pH-sensitive wormlike micelle and hydrogel formation by acylglutamic acid-alkylamine complex

Article abstract of DOI:10.1246/cl.160231

pH-sensitive viscoelastic fluids were obtained through the formation of wormlike micelles and hydrogels. These assemblies result from the 1:1 stoichiometric complex formation of acylglutamic acid (CnGlu) with tertiary alkylamine. The pH-sensitive nature reflects a change in the charge density around the CnGlu headgroups, controlling the curvature of the molecular assemblies. The longer chainCnGlu analogues yield the hydrogel in a narrow pH region. This study proposes a unique way to obtain stimulus-responsive viscoelastic fluids by means of gemini-like amphiphiles.

Full text of DOI:10.1246/cl.160231

Received: March 7, 2016 | Accepted: March 23, 2016 | Web Released: April 2, 2016
CL-160231
pH-sensitive Wormlike Micelle and Hydrogel Formation
by Acylglutamic Acid­Alkylamine Complex
Kenichi Sakai,*1,2 Masahide Sawa, Kazuyuki Nomura, Takeshi Endo, Koji Tsuchiya,2
1
1
1,2
1
2
1,2
Kazutami Sakamoto, Masahiko Abe, and Hideki Sakai
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
641 Yamazaki, Noda, Chiba 278-8510
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510
1
2
2
(
E-mail: k-sakai@rs.noda.tus.ac.jp)
pH-sensitive viscoelastic fluids were obtained through the
formation of wormlike micelles and hydrogels. These assemblies
result from the 1:1 stoichiometric complex formation of
acylglutamic acid (CnGlu) with tertiary alkylamine. The pH-
sensitive nature reflects a change in the charge density around the
CnGlu headgroups, controlling the curvature of the molecular
assemblies. The longer chain CnGlu analogues yield the hydrogel
in a narrow pH region. This study proposes a unique way
to obtain stimulus-responsive viscoelastic fluids by means of
gemini-like amphiphiles.
O
OH
O
CnGlu
N
H
n-2
O
OH
N
C12DMA
Figure 1. A typical chemical structure of CnGlu­C12DMA
complex.
Keywords: Wormlike micelle
| Hydrogel | pH sensitive
alkyl chain length. We abbreviate the acid to “CnGlu”, where n
refers to alkyl chain lengths of 8, 10, 12, 14, and 16. A typical
chemical structure of the complex is shown in Figure 1.
Rheology control is a key process in developing various
industrial products such as paints, foods, cosmetics, shampoos,
dishwashing/laundry detergents, and so on. The addition of
water- or oil-soluble polymers usually increases their solution
viscosity, and hence they have been used as thickeners of liquid-
based products. The use of wormlike micelles is also a potential
method to yield highly viscoelastic aqueous or non-aqueous
solutions. Here, wormlike micelles are formed as a result of the
The CnGlu analogues were synthesized in our laboratory via
a reaction of glutamic acid with octa-, deca-, dodeca-, tetradeca-,
or hexadecanoyl chloride in the presence of triethylamine.
C12DMA was purchased from Tokyo Chemical Industry and
used without further purification. The acid-type CnGlu (1 equiv)
was mixed with C12DMA (3 equiv) in ethanol at room temper-
ature for 48 h under magnetic stirring. Then, the solvent was
evaporated. The residue was recrystallized from either acetone
or a mixture of hexane/ethyl acetate. After filtration, the residue
