Nonionic diethanolamide amphiphiles with unsaturated C18 hydrocarbon chains: Thermotropic and lyotropic liquid crystalline phase behavior

Article abstract of DOI:10.1039/c1cp21808e

The neat and lyotropic liquid crystalline phase behavior of three nonionic diethanolamide amphiphiles with C18 hydrocarbon chains containing one, two or three unsaturated bonds has been examined. This has allowed the effect of degree of unsaturation on the phase behavior of diethanolamide amphiphiles to be investigated. Neat linoleoyl and linolenoyl diethanolamide undergo a transition from a glassy liquid crystal to a liquid crystal at ～-85 °C, while neat oleoyl diethanolamide undergoes a transition at ～-60 °C to a liquid crystalline material before re-crystallizing at -34 °C. Oleoyl diethanolamide then undergoes a third transition from a crystalline phase to a smectic liquid crystalline phase at ～5 °C. In the absence of water, the transition temperature from a smectic liquid crystal to an isotropic liquid decreases with increasing unsaturation. The addition of water results in the formation of a lamellar phase (L_α) for all three amphiphiles. The lamellar phase is stable under excess water conditions up to temperatures of at least 70 °C. Approximate partial binary amphiphile-water phase diagrams generated for the three unsaturated C18 amphiphiles indicate that the excess water point for each amphiphile occurs at ～60% (w/w) amphiphile. the Owner Societies 2011.

Full text of DOI:10.1039/c1cp21808e

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PAPER
Nonionic diethanolamide amphiphiles with unsaturated C18 hydrocarbon
chains: thermotropic and lyotropic liquid crystalline phase behaviorw
Sharon M. Sagnella,a Charlotte E. Conn,b Irena Krodkiewskab and
Calum J. Drummond*b
Received 3rd June 2011, Accepted 6th June 2011
DOI: 10.1039/c1cp21808e
The neat and lyotropic liquid crystalline phase behavior of three nonionic diethanolamide
amphiphiles with C18 hydrocarbon chains containing one, two or three unsaturated bonds has
been examined. This has allowed the effect of degree of unsaturation on the phase behavior of
diethanolamide amphiphiles to be investigated. Neat linoleoyl and linolenoyl diethanolamide
undergo a transition from a glassy liquid crystal to a liquid crystal at BÀ85 1C, while neat oleoyl
diethanolamide undergoes a transition at BÀ60 1C to a liquid crystalline material before
re-crystallizing at À34 1C. Oleoyl diethanolamide then undergoes a third transition from a
crystalline phase to a smectic liquid crystalline phase at B5 1C. In the absence of water, the
transition temperature from a smectic liquid crystal to an isotropic liquid decreases with
increasing unsaturation. The addition of water results in the formation of a lamellar phase (La)
for all three amphiphiles. The lamellar phase is stable under excess water conditions up to
temperatures of at least 70 1C. Approximate partial binary amphiphile-water phase diagrams
generated for the three unsaturated C18 amphiphiles indicate that the excess water point for each
amphiphile occurs at B60% (w/w) amphiphile.
headgroup can interact favorably with polar solvents while
the hydrocarbon chains exhibit hydrophobic interactions.
These interactions combined with local and global constraints
Introduction
Nonionic amphiphiles, such as surfactants, are integral
components of many industrial and consumer products.
dictated by shape, polarity, and molecular geometry drives the
Diethanolamide amphiphiles, and in particular those derived
self-assembly of these molecules in the presence and absence of
solvent.8,9 For lyotropic phases, which are formed by amphi-
from lauric acid and coconut oil, have been widely used in the
cosmetic industry, in a variety of household detergents, and
more recently in an insecticide.1–5 Their properties have
amphiphile can be explained with reference to the critical
philes in polar solvents such as water, the phase formed by the
additionally been exploited as foam boosters, thickening
packing parameter (CPP = n/(lca0)), where lc is the effective
length of an amphiphile chain, a0 is the effective amphiphile
agents, and corrosion inhibitors.1–7 Coconut oil, from which
coconut diethanolamide is derived, consists of a range of both
headgroup area, and n is the average volume occupied by the
amphiphile chain.9 ‘‘Rod-like’’ amphiphiles, where the molecule
saturated fatty acid components (ranging from C6–C18) and
unsaturated fatty acid components including monounsaturated
tends to be fairly cylindrical in shape and n/(lca0) has a value close
to one, form flat bilayer structures. The fluid lamellar (La)
oleic acid and polyunsaturated linoleic acid, thus coconut
DEA contains diethanolamides of these different fatty acid
phase consists of a one-dimensional stack of flat amphiphilic
components. While lauroyl diethanolamide has been studied
bilayers separated by water layers and is ubiquitous in nature
in detail, physicochemical characterization of other coconut
as it forms the basic building block of biological membranes.
DEA components has been limited.
The unique physicochemical properties of the diethanolamide
However, for molecules where n/(lca0) o 1, i.e. the headgroup
area is large compared to that of the hydrocarbon chain,
surfactants results from their amphiphilic nature; the hydrophilic
normal (oil in water) phases tend to form. Conversely inverse
(water in oil) phases form when the hydrophobic chain
a CSIRO Materials Science and Engineering, PO Box 184,
North Ryde, NSW, 1670, Australia
b CSIRO Materials Science and Engineering, Bag 10, Clayton South,
VIC, 3169, Australia. E-mail: Calum.Drummond@csiro.au
w Electronic supplementary information (ESI) available: Polarized
optical microscopy images of neat linoleoyl diethanolamide; Addi-
tional DSC heating scan of oleoyl diethanolamide with the different
transition states indicated on the scan. See DOI: 10.1039/c1cp21808e
occupies a larger volume than the head group, (n/(lca0) 4 1)
thereby producing a molecule with an effective reverse wedge
shape. As the effective reverse wedge-shape of the molecule
increases, there is an increased desire for curvature of the
interface resulting in the formation of inverse mesophases such
as inverse bicontinuous cubic phases (QII)10 and the inverse
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hexagonal phase (HII).11 An extremely high desire for curvature
may result in the formation of inverse micelles where the
hydrophilic headgroups are arranged towards water cores with
hydrophobic chains radiating outwards. These micelles can be
arranged in a disordered manner resulting in a fluid inverse
micellar phase, the L2 phase.12
the energy minimized structures of the previously examined
unsaturated monoethanolamides,30 as well as the unsaturated
diethanolamide amphiphiles examined in the current study.
