Comparison of structure/function correlational property of three kinds of gemini-type thixotropic surfactants capable of forming crystalline nanofiber based on hydrogen bonding-solid-state structure, two-dimensional molecular film forming, and epitaxial growth behavior

Article abstract of DOI:10.1246/bcsj.20170406

In this study, we compare and investigate both microscopic molecular packing and mesoscopic morphogenetic behavior in two-dimensional (2D) organized films/three-dimensional (3D) solids of three kinds of Gemini-type diamide surfactants that systematically differ in terms of their chemical structure. The gelation of the surrounding medium is promoted by growing crystalline nanofibers of these surfactants, and the disappearance of these nanofibers and solvent reflow are caused by the application of a force on the corresponding gel because these surfactant molecules are considered potential thixotropic agents. The layer structure and sub-cell in the 3D crystals of surfactants are formed by the association of van derWaals force and hydrogen bonding because the mechanism responsible for crystalline nanofiber formation is intermolecular hydrogen bonding. In the monolayer on the water surface, only surfactant molecules having a hydroxyl group in the hydrophobic chain formed crystalline nanofibers. The introduction of hydroxyl groups into the hydrophobic chains also promoted a singlephase molecular conformation in 3D crystals. However, the absence of hydroxyl groups in the hydrophobic chain promoted the growth of the nanofibers in the cast film, whereas the introduction of hydroxyl groups in the hydrocarbons improves the thixotropic property itself. In addition, the epitaxial growth of nanofibers upon the addition of a growth aid was promoted by the absence of hydroxyl groups.

Full text of DOI:10.1246/bcsj.20170406

Selected Paper
Comparison of Structure/Function Correlational Property of Three
Kinds of Gemini-Type Thixotropic Surfactants Capable of Forming
Crystalline Nanofiber Based on Hydrogen Bonding ® Solid-State
Structure, Two-Dimensional Molecular Film Forming, and Epitaxial
Growth Behavior ®
Manami Iizuka,1 Yuto Nakagawa,1 Yuma Moriya,2 Eiichi Satou,3 and Atsuhiro Fujimori*1
1Graduate School of Science and Engineering, Saitama University,
255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan
2Faculty of Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan
3R & D Dept. Laboratory Additive Unit, Kusumoto Chemicals Ltd.,
4-18-6 Benten, Soka, Saitama 340-0004, Japan
E-mail: fujimori@fms.saitama-u.ac.jp
Received: December 11, 2017; Accepted: February 12, 2018; Web Released: April 24, 2018
Atsuhiro Fujimori
Dr. Atsuhiro Fujimori received his Ph.D. (2002) from Saitama University under the guidance of Prof. Hiroo
Nakahara in the field of formation and structure of Langmuir-Blodgett films. After his experience as a post-
doctoral researcher of JSPS, he became an assistant professor in Prof. Toru Masuko’s Laboratory at
Yamagata University in 2003 and widened her research to solid-state structure of crystalline polymers. He
joined the department of graduate school of science and engineering, Saitama University, as an associate
professor (2011).
Abstract
nanofibers upon the addition of a growth aid was promoted by
the absence of hydroxyl groups.
In this study, we compare and investigate both microscopic
molecular packing and mesoscopic morphogenetic behavior in
two-dimensional (2D) organized films/three-dimensional (3D)
solids of three kinds of Gemini-type diamide surfactants that
systematically differ in terms of their chemical structure. The
gelation of the surrounding medium is promoted by growing
crystalline nanofibers of these surfactants, and the disappear-
ance of these nanofibers and solvent reflow are caused by the
application of a force on the corresponding gel because these
surfactant molecules are considered potential thixotropic
agents. The layer structure and sub-cell in the 3D crystals of
surfactants are formed by the association of van der Waals force
and hydrogen bonding because the mechanism responsible for
crystalline nanofiber formation is intermolecular hydrogen
bonding. In the monolayer on the water surface, only surfactant
molecules having a hydroxyl group in the hydrophobic chain
formed crystalline nanofibers. The introduction of hydroxyl
groups into the hydrophobic chains also promoted a single-
phase molecular conformation in 3D crystals. However, the
absence of hydroxyl groups in the hydrophobic chain promoted
the growth of the nanofibers in the cast film, whereas the intro-
duction of hydroxyl groups in the hydrocarbons improves the
thixotropic property itself. In addition, the epitaxial growth of
Keywords: Gemini-type diamide surfactant
j
Crystalline nanofiber Hydrogen bonding
j
1. Introduction
Thixotropy is defined as “a phenomenon in which fluidity
temporarily increases when strain and stress are applied, viscos-
ity decreases and returns to its original state when flow stops.”1,2
The thixotropy phenomenon is observed in foods such as
mayonnaise and tomato ketchup, as well as waste-oil treatment
agents, ballpoint pen inks, and bone marrow spinal fluid. On
the contrary, dilatancy3,4 is a phenomenon whereby solidifica-
tion phenomenon occurs only when an external force is applied,
whereas a flow is observed by standing still. In recent years,
both thixotropy and dilatancy have tended to be recognized as
macroscopic displacement phenomena, although attempts have
been made to obtain control using a collaborative phenomenon
from a molecular assembly level that transcends hierarchy.5
When actively using this phenomenon in industrial fields
