Self-Aggregation and liquid crystalline behavior of new ester-functionalized quinuclidinolium surfactants

Article abstract of DOI:10.1021/la502098h

A new type of ester-based cationic surfactant having a quinuclidinolium headgroup has been synthesized starting from linear fatty alcohols and has been characterized using spectroscopic techniques. The self-Aggregation and thermodynamic properties of these surfactants have been investigated by pendant-drop surface tensiometry and conductivity measurements. The liquid crystalline behaviors of these surfactants were investigated by small-Angle X-ray scattering (SAXS) technique. The quinuclidinolium headgroup demonstrated a unique ability to interlock among themselves thus affecting the physicochemical properties of surfactants in aqueous solution. The current research finding supports the new concept of headgroup interlocking which is supported by 1D and 2D NMR studies.

Full text of DOI:10.1021/la502098h

Article
pubs.acs.org/Langmuir
Self-Aggregation and Liquid Crystalline Behavior of New Ester-
Functionalized Quinuclidinolium Surfactants
Avinash Bhadani,* Takeshi Endo, Setsuko Koura, Kenichi Sakai, Masahiko Abe, and Hideki Sakai*
Department of Pure and Applied Chemistry in Faculty of Science and Technology and Research Institute for Science and
Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
S
* Supporting Information
ABSTRACT: A new type of ester-based cationic surfactant having a
quinuclidinolium headgroup has been synthesized starting from linear
fatty alcohols and has been characterized using spectroscopic
techniques. The self-aggregation and thermodynamic properties of
these surfactants have been investigated by pendant-drop surface
tensiometry and conductivity measurements. The liquid crystalline
behaviors of these surfactants were investigated by small-angle X-ray
scattering (SAXS) technique. The quinuclidinolium headgroup
demonstrated a unique ability to interlock among themselves thus
affecting the physicochemical properties of surfactants in aqueous solution. The current research finding supports the new
concept of headgroup interlocking which is supported by 1D and 2D NMR studies.
exists in several plant-based natural products as an alkaloid and
is naturally found in many Cinchona species. Synthetic
derivatives of quinuclidine have medicinal properties9,10 and
are commonly used as catalysts for organic reactions.11 Several
reports are available regarding heterocyclic cationics containing
pyridinium,12,13 imidazolium,14 pyrrolidinium,15 and piperidi-
nium16 as a hydrophilic headgroup; however, more complicated
and bulky headgroups like quinuclidinolium have never been
investigated for their self-aggregation and physicochemical
properties.
We expect that the unique molecular design of quinuclidi-
nolium headgroups enables them to interlock among
themselves thus greatly effecting the physicochemical proper-
ties of the surfactants in water. The surface and thermodynamic
parameters of the new ester-functionalized quinuclidinolium
surfactants such as surface excess concentration (Γcmc), surface
area occupied by molecule at the air/water interface (Amin),
efficiency in surface tension reduction by 20 mN/m (C20), the
effectiveness of surface tension reduction (γmin), critical micelle
concentration (cmc), degree of counterion binding (β), and
standard free energy of micellization (−G0mic) and adsorption
(−G0ads) have been evaluated by surface tension measurements
and conductivity method. The lattice parameter (α′) for liquid
crystalline phases of the quinuclidinolium surfactant−water
system and other important parameters, i.e., the radius of the
cylinder units (dH), cross-sectional area per surfactant at the
interface (aS), and the distance between the cylinders (dW),
have been determined by SAXS experiments.
1. INTRODUCTION
Cationic surfactants are an important category of amphiphilic
molecules which are an important constituent of several
industrial formulations and consumer products. The most
common headgroup being used in many technical applications
is the quaternary ammonium group, among the other
heterocyclic cationic surfactants.1,2 The surface active portion
or the hydrophilic group of these surfactants bears a positive
charge, and they dissociate in water as an amphiphilic cation
and an anion. Since this category of surfactants consists of
acyclic or heterocyclic positively charged quaternary ammo-
nium headgroups, they are able to adsorb on most of the
negatively charged surfaces.3 Because of this predominant
character, these surfactants have been used in many technical
and nontechnical areas as fabric softeners, electrodip coatings,
and in the stabilization of adhesive polymer latexes as well as in
mining and paper manufacturing. In recent years the environ-
mental concerns have shifted the focus of the surfactant chemist
toward developing and investigating biocompatible cationic
surfactants having significant portion of functional groups that
can be easily degraded in the environment after use. Most
recently several ester-functionalized heterocyclic, i.e., pyridi-
nium,4 imidazolium, piperidinium,5 and pyrrolidinium,6 surfac-
tants have demonstrated superior physicochemical properties
compared to conventional monomeric cationic surfactants.
