This document discusses the complexation and phase behavior of DNA with cationic surfactants and lipids in aqueous mixtures. When DNA forms a complex with a cationic surfactant, replacing the sodium counterions, an insoluble "complex salt" is formed (CSDNA). CSDNA can form liquid crystalline phases when mixed with water and additives like alcohols, lipids, or cyclodextrins. The phase diagrams of these ternary mixtures demonstrate a rich liquid crystallinity determined by factors like the components' hydrophobicity, CSDNA/surfactant ratio, and DNA packing constraints. Small-angle X-ray scattering was used to investigate the molecular arrangements in these phases.

1. Complexation between DNA and surfactants and lipids: phase behavior and
molecular organization
Azat Bilalov,†*a
Ulf Olssona
and Bj€orn Lindmanab
Received 4th July 2012, Accepted 27th July 2012
DOI: 10.1039/c2sm26553b
The interaction between DNA and various cationic species, e.g. cationic surfactant (CS), has a broad
biological and biotechnological significance. In the cell nucleus as well as in transfection formulations,
other species, mainly zwitterionic lipids, are also present but their exact role needs elucidation. A closer
investigation of the stability of structures formed as well as the molecular arrangements is hampered by
the complexity of the systems with respect to the number of components. A powerful way for reducing
the number of components is to base studies on the stoichiometric (1 : 1) compound CSDNA,
where the simple (sodium) counterions have been ion-exchanged by a cationic amphiphile ion. CSDNA
is typically insoluble in water but is able to form liquid crystalline phases in aqueous mixtures with
many additives capable of associating with the amphiphilic counterions (alcohols, non-ionic
surfactants, lipids, cyclodextrins, etc.). Mixtures of CSDNA with a number of components have been
investigated in detail with respect to phase behavior. The phase diagrams demonstrate a rich liquid
crystallinity. The organization of DNA and the surfactant–lipid self-assemblies is controlled by
different factors for different cases, mainly (i) the lipophilic characteristics of the components, (ii) the
[CSDNA]/[amphiphile] ratio and (iii) DNA packing constraints, due to the large persistence length. A
summary of phase diagrams is presented together with structural investigations based mainly on small-
angle X-ray scattering. The role of DNA rigidity is illustrated in a comparison with analogous systems
based on flexible polyanions.
Azat Bilalov
Azat Bilalov is a professor in
physical chemistry at Kazan
National Research Technolog-
ical University, Russia. He
obtained ‘‘kandidat nauk’’
(PhD) in 1995 and ‘‘doktor
nauk’’ (full doctor of science) in
2007 in Kazan. For the last 10
years he periodically worked as
a post-doctoral fellow at the
division of Physical Chemistry
at Lund University, Sweden. His
research interests are in the field
of polymer solutions and poly-
mer–surfactant systems
including biopolymers and lipids.
Recently, his research interests also include controlled growth of
metal particles on crystals and liquid crystals, and tuning of the
properties of the carbon particles by surfactants and polymers.
Ulf Olsson
Ulf Olsson is a professor of
physical chemistry at Lund
University, Sweden. He
obtained his PhD in Lund in
1988. After a year as a post-
doctoral fellow at Centre Paul
Pascal, Pessac (Bordeaux), he
returned to Lund at the end of
1989, and has remained there, at
the division of Physical Chem-
istry, ever since. His research
interests are focused on surfac-
tant self-assembly and micro-
emulsions involving self-
assembly structure, phase equi-
libria and structural trans-
formation kinetics. Experimental methods involve mainly various
NMR and scattering methods. His research interests also include
the self-assembly of peptides and other biomolecules.
a
Physical Chemistry, Lund University, POB 124, SE-22100 Lund,
Sweden. E-mail: Azat.Bilalov@fkem1.lu.se
b
Department of Chemistry, University of Coimbra, 3004-535 Coimbra,
Portugal
† On leave from Physical and Colloid Chemistry, Kazan National
Research Technological University, Russia.
11022 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012
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2. 1. Introduction
Cationic lipid (CL)–DNA complexes or compounds have
received significant attention because of potential applications in
non-viral gene therapy.1
The controlling factor in lipid-mediated
DNA delivery is the structural evolution and eventual dissolu-
tion of the lipid–DNA complexes.2–9
These complexes, lip-
oplexes, can exist in a variety of mesoscopic structures (lamellar,
normal and reversed hexagonal, cubic, etc.) depending on the
lipid composition.10
To design advanced lipoplex formulations,
suitable additives, like surfactants, polymers or co-solvents, can
be used to modulate the lipoplex structure.10
In the presence of additives that form mixed self-assemblies
with the amphiphiles and enhance micellization, the combination
of DNA and a cationic surfactant can form different liquid
crystalline phases.2,11
Here, the liquid crystalline structure is
determined by the packing of the surfactant or lipid aggregates,
in which the DNA is tightly packaged and shielded from the
external aqueous phase. It has a beneficial effect on the efficiency
of transfection of plasmid DNA into mammalian cells.1,3,12
Attard and co-workers13
have reported that cell nuclei, strip-
ped of their nuclear membranes (so-called naked nuclei), contain
substantial amounts of lipids. Whereas the precise role that lipids
play in nuclear function appears to be unclear, a recent paper14
from Attard’s group discusses several interesting matters related
to endonuclear lipid composition and the functional competence
of nuclei. Attard’s work also discusses the details of lipid orga-
nization suggesting that endonuclear lipids might form self-
organized structures of liquid crystalline nature, including such
of bicontinuous cubic type. As we demonstrate in the present
review there is indeed a very rich liquid crystallinity in DNA–
lipid systems, including bicontinuous phases.
The phase diagram approach has received little attention for
investigations of DNA–lipid interactions and, in particular, also
in the presence of a cationic cosolute, which may be a surfactant,
a protein or a polymer. In this review, we address this type of
systems using a simple cationic surfactant. Aqueous mixtures of
a polyelectrolyte and an oppositely charged surfactant are, like
any aqueous system of two electrolytes, complicated from a
phase diagram point of view in that they, with 4 different elec-
trolytes, have (at constant temperature) to be described by a 3-
dimensional phase diagram; adding a further component, like a
lipid, adds one more dimension. A very suitable remedy to these
problems is to eliminate the small ions, i.e. the counterions of the
polyelectrolyte and the surfactant.15
We have, therefore, chosen
to review the true ternary systems of DNA with a cationic
surfactant as the counterion (the ‘‘complex salt’’),16
an additive
(n-alcohol, nonionic lipid, zwitterionic lipid, cyclodextrin, etc.)
and water. These are genuine ternary systems, the phase
diagrams of which, at constant temperature, have a two-dimen-
sional representation. For the general case of a complex salt of
DNA with a cationic surfactant ion as counterion we use the
notation CSDNA, whereas for specific cases we indicate the
common abbreviation of the surfactant ion; thus DTADNA
refers to the complex salt of dodecyltrimethylammonium
and DNA.
