Thermal behavior of long-chain alkanoylcholine soaps

Article abstract of DOI:10.1039/c3ra47049k

Long-chain alkanoylcholines prepared from fatty acids (nACh) are fully sustainable cationic surfactants that are known for their biological and medicinal properties. In the present work the thermal behavior of the homologous series of alkanoylcholine iodides with n = 12, 14, 16 and 18, has been examined within the 25-200 °C range of temperatures. Up to three thermotropic phases have been identified, and the thermal transitions implied in their interconversion have been characterized by DSC and simultaneous WAXS and SAXS analysis carried out in real-time. All three phases consist of a bilayered structure with alkanoyl chains confined in the space between the head group layers and interdigitated to a greater or lesser extent. Melting-crystallization of either the polymethylene segments or the choline iodide groups is involved in such transitions. Additionally, a crystal phase consisting also of a bilayered structure but excluding side chain interdigitation was observed upon crystallization from solution and its structure was elucidated by single-crystal X-ray diffraction direct methods. The close correlation existing between thermal properties, phase structure and n has been brought into evidence.

Full text of DOI:10.1039/c3ra47049k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Thermal behavior of long-chain alkanoylcholine
soaps†
Cite this: RSC Adv., 2014, 4, 10738
Ainhoa Tolentino,a Abdelilah Alla,a Antxon Martınez de Ilarduya,a Merce Font-Bardıa,b
´
a
`
´
c
*
Salvador Leon and Sebastian Munoz-Guerra
´
´
˜
Long-chain alkanoylcholines prepared from fatty acids (nACh) are fully sustainable cationic surfactants that
are known for their biological and medicinal properties. In the present work the thermal behavior of the
homologous series of alkanoylcholine iodides with n ¼ 12, 14, 16 and 18, has been examined within
the 25–200 ꢀC range of temperatures. Up to three thermotropic phases have been identified, and the
thermal transitions implied in their interconversion have been characterized by DSC and simultaneous
WAXS and SAXS analysis carried out in real-time. All three phases consist of a bilayered structure with
alkanoyl chains confined in the space between the head group layers and interdigitated to a greater or
lesser extent. Melting–crystallization of either the polymethylene segments or the choline iodide groups
is involved in such transitions. Additionally, a crystal phase consisting also of a bilayered structure but
excluding side chain interdigitation was observed upon crystallization from solution and its structure was
elucidated by single-crystal X-ray diffraction direct methods. The close correlation existing between
thermal properties, phase structure and n has been brought into evidence.
Received 26th November 2013
Accepted 3rd February 2014
DOI: 10.1039/c3ra47049k
www.rsc.org/advances
in detail for long time.10 It is well known that metallic salts of
alkanoates display a complex behavior characterized by the
Introduction
occurrence of several thermal transitions involving crystalline,
semicrystalline and liquid-crystalline phases.11,12 The tempera-
ture, nature and number of transitions taking place in these
systems are critically dependent on both the length of the alkyl
chain and the size of the counter cation.13 Cholinium alkanoate
soaps are in line with the trend observed for metallic alkanoates
according to the high voluminosity of the cholinium cation, i.e.
they adopt a relatively low number of mesophases, and their
melting transitions occur at so low temperature that they may
be catalogued as ionic liquids.9 The lyotropic behavior of these
systems has been also examined in deep for both short and long
alkyl chains.14
Surfactants based on fatty acid choline esters are of more
recent interest for their potential as suitable components of
novel green materials. This class of compounds however has
been known for quite some time and shown to display certain
biological activity,15,16 as well as highly appreciated medical
properties.17–21 Long-chain alkanoylcholines are cationic lipids
due to the quaternary ammonium functionality in the head-
group and they are provided with a high molecular exibility
because the easy rotation of the inserted ester linkage. Although
all choline salts are classied as health dangerous compounds
just because they are quaternary ammonium ions, a recent
toxicity and biodegradability study of choline carboxylate
surfactants has proved the innocuity of these products.22 Alka-
noylcholines have in addition the property of being hydrolyzed
rapidly by the action of mammalian enzymes to produce fatty
Choline derivatives are extensively investigated due to multiple
reasons. Choline itself, (2-hydroxyethyl)trimethyl ammonium
salt, is a naturally occurring compound of extreme biological
relevance as a precursor of neurotransmitters and membrane
constituents.1,2 Choline is biocompatible and able to biode-
grade both in living organisms3 and in the environment,4
a
faculty that makes its derivatives very promising candidates for
the preparation of sustainable eco-friendly materials. Such
behavior has in fact incentivized the use of choline as an
additive for eutectic solvents5 or as a counter-ion in ionic
liquids, for which it is able to bring down their melting points.6
On the other hand, choline based ionic surfactants are distin-
guished by displaying low Kra temperatures.7,8 Recent works
have shown that it is possible to have green surfactants with
good water solubility from long-chain alkanoates by using
choline as counterion.9
The work reported in this paper is related to the use of
choline in the design of surfactants derived from fatty acids.
The thermotropic behavior of alkanoate soaps has been studied
a
´
`
Departament d’Enginyeria Quımica, Universitat Politecnica de Catalunya, ETSEIB,
Diagonal 647, Barcelona 08028, Spain. E-mail: sebastian.munoz@upc.edu
b
`
Centre Cientic i Tecnologic (CCiTUB), Universitat de Barcelona, Sole Sabaris 1-3,
Barcelona 08028, Spain
c
´
´
´
Departmento de Ingenierıa Quımica, Universidad Politecnica de Madrid, ETSIIM,
Gutierrez Abascal 2, Madrid 28006, Spain
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47049k
10738 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
acids and choline, both compounds being common human to involve semicrystalline phases and liquid-crystal phases
metabolites. This property makes them potentially valuable to while the thermotropic structural behavior of alkanoylcholines
replace traditional tetraalkylammonium compounds in has not been examined so far. In addition to contributing to the
personal care products.23 To our best knowledge, no study basic knowledge on choline-based surfactants, the present work
addressed to examine the thermotropic properties of alka- was undertaken with the aim of providing the information
noylcholines has been reported to date. Notwithstanding, the about alkanoylcholines necessary to evaluate their inuence on
Hofmeister anion effect on alkanoylcholines in the hydrated the structural properties of the complexes that they generate
state has been evaluated as a function of the counter anion for upon coupling to anionic polyelectrolytes.
their interest as bioactive surfactants.24 Although this study was
mainly centered in the gel behavior of the whole alkanoylcho-
line series, the crystal structure of hydrated stearoylcholine
iodide was solved. It was reported to consist of a bilayered
Experimental
Materials
arrangement with alkanoyl chains aligned and interdigitated in
a paraffinic phase that separates the polar layers integrated by
the cholinium iodide head groups.
Lauroyl (98%), miristoyl (97%), palmitoyl (97%) and stearoyl
(90%) chlorides, 2-(dimethylamino)ethanol (98%) and methyl
iodide (99%) were supplied from Sigma-Aldrich and used as
received. Chloroform, dichloromethane, pentane, hexane and
acetone were supplied from Panreac and used without further
purication.