Ester functional group is considered biocompatible and is easily
degraded in the environment, and their quaternary ammonium
derivatives have been found to be hydrolyzable.7,8
In the present work new ester-functionalized quinuclidino-
lium surfactants have been synthesized starting from linear fatty
alcohols (Scheme 1). These new surfactants consist of
quinuclidinolium moiety as a hydrophilic headgroup and linear
fatty alcohol ester as a hydrophobic tail. Quinuclidine moiety
Received: January 21, 2014
Revised: July 15, 2014
Published: July 16, 2014
© 2014 American Chemical Society
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Scheme 1
Master 700, Tokyo, Japan) at 25 °C. An inverted 16-gauge needle is
submerged in the aqueous phase such that the tip is visible in the
frame of capture. A gas-tight syringe is mounted in a microsyringe
pump to ensure instantaneous creation of a droplet of a preset volume.
Before the droplet is formed, the image capture software is triggered,
collecting images at 20 frames for the first 10 min and 1 frame/min
thereafter, for a total aging time of 3600 s (1 h). Edge detection is used
to identify the droplet shape, with the surface tension determined
using the Young−Laplace equation. Experimental runs of 3600 s are
chosen as surfactant solution attains equilibrium within the chosen
time frame.
2. MATERIAL AND METHODS
2.1. Materials. Lauryl alcohol, myristyl alcohol, chloroacetic acid,
p-toluenesulfonic acid monohydrate, and 3-quinuclidinol were
purchased from TCI, Tokyo, Japan. The alkyl-2-chloroacetates were
prepared according to the modified procedure of a previously reported
literature report.6
2.2. Synthesis. Lauryl alcohol (1; 9.32 g, 50 mmol) or myristyl
alcohol (2; 10.72 g, 50 mmol) were reacted with chloroacetic acid (3;
5.20 g, 55 mmol) in the presence of a catalytic amount of p-toluene
sulfonic acid monohydrate (4; 951 mg, 5 mmol) under solvent-free
conditions for 4 h at 80 °C. The reaction mixture was then allowed to
cool at room temperature and then was dissolved in 100 mL of
chloroform and washed twice with 100 mL of water. The chloroform
layer was separated with a separating funnel and was removed from the
crude reaction mixture under reduced pressure in a rotary flash
evaporator at 40 °C. The crude reaction mixture was then washed with
100 mL of warm aqueous methanol (92:8, methanol:water). The
lower layer consisting of alkyl 2-chloroacetates (5−6) was allowed to
separate in the separating funnel. It was then separated, dissolved in
hexane, dried using Na2SO4, and filtered. The solvent was removed in
vacuum rotary flash evaporator at 80 °C for 30 min. The resulting
intermediates alkyl 2-chloroacetate, i.e., dodecyl 2-chloroacetate (5;
6.57 g, 25 mmol) or tetradecyl 2-chloroacetate (6; 7.27 g, 25 mmol),
were then reacted with 3-quinuclidinol (7; 3.18 g, 25 mmol) at 50 °C
for 4 h in 20 mL of CHCl3 (Scheme 1). The chloroform was removed
by rotary evaporator at 60 °C. The resulting crude mixture was cooled
to 25 °C. The product was washed thrice with 100 mL of diethyl ether
and then cold precipitated in 100 mL of acetone to get new ester-
functionalized quinuclidinolium surfactants 12QuinucEsCl (8) and
14QuinucEsCl (9). The structures of these surfactants were confirmed
by NMR and high-resolution mass spectroscopy (HRMS). Mass
spectra of surfactants were recorded on a JEOL JMS-T100CS (JEOL
2.4. Conductivity Measurements. Conductivity was measured
on auto-temperature electrical conductivity meter CM-25R (DKK-
TOA Corporation) equipped with a conductivity cell having a cell
constant of 1. The solutions were thermostated at 25.0 0.1, 30.0
0.1, 35.0
0.1, and 40.0
0.1 °C in a thermostated glass vessel
controlled by temperature controller. For the determination of cmc, an
adequate quantity of a concentrated surfactant solution was added to
water in order to change the surfactant concentration from
concentrations well below the critical micelle concentration (cmc)
to up to at least 2−3 times the cmc. Degree of counterion binding (β)
has been calculated as (1 − α), where α = Smicellar/Spremicellar, i.e., ratio of
the slope after and before cmc.
