Spectral insights into gelation microdynamics of N-octyl-D-gluconamide in water

Article abstract of DOI:10.1039/c1sm05548h

Near infrared spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) technique is employed to illustrate the gelation microdynamic mechanism of hydrogelator N-octyl-D-gluconamide (8-GA), which can rapidly self-agglomerate into helical bilayer micellar fibers upon cooling from spherical micelles. Boltzmann fitting and PCMW easily determined the gelation temperature to be ca. 72°C and the transition temperature range to be 70-75°C. Moreover, band shifting and splitting phenomena can be observed for CH-related overtones, indicating the formation of much ordered and tight hydrophobic core from octyl tails. On the other hand, 2Dcos was used to discern the sequential orders during the gelation process and concluded that all the group motions have a continuous transfer from the octyl tail to the chiral carbohydrate head followed by the final immobilization of the solvent, which meanwhile, is actually a continuous dehydration process from the hydrophobic core to the outer hydrophilic chiral head. The driving force of the gelation process in microdynamics can only be the dehydration process of hydrophobic octyl chains, but with final helical superstructures being stabilized by amide-associated hydrogen bonding and the "chiral bilayer effect" of carbohydrate heads.

Full text of DOI:10.1039/c1sm05548h

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PAPER
Spectral insights into gelation microdynamics of N-octyl-D-gluconamide in
water†
*
Shengtong Sun and Peiyi Wu
Received 30th March 2011, Accepted 27th April 2011
DOI: 10.1039/c1sm05548h
Near infrared spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos)
and perturbation correlation moving window (PCMW) technique is employed to illustrate the gelation
microdynamic mechanism of hydrogelator N-octyl-D-gluconamide (8-GA), which can rapidly self-
agglomerate into helical bilayer micellar fibers upon cooling from spherical micelles. Boltzmann fitting
and PCMW easily ꢀdetermined the gelation temperature to be ca. 72 ꢀC and the transition temperature
range to be 70–75 C. Moreover, band shifting and splitting phenomena can be observed for CH-
related overtones, indicating the formation of much ordered and tight hydrophobic core from octyl
tails. On the other hand, 2Dcos was used to discern the sequential orders during the gelation process
and concluded that all the group motions have a continuous transfer from the octyl tail to the chiral
carbohydrate head followed by the final immobilization of the solvent, which meanwhile, is actually
a continuous dehydration process from the hydrophobic core to the outer hydrophilic chiral head. The
driving force of the gelation process in microdynamics can only be the dehydration process of
hydrophobic octyl chains, but with final helical superstructures being stabilized by amide-associated
hydrogen bonding and the ‘‘chiral bilayer effect’’ of carbohydrate heads.
as amphiphile lipids,4 sugar-based lipids,5 boloamphiphiles,6
gemini surfactants7 and two or three component systems.8
Chiral amphiphiles incorporating secondary amide groups
often form fluid, spherical micelles in water above 80 ꢀC, and
quickly agglomerate upon cooling to give long helical micellar
fibers with length-to-diameter ratio higher than 104.9 For
N-octylgluconamide (8-GA, as shown in Scheme 1), quadruple
helices constructed from bilayer micellar cylinders can be
observed by TEM, which is also the only known amphiphilic
assembly of bimolecular thickness without a charged surface.10
However, the surface energy of such fibers was so large that they
would rearrange quickly within several days to more stable
crystals without curvature. Addition of small amounts of sodium
dodecyl sulphate (SDS) proved to make it stable for years.11
Moreover, curved fibers are stable only if the amphiphiles are
enantiomers, and their racemates crystallize as planets.9c This
phenomenon is well-known as ‘‘chiral bilayer effect’’.12 As
1
Introduction
Over recent years, immense interests have been stimulated in low-
molecular-weight (LMW) hydrogelators capable of gelling
aqueous solvents due to their potential applications in templating
nanostructured materials, controlled drug delivery, tissue engi-
neering, and so on.1 The gelation is generally considered to arise
from nano- or mircofibers immobilizing and trapping solvents
via surface tension.2 Wherein, in contrast to inorganic and
polymer gels, the solid 3D entangled network formed by LMW
hydrogelators is linked solely via noncovalent interactions, such
as hydrogen bonding, p–p stacking, coordination, hydrophobic
and electrostatic interactions. Thus the gelation process is
completely thermoreversible. Some basic rules have also been
proposed to enable the design of new LMW gelators, e.g. the
existence of strong intermolecular interactions to enforce 1D self-
assembly, control of fiber-solvent energy to prevent crystalliza-
tion and some other factors to intertwining the aggregates.3 In
water, amphiphilic molecules with hydrophobic groups to
promote aggregation and hydrophilic groups to provide solu-
bility are most competent for hydrogelators.2 Up to now,
numerous amphiphile hydrogelators have been developed, such
The Key Laboratory of Molecular Engineering of Polymers, Ministry of
Education, Department of Macromolecular Science, Laboratory of
Advanced Materials, Fudan University, Shanghai, 200433, China.