was dried under reduced pressure. We confirmed the formation
of a 1:1 stoichiometric complex of CnGlu­C12DMA on the basis
1
self-assembly of surfactants. This means that wormlike micelles
are reformable against shear motion, which is a fundamental
difference from viscous polymer systems.
Wormlike micelles are typically formed in aqueous surfac-
tant solutions in the presence of organic/inorganic salts or
cosurfactants.2 These additives control the packing geometry
of surfactant molecules within micelles, i.e., the addition results
in decreased head-to-head repulsion and/or an increased tail
volume, achieving a suitable packing parameter for wormlike
micelles. Interestingly, under appropriate conditions, gemini (or
dimeric) surfactants can form wormlike micelles even in the
­5
1
of H NMR (JEOL-ECP 500 MHz) and ESI-MS (FT-ICR MS
Varian 910-MS) data. These characterization data are shown in
Supporting Information, Table S1. In typical experiments, the
complex (3 wt %) was mixed with a NaOH aqueous solution, and
the system was heated up to 80 °C. Then the solution was stirred
and finally kept for 4 days at a constant temperature of 25 °C. The
sample pH was controlled by changing the NaOH concentration.
Figure 2 shows the maximum viscosity (©max) of the CnGlu­
C12DMA complexes in aqueous media as a function of pH.
These data were obtained through steady shear rate measurements
by using a stress-controlled rheometer AR-G2 (TA Instruments)
6
­9
absence of such additives.
1
0
In our previous paper, we demonstrated a possible method
to form “gemini-like” amphiphiles through a very simple and
handy process, i.e., a mixture of alkyl dicarboxylic acid and
alkylamine yields a stoichiometric complex in aqueous solutions
as a result of a proton transfer from the acid to the amine. We have
1
2
with a cone-plate. The resultant viscosity against shear rate
11
13
also applied this concept to the formation of wormlike micelles.
In this earlier work, we employed an amino acid-based surfactant
dodecanoylglutamic acid, C12Glu) as an alkyl dicarboxylic acid
compound and dodecyldimethylamine (C12DMA) to yield their
:1 stoichiometric complex. Importantly, the rheological proper-
curves is given in Supporting Information, Figure S1. It is clear
in Figure 2 that the ©max data are strongly dependent not only on
the pH but also on the alkyl chain length of CnGlu. As discussed
(
11
in our previous paper, the pH-dependent rheological behavior
results from a change in the charge density (or acidity) around
the carboxylic acid headgroups, i.e., the decreased pH results in
a decreased charged density around the headgroups, leading to
decreased head-to-head repulsion; hence, lower positively curved
aggregates such as wormlike micelles tend to be formed at low
pH values. The highest ©max observed at pH 5.7 in the C12Glu­
1
ties of their aqueous solutions were found to be strongly
dependent on pH, resulting from the pH-dependent structural
transformation of spherical­rodlike­wormlike micelles.
In this letter, we demonstrate the rheological properties of a
series of acylglutamic acid­C12DMA complexes as a function of
Chem. Lett. 2016, 45, 655–657 | doi:10.1246/cl.160231
© 2016 The Chemical Society of Japan | 655