2.5. Sample Preparation and SAXS Measurements. Samples
were prepared in screw-capped glass tubes by weighing appropriate
surfactant and water. The tubes were sealed and kept at 80 °C for 1 h,
then vortexed for 10 min, and centrifuged at 3500 rpm for 30 min. The
procedure was repeated thrice, and the samples were kept for 1 week
to attain equilibrium. Measurements were performed using a SAXSess
camera (Anton Paar, PANalytical) attached to a PW3830 laboratory X-
ray generator with a long fine focus sealed glass X-ray tube (KR
wavelength of 0.154 nm; PANalytical). The apparatus was operated at
40 kV and 50 mA. The SAXSess camera is equipped with focusing
multilayer optics and a block collimator for an intense and
monochromatic primary beam with low background, and a translucent
beam stop for the measurement of an attenuated primary beam at zero
scattering vector (q = 0). Samples were enclosed in a vacuum-tight
thin quartz capillary with an outer diameter of 1 mm and thickness of
10 μm. Sample temperature was controlled with a thermostated
sample holder unit (TCS 120, Anton Paar). The 2D scattering pattern
was collected on an image plate (IP) detection system Cyclone
(PerkinElmer) and was finally integrated into one-dimensional
scattering curves as a function of the magnitude of the scattering
vector q = (4π/λ)sin(θ/2) using SAXSQuant software (Anton Paar),
where θ is the total scattering angle and λ is the wavelength of the X-
ray. All data were normalized to the same incident primary beam
intensity for the transmission calibration and were corrected for
background scattering from the capillary and the solvent.19,20
1
Japan) using ESI as ion source. H, 13C NMR, 2D COSY, and 2D
NOESY spectra were recorded on a JEOL-ECP500 (JEOL Japan) as a
solution in D2O.
1-(2-(Dodecyloxy)-2-oxoethyl)-3-hydroxyquinuclidin-1-ium
chloride (8). White solid, yield 83%; 500 MHz H NMR (D2O) δ
1
ppm: 0.88 (m, 3H), 1.26 (br s, 18H), 1.70 (m, 2H), 2.00 (m, 2H),
2.16 (m, 1H), 2.31 (m, 1H), 2.35 (m, 1H), 3.59 (m, 2H), 3.70 (m,
2H), 3.78 (m, 1H), 3.95 (m, 1H), 4.22 (br s, 2H), 4.35 (br s, 3H). 13C
NMR (D2O) δ ppm: 12.70, 16.27, 19.79, 21.56, 24.56, 27.02, 28.37,
28.53, 28.88, 28.95, 29.05, 30.94, 54.21, 54.45, 60.17, 61.95, 62.91,
65.17, 163.85. ESI-HRMS positive ions m/z: calculated 354.3003 for
+
C21H40NO3 (M−Cl or M+), found 354.3004.
1-(2-(Tetradecyloxy)-2-oxoethyl)-3-hydroxyquinuclidin-1-ium
1
chloride (9). White solid, yield 84%; 500 MHz H NMR (D2O) δ
ppm: 0.88 (m, 3H), 1.30 (br s, 22H), 1.70 (br s, 2H), 1.99 (br s, 2H),
2.30 (m, 1H), 2.35 (m, 1H), 2.30 (m, 1H), 3.59 (m, 2H), 3.70 (m,
2H), 3.79 (m, 1H), 3.95 (m, 1H). 13C NMR (D2O) δ ppm: 12.73,
16.28, 19.82, 21.57, 24.55, 27.03, 28.32, 28.50, 28.77, 28.83, 30.93,
54.28, 54.49, 60.23, 61.98, 62.95, 65.28, 163.87. ESI-HRMS positive
ions m/z: calculated 382.3316 for C23H44NO3+ (M−Cl or M+), found
382.3317.
2.3. Surface Tension Measurements Using Pendent Drop
Tensiometer. The surface tension at the water−air interface was
investigated using the pendant drop technique17,18 (Kyowa Drop
3.0. RESULTS AND DISCUSSION
3.1. Molecular Characterization. Linear fatty alcohols
(lauryl alcohol and myristyl alcohol) were reacted with
chloroacetic acid in the presence of a catalytic amount of p-
toluene sulfonic acid monohydrate under solvent-free con-
ditions to get alkyl-2-chloroacetate, which was then reacted
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with 3-quinuclidinol to get ester-functionalized quinuclidino-
lium surfactants. The structures of these new quinuclidinolium
surfactants: dodecyltrimethylammonium chloride and tetrade-
cyltrimethylammonium chloride.21 The cmc values of these
surfactants have also been found to be less compared with the
heterocyclic pyridinium and imidazolium surfactants: 1-
alkylpyridin-1-ium chloride and 1-methyl-3-alkyl-1H-imidazol-
3-ium chloride having similar hydrophobic alkyl chain
length.22−25
The maximum surface excess concentration at the air/water
interface, Γmax, is calculated by applying the Gibbs adsorption
isotherm equation:
1
surfactants have been established by H, 13C NMR, and high-
resolution mass spectroscopy (HRMS). The resonance for the
methylene protons directly attached to the heterocyclic
positively charged quinuclidinolium nitrogen and −CH−
proton of the quinuclidinolium ring attached to the hydroxyl
group were observed at δ 4.35 and 4.37 ppm as broad singlet
for 12QuinucEsCl and 14QuinucEsCl, respectively. The
chemical shifts for six methylene protons of the quinuclidino-
lium ring adjacent to the positively charged nitrogen are
diastereotopic in nature and were observed overlapping
between δ 3.59 and 3.95 ppm integrating for total six protons.
