Structural Analysis of Aggregates Formed by Linear- and Star-type Quaternary Ammonium Salt-Based Trimeric Surfactants Using Rheology and Small-Angle X-ray Scattering.

We performed a structural analysis of aggregates formed by two types of trimeric surfactants based on quaternary ammonium salts─linear-type 3Cnlin-s-Q and star-type 3Cntris-s-Q─featuring varying alkyl chain lengths (n) and spacer chain lengths (s) in aqueous solutions. We performed rheology, dynamic light scattering, and small-angle X-ray scattering measurements on the trimeric surfactants and investigated the effects of the alkyl chain length, spacer chain length, spacer skeleton structure, and surfactant concentration on their aggregation behavior. Linear-type 3C12lin-3-Q transitioned from gel solutions to worm-like micelles at high concentrations, and 3C14lin-3-Q became gel solutions over a wide range of concentrations. In contrast, all other studied surfactants formed ellipsoidal micelles. The minor and major axes of the ellipsoidal micelles formed by liner-type 3Cnlin-3-Q increased with the increasing alkyl chain length. As the spacer chain length of 3Cnlin-s-Q increased from 3 to 6, and as the spacer skeleton expanded from linear-type 3Cnlin-s-Q to star-type 3Cntris-s-Q, the surfactants formed ellipsoidal micelles without the formation of aggregates with a high-order structure, demonstrating this behavior over a broad concentration range.

Micelle formation of sodium taurolithocholate.

The proportion of sodium taurolithocholate (NaTLC) is extremely low in human bile salts. NaTLC forms aggregates with other lipids in the bile and functions as an emulsifying and solubilizing agent. The molecular structure of NaTLC contains hydrophilic hydroxyl and sulfonic acid groups at both ends of the steroid ring. This molecular structure is similar to bolaform amphiphilic substance having hydrophilic groups at both ends due to the characteristics of its molecular structure. This study investigated the aggregate properties of the NaTLC using surface tension measurements, light scattering, small-angle X-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM). Surface tension measurement showed that the surface tension of the NaTLC solution decreased to 54 mN m-1. The concentration that showed the minimum surface tension corresponded to the critical micelle concentration (CMC: 0.6 mmol L-1, 308 K) determined by the change in light scattering intensity. On the other hand, the degree of counterion (sodium ions) binding to the micelles increased with increasing NaTLC concentration. SAXS and cryo-TEM measurements showed that the NaTLC formed large string-like micelles. The surface activity and large aggregates showed the potential for use as biosurfactants. However, because of the relatively low solubility of NaTLC in water, its use as a biosurfactant is limited to a narrow concentration range.

Stability and Structural Analysis Using Small-Angle Neutron Scattering for Foam of Homogeneous Polyoxyethylene-Type Nonionic Surfactants with Multibranched Chains.

Small-angle neutron scattering can provide insight into the microstructure of the surfactant-stabilized foam. In this study, small-angle neutron scattering in combination with other techniques was employed to determine the microstructure of the foams stabilized using novel homogeneous polyoxyethylene (EO) alkyl ether-type nonionic surfactants with multibranched double chains (bC7-bC9EO12). Similarly, homogeneous EO-type nonionic surfactants with linear double chains (C8-C8EO12) and a linear single chain (C18EO12) were used. bC7-bC9EO12 and C8-C8EO12 surfactants with branched hydrophobic chains or double chains increased the foam stability and suppressed the draining. Furthermore, they formed rod-like micelles, and C18EO12 formed spherical micelles in the bulk solution. The foam film containing the plateau border contained micelles identical with those found in the bulk solution. For bC7-bC9EO12 and C8-C8EO12, the average radius of the bubbles immediately after foaming was of the order of hundreds of µm. Finally, these radii grew to the order of thousands of µm. Thus, a significant correlation was observed between the micellar structure and the stability of these foams.

Micelle Formation of Dodecanoic Acid with Alkali Metal Counterions.

