph Symbol

Structural form

Crystallization conditions

i Interdigitated Solution crystallization, room temperature

k Kinked Solution crystallization, room temperature

h ? Solution crystallization, high temperature

Non-interdigitated

and tilted

Crystallization from the melt

n-Octadecylammonium Chloride Kinked k Form

C18Cl-k single crystals grown from methanol at room tempSMART CCD data, structure refined to an R-factor of 0.083

crystal system: orthorhombic, space group: Pna21

cell: 70.90 x 5.45 x 5.36 Å, Z=4

• 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 %

n-Octadecyl Ammonium Chloride Fully Extended i Form

• 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

n-Octadecyl Ammonium Bromide Hydrate

n-Octadecyl Ammonium Iodide

Triclinic, P1bara = 6.4799, b = 7.1515, c = 22.941, = 98.610, = 90.763, = 91.466

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

Packing in the Polymorphs

Observed crystal forms: i m k a

Non-interdigitated C18Cl-k packing

Interdigitated C18Cl-i packing

Interdigitated C18Br hydrate packing

Interdigitated C18I packing

Typical Chain Tilting

C18Cl-k

Packing Examples

C18clk.mry C18cli.mry

C18ci.mry C18br-m-lt.mry

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°

B r

N H 3

H O2

Two N-H...Br interactions, one N-H...O interaction

and two O-H...Br interactions

3 .3 6 9 Å 3 .3 4 7 Å

2 .8 6 1 Å

3 .3 8 4 Å 3 .3 5 4 Å

Interaction distances in C18Br.H2O

Hydrogen Bonding Network in the Bromides

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 °)

C18I Ionic Layer

3 .5 5 3 Å

3 .4 9 5 Å

3 .5 7 1 Å

3 .6 7 0 Å

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

• 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 %

PXRD Structure of the i form of n-Octadecyl Ammonium Chloride

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

 

Lamellarreflections

Preferred Orientation will often help us

Blue: measuredRed: calculated

For flat-plate samples the lamellar reflections (h00)

are very intense and easy to spot

400

600

800

Fingerprinting by XRD Patterns Identification of C18Cl Phases

Melt-fresh Melt-aged Interdigitated Non-

interdigitated, kinked

Powder patterns of the CnBr phases – effect of chain length

C18Br C16Br C15Br C14Br C13Br (all mono

hydrated phases)

i-forms:

C16Cl(22.4Å)

C16Br(24.1Å)

C16I(20.4Å)

Powder Diffraction– effect of anion – C16X, X=Cl-, Br-, I-

10

20

30

40

50

60

70

80

7 9 11 13 15 17 19 21

No of C atoms

Lo

ng

sp

aci

ng

(Å

)

epsilon form new polymorph monohydrate form i form k form

Series of n-Alkyl Ammonium Chloride Polymorphs by XRD

Thermal Analysis – DSC of the C18Cl Phases

DSC of the n-Octadecyl Ammonium Halides

DSC – Effect of Chain Length

DSC of the phase transitions of the form of melt-crystallized n-alkylammonium bromides

exo

TGA of One of the n-Alkylammonium Bromide Monohydrates

m m e ltl iq u id c ry s ta l

Phase transition temperatures as observed by DSC are indicated by dotted lines.

0

50

100

150

200

250

11 12 13 14 15 16 17 18 19

No of C atoms

Tem

per

atu

re (

°C)

epsilon to delta delta to beta beta to alpha

alpha to liquid crystal liquid crystal to melt

Series of n-Alkyl Ammonium Chloride Polymorphs by DSC

Series of n-Alkyl Ammonium Chloride Polymorphs by DSC

0

2

4

6

8

10

12

14

11 12 13 14 15 16 17 18 19

Number of carbon atomsE

nth

alp

y (k

J/m

ol)

epsilon' to epsilon epsilon to delta delta to beta

0

5

10

15

20

25

30

35

40

45

50

11 12 13 14 15 16 17 18 19

Number of carbon atoms

En

thal

py

(kJ/

mol

)

i to beta beta to alpha

The transition enthalpies of the i transitions range from 25 to 45 kJ/mol, and are much larger than the enthalpy values of the high temperature transitions.

This high transition enthalpy is due to the postulated mechanism of the transition, namely that the molecules undergo chain separation and that the packing

changes from the interdigitated to the non-interdigitated state.

Thermal microscopy

Visual confirmation of phase changes Crystals on hot stage change with heating

C18Cl k phase

Room temperature liquid crystal at 162°C Melt at 196 °C

Variable Temperature PXRD with a heating stage

Use the Phase Relations in the Iodides as an Example

m elt

liq u idc ry s ta l

i

ev e n ch a in

Tem

pera

ture

a l l ch a in len g th s

x

n-Alkyl Ammonium Iodide Polymorphs by XRD

10

15

20

25

30

35

40

45

9 10 11 12 13 14 15 16 17 18 19

Number of C atoms

Lon

g sp

acin

g (Å

)

i form epsilon form y form m form b form

C18I - DSC

C18I – i form – 1st heating cycle Phase

changes during one cycle of heating and cooling – top to bottom

Form i changes to form epsilon when cooled to room temp

C18I – i form – 2nd heating cycle

Phase changes during one cycle of heating and cooling – top to bottom –

epsilon form returns to epsilon form

C18I – epsilon form – variable temp

Peak shifts and changes show epsilon to gamma phase conversions

C18NI patterns: i form (exp from solvent) & calculated (from single xtal) – different!

C18NI patterns: epsilon (from melt) & calculated (from single xtal) – the same!

The Superiority of Capillary PXRD Data - C18Cl forms

C18NCl - Capillary and Calculated Data Confirms:

Kinked form - kInterdigitated form - i

Hybrids:c6pbi

Low & Room Temperature Forms

c6pbi - heat and cool

Conclusion1: Intermolecular interactions observed

Typical parallel chain packing (like alkanes) Formation of H-bonding anion layers Digitated or non interlaced packing (as a result

of anion effects?) Chlorides: three anions surround NH3 group at

H-bonding distance and geometry Bromides: Water inclusion in hydrates Iodides: different NH3 group geometry Lead iodides: Layered packing retained

Conclusion2: Polymorphs and Probes

Polymorphism occurs widely in the long-chain alkylammonium complexes

Solid-solid phase changes take place when the layers realign when the conformations of the chain-like

molecules themselves change XRD (in its many forms) and Thermal

Analysis Techniques are excellent and complementary structural probes

Acknowledgements

Colleagues who did most of the work: Dave Billing (WITS) Melanie Rademeyer

(UND) Erie Reynhardt

(UNISA) Rosalie (Rothner)

Scholtz (UNISA) Finances – RAU/BGU

Eric Samson Fund

RAU

Students at RAU – soon to be University of Johannesburg

Thanks for helping us to light the candle, the light in Africa

Thanks for your attention