In this paper we study the thermal behavior of the homolo-
gous series of alkanoylcholine iodides (nACh$I) derived from
lauric, myristic, palmitic and stearic acids (n ¼ 12, 14, 16 and
18, respectively), which are represented in Scheme 1. The study
examines the thermal transitions occurring in these
compounds along the 20–200 ꢀC range both at heating and
cooling. The diverse phases appearing or disappearing by effect
of temperature are characterized by DSC, real-time XRD, solid-
sate 13C CP/MAS NMR and optical microscopy.
Synthesis of alkanoylcholine iodides
The synthesis of the alkanoylcholine iodide surfactants (nACh$I)
was carried out from 2-(dimethylamino)ethanol, fatty acids of 12,
14, 16 and 18 carbon atoms, and methyl iodide through two
concatenated reactions, esterication and quaternization,
without isolation of the intermediate aminoethyl ester. The
procedure was the following: a solution of 0.06 mol of 2-(dime-
thylamino)ethanol (DMAE) in chloroform (50 mL) was added
drop-wise to a solution (100 g Lꢁ1) of 0.04 mol of the fatty acid
chloride in the same solvent at a temperature between 0 and 20
ꢀC. Aer neutralization with aqueous NaHCO3, the organic phase
containing the 2-(dimethylamino)ethyl-alkanoate (DMAE-nA)
was separated as oil. For quaternization, DMAE-nA was made to
react with an excess of methyl iodide at room temperature. Upon
standing overnight the resulting alkanoylcholine iodide (nACh$I)
was obtained as a white precipitate that was puried by repreci-
pitation from chloroform with acetone. Yields were around 90%
except for n ¼ 12, which was only 25% due to isolation/
purication difficulties associated to the higher solubility dis-
played by this compound. The alkanoylcholine iodides were
soluble in a variety of organic solvents such as chloroform,
dichloromethane and methanol, and also in hot water. All they
were obtained as white powders and their chemical constitution
and purity were ascertained by NMR. A detailed account of the
conditions used in the synthesis of each compound together with
the NMR spectra of the whole series is provided in the ESI† le.
It is worthy to mention that a good number of studies
dealing with the thermal-structural behavior of a wide diversity
of cationic surfactants, both in solution and in the pure state,
are found in the literature. The occurrence of mesophases of
smetic type is a very common feature of all these systems.11
Among them, the ones closest to the ours are those dealing with
tetralkylammonium halides bearing long alkyl groups. In these
ꢀ
studies a transition taking place in the 70–130 C temperature
range and leading to a layered semicrystalline structure with the
alkyl chains in the molten state was described. The thermal
instability of such compounds hampered the detailed study of
high-temperature phases.25 Attachment of longer alkyl or
hydroxyalkyl groups to the nitrogen allowed to observing broad
and stable smectic phases.26
The alkanoylcholine iodides series object of the present
study has been recently used for ionic coupling with microbial
poly(g-glutamic acid) in order to obtain amphiphilic polymeric
complexes.27 These bio-based comb-like polymers are well
stable and readily able to self-assemble in supramolecular
structures with features similar to the lamellar arrangement
described for cationic surfactants bearing long alkyl tails. The
thermal transitions taking place in these complexes were found
NMR
1H- and 13C NMR spectra were registered at 300.1 and 75.5 MHz,
respectively, on a Bruker AMX-300 NMR instrument equipped
with a variable temperature unit. The samples were dissolved in
chloroform and TMS was used as internal reference. 128 FIDs
for 1H NMR spectra were recorded with 2.3 ms (30ꢀ) pulse width,
3.4 s acquisition time, 20 s relaxation delay, and 4.9 kHz spectral
width. For 13C NMR spectra, 1000 to 10 000 FIDs were recorded
using pulse and spectral widths of 4.3 ms (90ꢀ) and 18 kHz,
respectively. 13C CP/MAS NMR spectra were recorded within the
Scheme 1 Chemical structure of the alkanoylcholine iodides nACh$I
studied in this work (n ¼ 12, 14, 16 and 18).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10739

View Article Online
RSC Advances
Paper
ꢀ
temperature range of 25 to 95 C. Samples were heated rst at and SAXS, respectively. X-ray diffraction patterns produced by
190 ꢀC and then cooled to room temperature before spectral using conventional light were recorded on the PANalytical
acquisition. Around 200–250 mg sample were spun at approxi- X’Pert PRO MPD q/q diffractometer using the Cu-Ka radiation of
mately 4 kHz in a cylindrical ceramic rotor. All the spectra were wavelength 0.1542 nm.
acquired with contact and repetition times of 2 ms and 5 s,
respectively. Around 640 transients were accumulated. The
spectral width was 31.2 kHz, and the number of data points was
4 K. Chemical shis were externally calibrated against the
carbonyl peak of glycine appearing at 176 ppm relative to TMS.
Optical microscopy
Optical polarizing microscopy was performed on an Olympus
BX51 Orthoplan microscope with an Olympus digital camera
attached. A Linkham THMS-600 hot stage provided with a CS-
196 nitrogen cooling system, and equipped with TMS90
Critical micelle concentration (cmc) and Kra temperature
determinations
ꢀ
temperature controller (ꢂ0.5 C) was used for this study. Thin
lms of alkanoylcholines were prepared by casting and trapped
between glass cover slides. Observations were carried out under
a nitrogen atmosphere to avoid contact with air humidity, and
at least two heating–cooling cycles were applied for each sample
The cmc of nACh$I in D2O was determined by 1H NMR by
following the evolution of the chemical shis of specic signals
of the surfactant with increasing concentration according to the
procedure described in the literature.28,29 Samples were dis-
solved in D2O, and 1H NMR spectra were recorded at the
selected temperature using the sodium salt of the 3-
(trimethylsilyl)-propanesulfonic acid as internal reference.
The Kra temperatures were estimated both visually and
conductimetrically. For visual estimation, 1% mixtures of
nACh$I in water were heated until dissolution and then cooled
down to room temperature and kept in a refrigerator at 5 ꢀC for
24 hours. The cooled samples were then heated up in steps of
ꢁ1
ꢀ
at a rate of 5 C min
.
Stearoylcholine iodide (18ACh$I): single-crystal analysis
Crystallization of 18ACh$I addressed to grow crystals suitable
for single-crystal X-ray diffraction analysis was performed by the
vapor-diffusion technique. In brief, a solution of 0.5 mg of
18ACh$I in 0.5 mL of chloroform was placed in a capped 2 mL
glass vial provided with a needle and the vial put inside a sealed
glass ask containing 5 mL of hexane. The solution was then
1
ꢀC every 15 min in a water bath provided with a magnetic
ꢀ
le undisturbed at 25 C for a few days until a ne crystalline
precipitate was observed.
stirring, and the temperature at which turbidity disappeared
was taken as the Kra temperature. Conductimetry measure-
ments were performed in a Mettler Toledo conductivity meter
S30, provided with an lnlab720 sensor with a 0.06 cell constant.