Alkali series with different atomic numbers affect the physicochemical properties of aqueous solutions. The micellar properties of aqueous solutions of dodecanoate as surfactants were measured by changing the counterions (C12-Na, C12-K, C12-Rb, and C12-Cs). A plot of Krafft temperature vs. alkali metal atomic number showed a downward convex curve, with its minimum temperature (20°C) in the C12-K system. By contrast, a plot of the critical micelle concentration (CMC) vs. alkali metal atomic number exhibited an upward convex curve with the maximum CMC (25.6 mmol L-1) at C12-K. Furthermore, the minimum surface tension (γ min ) of the solution at the CMC increased with increasing atomic number (C12-Na ≈ C12-K < C12-Rb < C12-Cs). The size of the dodecanoate micelles decreased with increasing atomic number. The ionization degree of the micelles also increased with increasing atomic number of the alkali metal. Small-angle X-ray scattering (SAXS) measurements revealed that alkali dodecanoate micelles formed spherical to ellipsoidal structures. In addition, micelles from the shell region showed large electrostatic repulsion, judging from the shape of the spectrum in the higher Q -1 region. From the measurement results of the solubilization of naphthalene into the micelles, the size of the micelles corresponded to the maximum solubilization quantity of naphthalene.

Catalytic activity of gold nanoparticles protected by quaternary ammonium salt-based gemini surfactants with different spacer structures.

The protection and stabilization of metal nanoparticles with gemini surfactants can greatly improve their catalytic and reductive activities as well as stability, expanding their practical applicability. In this study, gold nanoparticles were prepared using three quaternary ammonium salt-based gemini surfactants with different spacer structures (2C12(Spacer)) as protective agents, and their structures and catalytic activities were investigated. The size of the 2C12(Spacer)-protected gold nanoparticles decreased as the ratio of [2C12(Spacer)] to [Au3+] ([2C12(Spacer)] : [Au3+]) increased from 1 : 1 to 4 : 1. Furthermore, the stability of the gold nanoparticles was affected by the spacer structure and surfactant concentration. The gold nanoparticles protected by 2C12(Spacer) with a diethylene chain and oxygen atom in the spacer were stable even at low surfactant concentrations because the gemini surfactants sufficiently covered the surface of the gold nanoparticles, and the aggregation between the nanoparticles was suppressed. In addition, the gold nanoparticles protected by 2C12(Spacer) with an oxygen atom in the spacer exhibited high catalytic activities for the reduction reaction of p-nitrophenol and 1,1-diphenyl-2-picrylhydrazyl radical scavenging reaction because of their small size. Thus, we elucidated the effect of spacer structure and surfactant concentration on the structure and catalytic activities of gold nanoparticles.

Effect of Spacer Structures on the Interfacial Adsorption and Micelle Properties of Quaternary Ammonium Salt-Based Gemini Surfactants.

In this study, we synthesized quaternary ammonium salt-based gemini surfactants, 2C12(Spacer), with different spacer structures using ethylenediamine derivatives, and investigated their adsorption and aggregation properties by measuring their electrical conductivity, surface tension, fluorescence, and viscosity in conjunction with dynamic light scattering and small-angle X-ray scattering studies to investigate the effect of spacer structures on the properties of the gemini surfactants. The gemini surfactants with spacers containing nitrogen and oxygen atoms were highly soluble in water, whereas those with rigid spacers containing diethylene and triethylene chains exhibited low water solubility. The adsorption and orientation of the gemini surfactants at the air/water interface were significantly affected by the spacer length. Among the synthesized gemini surfactants, the one with the N,N'-dimethylpiperazine spacer showed the highest surface activity. In contrast, the gemini surfactant with the 1-methyl-4-[2-(N,N-dimethylammonio)ethyl]piperazin-1-ium spacer containing an ethylene chain attached to the amino group in the N,N'-dimethylpiperazine spacer (2C12(2/2-N-2)) adsorbed efficiently. However, due to the increased spacer length, this surfactant was unable to orient efficiently at the air/water interface.