Experimental section
The introduction of unsaturation into the hydrophobe has
been shown to be an effective method of promoting the
formation of inverse lyotropic phases in a variety of amphiphile
families.13–25 The most widely studied of the unsaturated
amphiphile families is the monoacylglycerides, which have
been shown to form a variety of non-lamellar phases including
multiple inverse cubic phases and the inverse hexagonal phase
highlighting the effect of chain length, degree of unsaturation and
double bond positions on inverse phase formation.13–15,19–21,23
Furthermore, unsaturated ethylene oxide amphiphiles containing
a cis double bond form inverse bicontinuous cubic phases and
an L3 sponge phase.18,24 Our group has previously examined
in detail the structure-property correlation via modifications
to the hydrophobe and headgroup of nonionic urea based
surfactants demonstrating that unsaturation was one method
for promoting inverse phase formation in this amphiphile
family.16,17,25–28 In addition, we and others have recently
reported on the effect of hydrocarbon chain modification on
the physicochemical properties of monoethanolamide amphi-
philes demonstrating that hydrophobe unsaturation promotes
inverse cubic phase formation within the monoethanolamide
family.22,29–31 Here, similar to the urea based amphiphiles, we
have aimed to further extend our understanding of structure-
property correlation of ethanolamide based amphiphiles by a
systematic examination of the self-assembly, including neat
and lyotropic liquid crystalline phase behavior of a series of
C18 diethanolamides with one, two, and three carbon double
bonds. Fig. 1 provides the chemical structures and compares
Materials
All reagents were obtained from Sigma-Aldrich. Organic
solvents were either of analytical or spectroscopic grade and
used as received. A Milli-Q Plus Ultrapure water system
(Millipore; Australia) was used to filter deionized tap water
to obtain high purity water.
Diethanolamide amphiphile synthesis
All diethanolamides reported in this paper were prepared by
reaction of diethanolamine with the corresponding acid chloride.
Diethanolamine (99.5% purity) was purchased from Fluka.
Acid chlorides were formed from carboxylic acids, by using
oxalyl chloride and catalytic dimethylformamide. The crude acid
chlorides can be vacuum distilled but adequate purity can also
be achieved by treatment with petroleum spirit (b.p. 30–40 1C),
upon which most of the impurities separate as an insoluble oil.
As an example, a detailed synthesis of oleoyl diethanolamide
follows.
N,N-bis(2-hydroxyethyl)oleamide (oleoyl diethanolamide)
Step 1: Oleoyl chloride. 10 g (35.4 mmoles) of oleic acid
(Sigma, 99% purity), 50 ml of dry dichloromethane and a
small drop of DMF were placed in a 100 ml RB flask equipped
with a dropping funnel, magnetic stirrer and air condenser
under argon. The reaction flask was immersed in a water bath.
A 2 M solution of 5.39 g (1.2 eq., 42.5 mmols) of oxalyl
chloride in dry dichloromethane was added drop wise at room
temperature (21 1C), and stirring was continued until all the
acid had reacted as shown by NMR. The reaction mixture
became pale yellow. Dichloromethane and excess of oxalyl
chloride were removed on a rotary evaporator, and a light
yellow crude oleoyl chloride was vacuum distilled, resulting in
10.08 g of a colorless liquid; 94.6% yield.
Step2: Oleoyl diethanolamide (ODE). The preparation of the
amide is carried out in a two-necked 500-ml flask equipped
with a magnetic stirrer, a thermometer and a pressure equalizing
dropping funnel maintained in an argon atmosphere. Diethanol-
amine 14.12 g (4 eq.) was mixed into 150 ml of dry dichloro-
methane and cooled to À10 1C, not allowing the mixture to
freeze. 10.08 g of oleoyl chloride (33.49 mmoles; 1 eq) diluted
with 50 ml of dry dichloromethane was added drop wise, over
3 h. The temperature was initially À10 1C , and as the reaction
was progressing, the temperature could be lowered to À30 and
À40 1C. Stirring at this low temperature was continued for
another hour, then the reaction mixture was left in the freezer
until the next day. NMR of an aliquot taken at this point
showed that the reaction was complete. In the freezer the
excess of diethanolamine formed a solid upper layer. The flask
was allowed to warm to room temperature, reaction mixture
was transferred to a separatory funnel and cooled in the fridge
to the point when the mixture separated into two clear phases:
upper layer being an excess of diethanolamine. The DCM
Fig. 1 Molecular models and chemical structures of unsaturated C18
mono and diethanolamide amphiphiles.
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phase was separated and washed with 10% NaCl aqueous
solution; causing formation of an emulsion which was very
slow to separate. For that reason DCM was evaporated off
and the residue was redissolved in diethyl ether. NMR spectrum
of this solution showed tiny amounts of impurities present in
the material, therefore the etheral solution was further washed
with 50 ml of 0.2N hydrochloric acid, followed by 40 ml of
2.5% KOH solution and finally with 50 ml water. Solvent was
removed on rotary evaporator at room temperature, and the
resulting colorless oil was further dried under vacuum. 9.34 g
of a heavy colorless oil was obtained; yield 75.5%. HPLC
anlaysis indicated that the purity of all three diethanolamide
amphiphiles was 499%.
range of 2–601 2y with a step size of 0.051 2y, with each step
measured for 60 s. The raw data was exported and plotted in
Microsoft Excel as q value (nmÀ1) vs. intensity. The q value
(nmÀ1) was calculated from 2y based on Bragg’s law (lCu–Ka
0.154 nm).
=
Water penetration into diethanolamide amphiphiles
A small amount of neat diethanolamide amphiphile was
placed onto a microscope slide and heated to melting. A
coverslip was placed on top of the melted amphiphile and
then cooled to room temperature prior to addition of water.