The other four methylene protons attached to the β carbon of
the quinuclidinolium ring are also diastereotopic in nature and
were observed along with the −CH− proton attached to the γ
carbon of the quinuclidinolium ring between δ 1.99 and 2.35
ppm. The signal for this proton adjacent to ester functionality
in the hydrophobic alkyl chain was observed at δ 4.21 ppm.
The structures of these quinuclidinolium surfactants have
been further established by high-resolution mass spectroscopy
(ESI-HRMS positive ion). The parent ion peak for the
surfactants has been observed for positive ion minus chloride
ion from each surfactant molecule. The calculated m/z values
(354.3003 for 12QuinucEsCl and 382.3316 for 14QuinucEsCl)
closely matched the observed values (354.3004 for 12Quinu-
cEsCl and 382.3317 for 14QuinucEsCl) establishing high purity
of the synthesized surfactant molecules.
⎛
⎜
⎞
⎟
⎠
1
dγ
Γmax = −
2.303nRT dlogC
⎝
(1)
T
Here, γ denotes the surface tension, R is the gas constant, T is
the absolute temperature, and C is the surfactant concen-
tration.6 The value of n is taken as 2. The area occupied per
surfactant molecule (Amin) at the air water interface is obtained
by using the following equation:
Amin = 1/NΓmax
(2)
where N is Avogadro’s number and Amin is in nm2. The values
of Γmax and Amin are shown in Table 1. The Amin values of
quinuclidinolium surfactants have been found to be exception-
ally low. Both 12QuinucEsCl and 14QuinucEsCl form tighter
packing at the air−water interface.
The quinuclidinolium headgroup is bulky and structurally
different compared to other types of heterocyclic cationic
headgroups. Both hydrophobic alkyl chain and hydrophilic
headgroups influence the self-aggregation behavior of the
surfactant molecule in water and the molecular arrangement of
surfactant monomers at the air−water interface. However, the
role of different types of cationic headgroup is seldom
investigated. We have found that quinuclidinolium headgroup
dramatically influences the aggregation behavior at both low
and high concentrations. To understand the forces regulating
the self-aggregation behavior of the new quinuclidinolium
surfactants, we have to compare quinuclidinolium headgroup
with other categories of heterocyclic cationic headgroups
(Figure 2).
3.2. Dilute Aqueous Solution Properties. Figure 1
shows the surface tension plot of the quinuclidinolium
The pyridinium cation can be described as an aromatic ring
system where the positive charge is delocalized throughout the
ring and not centric to positively charged quaternary nitrogen.
This is supported by the fact that all the pyridinium protons are
1
highly deshielded and often observed downfield in H NMR
Figure 1. Surface tension vs log C plot for the quinuclidinolium
surfactants.
spectra.26 The delocalized positive charge is counterbalanced by
an oppositely charged couterion; however, the intramolecular
distance between two headgroups is also regulated by repulsive
forces operating between the two positively charged pyridinium
headgroups27 although the hydrophobic interaction plays a
dominant role in such arrangement. On the contrary, the
imidazolium headgroup is totally different compared to others
as the −NCHN− proton of imidazolium cation is strongly
deshielded and acidic in nature since imidazolium cation forms
CN π bond, leaving the carbon in the center between two
surfactants. The surface parameters of these surfactants are
given in Table 1. The cmc values of ester-functionalized
quinuclidinolium surfactants decrease with the increase in
hydrophobic alkyl chain length. The cmc values of
12QuinucEsCl and 14QuinucEsCl have been found to be
5.11 and 1.41 mM, respectively which is almost four times less
compared to the commercially available quaternary ammonium
Table 1. Surface Properties of 12QuinucEsCl and 14QuinucEsCl at 25 °C
a
b
surfactant
cmc (mM)
cmc (mM)
γcmc (mN/m)
106 Γmax (mol/m2)
Amin (nm2)
C20 (M)
12QuinucEsCl
14QuinucEsCl
5.11 ( 0.01)
1.41 ( 0.01)
5.33 ( 0.01)
1.45 ( 0.01)
28.9 ( 0.1)
38.0 ( 0.1)
3.01 ( 0.02)
3.16 ( 0.02)
0.55 ( 0.02)
0.53 ( 0.02)
9.12 × 10−4
5.62 × 10−4
a
b
cmc determined by surface tension method. cmc determined by conductivity method.
