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Cite this: DOI: 10.1039/c3ob41467a
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Template-directed self-assembly by way of molecular
recognition at the micellar–solvent interface:
modulation of the critical micelle concentration†
Mark A. Olson,* Jonathan R. Thompson, Trenton J. Dawson,
Christopher M. Hernandez, Marco S. Messina and Travis O’Neal
By incorporating the concepts of structural preorganisation and complementarity in concert with noncovalent donor–acceptor [π⋯π] and hydrophobic interactions, a duo of π-electron deficient bipyridiniumbased linear and gemini amphiphiles capable of responding to molecular templation have been
designed and synthesised. When combined with π-electron rich di(ethylene glycol)-disubstituted 1,5dihydroxynaphthalene, a dramatic decrease in the critical aggregation concentration by ≈66% was
observed with concomitant increases in the hydrodynamic diameter, ζ-potential, and Langmuir surface
pressures of the micellar solutions—thus enhancing the detergents’ efficiency and effectiveness at lowering the surface tension of water. By employing a phase separation model that takes into account the
Received 16th July 2013,
Accepted 8th August 2013
degree of counterion binding to the micellar aggregate superstructure, the effects of donor–acceptor
templation on the Gibb’s free energy of micellisation (ΔG°M) for the amphiphiles was quantified. It was
found that donor–acceptor templation was capable of lowering ΔG°M by up to 1.75 kcal mol−1 at which
DOI: 10.1039/c3ob41467a
point it was observed, while under the influence of molecular templation, that linear single hydrophobic
tailed detergent molecules exhibit properties characteristic of double-tailed phospholipid-like gemini
www.rsc.org/obc
surfactants.
Introduction
The controlled self-assembly of organic molecules into discrete
supramolecular architectures continues to impact research
and development in the areas of chemistry, materials science,
and molecular nanotechnology.1 By making use of a clutch of
non-covalent bonding interactions—in which donor–acceptor
[π–π], [CH–O], [CH–π], and hydrophobic interactions are all
orchestrated to a high degree of precision—it is now possible
to design synthetic surfactant systems whose adjustable properties are now starting to more closely resemble and mirror
the adaptable physiological dynamics of their biological
counterparts, for example pulmonary surfactant matrixes and
various other phospholipids.2 By utilising established structure–property relationships3 whereby subtle synthetic manipulations can have a huge impact on performance and
molecular functionality, the stage has been set to usher in
new classes of surfactants whose properties and performance
Department of Physical & Environmental Sciences, Texas A&M University-Corpus
Christi, 6300 Ocean Drive, Corpus Christi, Texas 78412-5774, USA.
E-mail: mark.olson@tamucc.edu
† Electronic supplementary information (ESI) available: 1H and 13C and 1H–1H
g-DQF-COSY spectra of compounds 12+ and 24+. See DOI: 10.1039/c3ob41467a
This journal is © The Royal Society of Chemistry 2013
characteristics can be easily modulated without the need of
covalent modifications.4
The detergency effect of surfactants is manifested by their
self-assembly into higher ordered structures known as
micelles. Of the many non-covalent interactions that play a
role in surfactant self-assembly, hydrophobic interactions are
the primary driving force whereby the entropic penalty for
micelle assembly is less than the entropic penalty for solvating
the entire surfactant with water molecules. The conditions for
surfactant self-assembly thus are governed by the concentration of the amphiphiles in solution, the concentration at
which the first aggregation process occurs being known as the
critical micelle concentration (CMC) and subsequent aggregation occurring at the critical aggregation concentration (CAC).
The process of micellisation can be modeled5 most easily
using a molecular thermodynamic (MT) theoretical approach
in which the free energy for micellisation (ΔG°M) can be
expressed as the sum of at least six different free energy contributions – namely the transfer free energy of the surfactant tail,
interfacial micellar aggregate core-water free energy, packing
free energy of the surfactant tail, as well as the surfactant
head-group steric, head-group electrostatic, and head-group
dipole interactions – expressed per mole of surfactant. Given
this thermodynamic treatment of surfactant self-assembly,
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only the transfer free energy is large, negative and favourable.
With exception to the aggregate core-water interfacial free
energy – which decreases with increasing aggregation number
providing positive cooperativity favoring micellar growth to
larger sizes – all of the other free energy contributions to ΔG°M
are positive. It is these free energy contributions which grow
more positive with increasing aggregation number leading to
negative cooperativity that ultimately limits the micellar aggregates to definite sizes. For someone attempting to exert control
over the micellisation process, its onset, and the size and
shape of the aggregate superstructures, these are the most
important free energy contributors which must be targeted for
modulation.
The electrostatic free energy contribution to micellisation—
which is typically opposing and repulsive in nature stemming
from the charged (anionic or cationic) surfactant head group—
is arguably6 one of the easiest free energy contributors to
target for modulation. It becomes clear then that any external
stimuli driven process which can serve to decrease the repulsive electrostatic interactions between contiguous surfactant
polar head groups can serve as the foundation for the molecular modulation of the critical micelle concentration as well
as both the micellar size and shape. Supramolecular donor–
acceptor complexes are a perfect candidate for minimising
electrostatic repulsions among surfactant head groups.