Dispersions were heated in a jacketed vessel with a heating rate
of 0.5 ꢀC every 5 min and conductivity was measured. The Kra
temperature was determined as the inection point of the
conductivity values versus temperature plot.
A prismatic crystal (approx. 0.2 ꢃ 0.1 ꢃ 0.07 mm) was
selected and mounted on a MAR345 diffractometer using
graphite monochromatized Mo-Ka radiation (l ¼ 0.071073 nm)
and provided with an image plate detector. Unit-cell parameters
were determined from 1141 reections located in the 3ꢀ < q <
31ꢀ range, and rened by the least-squares method. Intensities
of 6616 reections in the range 1.33ꢀ # q # 32.32ꢀ were
collected and the structure was solved by direct methods and
rened by using SHELXS computer programs. A fully detailed
description of the methodology used for this analysis is
provided in the ESI† le attached to this paper.
Thermal measurements
Decomposition temperatures (Td) were measured using a
Perkin-Elmer TGA6 thermobalance. Data were collected under a
ꢁ1
ꢀ
constant nitrogen ow at a heating rate of 10 C min within
the 30 to 600 ꢀC temperature interval. Differential scanning
calorimetry (DSC) measurements were performed on a Perkin-
Results and discussion
Elmer Pyris1 DSC instrument provided with an Intracooler cmc and TKra of nACh$I
device, and calibrated with indium and zinc. Sample weights of
The changes taking place at 50 ꢀC in the 3.15–3.30 ppm region
about 2–5 mg were examined within a temperature range of ꢁ30
to +200 ꢀC at heating and cooling rates of 10 ꢀC minꢁ1 under a
nitrogen atmosphere. Several cycles were recorded for each
compound and raw data were corrected in order to attain a at
baseline.
of the 1H NMR spectra of 12ACh$I, which is the region in which
the resonance of the methyl protons appears, as a function of
concentration are depicted in Fig. 1a, and the chemical shi of
the methyl peak is plotted against the inverse of concentration
in Fig. 1b. This plot shows that the position of the methyl signal
is independent of the amount of surfactant present in the
solution until the cmc is reached, aer which it moves upeld
X-ray scattering
X-ray real time diffraction studies of powdered samples were linearly with the inverse of concentration. It is interpreted that
carried out using X-ray synchrotron radiation in both WAXS and below cmc the observed resonance signal arises from free
SAXS modes at the A2 beam-line of light source DORIS III at surfactant molecules whereas above cmc the signal is attributed
DESY in Hamburg. Variable temperature experiments were to the equilibrium mixture of free and aggregated molecules
performed at heating and cooling rates of 5 ꢀC minꢁ1. The distributed between the solution and the micelles, respectively.
energy employed corresponded to a 0.15 nm wavelength, and The cmc values determined by this method for the four nACh$I
spectra were calibrated with PET and rat tail tendon for WAXS studied in this work together with the values reported in the
10740 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
1
Fig. 1 (a) Evolution of the H NMR methyl signal of 12ACh$I with concentration. (b) Plot of the methyl chemical shift against concentration at
50 ꢀC with indication of the cmc.
literature for the widely known alkyltrimethylammonium
bromides (nATMA$Br) are given in Table 1. Note that measuring
cmc of nACh$I in water required heating the samples above
room temperature to reach the needed concentrations. TKra
values determined by visual and conductimetric measurements
are presented in Table 1. A satisfactory coincidence was found
between the two methods. In all cases these temperatures are
well above the room temperature and much higher than those
reported for nATMA$Br. As it could be anticipated the Kra
temperatures steadily increased with the number of methylenes
of the alkyl chain.
Thermal decomposition
The TGA traces recorded from nACh$I surfactants under an
inert atmosphere are depicted in Fig. 2, and the decomposition
Fig. 2 TGA traces of nACh$I recorded under an inert atmosphere.
temperatures together with the weights remaining aer heating
ꢀ
at 600 C are listed in Table 2. Decomposition of nACh$I was
Table 1 Values of cmc and Krafft temperatures of nACh$I compared
found to start around 200 ꢀC with onset temperatures
with nATMA$Br
increasing slightly with the length of the alkyl chain. TGA
derivative curves (see ESI†) revealed that the decomposition
process evolves through a mechanism in two-steps that have
their respective maximum temperature rates located within the
225–250 C and 320–325 C intervals. The sample weight loss
aer the rst decomposition step increased steadily from 18%
up to 56% with n, whereas no residue was le at the end of the
heating treatment. Alkanoylcholines bearing long chains are
less thermally stable than acetylcholine in about 30 ꢀC but
compares well to the choline carboxylate soaps,9 as well as to
other cationic surfactants derived from alkylamines.38
Critical micelle
concentration (mM)
Kra
temperature (ꢀC)
T
Conducti-
metry
ꢀ
ꢀ
Surfactant (ꢀC) cmc
Solvent Method
Visual
38.5
nACh$I
12ACh$I
12ACh$I
14ACh$I
16ACh$I
18ACh$I
50
5.3 D2O
NMR
NMR
NMR
NMR
NMR
37.9
80 10.5 D2O
60
80
95
1.9 D2O
1.0 D2O
1.2 D2O
48.4
57.0
61.5
50.0
55.5
61.5
nATMA$Br
12ATMA$Br 35 16.130 D2O
NMR
NMR
<033
<035
Differential scanning calorimetry
14ATMA$Br 35
16ATMA$Br 25
18ATMA$Br 40
4.130 D2O
9.5–10.533 13.236
0.931 Water Ultrasounds
2634
2637
—
Since nACh$I compounds start to decompose in the proximities
of 200 ꢀC, the DSC study has been restricted to temperatures
0.332 Water Conductimetry 35–3633
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10741

View Article Online
RSC Advances
Paper
Table 2 Thermal properties of nACh$I surfactants
DSCb
Transition temperatures (ꢀC) and enthalpies in parenthesis (kJ molꢁ1
)
TGAa
First heating
Cooling
Second heating
nACh$I
oTd (ꢀC)
197
maxTd (ꢀC)
W (%)
Ia/II
II/III
II/Ib
III/II
Ib/II
II/III
12ACh$I
14ACh$I
16ACh$I
18ACh$I
2ACh$Ic
231–243
303
236–247
318
233–254
303
225–247
322
18
0
30
0
37
0
56
0
83
(25.9)
93
(33.4)
99
(41.4)
104
169
(14.2)
164
(14.6)
163
(14.6)
161
41
165
52
(10.9)
59
(15.5)
70
(22.1)
80
168
(13.8)
162
(13.8)
161
(14.2)
160
(ꢁ11.3)
(ꢁ13.8)
207
47
161
(ꢁ16.7)
59
(ꢁ13.0)
160
209
(ꢁ23.0)
(ꢁ13.8)
209
70
157
(50.2)
165
(15.0)
(ꢁ29.3)
94
(ꢁ14.6)
(24.7)
164
(14.6)
229
261
9
322
0
(32.6)
(ꢁ24.2)
(30.9)
Onset decomposition temperature at 5% weight lost (oTd), maximum rate decomposition temperature (maxTd), and remaining weight (W) for each
decomposition step. Transition temperatures and enthalpies recorded at the rst DSC run. Data for this compound refer to its melting–
a
b
c
crystallization transition.