Water placed at the edges of the coverslip was drawn between
the two glass surfaces to surround the ‘‘solidified’’ material by
capillary action. The microscope slide was placed into a Linkam
PE94 hot stage (Linkam Scientific Instruments Ltd; Surry,
England) and heated at 1 1C/min or less. The interaction of
water and the monoethanolamide amphiphile was observed
with an Olympus GX51 inverted optical microscope (Olympus
Australia Pty. Ltd.; Melbourne, Australia) in the presence and
absence of crossed polarizing lenses. Images were captured
with an Olympus c-5060 digital camera (Olympus Australia
Pty. Ltd.; Melbourne, Australia).
Oleoyl diethanolamide. 1H NMR (CDCl3, 400 MHz) d 5.34,
m, 2H,QCH; 3.87, t, J 4.9Hz, 2H, CH2OH; 3.79, t, J 5.1 Hz,
2H, CH2OH; 3.57, t, J 4.9 Hz, 2H, CH2N; 3.51, t, J 5.3Hz,
2H, CH2N; 3.08, br, 1H, OH; 2.87, br, 1H, OH; 2.39, t,
À
À
J 7.7Hz, 2H, CH2CO; 2.01, m, 4H, CH2CHQ; 1.63, m, 2H,
CH2CH2CO; 1.22–1.38, m, 20H, CH2; 0.88, t, J 6.8 Hz, CH3.
À
À
Linoleoyl diethanolamide. 1H NMR (CDCl3, 400 MHz)
d 5.28–5.43, m, 4H, QCH; 3.87, t, J 4.8Hz, 2H, CH2OH;
3.79, t, J 5.1 Hz, 2H, CH2OH; 3.57, t, J 4.9 Hz, 2H, CH2N;
3.50, t, J 5.3Hz, 2H, CH2N; 3.12, br, 1H, OH; 2.93, br, 1H,
À
À
Binary phase behavior of diethanolamide amphiphiles
OH; 2.77, t, J 6.6 Hz, 2H, QCHCH2CHQ; 2.39, t, J 7.7 Hz,
À
Diethanolamide amphiphile–water mixtures were prepared by
weighing out an amount of the diethanolamide amphiphile in
glass ampoules, and then adding the necessary quantity of
water by weight. Samples ranging from 5 wt% amphiphile in
water to 95 wt% amphiphile in water were prepared. The glass
ampoules were sealed, and the amphiphile was initially dissolved
(as much as possible) by heating the samples followed by rapid
cooling. Samples were then submerged into a glass water bath
with crossed polarizing film attached to the outside of the water
bath. The water temperature was controlled and monitored by
a polystat cc1 water bath heater (Crown Scientific Pty. Ltd.;
Sydney, Australia) equipped with an internal digital thermo-
meter. The temperature was incrementally increased and then
left to allow the samples to equilibrate properly. Samples were
illuminated and examined through the cross polarizing filters
to determine phase transitions.
2H, CH2CO; 2.05, m, 4H, CH2CHQ; 1.64, m, 2H,
CH2CH2CO; 1.24–1.41, m, 14H, CH2; 0.89, t, J 6.9 Hz, 3H,
CH3.
À
À
1
Linolenoyl diethanolamide. H NMR (CDCl3, 400 MHz) d
5.30–5.45, m, 6H,QCH; 3.87, t, J 4.9 Hz, 2H, CH2OH; 3.79,
t, J 5.2 Hz, 2H, CH2OH; 3.57, t, J 4.9Hz, 2H, CH2N; 3.50,
t, J 5.3 Hz, 2H, CH2N; 2.85–3.25, br, 2H, OH; 2.81, t, J 5.7
À
À
Hz, 4H, QCHCH2CHQ; 2.41, t, J 7.6 Hz, 2H, CH2CO;
À
2.00–2.15, m, 4H, CH2CHQ; 1.67, m, 2H, CH2CH2CO;
1.25–1.47, m, 8H, CH2; 0.89, t, J 6.9 Hz, 3H, CH3.
À
À
Differential scanning calorimetry (DSC)
DSC was performed using a Mettler DSC822e system with
a
Mettler TSO 801RO sample robot (Mettler Toledo;
Melbourne, Australia). Samples were run at scan rates of 10
and 2.5 1C/min and collected using the STARe software
package (Mettler Toledo; Melbourne, Australia). Temperature
calibration (Æ0.3 1C) of the ceramic sensor was performed
using octane, water, indium, and zinc. Integration of a stan-
dard indium peak was used for thermal calibration. The
energies and peak temperatures of the endotherms/exotherms
of the diethanolamide amphiphiles were determined using the
STARe software package.
SAXS
Samples for SAXS analysis were prepared by weighing
approximately 50 mgs of lipid into a vial and adding the
appropriate amount of deionised water by weight. Samples
were then re-weighed so the water content could be calculated
accurately. Samples were partially homogenized by using a
mechanical vortex but this did not fully homogenize the
samples. It is common to homogenize a binary lipid-water
sample by heating to fluidity then cooling to room tempera-
ture; however, in this case, heating was avoided due to the
labile nature of the chemicals. After one week equilibration,
samples were still visually inhomogeneous and were therefore
stirred vigorously with a spatula prior to SAXS measurements
being taken. All SAXS measurements showed sharp diffraction
peaks consistent with homogeneous samples.
X-ray diffraction (XRD)
XRD analyses were performed on a PANalytical X’pert PRO
X-ray diffractometer. Incident X-ray radiation was produced
from a line-focused PW3373/00 Cu X-ray tube operating at 45 kV
and 40 mA. The solid sample was placed and pressed into a
circular metal sample. The sample stage was Spinner PW3064
with reflection but not moving. The diffracted beam produced
by interaction with the sample was detected by an X’Pert data
collector. Samples were analyzed at room temperature over a
SAXS measurements were carried out using the SAXS/
WAXS beamline at the Australian Synchrotron. The experiments
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used a beam of wavelength l = 1.24 A (10.00 keV) with
dimensions 700 mm  500 mm and a typical flux of 1.2  1013
photons/s. 2-D diffraction images were recorded on a Pilatus
1M detector, which offers very low noise, a large dynamic
range and rapid data collection over a large active area. Dead
space due to intermodule gaps is overcome by radial integration
with the detector slightly offset. Calibration of the q-axis of the
1-D diffraction patterns was performed using silver behenate
as standard.
