Structure-property relationships in metallosurfactants

Article abstract of DOI:10.1039/b920143b

The morphology of micelles formed by three sub-classes of metallosurfactants - those with macrocyclic, linear and gemini head groups - has been studied by small-angle neutron scattering (SANS) for a series of metal- and counter-ions. All the data may be described by a model that invokes a globular micelle morphology in which the dimensions of the micelle are consistent with the known chemical structure of the constituent groups within the metallosurfactant. For two macrocyclic head group metallosurfactants, viz. 1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane that forms predominantly spherical micelles and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane that forms disc-like micelles, the metal ion and its counter-ion have a negligible effect on the morphology of the micelle. Binary mixtures of surfactants with these two macrocyclic head groups (with homo- or hetero-metal ions/counter-ions) form mixed micelles whose morphology is an average of the two single component micelles. Further, as found for the single surfactant solutions, the metal and counter-ion had no effect on the morphology of the mixed surfactant micelle. Lastly, the micelle morphology of two gemini surfactants was also shown to be insensitive to the number and nature of the metal and counter-ions present, but sensitive to the structure of the head group. These observations considerably extend our understanding of the relationship between chemical structure and micelle morphology for these interesting molecules.

Full text of DOI:10.1039/b920143b

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Volume 6 | Number 9 | 7 May 2010 | Pages 1835-2094
ISSN 1744-683X
PAPER
REVIEW
Peter C. Griffiths et al.
Structure–property relationships
in metallosurfactants
I. W. Hamley
Liquid crystal phase formation
by biopolymers
1744-683X(2010)6:9;1-I

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PAPER
www.rsc.org/softmatter | Soft Matter
Structure–property relationships in metallosurfactants†
a
a
Peter C. Griffiths, Ian A. Fallis, Craig James,a Ian R. Morgan,a Gemma Brett,a Richard K. Heenan,b
*
*
c
Ralf Schweins, Isabelle Grillo and Alison Paul
c
a
*
Received 30th September 2009, Accepted 14th January 2010
First published as an Advance Article on the web 12th February 2010
DOI: 10.1039/b920143b
The morphology of micelles formed by three sub-classes of metallosurfactants—those with
macrocyclic, linear and gemini head groups—has been studied by small-angle neutron scattering
(SANS) for a series of metal- and counter-ions. All the data may be described by a model that invokes
a globular micelle morphology in which the dimensions of the micelle are consistent with the known
chemical structure of the constituent groups within the metallosurfactant. For two macrocyclic head
group metallosurfactants, viz. 1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane that forms
predominantly spherical micelles and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane that
forms disc-like micelles, the metal ion and its counter-ion have a negligible effect on the morphology of
the micelle. Binary mixtures of surfactants with these two macrocyclic head groups (with homo- or
hetero-metal ions/counter-ions) form mixed micelles whose morphology is an average of the two single
component micelles. Further, as found for the single surfactant solutions, the metal and counter-ion
had no effect on the morphology of the mixed surfactant micelle. Lastly, the micelle morphology of two
gemini surfactants was also shown to be insensitive to the number and nature of the metal and counter-
ions present, but sensitive to the structure of the head group. These observations considerably extend
our understanding of the relationship between chemical structure and micelle morphology for these
interesting molecules.
surfactants form small globular micelles whose scattering could
Introduction
be described by a core–shell morphology; 1 was shown to form
predominantly spherical micelles which became slightly more
elongated with increasing ionic strength, whereas micelles
formed from 2 were more disc-like. The critical micelle concen-
trations (CMCs) of 1 and 2 were shown to follow the classical
behaviour observed for ionic surfactants—a decrease of
approximately ꢀ2 for each additional CH2 group inserted into
the hydrophobic tail. Further, the CMC values of 1 were found
to be approximately an order of magnitude lower than those of 2,
but otherwise insensitive to the nature of the metal ion.
Here, we elaborate the study of these interesting surfactant
micelles, and present exemplary scattering studies that quantify
the effect of the metal counter-ion on the structure of the micelles
formed from 1 and 2. In addition, we expand the study to include
homo- and hetero-metal surfactant mixtures of 1 and 2 and
finally, draw comparisons between the structural analogues of
macrocyclic, acyclic (3) and related gemini surfactants (4),
Scheme 1.
Metallosurfactants represent a class of surfactant in which the
polar head group of the surfactant molecule contains a d- or
f-block metal complex as an integral structural component.1,2
The incorporation of transition metal into a surfactant structure
facilitates new functionality, such as catalytic activity, to be
concentrated at interfaces.3 Further, the self-assembly of metal-
losurfactants provides a simple means of preparing well-defined
aggregates of metal complexes with nuclearities equal to the
aggregation numbers of the micelles themselves, and these are
beginning to find application in, for example, magnetic reso-
nance imaging, templating of mesoporous materials, thin-film
optoelectronics, interfacial photophysics and homogeneous
catalysis.4–12
Metallosurfactants have been shown to follow the same clas-
sical behaviour as conventional surfactants, viz. surface activity
and self-assembly.13–21 Our previous work focused principally on
dodecyl tailed derivatives of 1,4,7-triazacyclonane and 1,4,7,10-tet-
raazacyclononane, viz. 1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane
(1) and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane
(2), in conjunction with selected structural analogues.2,15 These
Results and discussion
The syntheses of ligands 3, 4a and 4b are illustrated in Schemes 1
and 2. Ligand 3 was prepared in good yield by the nucleophilic
ring opening of 1,2-epoxydodecane and was purified by distilla-
tion. Ligands 4a and 4b were prepared via the secondary amine
intermediate 40, which was obtained by the addition of a large
excess of N,N-dimethylethylenediamine to 1,2-epoxydodecane.