Piculell and co-workers have summarized recent progress in
the understanding of aqueous ‘‘complex salts’’ of ionic surfac-
tants with flexible acrylate-based polyions17
whereas Hansson
has made a theoretical analysis of the phase behavior of aqueous
polyion–surfactant complex salts.18
DNA is an amphiphilic highly charged polyelectrolyte. The
association of individual DNA strands into the double helix is
driven by the hydrophobic interactions between the bases. There
is, in this DNA self-assembly, a delicate balance between the
hydrophilic and hydrophobic interactions, as can be noted from
the effect of electrolyte. Thus in the absence of electrolyte, the
double helix stability is lost and the DNA strands dissociate.
Stabilization of the double helix can also be achieved by the
binding of a cationic species.
Double helix DNA is a highly charged polyelectrolyte, which
also is quite rigid. Double helix DNA, therefore, owes its
aqueous solubility to the entropy of the dissociated counterions,
whereas the contribution from configurational entropy is very
low. DNA associates strongly with multivalent cationic species,
which can be of many types, metal ions, polyamines (spermine,
spermidine, etc.), polycations and cationic surfactants or lipids,
which associate into multivalent self-assemblies. In the present
review, we have focused on studies of the latter category and
describe the behavior of systems with a complex salt of double
helix DNA in ternary mixtures with water and various additives.
2. Binary mixtures of the complex salt CSDNA with
water
The cationic surfactants considered here for the formation of the
complex salt, CSDNA, self-assemble in mixtures with water
alone into a large variety of structures, giving micellar solutions
as well as various liquid crystalline phases, cubic, hexagonal and
lamellar. The self-assembly of an ionic surfactant is facilitated in
the presence of an oppositely charged polyelectrolyte. This is due
to the reduction in the entropy decrease associated with
concentrating the counterions in the vicinity of the aggregates.
Polyelectrolyte-oppositely charged lipid–surfactant complexes
are water insoluble. The surfactant ions self-assemble in water
into highly charged micellar macroions that are strongly attrac-
ted to the similar highly charged polyelectrolyte. This results in a
Bj€orn Lindman
Bj€orn Lindman is a professor of
physical chemistry at Lund
University, Sweden since 1978
and for the last 10 years he
shares his appointment with
Coimbra University, Portugal.
His research interests are in the
field of colloid science, including
surfactant and polymer self-
assembly, polymer–surfactant
systems, polymer adsorption,
amphiphilic polymers and
biopolymers like DNA and
cellulose. He consults and gives
training courses for industry on
formulations and is also a co-
founder of spin-offs in this area. He is a fellow of the Swedish
Royal Academy of Sciences, the Swedish Royal Academy of
Engineering Sciences and the Portuguese Academy of Science.
This journal is ª The Royal Society of Chemistry 2012 Soft Matter, 2012, 8, 11022–11033 | 11023

3. precipitation or associative phase separation into a concentrated
micelle–polyelectrolyte phase, often a liquid crystal. Because of
the strong electrostatic polyelectrolyte–micelle attraction,
possible micellar shapes in the precipitated phase depend on the
flexibility of the polyelectrolyte chain.
A flexible polyelectrolyte, like polyacrylate (PA), can associate
with spherical micelles. It can wrap around a micelle adsorbing
at the interface, matching the micellar charges. An example is PA
with dodecyltrimethylammonium (DTA) counterions, DTAPA.
This medium chain surfactant ion forms (quasi) spherical
micelles and the complex precipitates as a micellar cubic
phase. The phase diagram of the binary water–DTAPA is shown
in Fig. 1.
The rigid DNA, on the other hand, cannot associate with
spherical micelles but requires cylindrical micelles in a parallel
arrangement. However, depending on the counterion, the
preferred packing and phase structure will be different. With
cetyltrimethylammonium (CTA) counterions, the hydrated
CTADNA has a hexagonal symmetry, where the micelles are
hexagonally ordered and each cylindrical micelle has six DNA
strands as their nearest neighbors, as illustrated in Fig. 2a. Here,
the micellar and DNA cylinders form a 1 : 3 compound (3 DNA
helices for every cylindrical micelle). The significance of the
particular compound i:j is that it reflects the ratio of the linear
charge density of the micelles, rmic, and DNA, rDNA, respectively
with j/i ¼ rmic/rDNA. For B-DNA, rDNA ¼ 0.6 AÀ1
. For cylin-
drical micelles of radius Rmic, rmic is given by
rmic ¼
2pRmic
As
¼
pRmic
2
Vs
(1)
where As is the average area that the surfactant headgroup
occupies at the micelle surface, Vs is the volume of the surfactant
ion and the second equality follows from the cylindrical geom-
etry for which we have Rmic ¼ 2Vs/As. For single chain ionic
surfactant micelles, one often finds a radius approximately equal
to the extended surfactant length. Thus, to a first approximation,
we expect Rmic $ Vs and hence that the charge density increases
approximately linearly with the surfactant chain length.
By decreasing the surfactant chain length from 16 carbons
(CTA) to 12 carbons (DTA) we indeed find a different structure
of the hydrated complex. The SAXS pattern of hydrated
DTADNA in excess water (maximum hydration) is presented in
Fig. 3. The two broad reflections, with relative positions 1 : O2,
suggest a square lattice and from their positions we obtain the
lattice parameter a ¼ 37 A. The SAXS data of Fig. 3 were
recorded in excess water, hence corresponding to maximum
hydration. Packing DTA cylindrical micelles of radius Rmic and
volume fraction fDTA on a square lattice we have
fDTA ¼
pRmic
2
a2
(2)
In the dry DTADNA complex, fDTA ¼ 0.57. Assuming a
similar maximum hydration as for CTADNA,19
ca. 30% water,
we have approximately fDTA z 0.4. With this value and a ¼ 37
A we obtain with eqn (2) Rmic ¼ 13 A. With this value, together
with Vs ¼ 464 A3
as the volume of the DTA ion,20
in eqn (1) we
obtain rmic ¼ 1.2 AÀ1
. This is twice the linear charge density of
the DNA, suggesting a 1 : 2 micelle–DNA compound. The
proposed structure of this phase is illustrated in Fig. 2b.