Thus in our own laboratory we have turned our attention to
this concentration dependent aggregation behaviour of amphiphiles in aqueous solvent in an effort to control precisely their
intermolecular interactions at the air–water interface and the
emergence of supramolecular aggregates. Our efforts were
motivated in part by recent advances7 in developing donor–
acceptor supramolecular systems based on the binding interactions of π-electron deficient bipyridiniums with π-electronrich molecular guests and the recent reemergence of bipyridinium-based detergents both in aqueous environments and
mixed aqueous–organic solvent systems in which, for example,
the complexation of the bipyridinium head group of the surfactant with cucurbit[6]uril,8 cyclodextrins,9 and electron rich
groups6 have been used to trigger the spontaneous formation
of vesicles and ultralong nanofibers. While these examples
have focused on the characterisation of the supramolecular
aggregate species, we have become more interested in these
similar donor–acceptor interactions and their effects and behaviour at the air–water interface and the thermodynamics
which govern the assembly of increasingly larger templated
micellar aggregates as a result of modulating the CMC. The
research reported herein describes a detergent binary blend in
which the surface activity and the CAC of two bipyridiniumbased amphiphiles – a dicationic linear surfactant 12+ and a
tetracationic gemini10 surfactant 24+ – can be easily modulated
by employing the principles of preorganisation1 and templatedirected self-assembly1 in aqueous solutions upon the
addition of an electron rich additive. Characterisation of the
micellar nanostructures and their self-assembly by surface
tension (ST), conductometry, NMR spectroscopy, dynamic
light scattering (DLS), and ζ-potential measurements
Org. Biomol. Chem.
Organic & Biomolecular Chemistry
confirmed the presence of two aggregation processes of which
only one, occurring at higher detergent concentrations,
responds to template-directed coercion resulting in dramatic
decreases in the CAC concomitant with increases in the hydrodynamic diameter and ζ-potential of the micellar aggregates,
as well as increases in the Langmuir surface pressures.
Experimental
General
Starting materials and reagents were purchased from commercial suppliers and used as received. Compounds 3+ and
di(ethylene glycol)-disubstituted 1,5-dihydroxynaphthalene
(DNP-DEG) were synthesised following procedures reported11,12
in the literature. All reactions were performed under an argon
atmosphere and in dry solvents unless otherwise noted.
Analytical thin-layer chromatography (TLC) was performed on
aluminum sheets, precoated with silica gel 60-F254 (Merck
5554). Flash chromatography was carried out using silica gel
60 (230 − 400 mesh) as the stationary phase. Deuterated solvents (Cambridge Isotope Laboratories) for NMR spectroscopic
analyses were used as received. 1H and 13C NMR spectra were
recorded on a Bruker Avance 300 MHz spectrometer. Chemical
shifts are reported in ppm relative to the residual signal of the
solvent (D2O: δ 4.79 ppm). High resolution electrospray ionisation (HRESI) mass spectral analyses were performed by the
Mass Spectrometry Facility of the Department of Chemistry
and Biochemistry at the University of Texas at Austin. Surface
tension measurements were performed on a CSC Precision
DuNoüy ring tensiometer. All surface tension samples were
allowed to equilibrate for 24 hours. UV-Vis spectra were
recorded on a Varian CARY 100BIO temperature-controlled
spectrophotometer in a 1 cm disposable cuvette with a 5 mm
path length at 298 K. Dynamic laser light scattering, and laser
Doppler electrophoresis (for the determination of ζ-potential
and electrophoretic mobility) measurements were performed
using a Malvern Nano Series Zetasizer Nano-ZS at a scattering
angle of (θ) of 173°. All solvents used for dynamic laser light
scattering and laser Doppler electrophoresis were filtered
through a Whatman Anotop 25 inorganic membrane filter
with a 0.02 μm pore size prior to sample preparation. Conductivity measurements were performed on a Thermo Orion 550
equipped with a conductivity cell.
Synthesis of 12+ and 24+
12+: monomethylated viologen 3+ (10.0 g, 33.5 mmol) and
11-bromo-1-undecene (15.56 g, 67.0 mmol, 14.64 mL) were added
to dry degassed DMF (130 mL) and heated at 80 °C while stirring for 24 hours. The reaction mixture was cooled and a red
precipitate was collected by filtration. The red precipitate was
washed with DMF and ethyl ether three times each and dried
in vacuo to afford the dicationic product 12+ (10.56 g, 59%) as
an orange/red bromide–iodide mixed counterion salt. Counterion exchange: the solid was then taken up in hot H2O and a
saturated solution of NH4PF6 in H2O was added dropwise until
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no further precipitate formed. The white precipitate was then
filtered and dissolved in a minimal amount of MeCN. A saturated solution of tetrabutylammonium bromide (TBABr) in
MeCN was then added dropwise until no further precipitate
formed. The precipitate was then filtered and dried in vacuo to
afford the dibromide salt as a yellow solid. Mp: 259–262 °C.