below this limit. An exploratory essay in which samples were of crystal polymorphs that could be simultaneously generated
ꢀ
heated up to 250 C did not show any sign of heat exchange in in the precipitation from solution step that is applied for puri-
the 200–250 ꢀC range. DSC traces of successive heating–cooling cation in the synthesis stage. On the contrary, only two
ꢀ
cycles of nACh$I along the 0–175 C range of temperatures are exothermic peaks are observed on the cooling traces of nACh$I,
depicted in Fig. 3, and temperatures and enthalpies recorded which are respectively associated to the two endothermic peaks
for the thermal transitions observed on rst cycle traces are that are displayed on the second heating traces. These two exo/
compared in Table 2. In the four cases, the rst heating trace endo peak pairs are made to correspond to two reversible
displays a group of two or three endothermic peaks within the transitions taking place in nACh$I along the examined
70–110 ꢀC interval, in addition to an isolate sharp peak located temperature interval. Notice that the transition occurring in the
between 160 and 170 ꢀC. The multiple heat adsorption observed low temperature region (below 110 ꢀC) implies a quite large
in the lower temperature range strongly suggests the presence supercooling (ꢄ40–50 ꢀC) and produces a material that shows at
Fig. 3 DSC traces of successive heating–cooling cycles (from top to bottom) of nACh$I along the 0–175 ꢀC range of temperatures.
10742 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
ꢀ
the second heating an endothermic peak at about 30 C lower
than in the rst heating. On the contrary, the high temperature
transition is well reproduced in the second heating trace with
both position and intensity of the endothermic peak essentially
preserved at the initial values. Furthermore, this transition
entails a very low supercooling indicating that a very effective
nucleation must operate in the recovery of its lower temperature
phase. These features strongly suggest that the low temperature
exo/endo peaks must involve a crystal melting process whereas
the heat exchange taking place at high temperature must arise
from a transition implying two structurally close liquid-crystal
phases. The DSC recorded from acetylcholine iodide (2ACh$I)
(shown in the ESI† le) is of valuable help in this regard. The
rst heating trace of this compound consists of only one sharp
endothermal peak at 165 ꢀC characteristic of melting that is well
reproduced in the second heating trace; crystallization at cool-
ing from the melt takes place at about 70 ꢀC supercooling. The
melting peak observed for 2ACh$I is comparable in location to
the high temperature transition endothermic peak observed for
nACh$I but it entails a much larger enthalpy and its recovery by
cooling from the melt requires much larger supercooling. Such
dissimilarities clearly indicate that the structural changes
implied in each case must be different. Since no liquid-crystal
phase is conceivable for the relatively short 2ACh$I molecule,
the endothermic peak observed for this compound doubtlessly
arises from the melting of the crystal phase into an isotropic
liquid phase. On the contrary, heating of nACh$I in the prox-
imities of 160–170 ꢀC would generate a partially ordered phase
where the long amphiphilic molecules still maintain a consid-
erable degree of side-by-side alignment.
Scheme 2 Interrelation between the nACh$I phases and their corre-
sponding transition temperatures (h and c denote heating and
cooling).
nACh$I: two phases (Ph-Ia and Ph-Ib) with existence at low
temperatures, one phase occurring at high temperatures (Ph-
III), and one phase with a existence domain in the interme-
diate temperature region (Ph-II). Ph-Ia is assumed to be a well
stable phase formed in the crystallization from solution
whereas Ph-Ib is a metastable phase that is generated by cooling
the previously heated samples. Upon standing at room
temperature for hours or by annealing, Ph-Ib is converted in Ph-
Ia. The interrelations among these phases are depicted in
Scheme 2 and their structural details are discussed below.
Thermal transitions
Several reversible thermal transitions have been characterized
Coming back to the heat exchange taking place at tempera- for nACh$I by DSC along the 0–200 ꢀC interval of temperatures.
tures below 120 ꢀC, it should be noticed that the behavior I/IITh(a) (measured at the rst heating) and I/IITh(b) (measured at
observed for both 18ACh$I and 16ACh$I at the second heating the second heating) are the temperatures at which Ph-Ia and
run was different from that displayed by their lower homo- Ph-Ib, respectively, are converted into Ph-II, and II/IIITh is the
logues. Both compounds underwent a complex heat exchange temperature of conversion of Ph-II into Ph-III. II/ITc(b) and III/IITc
process that ended in a material showing an endothermic peak are the temperatures measured on cooling for the Ph-II to Ph-Ib
at a temperature similar to that observed for their respective and Ph-III to Ph-II transitions, respectively. The plot of transi-
pristine samples. Such pattern is characteristic of a melting– tion temperatures against n is shown in Fig. 4a revealing that a
recrystallization process involving a structural rearrangement linear trend is followed for all nACh$I although noteworthy
from a less stable phase to a more stable one. This process differences are observed among them. (a) Transitions involving
however lost relevance with the number of applied heating– Ph-I and Ph-II take place at temperatures increasing with a
cooling cycles. In fact, the initial endothermic peak increased in similar slope but showing noticeable higher values for I/IITh(a).
intensity at the same time that the heat release decreased to (b) The supercooling (I/IITh(b)–II/ITc(b)) associateꢀd to the lower
fully disappear on the fourth heating trace. It should be stressed temperature transition is noticeable (around 20 C) and essen-
that this process was never observed neither for 14ACh$I nor for tially constant along the series. (c) Both melting and crystalli-
12ACh$I indicating their dependence on the alkyl chain length. zation involving Ph-II and Ph-III occur at temperatures slightly
The differences between higher (n ¼ 18 and 16) and lower (n ¼ decreasing with n but that are practically coincident along the
14 and 12) nACh$I compounds remained for samples that had whole series. The supercooling required for reversing this
been previously heated at temperatures below the temperature transition is practically negligible.
at which the second transition takes place (160–170 ꢀC).