40 was sufficiently volatile to be purified by bulb-to-bulb distil-
lation and hence the gemini ligands 4a and 4b were prepared in
a high state of purity as a mixture of diastereomers by the
aSchool of Chemistry, Cardiff University, Main Building, Park Place,
Cardiff, UK CF10 3AT. E-mail: griffithspc@cardiff.ac.uk; Fax: +44
(0)29 20874030; Tel: +44 (0)29 20875858
bSTFC ISIS Facility, Rutherford Appleton Laboratory, Harwell Science &
Innovation Campus, Didcot, UK OX11 0QX
cInstitut Laue-Langevin, BP 156, 6, rue Jules Horowitz, 38042 Grenoble
Cedex 9, France
† This work was presented at Association In Solution II, Structure,
Function & Performance held in Tomar, Portugal, July 2009.
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Soft Matter, 2010, 6, 1981–1989 | 1981

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addition of excess 40 to the appropriate diepoxides with the
subsequent distillation of excess 40. In general, attempts to isolate
complexes of ligands 1, 2, 3, 4a and 4b were thwarted by the
extreme solubility of these materials in polar solvents, including
water, with slow evaporation affording glasses in all instances.
The two exceptions to this were that the Cu(AcO)2 complex of 4a
slowly formed an ill defined precipitate on standing and that
NiCl2 complexes of racemic samples of 1 have been shown
previously to crystallise in alternating layers of R and S materials
as the well-defined [Ni(1)(H2O)2]Cl2.23 As amino alcohol donors
the electronic spectra of the metallosurfactants are dominated in
the visible region by d ) d transitions. Hence, the electronic
II
II
spectra for the Ni and Cu metallosurfactants derived from 1, 2,
3, 4a and 4b were measured as a means of determining their
coordination geometries in solution (Table 1). Given the asym-
metric nature of the ligands involved, the coordination geometry
II
of the Ni metallosurfactants is best described as pseudo-octa-
hedral in which the coordination spheres logically assigned to
consist of the ligand amine donors, pendant alcohol groups and
with coordinative saturation achieved via additional solvent
II
water molecules. For convenience the Ni spectra have been
Scheme 1 Syntheses and structures of (a) macrocyclic ligands and
(b) mononuclear metallosurfactants studied here. Reagents and condi-
tions: (i) EtOH, N,N,N0-trimethylethylenediamine (1.2 equiv.), room
assigned using the parent octahedral terms. The extinction
coefficient values for all these samples were well within the
normal range for d ) d transitions of pseudo-octahedral species
II
temperature, N2, 5 days, quantitative; (ii) M (X)2$nH2O, H2O (M ¼ Ni,
II
in aqueous solution.22 The Cu complexes of ligands 1, 2, 3, 4a
Cu; X ¼ Cl, NO3, SO4, and AcO).
and 4b all display absorption spectra typical22 of Jahn–Teller
distorted 6 coordinate species whilst that of ligand 2, as previ-
ously reported23 is 5 coordinate. The absorption spectra for all
the complexes examined were found to be insensitive to changes
in the nature of the counter-ion, indicating that the counter-ions
do not coordinate to the metal centres and that these species are
fully ionised in solution and may be regarded as 1 : 2 electrolytes
or a 1 : 1 electrolyte in the sulfate case. Thus, and in contrast to
those amphiphiles in which counter-ions occupy positions in the
primary coordination sphere,24 these materials might be expected
to be insensitive to changes in the nature of the counter-ions. The
pH values of 25 mM stock solutions of the metallosurfactants
II
were found to be in the range of 6.0–7.4 for the Ni species and 4.9–
II
5.9 in the case Cu . These pH values are significantly lower than
II
II
the acid dissociation values for Ni (ꢁ10.2) and Cu (ꢁ7.3– 8.0)
aqua species,25 and are thus presumably also lower than those of
the pendant secondary alcohols. Hence we may assume that the
coordinated water molecules and pendant donor alcohols remain
protonated, resulting in the formulae depicted in Table 1. This
contrasts the behaviour of the system reported by Lesh et al.26
where pendant phenolic donors could be deprotonated and
II
coordinated, giving octahedral Ni square planar species.
The critical micelle concentrations (CMCs) of both 1 and 2
show a very weak variation with metal ion or its counter-ion, e.g.
for 1, CMC([Cu(1)(H2O)n]Cl2) ¼ 0.50 ꢂ 0.06 mM; CMC([Cu(1)
(H2O)n](NO3)2) ¼ 0.80 ꢂ 0.05 mM; CMC([Cu(1)(H2O)n]acetate2) ¼
0.65 ꢂ 0.03 mM, whilst CMC([Cu(1)(H2O)n]SO4) ¼ 0.45 ꢂ
0.07 mM, whereas CMC([Ni(1)(H2O)n]Cl2) ¼ 0.45 ꢂ 0.06 mM. The
CMC is determined by a subtle balance of the hydrophilic (head
group) and hydrophobic (tail) characteristics of the molecule, the
former containing a term arising from the electrostatic nature of
the head group. The fact that for fixed hydrophobe size, the
CMC varies only slightly with counter-ion (indeed, only just
outside experimental error) implies that the hydrophilic
Scheme 2 Synthesis and nomenclature of gemini metallosurfactants
from ligands 4a and 4b. Reagents and conditions: (i) N,N-dimethylethy-
lenediamine (20 equiv.), room temperature, N2, 5 days, quantitative; (ii)
II
a,u bis-epoxide (0.4 equiv.), EtOH, quantitative; (iii) M Cl2$nH2O, H2O
(M ¼ Ni, Cu).