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the ratio of the slopes in two regions gives a degree of
counterion dissociation (α). The β value has been calculated
from the following relationship: β = 1 − α. The β values of
these surfactants slightly decrease with increase in temperature
of the surfactant solutions; however, change in length of
hydrophobic alkyl chain does not significantly affect this value.
This anomalous behavior is in contrast with the trend observed
for the previously reported β values of ester-functionalized
heterocyclic surfactants.4,6
The thermodynamics of these surfactants according to
pseudophase model of micellization6 and Gibbs free energy
for micellization (ΔG°mic) have been calculated from the
following equation
Figure 2. Energy-minimized structures of different positively charged
heterocyclic positively charged cations. The energy minimization was
performed with ChemBio3D Ultra 13.0 software (ChemOffice,
CambridgeSoft Corp.) using MM2 molecular mechanics.
ΔG°mic = RT(2 − β)ln Xcmc
(3)
where Xcmc is the cmc in molar fraction, Xcmc = cmc/55.4,
where cmc is in mol/L, and 55.4 comes from 1 L of water
corresponding to 55.4 mol of water at 25 °C. Similarly the
Gibbs free energy of adsorption (ΔG°ads) is calculated by
following equation:
nitrogens with concentrated positive charge.28 The sphere of
positive charge keeps the adjacent imidazolium moiety at a
certain distance; however, again the dominating force is
regulated by hydrophobic interaction. The nonaromatic
heterocyclic headgroup, i.e., pyrrolidinium and piperidinium,
arranges itself in such a way that the essence of carbon−carbon
single bond angle is maintained although the conformation in
the case of piperidinium cation may be influenced by charge to
some extent.29 Quinuclidinolium headgroup is completely
different from the previously discussed heterocyclic head-
groups. The structural design of this bulky headgroup creates
molecular cavities between the extended carbon arms of this
moiety (Figure 2), and the presence of additional hydroxyl
group enhances the hydrophilic nature of the headgroup. The
carbon arm of adjacent quinuclidinolium moiety arranges very
closely into the cavity formed by similar moiety, and this is
responsible for lower Amin values of these surfactants and
compact packing at the interface.
The cmc values of 12QuinucEsCl and 14QuinucEsCl
surfactants have further been evaluated by conductivity method
at vairous temperatures. The experimental conductivity has
been plotted as a function of surfactant concentration at various
temperatures (Figure 3).
The cmc values of quinuclidinolium surfactants increase with
increase in temperatures. The cmc value follows the same trend
as previously investigated by surface tension measurements.
The degree of counterion binding (β) for the surfactants has
been calculated from the slopes of two linear regimes. Initially
πcmc
Γ
ΔG°ads = ΔG°mic
−
(4)
Here, πcmc denotes the surface pressure at the cmc (πcmc = γo −
γcmc, where γo and γcmc are the surface tensions of water and the
surfactant solution at cmc, respectively). The standard enthalpy
change for micellization process (ΔH°mic) can be determined
using the Gibbs−Helmholtz equation:
⎛
⎞
∂(ΔG°mic/T)
∂(1/T)
ΔH°mic
=
⎜
⎝
⎟
⎠
(5)
(6)
ΔH°mic = −RT2(2 − β)ln Xcmc/dt
The standard entropy of micelle formation (ΔS°mic) can be
calculated according to the following equation:
ΔS°mic = (ΔH°mic − ΔG°mic)/T
(7)
The thermodynamic parameters, i.e., ΔG°mic, ΔH°mic, and
ΔS°mic, of ester-functionalized quinuclidinolium surfactants at
different temperatures are shown in Table 2.
It has been observed that the ΔG°mic values for the
quinuclidinolium surfactants under investigation are negative
and generally decrease with increase in temperature which
manifests that the formation of micelle is a spontaneous
Figure 3. Specific conductivity (κ) versus the concentration of (a) 12QuinucEsCl and (b) 14QuinucEsCl at different temperatures.
