Elucidation of the Origin of Thixotropic-Inducing Properties of Additive

Invited Feature Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Elucidation of the Origin of Thixotropic-Inducing Properties of Additive Amphiphiles and the Creation of a High-Performance Triamide Amphiphile Yuto Nakagawa,† Kaede Watahiki,‡ Eiichi Satou,§ Yuji Shibasaki,∥ and Atsuhiro Fujimori*,†

Langmuir Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/18/18. For personal use only.

†

Graduate School of Science and Engineering, and ‡Faculty of Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan § R & D Department Laboratory Additive Unit, Kusumoto Chemicals Ltd., 4-18-6, Benten, Soka-shi, Saitama 340-0004, Japan ∥ Department of Chemistry & Biological Sciences, Faculty of Science & Engineering, Iwate University, Ueda 4-3-5, Morioka, Iwate 020-8551, Japan S Supporting Information *

ABSTRACT: The spontaneous growth of helical fibers of amphiphilic diamide derivatives containing hydrocarbons with asymmetric carbon centers in their constituent hydrocarbons was investigated. 12-Hydroxystearic acid and a gemini-type surfactant obtained by the bimolecular condensation of this compound with hexamethylenediamine both impart thixotropic ability to a solvent. Although this thixotropic behavior is based on the growth of hierarchical crystalline nanofibers in the solvents, the degree of fiber growth itself was not the origin of the thixotropy. In this study, it has adopted the methods of the Langmuir monolayer and Langmuir−Blodgett films as technique to selectively and individually evaluate the behavior of 12-hydroxyl stearyl and/or stearyl chains themselves. The ability to impart thixotropy to the solvent via fiber organization was related to the intermolecular hydrogen bonding between the added amphiphiles. Additionally, homogeneous right-handed helical fibers were formed in the spin-cast films of the diamide derivatives, and a positive Cotton effect was observed in their circular dichroism spectra. It is suggested that fibers that do not form helical arrangements cannot impart sufficient thixotropy to the solvent even when extensive fiber growth is achieved, and the structure-dependent development of chirality is the driving force. In addition, to further the development of highly functional thixotropic agents, a trefoil-like triamide derivative containing three chains was synthesized. By using this molecule, solvent gelation occurred at 78% as an addition to the diamide case, and a supramolecular assembly was formed in the corresponding two-dimensional film.

1. INTRODUCTION Many biopolymers, such as DNA and proteins, have helical forms that play important roles in maintaining life.1,2 Recently, a basic concept in which “the key to the expression of the function of life is the “helix”, and if molecules or polymers can form helices, they will obtain functionalities approaching or exceeding those of a biopolymer” has been proposed.3 Accordingly, many attempts have been made to synthesize new molecules and macromolecules with a helical structure in order to study the correlation among their shape, physical properties, and function and to explore the boundary between chemistry and life science.4−7 Previously, molecules with a right-handed double helix and a catalytically active complementary strand intended to achieve both the information storage function of DNA and the catalytic function of proteins have been synthesized, and the perfect chain length and sequence recognition properties of DNA have been achieved by using their oligomers.8,9 Also, to demonstrate that durable © XXXX American Chemical Society

macromolecules can be formed by twisted structures entangled by helical polymers, methods of directly observing helical shapes and orientations via scanning probe microscopy have also been developed.10,11 In the chemistry of thixotropic additive molecules, it is necessary to understand the two conflicting concepts of hierarchy and cooperativity.12 Molecules that can induce thixotropy in the surrounding solvent form crystalline nanofibers in the solvent.13,14 These fibers are based on intermolecular hydrogen bonds derived from amide or hydroxyl groups. At the mesoscopic level, these molecules show fiberization behavior; at the macroscopic level, repeated gelation/flow with applied pressure is observed; and at the microscopic level, they exhibit a crystalline arrangement.12,13 Received: June 19, 2018 Revised: August 16, 2018 Published: August 20, 2018 A

DOI: 10.1021/acs.langmuir.8b02090 Langmuir XXXX, XXX, XXX−XXX

Langmuir

Invited Feature Article

evaluated with respect to the influence of hydrogen bonding between 12-hydroxy stearyl chains. In this study, we mainly evaluated a compound having both hydrophobic chains with a hydroxyl group at the 12 position and a hydrophilic group with an amide bonding site. Intermolecular hydrogen bonding seems to be the direct origin of nanofiber formation in the medium. Nanofiber formation directly/indirectly produces the thixotropy-inducing ability. By utilizing the method of a monolayer on the water surface, the amide moiety of the hydrophilic group can be immobilized under the water surface and the pure influence of the interaction of the hydroxyl group in the hydrophobic chain can be examined. Since the carbon to which the hydroxyl group at the 12 position is bonded is an asymmetric carbon, the relationship between the interaction of hydrophobic chains only and the fiberization ability should be clarified in order to know the correlation between chirality and fiber growth. Therefore, this study effectively utilizes the method of the interfacial molecular film. In addition, the results of the investigation into the origin of these physical properties were fed back to attempt to develop a higher-performance thixotropic additive molecule. Increasing the number of asymmetric-carbon-containing 12-hydroxyl stearyl chains in the molecular structure promotes the formation of an intermolecular hydrogen-bonding network. In this study, chirality and crystalline fiberization by hydrogen bonding were considered to be the driving forces for inducing thixotropic physical properties. On the basis of this, a novel trefoil-like triamide derivative containing three hydrocarbons was synthesized (Figure 1), and its solvent gelation ability was also investigated. Additionally, the formation of a hierarchical supramolecular assembly in the two-dimensional organized films of the synthesized triamide derivative was investigated.