E-mail: peiyiwu@fudan.edu.cn
Scheme 1 Chemical structure of N-octyl-D-gluconamide (8-GA). The
arrows represent both the sequential group motion and the dehydration
directions in the gelation process according to 2Dcos results.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1sm05548h
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reported, this effect is caused by the slowness of rearrangements
from tail-to-tail hydrophobic bilayers to head-to-tail crystals.
Further structural functionalization of gluconamides can
produce organogels,13 which have even been used in liquid
crystalline physical gels.14
Japan, 2004–2005). The final contour maps were plotted by
Origin program, ver. 8.1, with warm colors (red and yellow)
defined as positive intensities and cool colors (blue) as negative
ones. An appropriate window size (2m + 1 ¼ 9) was chosen to
generate PCMW spectra with good quality.
Several researches have been devoted to explore the molecular
origin of the bilayer helical assemblies of 8-GA, both by theo-
retical calculations15 and experimental observations.16 Obvi-
ously, there are three competing forces in the gelation process
corresponding to three binding sites: hydrophobic interactions of
octyl chains inclined to form spherical micelles, hydrogen
bonding of amide linkages inclined to transform the spheres to
discs, and the chiral bilayer stabilizing effect of open-chain
carbohydrate head groups.17 However, nearly all the previous
studies were focused on structural characterization of gels and
corresponding crystals. To the best of our knowledge, no inves-
tigations concerning the evolving gelation process or the micro-
dynamic mechanism have ever been reported. But it is really
helpful to understand the assembly phenomenon of gelators
because most gel systems are in a metastable state and hence their
formation is governed by thermodynamics and kinetics. What’s
more, solvents also involve the gelation process, but the role of
them is still poorly understood.18
2D correlation spectroscopy (2Dcos). FT-NIR spectra used for
PCMW analysis were also used to perform 2D correlation
analysis. 2D correlation analysis was carried out using the same
software 2D Shige ver. 1.3 (ª Shigeaki Morita, Kwansei-Gakuin
University, Japan, 2004–2005), and was further plotted into the
contour maps by Origin program ver. 8.0. In the contour maps,
warm colors (red and yellow) are defined as positive intensities,
while cool colors (blue) as negative ones.
3
Results and discussion
3.1 Conventional NIR
The whole gelation process of 8-GA was in situ monitored by NIR
spectroscopy upon cooling from 80 to 64 ꢀC (Fig. 1). For clarity,
only a 2 ꢀC interval is shown here. Two spectral regions are mainly
focused in this paper: NH- and OH-related overtones (7345 ꢁ
6035 cmꢁ1) and CH-related overtones (5940 ꢁ 5620 cmꢁ1). The
former region contributes from N–H in the amide linkage of 8-GA
and O–H from chiral carbohydrate head groups as well as the
solvent. The latter one all arises from C–H groups in 8-GA
In this paper, near infrared (NIR) spectroscopy was employed
to trace both the spectral variations of 8-GA and solvent in the
dynamic gelation process upon cooling. Additionally, two-
dimensional correlation spectroscopy (2Dcos)19 in combination
with perturbation correlation moving window (PCMW)20 tech-
nique was used to obtain information of sequential group
motions to further illustrate the final microdynamic mechanism.
2
Experimental section
2.1 Materials
8-GA was obtained by aminolysis of D-glucono-d-lactone with
n-octylamine (both from Aladdin Co. Ltd.) in methanol as
described in literature.9a The concentration of 8-GA aqueous
solution was fixed to be 30wt%.
2.2 Instruments and measurements
Calorimetric measurements were performed on a Mettler-Toledo
differential scanning calorimeter thermal analyzer. The sample of
8-GA solution for FT-NIR measurements was prepared by being
sealed in the sample cell (quartz glass, 1 mm width). The FT-NIR
measurements were performed on a Nicolet Nexus 470 spec-
trometer with a spectral resolution of 4 cmꢁ1 and 32 scans were
accumulated to obtain an acceptable signal-to-noise ratio. The
spectra ꢀwere recorded in a variable temperature cell between 80
ꢀ
ꢀ
and 64 C in intervals of 1 C with an accuracy of 0.1 C.