The enthalpy and entropy values involved in the transitions
Furthermore, it was also found that the reverse behavior of are plotted against n in Fig. 4b and c, respectively. The values
18ACh$I was dependent on the length of time that the sample associated to the low temperature transitions increase linearly
was maintained under heating. DSC traces demonstrative of with n as it should be expected provided that the alkanoyl chain
this behavior are provided in the ESI† document.
is taking active part in the molecular rearrangement. Moreover
At the light of DSC observations and disregarding at this the enthalpy associated to the conversion of Ph-Ia into Ph-II
moment the possible polymorphs present in the samples crys- (rst heating) takes values between 15 and 20 kJ molꢁ1 higher
tallized from solution, four phases may be identied for than for Ph-Ib, and its variation with n follows a slope of near
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10743

View Article Online
RSC Advances
Paper
18ACh$I: a single crystal study
nACh$I were found to yield well shaped microcrystals upon
casting from chloroform indicating that the phase Ph-Ia
obtained by precipitation from this solvent at room temperature
is highly crystalline. Illustrative pictures of the microcrystals
made of 18ACh$I are included in the ESI† le. With the aim at
elucidating in detail the structure present in this phase, a single
crystal suitable for X-ray diffraction analysis (0.2 ꢃ 0.1 ꢃ 0.07
mm size and exempt of signicant imperfections such as cracks
or twinning) was grown using the vapor-diffusion method in
absence of humidity. A picture of the crystal used for the anal-
ysis along with a full account of the crystallographic data
collected and handled in this study is given in the ESI† le. The
structure could be resolved with an R factor of 0.052. It was
found that 18ACh$I crystallized in a triclinic lattice with space
group P1 and unit cell dimensions a ¼ 0.576 nm, b ¼ 0.767 nm,
c ¼ 3.09 nm, a ¼ 83.1ꢀ, b ¼ 87.0ꢀ, g ¼ 89.3ꢀ, containing two
molecules related by an inversion centre. A representation of a
portion of the crystal lattice including ten unit cells as viewed
along the b-axis is depicted in Fig. 5. In this crystal the stear-
oylcholine molecules are arranged in a bilayered structure with
the polar trimethylammonium iodide head groups arranged
along the C-plane and the ethyloxystearoyl tails protruding from
this plane and following a bended trajectory caused by the kink
introduced by the carboxylate unit. As a result the alkyl tails
become tilted about 45ꢀ respect to the c-axis to x an interlayer
distance of ꢄ3.1 nm without appreciable interdigitation. An
ORTEP representation of the 18ACh$I molecule in the confor-
mation adopted in the crystal together with a complete account
of their atomic coordinates, bond distances and torsion angles
are included in the ESI† le.
Tarafdar et al.24 have recently reported on the crystal
structure of 18ACh$I. The structure found by these authors is
also triclinic P1 with unit cell dimensions a ¼ 0.799 nm, b ¼
0.963 nm, c ¼ 3. 5413 nm, a ¼ 86.40ꢀ, b ¼ 84.79ꢀ, g ¼ 89.96ꢀ
and contains 4 molecules. In this structure the alkyl chains are
aligned practically parallel to the c-axis (tilt angle ¼ 1.7ꢀ) and
they are deeply interdigitated. Obviously these authors are
dealing with a crystal form of 18ACh$I different from that
described above by us. It was noticed that one water molecule
every two surfactant molecules was inserted in the crystal
Fig. 4 Phase transition temperatures (a), enthalpies (b), and entropies
(c) of nACh$I compounds as a function of n. In (b) the negative
enthalpies registered at cooling are represented in positive values for a
closer comparison with the enthalpies registered at heating.
25% greater. This is a clear indication of that not only crystal-
linity of Ph-Ia is higher than of Ph-Ib but also that the individual
methylene contribution to the stability of Ph-I is greater in this
phase, as it should be expected for a more efficiently packed
crystal phase. With regards to the transition taking place at high
temperature, both enthalpy and entropy appear to be practically
independent of n. This result is fully consistent with the
invariability found in both II/IIITh and III/IITc along the series,
and strongly suggests that the alkanoyl chain must not partic-
ipate actively in the molecular rearrangement involved in this
transition.
Fig. 5 View of the 18ACh$I crystal (Ph-Ia) projected along the b-axis.
Ten unit cells are visualized.
10744 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
lattice described by these authors, which is consistent with the amount of water involved in this study is much larger than
solvent they used for crystal growth (a mixture of chloroform, that adsorbed in the 18ACh$I crystal described by Tarafdar
methanol and water). The presence of water in 18ACh$I could et al.24 However the aqueous environment used by these
be reasonably invoked to explain the different lattice dimen- authors for crystallization could favor initially a hydrated gel
sions found in each case and the consequent difference in the interdigitated structure that is retained aer most of water is
arrangement adopted by the alkyl chains to ll the interlayer released to form the crystal.
space in the two crystal forms. The critical dependence of the
structural behavior of choline-based lipid bilayers on hydra-
tion has been accounted in the study of hydrated dihexadecyl
Ph-Ia phase
phosphatidylcholine in a range of 5–70 wt% of water.39–41 It
was found there that the structure of this compound rear-
ranged from a two-separated sheets bilayer to a deeply inter-
digitated bilayer as the content in water increased. Different
factors including hydrogen bonding were claimed to be
responsible for the described water-mediated transition. The
The WAXS and SAXS proles recorded from Ph-Ia of nACh$I are
compared in Fig. 6 for the whole series, and the most charac-
teristic spacings measured on such patterns are listed in Table
3. The samples subjected to this analysis were those coming
from synthesis, i.e. generated by precipitation from chloroform
by adding acetone, aer being subjected to annealing in order
to improve crystallinity; according to their melting tempera-
tures they consist of Ph-Ia. The presence of a layered structure
in these samples is ascertained by the strong reection
appearing in the SAXS region (Fig. 6) at a spacing moving from
2.5 nm to 3.1 nm as the alkanoyl chain carbon number
increases from 12 to 18. In the WAXS region, all the nACh$I
displayed similar patterns consisting of a fair number of sharp
peaks concentrated in the 0.50–0.30 nm interval with the
strongest one corresponding to the 0.455 nm spacing. Most of
the other peaks appearing at lower spacings can be grouped in
three groups centered around 0.40, 0.36 and 0.30 nm. The
similitude of the groups for the whole nACh$I series is
remarkable with main discrepancies concerning the reections
with l s 0 as it should be expected from differences in the
alkanoyl chain length.
When the XRD pattern experimentally recorded from
18ACh$I powder was compared with the XRD pattern simulated
for the crystal structure shown in Fig. 5, an extremely close
coincidence was found in both SAXS and WAXS regions (see
Fig. 6 X-ray diffraction profiles of Ph-Ia phase of nACh$I. The peak
arising from the intersheet spacing (001) of the layered structure is
displayed in the SAXS region (2q < 5ꢀ). The WAXS scattering (2q > 15ꢀ)
contains the ꢄ0.45 nm peak characteristic of this phase.