Double-chained surfactants can generally not form normal
(direct) micelles in water. Instead, bilayer or reverse structures
are preferred. The diffraction pattern of the hydrated complex in
excess water, presented in Fig. 4, is consistent with a two
dimensional hexagonal structure. Three reflections are observed
with relative positions 1 :
ffiffiffi
3
p
: 2, that can be indexed to [hk] ¼
[10], [11] and [20] of the two-dimensional hexagonal lattice with
the first order peak at q10 ¼ 0.22 AÀ1
. The volume of a didode-
cyldimethylammonium (DDA) ion is essentially twice the
volume of DTA. Assuming that DDA forms reverse cylindrical
micelles, as illustrated in Fig. 2c, with DNA and hydration water
Fig. 1 Phase diagram of the binary DTAPA–water system. Redrawn
and adapted from Piculell et al.17
Top: schematic representation of the
hydrated DTAPA complex structure in the hexagonal (the upper left) and
cubic (the upper right) phases.
Fig. 2 Schematic representation of the hydrated CSDNA structures: (a)
the hydrated CTADNA (redrawn from Leal et al.19
), (b) the hydrated
DTADNA, and (c) the hydrated DDADNA.
Fig. 3 The SAXS pattern of hydrated DTADNA. The relative
positions of the Bragg peaks at 1 : O2 confirm the two dimensional square
lattice. The nearest neighbor distance between the columns, a, can be
obtained from the SAXS pattern where for the square structure the Bragg
reflections occur at qhk ¼
2p
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðh2 þ k2Þ
p
a
.
11024 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012

4. contained in the core, with radius Rcore, we can write for the first
order diffraction peak
q10 ¼
8p
O3
ðfDNAÞ
1=2
RDNA
¼
8p
O3
ðfDNA þ fwÞ
1=2
Rcore
(3)
With RDNA ¼ 10 A we obtain from the first equality of
eqn (3) fDNA ¼ 0.33. Since in dry (pure) DDADNA fDNA ¼
0.40, this corresponds to a hydration fw ¼ 0.17. With the sum
(fDNA + fw) ¼ 0.50 we obtain from the second equality of eqn (3)
Rcore ¼ 12 A.
For the reverse cylindrical geometry we also have
Rcore ¼
2ðfDNA þ fwÞ
fDDA
Vs
As
(4)
where for the fully hydrated compound fDDA ¼ 0.60 (1 À fw) ¼
0.50. For DDA, Vs z 790 A3
. We can then solve eqn (4) for As,
obtaining As ¼ 130 A2
. The proposed reverse hexagonal struc-
ture corresponds, in a way, to a 1 : 1 (inclusion) compound.
Hence, the linear charge density of the reverse micelles rmic ¼
2pRcore/As (cf. eqn (1)) must equal that of DNA, rDNA ¼ 0.6
AÀ1
. With the values of Rcore ¼ 12 A and As ¼ 130 A2
, we obtain
rmic ¼ 0.58 AÀ1
, providing strong support for a reverse hexag-
onal structure.
3. Ternary mixtures
CSDNA compounds are water insoluble. The reason for this is
that the amphiphilic counterions self-assemble in water into
highly charged macroion micelles. Due to the strong electrostatic
attraction between these macroions and the highly charged
polyelectrolyte, the ions will not disperse and CSDNA precipi-
tates from solution. By adding a suitable third component that
can disperse the amphiphilic counterions it is possible to solu-
bilize CSDNA. This third component can be another lipid
aggregate in which the CS can dissolve or it can be a cyclodextrin
that by forming an inclusion complex with CS prevents micelle
formation. Below we present examples of ternary phase diagrams
that illustrate this solution behavior.
3.1. CSDNA–water–alcohol systems
The CSDNA is insoluble in water but is soluble in some
nonpolar solvents21
and n-alcohols,22,23
where there is an
increased solubility from decanol to ethanol. The phase
behavior of DTADNA in mixtures of water and n-alcohols
(decanol, octanol, hexanol, butanol, and ethanol) shows a
regular pattern with respect to the occurrence of liquid crys-
talline regions.23
For example, in the phase diagram of the
DTADNA–n-decanol–water system shown in Fig. 5, there is a
hexagonal phase (of the reversed type) in the alcohol-rich side
whereas there is a lamellar phase in the water-rich side.23
For
balanced proportions of the components, there is a coexistence
of the lamellar and the hexagonal phases.
In contrast to surfactants, simple alcohols by themselves do
not self-assemble. On the other hand, the n-alcohols act as
cosurfactants mixing into the surfactant films thereby changing
the curvature of the films. Depending on the curvature of the
surfactant–alcohol films, different structures result. The curva-
ture changes from positive to less positive or negative on adding
n-alcohol to a hydrated CSDNA. The film is curved around the
hydrophobic part in the binary CSDNA–water system and it is
curved toward the polar parts in the reversed hexagonal phase of
the DTADNA–n-alcohol–water system. A planar film results on
small additions of n-alcohol to the hydrated CSDNA. This
behavior is common for many surfactant/cosurfactant systems.
For example, a cationic surfactant–n-alcohol–water system
shows a similar phase behavior, giving normal hexagonal–
lamellar–reversed hexagonal (H1–L–H2) or micellar–lamellar–
reversed micellar (I1–L–I2) phase transitions (Fig. 6).24
The
curvature can vary essentially from the inverse surfactant mole-
cule length to a similar negative value. (The curvature is the
inverse of the radius of curvature.)
Thus, the amphiphilic characteristics of the additive, mainly
hydrophilicity and packing, are important factors for tuning the
CSDNA structure. In the CSDNA systems, simple alcohols are
expected to show the same phase behavior as a single-chain non-
ionic surfactant with a short head group, in lowering the head-
group interfacial area. The effect of the interfacial area of the
head-group is strong, as will be now shown in the study of
nonionic lipids.