1
00
10
1
100
10
1
00000
0000
000
00
G’
n = 16
G’’
1
1
n = 14
G’’
1
G’
0
.1
0.1
0
.01
0.01
1
(
a) pH 5.6
(b) pH 5.8
0
.001
0.001
0
.01
0.1
1
10
100
0.01
0.1
1
10
100
10
n = 12
Angular frequency/rad s−1
Angular frequency/rad s−1
η
1
1
00
100
10
n = 10
n = 8
G’
0
.1
.01
.001
1
0
1
G’’
1
0
G’’
G’
0
.1
0.1
0.01
0
0
.01
4
5
6
7
8
9
10
(
c) pH 6.0
(d) pH 6.1
pH
0.001
0
.001
0.01
0
.01
0.1
1
10
100
0.1
1
10
100
Angular frequency/rad s−1
Angular frequency/rad s−1
Figure 2. ©max of the CnGlu­C12DMA complexes in aqueous
media as a function of pH.
Figure 3. G¤ and G¤¤ data obtained for the C14Glu­C12DMA
complex system at various pHs.
C12DMA system reflects the formation of a highly entangled
11
(a)
(b)
transient network structure of wormlike micelles. We also
suggested, on the basis of the dynamic viscoelastic data, that
a further decrease in pH from 5.7 resulted in branching or
interconnection of the wormlike micelles, inducing the decreased
viscosity.11
1
μm
1 μm
Hereafter we focus on the effects of the alkyl chain length of
CnGlu on the pH-dependent rheological behavior. The shortest
analogue, C8Glu­C12DMA, yields a low viscous Newtonian
fluid over the whole range of pH investigated, as shown in
Figure S1a. In the case of C10Glu­C12DMA, the solution
viscosity is dependent on pH, and a maximum ©max value is seen
at pH 5.4. Non-Newtonian fluids were obtained in the pH range
of 5.2­5.6 (Figure S1b); however, shear-birefringence was not
observed for these relatively low viscous samples. These results
indicate that both of the analogues form either spherical or
rodlike micelles in the solutions, and further one-dimensional
growth into wormlike micelles is not evidenced.
The longer chain analogues, C14Glu­C12DMA and
C16Glu­C12DMA, yield non-Newtonian fluids in the pH ranges
of 5.6­6.1 (C14) and 5.9­6.9 (C16), as shown in Figures S1c and
S1d. In these pH regions, the viscosity drastically increased, and
maximum ©max values were obtained at pH 5.8 (C14) and pH 6.1
Figure 4. SEM images of the hydrogel samples: (a) C14Glu­
C12DMA at pH 5.8 and (b) C16Glu­C12DMA at pH 6.1.
range of 5.9­6.2 and the wormlike micellar behavior in the pH
range of 6.4­6.5.
The morphology of the hydrogels formed by C14Glu­
C12DMA (at pH 5.8) and C16Glu­C12DMA (at pH 6.1) was
observed using a JEOL JSM-7600F scanning electron micro-
scope (SEM). A droplet of the hydrogel sample was placed on a
SEM grid coated with carbon tape, and then the grid was dried at
25 °C for 1 day under reduced pressure. Finally, the dried sample
was sputtered with platinum. As shown in Figure 4, a three-
dimensional network structure formed by fibrous molecular
assemblies was observed. A similar network structure has been
reported in the hydrogel system consisting of stearic acid and
1
5
(C16), respectively. In order to assess the viscoelastic properties
amines.
of these samples, we performed oscillatory-shear rheological
measurements. The results obtained for the C14Glu­C12DMA
system are shown in Figure 3. At pH 5.8, over the whole range
of angular frequencies, elastic behavior (storage modulus, G¤)
dominates over viscous behavior (loss modulus, G¤¤), indicating
hydrogel formation at this pH. In contrast, at pH values of 5.6,
In summary, we have demonstrated the pH-sensitive for-
mation of wormlike micelles and hydrogels by changing the
alkyl chain length of the CnGlu­C12DMA complex. The pH-
dependent nature primarily results from a change in the charge
density around the carboxylic acid headgroups of CnGlu,
controlling the curvature of the molecular assemblies in aqueous
media. The longer chain analogues experience structural trans-
formation from/into wormlike micelles and hydrogels in the
narrow pH region.
6.0, and 6.1, G¤ dominates at high frequencies, whereas G¤¤
dominates at low frequencies. This behavior indicates the
formation of highly entangled molecular assemblies in the
viscoelastic fluids, and is typically seen in wormlike micellar
systems. Indeed, we observed shear-birefringence for the three
samples. On the basis of these results, we suggest wormlike
micelle formation at these pH values, although the Maxwell
model14 does not always fit with the G¤ and G¤¤ data. The
dynamic rheology data obtained for the C16Glu­C12DMA
system are given in Supporting Information, Figure S2, where
one can see the hydrogel-type viscoelastic behavior in the pH
The development of stimulus-responsive “smart” wormlike
micellar systems is an interesting research topic in the field of
1
6
surfactant chemistry. pH is a typical trigger in such systems.
Recently, pH-responsive wormlike micellar systems have been
proposed in mixtures of tertiary alkylamine with dicarboxylic
1
7­19
20
acid
or tricarboxylic acid. These stoichiometric complexes
are sometimes called “pseudo-gemini (dimeric)” or “pseudo-
trimeric” surfactants. In our case, the wormlike micelles can be
656 | Chem. Lett. 2016, 45, 655–657 | doi:10.1246/cl.160231
© 2016 The Chemical Society of Japan

transformed into hydrogels in the narrow pH region, and hence
the observed change in the viscoelastic properties is greater than
the results reported in the previous papers. These stoichiometric
complexes, including ours, can be obtained through a very
simple, handy, and cost-effective process without difficulty of
organic synthesis. We expect, therefore, that this concept is
versatile in developing a wide variety of stimulus-responsive
materials as well as “pseudo” polymeric surfactants.
10, 1714.
8
9
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13 Basically, we equated the ©max with the zero-shear viscosity
in the observed shear rate curve. However, it was not possible
to estimate the zero-shear viscosity in highly viscoelastic
samples. In such case, the highest viscosity measured in
the low shear rate (or low angular frequency) region was
employed as the ©max.
This work was financially supported by The Cosmetrogy
Research Foundation.
Supporting Information is available on http://dx.doi.org/
1
0.1246/cl.160231.
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Chem. Lett. 2016, 45, 655–657 | doi:10.1246/cl.160231
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