Liquid crystalline mesophases give rise to distinct diffraction
patterns which may be used as an unambiguous identification
for each phase, provided an adequate number of reflections are
observed. All images were analyzed using AXcess, a custom-built
SAXS analysis program written by Dr Andrew Heron: details
on the use of AXcess for SAXS analysis are reported in a
review article.32
Molecular modeling
Chem 3D pro v11 (Cambridgesoft Corporation) was used
to obtain the energy minimized conformation using its
MM2 energy minimization routine as well as theoretical
molecular lengths. This software package ceases optimization
at local minimums. Therefore, when a change from linearity
occurred, bonds were rotated manually, and minimization was
continued. Thus, an optimum energy minimum conformation
was obtained. Molecular lengths were measured from the
minimized conformations as the longest distance between
the end of the hydrocarbon chain and the furthest atom in
the diethanolamide headgroup.
Fig. 2 Differential Scanning Calorimetry of oleoyl diethanolamide,
linoleoyl diethanolamide and g-linolenoyl diethanolamide. (A). Scan
Rate 2.5 1C/min, (B). Scan Rate 2.5 1C/min, enthalpy x10 to resolve
low energy transitions.
This trend changes slightly for the C18 amphiphiles with three
unsaturated bonds (Fig. 3). The amides and ureas have the
highest melting points due to the strong H-bonding network
within the headgroups of these amphiphiles.16,25–28 As the
degree of unsaturation is increased, the decrease in melting
points of these C18 amphiphile families tends to level out. A
similar trend is observed for the fatty acids and alcohols,
which have much lower melting points than the amides and
ureas. In contrast, the melting points of the diethanolamide
amphiphiles, the monoethanolamides amphiphiles30 and the
monoglycerides display a more linear dependence on the
degree of unsaturation. We note that the melting points of
the C18 diethanolamide amphiphiles are almost identical to
their monoglyceride counterparts. This is most likely due to the
close similarities in molecular structure of the diethanolamides
and the monoglycerides, which results in a similar degree of
headgroup H-bonding.
Results and discussion
Thermal behavior of unsaturated diethanolamide neat
amphiphiles
DSC scans were run at 2.5 1C/min (Fig. 2) and 10 1C/min (not
shown) for all three unsaturated diethanolamide surfactants.
DSC scans run at 10 1C/min were started at À130 1C, and no
additional peaks were observed at temperatures below those
shown. To ease visualization of low enthalpy transitions, the
curves of linoleoyl and linolenoyl diethanolamide, as well as
oleoyl diethanolamide at temperatures above 10 1C, have been
magnified x10 and are shown in Fig. 2B. Multiple heating/
cooling cycles were performed for each sample in order to
resolve any polymorphisms in the sample. Analysis of the
DSC curves to determine transition temperatures and their
associated enthalpies is provided in Table 1. Phase transition
temperatures are reported as the peak maxima of the appropriate
endo- or exotherm. Enthalpies were obtained by integration of
the transition peaks.
In terms of thermal behavior, one of the main differences
between the unsaturated C18 diethanolamides and their mono-
ethanolamide counterparts is that while the monoethanolamides
all exhibit a distinct crystal-isotropic transition,30 the unsaturated
diethanolamides instead undergo an extremely low enthalpy
transition (C18:1, 37.5 1C; C18:2, 20 1C;.C18:3, À47 1C).
These low enthalpy transitions may be attributed to the
transition from a liquid crystal to an isotropic fluid, also called
the clearing point. The presence of a clearing point for the neat
amphiphile has also been observed for the H-farnesoyl and
phytanoyl monoethanolamide and urea amphiphiles as well as
certain alkyl glucosides.26,29,37
The melting transition temperature has been plotted as a
function of the degree of unsaturation for the C18 diethanol-
amide amphiphiles in Fig. 3.25,26,30,33–36 Literature values
for the corresponding fatty acids, ureas, amides, alcohols,
monoglycerides, and monoethanolamides have been plotted
on the same graph for comparison. The melting temperature
of C18 amphiphiles with different polar head groups follows
the general trend: ureas 4 amides 4 monoethanolamides 4
monoglycerides E diethanolamides 4 fatty acids 4 alcohols.
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Table 1 Thermal transition for unsaturated C18 diethanolamide amphiphiles determined from DSC scanned at 2.5 1C/min
Glass
Transition Isotropic Melting LC–Isotropic
Smectic LC–
Smectic
Devitrification Devitrification
Temp (1C)
Crystal–Smectic LC
Melting Temp (1C)
Crystal–Smectic
Enthalpy
(kJ/mol)
LC Melting
Enthalpy
(kJ/mol)
Temp (1C)
Temp (1C)
Melting
Enthalpy
(kJ/mol)
Stearoyl
Diethanolamide62
(C18:0)a
—
—
—
—
—
11.7 Æ 0.4
—
73.4 Æ 0.2b
66.2 Æ 2.4b
Oleoyl
Diethanolamide
(C18:1)
À60.4 Æ 0.3
À85.7 Æ 0.6
À87.2 Æ 3.2
39.9 Æ 3.4
19.9 Æ 8.3
À46.7 Æ 6.2
À0.22 Æ 0.2
À0.3 Æ 0.2
À34.3 Æ 0.6
(À21.8)a,
5.3 Æ 0.7
(À6.5)a,
À19.2 Æ 6.2
Linoleoyl
Diethanolamide
(C18:2)
—
—
—
—
—
g-Linolenoyl
Diethanolamide
(C18:3)
À0.25 Æ 0.02
—
—
a
b
crystal-crystal transition (data obtained from Fig. S2, ESIw). crystal-isotropic liquid melting.
Fig. 3 Melting behaviour of unsaturated C18 amphiphiles with
different headgroups.
Fig. 4 Polarized optical microscope images of neat oleoyl diethano-
lamide. A. Image acquired at 5 1C showing a crystalline solid, B. and
C. Image acquired at 30 1C showing typical smectic liquid crystalline
texture. D. Image acquired at 35 1C, an isotropic band is present at the
edge of the amphiphile indicating melting of the liquid crystalline
phase to a fluid isotropic phase. These transitions occur at similar
temperatures to the transitions indicated by DSC (mag 100x).