1
H NMR (D2O, 300 MHz, 25 °C): δ = 1.29 (s, 6H), 1.37 (s, 6H),
2.00 (m, 4H), 4.51 (s, 3H), 4.73 (t, J = 7.37 Hz, 2H), 4.97 (d, J =
10.02 Hz, 2H), 5.01 (d, J = 17.18 Hz, 2H), 5.91 (m, 1H), 8.54 (t,
J = 6.09 Hz, 4H), 9.06 (d, J = 5.81 Hz, 2H), 9.12 (d, J = 5.81 Hz, 2H);
13
C NMR (D2O, 100 MHz, 25 °C): δ = 26.03, 28.86, 28.91, 29.09,
29.32, 29.35, 31.16, 33.72, 48.58, 62.04, 114.42, 126.88, 127.27,
139.13, 145.56, 146.39, 149.47, 149.92; LRMS (ESI): m/z calcd
for C22H32N2: 324.2560; found: 324.3 [M − BrI]+; HRMS (ESI):
m/z calcd for C22H32N2: 324.2560; found: 324.2561 [M − BrI]+.
Mono-undecylated bipyridine
4,4′-Dipyridyl (20.0 g, 128 mmol) was dissolved in dry degassed
DMF (80 mL), and the solution was heated at 90 °C. 11-Bromo1-undecene (29.7 g, 128 mmol) in dry degassed DMF (40 mL)
was then added dropwise to the reaction. The reaction was
stirred for an additional 24 h. The reaction mixture was cooled
and a yellow precipitate was removed by filtration. The DMF filtrate was evaporated under reduced pressure, resulting in a
brown oil out of which the title compound crystallised upon
standing. This solid was taken and pulverised in toluene
(150 mL) via sonication. The toluene–solid mixture was filtered
and the solid was then washed with toluene and diethyl ether
three times each and dried in vacuo to afford the monocationic
product 4+ (31.85 g, 64%) as a light tan solid. Mp: 66–67 °C.
1
H NMR (D2O, 300 MHz, 25 °C): δ = 1.22 (s, 6H), 1.31 (s, 6H),
1.80–2.00 (m, 4H), 4.65 (t, J = 7.22 Hz, 2H), 4.89–5.00 (m, 2H),
5.84 (m, 1H), 7.91 (d, J = 6.25 Hz, 2H), 8.41 (d, J = 6.25 Hz, 2H),
8.77 (d, J = 6.25 Hz, 2H), 8.96 (d, J = 6.25 Hz, 2H); 13C NMR
(D2O, 100 MHz, 25 °C): δ = 25.02, 27.88, 28.09, 28.15, 28.22,
28.25, 30.35, 33.03, 61.68, 113.91, 122.44, 125.95, 140.24,
142.53, 144.74, 149.99, 153.68; LRMS (ESI): m/z calcd for
C21H29N2: 309.2325; found: 309.3 [M − Br]+; HRMS (ESI): m/z
calcd for C21H29N2: 309.2325; found: 309.2327 [M − Br]+.
24+: mono-undecylated bipyridine (10.0 g, 25.7 mmol) and
α,α′-dibromo-p-xylene (1.70 g, 6.44 mmol) were combined in
dry degassed DMF (100 mL) and the mixture was heated at
80 °C while stirring for 24 h. The reaction mixture was cooled
and a yellow precipitate was collected by filtration. The yellow
precipitate was then washed with DMF and ethyl ether three
times each and dried in vacuo to afford the tetracationic
product 24+ (6.45 g, 96%) as a yellow solid. Mp: 292–293 °C.
1
H NMR (D2O, 300 MHz, 25 °C): δ = 1.22–1.32 (br d, 24H),
1.91–2.05 (br m, 8H), 4.71 (t, J = 7.44 Hz, 4H), 4.82–4.96 (m,
4H), 5.80 (m, 2H), 5.98 (s, 4H), 7.64 (s, 4H), 8.53 (t, J = 7.10 Hz,
8H), 9.10 (d, J = 7.10 Hz, 4H), 9.17 (d, J = 7.10 Hz, 4H)
13
C NMR (DMF-d7, 100 MHz, 25 °C): δ = 24.97, 27.80, 28.09,
28.12, 28.14, 28.19, 30.40, 33.02, 62.34, 64.19, 113.88, 126.95,
127.10, 127.23, 130.35, 134.22, 140.33, 145.63, 149.70, 150.54;
HRMS (ESI): m/z calcd for C50H66Br2N4: 440.1828; found:
440.1825 [M − 2Br]2+.