such as paints, a fiber-growth technique is employed in solvents
using additive molecules.6 This fiber is a characteristic crys-
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talline fiber derived from hydrogen bonding,7 and the reflow of
the solvent is caused by the decay of nanofiber growth and
breaking hydrogen bonds (Figure S1). Hydrogen bonds that are
generated during the regrowth of nanofibers are weaker com-
pared to universal gel formation,8,9 and they change coopera-
tively and hierarchically by external force induction.10 Previ-
ously, 12-hydroxystearic acid11,12 which is an ecological mate-
rial obtained from castor oil, has been successfully utilized as a
thixotropic agent, and has been commercialized.13 However, the
use of a low-molecular-weight compound having a low vapor
pressure has limitations in terms of its application14 as a drip-
ping preventive agent or anti-settling agent for paints. However,
molecules having hydrogen-bonding capabilities can improve
thermal stability without the need for polymerization by increas-
ing the number of hydrogen bonding sites.15,16 Therefore, the
three Gemini thixotropic molecules evaluated in this study are
gemini-type17,18 diamides having hydrocarbons. Actually, its
performance has already been determined, and thixotropic agent
molecules with the -OH group at the 12-position in a hydro-
phobic stearyl chain6 are not inferior to other derivatives used in
this study for all cases. Therefore, there is a possibility that the
degree of microscopic hydrogen bonding will affect thixotropic
physical properties. However, changes in the molecular level/
functional group level cannot directly affect macroscopic visco-
elasticity beyond the scale hierarchy.19,20 On the medium scale,
it appears that the molecular arrangement and packing, aggre-
gation state, and morphology are hidden. In a previous study, we
evaluated the structure and growth behavior of nanofiber in the
cast film of diamide derivatives with two hydrocarbons obtained
from castor oil,6,21 a charge-transfer complex including tetra-
thiafulvalene (TTF),22 and topological polymer.23 In particular,
it has been found that the addition of montmorillonite24,25 parti-
cles subjected to organo-modification26,27 at the surface induces
remarkable fiber growth for Gemini-type diamide nanofibers
having two hydrocarbons. However, the degree of growth of
the nanofibers may not necessarily have a significant influence
on the gelling of the contacting solvent. Hence, because we
considered that it is important to explain the higher-order
structure and its influence on the functionality caused by the
difference in molecular structure, in this study, we compared the
organization behavior of molecular species using the method
involving interfacial molecular films.
By the way, diamide-based surfactants containing two
hydrocarbons used in this study are materials that have already
been put to practical use as “thixotropic additives” for automo-
bile paints. The required function is to be able to apply a paint
containing it and to prevent dripping after application. Since
it is an amphiphilic chemical structure, it can be dispersed
without phase separation both in the aqueous and in the oil
systems and can be effectively used particularly for oil-based
paints containing moisture such as paint.28 In addition to the
nanofiber formation in the medium by the formation of a
hydrogen bonding network, the presence of the hydroxyl group
is more easily dispersed in an aqueous system than a general-
purpose surfactant, and the molecular design corresponds to
chemical structure for which stability of the interface mono-
layer is relatively low. The difference from the gelator is that
it includes the industrial requirement that “the solvent contain-
ing it can be applied to a paint”. On the other hand, these like
hydrogen bonding networks tend to be broken and easily
reproduced. Further, since the thermal stability of the com-
pound itself is required, amide-based compounds are often
formed by dehydration condensation.
In this study, we compare three Gemini-type diamide deriv-
atives having two hydrocarbons obtained by condensation.
There are 2:1 condensates of 12-hydroxystearic acid:hexa-
methylenediamine, stearic acid:hexamethylenediamine, and
stearylamine:octanedioic acid. As an evaluation, the influence
of the thermal behavior and solid-state structural formation
exerted by intermolecular hydrogen bonds are investigated
using thermal analysis, infrared (IR), and powder X-ray dif-
fraction (XRD). Furthermore, the development of an organized
molecular film was carried out at the air/water interface, and
the fiberizing ability in the monolayer is evaluated from the
viewpoint of the presence or absence of the hydroxyl group in
the hydrophobic chain, and the crystallinity of the textured film
was also examined. In addition, pseudo-thixotropic behavior
was observed based on the state of spontaneous fiber growth in
these cast films, and we compared the fiber-growth behavior
when adding an effective growth aid to them (Figure 1). This
study focuses on the comparison of the solid-state structure and
the behavior/molecular arrangement in the organized films of
three kinds of amphiphiles having the ability to impart thixot-
ropic properties to the contacting solvent. Although this paper
includes discussion of predicting the origin of thixotropy on
the way, details will be discussed the next report. Discussions
are being developed in this paper assuming that the high
thixotropic performance and the degree of growth of nanofibers
have almost equivalent implications.