A combination of SAXS and polarized optical microscopy
measurements were carried out on the neat oleoyl (Fig. 4)
and linoleoyl (Fig. S1, ESIw) diethanolamide amphiphiles at
different temperatures, to confirm the existence of a liquid
crystalline phase. The data indicate the formation of a layered
smectic liquid crystal for both samples and are described in
detail in subsequent sections. Similar data were not obtained
for linolenoyl diethanolamide as the liquid crystal–isotropic fluid
transition for this amphiphile occurs at sub-zero temperatures.
As the temperature is increased above the clearing point, both
oleoyl (Fig. 4D) and linoleoyl (Fig. S1B, ESIw) diethanolamide
begin to become an isotropic melt, as demonstrated by the dark
isotropic band at the edge of the amphiphile. Upon standing at
these temperatures for 10 min, the liquid crystalline phase
transitions fully to an isotropic melt (not shown).
unfavorable interactions with either the hydrophilic head-
groups or, in the case of the fluid lamellar phase, with a polar
solvent. As described in the introduction the formation of this
type of structure is common for ‘‘rod-like’’ amphiphiles.8,38–40
As shown in Fig. 1, the diethanolamide headgroup occupies a
much larger area than the monoethanolamide headgroup thus
resulting in a more cylindrical molecular geometry which
favors smectic liquid crystalline formation.
The smectic liquid crystalline phase consists of a stack of
lipid bilayers with hydrophilic headgroups sandwiched between
the hydrophobic tails. It is similar in structure to the hydrated
lyotropic fluid lamellar (La) phase but without the water
layers. In both cases the formation of the bilayer structure
acts to shield the hydrophobic hydrocarbon chains from
Another unique thermal property of the unsaturated diethanol-
amides is that all three exhibit a glass transition temperature.
This transition occurs at almost identical temperatures (BÀ85 1C)
for the linoleoyl and linolenoyl diethanolamide and at a higher
temperature (À60 1C) for oleoyl diethanolamide. Glass transi-
tions, while commonly seen in polymers, are not as common
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in amphiphiles. Due to the size and complexity of polymers, in
most cases they are unable to form crystals and instead their
polymer chain motion ceases below their Tg becoming an
amorphous solid or glass. We have noted the presence of
glass transition temperatures in isoprenoid-type amphiphiles
containing both monoethanolamide and urea headgroups
at temperatures ranging from À72 to À77 1C26,29 and in
phytantriol at BÀ40 1C (unpublished data). In addition,
phytanyl and farnesyl ethylene oxide, mono and bis-phytanyl
EDTA, certain endogenous arachidonyl chained amphiphiles,
thymidine phytanyl, 5FC-phytanyl, and some n-alkyl maltosides
exhibit glass transitions.41–50
Unlike crystals, amorphous solids lack long range transla-
tional order. In the case of the amphiphiles mentioned above,
however, instead of forming an amorphous solid, below
their Tg they form a glassy liquid crystal.26,29,46,40–42 A glassy
liquid crystal is one in which the glass possesses the same
structure as its corresponding liquid crystal, but molecular
motion is ‘‘frozen.’’ The ability of an amphiphile to form a
glassy liquid crystal is reliant on extensive H-bonding within
the headgroup. Typically, these types of amphiphiles first
undergo an initial low temperature transition from a glassy
state to one or more liquid crystalline states, before finally
becoming an isotropic fluid at the molecule’s clearing point,
however, we have noted that the DSC and POM data suggests
that this type of direct transition does not in fact occur for
oleoyl diethanolamide.
Fig. 5 1-D XRD plot of intensity vs. q for neat oleoyl diethanol-
amide. The first and second order reflections of a lamellar phase are
indicated. A broad peak in the wide angle region is indicative of
disordered hydrocarbon chains.
(data not shown). Based on comparison with POM results we
suggest this represents the first-order reflection of a lamellar
phase of d-spacing 33.0 A. The presence of this lamellar phase
was confirmed by SAXS analysis of the neat sample and is
discussed in detail in a later section. In both cases the XRD
patterns display a broad peak centered around a q-value of
B14 nmÀ1 (ca. 4.5 A) indicative of fluid hydrocarbon
chains41,42,54 In contrast, ‘‘gel-like’’ lamellar phases, i.e. where
the chains are packed into an all-trans configuration but the
headgroups are disordered, show a narrow, well defined peak
at ca. 4.2 A that is typical for packed, all-trans alkyl
chains.41,55 Crystalline chain packing leads to many well-
defined peaks in this region. Thus, the broad peak is attributed
to the unsaturated chains residing in a disorganized or molten
state, while the distinct reflections in the low angle region arise
due to the long-range layered lamellar structure of the amphi-
phile molecules.41,42,54,55
Despite the fact that oleoyl diethanolamide forms a glassy
liquid crystal, it does not transition in a direct manner from
glassy liquid crystal–liquid crystal–isotropic fluid upon heating,
but instead undergoes devitrification, or crystal formation at
À34 1C followed by liquid crystal formation at 5 1C. Thus,
oleoyl diethanolamide undergoes the following transition order:
glassy liquid crystal–liquid crystal–crystalline–liquid crystal–
isotropic fluid. In addition, in the first heating scan of the
oleoyl diethanolamide sample (Fig. S2, ESIw), an additional
crystalline pre-transition is present that occurs after the devi-
trification peak and before the main crystalline melting point,
and can be attributed to a crystal-crystal transition. This pre-
transition is highly dependent on the sample history and
disappears in subsequent heating scans. Similar polymorphic
crystal transitions occur in oleoyl monoethanolamide.30 Pure oleic
acid displays a single pre-transition which has been ascribed
to a well characterized polymorphic transition from the g- to
a-form. The difference between the two forms has to do with
the conformation of the section of the hydrocarbon chain
between the double bond and the terminal methyl group which
retains an all trans structure in the g-form, while possessing a
disordered liquid like conformation in the a-form.51–53
Due to increased chain splay associated with unsaturated
hydrocarbon chains, the theoretical length of unsaturated
hydrocarbon chains will be less than that predicted via
Tanford’s formula. Theoretical molecular lengths were therefore