Although the material hierarchy is strictly established, collaborative molecular organization to impart thixotropy also occurs. However, the growth of fibers alone does not directly contribute to thixotropy.15 In 12-hydroxystearic acid, which is traditionally used as a waste oil treatment agent, the carbon at the 12 position in the hydrophobic chain is asymmetric.16 Conventionally, thixotropy-imparting agents containing 12-hydroxy stearyl chains have been highly effective for drip prevention and antisettling applications.17 Therefore, in this study, the chirality and fiber morphology of stearic acid, 12-hydroxystearic acid, a gemini-type diamide derivative containing two stearyl chains, and a gemini-type diamide derivative containing two 12-hydroxy stearyl chains have been compared. Stearic acid is the most abundant fatty acid in nature, and it is also commonly used to form a monolayer on the surface of water.18−20 There is no possibility that this compound will exhibit thixotropy, and it is expected to show poor fiberizing ability and few hydrogen bonding sites. However, the gemini-type diamide derivative containing two stearyl chains is known to exhibit very slight thixotropic ability, and it is expected to show fiberizing ability based on the hydrogen bonding between amide groups. Films were spuncast from solutions of these four compounds, and morphological observations and analysis by circular dichroism spectroscopy were carried out. Furthermore, in order to observe the unique hydrogen bond network formation behavior of the hydroxyl group at the 12 position and the resulting mesoscopic structures, monolayers of the compounds containing 12-hydroxy stearyl chains were placed on surface of water (Figure 1), and their morphogenetic behavior and molecular arrangement were examined. The reason for effectively utilizing the methods of a monolayer on the water surface (Langmuir monolayer) and a Langmuir−Blodgett (LB) film21−23 in this study is selectively

2. EXPERIMENTAL SECTION 2.1. Materials. Stearic acid and 12-hydroxystearic acid were purchased from Wako Chemical Industries Co. Ltd. Condensation reactions of (R)-12-hydroxystearic acid (Figure 2a) and hexamethylenediamine and of stearic acid (Figure 2b) and hexamethylenediamine (2:1 mol ratio) were performed. During the synthesis, triphenyl phosphite and pyridines were used as the condensing agent and catalyst, respectively. The trefoil-like triamide derivative containing three alkyl chains was obtained by dehydration condensation according to the following procedure. 1,3,5-Tris(4aminophenyl) benzene (0.5 mL) and 12-hydroxystearic acid (1.65 mmol) were added to a 50 mL eggplant-type flask containing 20 mL of the N-methyl-2-pyrrolidone (NMP) solvent. Furthermore, an excess of 1.65 × 1.2 mmol of both triethylamine (TEA) and phenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl) phosphonic acid (DBOP) was added to this flask as a condensing agent. TEA was required as a counterion for the DBOP condensation agent. The mixture was stirred for 30 min while cooling. Cooling was utilized, as excessive heating of the reaction might cause the NH2 group to bond to the hydroxyl group rather than to the carboxyl group. The reaction was carried out by increasing the concentration of 12-hydroxystearic acid and the condensing agent to 10% relative to that of the core material to ensure that the reaction proceeded at all NH2 groups of the (4-aminophenyl) benzene core. The mixture was heated in stages to 50 °C and stirred. Subsequently, a 30% NaHCO3 aqueous solution (400 mL) was introduced. After washing the mixture under stirring to remove unreacted substances, vacuum filtration, and drying, 20 mL of THF was added, and the solution was concentrated using an evaporator. Thereafter, the mixture was stirred and washed with 400 mL of a 3 wt % K2CO3 aqueous solution, followed by vacuum filtration and drying to obtain the final compound in a yield of 42%. The resulting compound was identified via nuclear magnetic resonance (NMR, Figure S1), mass spectrometry, and elemental

Figure 1. Research strategy of this study. B


Langmuir


Langmuir monolayers of the gemini-type diamide derivatives, trefoil-like triamide derivative, and corresponding raw surfactant materials (∼1.0 × 10−4 M) were formed by spreading a CHCl3 solution containing a small amount of trifluoroacetic acid (TFA) on the surface of distilled water (resistivity ≈ 18.2 MΩ·cm). After waiting 5 min to allow the CHCl3 to evaporate, surface pressure−area (π−A) isotherms were recorded at a compression speed of 4.8 cm2·min−1. The air/water interface was maintained at a constant temperature through the circulation of temperature-controlled water around the trough. Measurements of the monolayer properties and LB film transfer were carried out using a USI-3-22 Teflon-coated LB trough (USI Instruments). The monolayer on the water surface of (12-OHC18A)2-C6-amide was transferred to a solid substrate at 25 mN m−1, which is a solid condensed phase transferred from the expanded film. Since it was clear that the (C18A)2-C6-amide monolayer had a steep rise in the π−A curve and it was clear that a stable solid film was formed, the transferring was performed at 30 mN m−1. 12Hydroxystearic acid was transferred onto a solid at 20 mN m−1, which is a stable state just before the slope of the π−A isotherm became gentle. In addition to the van der Waals interaction between hydrophobic chains, 12-hydroxystearic acid has a strong interaction in the in-plane direction due to the addition of a hydrogen-bonding interaction between the hydroxyl groups at the 12 position. Therefore, from the strength of the interaction between the molecules, it becomes possible to transfer the monolayer onto the substrate beyond the interaction between the hydrophilic group and the subphase. On the other hand, stearic acid is difficult to transfer onto a solid substrate unless it is under high pressure. In the case where metal ions and the like are not contained in the subphase, it was necessary to enhance the two-dimensional crystallinity by compression and to prompt the rapid transfer onto the substrate.25 The transfer rate was 0.03 cm min−1, and this value was made constant in all transferring processes. The transfer started from the upstroke process and with the process of attaching the hydrophilic group side to the solid substrate. The total number of layers is 20 (LB multilayers, Y-type films). These samples were used for XRD and IR measurements. In atomic force microscopy (AFM) observations, a Z-type monolayer on a solid is evaluated. The drying time is 10 min. In addition, mixed monolayers on the aqueous buffer solution, including Na+ ions of the diamide derivative and organo-MMT, were formed by the cospreading of a CHCl3 solution and a small amount of TFA and toluene, respectively. The surface morphologies of the transferred monolayers were observed using a scanning probe microscope (AFM, SII Nanotechnology, SPA300 with an SPI-3800 probe station) and microfabricated rectangular Si cantilevers with integrated pyramidal tips by applying a constant force of 1.4 N·m−1. In this study, AFM observations were performed in tapping mode (dynamic force mode). The in-plane spacing of the two-dimensional lattice of the LB films was determined using an X-ray diffractometer with different geometrical arrangements25,26 (Bruker AXS, MXP-BX, Cu Kα radiation, 40 kV, 40 mA, a customized instrument) equipped with a parabolically graded multilayer mirror. Circular dichroism (CD) spectra were obtained using an Applied Photophysics qCD. The solution CD spectra were measured by dissolving each sample in hexane containing 0.03% trifluoroacetic acid. A scanning electron microscope (SEM) observation was performed using a Hitachi SU 8030 ultra-high-resolution field-emission SEM. Transmission electron microscope (TEM) measurements were carried out using a JEM-1400 Plus (JEOL) operated at an accelerating voltage of 120 kV.