Adsorption and Aggregation Behavior of Mixtures of Quaternary-Ammonium-Salt-Type Amphiphilic Compounds with Fluorinated Counterions and Surfactants.

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C4 FSA, C8 FSA, and C4 NTf2) or gemini compounds (C10-2-C4 FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C12EO6), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C12TAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C12Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA-) or bis(trifluoromethanesulfonyl)imide (NTf2-) as counterions. The mixtures were analyzed for surface tension, viscosity, electrical conductivity, and pyrene fluorescence, in addition to evaluation by cryogenic transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. Our results showed that the surface tension depended on the surfactant structure. For the mixture of C8 FSA and SDS, as the SDS concentration increased, the surface tension first decreased and became constant at the critical micelle concentration (CMC). In this concentration range, C8 FSA and SDS were approximately equimolar (2.5 mmol dm-3), the mixture adsorbed efficiently at the air-water interface, and vesicles and linear-type micelles were formed in the solution owing to the decreased electrostatic repulsion between the hydrophilic groups. As the SDS concentration further increased, the surface tension increased and reached another constant value. The C8 FSA at the interface was replaced by SDS and the aggregates transformed into spherical micelles. The surface tension plot of the mixture of the amphiphilic compounds and C12Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C10-2-C4 FSA, indicating that the gemini compounds offer excellent adsorption and orientation at the air-water interface. It was revealed that the quaternary-ammonium-salt-type amphiphilic compounds in this study acted as ionic liquids on their own and as surfactants in aqueous solution. Further, they could improve the surface activity of conventional ionic surfactants.

Micelle Formation of Monoammonium Glycyrrhizinate.

Monoammonium glycyrrhizinate is produced by the neutralization of glycyrrhizic acid from plant licorice with ammonia. In this study, the physicochemical properties of aqueous monoammonium glycyrrhizinate were investigated from the viewpoint of surface chemistry. The structure of the amphiphilic molecule is bola type, comprising two glucuronic acid moieties having two carboxylic acids groups and an aglycone part having a carboxylic acid at the opposite end of the molecule from the glucuronic acids. We found that the physicochemical properties of aqueous monoammonium glycyrrhizinate are dependent on the ionization of the carboxylic acid groups. The solubility of monoammonium glycyrrhizinate gradually increased above pH 4 in the buffer solution. The critical micelle concentration (CMC) and surface tension at the CMC (γCMC) of monoammonium glycyrrhizinate were determined by the surface tension method to be 1.5 mmol L-1 and 50 mN m-1 in pH 5 buffer and 3.7 mmol L-1 and 51 mN m-1 in pH 6 buffer, respectively. The surface tension gradually decreased with increasing concentration of monoammonium glycyrrhizinate in the pH 7 buffer, but the CMC was not defined by the curve. Light scattering measurements also did not reveal a clear CMC in the pH 7 buffer. The ionization of the carboxylic acid groups in the bola-type amphiphilic molecule with increasing pH is disadvantageous for micelle formation. Cryo-transmission electron microscopy showed that monoammonium glycyrrhizinate forms rod-like micelles in pH 5 buffer, and small angle X-ray scattering experiments confirmed that the average micellar structure was rod-like in pH 5 buffer. Thus, it was found that monoammonium glycyrrhizinate can form micelles only in weakly acidic aqueous solutions.

Fine Tunable, Redox Active Octapalladium Chains Supported by Linear Tetraphosphines, Leading to Dynamically 1D Self-Assembled Coordination Polymers.