Fig. 4 The SAXS pattern of hydrated DDADNA. The relative positions
of the Bragg peaks at 1 : O3 : O4 confirm the two dimensional hexagonal
lattice. The nearest neighbor distance between the columns, a, can be
obtained from the SAXS pattern where for the hexagonal structure the
Bragg reflections occur at qhk ¼
4p
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðh2 þ k2 þ hkÞ
p
a$
ffiffiffi
3
p .
Fig. 5 Phase diagram of the ternary DTADNA–n-decanol–water
system. Key: L – lamellar phase; H2 – hexagonal phase (reversed type);
and I2 – isotropic solution. The phase diagram was established and the
different regions of the phase diagram characterized with respect to
microstructure by 2
H NMR, small-angle X-ray scattering (SAXS), and
other techniques. Redrawn from Bilalov et al.22
This journal is ª The Royal Society of Chemistry 2012 Soft Matter, 2012, 8, 11022–11033 | 11025

5. 3.2. CSDNA–water–nonionic lipid systems
The most common type of nonionic surfactant is that with an
oligo(oxyethylene) group as the polar head. For the most
important surfactants containing oxyethylene groups the volume
of the polar head group is similar to or larger than the volume of
the non-polar alkyl chain. For these non-ionic surfactants
(abbreviated as CmEn), the polyoxyethylene chain length is the
prime factor in determining the phase behavior. For example,
dodecyl tetraethylene glycol (C12E4) forms a lamellar phase in
water at room temperature whereas dodecyl octaethylene glycol
(C12E8) preferably forms globular micelles and hexagonal and
cubic phases of the normal type.25
Their aqueous phase diagrams
with DTADNA are shown in Fig. 7a and b, respectively.
In the DTADNA–C12E4–water system26
there is a monotonic
decrease in curvature, from normal hexagonal to lamellar, when
moving from left to right in the ternary diagram (Fig. 7a). In the
case of C12E8, increasing amounts of this nonionic surfactant
result in an evolution from square ordered DNA via a normal 2D
hexagonal phase into a lamellar phase.
The very low solubility of DTADNA in the C12E8 micellar
phase is striking. One could expect that DTA would solubilize
into globular C12E8 micelles and thereby get properly dispersed.
This is also what happens, but only to a certain limit. The
solubility corresponds approximately to a DTA–C12E8 molar
ratio of 1/75. As C12E8 forms globular micelles with an aggre-
gation number of ca. 75,27
this means that the solubility limit
corresponds to approximately one DTA per micelle. Hence
DTADNA is soluble as long as the micelles are monovalent
counterions. On increasing the DTADNA concentration, the
micelles become effectively divalent, trivalent, etc. counterions
with which DNA precipitates. This implies the presence of a
‘‘gas–liquid’’ phase transition where the solution phase splits into
a dilute and concentrated phase, respectively, with a critical
point. This particular phase behavior was investigated in more
detail in the related CTAPA–water–C12E8 system28
where the
liquid–liquid phase separation was clearly identified.
DTADNA is essentially insoluble in the C12E8 rich hexagonal
phase. This is another example of the constraints involved in the
dictation of the phase behavior. As was discussed above for the
hydrated CSDNA compound, a structure composed of parallel
and oppositely charged DNA and surfactant rod aggregates
requires a particular matching of the surfactant rod aggregate
and DNA linear charge densities. At low DTA–C12E8 ratios, this
cannot be achieved. As demonstrated by the DTADNA–C12E4–
water phase diagram, the situation is less constrained in the
lamellar phase. Here, DNA is confined between bilayers, and
concentrations from zero up to close packing are possible. The
solubility of CSDNA in lamellar phases is discussed in more
detail below.
Another important class of nonionic lipids is given by the
monoglycerides, with monoolein (MO) being a frequently
studied example.29,30
MO favors the formation of inverse struc-
tures because of the cis double bond in the hydrocarbon tail. The
MO–water system gives bicontinuous cubic phases, which are
stable over wide ranges of concentration and temperature; one of
the cubic phases coexists with excess water. MO in water also
gives a lamellar phase but only in a limited range of concentra-
tions and temperature.
The isothermal phase diagram of the DTADNA–MO–water
system (Fig. 8a) contains four LC phase regions (hexagonal,
lamellar and two cubic phases classified as Pn3m and Ia3d
structures).31
The supramolecular assemblies in this system
evolve from a bicontinuous cubic structure of the reversed type
to the two-dimensional hexagonal phase as the content of
DTADNA is increased. Whereas the dodecyltrimethyl-
ammonium ions alone tend to be incorporated into the lamellar
phase formed by MO and water, then giving rise to a swelling and
Fig. 6 Phase diagram of the ternary CTAB–n-decanol–water system.
Key: L – lamellar phase; H1 – hexagonal phase (normal type); H2 –
hexagonal phase (reversed type); I1 – isotropic solution (normal micelles);
I2 – isotropic solution (reversed micelles); and K – liquid crystalline
phase, presumably with rod-like reversed micelles (non-hexagonal
packing). Redrawn from Fontell et al.24
Fig. 7 Partial phase diagrams of the DTADNA–C12E4–water system (a)
and the DTADNA–C12E8–water system (b) at 25
C. Key: (L) lamellar
liquid crystalline phase; (Cub1) bicontinuous cubic Ia3d liquid crystalline
phase; and (H1, H1
0
and H1
00
) hexagonal liquid crystalline phases. The
phase diagrams were obtained by visual inspection, microscopic exami-
nation under polarized light, small-angle X-ray scattering (SAXS) and
deuterium NMR (2
H NMR) at 298 K and normal pressure. Redrawn
from Leal et al.26
11026 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012

6. to a significant extension of the lamellar phase region (as inferred
from the phase diagram of the DTAB–MO–water system shown
in Fig. 8b), DTADNA tends to give a reversed hexagonal phase
(Fig. 8a).31
This difference in the phase behavior can be explained
mainly by a difference in the counterion entropy between the
cases of monovalent and polyvalent counterions. Thus the phase
behavior of single-chain ionic surfactants with small monovalent
counterions is strongly influenced by electrostatic repulsions
arising from the large contribution from the counterion entropy.