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Paper
Determination of the free energies of micellisation (ΔG°M)
Free energies of micellisation were calculated using the
method described by Zana13 and are reported per hydrophobic
tail. The following equation was used for all free energy calculations:
1
i Zs i Zs i Zs ln j
β
ln
ΔG°M ¼ RT
þ β ln CMC þ RT
j
j Zc
j Zc j Zc j
where R is the gas constant, T is the temperature in Kelvin, i is
the number of charged groups in the molecule, j is the
number of hydrophobic tails in the molecule, zs is the charge
per charged group, zc is the charge per counterion, β = (m1 −
m2)/m1 where m1 is the slope of the graph of conductivity
versus concentration below the critical micelle concentration,
m2 is the slope of the graph of conductivity versus concentration above the critical micelle concentration, and CMC is
the critical micelle concentration.
Results and discussion
Design and synthesis of the bipyridinium-based surfactants
In an effort to investigate the modulation of the CMC/CAC
using donor–acceptor interactions, we designed two π-electron
deficient bipyridinium-based surfactants capable of forming
donor–acceptor complexes with π-electron rich guests. Compound 12+ was designed to bind a single molar equivalent of
π-electron rich guest when situated alongside and packed
amongst other molecules of 12+ at the micelle–solvent interface. Compound 24+, a gemini surfactant, was designed to
bind up to two molar equivalents of π-electron rich guests,
with a single guest residing within the bipyridinium pocket
and the other equally engaged in along-side binding interactions at the packed micellar–solvent interface. Terminal
unsaturation of the hydrophobic tails was employed for future
polymerisation and/or oligomerisation, the results of which
will be discussed in a future communication. However, it must
be noted here that the effects of terminal unsaturation on
hydrophobic surfactant tails are reported14 to result in a clear
decrease in hydrophobicity equivalent to the removal of 1–1.5
CH2 groups.
Synthetic routes to obtain compounds 12+ and 24+ were
devised as shown in Scheme 1. The dicationic amphiphile 12+
was obtained in by the quaternisation of a known10 monomethyl viologen derivative with excess 11-bromo-1-undecene in
dry degassed DMF at 80 °C for 24 hours. Counterion exchange
of the mixed I−Br− counterion to the hexafluorophosphate salt
using NH4PF6 in water followed by a subsequent counterion
exchange to the dibromide salt using TBABr in MeCN gave the
desired dicationic surfactant 12+ in 59% yield. The tetracationic amphiphile 24+ was made in 61% overall yield over two
steps beginning with monoquaternisation of 4,4′-bipyridine
with equimolar 11-bromo-1-undecene in dry degassed DMF at
90 °C for 24 hours. A two-fold quaternisation of the monoundecylated bipyridine, obtained in 64% yield in the first step,
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tension of water. We were surprised to discover that the CMCs
obtained at higher concentrations actually reflected a second
critical aggregation concentration (CAC)16 when variable concentration conductivity experiments at 298 K revealed transitions at much lower concentrations for 12+ and 24+ at 2.73 ±
0.24 mM and 2.77 ± 0.15 mM respectively. It is known17 that
amphiphiles typically undergo several concentration-based
self-assembly processes—the first yielding spherical-like
micelles and a second at much higher concentrations yielding
larger cylindrical-type structures. However earlier reports18 on
similar bipyridinium-based amphiphilic compounds did not
reveal the existence of both a lower CMC and a higher CAC.
Probing the donor–acceptor interactions
Scheme 1 Graphical representations (top left) and the synthesis of the bipyridinium-based single-tailed dicationic amphiphile 12+ (top right) and doubletailed gemini tetracationic amphiphile 24+.
with α,α′-dibromo-p-xylene in dry degassed DMF at 80 °C over
24 hours gave the desired tetracationic product 24+ in 96%
yield.
All of the surfactants and their precursors were characterised by 1H NMR, 13C NMR, and HRMS.