A series of the octapalladium chains supported by meso-Ph2 PCH2 P(Ph)CH2 P(Ph)CH2 PPh2 (meso-dpmppm) ligands, [Pd8 (meso-dpmppm)4 (L)2 ](BF4 )4 (L=none (1), solvents: CH3 CN (2 a), dmf (2 b), dmso (2 c), RN≡C: R=Xyl (3 a), Mes (3 b), Dip (3 c), t Bu (3 d), Cy (3 e), CH3 (CH2 )7 (3 f), CH3 (CH2 )11 (3 g), CH3 (CH2 )17 (3 h)) and [Pd8 (meso-dpmppm)4 (X)2 ](BF4 )2 (X=Cl (4 a), N3 (4 b), CN (4 c), SCN (4 d)), were synthesized by using 2 a as a stable good precursor, and characterized by spectroscopic (IR, 1 H and 31 P NMR, UV-vis-NIR, ESI-MS) measurements and X-ray crystallographic analyses (for 1, 2 a, b, 3 a, b, e, f, 4 a-d). On the basis of DFT calculations on the X-ray determined structure of 2 b ([2b-Pd8 ]4+ ) and the optimized models [Pd8 (meso-Ph2 PCH2 P(H)CH2 P(H)CH2 PH2 )4 (CH3 CN)2 ]4+ ([Pd8 Ph8 ]4+ ) and [Pd8 (meso-H2 PCH2 P(H)CH2 P(H)CH2 PH2 )4 (CH3 CN)2 ]4+ ([Pd8 H8 ]4+ ), with and without empirically calculating dispersion force stabilization energy (B3LYP-D3, B3LYP), the formation energy between the two Pd4 fragments is assumed to involve mainly noncovalent interactions (ca. -70 kcal/mol) with four sets of interligand C-H/π interactions and Pd⋅⋅⋅Pd metallophilic one, while electron shared covalent interactions are almost canceled out within the Pd8 chain. All the compounds isolated are stable in solution and exhibit characteristic absorption at ∼900 nm, which is assignable to a spin allowed HOMO to LUMO transition, and shows temperature dependent intensity change with variable absorption coefficients presumably due to coupling with some thermal vibrations. The structures and electronic states of the Pd8 chains are found finely tunable by varying the terminal capping ligands. In particular, theoretical calculations elucidated that the HOMO-LUMO energy gap is systematically related to the central Pd-Pd distance (2.7319(6)-2.7575(6) Å) by two ways with neutral ligands L (1, 2, 3) and with anionic ligands X (4), which are reflected on the NIR absorption energy of 867-954 nm. The isocyanide terminated Pd8 complexes (3) further reacted with excess of RNC (6 eq) to afford the Pd4 complexes, [Pd4 (meso-dpmppm)2 (RNC)2 ](BF4 )2 (13), and the cyclic voltammograms of 2 a (L=CH3 CN), 3, and 13 (R=Xyl, Mes, t Bu, Cy) demonstrated wide range redox behaviors from 2{Pd4 }4+ to 2{Pd4 }0 through 2{Pd4 }2+ ↔{Pd8 }4+ , {Pd8 }3+ , and {Pd8 }2+ strings. The oxidized complexes, [Pd4 (meso-dpmppm)2 (RNC)3 ](BF4 )4 (16), were characterized by X-ray analyses, and the two-electron reduced chain of [Pd8 (meso-dpmppm)4 ](BF4 )2 (7) was analyzed by spectroscopic and electrochemical techniques and DFT calculations. Reactions of 2 a with 1 equiv. of aromatic linear bisisocyanide (BI) in CH2 Cl2 deposited insoluble coordination polymers, {[Pd8 (meso-dpmppm)4 (BI)](BF4 )4 }n (5), and interestingly, they were soluble in acetonitrile, 31 P{1 H} and 1 H DOSY NMR spectra as well as SAXS curves suggesting that the coordination polymers may exist in acetonitrile as dynamically 1D self-assembled coordination polymers comprising ca. 50 units of the Pd8 rod averaged within the timescale.

Maltotriose-Chlorin e6 Conjugate Linked via Tetraethyleneglycol as an Advanced Photosensitizer for Photodynamic Therapy. Synthesis and Antitumor Activities against Canine and Mouse Mammary Carcinoma Cells.