Figure 2. Chemical structures and 3D models of the amphiphilic additives used in this study. analysis. Since the solubility of the synthesized sample in deuterochloroform was low, the NMR measurement was carried out at a controlled temperature of 50 °C. The NMR spectrum confirmed the appearance of the hydrogen signal of a N−H group clearly derived from amide bond formation and the disappearance of the hydrogen signal of the carbonyl group. The obtained materials (Figure 2, N,N′1,6-hexanediyl-bis-12-hydroxy octadecanamide (Figure 2c), N,N′hexane-1,6-diyl dioctadecanamide (Figure 2d), and 1,3,5-tris[4-(12hydroxy octadecanamide)phenyl] benzene (Figure 2e) were purified by recrystallization, and their purity was confirmed by performing thermal analysis. To estimate the sublimation or thermal-degradation behavior, we performed thermogravimetric (TG) analysis using an SII TG/DTA 3200 in N2. Furthermore, the phase-transition behavior was estimated by differential scanning calorimetry (DSC, SII DSC6200). 2.2. Procedures. The spin-cast films of the two gemini-type diamide derivatives containing two hydrocarbons and their mixtures with an organo-modified layered silicate were formed from a xylene/ ethanol (9:1 v/v) mixed solution at 140 °C; these samples dissolve at 100 °C. Organo-modified montmorillonite (MMT) was prepared using a surface-modification method from natural MMT and longchain quaternary ammonium cations at an oil/water interface.24 Spincast films of stearic acid and 12-hydroxystearic acid were formed from their respective hexane solutions at room temperature. Powder X-ray diffraction (XRD) measurements of both the bulk and cast film samples were obtained using an X-ray diffractometer (Rigaku, RintUltima III, Cu Kα radiation, 40 kV, 30 mA) equipped with a graphite monochromator. High-power wide-angle X-ray diffraction (WAXD) was performed using a Rigaku R-axis Rapid (Cu Kα radiation, 40 kV, 200 mA) with a rotating anode source and an imaging-plate detector. Infrared (IR) spectra of the samples were obtained using an IR spectrometer (Bruker AXS TENSOR II).

3. RESULTS AND DISCUSSION 3.1. Elucidation of the Origin of the Thixotropic Properties by Additive Molecules. The solvent gelation ability of molecular additives that can undergo hydrogen bonding is generally believed to originate from the formation of crystalline nanofibers in the solvent, and the reflow phenomenon upon the application of an external force occurs due to the disappearance of these nanofibers.12 Figure 3 shows the solvent gelation ability of the gemini-type thixotropic C


Langmuir


Figure 3. Photographs of the drop formation tendency of solvents containing amphiphile additives from a tilted medicine spoon.

Figure 4. AFM images of the cast films and powder X-ray diffraction profiles of the amphiphile additives used in this study.

agents and their corresponding raw surfactant materials. In this case, the critical gelation concentration was 2.10 wt %. The physical properties of these additives are well known. Geminitype diamides having two hydrocarbons with hydroxyl groups have excellent thixotropic ability with respect to the surrounding solvent and also have extreme durability as materials.13 This molecule has previously been utilized as a drip prevention agent for automobile body paints. 12Hydroxystearic acid, which is derived from castor oil, is also used as a household waste oil treatment agent but has inferior long-term stability.14 Although gemini-type diamides with stearyl chains exhibit minimal performance deterioration over time, their performance is inferior to those of other agents.13 Stearic acid, which is a standard substance that forms a monolayer on the surface of water and LB films, does not demonstrate solvent gelation ability by itself. Gelation agents are also specific to the solvent, gemini-type diamides with two hydrocarbons are effective gelation agents for phenyl-type solvents such as xylene, and 12-hydroxystearic acid is effective for the gelation of hexane. It is very important to note that because the gelation of a solvent is a macroscopic phenomenon it is not directly influenced by the change in the molecular species used as the gelation agent but instead corresponds to the existence of aggregates formed by the additive molecules. Figure 4 shows the nanofiber formation behavior and crystallinity of the gemini-type thixotropic agents and their corresponding raw surfactant materials. The AFM images shown in this figure demonstrate the structure of the cast film of each sample. The high-power WAXD results further indicated that all of the fibers were crystalline. All of the molecules other than stearic acid showed fiberization. In order to confirm the crystallinity, profiles are shown only in the middle-angle region. Therefore, it can be predicted that the solvent gelation ability of a gelation agent is derived from the formation of nanofibers based on intermolecular hydrogen bonding. Also, the packing structure of hydrophobic chains in

two diamide compounds is briefly described. Although there is the existence of a wide-angle shoulder of a 3.9 Å diffraction peak in the case of (12-OH-C18A)2-C6-amide and the existence of fine peaks in the wide angle range in the case of (C18A)2-C6amide, we considered that the presence of in-plane spacings of 4.6 and 3.9 Å in the former and of 4.6 Å in the latter as the main structure, respectively. Although it seems that the 3.9 Å diffraction peaks disappears at first glance, in fact it is due to the difference in the in-plane packing structure. As shown in the inset, it seems that the compound of the former forms a two-dimensional orthorhombic subcell and that the latter forms a two-dimensional hexagonal subcell.27,28 Fiberization/ gelation is believed to occur whether the hydrogen bonding site is a hydroxyl group in the hydrophobic chain or an amide bond site. Additionally, in the previous study,12 gemini-type diamides with two hydrocarbons were found to show extensive fiber development when 1 wt % organoclay was added as a growth aid. However, it is difficult to find a correlation between the length and/or thickness of the fibers and the thixotropic performance. Essentially, the microscopic fiber length and thickness might not have a strong effect on solvent gelation. Figure 5 shows the IR spectra of all of the samples, which provide evidence of hydrogen bonding. The spectra were recorded in two states: as KBr disks and cast films. Details of the detected bands are shown in Table 1. At the same time, its assignment is also shown. For example, detected wavenumbers of a carbonyl group in a fatty acid are confirmed at a reasonable position. The hydroxyl group and amide NH2 group bands of the gemini-type diamide with two hydrocarbons containing a hydroxyl group, the amide NH2 group band of the gemini-type diamide with two stearyl chains, and the hydroxyl group band of 12-hydroxystearic acid were shifted from 3700 cm−1 (representing the free groups) to 3200−3300 cm−1 (corresponding to the hydrogen-bonded groups). The D


Langmuir


Figure 6. (a) π−A isotherms of monolayers of the amphiphile additives on the water surface (15 °C) and (b) corresponding AFM images of the transferred monolayers on a solid with (transferring surface pressures of (12-OH-C18A)2-C6-amide, (C18A)2-C6-amide, 12hydroxy stearic acid, and stearic acid of 25, 30, 20, and 40 mN m−1, respectively).