1982 | Soft Matter, 2010, 6, 1981–1989
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II
Table 1 Electronic spectroscopy data for Ni and Cu complexes of ligands 1, 2, 3, 4a and 4b
II
l/cmꢃ1 (3/L molꢃ1 cmꢃ1
)
3
3
(n2) T1g(F)
3
(n3) T1g(P)
Metallosurfactant
Colour
Chromophore
(n1) T2g z D
1Eg
B
930a
995a
866a
834a
892a
II
Ni N3O3
[Ni(1)(H2O)2](NO3)2
[Ni(2)(H2O)](NO3)2
[Ni(3)(H2O)3](NO3)2
[Ni2(4a)(H2O)3](NO3)2
[Ni2(4b)(H2O)3](NO3)2
Blue purple
Purple
Blue-green
Blue-green
blue-green
10530 (10.3) 9730 (sh)
10395 (19.5)
9690 (6.1)
9810 (11.4)
9770 (11.1)
12800 (sh)
12660 (sh)
13315 (sh)
13368 (sh)
Not observed
17540 (4.7)
17860 (9.8)
15750 (5.8)
15780 (8.8)
15770 (7.6)
28010 (6.9)
28250 (24.9)
26310 (11.8)
27030 (26)
26040 (16)
II
Ni N4O2
II
Ni N2O4
II
Ni N2O4
II
Ni N2O4
a
b
3
Racah B ¼ (n2 + n3 ꢃ 3D)/15. Ground state is A2g
.
See ref. 15.
Metallosurfactant
Colour
Chromophore
l/cmꢃ1 (3/L molꢃ1 cmꢃ1
)
b
II
Cu N3O3
[Cu(1)(H2O)2]Cl2
[Cu(2)(H2O)]Cl2
Blue
Purple
Blue
Blue
Blue
664 (52)
568 (199)
700 (77)
690 (71)
706 (72)
b
II
Cu N4O
II
Cu N2O4
[Cu(3)(H2O)3]Cl2
[Cu(3)(H2O)3](NO3)2
[Cu(3)(H2O)3](SO4)
II
Cu N2O4
II
Cu N2O4
character and hence charge on the head group is also largely
invariant with counter-ion, consistent with their electronic
spectra. The surfactants therefore do bear some charge—reflec-
ted in the scaling of the CMC with hydrophobe size—but the
magnitude of the CMC, which is lower than the equivalent
dodecyl sulfate (CMC ¼ 8 mM) or dodecylammonium halides
(CMC(Brꢃ) ¼ 2 mM), implies that there is considerable counter-
ion binding to the micelle surface.
For [Cu(2)(H2O)n]Xn—Fig. 1b—differences are observed in
the datasets in both low and high Q regions. The acetate con-
taining surfactant was not fully soluble and is hence omitted
from the series. At low Q, it is clear that the data do not show the
weak peak observed for 1 suggesting the absence of a structure
factor, and thus, that these surfactants are somewhat less
charged than those formed from 1. There is an upturn at low Q,
a signature of an attractive interaction associated with the
proximity of the concentration of this sample to the phase
boundary, in which no doubt contributes to the insolubility of
acetate sample and various other metal salts that could not be
explored here due to their insolubility. The differences at high
Q indicate that the smaller dimension associated with the micelle
morphology is increasing across the counter-ion series, in the
order: sulfate > nitrate z chloride, but these changes are
approaching the limit of the resolution of the data.
Typical scattering data from 1 and 2 are presented in Fig. 1
and 2, as functions of metal ion (copper, zinc and nickel) and its
counter-ion (chloride, nitrate, acetate and sulfate). Fig. 3 shows
scattering from binary surfactant mixtures of 1 and 2 with copper
(Fig. 3a), nickel (Fig. 3b) and mixed metal (Fig. 3c) head groups.
In all plots, the intensities have been normalised to (divided by)
the concentration of micellised surfactant after subtracting the
unimer term (i.e. a concentration equivalent to the CMC).
A simple interpretation of the effects of these variables may be
gained by a straightforward comparison of different datasets.
Notwithstanding any moderation of the data at low Q by
a structure factor (S(Q)), it should be remembered that the Q
range is an inverse distance scale, and therefore smaller dimen-
sions affect the scattering at high Q, whereas overall or larger
dimensions affect the scattering at lower Q values.
Further insight into the interplay of head group structure/
composition may be gained by considering the comparisons
presented in Fig. 2a and b, for 1 as a function of metal ion with
II
nitrate or sulfate counter-ion viz. [M (1)(H2O)n](NO3)2
a
II
(Fig. 2a) and [M (1)(H2O)n](SO4)2 (Fig. 2b). Clearly, for both
series, there is a negligible change in scattering, and hence
morphology. One may conclude therefore, that the micelle
morphology for both 1 and 2, whilst mutually very different, is
insensitive to both the metal ion and its counter-ion.