With a polyion as counterion, this entropic effect is essentially
eliminated, and, furthermore, the rigidity of double-stranded
DNA results in a low conformational entropy. Therefore,
dissociation of the counterions is limited, resulting in a smaller
interfacial area of the headgroups, which allows for reversed
structures. As illustrated in Fig. 9, this arrangement would
consist of infinitely long surfactant cylinders with an aqueous
core containing the DNA rods and the hydrophobic tails
pointing toward the exterior. This structure is stable only in a
limited range of surfactant volume fractions because of two
constraints. Firstly, the maximum interdigitation of the surfac-
tant hydrophobic chains in the bilayers, at the shortest distance
between the aqueous cylinders, is limited by the surfactant
length. Secondly, MO is soluble in the hydrophobic environment
of this inverted hexagonal phase.31
There is only a small tendency of the reversed phases existing
in the binary MO–water system to accommodate DTAB or
DTADNA. Monoolein at room temperature gives two reversed
phases in water, both phases being bicontinuous cubic (Pn3m
and Ia3d), whereas CSDNA as well as DTAB favors formation
of cubic structures of the normal type. In contrast to the behavior
of MO, as we will see now, investigations of mixed phospholipid–
DNA systems show a rather different pattern.32
3.3. CSDNA–water–phospholipid systems
As a typical example of a phospholipid we consider the zwit-
terionic lipid lecithin. Phospholipids of this type behave similarly
to double-chain surfactants in showing a strong preference to
form a lamellar phase. The lamellar phase of lecithin can take up
water and swell to ca. 45 wt% water. At higher water contents,
there is a coexistence of the lamellar phase and a very dilute
aqueous solution.
The two phase diagrams presented in Fig. 8 can be compared
to those of the two systems in which MO is replaced by lecithin
(Fig. 10). The DTAB–lecithin–water system shows20
a more
extended lamellar phase and no reversed phases (Fig. 10a). In
this case, the lamellar spacing is large enough to accommodate
the DNA rods, and thus the DTADNA–lecithin–water system
also shows a lamellar phase (Fig. 10b). Only at lower water
contents, there is the same type of reversed hexagonal packing as
in the DTADNA–MO–water system. Both DTADNA–lecithin–
water and DTADNA–MO–water exhibit cubic phases, although
in different regions of the phase triangle. Whereas the cubic
phase of the system containing lecithin is of the normal type as in
the DTAB–water system, the two cubic phases of the MO-con-
taining system are of the reversed type. The Ia3d phase formed by
MO in water can contain up to 7% (w/w) of the DTADNA
complex, whereas up to 70% (w/w) of the complex can be
incorporated into the Ia3d matrix formed on adding lecithin.
The ability of amphiphilic molecules to form lyotropic liquid
crystals depends on their structure; thus, the aqueous phase
behavior of CSDNA, dispersed by the aid of lecithin, is expected
to depend on the amphiphilic counterion structure. The spon-
taneous curvature decreases on replacing a single-chained
counterion by a double-chained one, e.g. on replacing DTA by
DDA.33
The ternary phase diagram of the DDADNA–lecithin–water
system is shown in Fig. 10c. When comparing the phase diagrams
Fig. 8 Phase diagrams of the DTADNA–MO–water system (a) and the
DTAB–MO–water system (b) at 25
C. Key: (I1 and I2) fluid isotropic
phases; (L) lamellar liquid crystalline phase; (Ia3d) bicontinuous cubic
Ia3d liquid crystalline phase; (Pn3m) bicontinuous cubic Pn3m liquid
crystalline phase; (Cub1) cubic phase; and (H1 and H2) hexagonal liquid
crystalline phases. The phase diagrams were obtained by visual inspec-
tion, microscopic examination under polarized light, small-angle X-ray
scattering (SAXS) and deuterium NMR (2
H NMR) at 298 K and normal
pressure. Redrawn from Bilalov et al.31
Fig. 9 A cationic surfactant with DNA as a polyvalent counterion
preferably forms a reversed hexagonal phase in the mixtures with mon-
oolein and water, whereas with monovalent counterion the same
surfactant is simply incorporated into the existing lamellar phase formed
by monoolein in water. Redrawn from ref. 31.
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7. of the single- and double-chain surfactant systems (Fig. 10b and
c), the following two differences appear. (i) DTA with lecithin
forms a cubic liquid crystalline phase (gyroid symmetry, normal
type), where the DNA duplexes are incorporated within the
water domains32
(Fig. 11a). This phase is not present in the case
of DDA, obviously because this double-chained surfactant does
not form worm-like surfactant micelles in mixtures with lecithin
(Fig. 11b). (ii) The aqueous lecithin lamellar phase can solubilize
up to 55 weight% DDADNA, where DDA and lecithin form
mixed bilayers. On the other hand, only 25 wt% of DTADNA
can be incorporated into the lamellar phase of aqueous lecithin.
The comparison of the lecithin-based system with the MO-
based system shows that the choice of the amphiphilic additive
has a strong influence on the phase behavior. In contrast to the
MO-based system, in the lecithin-based systems, CSDNA can be
incorporated into the lamellar phase formed by the amphiphilic
additive in water.
Lamellar phases in aqueous systems of DNA and amphiphiles
have been found for different kinds of lipids and surfac-
tants,1,10,26,34,35
and they consist of surfactant bilayers intercalated
with DNA rods as represented in Fig. 11. The DNA molecules
are confined to 2 dimensions within the water layers in the
lamellar structure. For very low amounts of DNA, the DNA
molecules are expected to be randomly distributed without
orientational and translational correlations forming essentially
an isotropic phase within the water layers, as illustrated in
Fig. 12a. However, with increasing DNA concentration, we
expect the DNA molecules to orientationally order and form a
Fig. 10 Phase diagrams of the ternary (a) DTAB–lecithin–water system,
(b) the DTADNA–lecithin–water system and (c) the DDADNA–leci-
thin–water system at 25
C. Compositions are given as weight%. Cub1 is a
bicontinuous cubic phase (Ia3d), H2 is a 2D hexagonal phase of the
reversed type, and L is a lamellar phase. Figure is redrawn from
ref. 20 and 33.
Fig. 11 Schematic illustrations of the transformation of the LC struc-
tures found in the DTADNA–lecithin–water system (a) and DDADNA–
lecithin–water system (b).