Determination of the CMC and the CAC
Critical micelle concentrations (CMCs) were determined at
298 K by means of surface tension (ST) and conductivity
measurements (Table 1). Plots of ST versus the log concentration for both 12+ and 24+ revealed sharp inflections corresponding to CMCs of 29.8 ± 2.0 mM and 9.0 ± 1.0 mM and
Langmuir surface pressures of 15.7 ± 0.3 mN m−1 and 25.5 ±
0.3 mN m−1 respectively. As expected, the covalent preorganisation of two amphiphile chains in gemini8 surfactant 24+
resulted in a dramatic decrease in its CMC and an increase in
the Langmuir surface pressure producing a detergent which
was more efficient15 and effective15 at lowering the surface
π-Electron deficient bipyridinium-based derivatives such as 12+
and 24+ are known19 to form stable donor–acceptor π–π stacking complexes with π-electron rich dihydroxynaphthalene
(DNP) containing compounds. The DNP-bipyridinium binding
motif has been exploited in the template-directed synthesis of
mechanically interlocked compounds20 and the formation of
host–guest complexes in solution,19 at the side-chains of polymers,21 and at the metal nanoparticle–solvent interface.22
Di(ethylene glycol)-disubstituted 1,5-dihydroxynaphthalene
(DNP-DEG) was thus chosen as an appropriate agent capable
of lowering the Gibbs free energy of micellisation (ΔG°M)5b,13,23
by mitigating Coulombic repulsion among contiguously
assembled amphiphiles acting as molecular glue for the hydrophilic amphiphile head groups (Fig. 1 process I–III). The
addition of the template results in the triggering of micellar
aggregate formation at substantially lower detergent concentrations, whereby 12+ can benefit by the addition of 1 molar
equivalent of DNP-DEG (Fig. 1 process I) and 24+ can bind up
to 2 molar equivalents of DNP-DEG (Fig. 1 process II and III)
with one equivalent residing within the bipyridinium pocket
and the other equivalent equally engaged in alongside intermolecular binding interactions.24
Upon addition of DNP-DEG to solutions of 12+ and 24+, a
dramatic colour change (Fig. 2b) occurred, stemming from the
donor–acceptor charge transfer interactions between the
Table 1 Critical micelle concentrations of 12+ and 24+ for the 1st and 2nd transitions and the corresponding changes in Gibb’s free energies of micellisation (ΔG°M),
Langmuir surface pressure (Π), hydrodynamic diameter (DH), and ζ-potential in the presence and absence of DNP-DEG in water at 298 K
Species
CMCa
[mM]
ΔG°M CMC b
[kcal mol−1]
CACa
[mM]
CACc
[mM]
ΔG°M CAC b
[kcal mol−1]
Πd
[mN m−1]
D°H e
[nm]
ζ-Potential f
[mV]
12+
12+ + 1 eq. DNP-DEG
24+
24+ + 1 eq. DNP-DEG
24+ + 2 eq. DNP-DEG
2.73 ± 0.24
2.31 ± 0.32
2.77 ± 0.15
1.67 ± 0.18
1.61 ± 0.23
−4.15 ± 0.08
−4.20 ± 0.11
−3.13 ± 0.15
−4.00 ± 0.12
−3.82 ± 0.15
22.3 ± 7.8
—
8.2 ± 1.4
5.9 ± 0.3
4.9 ± 0.7
29.8 ± 2.0
10.2 ± 1.0
9.0 ± 1.0
5.5 ± 1.0
4.5 ± 1.0
−3.35 ± 0.33
−4.51 ± 0.54
−3.27 ± 0.13
−4.69 ± 0.07
−5.02 ± 0.10
15.7 ± 0.3
25.5 ± 0.3
27.8 ± 0.8
27.4 ± 0.5
26.9 ± 0.4
93 ± 3
174 ± 3
112 ± 19
158 ± 5
279 ± 37
21.0 ± 6.8
33.6 ± 7.6
17.3 ± 0.4
35.0 ± 4.5
35.8 ± 1.1
a
Obtained by variable-concentration conductivity measurements. b Obtained by the method described by Zana.13 The values represent the free
energy of transferring over a single hydrophobic chain from water to the micellar pseudophase. For the purposes of comparison, the free
energies reported here for the two-tailed gemini surfactant 24+ are for a single chain transfer. Doubling the ΔG°M for compound 24+ will yield a
more accurate measure for the whole molecule. c Obtained by variable-concentration surface tension measurements using the Du Noüy ring
method. Only the 2nd critical micelle concentration occurring at higher concentrations was observed using this method. d Determined by Du Noüy
ring method using equation Π = γo − γCMC where γo is the surface tension of water at 298 K and γCMC is the surface tension for the
detergent solution at the critical micelle concentration. e Obtained using dynamic laser light scattering. f Obtained using laser Doppler
electrophoresis.
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Fig. 1 Molecular structure of DNP-DEG and the graphical representation of the
donor–acceptor binding interactions between 12+ ( process I) and 24+ ( process II
and III) with DNP-DEG.
Paper
shifts19 of the α and β protons of the bipyridinium recognition
units were observed along with the simultaneous upfield shifts
of DNP-DEG aromatic protons. This effect arises from aromatic
π-electron shielding of the bipyridinium units and strongly
suggests19a that face-to-face π–π stacking25 of DNP-DEG with
the bipyridinium units has occurred. The NMR spectra taken
during the titration of 12+ and 24+ with DNP-DEG (Fig. 3)
suggest that the binding of DNP-DEG to 12+ differs substantially from the binding of DNP-DEG to 24+. Increasing the
guest concentration of a 2 × 10−2 M solution of 12+ in D2O
resulted in a consistent, fairly linear change in ppm shifts for
protons in both the host and the guest. The two α proton
peaks remain within 0.2 ppm of one another as the DNP-DEG
concentration increases.