Fig. 1a presents the scattering from [Cu(1)(H2O)n]Xn (nor-
malised to the concentration of micellised surfactant, [Ctotal
ꢃ
CMC]) for a series of counter-ions, Xn: chloride, nitrate, acetate
and sulfate. These four datasets are very similar at high Q,
allowing us to conclude that the counter-ion does not have
a significant effect on the smaller dimension describing the
micelles, but the subtle differences at low Q values may arise
either due to a change in the larger dimension characterising the
micelle morphology, or due to a change in the micellar charge.
The presence of the peak in the data (or more precisely, the
decrease in intensity towards Q ¼ 0) suggests these differences
arise from a structure factor, and hence the charge on the micelle,
in the order: acetate > nitrate > chloride > sulfate. This
conclusion—that micelles formed from 1 are weakly charged and
that this character does not vary with metal or its counter-ion—is
entirely consistent with the observed CMC behaviour.
In order to more fully analyse these data, and to correctly
identify the most appropriate model with which to interpret these
data, one starts first with a coarse model e.g. solid sphere/ellipse,
and refine to
a more complex parameterisation of the
morphology based on a ‘‘goodness of fit’’, the physical signifi-
cance of the parameters obtained, and a check against the
absolute scattering intensity using the known concentration and
composition of the materials under investigation. For 1 and 2,
the best model is that of a core–shell morphology in which one is
able to arbitrarily delineate the head group (polar shell) and tail
(core) regions of the micelle, and to constrain the micelle
curvature and size based on calculated lengths and volumes of
the respective fragments of the molecule. These constraints and
how they are defined in the fitting routine are listed in the
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Fig. 1 (a) Concentration normalised small-angle neutron scattering data
II
for [Cu (1)(H2O)n]Xn (25 mM) and fit to the core–shell ellipse model
Fig. 2 (a) Concentration normalised small-angle neutron scattering data
II
for [M (1)(H2O)n](NO3)2 (25 mM) and fit to the core–shell ellipse model
where X ¼ chloride (open circles), nitrate (filled circles), acetate (hexa-
where M ¼ copper (circles) and nickel (squares) in D2O. The zinc sample
gons) and sulfate (triangles) in D2O. (b) Concentration normalised small-
II
angle neutron scattering data for [Cu (2)(H2O)n]Xn (50 mM) and fit to
is not shown as it was unstable. (b) Concentration normalised small-angle
II
neutron scattering data for [M (1)(H2O)n]SO4 (25 mM) and fit to the
the core–shell ellipse model where X ¼ chloride (open circles), nitrate
(filled circles) and sulfate (triangles) in D2O. The acetate sample did not
dissolve.
core–shell ellipse model where M ¼ copper (triangles), zinc (circles) and
nickel (squares) in D2O.
Experimental section. The principle fitting parameters then
become the micelle ellipticity (X) defining the shape and the
volume fraction of water in the polar shell. For 1, one also needs
to include a contribution from a structure factor to account for
the weak peak in the data. In essence, the micelles formed from 1
are predominantly spherical; an ellipticity of 1.2 < X < 1.5 best
describes the data given the constraints applied in the fitting
routine. This correctly accounts for the absolute intensities, and
based on better estimates for the molar volumes, represents
a significantly more developed analysis of our earlier data.15 The
charge on the micelle is very small, consistent with a degree of
counter-ion dissociation of around 10–15%. Across the counter-
ion series, the only fitting parameter that varies is the charge in
the micelle, and this largely follows the order stated previously:
acetate > nitrate > chloride > sulfate, although any interpreta-
tion of this parameter is at the limit of the resolution of the data.
For 2, the micelles are predominantly disc-like (0.3 < X < 0.5)
but there is a slight variation in micelle morphology with
counter-ion; X(sulfate) ¼ 0.3 (ꢂ0.05), whereas X(chloride,nitrate) ¼ 0.5
(ꢂ0.07). This observation, combined with the differences in
scattering length density and CMCs, accounts for the nature of
the scattering at low Q.
Fig. 3a–c present representative data for equi-concentration
mixtures of 1 and 2, for homo- (Fig. 3a and b) and hetero-metal
salts (Fig. 3c). Clearly, the three datasets in Fig. 3a and b are
mutually different, implying substantial changes in micelle
1984 | Soft Matter, 2010, 6, 1981–1989
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morphology with head group structure/composition. It is worth
remembering that the single surfactant micelles 1 and 2 have
quite different shapes; 1 forms largely spherical micelles (1.3 < X
< 1.5), whereas 2 forms disc-like micelles (X ¼ 0.5). For both
systems presented in Fig. 3a and b, the behaviour of the scat-
tering at high Q is (mutually) very different, with the blend data
displaying more of the characteristics of the scattering from 1,
indicating that the smaller dimension for the mixture not only lies
between the individual cases 1 and 2, but that this dimension
increases in the blend. Fitting the data to the core–shell model as
described previously—invoking now constraints based on the
composition weighted average for the ratio of surfactant head
group to tail volume—allowed the data to be described by
a micelle with a morphology with X ¼ 1.0 (ꢂ0.1) and shell
II
˚
thickness of 12 (ꢂ1) A for the Cu system in Fig. 3a and X ¼ 0.7
II
˚
(ꢂ0.1) and shell thickness of 10 (ꢂ1) A for the Ni system in
Fig. 3b i.e. by ‘‘blending’’ surfactants that prefer different
curvatures, one arrives at an ‘‘average’’ curvature, consistent with
the known behaviour for traditional surfactants.27
The hetero-systems in Fig. 3c follow an analogous trend—
namely, by blending two surfactants that form micelles with
different preferred curvatures results in a morphology consistent
with the average curvature. Since the morphology of the micelles
formed from 1 and 2 is insensitive to the metal ion and its
counter-ion, it perhaps should come as no surprise that micelles
formed from mixtures of 1 and 2 also exhibit the same insensi-
tivity to the metal and its counter-ion.