11028 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012

8. two dimensional nematic phase (Fig. 12b). Simulations36–38
as
well as density functional theory calculations39
indicate that the
isotropic-to-nematic transition is of second order with a
continuously increasing order parameter. Thus the order
parameter depends on the concentration of DNA in the water
layer. Several ordered phases of 2D confined DNA have been
predicted.40
There is a strong electrostatic attraction between the DNA
polyions and the cationic surfactant that forms a mixed bilayer
together with the zwitterionic lipid. Hence, DTA+
ions and
lecithin will not mix ideally. In contrast, the ordering of the DNA
polyions is accompanied by a corresponding ordering of DTA+
ions within the bilayers, with a sinusoidal-like concentration
profile perpendicular to the DNA strands and with the maxima
coinciding with the positions of DNA molecules (Fig. 13).41
Furthermore, a significant positional order of the DNA
molecules is expected, in particular because of the long-range
electrostatic repulsion between DNA polyions within the water
layers in the lamellar structure. This repulsion is partly eliminated
by a strong electrostatic attraction between the negatively
charged DNA polyions and the positively charged surfactant self-
assemblies. CSDNAs are water insoluble because the counterions
self-assemble into highly charged aggregates that strongly asso-
ciate with the equally highly charged polyions. Different additives
can have a pronounced effect on the surface charge density due to
the interaction between the additive with the DNA polyion or
with the aggregate. By adding alcohols, nonionic lipids, etc., we
decrease the surface charge density of the aggregates and the
specific surface energy with a concomitant homogenization of the
system (increase of the solubility in water), which is accompanied
by formation of the LC phase. In the absence of surfactant
micelles, an ordered structure, e.g. a true columnar phase, is not
expected. However, at a high DNA density we do indeed expect a
high degree of orientational order due to the long-range elec-
trostatic repulsion between rigid DNA polyions.
3.4. CSDNA–water–cyclodextrin systems
All the additives considered so far co-assemble with the cationic
surfactant ions, which are the counterions of DNA. We will now
consider additives with a very different mode of action, cyclo-
dextrins. Cyclodextrins (CDs) are cyclic polyglucoses with a
polar exterior and a nonpolar interior. Cyclodextrins (CDs) can
thus bind hydrophobic molecules and form water-soluble inclu-
sion complexes with e.g. surfactants and lipids, with a very high
binding constant. As the inclusion complexes are water-soluble,
this is an efficient way for solubilizing hydrophobic molecules in
water whereas, in the case of surfactants and lipids, at the same
time preventing micelle formation.42–57
Individual surfactant
molecules thus constitute examples of location in the nonpolar
cavity of CDs and, therefore, on CD addition to a surfactant
solution, surfactant aggregates can be disassembled.
CDs have been found to increase the efficiency of transferring
DNA into eukaryotic and bacterial cells.58
It was shown that
CDs can extract membrane components, making membranes
more permeable for DNA, without lysing the cells. Otherwise,
the most common applications of CDs in pharmaceutical
formulations are to enhance the solubility, stability, and
bioavailability of drug molecules.59,60
DNA compaction in vitro can be controlled by cationic lipids.
CDs can disperse lipids in the form of water soluble inclusion
complexes, thus tuning their self-assembly. CDs form strong
inclusion complexes with DTA, with an essentially infinite
binding constant, and in the presence of CDs the self-assembly
behavior is controlled by the molar ratio between lipid and CD,
in addition to the DNA concentration.61–63
This behavior is illustrated here by the case of hydroxypropyl-
b-cyclodextrin, HPbCD. The phase diagram of the DTADNA–
HPbCD–water system62
is shown in Fig. 14a. In the presence of
Fig. 12 Schematic drawing of the DNA molecules (rods) confined to 2
dimensions within the water layers in the lamellar structure at very low (a)
and at high (b) amounts of DNA. Redrawn from ref. 20.
Fig. 13 Schematic drawing of the ordering of DTA ions within the lipid
bilayers in the lamellar structure at high amounts of DNA.
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9. HPbCD, DTADNA can be solubilized in the isotropic liquid
phase of water and HPbCD when the ratio R ¼ [DTA]/[CD] # 1.
This solution phase does not contain micelles and from the sharp
phase boundary at R ¼ 1 it is concluded that HPbCD does not
associate with DNA, only with DTA. Increasing the DTADNA
concentration, keeping R 1, leads to the formation of a liquid
crystal with 2D hexagonally ordered DNA molecules.
With a further increase of the DTADNA concentration and at
R 1.5–2, a second anisotropic phase, having a tetragonal
lattice, is formed. The tetragonal phase corresponds to a unique
stoichiometric compound. In this phase, the DNA duplexes are
still parallel but with a simple square rather than a hexagonal
packing. The periodicity in the direction of the DNA duplexes is
coupled to the DNA pitch length, 3.3 nm.
A schematic representation of the structure evolution is shown
in Fig. 15, from DTADNA in the absence of HPbCD, through
the tetragonal phase, and the hexagonal phase up to the isotropic
liquid phase of the solution.
In the 2D hexagonal phase, the DNA duplexes can rotate
freely around their long axes and the phases of their helical
pitches are uncorrelated. In the tetragonal phase, the neighboring
DNA duplexes have at least their helices in phase. The locking of
the helical phase and the transition from a hexagonal to square
packing of the DNA molecules correlate with the introduction of
the micellar macroions. Because of their high charge, micelles
associate with more than one DNA duplex. Thus, a likely
explanation for the transition from a hexagonal to a square
packing is that there is a preference for the highly charged
micelles to coordinate four DNA duplexes rather than three. The
micelles are expected to have a stronger attraction to the minor
groove, where the negative charge density is higher, and this may
explain the lateral correlation of the helical pitch. The picture
that arises is that a central micelle in a plane locally coordinates
the minor grooves of four parallel DNA duplexes. Hydrated
CSDNA with rod-like surfactant micelles in the absence of CD
has a 2D square symmetry similar to the tetragonal phase of
CSDNA, with spherical surfactant micelles in the presence of CD
(Fig. 15).
The aqueous phase behavior of polymer–surfactant
complexes, dispersed by the aid of CDs, is expected to depend on
the polyion rigidity.64
The partial ternary phase diagram of the
DTAPA–HPbCD–water system is presented in Fig. 14b The
linear charge densities of the polyions are similar but the average
contour length of PA (ca. 1.5 mm) is approximately 6 times longer
than the average DNA contour length (0.24 mm, assuming B-
form DNA).