Increasing the guest concentration causes the β peaks to
split, however the peak splitting never exceeds 0.2 ppm. This
suggests that DNP-DEG, when bound and intercalated between
contiguous molecules of 12+, resides centered between the
bipyridinium units. Furthermore, the consistent ppm shifts
brought about by changes in guest concentration suggest that
a single binding process is being observed between the host
and the guest. Changes in guest concentration produce
gradual ppm shifts by virtue of shifting the thermodynamic
equilibrium of the binding process toward the host–guest complexation. Contradistinctively, the spectra obtained from titrating 24+ with DNP-DEG in D2O (Fig. 3b) reveal the presence of
two distinct binding processes.24 Upon addition of DNP-DEG,
the ppm shifts of the signals corresponding to the α protons
adjacent to the nitrogens which have been alkylated by the
undecylenic tail increase abruptly, as do the signals of the β
protons. However, once the sample contains equal concentrations of host and guest, the rate of change for these ppm
shifts slows. For example, the signals for the ρ-xylyl aromatic
protons shift from 7.65 ppm to 8 ppm as the ratio of 24+ to
Fig. 2 (a) UV-Vis absorption spectra of 12+ (1 × 10−2 M) with the addition of
1 molar equivalent of DNP-DEG (black trace) and 24+ (3 × 10−3 M) with the
addition of 1 (red trace) and 2 (green trace) molar equivalents of DNP-DEG
recorded in H2O at 298 K; (b) photographs of vials containing 1 × 10−2 M solutions of 12+ (A) and 12+ with 1 molar equivalent of DNP-DEG (B) and 5 × 10−3 M
solutions of 24+ (C) and 24+ with 1 (D) and 2 (E) molar equivalents of DNP-DEG
in H2O taken 5 minutes after vigorous shaking. The visible colour change is due
to donor–acceptor charge-transfer between the π-electron deficient bipyridinium amphiphile head groups and the π-electron rich DNP-DEG template. The
stable formation of closed-cell foam indicates that the Marangoni effect and the
detergency of the solution have been augmented.
bipyridinium surfactant head groups and DNP-DEG – strong
evidence19 indicating that molecular recognition and complexation has taken place. The band gaps for the charge transfer interaction between the detergents and the template were
measured at 2.79 eV, corresponding to absorption maxima
(Fig. 2a) at 443 nm for 12+ (Fig. 2b, B) and 2.61 eV at 475 nm
for 24+ (Fig. 2b, D and E).
The binding of DNP-DEG was further characterised in D2O
using 1H-NMR spectroscopy (Fig. 3), whereby downfield
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Fig. 3 Stacked 1H NMR spectra (300 MHz, D2O, 298 K) following the titration
of a 2 × 10−2 M solution of 12+ (a) and a 1.4 × 10−2 M solution of 24+ (b) with
DNP-DEG. The observed upfield shifts of the signals corresponding to the π-electron deficient bipyridinium head group aromatic protons are a result of the
donor–acceptor charge transfer interactions with π-electron rich DNP-DEG.
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DNP-DEG increases from 1 : 0 to 1 : 1, but as that ratio
increases from 1 : 1 to 1 : 2, the signals only shift from 8 ppm
to 8.05 ppm. This change in behavior suggests a transition in
the binding interactions between the DNP-DEG and the bipyridinium units of 24+—where first and foremost the DNP-DEG
engages the bipyridinium units intramolecularly, where it is
nestled in the pocket of 24+, followed by intermolecular “alongside” binding interactions as the ratio of host to guest
increases. Evidence for this binding sequence is also supported by the shifts of the signals corresponding to the aromatic protons of DNP-DEG (Fig. 4). Initially the signals for
aromatic protons H3/7, H4/8, and H2/6 experience a classical19,24
upfield shift upon complexation with 24+. As the ratio of host
to guest increases, these protons suddenly start to experience
dramatic downfield shifts as each bipyridinium unit now
starts to engage in binding interactions with two molecules of
DNP-DEG—one in which DNP-DEG resides within the pocket
of 24+ and the second in which DNP-DEG resides between contiguous molecules of 24+ serving as intermolecular “glue”
further stabilising the micellar aggregate.
Notably, while the signals for the different α protons of 12+
experienced similar shielding effects with the addition of
guest, the signals for the different α protons of 24+ experienced
very different shielding changes. The signals for the α protons
adjacent to the nitrogens bound to the alkyl tails experienced
shifts from 9.15 ppm to 8.7 ppm, while the signals for the α
protons adjacent to the methylene p-xylyl bridge shifted by
only 0.1 ppm. This indicates that DNP-DEG, when bound to
24+, resides closer to the hydrophobic core of the micelle and
does not sit perfectly centered between the bipyridinium units
as with 12+. As a result we further conclude that the aromatic
protons of DNP-DEG do not engage in edge-to-face π-interactions with the aromatic ring of the p-xylyl bridge as has been
reported19,24 for similar donor–acceptor systems in polar aprotic
Fig. 4 Stacked 1H NMR spectra (300 MHz, D2O, 298 K) following the titration
of a 1.4 × 10−2 M solution of 24+ (b) with DNP-DEG illustrating the downfield
shifts of the signals for the DNP-DEG aromatic protons as the concentration of
DNP-DEG is increased. The observed downfield shifts of the signals corresponding to the π-electron rich DNP-DEG aromatic protons result from the
growing donor–acceptor polar stacks across the micellar surface as successively
more DNP-DEG molecules intercalate themselves between contiguous π-electron
deficient bipyridinium head groups.