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Table 2. Surface and Thermodynamic Parameters of Quinuclidinolium Surfactants Determined by the Electrical Conductivity
Method at Different Temperatures
T
cmc
β
ΔG°mic
ΔG°ads
ΔH°mic
−TΔS°mic
ΔS°mic
surfactant
(°C)
(mmol·L−1
)
(%)
(kJ mol−1
)
(kJ mol−1
)
(kJ mol−1
)
(kJ mol−1
)
(J mol−1·K−1
)
12Quinuc EsCl
25 0.1
30 0.1
35 0.1
40 0.1
25 0.1
30 0.1
35 0.1
40 0.1
5.33 ( 0.01)
5.41 ( 0.01)
5.48 ( 0.01)
5.55 ( 0.01)
1.45 ( 0.01)
1.50 ( 0.01)
1.55 ( 0.01)
1.60 ( 0.01)
57.2 ( 0.1)
56.3 ( 0.1)
55.3 ( 0.1)
54.8 ( 0.1)
57.1 ( 0.1)
56.0 ( 0.1)
54.6 ( 0.1)
53.8 ( 0.1)
−32.74 ( 0.05)
−33.44 ( 0.05)
−34.19 ( 0.05)
−34.81 ( 0.05)
−37.37 ( 0.05)
−38.17 ( 0.05)
−39.05 ( 0.05)
−39.79 ( 0.05)
−47.06 ( 0.05)
−2.83 ( 0.05)
−2.95 ( 0.05)
−3.07 ( 0.05)
−3.18 ( 0.05)
−6.93 ( 0.05)
−7.22 ( 0.05)
−7.53 ( 0.05)
−7.81 ( 0.05)
−29.91 ( 0.05)
−30.50 ( 0.05)
−31.12 ( 0.05)
−31.63 ( 0.05)
−30.44 ( 0.05)
−30.95 ( 0.05)
−31.52 ( 0.05)
−31.96 ( 0.05)
100.31 ( 0.1)
100.60 ( 0.1)
100.99 ( 0.1)
101.02 ( 0.1)
102.11 ( 0.1)
102.09 ( 0.1)
102.29 ( 0.1)
102.07 ( 0.1)
14Quinuc EsCl
−48.13 ( 0.05)
Figure 4. SAXS pattern for the hexagonal phases for (a) 12QuinucEsCl:water and (b) 14QuinucEsCl:water systems at different weight percents at
25 °C.
process. The values for ΔH°mic are negative indicating that the
micelle formation is an exothermic process and decreases with
the increase in temperature for each individual surfactant under
investigation. The absolute values of both ΔG°mic and ΔH°mic
increase with the increase in hydrophobic alkyl chain for the
suggesting the existence of cylindrical self-assembly crystals in a
hexagonal lattice.
The lattice parameter α′ is calculated from the first peak
position by α = (4π/√3)/q100. Volume of the solvophobic
chains (υL) has been calculated by the Tanford equation.30
quinuclidinolium surfactants. The entropy change (ΔS°mic
)
υL = 27.4 + 26.9N
(8)
with increasing temperature as well as increase in hydrophobic
alkyl chain has little effect on the micellization process. This
anomalous behavior can be explained on the basis of
interlocking of the quinuclidinolium headgroup which invalid-
ates the effect of transfer of hydrocarbon chains from water into
the interior of micelle.
where N is the number of carbon atoms in the hydrocarbon
chain. The volume fraction (ϕL) has been deduced by the
modifying equation mentioned by Yue et al. as water was used
instead of ionic liquid.31
From the lattice parameter and volume fraction, ϕL, of the
hydrophobic part of the surfactant in the system, some
characteristic parameters are deduced.32,33 The radius, dH, of
the hydrophobic part of the cylinders in the hexagonal
structures was calculated using the following equation:
3.3. Characterization of LC. The quinuclidinolium
surfactants (12QuinucEsCl and 14QuinucEsCl) were inves-
tigated up to 66 and 57 wt % respectively in water. Above these
concentrations, the phases were not found to be stable, and
reproducibility does not offer consistent results. Below 48 wt %
(in the case of 12QuinucEsCl) and 44 wt % (in the case of
14QuinucEsCl), these surfactants exist as a micellar solution.
12QuinucEsCl forms a hexagonal H1 LC phase at 49−66 wt %
while 14QuinucEsCl forms a similar phase at 45−57 wt % in
water at 25 °C. The SAXS data give information about the LC
phase arranged in a particular fashion. The Bragg reflections
corresponding to particular arrangement demonstrated the
existence of hexagonal packing. Figure 4 shows the SAXS
patterns for the representative samples of H1 phase of
12QuinucEsCl−H2O and 14QuinucEsCl−H2O systems at 25
°C. The value of scattering vector q was found to be in the ratio
q1:q2:q3 = 1:√3:2 for both 12QuinucEsCl and 14QuinucEsCl
3 ϕL
2π
dH = α′
(9)
The distance between the cylinders, dW, is given by the relation
dW = α′ − 2dH
(10)
The effective cross-sectional area per surfactant at the interface,
aS, is calculated by the following equation
2υL
dH
aS =
(11)
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where υL is the volume of the apolar part of one surfactant
molecule. The results of parameters α′, dH, dW, and aS are
summarized in Table 3.
becomes unstable and often separates as solid surfactant above
66 wt % (in the case of 12QuinucEsCl) and 57 wt % (in the
case of 14QuinucEsCl).