Fig. 4a–c display the effect of metal ion concentration,
[Ni(NO3)2)], on the scattering from three analogous surfactants
with a linear head group 3 (Fig. 4a), an ethylene oxide 4a
(Fig. 4b) and an ethylene linked 4b (Fig. 4c) gemini surfactant.
The linear head group of 3 can be regarded as a monomeric form
of 4a and 4b. Significant differences exist in the scattering from
the three concentrations of [Ni(NO3)2] for the linear surfactant 3,
but the scattering (and hence, micelle morphology) is largely
invariant as a function of ionic strength for the two gemini cases.
One may conclude therefore that the presence of the inter-head
group linker in the gemini cases effectively restricts any
morphological variation in the micelle induced by the additional
metal in the linear head group case. For the gemini cases, the
metal concentrations (25, 50 and 125 mM) correspond to a head
group population of 1 metal per surfactant head group (25 mM
metal and 25 mM surfactant), 1 metal per chelating group
(2 metals per head group), and an excess of metal ion.
The same core–shell model has been invoked to analyse these
data where the head group volumes have been calculated based on
the constituent units. For the linear head group, constraining the
core radius to the dimensions of the alkyl chain, yields an elliptical
micelle, X ¼ 2, consistent with that observed for 1. On addition of
an excess of Ni(NO3)2, the micelles formed from 3 become less
elliptical, and there is a concomitant decrease in the shell thickness
Fig. 3 (a) Concentration normalised small-angle neutron scattering data
II
for a 1 : 1 25 mM mixtures of [Cu (1)(H2O)n]SO4 and [Cu (2)(H2O)n]SO4
II
(grey squares) and fit to the core–shell ellipse model in D2O. Also shown
II
are the single component surfactant solutions for comparison, [Cu
II
(1)(H2O)n]SO4 (open circles) (25 mM) and [Cu (2)(H2O)n]SO4 (open
˚
from17to 12A. Theionic strength inducedshape changeobserved
for 3 is consistent with that observed for 1.15 For the gemini
squares) (50 mM). (b) Concentration normalised small-angle neutron
surfactants, the ethylene oxide linked gemini fits better to an
II
scattering data for a 1 : 1 25 mM mixtures of [Ni (1)(H2O)n]SO4 and [Ni
II
(2)(H2O)n]SO4 (grey squares) and fit to the core–shell ellipse model in
II
II
(1)(H2O)n]SO4 and [M(2) (2)(H2O)n]SO4 and fit to the core–shell ellipse
D2O. Also shown for comparison are the single component surfactant
II
II
solutions [Ni (1)(H2O)n]SO4 (25 mM) (open circles) and [Ni
II
model in D2O; two homo-metal systems, M(1) ¼ M(2) ¼ Cu (squares) and
II
(2)(H2O)n]SO4 (open squares) (50 mM). (c) Concentration normalised
M(1) ¼ M(2) ¼ Ni (diamonds), plus the two hetero-metal systems, M(1)
¼
Ni and M(2) ¼ Cu (triangles) and M(1) ¼ Cu and M(2) ¼ Ni (circles).
II
II
II
II
small-angle neutron scattering data for 1 : 1 25 mM mixtures of [M(1)
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This work provides a detailed indication of the behaviour of
metallosurfactants under a range of metal ion, counter-ion and
ionic strengths. The relative insensitivity in these behaviours
indicates a robustness of structure under conditions likely to be
required for any practical application of these materials in,
e.g., catalysis, where parameters such as surface charge, head
group hydration and curvature may dictate functional activity.
Hence, these systems combine elements of classical surfactant
behaviour, favouring predictive formulation based on existing
knowledge, and introduce active functionality in a manner
amenable and resilient to changing processing conditions
required for many potential applications.
Experimental
General
The syntheses of ligands 1 and 2 are described elsewhere.15 1,2-
epoxytetradecane was prepared as previously reported.15 Digly-
cidyl ether was prepared as a mixture of diastereomers by the
m-CPBA oxidation of racemic allyl glycidyl ether and distilled
twice prior to use according to the method of Cassel et al.28 All
other reagents (Aldrich or Lancaster) were used as received.
Solvents were purified by standard literature methods.29 Mass
spectra were obtained in electrospray mode on a Waters Q-tof
Micro spectrometer equipped with an Aquity UPLC autosam-
pler. IR spectra were recorded on KBr discs as thin films using
a Perkin Elmer 1600 spectrometer. Electronic spectra were
recorded in aqueous solution using Jasco V-570 NIR-UV-vis or
Perkin Elmer Lambda 900 spectrometers. NMR spectra were
€
obtained on Bruker AMX 400, AMX 500 or Jeol Eclipse 300
spectrometers and referenced to external TMS.