Table 3 Temperature domains and XRD data for the nACh$I phasesa
Ph-Ia
Ph-Ib
Ph-II
Ph-III
T
(ꢀC)
SAXS
(nm)
WAXS
(nm)
T
(ꢀC)
SAXS
(nm)
WAXS
(nm)
T
(ꢀC)
SAXS
(nm)
WAXS
(nm)
T
(ꢀC)
SAXS
(nm)
WAXS
(nm)
n
12
<83
<93
<99
<104
2.5
2.7
2.9
3.1
0.30
0.36
0.45
<52
<59
<70
<103
3.3
3.6
4.0
4.2
0.32
0.35
0.40
0.55
0.32
0.35
0.40
0.55
0.32
0.35
0.40
0.55
0.32
0.35
0.40
0.55
52–169
59–162
70–161
103–160
3.5
3.8
4.2
4.5
0.30
0.36
0.40
0.55
0.30
0.36
0.40
0.55
0.30
0.36
0.40
0.55
0.30
0.36
0.40
0.55
>169
>162
>161
>160
3.1
3.3
3.7
3.9
—
—
—
—
14
16
18
0.30
0.37
0.45
0.30
0.37
0.45
0.30
0.37
0.45
a
The strongest reection observed in each case is in bold.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10745

View Article Online
RSC Advances
Paper
ESI†), which conrmed that the structure determined for the structural modications involved in these transitions were fol-
monocrystal is that present in Ph-Ia. Furthermore, given the lowed by X-ray diffraction in real time using synchrotron radi-
similar scattering displayed by the Ph-Ia phase in all ation with simultaneous recording of the scattering changes
the nACh$I, it can be reasonably assumed that same structure is occurring in the small and wide angle regions. A list of the
shared by the whole series. Taking as reference the 18ACh$I spacings observed for the whole series in the two regions with
crystal structure, similar lattices were built for the other precise indication of the temperatures at which they appear is
members of the series (see ESI† le). The same crystallographic given in Table 3. For illustration the sets of SAXS plots registered
parameters but with the c-axis shortened according to the from 16ACh$I at heating, cooling and reheating are depicted in
observed 001 spacing length were used for building, and the Fig. 7, and the whole collection of plots recorded from the other
same molecular arrangement was maintained in all the cases.
members of the series including also those obtained from
2ACh$I are provided in the ESI† le.
At rst glance the two thermal transitions dening the
existence domains of Ph-I, Ph-II and Ph-III phases are clearly
denoted in the SAXS plots by jumps in the positions of their
respective characteristic peaks. It is remarkable that whereas
Ph-II and Ph-III are faithfully reproduced in second heating
plots aer cooling, the low-temperature peak (initially Ph-Ia)
reappears in the second heating largely displaced towards
higher spacings. Furthermore, the three peaks initially observed
for 12ACh$I merged into one aer cooling from the melt. These
results are in concordance with the occurrence of two phases
(Ph-Ia and Ph-Ib) at low temperature, and prove the relative
instability of Ph-Ib respect to Ph-Ia. According to antecedents
on other alkylammonium surfactants containing long alkyl
chains, the long-spacings observed for nACh$I should be asso-
ciated to the interlayer distance L characteristic of the lamellar
structure present in the crystal, semicrystalline or liquid crys-
talline phases eventually present in these systems.25 In fact, no
discrete SAXS at all was observed for 2ACh$I as it should be
expected from the small acetyl group. The L values for the four
phases that have been identied for nACh$I are plotted against
n in Fig. 8. Remarkable features of this plot are the following: (a)
a linear trend is obtained for all the phases and extrapolation to
n ¼ 0 yields a similar value in the four cases. (b) The straight
lines resulting for Ph-Ib, Ph-II and Ph-III are close to each other
and have a very similar slope. On the contrary, much lower
values of L are found for Ph-Ia and its increment with n occurs
at a signicant lower ratio.
Ph-Ib, Ph-II and Ph-III phases
Heating of Ph-Ia of nACh$I at temperatures between 80 and 105
ꢀC led to Ph-II, and further heating of Ph-II above 160–170 ꢀC led
to Ph-III. The exact temperatures at which the phase transitions
take place for each surfactant are listed in Table 2. The
A preliminary analysis of the SAXS data taking into account
conformational considerations and assuming some approxi-
mations regarding molecular volumes allows outlining a rst
Fig. 7 SAXS plots from 16ACh$I registered at heating from room Fig. 8 Lamellar interlayer spacing L of nACh$I as a function of the
temperature (a), cooling (b) and reheating (c).
number of carbon atoms in the alkanoyl chain.
10746 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
picture of the molecular arrangement present in each phase.
First, the difference between the lamellar thickness and the L-
intercept yields an estimation of the thickness (L0) of the alka-
noyl core in the structure; a comparison of this value with the
length of the alkanoyl tail calculated for an all-trans confor-
mation (l) gives indication of the extent of shortening under-
gone by the structure in terms of chain tilting, degree of
interpenetration or gauche conformation present in each phase.
The results of these calculations are compared in Table 4. The
low (L ꢁ L0)/l ratio (0.8) found for Ph-Ia is consistent with the
tilting of the alkanoyl chains and in fully agreement with
the crystal model resulting from the structural analysis of the
monocrystal. The (L ꢁ L0)/l ratio estimated for Ph-Ib is between
1.25 and 1.30, which is noticeably higher than for Ph-Ia.
Assuming that Ph-Ib appears as a consequence of the normal
orientation respect to the layer plane adopted by the alkanoyl
chains upon heating while preserving their fully-extended
conformation, the calculated (L ꢁ L0)/l ratio for this phase is
consistent with an interdigitated chain arrangement with
around 75% of interpenetration. In Ph-II, the (L ꢁ L0)/l ratio is
essentially constant along the whole series with a value close to
1.4 according to an interpenetration of 60% provided that both
orientation and conformation of the alkanoyl tail are the same
as in Ph-Ib. Regarding Ph-III, the (L ꢁ L0)/l ratio takes values
(ꢄ1.1) signicantly lower than in Ph-II, and even lower than in
Ph-Ib, which reveals a maximum degree of interdigitation for
this phase.
The real-time WAXS analysis of nACh$I afforded invaluable
information on the short distance rearrangements, i.e. those
involving the head groups and alkanoyl tails side-by-side
packing, that take place by effect of temperature. The three
sets of WAXS proles registered from 16ACh$I at heating,
cooling and reheating are depicted in Fig. 9 for illustration, and
those recorded from the other members of the series including
also those obtained from 2ACh$I, are provided in the ESI† le.
The spacings of the characteristic peaks of the different phases
occurring for nACh$I are listed in Table 3. Inspection of Fig. 9
brings to light the following relevant points: (a) the initial
proles registered along the rst heating run show the peaks
characteristic of Ph-Ia, in particular the prominent one at 0.45
nm. (b) When temperature overpasses 100 ꢀC, the proles
become simpler and consist of one main peak located near to
0.40 nm accompanied by two other much weaker peaks at 0.36
and 0.30 nm; this is the pattern characteristic of Ph-II. (c) At
Fig. 9 WAXS plots from 16ACh$I registered at heating from room
temperature (a), cooling (b) and reheating (c).
