Polymorphism in the Longchain 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
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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
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Candles
Polishes
Cosmetics
Fruit coatings
Waxes and their Components
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Natural Waxes
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Synthetic Waxes
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This group includes plant, animal, mineral waxes
They contain alkanes but also esters, alcohols, acids
From Fischer-Tropsch and other synthetic routes
Contain normal alkanes, isoalkanes, cycloalkanes
Petroleum waxes
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A similar blend of paraffins from crude oil
Our aims
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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?
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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 chain
Minimum 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 plane
b) Single-layer structures give skew unit cell
c) Double-layer structures give orthorombic cells
Alkane Packing Example
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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 Orthorhombic, n odd Monoclinic, n even
(CnH2n+2 6<n<26)
(11<n<39)
(28<n<36)
n-Alkane Subcell
Orthorhombic O
Polymorphism in long-chain
compounds
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Exhibited by most long-chain compounds
Types:
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Stacking differences
Conformational polymorphism
Solvates
Polymorph-dependent physical properties include:
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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:
X-
H
+
N
H
H
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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.
Why Study n-Alkyl Ammonium Halides if
we are really interested in Waxes?
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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
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They crystallize with ammonium and halide
layers; hydrocarbon layers
Crystallization Strategies
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Two-fold aim:
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Crystallize at different temperatures
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to obtain good quality single crystals
and as many polymorphic forms as possible.
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
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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
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Many authors contributed to the rich literature on the subject, mostly
work on the short-chain chlorides
Solid-solid phase transitions on heating:
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Structural information:
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Chlorides: Tsau and Gilson (1968); Busico et al, (1983); Terreros et al,
(2000)
Bromides: Tsau and Gilson (1968)
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
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Phase transitions
similar to those of nparaffins
Chain kinks give
additional lowtemperature
conformational
polymorphs
Temp
Polymorphic Forms of nAlkylammonium Halides at Room Temp
lamellar thickness – long spacing
i – tilted,
interdigitated
k – kinked,
 - tilted,
non-interdigitated non-interdigitated
Polymorphic Forms of nAlkylammonium Halides at High Temp
Temperature
- perpendicular,
non-interdigitated
- perpendicular,
non-interdigitated,
rotating
Liquid crystal,
hydrocarbon chains
melted
Our Single Crystal Structure
Determinations
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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

Noninterdigitated
and tilted
Crystallization from the melt
n-Octadecylammonium
Chloride Kinked k Form
C18Cl-k single crystals grown from methanol at room temp
SMART 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
n-Octadecyl Ammonium
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 %
n-Octadecyl Ammonium
Bromide Hydrate
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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 Iodide
Triclinic, P1bar
a = 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:
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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
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°
Hydrogen Bonding Network in
the Bromides
Br
Interaction distances
in C18Br.H2O
3.369Å
3.347Å
NH3
H2O
2.861Å
3.354Å
Two N-H...Br interactions, one N-H...O
interaction
and two O-H...Br interactions
3.384Å
N-H…I interactions
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Average N-H-I
bond values:
H-I = 2.7 Å
N-I = 3.5 Å
N-H-I = 169°
(136 °)
C18I Ionic Layer
3.553Å
3.495Å
3.571Å
3.670Å
Remarks on the use of Powder
XRD and Thermal Analysis
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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
PXRD Structure of the i form of nOctadecyl Ammonium Chloride
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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 %
Typical powder pattern - C18Br.H2O
Capillary sample – Cu radiation
Lamellar
reflections
Preferred Orientation will often
help us
Blue: measured
Red: calculated
For flat-plate samples the
lamellar reflections (h00)
are very intense and easy to
spot
600
400
800
Fingerprinting by XRD Patterns
Identification of C18Cl Phases
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Melt-fresh
Melt-aged
Interdigitated
Noninterdigitated,
kinked
Powder patterns of the CnBr
phases – effect of chain length
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C18Br
C16Br
C15Br
C14Br
C13Br
(all mono
hydrated
phases)
Powder Diffraction– effect of
anion – C16X, X=Cl-, Br-, Ii-forms:
C16Cl
(22.4Å)
C16Br
(24.1Å)
C16I
(20.4Å)
Series of n-Alkyl Ammonium
Chloride Polymorphs by XRD
80
Long spacing (Å)
70
60
50
40
30
20
10
7
9
11
13
15
17
19
No of C atoms
epsilon form
new polymorph
monohydrate form
i form
k form
21
Thermal Analysis – DSC of
the C18Cl Phases
DSC of the n-Octadecyl
Ammonium Halides
DSC – Effect of Chain Length
exo
DSC of the phase transitions of the  form of melt-crystallized n-alkylammonium bromides
TGA of One of the n-Alkylammonium
Bromide Monohydrates
m



liquid
crystal
melt
Phase transition temperatures as observed by DSC are indicated by dotted lines.
Series of n-Alkyl Ammonium
Chloride Polymorphs by DSC
250
Temperature (°C)
200
150
100
50
0
11
12
13
14
15
16
17
No of C atoms
epsilon to delta
delta to beta
alpha to liquid crystal
liquid crystal to melt
beta to alpha
18
19
50
45
40
35
30
25
20
15
10
5
0
14
12
Enthalpy (kJ/mol)
Enthalpy (kJ/mol)
Series of n-Alkyl Ammonium
Chloride Polymorphs by DSC
10
8
6
4
2
0
11
12
13
14
15
16
Number of carbon atoms
i to beta
beta to alpha
17
18
19
11
12
13
14
15
16
17
18
Number of carbon atoms
epsilon' to epsilon
epsilon to delta
delta to beta
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.
19
Thermal microscopy
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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
melt
Temperature
liquid
crystal




x
i
even chain

all chain lengths
n-Alkyl Ammonium Iodide
Polymorphs by XRD
45
Long spacing (Å)
40
35
30
25
20
15
10
9
10
11
12
13
14
15
16
17
18
Number of C atoms
i form
epsilon form
y form
m form
b form
19
C18I - DSC
C18I – i form – 1st heating
cycle
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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
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Phase
changes
during
one
cycle of
heating
and
cooling –
top to
bottom –
epsilon
form
returns
to
epsilon
form
C18I – epsilon form – variable
temp
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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 - k
Interdigitated form - i
Hybrids:
c6pbi
Low & Room
Temperature Forms
c6pbi - heat and cool
Conclusion1: Intermolecular
interactions observed
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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
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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
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
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