obtained via molecular modeling from energy minimized
structures of the amphiphiles. The theoretical length of one
oleoyl diethanolamide molecule was determined to be 24.7 A
(d = 49.4 A), while for linoleoyl diethanolamide it was 22.2 A
(d = 44.4 A). We note that both the predicted and measured
lengths of the linoleoyl derivative are slightly shorter than
those for its oleoyl counterpart, reflecting the increased degree
of unsaturation of the linoleoyl chain, which results in an
effective shortening of the alkyl chain. The d-spacing obtained by
XRD for both linoleoyl and oleoyl diethanolamide is signifi-
cantly less than the predicted d-spacing for non-tilted and non-
interdigitated bilayers. Similar results have been demonstrated
for carbohydrate amphiphiles in which the measured d-spacing is
typically B1.3–1.4 times the predicted length of a monolayer,
and in diols where the difference is B1.6–1.7 times the
predicted length.41,42,54 For alkylglucosides/maltosides, the
difference in measured versus predicted distance is accounted
for by partial interdigitation of the alkyl chains, although in
XRD of neat unsaturated diethanolamide amphiphiles
A typical 1-D X-ray powder diffraction pattern, for neat oleoyl
diethanolamide at 25 1C, is shown in Fig. 5. Two low-angle
peaks are observed at q-values of 1.83 and 3.65 nmÀ1, consistent
with the first and second order reflections of a lamellar phase
of lattice parameter 34.4 A. For linoleoyl diethanolamide a
single distinct low angle peak was observed at a q-value of
1.90 nmÀ1 close to the low-angle resolution of the beamline
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some cases tilting also occurs.41,42,56 With the diols, the alkyl
chains are in an extremely disordered state, and thus little
interdigitation occurs. Instead, H-bonding in the diol head-
groups is responsible for smectic layer formation, with disordered
chains sandwiched between the ordered headgroups. The
extremely disordered chains combined with a predicted tilt
account for the difference in measurements for this class of
amphiphiles.54 It has been demonstrated that the alkyl chain
length is reduced B20% in the disorganized liquid crystalline
phase.42,57 In the case of the unsaturated diethanolamides, the
location of the two OH moieties in the headgroup is similar to
that of the diols and thus it is likely a similar smectic liquid
crystalline structure occurs in these amphiphiles as predicted
for the diols thus resulting in the difference between the measured
length and predicted length of the amphiphiles. However, further
structural studies are necessary in order to fully describe the
exact structure of the smectic liquid crystalline phase.
Lyotropic phase behavior of unsaturated diethanolamide
amphiphiles
Cross-polarized light microscopy (POM) was used to provide
a simple and rapid assessment of the lyotropic phase behavior
of the three unsaturated diethanolamide amphiphiles, Fig. 6.
Under cross polarizers, cubic and micellar phases appear as
dark bands while anisotropic phases such as lamellar and
hexagonal phases are birefringent with well characterized
textures.58 The sequence of phases formed from the water/
amphiphile interface along with their evident viscosity and
texture formation provide circumstantial evidence for phase
identification. Amphiphile/water mixtures ranging from 5 wt%
water to 95 wt% water were then made in sealed ampoules and
examined over a range of temperatures to construct a preliminary
partial binary phase diagram. SAXS experiments were run at
various concentrations sampling the region of interest determined
from the partial binary phase diagram in order to provide
definitive phase assignment and lattice parameters.
POM measurements indicated that all three unsaturated
diethanolamides form self-assembly phases in water. The
phase behavior observed for all three compounds was quite
similar with each forming an La phase in excess water over a
large temperature range. The temperature range over which
the lamellar phase was observed for each diethanolamide are
presented in Table 2.
The presence of a fluid lamellar phase was confirmed by SAXS
measurements, Fig. 7, and lattice parameters are presented as a
function of water content and temperature in Fig. 8.
Fig. 6 Polarized optical microscope images of water penetration of A.
oleoyl diethanolamide, B. linoleoyl diethanolamide, and C. linolenoyl
diethanolamide All images were acquired at 25 1C (mag 100x).
Oleoyl diethanolamide
sealed in glass ampoules allowing approximate phase bound-
aries in both composition and temperature to be defined as
shown in Fig. 9A. Below 5 1C the system adopts a fully
crystalline phase at all water compositions studied. Above this
temperature the system was anisotropic at all water contents
consistent with the formation of a lamellar phase. A fluid
isotropic phase begins to form at low water contents at
temperatures above 37 1C. The excess water point for the La
phase is approximately 40 wt% water.
Initial water penetration scans using cross-POM indicated that
oleoyl diethanolamide forms an La phase at B5 1C, stable up
to 85 1C, Fig. 6A. In addition to the La phase, the neat
surfactant resides as a smectic liquid crystal. The smectic
liquid crystalline phase melts around 37 1C to form an
isotropic liquid phase in addition to the lamellar phase. Above
85 1C, the entire amphiphile/water mixture becomes an isotropic
melt. An approximate partial binary phase diagram of oleoyl
diethanolamide was constructed from samples with a controlled
water content ranging from 5 wt% water to 95 wt% water
SAXS experiments were therefore carried out to allow
definitive phase assignments to be made. Experiments, using
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Table 2 Phases observed via optical microscopy, and the temperatures at which they appear. All three unsaturated C18 diethanolamides form an
La phase in excess water at room and physiological temperature
Amphiphile
Lamellar
Hexagonal
Isotropic
Isotropic Melt
Stearoyl Diethanolamide (C18:0)b
Oleoyl diethanolamide (C18:1)
Linoleoyl diethanolamide (C18:2)
g-linolenoyl diethanolamide (C18:3)
55–75 1C
5–85 1C
—
—
—
—
—
—
—
—
75 1C
85 1C
75 1C
70 1C
15–75 1C
5–70 1Ca
a
b
Not examined below this temperature. Saturated C18 Diethanolamide data added for comparison.62
the SAXS/WAXS beamline at the Australian Synchrotron,
were run on oleoyl ethanolamide samples at 0, 5, 10, 20, 30
and 50 wt% H2O and at temperatures between 10 and 70 1C.