2. Experimental
2.1 Synthesis and Characterization of Three Kinds of
Amphiphilic Diamide Derivatives.
Condensation reac-
tions of 12-hydroxystearic acid:hexamethylenediamine, stearic
acid:hexamethylenediamine, and stearylamine:octanedioic acid
(2:1 mole ratio) were performed, respectively. During the
synthesis, triphenyl phosphite and pyridines were used as the
condensing agent and catalyst, respectively. The resulting com-
pounds were identified via nuclear magnetic resonance (NMR),
mass spectroscopy, and elemental analysis.6 The obtained
materials (Figure 2, N, N¤-1,6-hexanediyl-bis-12-hydroxy-octa-
decanamide, N, N¤-hexane-1,6-diyldistearamide, and N, N¤-
dioctadecyl octanediamide) were purified by recrystallization,
and their purity was confirmed by performing thermal analysis.
In order to estimate the sublimation or thermal-degradation
behavior, we performed thermogravimetric (TG) analysis with
an SII TG/DTA 3200 in N2. Furthermore, the phase-transition
behavior was estimated by differential scanning calorimetry
(DSC, SII DSC6200). Figure S2 shows the gelation test of the
monomers and synthetic samples at solvent introduction, and
the drop experiment by visual observation. Although stearic
acid had no gelling ability, 12-hydroxystearic acid, its con-
densate, and diamide derivative of stearic acid showed gelling
ability in the solvent.
2.2 Characterization of Three-Dimensional Structures in
Bulk and Their Spin-Cast Films. The spin-cast films of three
kinds of Gemini-type diamide derivatives with two hydro-
carbons and a mixture with organo-modified layered silicate
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Figure 1. Research strategy of this study.
was used to estimate the long periods of the layer structure.
The apparatus (Nano-Viewer, Rigaku Co. Ltd.) was operated at
40 kV and 30 mA to generate CuKα radiation (λ = 0.1542 nm).
2.3 Monolayer Formation for Three Kinds of Gemini-
Type Diamide Derivatives with Two Hydrocarbons at the
Air/Water Interface.
Monolayers of the Gemini-type
diamide derivates (³1.0 © 10¹4 M) were formed by spreading
from a CHCl3 solution comprising a small amount of trifluoro-
acetic acid (TFA), on the surface of distilled water (resistivity:
approximately 18.2 M³¢cm). After evaporation of the CHCl3
for 5 min, π-A isotherms were recorded at a compression speed
of 4.8 cm2¢min¹1. The air/water interface was kept at a con-
stant temperature of 12 °C by the circulation of temperature-
controlled water around the trough. Measurements of the
monolayer properties and Langmuir-Blodgett (LB) film transfer
were obtained using a USI-3-22 Teflon-coated LB trough (USI
Instruments).
2.4 Study of Surface Morphology and Estimation of
Molecular Arrangement. The surface morphologies of the
transferred monolayers were observed using a scanning probe
microscope (atomic force microscope (AFM), SII Nanotech-
nology, SPA300 with an SPI-3800 probe station) and micro-
fabricated rectangular Si cantilevers with integrated pyramidal
tips, by applying a constant force of 1.4 N¢m¹1. In this study,
AFM observations were performed in the tapping mode
(dynamic force mode). XRD samples were then transferred
onto a glass substrate using the LB method (20 layers, a sub-
phase temperature of 15 °C, and a surface pressure of 35
mN¢m¹1). The large spacing between the layers in the films
was measured using an out-of-plane X-ray diffractometer
(Rigaku, Rint-Ultima III, Cu-Kα radiation, 40 kV, 30 mA)
equipped with a graphite monochromator. We determined the
in-plane spacing of the two-dimensional (2D) lattice of the
films using an X-ray diffractometer with different geometrical
arrangements29,30 (Bruker AXS, MXP-BX, Cu-Kα radiation,
40 kV, 40 mA, a customized instrument), and it was equipped
with a parabolic graded multilayer mirror. The X-rays had an
Figure 2. Chemical structure and 3D models of three kinds
of Gemini-type amphiphiles used in this study.
were formed from a xylene:ethanol (9:1, v/v) mixed solution at
100 °C. From the evaluation of solution temperature dependen-
cy at spin-casting, uniform morphology was always obtained
without dependence at over 100 °C. Organo-modified mont-
morillonite (MMT) was prepared using a surface-modification
method from natural MMT (Figure S3(a)) and long-chain
quaternary ammonium cations (Figure S3(b)) at an oil/water
interface.26 Powder XRD measurements of both the bulk and
cast film samples were obtained using an X-ray diffractometer
(Rigaku, Rint-Ultima III, Cu-Kα radiation, 40 kV, 30 mA)
equipped with a graphite monochromator. IR spectra of the
samples were obtained using an IR spectrometer (Bruker AXS
TENSOR II). Further, small-angle X-ray scattering (SAXS)
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incident angle of 0.2°, and the films were scanned at a speed of
0.05°/20 s, as a result of which the in-plane XRD measure-
ments had monomolecular resolution.