2.3 Investigation methods
Perturbation correlation moving window (PCMW). FT-NIR
ꢀ
spectra collected with an increment of 1 C during cooling were
used to perform 2D correlation analysis. Primary data processing
was carried out with the method Morita provided and further
correlation calculation was performed using the software 2D
Shige, ver. 1.3 (ª Shigeaki Morita, Kwansei-Gakuin University,
Fig. 1 Temperature-dependent overlaid NIR spectra of 8-GA in water
(30wt%) during gelation upon cooling from 80 to 64 ꢀC. For clarity, only
a 2 ꢀC interval is shown here. The arrows represent the main tendency of
spectral variations.
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skeletons. Thus nearly all the group motions of the gelator and
solvent can be primely traced by NIR spectroscopy.
From Fig. 1, obvious spectral changes can be observed.
NH-related overtones from 8-GA at lower frequencies show
continuous increase during cooling, while all CH-related over-
tones show similar spectral increase in combination with
apparent band shift and band splitting. The band shift of CH-
related overtones can be explained by a hydrophobic interaction
with neighboring water molecules of the solution. The higher the
number of water molecules surrounding C–H groups is, the
higher the vibrational frequency is.21 Similar phenomenon can be
easily found in the coil-to-globule phase transition of poly(N-
isopropylacrylamide) in water.22 It reveals that the octyl chains of
8-GA experienced a dehydration process during gelation.
Upon cooling, the band at 5795 cmꢁ1 continuously splits into
two bands at 5824 and 5772 cmꢁ1, which all come from the
vibrational overtones of octyl chains. As reported,4a,17 at the
temperature above the gelation point (Tgel), 8-GA molecules are
inclined to form spherical micelles in which all NH and OH
groups are hydrated outside while hydrophobic octyl chains are
inside. Although the cross-section structure of spherical micelles
seems to be similar to the structure of micellar fibers after gel
formation, our NIR analysis shows that the conformation of
hydrophobic octyl chains in micellar fibers are much more
ordered than that in spherical micelles, which can only explain
the C-position-induced band splitting of C–H groups surrounded
by water molecules. This result is also in good conformity with
previous judgement about the ‘‘tight hydrophobic cores’’ in
curved bilayer superstructures.11
Fig. 2 Temperature-dependent integral absorption of 8-GA in the
regions 6672–6035 cmꢁ1 and 5940–5620 cmꢁ1 respectively during gelation.
The solid lines represent Boltzmann fitted curves.
To examine the spectral variations to further determine Tgel
,
Fig. 3 presents PCMW synchronous and asynchronous spectra
ꢀ
temperature-dependent integral absorption of NH-related over-
tones as well as CH-related overtones of 8-GA have been pre-
sented in Fig. 2. Interestingly, all the points obey S-shaped
variations, thus Boltzmann fitting method was employed for an
accurate determination of the transition temperature. The cor-
responding equation is as follows:
of 8-GA during cooling from 80 to 64 C. PCMW synchronous
spectra are very helpful to find transition points. Thus for NH-
ꢀ
related overtones, Tgel ꢂ 71 C, and for CH-related overtones,
Tgel ꢂ 72 ꢀC. The gel formation temperatures obtained by
A1 ꢁ A2
y ¼
þ A2
(1)
1 þ eðxꢁx Þ=dx
o
In eqn (1), A1 is the minimum value of the function; A2 is the
maximum value of the function; xo is the value on the x axis
corresponding to the inflection of the curve, which also equals to
the transition temperature; and dx is the domain where this value
lies.23 Therefore Tgel can be determined to be 71.1 C for NH-
ꢀ
ꢀ
related overtones and 72.2 C for CH-related overtones, which
are all close to DSC results (66.7 ꢀC, shown in the ESI).† Thus we
can preliminarily conclude that CH-related dehydration occurs
earlier than NH-related hydrogen bonding changes.
3.2 Perturbation correlation moving window
PCMW is a newly developed technique, whose basic principles
can date back to conventional moving window proposed by
Thomas.20a Later in 2006, Morita20b improved this technique to
much wider applicability through introducing the perturbation
variable into correlation equation. In addition to its original
ability in determining transition points as conventional moving
window did, PCMW can additionally monitor complicated
spectral variations along the perturbation direction.
Fig. 3 PCMW synchronous and asynchronous spectra of 8-GA gener-
ated from all the spectra between 80 and 64 ꢀC. Wherein, warm colors
(red and yellow) are defined as positive intensities, while cool colors (blue)
as negative ones.