A typical 1-D diffraction pattern for oleoyl di-ethanolamide
(10 wt% water) at 20 1C is shown in Fig. 7A, Four reflections
in the ratio 1 : 2 : 3 : 4 are observed, marked with an asterisk in
Fig. 7A, consistent with the formation of a lamellar phase. A
diffuse peak was also observed in the wide-angle region at a
q-value of 4.5 AÀ1 indicative of fluid hydrocarbon chains. The
combined phase diagram, showing results from POM and SAXS
is shown in Fig. 9(A). In general we note good agreement
between SAXS and POM experiments with a fluid lamellar
phase observed for all samples studied.
in Fig. 7B The first and second order reflections of a lamellar
phase are marked with an asterisk. Again a diffuse peak in the
wide-angle region is indicative of a fluid lamellar phase. We
note that the fluid micellar L2 phase observed at low water
contents (o 5 wt%) and at temperatures 4 B25 1C by POM,
was not observed using SAXS.
The lattice parameter of the lamellar phase formed by
linolenoyl di-ethanolamide is shown in Fig. 8B as a function
of water content at 10, 20, 25 and 37 1C. As for oleoyl di-ethanol-
amide we note that the lattice parameter of the fluid lamellar
phase increases significantly with increasing water content and
decreases with increasing temperature.
Linolenoyl ethanolamide
The lattice parameter of the fluid lamellar phase is plotted in
Fig. 8A as a function of water content, at 10, 20 and 37 1C. A
significant increase in the lattice parameter with increased
water content is observed, consistent with water uptake by
the fluid lamellar phase. In addition there is a small decrease in
lattice parameter with increasing temperature, reflecting increased
chain splay at higher temperatures. The observed decrease in
lattice parameter with temperature is significantly greater
under excess water conditions (50 wt% water) than under
limited hydration conditions (r30 wt% water). This may
reflect the increased constraint on hydrocarbon chain movement
under limited hydration conditions, preventing the chains
from adopting their preferred conformation.
Initial water penetration scans using cross-POM indicated that
linolenoyl diethanolamide forms an La phase at all temperatures
examined, which is stable up to 70 1C (Fig. 6C). Above 70 1C,
the entire amphiphile/water mixture becomes an isotropic melt.
A partial binary phase diagram for linolenoyl ethanolamide
was constructed from samples with a controlled water content
ranging from 5 wt% water to 95 wt% water sealed in glass
ampoules, allowing approximate phase boundaries in both
composition and temperature to be defined as shown in
Fig. 9C. The phase behavior of linolenoyl diethanolamide is
similar to that of the other two amphiphiles at temperatures
well above their smectic liquid crystalline melting temperatures.
At low water contents (r10 wt% water) the system is fluid
isotropic at all temperatures examined, while at water contents
above this, the entire system forms an La phase. As with the
other two amphiphiles, the excess water point for the La phase
is approximately 40 wt% water.
Linoleoyl diethanolamide
Initial water penetration scans using cross-POM indicated that
linolenoyl diethanolamide forms an La phase at B15 1C which
is stable up to 75 1C (Fig. 6B). In addition to the La phase, the
neat surfactant resides as a smectic liquid crystal. The smectic
liquid crystalline phase melts around 25 1C to form an isotropic
liquid phase in addition to the La phase. Above 75 1C, the
entire amphiphile/water mixture becomes an isotropic liquid.
A partial binary phase diagram of linolenoyl ethanolamide
was constructed from samples with a controlled water content
ranging from 5 wt% water to 95 wt% water sealed in glass
ampoules, allowing approximate phase boundaries in both
composition and temperature to be defined as shown in
Fig. 9B. SAXS experiments were then carried out on samples
at 0, 20, 30 and 50 wt% water at temperatures between 10 and
70 1C. The combined phase diagram, showing results from
POM and SAXS is shown in Fig. 9B.
As linolenoyl diethanolamide is particularly unstable,
SAXS experiments were only carried out on a sample under
excess water conditions. Experiments were carried out at the
Australian Synchrotron at 25, 50 and 70 1C. A typical
diffraction pattern, at 25 1C is shown in Fig. 7C. The calculated
lattice parameter is 45.4 Æ 0.1 A (25 1C), 42.9 Æ 0.1 A (50 1C),
and 41.7 Æ 0.1 A (70 1C), again displaying a decrease with
increasing temperature.
Structure-property comparisons
Unlike the unsaturated C18 monoethanolamides which possess
an effective wedge-shaped conformation due to the small head-
group and relatively large volume occupied by the hydrocarbon
chains resulting from the chain splay introduced via unsatura-
tion30 the unsaturated C18 diethanolamide molecules effec-
tively possess a more rod-like or cylindrical conformation due
to the larger area occupied by the diethanolamide headgroup
(Fig. 1). This effective molecular geometry has been shown to
The phase behavior of linoleoyl diethanolamide is similar to
that of oleoyl diethanolamide with the formation of a smectic
phase for the neat sample and a fluid lamellar phase in the
presence of water. A typical 1-D diffraction pattern, for
linoleoyl di-ethanolamide (30 wt% water) at 20 1C, is shown
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Fig. 8 The lattice parameter of the lamellar phase is plotted as a
function of water content for A. oleoyl diethanolamide and B.
linoleoyl diethanolamide. Data are shown at a range of temperatures
between 10 and 37 1C. In both cases the approximate excess water
point is marked by a dashed line.