In the case of AFM observation, mica substrate was used
for both single layer LB film and cast film. When the surface of
the glass substrate was observed by AFM, it was judged that
this substrate was inappropriate for morphological observation
because it was very rough. However, since information can
be averaged by XRD analysis, it was deposited on a glass
substrate comparing the periodic structure of the LB multi-
layers. In the case of LB films, it is thought that the factors that
mainly form the structure are large due to the intermolecular
interaction in the lateral direction. I thought that the influence
from the surface roughness of substrate would be less if multi-
layered. As mention above, considering the probing area, we
avoided the use of glass substrates for morphological obser-
vation, and we used glass substrates for XRD analysis of layer
period and in-plane spacing.
The experimental conditions used in this study are based on
the conditions recognized as well-defined in a report of K.
Blodgett in 1935 and the interfacial molecular film study.31 It
has assumed time, temperature, humidity, compound purity,
substrate type, and film formation conditions as prescribed
by Langmuir et al.,32,33 in order to compare with previously
reported data.
3. Results and Discussion
3.1 Thermal Behavior and Molecular Packing of Three
Kinds of Diamide Derivatives in Bulk. First, in order to
estimate the influence of the hydroxyl group at the 12-position
in the hydrophobic chain, we investigated the thermal behavior
and molecular arrangements of the three kinds of Gemini-type
amphiphiles in bulk. Figure 3(a) shows the thermogravimetric
(TG) curves of these three compounds. Under nitrogen atmos-
phere, all of the thermal degradation temperatures of these com-
pounds are 300 °C, which is very high for the degradation tem-
perature of a general low-molecular compound. In particular,
the thermal degradation temperature of a Gemini-type molecule
having a hydroxyl group at the 12-position in the hydrophobic
chain is very high. In this regard, when the TG measurement of
this compound is done in the atmosphere, a residue is generated,
and the value of the weight loss does not reach zero. In other
words, because of the influence of intermolecular hydrogen
bonding, these amide-based compounds exhibit behavior closer
to the thermal degradation behavior of general polymers than
those of the sublimation phenomenon of low-molecular-weight
compounds. In particular, the existence of a hydroxyl group at
the 12-position increases the hydrogen bonding site, and it is
expected that the thermal degradation temperature would be
enhanced with the development of the hydrogen bond network.
Figure 3(b) shows the DSC curves of these three kinds of
compounds. All of the compounds melt within the range of
135-160 °C, and the figure shows the crystallization behavior
within the range of 125-145 °C. Although no systematic differ-
ences have been observed based on the chemical structure, all
three of the compounds were crystalline compounds.
Figure 3. (a) TG curves of diamide derivatives having two
hydrocarbons used in this study under the N2 purge and in
air atmosphere. (b) DSC thermogram of purified diamide
derivatives having two hydrocarbons (scanning rate;
10 °C min¹1).
firmed in the low-angle region of the profiles of all compounds.
The fourth- or fifth-order reflections are confirmed, suggesting
the formation of a highly ordered layer structure. From the
comparison with the results obtained during the small-angle
X-ray scattering measurement described later, the values of the
primary periodic length are 48.5, 42.5, and 44.2 ¡ at this point.
Figure 4(b) shows the IR spectrum in the bulk of these three
kinds of compounds. Although the spectra of all of the com-
pounds clearly indicate the amide bands (1500-1600 cm¹1),
the bands on the high wavenumber side are also important
(Figure 4 (c)). It can be seen that both the O-H stretching
vibration and the N-H stretching vibration band, which exist
in the vicinity of 3700 cm¹1, are shifted to 3200 to 3300 cm
¹1
.
Examining previous literature34,35 and IR data books, it seems
likely that as a result of strong intermolecular hydrogen bond-
ing, free O-H and N-H stretching vibrations greatly cause a low
wavenumber shift. Although the degree of the shift is small
in the case of dimerization, it is known that the degree of shift
is large in the case of homogeneous multimer. In particular, the
molecule with the -OH group shows corresponding bands
around 3200 cm¹1, and it can be easily attributed. These results
indicate that all of the N-H groups in the amide group and the
-OH group in the hydrophobic chain are involved in inter-
molecular hydrogen bonding.
Figure 4(a) shows the powder XRD profile of these three
kinds of Gemini-type amphiphiles. The peaks that were devel-
oped based on the long period of the c-axis length are con-
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(a)
Figure 5. Schematic illustration of two kinds of potential
molecular arrangements of diamide derivatives having
two hydrocarbons in bulk. (a) Extended chain model.