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PCMW are almost the same to Boltzmann fitting results, and
once again we can conclude CH-related dehydration has an
earlier response during gelation process.
In addition to determine transition points, PCMW can also
monitor the spectral variations along temperature perturbation
combining the signs of synchronous and asynchronous spectra
by the following rules: positive synchronous correlation repre-
sents spectral intensity increasing, while negative one represents
decreasing; positive asynchronous correlation can be observed
for a convex spectral intensity variation while negative one can
be observed for a concave variation.20b Based on this point, we
can ascertain that both NH- and CH-related overtones of 8-GA
show an S-shaped spectral increase, consistent with the above
conventional NIR analysis. The transition temperature region
can also be determined by the peaks in asynchronous spectra
which are all turning points of the S-shaped curves. Then we can
conclude that 8-GA form entangled gels mainly between 70 and
ꢀ
75 C. Note that although to obtain the transition temperature
and region is important for gelators, it is usually very hard by
DSC measurements due to apparent solvent evaporation but
often roughly estimated by a tube inversion method. PCMW
proves to be a feasible method. It is also notable that OH-related
overtones at higher frequencies contributing from both chiral
head groups of 8-GA and large amounts of water in the system
are hardly distinguished by PCMW. Thus we would not discuss
the spectral variations of OH-related overtones in detail in this
section.
3.3 Two-dimensional correlation analysis
2Dcos is a mathematical method whose basic principles were first
proposed by Noda in 1986.19a Up to the present, 2Dcos has been
widely used to study spectral variations of different chemical
species under various external perturbations (e.g. temperature,
pressure, concentration, time, electromagnetic, etc).19b Due to the
different response of different species to external variables,
additional useful information about molecular motions or
conformational changes can be extracted which cannot be
obtained straight from conventional 1D spectra.
All the spectra during gel formation process between 80 and
64 ꢀC were used to perform 2D correlation analysis, as shown in
Fig. 4. Two spectra, synchronous and asynchronous spectra, can
also be generated. 2D synchronous spectra reflect simultaneous
changes between two given wavenumbers. The bands at 6880,
6763, 6580, 5907, 5824, 5772, 5664 cmꢁ1 all have positive cross-
peaks, indicating that they had similar response of spectral
intensities to temperature perturbation—that is, all increase
during cooling determined from raw spectra. On the other hand,
the bands at7076 cmꢁ1 havenegative cross-peakswith allthe other
bands, indicating spectral intensities decrease during cooling.
2D asynchronous spectra can significantly enhance the spec-
tral resolution. In Fig. 4, many subtle bands have be identified,
such as the splitting bands at 5811, 5758 cmꢁ1 attributed from
2vas(CH2) of octyl chains in the gel state. For the convenience of
discussion, all the bands found in asynchronous spectra and their
corresponding assignments are presented in Table 1.
Fig. 4 2D synchronous and asynchronous spectra of 8-GA generated
from all the spectra between 80 and 64 ꢀC. Wherein, warm colors (red and
yellow) are defined as positive intensities, while cool colors (blue) as
negative ones.
develop only if the intensities of two spectral features change out
of phase with each other (i.e., delayed or accelerated if time is the
external variable). The judging rule can be summarized as
Noda’s rule—that is, if the cross-peaks (v1, v2, and assume v1 >
v2) in synchronous and asynchronous spectra have the same sign,
the change at v1 may occur prior to that of v2, and vice versa. As
space is limited, the determination details of sequential orders
have been presented in supporting information, and only the
final specific order is given as follows (/ means prior to or
earlier than): 5859 cmꢁ1 / 5843 cmꢁ1 / 5795 cmꢁ1 / 5884
cmꢁ1 / 5685 cmꢁ1 / 5811 cmꢁ1 / 5758 cmꢁ1 / 5824 cmꢁ1
5772 cmꢁ1 / 5664 cmꢁ1 / 5907 cmꢁ1 / 6295 cmꢁ1 / 6561
cmꢁ1 / 5708 cmꢁ1 / 6912 cmꢁ1 / 6856 cmꢁ1 / 6698 cmꢁ1
/
Except for enhancing spectral resolution, 2D correlation
spectroscopy can also discern the specific order taking place
under external perturbation. An asynchronous cross-peak can
/
0
0
0
7116 cmꢁ1. That is, 2vas(C–H in C8 H3) / 2vas(C–H in C2 ꢁ7 H2)
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Table 1 Tentative band assignments of 8-GA according to 2Dcos during
gel formation process. Corresponding mid-IR spectra can be consulted in
the ESI
have larger surface than spherical micelles to be surrounded by
more water molecules, which may also account for the too large
surface energy to make the helices stable.10b
Wavenumber (cmꢁ1
)
Assignments
Therefore, we can draw a conclusion that the gelation process
of 8-GA upon cooling can be described in microdynamics as
follows: all group motions have a continuous transfer from the
octyl tail to the chiral carbohydrate head followed by the final
immobilization of the solvent, which meanwhile, is actually
a continuous dehydration process from the hydrophobic core to
the outer hydrophilic chiral head. Both the two directions have
been presented in Scheme 1.