When comparing the phase diagrams of the two systems, the
following two qualitative differences appear: (i) the two liquid
crystalline phases where the DNA duplexes are ordered parallel
with 2D hexagonal symmetry (hexagonal phase) or 2D square
symmetry (tetragonal phase) are not present in the case of PA
because this polyion is too flexible. (ii) DTAPA can swell with up
to 55 weight% water, where DTA forms small spherical micelles
at the swelling limit.
Both phases, a simple square of hydrated DTADNA and 2-D
hexagonal of hydrated DTAPA, consist of ordered parallel
cylindrical surfactant micelles. However, a cubic phase at higher
water contents is not formed with DNA. Also this can be
understood from differences in the stiffness of the two polymers.
Comparing cylinders and spherical micelles, only cylindrical
surfactant micelles can provide a homogeneous matching of the
rod-like DNA charge. PA, on the other hand, is flexible enough
to wrap around spherical DTA micelles.65
Fig. 14 Complete/partial phase diagrams of the ternary (a) DTADNA–
HPbCD–water system and (b) DTAPA–HPbCD–water system at 25
C.
Compositions are given as weight%. Key: I is an isotropic solution phase,
Cub1 is a micellar cubic phase (Pm3n), H1 is a micellar hexagonal
phase (cylindrical micelles), H is a 2D hexagonal phase formed by DNA,
and T is a tetragonal phase. The two-phase areas are white areas
without any label. The grey area indicates a three-phase region. The
molar ratio R ¼ [DTA]/[HPbCD] ¼ 1 is shown as a dotted line. Redrawn
from ref. 62 and 63.
Fig. 15 Illustrations of the CSDNA microstructure variations with
cyclodextrin (HPbCD) concentration: from the squared ordering of
cylindrical surfactant aggregates in the phase of hydrated DTADNA in
the absence of HPbCD (left) through the tetragonal phase (middle) and
the hexagonal phase (right) up to the isotropic liquid solution phase.
11030 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012

10. The phase behavior of the polyion–surfactant systems depends
on the flexibility of the polyion in both cases, in the absence as
well as in the presence of the surfactant aggregates. At the same
time, the polymer chain flexibility effect may be eliminated in the
presence of the surfactant self-assemblies. When comparing the
phase diagrams of the two systems, CTAPA–alcohol–water
(Fig. 16) and DTADNA–alcohol–water (Fig. 6), no marked
differences could be found. Both systems show a similar phase
behavior in giving a L–H2 transition.
Thus, whereas in the presence of the surfactant self-assemblies,
the flexibility of the polyion may have an insufficient effect on the
phase behavior; in the absence of the surfactant aggregates, the
polymer packing considerations and flexibility of the polymer
chain have a pronounced effect.
4. Understanding the ternary phase diagrams
The role of cyclodextrins in assisting the solubilization of
CSDNA in water is straightforward. By forming inclusion
complex with the surfactant ion, micellization is prevented and
the surfactant ions are dispersed as monovalent counterions. In
the other systems, the third component is another amphiphile
or a co-surfactant with which the CS ions co-aggregate. By
adding alcohols as co-surfactants, the charge density of the
surfactant aggregates can be decreased allowing for different
aggregate shapes. As discussed above for the binary system, an
important criterion for phase stability is the matching of the
aggregate charge density with the charge density of the DNA
polyion. With single chain nonionic surfactants, CmEn or
monoolein, the phase diagram depends on the choice of the
surfactant as they have different preferred curvatures. With the
lamellar forming lecithin we find that the lamellar phase can
solubilize substantial amounts of CSDNA at higher water
contents whereas at lower water contents there is a transition
to a reverse hexagonal phase. The phase diagrams with
DTADNA and DDADNA show large similarities and appear
to follow some general principles that deserve a more detailed
discussion.
4.1. The reverse hexagonal phase
In the reverse hexagonal phase, DNA is incorporated in the
aqueous core of the reverse cylindrical micelles. For a homo-
geneous phase, there is a constraint that the total length of
DNA essentially has to equal the total length of the reverse
cylindrical micelles. The contour length density of DNA is
given by
LDNA
V
¼
fDNA
ADNA
(5)
Here, ADNA ¼ pR2
DNA is the effective cross-sectional area of
DNA, with radius RDNA z 1.0 nm, and fDNA is the DNA
volume fraction. The total volume fraction of the aqueous
micellar core, fDNA + fw ¼ 1 À fs, can be written as
1 À fs ¼
pRw
2
Lcyl
V
(6)
where Rw is the radius of the aqueous cylinder core and Lcyl/V is
the length per unit volume of the cylindrical micelles. Assuming
that all surfactants occupy the interface of the aqueous
domains, the total interfacial area per unit volume is given by
fs/ls. Here, the effective surfactant length, ls, is strictly the
surfactant volume-to-area ratio ls ¼ vs/as, where vs is the
surfactant molecular volume and as is the average area that each
surfactant molecule occupies at the interface, here defined by Rw.
In analogy with eqn (6), this area density can be written as
fs
ls
¼
2pRwLcyl
V
(7)
Dividing eqn (6) by eqn (7) we obtain an expression for Rw
Rw ¼
2lsð1 À fsÞ
fs
(8)
which incorporated into eqn (6) gives
Lcyl
V
¼
fs
2
4pð1 À fsÞls
2
(9)
With eqn (5) and (9) the constraint LDNA/V ¼ Lcyl/V can be
expressed as
fDNA ¼
ADNAfs
2
4pð1 À fsÞls
2
(10)
Apart from eqn (10), there are additional constraints involved.
First of all, Rw RDNA z 10 A. At the same time, Rw can only be
a few Angstr€oms larger than RDNA. The cationic amphiphilic
counter-ions of DNA are anchored at the interface and there is a
significant attraction between the cylindrical interface and DNA
that gives an upper limit to Rw. From eqn (8) we see that con-
straining Rw to a narrow range around, say, 15 A sets a corre-
sponding constraint on fs and further, by eqn (10) on fDNA.
Hence, the reverse hexagonal phase should be limited to a narrow
‘‘island’’ in the phase diagram, the location of which can be
calculated from eqn (8) and (10) and a Rw constraint.