Figure 5. IR spectra of (a) bulk (KBr disc) and (b) cast films of the amphiphile additives.

Table 1. Detected Bands List and Corresponding Assignment by IR Spectra additive amphiphiles

mode of vibration

vibrational frequency (cm−1)

(R)-12-hydroxystearic acid

ν(O−H) ν(CO) ν(CO) ν(N−H) ν(O−H) ν(CO) (amide I) δ(N−H) (amide II) ν(N−H) ν(CO) (amide I) δ(N−H) (amide II)

3300−3100 1698 1703 3305 3300−3100 1641 1546 3311 1634 1536

stearic acid (12-OH-C18A)2-C6-amide

(C18A)2-C6-amide

The monolayer on the water surface of 12-hydroxystearic acid shows a tendency to expand with respect to the condensed film of the Langmuir monolayer of stearic acid, which is well known. The tendency, especially in the low-pressure region, is remarkable. This behavior seems to be based on the fact that the hydroxyl group at the 12th position acts as a hydrophilic group in the low-pressure region and forms an extreme tilted conformation as judged from the molecular structure. Therefore, this result is helpful for considering the behavior of gemini-type diamides having two hydrocarbons on the water surface. An extremely expanded phase is not observed in the gemini-type diamide derivative composed of stearic acid. On the other hand, in a gemini-type diamide derivative of 12hydroxystearic acid, in addition to the expanded behavior, several phase transitions accompanying compression are confirmed. In this case, intermolecular hydrogen bonding may be a phenomenon that occurs when compression progresses. Since the amide moiety is immobilized beneath the surface of the water, only two molecules, the gemini-type diamide with a hydroxyl group and 12-hydrostearic acid, could form hydrogen bonds between their hydrophobic chains in this environment. The gemini-type diamide with a hydroxyl group in its hydrophobic chains and 12-hydroxystearic acid showed distinct monolayer fiber morphologies in the condensed monolayer state. The gemini-type diamide without a hydroxyl group is formed of aggregates of microcrystals, and stearic acid formed a sheet. The dependence of surface pressure, subphase temperature, and concentration of trifluoroacetic acid (TFA), which

gelation of the solvent shown in Figure 3 exhibited reflow when an external pressure such as stirring was applied. This thixotropic property probably results from the relatively low degree of development of the hydrogen bond network. Reflow is believed to occur as a result of the breakage of hydrogen bonds and the disappearance of nanofibers when external pressure is applied. These four molecules contained two kinds of hydrogenbonding sites, that is, the hydroxyl group in the hydrocarbon chains and the amide group. This provides a means to selectively evaluate the influence of the hydroxyl group at the 12 position in the hydrophobic chain. Figure 6 shows the π−A isotherms of the monolayers of these four compounds on the surface of water and AFM images of the monolayers obtained by transferring these to the solid substrates by the LB method. E


Langmuir


disrupts hydrogen bonding, in the spreading solvent29 on the morphology of the fibrous monolayer of the gemini-type diamide containing a hydroxyl group was investigated (Figure 7). In the study of the dependence of the morphology on

intermolecular distance and remarkable hydrogen bonding accompanied by the increasing value of surface pressure. When hydrogen bonding at the molecular level cooperates to lead to mesoscopic nanofiber formation, negative compressibility before collapse may be observed on the π−A curve. Generally, the decrease in surface pressure with increasing monolayer molecular density is owing either to a prestate of collapse or to a rapid molecular rearrangement in which the compression is not fast enough to equilibrate. It seems that this is observed in the case where the subphase temperature is increased and the molecular mobility is improved or the case where hydrogen bonds are difficult to form as a result of the high TFA concentration. Therefore, this is probably due to the necessity of rearrangement in the preliminary stage in which the intermolecular distance approaches and fiberization due to hydrogen bond formation occurs. In other words, this negative compressibility behavior indicates that the rearrangement in the preliminary stage in which fiberization occurs is a rapid molecular rearrangement that is not fast enough for the compression to equilibrate. Although the hydrogen bonding is relative weak due to molecular motion and the effect of solvent before the appearance of the negative compressibility behavior, it seems possible to form a hydrogen bond network after rearrangement at this transition point. In other words, it seems that negative compressibility behavior will be observed in the case in which the hydroxyl group at the 12 position in the hydrophobic chain whose phases of groups are not aligned starts to interact within the proximity of the intermolecular distances. The same evaluation was also performed on monolayers of 12-hydroxystearic acid at the air/water interface (Figure S2). The resulting π−A isotherms confirmed the dependence of the fiber morphology on surface pressure, subphase temperature, and TFA concentration in this gelation agent as well. These results mean that fiberization takes place only with hydrogen bonding between the 12-hydroxy stearyl chains, with the amide moiety immobilized under the subphase. The fiberization is simultaneously inhibited by the widening of the intermolecular distance, the molecular motion, and the influence of the solvent inhibiting hydrogen bonding between hydroxyl groups. If this fiberization shows a direct/ indirect contribution to thixotropy induction, then it can be understood that the role of this hydrophobic chain having an asymmetric carbon is important. The carbon atom to which the hydroxyl group at the 12position is bonded is an asymmetric carbon. Although the 12hydroxystearic acid used in this study was the pure R form, we wished to investigate the effect of molecular chirality on fibrillation behavior and gelation behavior. Figure 8 shows the CD spectra of the solutions and cast films of these four molecules. In addition, microscopy images of the cast films are shown in Figure 4b. Initially, no signal was observed in the solution CD spectra of any of the samples. Molecular chirality itself was considered to be absent. In contrast, a clear righthandedness signal30 was observed for one of the cast films; a positive Cotton effect was confirmed only in the film of the gemini-type diamide containing a hydroxyl group. Accordingly, when the fiber structures of the cast films were observed in detail by microscopy techniques, the gemini-type diamide having a hydroxyl group was confirmed to be a right-handed helical fiber by SEM, TEM, and AFM observation. The observed chirality corresponded to the formation of the fiber structure. The gemini-type diamide derivatives containing stearyl chains formed linear fibers and had no CD signal.