(b) Double-layered structure model.
stabilized double layered structure of such surfactant mole-
cules, and support inference.6 In this case, the bimolecular
period corresponds to the first-order periodic value. It is gener-
ally well known that many surfactant molecules form such a
double-layer structure.36 Because the molecular length of
stearic acid is about 25 ¡, there is good agreement between
the molecular length and the long-period value. Hydrogen
bonds occur between amide groups along the c-axis at inter-
layers, and they occur between alkyl chains along the trans-
verse direction between hydroxyl groups. At first glance, these
results appear reasonable in Figure 5(b), but they are not
well confirmed. Therefore, this approach is considered together
with the result of the subsequent small-angle X-ray scattering
measurement.
Figure 6(a) shows the small-angle X-ray scattering patterns
and profiles of the three compounds used in this study. Because
all of the SAXS patterns are annular, it finds that these mea-
surements are performed for molecules in an isotropic state.
Interestingly, the compound with hydroxyl groups indicates
only a single period, while the other two compounds show two
long-period values. A long period spacing 50 ¡ could not be
confirmed because of the detection limit in WAXD. Compared
with the results of powder X-ray diffraction, it can be con-
sidered that the values of the XRD first period and the SAXS
period of the hydroxyl group-containing compound are the
same at the error level. However, in the other two compounds,
short-value periods on the high q value side correspond to the
XRD first-order periods, and the first-order periods exceeding
50 ¡ in both cases are also shown. From the XRD results, we
observe that scattering peaks on the high q-value side are based
on a developed layer structure showing XRD high-order reflec-
tion. On the contrary, from the high intensity of these SAXS
peaks, it is believed that these values of the period reflect
lengths with a large electron-density difference. Based on the
above, we estimated the model diagram shown in Figure 6(b).
Figure 4. (a) Powder X-ray diffraction profile of diamide
derivatives having two hydrocarbons in bulk. (b) IR
spectra of diamide derivatives having two hydrocarbons
in bulk. (c) Enlarged IR spectra of region of hydrogen
bonding O-H and N-H bands.
Here, we predict the molecular arrangement in the solid
phase, taking into consideration the formation of intermolec-
ular hydrogen bonds. Two conformations can be considered
simply. One is the formation of extended chain crystals
(ECCs). Figure 5(a) shows an example of an ECC conforma-
tion for three kinds of compounds. Amide groups and hydroxyl
groups individually interact with each other, and molecules
arrange and pack with the same phase, so that a highly ordered
layer structure is formed. In the compounds having no hydroxyl
group, the only interaction between the hydrophobic chains is
the van der Waals force. In this case, the extended molecular
length is much greater than 50 ¡ for each molecule. In other
words, based on the value of the XRD first-order period, a
molecular arrangement having a relatively large tilted angle is
predicted.
However, the molecular arrangement shown in Figure 5(b)
is also a potentially good candidate as inference. At first glance,
it seems that the model where there is a space between the alkyl
chains is irrational. However, in fact, the corresponding space
does not exist. This case indicates model in which two alkyl
chains are densely packed. In this model, the hexamethylenedi-
amine linker is random and does not stretch. This evidence is
supported from the emergence of multiple diffraction peaks
showing hydrocarbons densely packed in the lateral direction
in the vicinity of d = 4 ¡ in the powder XRD shown for the
following Figure S5. In addition, there are many reports of a
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solid-solid phase transition was confirmed in DSC thermo-
grams. In the 12-hydroxystearic acid derivative, a shift in the
layer period (Long Spacing) by heating was not confirmed. A
change was observed in the diffraction peaks near 2© = 20°
(d = 4 ¡) called Short Spacing. However, this seemed to be a
peak shift corresponding to the change in lattice size by heat-
ing. On the other hand, as the stearic acid and the stearylamine
derivatives heated, the long spacing peaks shifted by a small
angle side and the interlayer spacing broadened. Also, in each
case, a change in the short spacing appeared. This is probably
because the mixed crystal of extended chain crystal and double
layered crystals is formed, and the structure is likely to transfer
due to molecular motion by heating. Although the driving force
of structural formation is an enhanced van der Waals inter-
action between alkyl chains in any case, it seems that mixing
of different layer structures is likely to lead to disorder of
arrangement.
3.2 Monolayer Morphology and Molecular Arrangement
of Three Kinds of Diamide Derivative with Two Hydro-
carbons in Two-Dimensional Films. Up to the present, the
solid-state structure of three kinds of Gemini-type amphiphiles
capable of imparting thixotropic properties to the contact sol-
vent have been compared. The reversal of the amide bond and
the length of the linker length do not significantly affect the
structure formation. The presence of the hydroxyl group at the
12-position in the hydrophobic chain has a significant effect,
which is more owing to the influence of intermolecular hydro-
gen bonds that arise from this -OH group rather than the
presence of the hydroxyl group itself. In fact, the thixotropy
imparting property is also remarkable in the derivative of the
12-hydroxystearic acid. Subsequently, the state of comparing
these three compounds is at the air/water interface and in a
complete-layer structural organization. The thixotropy impart-
ing property is aimed to be expressed even at extremely thin
thickness as a monolayer. Because the induction of the thix-
otropic ability by additives is accompanied by fiber growth,
for introduction into painting media, it is desired to suppress
fiber growth in the thickness direction. In order to prevent the
nonuniformity of the coating film owing to contamination and
light scattering, fiberization in a monomolecular film state is
required. The aim is to determine whether it is possible to form
a monolayer fiber, and to clarify the correlation between the
monolayer fiberization and chemical structure.