3.4. 1D and 2D NMR Studies of Quinuclidinolium
Surfactants. The headgroup interlocking of the quinuclidino-
lium surfactants has been investigated in detail by 1D and 2D
NMR spectroscopies. The 1H NMR spectra of the
quinuclidinolium surfactant (12QuinucEsCl) were recorded at
different concentrations in D2O (Figure 5). At 0.25 wt %
(∼6.43 mM) the majority of the surfactant molecules exist as
monomers as the concentration is just above the cmc value and
most of surfactant molecules are completely exposed to the
solvent (D2O) surface. The methylene protons (He and Hf)
directly attached to the heterocyclic positively charged
quinuclidinolium nitrogen which is present in-between
quaternary nitrogen and ester bond undergo D2O exchange
(Figure 5a) at this concentration, and the proton signals are not
observed. However, increasing the surfactant concentration
causes gradual appearance of the He and Hf proton signals. At
0.40 and 0.50 wt % (∼10.30 and ∼12.88 mM, respectively)
approximately half of the surfactant molecules exist as
monomers whereas the rest exist in the form of micelles in
D2O. The interlocking of quinuclidinolium headgroup prevents
the solvent D2O from penetrating inside the micelle;
consequently, the surfactant solution demonstrates partial
D2O exchange in NMR (Figure 5 b and c), and He and Hf
proton signals are observed along with Hc and Hd proton
signals as a multiplet. The merger of signal causes only a slight
increase in the integration value accounting for the He and Hf
protons which are part of the micelle. Further increase in the
surfactant concentration results in the appearance of a new
Table 3. SAXS Data for Hexagonal Liquid Crystalline Phases
surfactant
(wt %)
water
α′
dW
dH
aS
(Å2)
surfactant
(wt %)
(Å)
(Å)
(Å)
12QuinucEsCl
50
55
60
65
45
50
55
50
45
40
35
55
50
45
54.3
52.2
49.4
47.8
64.2
61.5
59.0
16.1
13.6
11.4
9.40
20.0
16.7
13.8
19.1
19.3
19.0
19.2
22.1
22.4
22.6
36.7
36.3
36.9
36.5
36.4
36.1
35.8
14QuinucEsCl
The ability of these surfactants to selectively form the
hexagonal H1 LC phase at high concentartion in water can be
explained on the basis of interlocking of the quinuclidinolium
headgroup. The molecular design of the quinuclidinolium
headgroup enables them to self-aggregate in the aqueous
system in such a manner in which the quinuclidinolium
headgroups get interlocked among themselves.
The interlocking of the headgroups offers selectivity in only
forming hexagonal H1 LC phase in water since the flexibility to
transit into different phases is inhibited. This is justified by the
fact that the radius of the cylinder units (dH) and cross-
sectional area per surfactant at the interface (aS) do not
significantly change with increase in percent of surfactant
amount, and both the values have been found to be
independent of the length of the hydrophobic alkyl chain.
However, since the overall value of α′ decreases, the water
channel becomes more compact with increase in surfactant
concentration. Since the rearrangement into different phases is
restricted due to the headgroup interlocking of the surfactant
molecules, with the increase in surfactant amount the system
1
independent signal in H NMR for He and Hf protons which
merge with the signal of Hp proton above 2.5 wt %. At higher
concentrations (above 2.5 wt %), most of the surfactants are
present in the form of micelle and the interlocking of the
quinuclidinolium headgroup does not allow the solvent to
Figure 5. 1H NMR spectra of the quinuclidinolium surfactant (12QuinucEsCl) in D2O at different concentrations at 25 °C: (a) 0.25, (b) 0.4, (c) 0.5,
(d) 0.8, (e) 1.0, (f) 1.25, (g) 1.5, (h) 2.5, (i) 3.0, (j) 10.0, and (k) 30.0 wt %. Each individual proton is represented as colored balls as represented in
the model in the right-hand side of the figure.
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Figure 6. 2D NMR proton−proton correlation spectroscopy (COSY) of the quinuclidinolium surfactant (12QuinucEsCl) at (a) 1.5 wt % and (b) 30
wt % in D2O at 25 °C. Each individual proton is represented as colored balls as represented in Figure 5
Figure 7. 2D nuclear Overhauser effect spectroscopy (NOESY) spectra of the quinuclidinolium surfactant (12QuinucEsCl) at (a) 1.5 wt % and (b)
30 wt % in D2O at 25 °C. Each individual proton is represented as colored balls as represented in Figure 5
penetrate inside the micelle resulting in negligible D2O
exchange.
trations (i.e., 1.5 and 30 wt %, Figure 6a and b, respectively).