Molecules that can form both thermotropic and lyotropic liquid
crystalline phases are known as amphitropic compounds. In
the case of non-ionic amphitropic amphiphiles, polyhydroxy
and carbohydrate amphiphiles are the most widely investigated
compounds exhibiting this property.37,59 The diethanolamide
headgroup is one of the simplest examples of a polyhydroxy
headgroup consisting of two terminal –OH moieties. The
presence of the two hydroxyl groups in addition to the
amide linkage connecting the diethanolamide headgroup to
the unsaturated alkyl chain allows for extensive intermolecular
H-bonding among headgroups. The formation of smectic
phases in polyhydoxy and carbohydrate amphiphiles has been
attributed to extensive intermolecular H-bonding networks
between headgroups. The alkyl chains are described as being
sandwiched between the headgroup network.42,56,57,59,60 In all
cases the d-spacing corresponds to l o d o 2l, where l is the
length of the fully extended molecule and d is the bilayer
thickness. This result holds true for the unsaturated diethanol-
amide amphiphiles as well. Thus, the multiple hydroxyl
groups combined with the XRD and SAXS data support a
smectic bilayer structure for this class of amphiphiles with
Fig. 7 1-D SAXS plots of intensity vs. q for A. oleoyl diethanol-
amide (10 wt% water), B. linoleoyl diethanolamide (30 wt% water)
and C. linolenoyl diethanolamide (50 wt% water). A and B were run at
20 1C and C at 25 1C. In A the first, second, third and fourth order
reflections of a lamellar phase are observed. In B and C the first and
second order reflections of a lamellar phase are indicated. A broad
peak observed at a q-value of approximately 0.4 AÀ1 in A and B is a
kapton signal from the kapton tape used to contain the samples within
the paste cell.
favor the formation of both lyotropic lamellar phases and
thermotropic smectic liquid crystal phases.
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disorganized interdigitating alkyl chains at the core. The
transition between thermotropic and lyotropic phases for this
family of molecules should be continuous since they are non-ionic
and thus the addition of water will not significantly alter the
intermolecular H-bonding.59 Thus, the addition of water sees
the smectic liquid crystal phase of the unsaturated diethanol-
amides transition to a fully swollen lyotropic lamellar liquid
crystal phase. The one difference that occurs with the addition
of water is that it imparts a greater degree of thermal stability
to the H-bonding network thus allowing stabilization of the La
phase up to much higher temperatures than the smectic liquid
crystalline phase.
As the degree of unsaturation increases the lattice parameter
of the fully swollen fluid lamellar phase decreases. For example,
at room temperature all three diethanolamide amphiphiles
adopt a fluid lamellar phase with lattice parameters of 49.6 A
(20 1C), 45.4 A (25 1C) and 42.9 A (25 1C) for oleoyl, linoleoyl
and linolenoyl di-ethanolamide respectively. This reflects the
increased ‘kinking’ introduced into the chain with higher
degree of unsaturation which acts to increase chain splay
and hence reduce the bilayer thickness.
The temperature at which the La phase eventually becomes
fluid isotropic also increases linearly with decreasing degree of
unsaturation. Similarly, the temperature at which the smectic
liquid crystalline phase melts to a fluid isotropic phase increases
somewhat linearly with a decreasing degree of unsaturation.
These results indicate that the chain splay introduced via
unsaturation affect the chain packing ability, which in turn
affects the strength of the van der Waals interaction between
the unsaturated chains. Thus, the greater the number of
unsaturated bonds in the hydrocarbon chain, the lesser the
ability of the chains to pack in an ordered fashion, and thus
the thermal stability of both the thermotropic and lyotropic
phases formed will decrease. Furthermore, the oleoyl derivative is
the only member of the unsaturated C18 diethanolamide
family capable of forming a crystalline solid. The ability of
the amphiphile to crystallize indicates that the oleoyl chain is
still able to pack in a somewhat regular fashion thus imparting
a solid crystalline structure to the neat oleoyl diethanolamide at
lower temperatures. In contrast, the multiple unsaturated bonds
in the linoleoyl and linolenoyl derivatives prevent crystalline
packing resulting in a smectic liquid crystalline form at low
temperatures. This is in contrast to the crystalline solid formed
by both the neat unsaturated C18 monoethanolamide amphi-
philes30 and the neat unsaturated C18 urea amphiphiles.25
Both the urea and monoethanolamide headgroups exhibit very
strong hydrogen bonding, when contrasted with the van der
Waals interaction that exists between the hydrocarbon
Fig. 9 (A) Partial phase diagram for the oleoyl diethanolamide–
water system showing results from SAXS experiments and from
polarising microscopy. SAXS results show a smectic phase (’), a
fluid lamellar (La) phase (K) and a fluid lamellar phase co-existing
with excess water (m). Regions indicated by polarising microscopy
indicated include a crystalline phase region, a fluid isotropic inverse
micellar L2 phase, a fluid lamellar phase region, and a region of fluid
lamellar phase co-existing with excess water. (B) Partial phase diagram
for the linoleoyl diethanolamide–water system showing results from
SAXS experiments and from polarizing microscopy. SAXS results
show a smectic phase (’), a fluid lamellar (La) phase (K) and a fluid
lamellar phase co-existing with excess water (m). A fluid lamellar
phase region, including the excess water point is indicated by Polarizing
microscopy. (C) Partial phase diagram for the linolenoyl ethanol-
amide- water system showing results from SAXS experiments and
from polarizing microscopy. SAXS results at 50 wt% water show a fluid
lamellar phase co-existing with excess water (m). Regions indicated by
polarising microscopy include a fluid isotropic inverse micellar L2
phase and a fluid lamellar phase. As a guide to the eye, dotted lines
demarcate the various phase regions; these should not be considered as
true thermodynamic phase boundaries.
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moieties.25,30 These results indicate that the strength of the
H-bonding compared to the strength of other intermolecular
interactions has a major effect on a molecule’s ability to form
thermotropic liquid crystalline phases. In the case of the
unsaturated C18 diethanolamides, the role of H-bonding and
van der Waals interactions within the molecular arrangement
are more balanced than those in either their urea or mono-
ethanolamide counterparts. A similar result was seen in a
series of hexaalkoxytriphenylenes where the addition of either
an amide or urea group in the alkoxy tail resulted in suppression
of liquid crystalline behavior in the polymer due to strong
hydrogen bonding.61 Thus, when the stronger interactions within
an amphiphile dominate over the weaker ones then the tendency
to form liquid crystal mesophases is markedly reduced.
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Part of this research was undertaken on the SAXS/WAXS
beamline at the Australian Synchrotron, Victoria, Australia.
SMS and CEC were the recipients of CSIRO post-doctoral
fellowships. CJD was the recipient of an Australian Research
Council Federation Fellowship.
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