ꢀ
temperatures above 165 C, the scattering losses any vestige of
discrete diffraction indicating that Ph-III is fully devoid of
crystalline order. Crystallization from Ph-III invariably happens
at cooling below 160 ꢀC with recovery of the WAXS pattern
characteristic of Ph-II, which remains essentially unaltered
along the whole cooling run nishing at 10 ꢀC. In fact, no
appreciable differences between the WAXS patterns of Ph-II and
Ph-Ib were noticed indicating therefore that the arrangement in
Table 4 Interlayer distance (L), choline iodide head size (L0) and shrinkage ratio (L ꢁ L0)/l in the nACh$I phases
Ph-Ia
Ph-Ib
Ph-II
Ph-III
na
lb
L
L0
(L ꢁ L0)/l
L
L0
(L ꢁ L0)/l
L
L0
(L ꢁ L0)/l
L
L0
(L ꢁ L0)/l
12
14
16
18
1.5
1.75
2.0
2.5
2.7
2.9
3.1
1.3
1.3
1.3
1.3
0.80
0.80
0.80
0.80
3.3
3.6
4.0
4.2
1.4
1.4
1.4
1.4
1.27
1.26
1.30
1.25
3.5
3.8
4.2
4.5
1.4
1.4
1.4
1.4
1.40
1.37
1.40
1.38
3.1
3.3
3.7
3.9
1.4
1.4
1.4
1.4
1.13
1.09
1.15
1.11
2.25
a
Number of atoms in the alkanoyl tail. b Length of the alkanoyl tail in all-trans conformation assuming a C–C bond length of 0.125 nm.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10747

View Article Online
RSC Advances
Paper
the crystalline part of the structure must be the same in the two observed in the signals arising from internal methylene units
phases. It seems that differences between Ph-Ib and Ph-II contained in the alkanoyl chain, as well as those affecting to the
exclusively concern the periodical repeat of the layered array; carbonyl signal, indicate that the all-trans conformation initially
according to calculations, it must be the consequence of the present (Ph-Ia) is partially lost when the compound is heated
different interdigitation degree attained by the alkanoyl tails in withinthe 80–90 ꢀC interval.42 It is reasonable to interprettherefore
each one of these phases.
that Ph-II consists of a “semicrystalline” phase in which the polar
heads of the surfactant are crystallized whereas the alkanoyl tails
are side-by-side aligned but without crystalline order. This picture
of Ph-II is according to both SAXS and WAXS data and gives strong
support to the interpretation of the Ph-Ia to Ph-II transition as a
process entailing the melting of the alkanoyl chains accompanied
by a slight rearrangement of the ionic pair head groups.
NMR study of Ph-Ia and Ph-II phases
Additional information of valuable help for the elucidation of
the structural changes taking place in nACh$I upon heating was
afforded by solid-state 13C CP-MAS NMR. Unfortunately the
study of the high temperature interconversion of Ph-II and Ph-
III was unfeasible due to limitations in the temperature level
attainable by our equipment. The difficulty in the availability of
the metastable Ph-Ib phase in the amounts required for the
analysis is an additional shortcoming of this study.
The transition involving Ph-Ia and Ph-II was examined for the
whole nACh$I series. Best results were obtained for 12ACh$I
because the I/IITh (a) of this surfactant is well below the maximum
temperature allowed by the NMR probe head. All samples sub-
jected to study were initially in Ph-Ia as ascertained by both DSC
and WAXS. The spectra recorded from 12ACh$I when subjected to
aheating–cooling cycle over the 20–90 ꢀC intervalare reproducedin
Fig. 10 for illustration, and similar plots for the other members of
the series are provided in the ESI† document. The changes
It is worth noting moreover that a change in the NMR signal
was perceived in the 14–17 ppm before that Ph-Ia to Ph-II
transition takes place. Specically, the initial alkanoyl methyl
ꢀ
signal at 14.5 ppm disappeared upon heating in the 60–70 C
range, and a peak emerged at around 16.8 ppm instead, which
also disappeared when temperature reached 80 ꢀC. Such
changes are also related to variations in the polymethylene
chain packing, and according to observations reported for n-
alkanes, they must be ascribed to the occurrence of a crystal/
crystal transition that is induced by thermal effects.43
Polarizing optical microscopy
The thermal transitions taking place in nACh$I were examined by
polarizing optical microscopy at both heating and cooling. A
Fig. 10 13C CP/MAS NMR spectra of 12ACh$I registered at the indi- Fig. 11 Micrographs recorded by POM from 16ACh$I at the indicated
cated temperatures. (a) At heating and (b) at cooling.
temperatures (the bottom-right one at 50 ꢀC after cooling).
10748 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 12. At room temperature, a stable phase (Ph-Ia) made of
ꢀ
triclinic microcrystals in P1 space group prevails whereas at
temperatures above 80–100 ꢀC (depending on n) a semi-
crystalline layered phase with the alkyl tails in the molten state
but aligned and extensively interdigitated is formed (Ph-II). The
semicrystalline Ph-II phase becomes fully melted above ꢄ160 ꢀC
with formation of a smectic-A mesophase (Ph-III); in this phase
both choline iodide and alkyl tails are in a disordered state but
the layered arrangement is still preserved with a higher degree
of interpenetration of the alkanoyl chains. The structural
changes taking place by heating effect are reversible. Never-
theless a metastable elusive phase (Ph-Ib) appears when Ph-III
is cooled to room temperature; the structure of Ph-Ib is not
well characterized but presumably it is similar to Ph-II but with
a degree of side chain interpenetration comparable to Ph-III.
Fig. 12 Raw scheme of crystalline, semicrystalline and liquid-crystal
phases observed for nACh$I.
selection of the pictures taken from 16ACh$I is shown in Fig. 11
and a complete assortment of images illustrating the texture
changes occurring in the other members is afforded in the ESI†
le. Samples for observation were prepared by casting from chlo-
roform at room temperature so they were initially in Ph-Ia phase.
The high crystallinity of Ph-Ia previously evidenced by both DSC
and WAXS is reected in the crystal-mosaic texture present in the
rst pictures taken at low temperatures; this well-dened
geometrical morphology was lost aer heating above 100 ꢀC and
not recovered any more. The texture seen at 125 ꢀC corresponds to
Ph-II which was observed to evolve with expansion of the material
indicating that the Ph-Ia to Ph-II conversion takes place with a
decrease in density inagreement with theincrease in the interlayer
spacing revealed by SAXS. A close inspection of this picture
revealed the presence of the features characteristic of a smectic
mesophase, which is in full agreement with SAXS results. Further
Acknowledgements
This work received nancial support from MCINN of Spain with
Grants MAT2009-14053-C02-01 and MAT2012-38044-C03-03
and from AGAUR with grant 2009SGR1469. Thanks also given
to Dr Campos from UPC for her guidance in the single crystal
preparation method. Portions of this research were carried out
at the A2 beamline of light source DORIS III at DESY (Hamburg,
Ge); authors are indebted to Dr S. Funari for his invaluable
assistance. Thanks also to the Basque Government for the Ph.D.
grant awarded to Ainhoa Tolentino.
ꢀ
heating at temperatures above ꢄ165 C led to Ph-III which dis-
played a texture not very different from that of Ph-II. In fact,
according to WAXS/SAXS data, Ph-II and Ph-III phases will share
the same stratied patternwith alkanoyl chains inthe moltenstate
intercalated between the layers where the choline iodide counter-
parts are located. Differences between the two phases concern the
order degree of the ionic layer, which is crystallized in Phase II but
disordered in Phase III, and the interdigitation degree, which is
greater in Phase III. Aer cooling down to room temperature a
strongly birefringent material devoid of any distinctive texture was
generated; according to DSC and diffraction data it must corre-
spond to Ph-Ib, Ph-Ia of even a mixture of both.