The experiments gave direct information about the spin−spin
coupling of each individual proton which is connected through
chemical bonds. The assigned signals for the diastereotopic
protons of each methylene group of quinuclidinolium head-
group (Figure 6a and b) are based on the analysis of the COSY
cross-peaks. Additionally differentiation of each individual
proton could be made from the heteronuclear 1H−13C
correlated spectra (HETCOR). At low concentration (Figure
6a), each of the diastereotopic protons of the quinuclidinolium
headgroup directly attached to the positively charged
quaternary nitrogen (i.e., HgHh, HkHl and HqHr) shows
COSY cross-peaks demonstrating scalar couplings with the
adjacent protons (i.e. HiHj, HmHn, and Hp) as well as among
themselves. Such COSY cross-peaks are greatly affected with
the increase in surfactant concentration in micellar region as
HgHh, HkHl, and HqHr sets of protons demonstrate very weak
(Figure 6b) or negligible spin−spin coupling at surfactant
The increased concentration of the quinuclidinolium
surfactants causes the chemical environment of the micellar
surfactant solution to change which is evident from the
downfield chemical shift of the observed protons. The protons
directly attached to the quinuclidinolium headgroup experience
more chemical shift compared to the protons which are part of
1
the hydrophoblic alkyl chain. The results of the H NMR
spectroscopy strongly support the concept of quinuclidinolium
headgroup interlocking; however, to understand the nature of
such interlocking the quinuclidinolium surfactants were further
investigated by 2D NMR (i.e., correlation spectroscopy
(COSY) and nuclear Overhauser effect spectroscopy
(NOESY)).
Homonuclear proton−proton correlation spectroscopy
(COSY) experiments of the quinuclidinolium surfactant
(12QuinucEsCl) were investigated at two different concen-
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concentrations above 30 wt %. Since spin−spin coupling is a
molecular internuclear interaction and depends on the
molecular geometry of the molecule, the interlocking of the
quinuclidinolium headgroup greatly influences the spin−spin
coupling due to rigidness and change in chemical environment
of the interlocked headgroup in the micellar system at high
surfactant concentrations.
Nuclear overhauser effect spectroscopy (NOESY) provided
further insight into the nature of interaction exhibited by the
quinuclidinolium headgroups when they interlock among
themselves in micellar solution (Figure 7a and b) as the
associated cross-peaks in the spectra arise from the nuclei that
are spatially or physically close and not connected to each other
through chemical bond. At the low surfactant concentration
(Figure 7a, the absence of cross-peaks in the spectra
demonstrates that the nuclear Overhauser effect (NOE) is
not prominent in dilute micellar solution; however, micellar
solutions of higher concentrations above 30 wt % (Figure: 7b
demonstrate significant NOE effect due to the quinuclidinolium
headgroup interlocking.
The quinuclidine moiety can be considered having a twisted
C3-symmetry34 which has molecular cavities between the
extended carbon arms. The similar carbon arm of adjacent
quinuclidinolium moiety probably positions itself tightly in this
cavity maintaining a distance due to electrostatic repulsion
between similarly charged headgroups. Such arrangement in
spherical system such as micelle results in interlocking of
headgroups in a random manner where the protons of one
quinuclidinolium moiety are spatially very close (less than 5 Å)
to the other and may interact through dipole−dipole
interaction.35 However, the protons attached to carbon
adjacent to the positively charged nitrogen of quinuclidinolium
moiety (i.e., HgHh, HkHl, and HqHr) have been found to be
spatially very close to similar protons of another quinuclidino-
lium moiety establishing highly ordered three-dimensional
headgroup interlocking. Similarly, the protons attached to
carbon at the β position from the quaternary nitrogen (i.e.
HiHj, HmHn, and Hp) have been found to be close to similar
protons of another quinuclidinolium moiety. Interestingly, the
HOD proton has been found to be very close to Ho and Hp
protons. It may be due to the formation of hydrogen bonding
network possibly because quinuclidinolium moieties reorient
anisotropically during headgroup interlocking which is
supported by the fact that prominent downfield chemical
shift has been observed for such micellar solutions (Figure 5).
The free hydroxyl group of quinuclidinolium moiety may be
responsible for the formation of intermolecular hydrogen
bonding network between micellar systems with the help of
water molecules in aqueous system, and such network further
provides rigidity to the interlocked headgroups.
headgroups. Such phenomena are able to primarily control the
self-aggregation behavior of these surfactants in dilute aqueous
solution since the entropic effect of transfer of the hydrocarbon
chains from water into the interior of micelle is greatly affected.
At the higher concentration the interlocked headgroup offers
selectivity in forming a single type of liquid crystalline phase in
water. The surface properties of these surfactants have been
found to be better compared to commercially available
conventional cationic surfactants. The ease of synthesis
combined with biocompatible functional design and excellent
surface properties makes them useful for several industrial and
consumer applications.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C NMR spectra, and HRMS data of the surfactants. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: abhadani@rs.tus.ac.jp.
*E-mail: hisakai@rs.noda.tus.ac.jp.
Notes
The authors declare no competing financial interest
.
ACKNOWLEDGMENTS
■
A.B. is thankful to Tokyo University of Science, Tokyo, Japan
for a Postdoctoral Research Fellowship and research support.
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