Org. Biomol. Chem.
Organic & Biomolecular Chemistry
Fig. 5 Space filling molecular models of the proposed donor–acceptor stacking
interactions between DNP-DEG and 12+ (a) and 24+ (b) at the micellar–water
interface. Hydrogen atoms have been omitted for clarity.
organic solvents. In each case, the one-dimensional face-to-face
packing model25 between the π-electron deficient bipyridinium
units with the π-electron rich DNP-DEG (Fig. 5) is evident.
Effects of donor–acceptor templation on micellar
self-assembly
The addition of 1 molar equivalent of DNP-DEG to solution of
12+ and both 1 and 2 equivalents to solutions of 24+ produced
a substantial change in the micellar aggregation process as
determined by further ST (Fig. 6 and 7a), conductivity (Fig. 7b–
d), dynamic light scattering (Fig. 9 and 10), and ζ-potential
measurements. It is interesting to point out however that
changes observed for the CAC were more pronounced than
those measured for the lower concentration CMC where little
change was observed. The effects of template-directed micellar
aggregation on the CMC and CAC are summarised in Table 1.
The addition of 1 equivalent of template to a solution of 12+
was sufficient enough to decrease the CAC by almost 66% from
29.80 ± 1.0 mM to 10.2 ± 1.0 mM with a concomitant increase
in the Langmuir surface pressure (Fig. 6) to 25.5 ± 0.3 mN m−1
—mirroring surface pressure levels measured (Fig. 7a) for
24+ both with and without DNP-DEG added. The data suggest
that DNP-DEG behaves as a molecular glue stitching together
molecules of 12+ such that 12+ no longer behaves as a linear
single chain amphiphile but rather exhibits—under templation conditions—properties of a dual chained gemini surfactant. Similarly the addition of 1 equivalent of DNP-DEG to
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Fig. 6 The graphical representations of the Langmuir film packing at the air–water interface for 12+ and the corresponding Langmuir surface pressure (Π) in the
absence of DNP-DEG (a) and the increased Langmuir film packing density of 12+ upon addition of 1 molar equivalent of DNP-DEG and the corresponding Π. Variable
concentration surface tension plots for both processes (center) and the calculated ΔΠ taken at 298 K in H2O.
hydrodynamic diameter (DH) of 93 ± 3 nm was measured and
augmented upon the addition of 1 equivalent DNP-DEG to 174
± 3 nm (Fig. 8) with a concomitant increase in the ζ-potential
from 21.0 ± 6.3 mV to 33.6 ± 7.6 mV. 24+ at 5 mM experienced
much more substantial total gains in aggregate size from 112 ±
19 nm to 279 ± 37 nm (Fig. 9) with a simultaneous increase in
the ζ-potential from 17.3 ± 0.4 mV to 35.8 ± 1.1 mV following
Fig. 7 (a) Variable-concentration surface tension plots measured in H2O at
298 K illustrating the CAC and the average Langmuir surface pressure (Π) for 24+
(red) with 1 equivalent of DNP-DEG (black) and 2 equivalents of DNP-DEG (blue)
added. Variable-concentration conductivity plots measured in H2O at 298 K illustrating the CAC for 24+ (b) with 1 equivalent of DNP-DEG (c) and 2 equivalents
of DNP-DEG (d) added.
solutions of 24+ resulted in ≈39% decrease in the CAC from
9.0 ± 1.0 mM to 5.5 ± 1.0 mM (Fig. 7a). The addition of a second
equivalent of DNP-DEG to 24+ resulted in another net decrease
in the CAC albeit not as significant as the change observed
upon the addition of the first equivalent. In neither case did
the addition of DNP-DEG affect the maximum reduction in
surface tension for 24+ (Fig. 7a), thus no significant changes in
the Langmuir surface pressure were measured.
Dynamic light scattering (DLS) and ζ-potential measurements confirmed that template-directed micellisation results
in significant increases in the size and stability of the aggregate structures. For a 10 mM solution of 12+ an average
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Fig. 8 Hydrodynamic diameter (DH) distributions measured for a 1 × 10−2 M
aqueous solution of 12+ (a) with 1 equivalent of DNP-DEG (b) added using
dynamic laser light scattering at 298 K.
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Fig. 9 Hydrodynamic diameter (DH) distributions measured for a 5 × 10−3 M
aqueous solution of 24+ (a) with 1 equivalent of DNP-DEG (b) and 2 equivalents
of DNP-DEG (c) added using dynamic laser light scattering at 298 K.
the addition of 2 equivalents of DNP-DEG. The addition of
DNP-DEG to the detergent solutions also dramatically
increased the hydrodynamic size distribution of the aggregate
structures. We believe that this marked increase in the aggregate polydispersity supports our hypothesis that cylindricaltype species6,8,15,17 with larger length/diameter ratios are
perhaps being formed in which aggregates of various lengths
are responsible for the overall size distribution. In each case
the addition of DNP-DEG resulted in ζ-potentials greater than
30 mV indicating that molecular templation at the micellar–
solvent interface results in an increase of the colloidal stability
of these solutions.