Figure 7(a) shows the behavior of the monolayer on the
water surface for the three kinds of Gemini-type surfactant
molecules used in this study. As shown in previous studies,6 the
surface pressure-area isotherm of compounds with hydroxyl
groups shows clear 2D phase transitions. Though the π-A
isotherm of (12-OH-C18A)2-C6-amide is very complicated, this
result is reproducible as long as it is the same temperature
condition of subphase. Also, since the presence of impurities is
not confirmed in the results of NMR and thermal analysis, this
material is considered to be a pure material. It seems that the
cause of the tortuous π-A curve of this compound is the promo-
tion of intermolecular hydrogen bonding between the hydroxyl
groups present in the hydrophobic chain. As the surface
pressure rises, formation of intermolecular hydrogen bonds
becomes noticeable as the intermolecular distance approaches.
The morphological changes at this time, and the change of the
Figure 6. (a) Small-angle X-ray scattering patterns and
profiles of three kinds of Gemini-type diamide derivatives
having two hydrocarbons in bulk. (b) Expected phase
models of three kinds of Gemini-type diamide derivatives.
It is clear that the derivative of 12-hydroxystearic acid has a
double-layered single-phase structure. However, derivatives of
stearic acid and derivatives of stearyl amine are expected to be
mixed crystals. In this case, it is reasonable to assume that a
developed multilayer structure and elongated chain crystals are
mixed. Judging from the height of the vertical-axis intensity of
SAXS, it is expected that there would be repeated periods
between the high-density packed state of long alkyl chains and
the low-density aggregated state of linker moiety. In the forma-
tion of a double-layer structure, it is considered that the linker
part, which is randomized owing to strong interactions between
long chain alkyls, is elongated in the extended chain crystal to
form a low-density phase. In other words, it is considered that
the hydroxyl group at the 12-position has the effect of forming
a “monophasic” molecular group. This is an effect of inter-
molecular hydrogen bonding, which occurs between hydroxyl
groups.
Based on the above-mentioned results, it was aimed to
acquire related information by measuring temperature control
IR (Figure S4) and temperature control XRD (Figure S5).
Almost no change was confirmed in the temperature-controlled
IR measurement.
However, the temperature-controlled WAXD also showed
changes other than the octadecylamine derivative which the
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© 2018 The Chemical Society of Japan

Figure 7. (a) Surface pressure-area isotherms of monolayer
on the water surface of diamide derivatives having two
hydrocarbons transferred at several surface pressures at
12 °C. (b) AFM images of monolayer on solid substrate of
diamide derivatives.
isotherms due to the temperature dependence of subphase has
been shown in the previous report.6,30,33 It is expected that the
transition will be linked to the promotion of hydrogen bond-
ing because the dot-like domains become fibrillated and grow
while repeating the transition (corresponding AFM images of
Figure 7(b)). Interestingly, if the hydroxyl group at the 12-
position is lost, the phase-transition behavior cannot be seen on
the π-A curve. In this case, since it is confirmed that the AFM
image of the transferred monolayers on the solid indicates
sheet-like texture and aggregated crystallite film, these are
observed that the monolayer fiber cannot be formed. As shown
in the illustration in Figure 7(a), because the amide bond site
is present under the water surface and cannot participate in
intermolecular hydrogen bonding, it is expected that only
compounds with a hydrogen bonding site in the hydrophobic
chain can be made into fibers.
It should be noted that the interfacial conformation of these
molecules is considered to equally place two amide sites under
the water surface as a hydrophilic group. Considering amphi-
philicity, it is difficult to consider other possibilities. The height
information of the AFM in the low surface pressure region
almost corresponds to this prediction. However, the corre-
sponding height information in high surface pressure region
has not been accurately grasped from AFM observation. The
height information of the AFM as shown in Figure 7(b) is
the highest numerical value in the 2 © 2 ¯m2 probing area.
Actually, as a locally averaged height, it can reach a value
about 3 nm. However, especially in the high-pressure region of
transferred surface pressure, the grown single-layered fibers
and the crystallites themselves partially overlap, and the height
information is estimated to be large. Further examination and
experimentation may be necessary for this part. It may be
regarded as a feature of fiber/microcrystal growing together
Figure 8. Out-of-plane XRD profiles of LB multilayers of
diamide derivatives having two hydrocarbons transferred at
20 or 35 mN m and 12 °C.