Glycoconjugated chlorins represent a promising class of compounds that meet the requirements for the third-generation photosensitizer (PS) for photodynamic therapy (PDT). We have focused on the use of glucose (Glc) to improve the performance of the PS based on the Warburg effect-a phenomenon where tumors consume higher Glc levels than normal cells. However, as a matter of fact, Glc-conjugation has a poor efficacy in hydrophilic modification; thus, the resultant PS is not suitable for intravenous injection. In this study, a Glc-based oligosaccharide, such as maltotriose (Mal3), is conjugated to chlorin e6 (Ce6). The conjugation is assisted by two additional molecular tools, such as propargyl amine and a tetraethylene glycol (TEG) derivative. This route produced the target Mal3-Ce6 conjugate linked via the TEG spacer (Mal3-TEG-Ce6), which shows the required photoabsorption properties in the physiological media. The PDT test using canine mammary carcinoma (SNP) cells suggested that the antitumor activity of Mal3-TEG-Ce6 is extremely high. Furthermore, in vitro tests against mouse mammary carcinoma (EMT6) cells have been demonstrated, providing insights into the photocytotoxicity, subcellular localization, and analysis of cell death and reactive oxygen species (ROS) generation for the PDT system with Mal3-TEG-Ce6. Both apoptosis and necrosis of the EMT6 cells occur by ROS that is generated via the photochemical reaction between Mal3-TEG-Ce6 and molecular oxygen. Consequently, Mal3-TEG-Ce6 is shown to be a PS showing the currently desired properties.

Novel Photosensitizer ß-Mannose-Conjugated Chlorin e6 as a Potent Anticancer Agent for Human Glioblastoma U251 Cells.

A photosensitizer is a molecular drug for photodynamic diagnosis and photodynamic therapy (PDT) against cancer. Many studies have developed photosensitizers, but improvements in their cost, efficacy, and side effects are needed for better PDT of patients. In the present study, we developed a novel photosensitizer ß-mannose-conjugated chlorin e6 (ß-M-Ce6) and investigated its PDT effects in human glioblastoma U251 cells. U251 cells were incubated with ß-M-Ce6, followed by laser irradiation. Cell viability was determined using the Cell Counting Kit-8 assay. The PDT effects of ß-M-Ce6 were compared with those of talaporfin sodium (TS) and our previously reported photosensitizer ß-glucose-conjugated chlorin e6 (ß-G-Ce6). Cellular uptake of each photosensitizer and subcellular distribution were analyzed by fluorescence microscopy. ß-M-Ce6 showed 1000× more potent PDT effects than those of TS, and these were similar to those of ß-G-Ce6. ß-M-Ce6 accumulation in U251 cells was much faster than TS accumulation and distributed to several organelles such as the Golgi apparatus, mitochondria, and lysosomes. This rapid cellular uptake was inhibited by low temperature, which suggested that ß-M-Ce6 uptake uses biological machinery. ß-M-Ce6 showed potent PDT anti-cancer effects compared with clinically approved TS, which is a possible candidate as a next generation photosensitizer in cancer therapy.

Surface Adsorption Properties and Layer Structures of Homogeneous Polyoxyethylene-Type Nonionic Surfactants in Quaternary-Ammonium-Salt-Type Amphiphilic Gemini Ionic Liquids with Oxygen- or Nitrogen-Containing Spacers.

