M. Kozak*, S. Jurga
Department of Macromolecular Physics,
A. Mickiewicz University, Poznañ, Poland
* corresponding author;
e-mail: [email protected]
SAXS Study of Lipid Bi-Layer Model Systems.
Phase Diagram of N-Undecylammonium
Chloride/Water System
Abstract
Alkylammonium halides belong to the cationic surfactants. In water solutions they can
form bi-layer systems whose properties mimic those of the lipid membranes. The nundecylammonium chloride/water system was studied by small angle X-ray scattering
(SAXS) in concentrations of UDACl from 15 to 70% and in a temperature range from 20 to
70ºC. The results permitted a fragment of phase diagram to be drawn for this system.
Three main phases, the isotropic, lamellar and hexagonal, were observed and characterised.
Key words: n-undecylammonium chloride, SAXS, lipid bi-layers, liquid crystals.
brane structure. Lipids are complex
biomolecules, esters of glycerol, fatty
acids, phosphoric acid and nitrogen compounds such as serine (phosphatidylserines), ethanolamine (phosphatidylethanolamines) or choline (phosphatidylcholines). The occurrence of a hydrophilic
and a hydrophobic part in lipid molecules
enables them to self-assemble in such
solvents as water, in which they undergo
different structural phases (for a review
see Katsaras & Gutberlet, 2001).
Introduction
The main structural elements of the cell
membrane are lipids and their derivatives,
whose chemical and physical properties
ensure the formation of a unique memFIBRES & TEXTILES in Eastern Europe
Alkylammonium halides, which are among
the cationic surfactants, are described by the
general formula CnH2n+1NH3X (where X: Cl,
Br and n > 5). In solutions they can form
bi-layer systems whose properties mimic
those of lipid membranes. From the point
of view of biophysics, their water solutions are particularly important forming
a system of the liotropic liquid crystals
structure. The structural parameters of
alkylammonium halides have been studied by various techniques. The crystal
structures of n-alkylammonium halides
have been reported for three anhydrous
compounds: n-decylammonium chloride
[14, 11] n-dodecylammonium chloride
[11, 15] and n-dodecylammonium bromide [10]. A further two isostructural
crystal forms have been reported for
undecylammonium chloride monohydrate
[16] and undecylammonium bromide
monohydrate [8]. In the crystal structure,
the molecules of undecylammonium halides are packed as parallel and interdigitated ‘cylinders’, roughly perpendicular
to the layers.
The microstructure and molecular dynamics of decylammonium chloride in solid
state have been studied as a function of
January / December 2005, vol. 13, No. 5 (53)
temperature [5, 12]. At room temperature,
the alkyl chains in the decylammonium
chloride bi-layer are interdigitated, and
upon heating they transform to a non-interdigitated arrangement.
The most interesting system is the binary
mixture of alkylammonium chloride and
water as they best reproduce the properties of biomembranes. The phase diagrams of water solutions of decylammonium and dodecylammonium chlorides
have been characterised on the basis of
the SAXS results [1]. Moreover, a significant effect of inorganic salt addition on
their phase diagrams has been reported
[13]. The collective and individual dynamics of decylammonium and dodecylammonium chlorides in a water environment have been also investigated by NMR
relaxation as a function of surfactant concentration and temperature [18, 3, 17].
Phase diagrams of the systems of this type
have been also established on the basis
of polarisation light microscopy [2]. However, the exact structural parameters
characterising the existing phases can
mainly be obtained from the diffraction
techniques.
The aim of this paper is to characterise
the phase diagram of the hitherto poorly
recognised system of n-undecylammonium chloride (UDACl)/water in the
UDACl concentration range from 15 to
70% w/w. Recently, only small-angle neutron scattering (SANS) measurements
have been made to estimate the shape and
size of undecylammonium chloride micelles in D2O in the presence of NaCl at
two ionic strengths [9].
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Materials and methods
The n-undecylammonium chloride
(C11H23NH3Cl; UDACl) used in the study
was prepared in our laboratory from
undecylamine, as described previously [7,
3]. The n-undecylammine (purity 99.9%)
was purchased from Sigma-Aldrich.
The undecylammonium chloride water
solutions (15-70% w/w) were subjected to
X-ray scattering. The concentration of
undecylammonium chloride in water was
increased in 5% steps, but in the regions
where the phase transitions occurred, the
concentration step was 2.5%. The measurements were conducted using the
NanoSTAR system (Bruker-AXS) with a
pin-hole collimation and a two-dimensional detector (HiSTAR), mounted on a
microfocus X-ray tube with a copper an-
ode (CuKα, l=0.154178 nm), and
equipped with Göbel mirrors. The sampleto-detector distance was 650 mm. The
intensities were recorded within the range
of 0.15 nm -1 < s < 3.5 nm -1 , (where
s=4πsinq/λ, 2θ was the scattering angle
and l was the X-ray wavelength). The saxis was calibrated by the observation of
peaks from a silver behenate diffraction
pattern [4]. The series of expositions starting from 70 to 20°C in 2°C temperature
steps were recorded for each sample. The
sample temperature in all experiments
was controlled using a TCU-50 unit and
monitored with an accuracy of 0.2°C.
Before the test measurements, each
sample was incubated at the chosen temperature for a period of 15 min. The exposure time for a single frame was 90 min.
The positions of the maxima on the SAXS
curves were fitted by the TOPPAS program (Bruker-AXS).