Figure 7. π−A isotherms of monolayers of (12-OH-C18A)2-C6-amide on the water surface and the corresponding AFM images of transferred monolayers on a solid under various conditions. (a) Surface pressure dependence (15 °C, transferring surface pressures of 7, 12, and 25 mN m−1), (b) subphase temperature dependence (5, 15, 25, and 35 °C, transferring surface pressures of 12, and 15 mN m−1), and (c) TFA content dependence in spreading solutions (15 °C, transferring surface pressure of 15 mN m−1).

surface pressure, the surface pressure was increased, and when the intermolecular distance narrowed, an extensive development of fibers was observed. On the other hand, at low surface pressure, fiber formation was limited. Furthermore, when the subphase temperature was increased and the molecular mobility increased, the fiber morphology tended to disappear. In addition, when the concentration of TFA in the spreading solvent was increased, the development of the fibers was hindered. Overall, these evaluations of the monolayer made it clear that hydrogen bonding between the hydroxyl groups at the 12 position plays a major role in fiberization. Although a detailed explanation of π−A isotherms and AFM images other than the development and collapse of monolayer fiber was omitted due to the existence of previous reports,12,13 in this article several interesting behaviors are also confirmed in this figure. As shown in the previous report,12 it is thought that a mesoscopic fiber monolayer of the gemini-type diamide derivative used in this study will be formed via the equilibrium state as a two-dimensional transition based on the approach of F


Langmuir


Figure 8. CD spectra of (a) solutions and (b) cast films of the amphiphile additives used in this study. (c) Morphological images of the cast films of amphiphile additives observed by microscopy techniques. (d) Histograms of the helical nanofiber pitch and fiber width of (12-OH-C18A)2-C6-amide in its cast films.

Naturally, stearic acid did not form fibrils and did not show a CD signal. However, 12-hydroxystearic acid, which did not show a CD signal, formed a right-handed helical fiber. According to the literature,31 this compound shows a weak CD signal in the infrared region. A histogram of the helical pitch and fiber width of the gemini-type diamide containing 12-hydroxy stearyl chains is shown in Figure 9c. The helical pitch was distributed at the 220 nm center, and the fiber width was 120 nm on average. Figure 9 shows the CD spectra and AFM images of the cast films formed under different conditions from the gemini-type diamide containing 12hydroxy stearyl chains. The fibrous morphology was confirmed, and a clearer positive Cotton effect appeared when the solution casting temperature was reduced to 100 °C. Additionally, when organoclay as was added as a growth aid, helical fiber formation occurred regardless of concentration, and a right-handed CD signal was also confirmed. In this feature article, mostly the suprascale chirality and nanoscale chirality are discussed. Thus, it is reasonable that they could not observe the CD in solution but in the film or gels. This is because in a film the moleculess are packed to form a supramolecular structure.30 As mentioned above, gelation is thought to occur via association of the homogeneously right-handed helices to form nanofibrous bundles that efficiently incorporate solvent molecules into a spongelike structure (Figure 10).32,33 However, when twisting tissue or rubber to create such a

Figure 9. Casting temperature dependence and concentration dependence of organo-MMT as a growth aid of cast films of (12OH-C18A)2C6-amide with a view toward CD spectra and morphological images.

helix, considerable force is required (Figure S3), and the helical fiber itself is expected to exist in a very high density packing state. That is to say, the helix is also a kind of stable structure. For the question “why does it to form a helix?”, there are examples of plants that point toward the sun as shown in Figure S3, and there are also purposes in stabilizing the structure. The helix as a result of the tight packing of the molecular groups leads to dense filling locally. However, by forming a helix, at the same time the entire fiber becomes difficult to stretch and tends to be entangled by bending. The difference between the helical fiber and the linear fiber with only the amide moiety is manifested as the difference between the packing density and the solvent molecule uptake efficiency into the organized fibrous structures. That is to say, on the microscopic scale, a densely packed crystalline arrangement is formed, and CD-active right-wound helical fibers are formed on the mesoscopic scale. Apparently, the formation of a crystalline arrangement is a result of hydrogen bonding, and the crystallinity itself is not the origin of nanofiber formation G


Langmuir


Scheme 1. Condensation Reaction of a Trefoil-like Triamide Containing Three 2-Hydroxy Stearyl Chains as a New Thixotropic Additive

Figure 10. Model diagrams summarizing the elucidation of the origin of the thixotropic properties of the additive surfactants through hydrogen bonding.

and solvent-thixotropy-inducing properties. The direct cause of the thixotropy may be the difference in the uptake efficiency of the helical fiber aggregates toward the solvent molecules. The helical fiber cannot be extended along the long axis. On the other hand, the linear fibers are relatively elongated. The thixotropy based on mesoscopic helix formation originates from the ability of the nanofibers to become easily intertwined to form a spongelike organization capable of absorbing solvent molecules. However, the resulting hydrogen-bonding networks may have a low degree of development. Therefore, when external pressure is applied to the formed solvent gel, the hydrogen bonds are broken, the fibers disappear, the solvent molecules are released, and thus reflow occurs. 3.2. Development of a Triamide Derivative as a Thixotropic Additive and Its Formation of a Supramolecular Structure. As the next step, the results of the investigation into the origin the thixotropic property were fed back, and the development of a higher-performance thixotropic additive molecule was attempted. Increasing the number of asymmetric-carbon-containing 12-hydroxyl stearyl chains promotes the formation of an intermolecular hydrogenbonding network. For this reason, the synthesis of a triamide derivative containing three hydrocarbons, in which three 12hydroxy stearylates were introduced into the trefoil-like core by amide bonds (Scheme 1), was attempted, and the structure and physical properties of the resulting compound were evaluated. The trefoil-like triamide derivative with three 12-hydroxy stearyl chains showed distinct melting and crystallization peaks in the DSC measurement (Figure 11a). Its melting temperature was about 150 °C, and large hysteresis of the crystallization temperature was observed at about 100 °C. This molecule with a bulky core seemed to require a retention time for crystallization. In the thermogram of the first heating