¹1
with increasing surface pressure. In this case, if multilayers are
formed by the LB method, the formation of a double-layer
structure, which is inevitably predicted in the bulk state, can
be achieved. Figure 8 shows the out-of-plane XRD results for
these three kinds of compounds with respect to the 20 LB
layers. All of the multilayers exhibited a high degree of layered
regularity showing a third-order reflection. Here, because the
d-spacing value calculated from the high-order reflection is
more accurate than the peak position on the low-angle side, we
calculated the first-order period. The d001 period obtained here
is a distance not exceeding 50 ¡, which confirms that the esti-
mation of the double-layer structure period (estimated from the
powder XRD) in the bulk state was appropriate. Furthermore,
from the results of the in-plane XRD for these samples, crys-
talline peaks were all confirmed, and we determined that all
of the multilayers were crystalline films (Figure 9(a)). In this
case, since LB film of hydroxyl group-containing molecules
corresponds to multilayers of “a single-layer fiber film”, it is
found that this compound can form a crystalline monolayer
fiber. This result shows the potential for the development of
monolayer thixotropy because nanofibers that exhibit thixo-
tropic properties are recognized as fibers having crystalline
regularity in either case. Incidentally, as shown in the enlarged
views of Figures 9(b) and 9(c), all of the O-H groups and N-H
groups also form hydrogen bonds in the multilayers as a result
of the IR. During the formation of a monolayer on a water
surface, the amide group is present under the water surface
Bull. Chem. Soc. Jpn. 2018, 91, 813–823 | doi:10.1246/bcsj.20170406
© 2018 The Chemical Society of Japan | 819

Figure 10. Schematic models of layer structure and sub-cell
of hydrocarbons in LB multilayers of diamide derivatives
having two hydrocarbons.
film (Figure 11(a)). As mentioned above, the function as a
thixotropic additive of hydroxyl group-containing surfactant
molecules is excellent. In addition, the origin of thixotropy is
recognized as fiber growth in the medium. However, at first
glance, the degree of fiber formation appears to be more prom-
inent in the group of compounds having no hydroxyl group.
Although relatively fine nanofibers are formed in the hydroxyl
group-containing compound, the growth of the developed fiber,
which leads to microfibers, is confirmed in the other two
compounds. In the preceding study,6 owing to the addition of
organo-modified clay (montmorillonite) having a crystalline-
layer arrangement as well as these fiber-forming compounds,
the development of fibers by epitaxial growth was found from
the similarity of layer spacing. When 1 wt% of organo-
modified montmorillonite was added to all three compounds
using this technique, the growth of mesoscopic fiber morphol-
ogy was confirmed in all the compounds. Incidentally, as
shown in Figure S6, there is a clear dependence on the con-
centration of organo-clay that is to be added as a growth aid.
When 0.5 wt% was added, it is not easily distinguished from
neat materials. In the case of the 5-wt% addition, because the
growth progresses to such a degree that no significant differ-
ence can be confirmed for each compound, an additional 1 wt%
was adopted in this study. Even under the same condition, there
is a high degree of promotion of fiber growth of hydroxyl
group-free compounds (Figure 11(b)). Since it is presumed that
it may be due to the presence of hydrogen bonding network
along a horizontal direction derived from hydroxyl groups in
hydrophobic chain, it is assumed that there will be suppressed
growth along the c-axis direction from the surface of organo-
clay which is a layer structural material like a thixotropic agent
Figure 9. (a) In-plane XRD profiles and (b) IR spectra of
LB multilayers of diamide derivatives having two hydro-
¹1
carbons transferred at 20 or 35 mN m
and 12 °C.
(c) Enlarged IR spectra of region of hydrogen bonding
O-H and N-H bands.
and cannot contribute to intermolecular hydrogen bonding.
However, in the LB multilayers, it can form an interlayer
hydrogen bond with hydrophilic groups that face each other, so
all of the N-H groups are presumed to be involved in hydrogen
bonding.
Figure 10 shows a model diagram of the molecular arrange-
ment in LB multilayers based on the above discussion. All
of the molecules form a developed layer structure in the LB
film, and its first-order periodic value is less than 50 ¡. All of
the packing modes of molecular chains in the 2D plane are
2D orthorhombic systems,37 and the in-plane periodic value is
estimated to be 4.4 (or 4.5) ¡ and 4.1 ¡.
3.3 Nanofiber Formation in Cast Films and Their
Epitaxial Growth Behavior by Additive Growth Aid. In
this section, we discuss the formation behavior of fiber mor-
phology in the bulk state based on the above investigation.
Three kinds of Gemini-type amphiphiles used in this study are
thixotropic additive candidates. These additive molecules have
the ability to form crystalline nanofibers derived from inter-
molecular hydrogen bonds in a solvent. Fiber growth in a sol-
vent corresponds to a driving force for the development of
thixotropy. Therefore, as an evaluation of a pseudo fiber growth
in solvent, we carried out an AFM observation on a spin cast
820 | Bull. Chem. Soc. Jpn. 2018, 91, 813–823 | doi:10.1246/bcsj.20170406
© 2018 The Chemical Society of Japan

based surfactants that are inferior to other thixotropic prop-
erties, did not indicate the cotton effect in the CD spectra. In
this case, these diamide derivatives formed only linear fibers
in SEM images. That is to say, in compounds having excellent
thixotropy, chirality due to mesoscopic structure formation is
developed, and this is expected to be related to the origin of
enhancement of this physical property. The contents of this
research will be clearly indicated by research reports in the near
future.