Templation effects on the free energy of micellisation (ΔG°M)
By employing a phase separation model13 which takes into
account the degree of counter ion binding to the micellar
Org. Biomol. Chem.
Organic & Biomolecular Chemistry
aggregates, ΔG°M per hydrophobic tail was calculated for both
the CMC and the CAC for 12+ and 24+ in the presence of the
DNP-DEG template and without. Comparing the Gibbs free
energies (ΔG°M) for analogous micellisations (Table 1) of the
gemini and single tailed surfactants reveals a consistent trend:
that while the magnitude of ΔG°M changes per molecule of the
gemini surfactant 24+ exceeds the ΔG°M per single tailed surfactant molecule 12+ for each different micellisation process, the
ΔG°M per tail of the gemini surfactant is always smaller than
the single tailed surfactant’s ΔG°M per tail, except where molecular templation by DNP-DEG becomes more pronounced.
This would be the expected result if the micelles formed by
both surfactants are fundamentally similar and the principal
contribution of the covalent connection between the two
halves of the gemini surfactant is to effectively preorganise two
surfactant monomers into dimers. The ΔG°M per tail of the
gemini surfactant is smaller than the per tail ΔG°M of the
single tailed surfactant because the gemini surfactant’s micellisation process is already under way if we consider the
dimeric gemini surfactant as having been covalently preorganised at the p-xylyl bridge. When the single tailed surfactant
forms into micelles, many surfactant molecules come together
to form structures that minimise interaction between the
hydrophobic tails and water molecules, an entropically driven
process. The inclusion of each new surfactant molecule arguably makes some small contribution to the total ΔG°M. The
micellisation of the gemini surfactant 24+ has a smaller total
ΔG°M because there is no contribution to the total ΔG°M from
the joining of two free monomers—for the gemini surfactant,
the “monomers” are already joined by the p-xylyl bridge
between the two halves of the molecule. It can also be
observed (Table 1, Fig. 10) that the effects of molecular donor–
acceptor templation on ΔG°M become more pronounced for the
CAC transition occurring at higher concentrations. This
suggests that the 2nd transition leads to micellar aggregate
structures with a larger length-to-diameter ratio (e.g., oblate or
cylindrical type species) for which face-to-face π-stacking interactions between the π-electron deficient bipyridinium surfactant head groups and DNP-DEG are more favoured.
Conclusions
In summary we have developed a model detergent binary
blend in which the surface activity and the CAC of two bipyridinium-based amphiphiles can be modulated causing selfassembly to occur at significantly lower surfactant concentrations by employing the principles of preorganisation and
template-directed self-assembly in aqueous solutions. We have
also demonstrated rather unexpectedly that linear single
hydrophobic tailed detergent molecules under templation conditions behave as double-tailed phospholipid-like gemini surfactants. That is to say that the monomeric surfactants in this
study experienced a significant enhancement in their
efficiency and effectiveness in reducing the surface tension of
water – mirroring the efficiency and effectiveness of the
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4
Fig. 10 Graphical representation of the effects of donor–acceptor templation
on the micellar self-assembly of 12+ and 24+ and the Gibbs free energy of micellisation (ΔG°M) for both the critical micelle concentration (CMC) and the critical
aggregation concentration (CAC). Under templated conditions, it is the second
micellar transition, the CAC, occurring at higher concentrations rather than the
CMC occurring at much lower concentrations that is affected the most by
the addition of DNP-DEG. Favourable decreases in ΔG°M for the CAC exceed
° for the CMC do not exceed
1 kcal mol−1 whereas favourable decreases in ΔGM
1 kcal mol−1. See Table 1.
dimeric gemini species – upon donor–acceptor templation. The
experimental results further suggest that template-directed selfassembly at the micellar–solvent interface cannot augment to
any large extent, detergent efficiency or effectiveness at lowering
the surface tension of water and the onset of the CMC/CAC,
until the possibility for long range π–π stacking cooperativity
along the air–water interface and across larger micellar superstructures is established at concentrations well above the first
CMC. This leads us to believe that the native micellar aggregate
geometry plays a tremendous role in whether or not the addition
of π-electron rich molecular templates will result in a more
favourable ΔG°M. These findings auger well for the future development of programmable and switchable detergent blends
based on donor–acceptor interactions in aqueous environments.
5
6
7
8
9
Acknowledgements
This research is supported by a Welch Foundation Departmental Grant (BT-0041), a Texas A&M University Corpus Christi
research grant, and a grant from the Texas Research Development Fund. C. M. H. and M. S. M. gratefully acknowledge the
support of an NSF Louis Stokes Alliances for Minority Participation (LSAMP) grant (0703290).
10
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14
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