Figure 11. (a) DSC thermogram and (b) XRD profile with (c) corresponding packing model of the newly synthesized trefoil-like triamide derivative containing three hydrocarbons.

process, the compound showed two endothermic peaks, which coalesced on the higher-temperature side during the second heating process. It can be inferred that the endothermic peak on the low-temperature side during the first heating process was the phase-transition peak of rearrangement. The crystals formed after synthesis were believed to rearrange during the initial heating, with a more stable crystal system appearing only after the subsequent melting−crystallization process. In the structural investigation using powder XRD, a clear crystalline reflection was observed at 4.3 Å (Figure 11b). This distance was assigned as the hexagonal packing distance between alkyl chains rather than π−π stacking of the cores (Figure 11c). During the in situ XRD measurement, multiple long spacings appeared at around d002 = 26 Å at 135 °C (Figure S4a; the d001 peak can be confirmed in a lower-angle region by small-angle X-ray scattering in Figure 4b). With respect to the packing of diamide and triamide cases, the double-layer structure of the compounds is considered.12,13 Meijier34 and Liu35 reported that an extended conformation is typical of C3 molecules, as for a triamide case. When they form gels, they usually form a supramolecular polymer. In particular, why those molecules can be formed into a helical twist or nanofibers had been explained. In the case of our study, the estimated value of the long period should correspond to the spacing of the layer H


Langmuir


structure along the c-axis direction. This long spacing did not disappear after the melting−crystallization process. That is to say, the formation of a single-layer structure was induced during the first melting after synthesis in this compound, and this molecular arrangement subsequently remained stable. The multiple similar layered periods at close spacing were expected to be due to the presence of multiple similar periodic structures with slight distortions based on the complex molecular structure of this compound. Figure 12 shows the

compound has a chromophore, the UV−vis spectrum was simultaneously shown in this figure. This molecule has an absorption band in the ultraviolet region at 280 nm. In this case, the nanofiber form in this cast film that can be confirmed by SEM observation was not always linear. Although the nanofiber form is slightly unclear at this observation size in the cast film because it may be derived from a complicated molecular structure, there is almost no doubt that the CD signal appeared because of the formation of a higher-order structure. When this newly synthesized molecule was spread on the surface of water, a monolayer was formed in the low-surfacepressure region below 10 mN m−1 (Figure 13a). When this

Figure 12. (a) IR spectra of the trefoil-like triamide derivative in its cast film. (b) AFM image of the cast film and the gelation behavior of solvent upon addition of the trefoil-like triamide synthesized in this study. (c) CD spectra, SEM image, and UV−vis spectra of the cast film of the trefoil-like triamide derivative.

Figure 13. (a) π−A isotherm of the monolayer of the trefoil-like triamide derivative containing three alkyl chains on the water surface (15 °C). (b) AFM images of the monolayer of the trefoil-like triamide derivative on a solid (transferring surface pressure = 5 mN m−1). (c) In-plane XRD profile of Langmuir−Blodgett multilayers of the trefoillike triamide derivative.

IR spectrum of the trefoil-like triamide derivative with three 12-hydroxy stearyl chains. The band of the hydroxyl group at the 12 position of the hydrophobic chain and the −NH2 group of the amide group shifted from 3700 cm−1, corresponding to the free functional groups, to 3300 cm−1, corresponding to the hydrogen-bonded functional groups (Figure 12a). The cast film prepared from a solution of the compound in a xylene/ ethanol = 9:1 solvent mixture (140 °C) contained dense, fibrous structures, as shown in the AFM image of Figure 12b. In this case, the critical gelation concentration was 1.65 wt %. The amount of this molecule required to gel the organic solvent efficiently was only 78% with respect to the amount of gemini-type diamide containing 12-hydroxyl stearyl chains needed. In addition, the cast film showed a clear but complex CD spectral signal (Figure 12c). On the other hand, no signal appeared in the CD spectrum of the solution. Since this

organized film was transferred onto a solid substrate for morphological evaluation, a fibrous structure with monolayer thickness was observed (Figure 13b). This structure was thought to be formed by the interaction between hydrophobic chains via hydrogen bonding between the hydroxyl groups at the 12 position. From the LB film on which the fibrous monolayer was deposited, crystalline reflection by in-plane Xray diffraction was confirmed. This result seemed to provide evidence that crystalline nanofibers were formed (Figure 13c). In the IR spectrum of the X-type multilayers created from the corresponding monolayer, the appearance of a band originating from hydrogen bonding between the hydroxyl groups at the 12 I


Langmuir


position in the hydrophobic chain was confirmed (Figure 14a). In addition to this, a band shift corresponding to the change

Figure 14. (a) IR spectra of the LB multilayers of the trefoil-like triamide derivative. (b) Schematic models of the hydrogen-bonding network as a supramolecular assembly in the organized molecular films of the trefoil-like triamide derivative.