Fig. 16 Partial phase diagram of the ternary CTAPA–n-octanol–water
system at 25
C. Compositions are given as weight%. I is an isotropic
solution phase, H1 is a micellar hexagonal phase (cylindrical micelles), L
is a lamellar phase, and H2 is a reversed hexagonal phase. The two-phase
areas are white areas without any label. The grey area indicates a three-
phase region. Redrawn from ref. 66.
This journal is ª The Royal Society of Chemistry 2012 Soft Matter, 2012, 8, 11022–11033 | 11031

11. We can perform this analysis on the DDADNA–lecithin–
water system. Here, we have fDNA ¼ 0.40fDDADNA and the total
surfactant volume fraction fs ¼ flecithin + 0.60fDDADNA. Setting
Rw z 15 A and using ls ¼ 20 A, which is a reasonable value for
lecithin which is the majority lipid, we obtain with eqn (4) and (6)
flecithin z 0.55 and fDDADNA z 0.25 as the location of the
reverse hexagonal phase, in good agreement with the experi-
mental phase diagram. From corresponding calculations for the
DDADNA–lecithin–water system we find a similar good agree-
ment. In Fig. 17 we present a model phase diagram of the
DDADNA–lecithin–water system with the calculated location
(spot) of the reverse hexagonal phase shown as a filled circle.
4.2. The lamellar phase
In the lamellar phase, the DNA molecules are confined to a thin
water layer of thickness
dw ¼ 2ls
1 À fs
fs
(11)
where 2ls is the effective bilayer thickness. For the reversed
hexagonal phase we noted that the DNA-interface attraction
constrains Rw T RDNA. In the lamellar phase, for the same
reason, we expect dw to be constrained to values similar to but
slightly larger than 2RDNA. This implies that the lamellar phase
has essentially the shape of a line in the phase diagram. Focussing
again on the DDADNA–lecithin–water system, and assuming a
constant bilayer thickness 2ls ¼ 40 A and dw ¼ 30 A, the line
begins at flecithin ¼ 2ls/(2ls + dw) ¼ 0.57 on the binary lecithin–
water axis. As DDADNA is added, the water content in the
lamellar phase is slightly decreased as DNA partly replaces water
at constant dw. When DNA gets added to this lamellar phase,
confined to two dimensions, we first expect at low DNA
concentrations an isotropic phase where the DNA rods are free
to rotate in the plane (Fig. 12a). At higher DNA concentrations
we expect the rods to order into a nematic phase, with an order
parameter that increases with increasing DNA concentration.
This two dimensional ordering of the DNA rods in the lamellar
phase often results in a (broad) diffraction peak,20,34
from which
the separation, dDNA, between approximately parallel DNA rods
can be obtained.
We expect the lamellar line to terminate approximately when
the DNA rods get close packed, thus when dDNA z dw. Setting
this termination to dDNA ¼ dw ¼ 30 A corresponds to fDNA/fw ¼
0.54 and, for the case of DDADNA to fDNA/fw ¼ 1.3. In Fig. 17
the calculated location of the lamellar phase is shown as a line
defined by dDNA $ dw ¼ 30 A. At higher water contents this
lamellar phase coexists with pure water. So does also the fully
hydrated DDADNA complex. With an excess of DDADNA,
beyond DNA close packing in the lamellar phase, the hydrated
complex forms and we thus have the three phase coexistence of
hydrated complex (hexagonal), water and lamellar phase.
The calculated phase diagram, Fig. 17, clearly catches the
salient features of the DDADNA–lecithin–water phase diagram
(Fig. 10c). With the corresponding single chain surfactant
counterion, DTA, the situation is slightly different. For lower
lecithin concentrations, the lamellar phase becomes unstable
because the single chained DTA, unlike the double chained
DDA, is not a lamellae forming surfactant. Instead a cubic phase
having a weakly curved surfactant interface is formed.
5. Conclusions
In conclusion, we have summarized phase diagram studies for
the ternary systems of water, additive and rigid DNA duplexes
with amphiphilic counterions (cationic surfactants, CS) that act
as a salt of two macro-ions because of the tendency of the
amphiphilic counterions to self-assemble into highly charged
micelles. These ternary CSDNA–additive–water systems may
show a rich phase behavior with various liquid crystalline phases
that can be tuned by the choice of additive and by varying the CS/
additive ratio. We have recently determined several ternary
CSDNA–additive–water phase diagrams, for different surfac-
tants–lipids or cyclodextrins as additive, and dodecyl-
trimethylammonium, DTA, or didodecyldimethylammonium,
DDA, as the CS component. With a second lipid–surfactant–
alcohol as additive, the system is essentially dominated by
aggregated surfactants–lipids that act as the structure deter-
mining part and dictate the phase behavior. In the presence of
cyclodextrins, CD, as additives preventing micelle formation, the
DNA duplexes act as the main building blocks in the system. The
results are in qualitative agreement with experiments on ternary
systems for flexible polyacrylate (PA) chains, and it would be
useful to compare them with experiments on a flexible
Fig. 17 Calculated phase diagram of the DDADNA–water–lecithin
system showing the predicted locations and extensions of the lamellar and
reverse hexagonal phases, respectively. Redrawn from Krivtsov et al.33
Fig. 18 Illustrations of the polyion–surfactant complex microstructure
variations with different characters of additive: for the lipophilic additives
(cosurfactants) penetrating the surfactant aggregates (to the left) and for
the cyclodextrins dissolving the surfactant aggregates (to the right).
11032 | Soft Matter, 2012, 8, 11022–11033 This journal is ª The Royal Society of Chemistry 2012

12. single-stranded DNA or other biopolymers. CSDNAs in the LC
state are largely controlled by the following factors: (i) the
lipophilic characteristics of the additive, (ii) the [CSDNA]/
[additive] molar ratio and (iii) DNA packing considerations. We
summarize our conclusions in the scheme shown in Fig. 18.
Acknowledgements
Financial support from the Swedish Research Council (VR),
including support through the Linnaeus center of excellence
grant Organizing Molecular Matter (OMM) (239-2009-6794), is
gratefully acknowledged.
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This journal is ª The Royal Society of Chemistry 2012 Soft Matter, 2012, 8, 11022–11033 | 11033