Synthesis of materials
2,5-Dimethyl-7-hydroxy-2,5-diazaheptadecane (3). To a solu-
tion of 1,2-epoxydodecane (4.6 g, 25 mmol) in freshly distilled
dry ethanol (50 ml) was added N,N,N0-trimethylethylenediamine
(3.1 g, 30 mmol, 1.2 equivalents). The reaction mixture was
allowed to stand at room temperature for 5 days. Solvent and
excess reactant were removed using a rotary evaporator by the
addition of small volumes of toluene. The residue was distilled in
a short path distillation apparatus to afford the desired product
1
as a colourless mobile oil in essentially quantitative yield. H
NMR (400 MHz, CDCl3): d ¼ 0.81 (t, 3H, 6.6 Hz, -CH3), 1.11–
1.45 (m, 18H, -(CH2)9-), 2.16 (s, 6H, NCH3), 2.24 (s, 3H, NCH3),
2.18–2.27 (m, 3H, HCHN), 2.31–2.42 (m, 2H, HCHN), 2.56
(m, 1H, HCHN), 3.53 (m, 1H, CHOH). 13C NMR (100 MHz,
CDCl3): d ¼ 13.13 (CH3), 21.68 (CH2), 24.78 (CH2), 28.33 (CH2),
28.56 (CH2), 28.61 (CH2), 28.86 (CH2), 30.90 (CH2), 33.92
(CH2), 42.56 (N-CH3), 44.34 (N-CH3), 54.08 (N-CH2), 56.28
(N-CH2), 63.30 (N-CH2CH(OH)), 66.62 (CH(OH)). HR-MS
(ESI, MeCN–H2O) (m/z): calculated for +H+ (M + H+) 287.3057,
found 287.3065.
Fig.
4 (a) Small-angle neutron scattering data from 25 mM
II
[Ni (3)(H2O)n]NO3 as a function of Ni(NO3)2; 25 mM Ni(NO3)2 (circles),
50 mM Ni(NO3)2 (squares) and 125 mM Ni(NO3)2 (triangles). (b) Small-
II
angle neutron scattering data from 25 mM [Ni (4a)(H2O)n]NO3 as
a function of the concentration of Ni(NO3)2; 25 mM (circles), 50 mM
(squares) and 125 mM (triangles). (c) Small-angle neutron scattering data
II
from 25 mM [Ni (4b)(H2O)n]NO3 as a function of the concentration of
Rac/meso-1,2;9,10-diepoxydecane. 1,2-Epoxydec-9-ene (3.86 g,
25 mmol) and excess m-CPBA (17.3 g, 50 mmol assuming 50%
water content) were stirred in chloroform (150 ml) for 48 h. The
precipitated m-chlorobenzoic acid was filtered off and the
remaining solution washed with 1 M sodium thiosulfate
Ni(NO3)2; 25 mM (circles), 50 mM (squares) and 125 mM (triangles).
˚
elliptical micelle, 1.5< X < 2 witha shell thickness of 10(ꢂ2) A, but
the ether linked gemini fits better to a disc-like micelle, X ¼ 0.65
˚
˚
(ꢂ0.05) A with a slightly thicker shell thickness, 14 (ꢂ2) A.
1986 | Soft Matter, 2010, 6, 1981–1989
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(100 ml ꢀ 3). The organic layer at this stage tested negative for
peracids or peroxides. The chloroform layer was dried (MgSO4)
and reduced to a small volume and absorbed onto a short
(5 ꢀ 5 cm) silica column. Elution with CH2Cl2 and removal of
solvent affordedꢄ a residue that was purified by bulb-to-bulb
distillation (110 C, 0.5 mmHg), after discarding a short initial
fraction, to afford the desired product as a clear colourless liquid.
Yield 3.3 g (78%). Spectroscopic data were consistent with
reported by van As et al.30
H2O in volumetric flasks. The reaction mixtures were gently
warmed to effect complete dissolution of the ligands and diluted
to the appropriate concentration. Samples for SANS measure-
ments were made in a similar manner using D2O.
Small-angle neutron scattering (SANS)
Small-angle neutron scattering experiments were performed
either on (i) the fixed-geometry, time-of flight LOQ diffractom-
eter at the ISIS Spallation Neutron Source, Rutherford Appleton
Laboratory, Didcot, UK which spans a Q range (Q ¼ (4p/l)
2-Methyl-7-hydroxy-2,5-diazaheptadecane (40). To a solution
of 1,2-epoxydodecane (4.61 g, 25 mmol) in freshly distilled dry
ethanol (100 ml) was added N,N,-dimethylethylenediamine (22 g,
250 mmol, 10 equivalents). The reaction mixture was allowed to
stand at room temperature for 5 days. Solvent and excess N,N,-
dimethylethylenediamine were removed using a rotary evapo-
rator. The residue was purified twice by bulb-to-bulb distillation
ꢃ1
˚
sin(q/2)) 0.008 < Q < 0.25 A by using neutron wavelengths
˚
(l) spanning 2.2 to 10 A with a fixed sample-detector of 4.1 m or
(ii) on the D11 or D22 diffractometers at the ILL, Grenoble,
ꢃ1
˚
˚
which span a Q-range of 0.01 < Q < 0.25 A , (l ¼ 8 A) requiring
two or three separate instrument configurations (sample–
detector distances and collimation).