from a free −NH2 group to a hydrogen-bonded one was also confirmed. Since the multilayer film was an X-type film obtained by the horizontal lifting method, hydrogen bonding between the amide moieties of different layers was not possible. Therefore, it was predicted that the hydrogen bonding interaction between the hydrophilic groups developed in the in-plane direction (Figure 14b). That is to say, in the two-dimensional field at the air/water interface the core portion of the molecule was contiguously penetrated to form a supramolecular assembly close to each molecule, which then hierarchically grew into a mesoscopic nanofiber. The thixotropic ability became extremely strong due to the development of an extensive hydrogen-bonding network. The helical nanofibers became denser, chirality was imparted by the CD-active structures, and the solvent molecules were effectively incorporated into spongelike entangled fiber aggregates as a result of the increase in the number of hydrogen-bonding sites based on the use of three asymmetriccarbon-containing hydroxy stearyl chains. The above results are summarized in Figure 15. Diamidebased thixotropic additives influence physical properties through the growth of nanofibers derived from hydrogen bonding between 12-hydroxy stearyl chains. In this case, the thixotropic activity improves when these nanofibers intertwine to form a spongelike structure capable of efficiently capturing solvent molecules in which the fibers have high crystallinity on the microscopic scale and right-handed helical fibers form on the mesoscopic scale. The trefoil-like three-chain triamide into which three 12-hydroxy stearyl chains were introduced formed crystalline nanofibers with CD spectral activity. The amount of triamide required to achieve sufficient gelation was only twothirds that of the diamide. Overall, the thixotropic ability became extremely high due to the extensive development of hydrogen-bonding networks, the increased helical nanofiber density, the chirality imparted by the CD-active structure, and the effective incorporation of solvent molecules into the spongelike entangled fiber aggregates due to the increase in the

Figure 15. Schematic illustration of the summary of this study.

number of hydrogen bonding sites based on the use of triplehydroxy stearyl chains containing an asymmetric carbon. In addition, the formation of a two-dimensional crystalline supramolecular assembly derived from hydrogen bonding with the bulky planar core in the molecule was confirmed.

4. SUMMARY AND CONCLUSIONS In this study, the growth of helical nanofibers of amphiphilic diamide derivatives with two asymmetric carbons in their two 12-hydroxystearyl chains was investigated. A gemini-type surfactant obtained by the bimolecular condensation of 12hydroxystearic acid with hexamethylenediamine imparted thixotropic ability to the surrounding solvent. Although this property was also observed for 12-hydroxystearic acid, stearic acid could not impart thixotropic properties to the surrounding solvent. While the remarkable gelation was directly related to the hierarchical organization of the molecules into crystals and subsequently helical nanofibers in the solvents, the fiber growth itself was not the origin of the thixotropic ability. On the basis of the objective experimental results and circumstantial evidence, it is almost certain that the ability to impart thixotropy originates from the structure of the helical fiber. The thixotropic fiber organization was based on intermolecular hydrogen bonding between amphiphiles with 12-hydroxy stearyl chains. In particular, we find that hydrogen bonding between the −OH groups at the 12 position of the hydrophobic chains enabled the formation of fibers in the monolayers. This was confirmed by selectively evaluating the result of the interaction between the 12-hydroxy stearyl chains by experiments on monolayers on the water surface and LB J


Langmuir


films. With a study on helical nanofibers, even after the addition of organo-modified clay as a growth aid, a positive Cotton effect of CD spectra was still observed in the helical nanofiber film. It was shown that nanofibers without a helical conformation could not impart thixotropy to the solvent despite extensive fiber growth; thus, it seems that the chirality resulting from the mesoscopic structure is the driving force. In this case, it was unclear whether the right-handedness molecular chirality directly contributed to the mesoscopic growth of right-handed helices in a hyperhierarchical manner. On the other hand, because of molecular cooperation phenomena, the possibility that the chirality at the molecular level influenced morphological growth cannot be denied. The elucidation of this relationship will be an extremely interesting academic task. Furthermore, to further the development of highly functional thixotropic agents, a trefoil-like triamide derivative containing three chains was synthesized. By using this molecule, the solvent gelation ability was improved, and a supramolecular assembly was formed in the corresponding two-dimensional film.

■

(3) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Helical Polymers: Synthesis, Structures, and Functions. Chem. Rev. 2009, 109, 6102−6211. (4) Sun, R.; Xue, C.; Ma, X.; Gao, M.; Tian, H.; Li, Q. Light-Driven Linear Helical Supramolecular Polymer Formed by MolecularRecognition-Directed Self-Assembly of Bis(p-sulfonatocalix [4] arene) and Pseudorotaxane. J. Am. Chem. Soc. 2013, 135, 5990−5993. (5) Lee, E.; Hammer, B.; Kim, J. K.; Page, Z.; Emrick, T.; Hayward, R.; Ryan, C. Hierarchical Helical Assembly of Conjugated Poly(3hexylthiophene)-block-poly(3-triethylene glycol thiophene) Diblock Copolymers. J. Am. Chem. Soc. 2011, 133, 10390−10393. (6) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. A Helical Polymer with a Cooperative Response to Chiral Information. Science 1995, 268, 1860−1866. (7) White, C. T.; Robertson, D. H.; Mintmire, J. W. Helical and Rotational Symmetries of Nanoscale Graphitic Tubules. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 5485−5488. (8) Harada, Y.; Ohara, O.; Takatsuki, A.; Itoh, H.; Shimamoto, N.; Kinosita, K. Direct observation of DNA rotation during transcription by Escherichia coli RNA polymerase. Nature 2001, 409, 113−115. (9) Kim, J. L.; Nikolov, D. B.; Burley, S. K. Co-Crystal Structure of TBP Recognizing the Minor-Groove of a TATA Element. Nature 1993, 365, 520−527. (10) Sakurai, S. I.; Okoshi, K.; Kumaki, J.; Yashima, E. Twodimensional surface chirality control by solvent-induced helicity inversion of a helical polyacetylene on graphite. J. Am. Chem. Soc. 2006, 128, 5650−5651. (11) Kumaki, J.; Nishikawa, Y.; Hashimoto, T. Visualization of single-chain conformations of a synthetic polymer with atomic force microscopy. J. Am. Chem. Soc. 1996, 118, 3321−3322. (12) Iizuka, M.; Nakagawa, Y.; Ohmura, K.; Satou, E.; Fujimori, A. Two-dimensional Growth of Crystalline Nanofiber Fabricated from Gemini-type Amphiphilic Diamide Derivative Inducing the Thixotropic Property. J. Colloid Interface Sci. 2017, 498, 64−75. (13) Iizuka, M.; Nakagawa, Y.; Moriya, Y.; Satou, E.; Fujimori, A. 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−. Bull. Chem. Soc. Jpn. 2018, 91, 813−823. (14) Bhattacharjee, S.; Bhattacharya, S. Phthalate mediated hydrogelation of a pyrene based system: a novel scaffold for shapepersistent, self-healing luminescent soft material. J. Mater. Chem. A 2014, 2, 17889−17898. (15) Uskokovic, V. Composites comprising cholesterol and carboxymethyl cellulose. Colloids Surf., B 2008, 61, 250−261. (16) Tamura, T.; Suetake, T.; Ohkubo, T.; Ohbu, K. Effect of AlkaliMetal Ions on Gel Formation in the 12 Hydroxystearic acid Soybean Oil System. J. Am. Oil Chem. Soc. 1994, 71, 857−861. (17) Toro-Vazquez, J. F.; Morales-Rueda, J.; Mallia, V. A.; Weiss, R. G. Relationship Between Molecular Structure and Thermo-mechanical Properties of Candelilla Wax and Amides Derived from (R)-12Hydroxystearic Acid as Gelators of Safflower Oil. Food Biophys. 2010, 5, 193−202. (18) Kimura, F.; Umemura, J.; Takenaka, T. FTIR-ATR Studies on Langmuir-Blodgett-Films of Stearic-Acid with 1−9 monolayers. Langmuir 1986, 2, 96−101. (19) Kanicky, J. R.; Shah, D. O. Effect of degree, type, and position of unsaturation on the pK(a) of long-chain fatty acids. J. Colloid Interface Sci. 2002, 256, 201−207. (20) Thompson, M.; Arthur, C. L.; Dhaliwal, G. K. Liquid-Phase Piezoelectric and Acounstic Transmission Studies of Interfacial Immunochemistry. Anal. Chem. 1986, 58, 1206−1209. (21) Gaines, G. L., Jr. Insoluble Monolayers at Liquid Gas Interfaces; Wiley: New York, 1966. (22) Blodgett, K. Monomolecular films of fatty acids on glass. J. Am. Chem. Soc. 1934, 56, 495−495. (23) Petty, M. C. Langmuir-Blodgett Films; Cambridge University Press: New York, 1996.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02090. Temperature-controlled NMR spectra of the trefoil-like triamide containing three 12-hydroxy stearyl chains; π− A isotherms and corresponding AFM images of the monolayer of 12-hydroxystearic acid; formation of helical morphologies in nature; temperature-controlled XRD of the trefoil-like triamide containing three 12hydroxy stearyl chains in the bulk; and SAXS profile of the trefoil-like triamide derivative in the bulk (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel and Fax: +81 48 858 3503. E-mail: [email protected]. ORCID