7116
6912
6856
6698
6561
6295
2v(O–H in water)
2v(O–H in 8-GA)
Self-associated
H-bonded with water
H-bonded with C]O 2v(N–H in 8-GA)
H-bonded with water
0
2vas(C–H in C1 H2)
0
5907
5884, 5859, 5843
5824, 5811, 5772, 5758 In gel
2vas(C–H in C8 H3)
0
0
2vas(C–H in C2 ꢁ7 H2)
5795
5708
5685
5664
In solution
Conclusions
2v(C–H in C1ꢁ5H)
0
0
Hydrated
Dehydrated
2vs(C–H in C2 ꢁ7 H2)
In this paper, NIR spectroscopy in combination with 2Dcos and
PCMW is employed to illustrate the gelation microdynamic
mechanism of 8-GA in water. Two regions involving NH-, OH-
and CH-related overtones are focused on to trace nearly all the
group motions of 8-GA upon cooling. According to conven-
tional NIR analysis, all CH-related overtones exhibit an
apparent band shift to lower frequencies and a band splitting
phenomenon. It reveals that the octyl chains of 8-GA experi-
enced a dehydration process during gel formation resulting in
a much tight and ordered hydrophobic core. Boltzmann fitting
and PCMW have determined the transition temperature to be ca.
72 ꢀC and the gelation temperature range 70–75 ꢀC. According to
2Dcos results, we obtained such sequential orders as follows:
0
0
(in solution) / 2vs(C–H in C2 ꢁ7 H2) (hydrated) / 2vas(C–H in
0
0
0
0
C2 ꢁ7 H2) (in gel) / 2vs(C–H in C2 ꢁ7 H2) (dehydrated) /
0
2vas(C–H in C1 H2) / 2v(N–H in 8-GA) (H-bonded with water)
/ 2v(N–H in 8-GA) (H-bonded with C]O) / 2v(C–H in
C1ꢁ5H) / 2v(O–H in 8-GA) (self-associated) / 2v(O–H in
8-GA) (H-bonded with water) / 2v(O–H in water).
Not considering different vibrational modes and states (gel or
solution, hydrated or dehydrated, and H-bonding types), we
0
0
0
0
always have C8 H3 / C2 ꢁ7 H2 / C1 H2 / N–H / C1ꢁ5H /
O–H in 8-GA / O–H in water. It is notable that during the
gelation process of 8-GA, all group motions have a continuous
transfer from the octyl tail to the chiral carbohydrate head
through amide linkage followed by the final immobilization of
the solvent. This transfer of groups motions can be interpreted to
the sequential or successive response of specific groups (reflected
by changes of spectral intensities) to the cooling temperature.
That means the octyl tail has an earlier response to temperature
than amide linkage and chiral carbohydrate head during the
gelation process. Judging from earlier conventional NIR anal-
ysis, this response is actually a dehydration process. Therefore,
we can conclude that the driving force of the gelation process in
microdynamics can only be the dehydration process of hydro-
phobic octyl chains, which have not been paid enough attention
in previous studies. Due to the late response upon cooling, chiral
carbohydrate head has an obvious stabilizing effect on the gel
state, which can be attributed to the ‘‘chiral bilayer effect’’,12
responsible for the formation of helical aggregates. As the
linkage both structurally and in group motions, hydrogen
bonding of amide groups are also essential for the formation of
much ordered bilayer superstructures. As reported, no gel can be
formed in N-methylated 8-GA.9a
0
0
0
0
C8 H3 / C2 ꢁ7 H2 / C1 H2 / N–H / C1ꢁ5H / O–H in 8-GA
/ O–H in water. Thus the gelation microdynamic mechanism of
8-GA can be concluded that all the group motions have
a continuous transfer from the octyl tail to the chiral carbohy-
drate head followed by the final immobilization of the solvent,
which meanwhile, is actually a continuous dehydration process
from the hydrophobic core to the outer hydrophilic chiral head.
Acknowledgements
This work was financially supported by National Science
Foundation of China (NSFC) (No. 20934002, 51073043) and the
National Basic Research Program of China (No.
2009CB930000).
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