€
using a Kugelrohr apparatus to afford the desired product as
1
a white waxy solid. Yield 5.4 g (79%). H NMR (400 MHz,
Samples were contained in 2 mm path length, UV-spectro-
photometer grade, quartz cuvettes (Hellma, GmBh) and mounted
in aluminium holders on top of an enclosed, computer-
controlled, sample chamber. Sample volumes were around
0.6 cm3. All experiments were conducted at 25 ꢄC (unless
otherwise stated). Temperature control was achieved by using
a thermostatted circulating bath pumping fluid through the base
of the sample chamber, achieving a temperature stability of
ꢂ0.2 ꢄC. Experimental measuring times were approximately
40 min per sample.
CDCl3): d ¼ 0.82 (t, 3H, 6.6 Hz, -CH3), 1.12–1.43 (br m, 18H,
-(CH2)9-), 2.15 (s, 6H, NCH3), 2.36 (m, 3H, HCHN), 2.63
(m, 3H, HCHN), 3.52 (m, 1H, CHOH). 13C NMR (100 MHz,
CDCl3): d ¼ 13.12 (CH3), 21.68 (CH2), 24.76 (CH2), 28.33 (CH2),
28.61 (CH2), 28.79 (CH2), 28.86 (CH2), 33.72 (CH2), 44.33
(N-CH3), 45.88 (N-CH2), 54.58(N-CH2), 58.14(N-CH2), 68.47
(CH(OH)). HR-MS (ESI, MeCN–H2O) (m/z): calculated for
C16H36N2O + H+ (M + H+) 273.2900, found 273.2911.
All scattering data were (a) normalized for the sample trans-
mission, (b) background corrected using a quartz cell filled with
D2O, and (c) corrected for the linearity and efficiency of the
detector response using the instrument specific software package.
Ultimately, the model of the micelle adopted here is that of
a charged particle with an elliptical core–shell morphology and
therefore, the intensity of scattered radiation, I(Q), as a function
of the wave-vector, Q, is given by;
5,13-Bis-(2-hydroxydodecyl)-2,16-dimethyl-7,11-dihydroxy-2,5,13,16-
tetraaza-9-oxa-heptadecane (4a). To a solution of diglycidyl ether
(0.65 g, 5 mmol) in ethanol was added 40 (3.40 g, 12.5 mmol, 2.5
equivalents). The reaction mixture was allowed to stand at room
temperature for 14 days. The solvent was removed using a rotary
evaporator and the excess starting material was separated by
bulb-to-bulb distillation (oven temp 170 ꢄC, 0.05 mmHg) to
afford the desired product as a highly viscous pale yellow oil in
quantitative yield. 1H NMR (400 MHz, CDCl3): d ¼ 0.86 (t, 6H,
7.0 Hz, -CH3), 1.18–1.56 (m, 36H, C-CH2-C), 2.23 (s, 12H,
N-CH3), 2.15–2.82 (br m, 20H), 3.12–4.10 (m, 8H). HR-MS
(ESI, MeCN–H2O) (m/z): calculated for C38H83N4O5 + H+
(M + H+) 675.6358, found 675.6358.
2
2
I(Q) ¼ nm[S(Q)hF(Q)i + h|F(Q)|2i ꢃ hF(Q)i ] + Binc
(1)
where nm is the number of micelles per unit volume, S(Q) is the
structure factor and Binc is the incoherent background.
For an elliptical micelle, both F(Q) and F(Q)2 require numer-
ical integration over an angle g between Q and the axis of the
ellipsoid to account for the random distribution of orientations
of the ellipse. For clarity, we omit this.
5,16-Bis-(2-hydroxydodecyl)-2,19-dimethyl-7,14-dihydroxy-2,5,16,19-
tetraazaicosane (4b). This material was prepared by using the
method for 4a using rac/meso-1,2;9,10-diepoxydecane (0.850 g,
5 mmol) and 40 (3.40 g, 5 mmol, 2.5 equivalents). The yield was
F(Q) ¼ V1(r1 ꢃ r2)F0(QR1) + V2(r2 ꢃ rsolvent)F0(QR2) (2)
1
quantitative. H NMR (400 MHz, CDCl3): d ¼ 0.89 (t, 6H, 6.6
The first term in eqn (2) represents the scattering from the core
Hz, -CH3), 1.18–1.56 (m, 48H, C-CH2-C), 1.88 (br m, 4H, OH),
2.22 (s, 12H, N-CH3), 2.03–2.80 (8H, N-CH2), 3.15–3.55
(m, 8H), 3.77 (m, 4H, CHOH). HR-MS (ESI, MeCN–H2O)
(m/z): calculated for C42H91N4O4 + H+ (M + H+) 715.7035,
found 715.7054.
with radius R (subscript 1) and the second, the polar shell
4
3jiðQRiÞ
(subscript 2). Vi ¼ pR3i and F0ðQRiÞ ¼
(ji is the first-
3
QRi
order spherical Bessel function of the first kind). S(Q) represents
the spatial arrangement of the micelles in solution and n the
micelle number density.