Yuji Shibasaki: 0000-0002-9176-0667 Atsuhiro Fujimori: 0000-0002-2503-8529 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors appreciate the support of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) for funding provided by a Grant-in-Aid for Scientific Research (C, 17K05986 (A.F.)). Furthermore, the authors express their gratitude to Prof. Toru Mizuki at Toyo University for CD spectra and SEM observations. The authors deeply appreciate Professor Hiroshi Katagiri, Yamagata University, for useful discussions.

■

REFERENCES

(1) Harley, C. B.; Futcher, A. B.; Greider, C. W. Telomeres shorten during aging of human fibroblasts. Nature 1990, 345, 458−460. (2) Doyle, D. A.; Cabral, J. M.; Pfuetzner, R. A.; Kuo, A. L.; Gulbis, J. M.; Cohen, S. L.; Chait, B. T.; MacKinnon, R. The structure of the potassium channel: Molecular basis of K+ conduction and selectivity. Science 1998, 280, 69−77. K


Langmuir


(24) Fujimori, A.; Arai, S.; Kusaka, J.; Kubota, M.; Kurosaka, K. Formation and Structure of Langmuir-Blodgett Films of Organomodified Alminosilicate with High Surface Coverage. J. Colloid Interface Sci. 2013, 392, 256−265. (25) Fujimori, A.; Sugita, Y.; Nakahara, H.; Ito, E.; Hara, H.; Matsuie, N.; Kanai, K.; Ouchi, Y.; Seki, K. Change of molecular packing and orientation from monolayer to multilayers of hydrogenated and fluorinated carboxylates studied by In-plane X-ray diffraction together with NEXAFS spectroscopy at CK-edge. Chem. Phys. Lett. 2004, 387, 345−348. (26) Fujimori, A.; Araki, T.; Nakahara, H.; Ito, E.; Hara, M.; Ishii, H.; Ouchi, Y.; Seki, K. In-plane X-ray diffraction and polarized NEXAFS spectroscopic studies on the organized molecular films of fluorinated amphiphiles with vinyl esters and their comb-polymers. Chem. Phys. Lett. 2001, 349, 6−12. (27) Vand, V.; Vell, I. P. A direct determination of the crystal structure of the β form of trilaurin. Acta Crystallogr. 1951, 4, 465− 469. (28) Fujimori, A.; Saitoh, H.; Shibasaki, Y. Influence of Molecular Arrangement on the γ-Ray-Irradiation Solid-State Polymerization of 1-Octadecyl Vinyl Ether with a Characteristic Polymorphism. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3845−3853. (29) Pu, L.; Sun, Y.; Zhibing, Z. Hydrogen Bonding in Hydrates with one Acetic Acid Molecule. J. Phys. Chem. A 2010, 114, 10842− 10849. (30) Liu, M. H.; Zhang, L.; Wang, T. Supramolecular Chirality in Self-Assembled Systems. Chem. Rev. 2015, 115, 7304−7397. (31) Sato, H.; Sakurai, T.; Yamagishi, A. Comparison of Vibrational Circular Dichroism between the Langmuir−Blodgett Films and Gels of 12-Hydroxyoctadecanoic Acid. Chem. Lett. 2011, 40, 25−27. (32) Kudo, Y.; Sakuragi, M.; Hashida, S.; Kuwahara, R.; Ishi-I, T.; Masunaga, H.; Sakurai, K. Flexibility and local structure of a wormlike cylinder of self-assembled discotic triazine triamide. Polym. J. 2010, 42, 812−817. (33) Hanabusa, K.; Suzuki, D. Gelators Bringing About Gelation and Thickeners Raising Viscosity. Sen'i Gakkaishi 2009, 65, 159−165. (34) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167−170. (35) Huang, X.; Li, C.; Jiang, S. G.; Wang, X. S.; Zhang, B. W.; Liu, M. H. J. Am. Chem. Soc. 2004, 126, 1322−1323.

L


Elucidation of the Origin of Thixotropic-Inducing Properties of Additive

Recommend Documents