Results and discussion
During the whole series of measurements
the mass of the samples does not change,
which could indicate that the solvent had
evaporated, and so the concentration of
the system was changed. Figures 1-3
present example SAXS curves recorded
for the samples of undecylammonium
chloride at concentrations of 20%, 42.5%
and 45%, showing the characteristic regions of the phase diagram. The samples
of the UDACl concentrations of 15 to
30% reveal only the presence of the isotropic (micellar) phase. The values of d001
characterising this phase changed from
5.96 nm (T=25ºC, conc. 20%) to 5.11 nm
Figure 1. The SAXS data collected for the n-undecylammonium chloride/water system (concentration 20% w/w, temp. 20 - 70°C).
Figure 2. The SAXS data collected for the n-undecylammonium
chloride/water system (concentration 42.5% w/w, temp. 20 - 70°C).
Figure 3. The SAXS data collected for the n-undecylammonium
chloride/water system (concentration 45% w/w, temp. 20 - 70°C).
Figure 4. Schematic representation of the phase diagram of the nundecylammonium chloride/water system. The hexagonal phases
of n-dodecylammonium chloride (—) and n-decylammonum chloride/water (…) systems were also indicated.
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January / December 2005, vol. 13, No. 5 (53)
(T=70°C, conc. 30%). In the concentration range of 30 to 37.5%, the isotropic
micellar and mixed lamellar phases (with
a small contribution from the micellar
one) co-exist. The micellar phase is characterised by d001 values ranging from 4.13
nm (T=70°C, 35%) to 4.65 nm (T=64°C,
37.5%), while for the lamellar phase, d001
varies from 3.87 nm (T=50%, 35%) to
4.35 nm (37.5%, 40°C).
In the sample of the concentration of 40%
of UDACl, three phases were recorded.
Between 64 and 70°C there is the hexagonal phase, between 64 and 54°C the isotropic phase, and between 64 and 20°C the
lamellar phase. The hexagonal phase is
characterised by the lattice parameter
ahex=5.04 nm, which is equal to the distance between the rods. The lamellar
phase is characterised by d001=3.88 – 3.78
nm, while the isotropic phase by d001=4.43
nm.
A similar situation is observed at the concentration of 42.5%, but the hexagonal
phase occurs within a narrower temperature range (68-70°C) and is characterised
by ahex=5.0 nm. The lamellar phase occurs
in temperatures from 20°C to 54°C
(d001=4.10 – 3.88 nm). The isotropic phase
separating the hexagonal and lamellar
phases occurs in the range from 54°C to
68°C (d001=4.24 – 4.42 nm).
At the concentration of 45%, the hexagonal phase disappears and the two phases
lamellar and micellar coexist. The lamellar phase is present at lower temperatures
20-48°C), and is characterised by d001
from 4.03 nm (T=20°C) to 3.96 nm
(T=48°C). At 50°C the system undergoes
a phase transition to the isotropic phase,
which occurs above 52°C and is characterised by d001 from 4.22 nm (T=52°C) to
4.14 nm (T=70°C).
For the sample of the concentration of
50% w/w UDACl, the lamellar phase is
dominant throughout the whole temperature range studied. It is characterised by
the lattice constant d 001, whose value
slightly changes with temperature from
d001=3.89 nm at 25°C to d001=3.78 nm at
70°C.
With increasing concentrations of UDACl,
the lattice constant values d001 characterising the lamellar phase decrease, which
is related to the decreasing volume of the
solvent in the space between the bi-layer
systems. For the sample of UDACl conFIBRES & TEXTILES in Eastern Europe
centration of 55%, the values of d001 varied from 3.84 nm to 3.64 at 20°C and 70°C
respectively. A similar situation was observed for the samples of UDACl concentrations of 60% (d001=3.71 – 3.56 nm), 65%
(d001=3.73 – 3.57 nm) and 70% (d001=3.73
– 3.56 nm).
The above results permit a phase diagram
for this system to be drawn. The phase
diagram, presented in Figure 4, implies
the presence of three main phases: isotropic, lamellar and hexagonal. The
ranges of these phases are comparable
with those determined in the phase diagrams obtained for similar systems of
decylammonium and dodecylammonium
chlorides.
Previous PLM [13] and SAXS (Auvray
et al, 1984) studies of the DAC/water system show that for the decylammonium
chloride/water system, the hexagonal
phase is located on the phase diagram
between concentrations of DACl of 35%
and 60% in temperatures from 30 to 90°C.
A similar situation is observed for
dodecylammonium chloride/water system, but the hexagonal phase area on the
phase diagram [2] is significantly smaller
(concentrations of DDACl of 30-40%, a
temperature range of 50 - 90°C. The hexagonal phase for undecylammonium chloride/water system was observed for UAC
concentrations of 40% and 42.5% (a temperature range of 64-70°C), exactly between the hexagonal phases observed for
DACl and DDACl/water systems.
The studies of DACl/water system [13]
indicate the existence of the nematic
phase on the phase diagram (DACl concentration: 40-50%, temperature range:
10-20°C). Our results did not permit us
to draw the nematic phase on the phase
diagram of the UDACl/water system.
G
Acknowldgements
The authors wish to acknowledge the financial support of the State Committee for Scientific Research
under grant No. 3 T09A 050 27.
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G Received 08.12.2004
Reviewed 10.02.2005
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