The presence of the hydroxyl group at the 12-position in the
hydrophobic chain of the Gemini-type surfactant molecule
affects the distinct structure formation and morphological
growth. The monophasic formation of a layered structure and
thin nanofiber growth, and the formation of single-layer
nanofibers are realized by intermolecular hydrogen bonds that
are derived from hydroxyl groups. The absence of a hydroxyl
group leads to the microfiber formation of mixed crystals and a
2D microcrystalline monolayer, indicating that the presence or
absence of one functional group clearly influences hierarchi-
cally the molecular arrangement and mesoscopic morpholog-
ical formation. In this research, although it conducted a
multilateral analysis, there are various methods for analyzing
molecular packing and hydrogen bonding, and there is a
possibility that new information can be obtained from these
1
results. For example, since the result of H NMR in solution,
solid, or gel may be useful to understand the molecular packing
among surfactants, Figure S9 showed 1H NMR spectra of
Gemini-type diamides used in this study under heating. It is
presumed that this data makes it possible to pursue new facts
by proactively analyzing though the resolution is not sufficient
because of poor solubility of the sample.
Figure 11. AFM images of spin-cast films of (a) neat
diamide derivatives having two hydrocarbons and (b) their
composite with 1 wt% organo-modified MMT, fabricated
at room temperature.
molecule. However, it is also confirmed that the formation and
growth of such fine nanofibers can impart more pronounced
thixotropy to the contact medium. The relationship between the
presence of the microscopic hydroxyl group and the hydrogen-
bonding network formation, the growth style of mesoscopic
nanofibers, and macroscopic thixotropic properties remains
unclear. This is regarded as a molecular cooperative effect that
transcends the hierarchy of materials. Current studies aim to
determine whether the hydroxyl group at the 12-position is
bonded to the asymmetric carbon. Preliminary experiments
have confirmed the dextrorotatory behavior by circular dichro-
ism spectra, as well as the right-handed spiral fiber growth of
the molecules. Now, the explanation of the correlation between
molecular chirality and morphological growth38 is interesting,
and it is positioned as a future study topic. In this point, rather
growth entangled fibers estimated by a scanning electron and
transmission electron microscopy (SEM and TEM) have
already been observed (Figure S7). It was thought the origins
of thixotropy in a bundled fiber form developed in an organi-
zation like observed by electron microscopy, not a thin elon-
gated fiber as observed with AFM in the above data. Here, the
measurement result of circular dichroism (CD) spectrum was
also indicated (Figure S8(a)). As shown in Figure 8(b), no
cotton effect is observed in all Gemini-type diamide derivatives
and 12-hydroxy stearic acid in solution. However, in the cast
film, the positive cotton effect was clearly observed only in the
surfactant having 12-hydroxy stearyl chains with the highest
thixotropic ability. On the other hand, the other two diamide-
4. Conclusion
In this study, we compared the molecular arrangement/
packing on a microscopic scale and the morphological forma-
tion at a mesoscopic scale in 2D and 3D structures of three
kinds of Gemini-type diamide amphiphiles that are systemati-
cally modified in the chemical structure (Figure 12). The
amphiphiles used in this study are potential additives that
induce thixotropic properties to the contact media. The origin
of the expression of thixotropic properties is that additive mole-
cules form crystalline nanofiber morphology in the medium by
intermolecular hydrogen bonding. All three kinds of amphi-
phile used in this work are diamide derivatives with two
hydrocarbons. The amphiphile having the highest thixotropy-
imparting ability has a hydroxyl group at the 12-position of the
two hydrophobic chains. Although all three kinds of thixo-
tropic molecules are crystalline molecules forming highly
ordered layer structures, the presence of the hydroxyl group at
the 12-position improves the thermal degradation temperature,
becoming a monophasic layer structure, and forms single-layer
nanofibers at the air/water interface. The growth of fiber
morphology was confirmed in the cast-film formation of each
compound. The lack of hydroxyl groups in the long-alkyl chain
promoted the growth of the nanofibers in the cast film, whereas
the introduction of hydroxyl groups in the stearyl chains
improves the thixotropic property to the contact media. Further,
the epitaxial growth of nanofibers upon the addition of organo-
modified clay was promoted by the absence of hydroxyl
Bull. Chem. Soc. Jpn. 2018, 91, 813–823 | doi:10.1246/bcsj.20170406
© 2018 The Chemical Society of Japan | 821

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The authors appreciate the support by the Ministry of Educa-
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funding provided by a Grant-in-Aid for Scientific Research
(C, 17K05986 (A.F.)). The authors thank Prof. Yuji Shibasaki,
Iwate University for measurement of temperature-controlled
NMR spectra. Further, authors would like to express our grati-
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© 2018 The Chemical Society of Japan | 823