References
1 J. K. Blusztajn and R. J. Wurtman, Science, 1983, 221, 614–620.
2 J. K. Blusztajn, Science, 1998, 281, 794–795.
3 G. J. Kortstee, Arch. Mikrobiol., 1970, 71, 235–244.
4 J. T. Gorke, F. Srienc and R. Kazlauskas, Chem. Commun.,
2008, 1235–1237.
5 A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and
R. K. Rasheed, J. Am. Chem. Soc., 2004, 126, 9142–9147.
6 K. D. Weaver, H. J. Kim, J. Z. Sun, D. R. MacFarlane and
G. D. Elliot, Green Chem., 2010, 12, 507–513.
7 R. Klein, D. Touraud and W. Kunz, Green Chem., 2008, 10,
433–435.
Conclusions
A homologous series of alkanoylcholine surfactants (nACh$I)
has been prepared from fatty acids with n ¼ 12, 14, 16 and 18,
and their thermal and structural properties studied as a func-
tion of temperature. These surfactants are particularly inter-
8 R. Klein, M. Kellermeier, M. Drechsler, D. Touraud and
W. Kunz, Colloids Surf., 2009, 338, 129–134.
9 R. Klein, H. Dutton, O. Diat, G. J. T. Tiddy and W. Kunz,
J. Phys. Chem. B, 2011, 115, 3838–3847.
esting because they are bio-based compounds displaying 10 A. Skoulios and V. Luzzati, Nature, 1959, 183, 1310–1312.
biodegradable and biocompatible properties, as well as for their 11 K. Binnemans, Chem. Rev., 2005, 105, 4148–4204.
suitability to prepare bio-based comb-like polymers by ionic 12 A. R. Ubbelohde, H. J. Michels and J. J. Duruz, Nature, 1970,
coupling with naturally-occurring polyelectrolytes.
nACh$I surfactants are soluble in warm water showing cmc 13 K. Shinoda, Y. Minegishi and H. Arai, J. Phys. Chem., 1976,
within the 1 to 10 mM concentration range and Kra temper- 80, 1987–1988.
atures between 30 and 60 ꢀC depending on n. In the condensed 14 D. Rengstl, O. Diat, R. Klein and W. Kunz, Langmuir, 2013,
phases, nACh$I invariably adopt bilayered molecular
29, 2506–2519.
arrangement in which ionic choline iodide heads and alkyl tails 15 K. S. Cho and P. Proulx, Biochim. Biophys. Acta, 1971, 225,
are alternating counterparts. Upon heating, a diversity of pha- 214–223.
ses differing in order degree or/and mutual arrangement of 16 B. Ahlstrom, M. Chelminska-Bertilsson, R. A. Thompson and
228, 50–52.
a
¨
layers are taken up according to temperature, as illustrated in
L. Edebo, Antimicrob. Agents Chemother., 1995, 39, 50–55.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 10738–10750 | 10749

View Article Online
RSC Advances
Paper
17 R. Schneider and A. R. Timms, Br. J. Pharmacol., 1957, 12, 32 B. W. Barry and G. F. J. Russell, J. Colloid Interface Sci., 1972,
30–38. 40, 174–194.
18 V. Carelli, F. Liberatore, L. Scipione, M. Cardellini, 33 T. W. Davey, W. A. Ducker, A. R. Hayman and J. Simpson,
D. Rotiroti and V. Rispoli, US Pat., 0166721 A1, 2003.
19 H. Patel, WO Patent 2005/018631 A1, 2005.
20 J. Alexander and J. A. Fix, US Pat., 4,822,773, 1989.
21 J. Alexander, J. A. Fix, and A. J. Repta, US Pat., 4963556,
1990.
Langmuir, 1998, 14, 3210–3213.
34 S. Kanehina, H. Matsuki, R. Ichikawa and T. Kuwahara,
Proceedings of the International Conference on Colloid and
Surface Science, ed. Y. Iwasawa and Oyama H. Kunieda,
Elsevier, 2001, pp. 45–47.
¨
22 R. Klein, E. Muller, B. Kraus, G. Brunner, B. Estrine, 35 A. Renoncourt, Ph.D. dissertation, University of Regensburg,
D. Touraud, J. Heilmann, M. Kellermeier and W. Kunz,
RSC Adv., 2013, 3, 23347–23354.
23 M. K. Watson, U. Tezel and S. G. Pavlostathis, Water Res.,
2012, 46, 2947–2956.
2005.
36 K. S. Sharma, S. R. Patil, A. K. Rakshit, K. Glenn, M. Doiron,
R. M. Palepu and P. A. Hassan, J. Phys. Chem. B, 2004, 108,
12804–12812.
24 P. K. Tarafdar, S. T. Reddy and M. J. Swamy, J. Phys. Chem. B, 37 C. Vautier-Giongo and B. L. Bales, J. Phys. Chem. B, 2003, 107,
2013, 117, 9900–9909. 5398–5403.
25 K. Iwamoto, Y. Ohnuki, K. Sawada and M. Seno, Mol. Cryst. 38 J. A. Portilla-Arias, M. Garcıa-Alvarez, A. Martınez de Ilarduya
´
´
˜
Liq. Cryst., 1981, 73, 95–103.
and S. Munoz-Guerra, Polym. Degrad. Stab., 2007, 92, 1916–
26 A. Malliaris, C. Christias, G. Margomenou-Leonidopoulou
and C. M. Paleos, Mol. Cryst. Liq. Cryst., 1982, 82, 161–166.
1924.
39 M. J. Ruocco, D. J. Siminovitch and R. G. Griffin,
Biochemistry, 1985, 24, 2406–2411.
´
´
27 A. Tolentino, S. Leon, A. Alla, A. Martınez de Ilarduya and
˜
S. Munoz-Guerra, Macromolecules, 2013, 46, 1607–1617.
40 J. T. Kim, J. Mattai and G. G. Shipley, Biochemistry, 1987, 26,
6599–6603.
¨
¨
28 C. Chachaty, T. Ahlnas, B. Lindstrom, N. Nery and
A. M. Tistchenko, J. Colloid Interface Sci., 1988, 122, 406–417. 41 N. S. Haas, P. K. Sripada and G. G. Shipley, Biophys. J., 1990,
29 J. Zhao and B. M. Fung, Langmuir, 1993, 9, 1228–1231. 57, 117–124.
30 Y. S. Lee and K. W. Woo, J. Colloid Interface Sci., 1995, 169, 42 T. Yamanobe, M. Tsukahara, T. Komoto, J. Watanabe,
34–38.
I. Ando, I. Uematsu, K. Deguchi, T. Fujito and M. Imanari,
Macromolecules, 1988, 21, 48–50.
43 D. L. VanderHart, J. Magn. Reson., 1981, 44, 117–125.
31 E. Kudryashov, T. Kapustina, S. Morrissey, V. Buckin and
K. Dawson, J. Colloid Interface Sci., 1998, 203, 59–68.
10750 | RSC Adv., 2014, 4, 10738–10750
This journal is © The Royal Society of Chemistry 2014