Metallosurfactant formation and coordination
determination
r is the neutron scattering length density of the micellar core
(subscript 1), the polar shell (subscript 2) and the solvent
(subscript 0). These constants are combined into a single fittable
parameter used to ‘‘scale’’ the model intensity to the absolute
value. Post-fitting, this scalar is recalculated using the parameters
II
II
Ni and Cu complexes of 1, 2, 3, 4a and 4b were prepared by the
addition of 1.05 (or as stated explicitly) equivalents of the
appropriate divalent metal salt to a suspension of the ligand in
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describing the micelle morphology/composition and the molar
concentration of micelles to validate the fit. The calculated and
observed values should lie within ꢁ10%.
calculated based on the assumption of 20% dissociation of the
counter-ions.
In summary, the minor radius, the ratio of head group to tail
volumes, the scattering length densities of the anhydrous head
group and core regions, the volume fraction and ionic strength
are input as constants. The only fitting parameters describing the
form factor P(Q) are the ellipticity X of the micelle and the
volume fraction of water in the head group region. The thickness
and scattering length density of the hydrated head group region
are (re-)calculated within the analysis software, based on the
fraction of water in the shell, fwater. The only fitting parameters
in the structure-factor calculation are the charge and the hard-
sphere radius.
It is expedient here to limit the number of adjustable param-
eters within the core–shell fit and this is most conveniently
undertaken by first delineating the head group and tail regions to
comprise the macrocyclic group, metal ion and its counter-ion,
and the pendant hydroxyl group whilst the dodecyl (alkyl) group
is the tail. Hence, for 1 and 2, the minor radius of the ellipse
˚
defining the hydrophobic core can be set to 16.7 A, being the all-
trans length of a dodecyl tail, and thus, its scattering length
density set to be that of dodecane. In a likewise fashion, based on
this delineation, the model can be further refined by calculating
the volume and scattering length density of the anhydrous
surfactant head groups. As an estimate, we have computed the
surface area of the head group using a DFT routine ADF21 based
on a triple zeta basis set and polarization functions on all atoms
except the metal. From this surface area, an effective spherical
Conclusion
Small-angle neutron scattering has been used to quantify the
morphology of the micelles formed from a series of metal-
losurfactants as a function of metal and counter-ion. All the data
may be described by a model that invokes a globular micelle
morphology in which the dimensions of the micelle are consistent
with the known chemical structure of the constituent groups
within the metallosurfactant, i.e. classical surfactant behaviour.
Surfactants based on 1 form spherical micelles whereas those
formed from 2 are disc-like. Mixtures of 1 and 2 form micelles of
intermediate ellipticity, but both the single component and
binary surfactant micelles are insensitive to the metal ion and its
counter-ion. A similar conclusion may be drawn from the scat-
tering of two gemini based surfactants—the morphology of the
micelle formed from an ethylene bridged surfactant is substan-
tially different to that formed from a ethylene oxide bridged
surfactant, but the morphology of both was insensitive to the
metal and its counter-ion.
3
˚
volume is calculated having incorporated 30 A for the OH
3
˚
group, 27 A for each of the two –CH2– groups, and an appro-
priate volume for the metal ion and its counter-ions based on
their atomic radii.
Modelling of the structure of the head group region is slightly
complicated by the fact that the counter-ions may dissociate
from the micelle surface to some degree. This parameter is not
known so we have predicted, based on the insensitivity of the
scattering and CMC to change in counter-ion, that ꢁ80% of the
counter-ions are electrostatically bound to the micelle surface.
The maximum error in both the head group scattering length
density and head group volume introduced by using this
assumption is less than 10%, well within experimental error (for
II
example, for [Cu (2)]Cl2 (i.e. no dissociation); rhead group ¼ 0.90
3
II
+
ꢀ 10ꢃ6
A
ꢃ2, Vhead goup ¼ 475 A ; {[Cu (2)]Cl} (i.e. 50% disso-
˚
˚
ciation); rhead group ¼ 0.75 ꢀ 10ꢃ6
A
ꢃ2, Vhead goup ¼ 450 A ). We
3
˚
˚
have adopted a 20% degree of dissociation and thus, the volume
Acknowledgements
3
of the polar head is 465 A for [Cu (2)]Cl2 and 420 A for
II
3
˚
˚
II
[Cu (1)(H2O)2]Cl2. The corresponding scattering length densities
Funding support is gratefully acknowledged from EPSRC
(GR/S25456/01; GR/R58604/01; EP/C53090X/1; GR/L80447/01;
GR/M55732/01; GR/M81373/01; EP/G006156/1), Science
Facilities Technology Council (STFC) and The Leverhulme
Trust (F/00407/Q).
of the anhydrous head groups are very similar, 0.83 ꢀ 10ꢃ6
A
ꢃ2
˚
and 0.81 ꢀ 10ꢃ6
A
ꢃ2, respectively. Practically, these volumes are
˚
introduced into the fitting routine via the ratio of the anhydrous
surfactant head group volume to the volume of the hydrophobic
tail, corresponding to 1.1 for 1 and 1.3 for 2. For the gemini
surfactants, the ratio of the anhydrous surfactant head group
volume to the volume of the hydrophobic tail is calculated in
a similar fashion, and is fixed at 1.5.
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Soft Matter, 2010, 6, 1981–1989 | 1989