The amphiphilic ionic liquids containing an alkyl chain in molecules form nano-structure in the bulk, although they also show surface activity and form aggregates in aqueous solutions. Although insights into the layer structures of ionic liquids were obtained using X-ray and neutron scattering techniques, the nanostructures of ionic liquids remain unclear. Herein, the surface adsorption and bulk properties of homogeneous polyoxyethylene (EO)-type nonionic surfactants (CxEO6; x = 8, 12, or 16) were elucidated in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids with oxygen or nitrogen-containing spacers [2Cn(Spacer) NTf2; (Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), (3), (5), or (6); n = 10, 12, or 14 for (2-O-2) and n = 12 for all other spacers] by surface tension, small- and wide-angle X-ray scattering, cryogenic transmission electron microscopy, and viscosity measurements. The surface tension of C12EO6 in 2Cn(Spacer) NTf2 with oxygen-containing spacers increased with increasing concentration of C12EO6, becoming close to that of C12EO6 alone, indicating that the amphiphilic ionic liquid adsorbed at the interface was replaced with CxEO6. In contrast, both 2Cn(Spacer) NTf2 with nitrogen-containing spacers and nonionic surfactants remained adsorbed at the interface at high concentrations. In the bulk, it was found that 2Cn(Spacer) NTf2 formed layer structures, in which the spacing depended on the alkyl chain length of CxEO6. These insights are expected to advance the practical applications of amphiphilic ionic liquids such as ion permeation, drug solubilization, and energy delivery systems.

Microstructural Characterization of Foam Formed by a Hydroxy Group-Containing Amino Acid Surfactant Using Small-Angle Neutron Scattering.

Small-angle neutron scattering, which has not been extensively utilized for foam characterization, can provide important insights into the microstructure of surfactant-stabilized foam. Small-angle neutron scattering in combination with several other techniques was herein employed to determine the microstructure of foams stabilized by hydroxy group-containing (C12-EtOH-ßAla) and hydroxy group-free (C12-Me-ßAla) surfactants of the amino acid type. Hydroxy group introduction at the amide nitrogen had no effect on the foam film thickness (∼26 nm in both cases) but increased the foam stability and suppressed draining, as hydrogen bonding between hydroxy groups and carboxylate ions increased the foam film strength. Moreover, the obtained foam films were shown to contain micelles identical to those in the bulk solution.

Surface Adsorption and Bulk Properties of Surfactants in Quaternary-Ammonium-Salt-Type Amphiphilic Monomeric and Gemini Ionic Liquids.

The physicochemical properties of ionic liquids can be readily controlled. Currently, it is necessary to investigate the properties of different surfactants to elucidate the mixtures used in quaternary-ammonium-salt-type ionic liquids. Herein, the surface adsorption and bulk properties of homogeneous polyoxyethylene (EO)-type nonionic surfactant, quaternary-ammonium-salt-type cationic surfactant, and sulfobetaine-type zwitterionic surfactant are elucidated in quaternary-ammonium-salt-type amphiphilic monomeric ionic liquids and gemini ionic liquids with bis(fluorosulfonyl)imide or bis(trifluoromethanesulfonyl)imide as counterions. The monomeric amphiphilic ionic liquids that adsorbed at the interface were replaced with CxEOy (where x and y represent alkyl and EO chain lengths, respectively) as the concentration of CxEOy increased. On the other hand, in the gemini amphiphilic ionic liquids, the surface tensions of CxEOy were lower than those of the monomeric ionic liquids. Consequently, both gemini amphiphilic ionic liquids and CxEOy adsorbed efficiently at the interface and oriented themselves because of a synergistic effect. Furthermore, for ionic liquids with short alkyl chains, an orderly bulk nanostructure was not observed at low concentrations in CxEOy, while a layer structure formed at higher concentrations; in contrast, ionic liquids with long alkyl chains formed a layer structure. The alkyl chains, which were interlocked in the bilayer structure, resulted in a densely packed layer structure.

Adsorption and Aggregation Properties of Gemini-Type Amphiphilic Dendrimers.

Gemini-type amphiphilic dendrimers featuring two dodecyl chains and two poly(amidoamine) dendrons (2C12-2denGn, where n is the dendron (den) generation (G) number (1, 2, 3, 4, or 5)) were synthesized using ethylenediamine, alkyl bromide, and methyl acrylate. These gemini-type dendrimers were characterized by surface tension, pyrene fluorescence, static light scattering, and small-angle X-ray scattering. The results showed clear breakpoints in the surface tension versus concentration plots, which indicated adsorption at the air/water interface and micelle formation in solution despite the bulky dendron structure (e.g., generations 3 to 5), contrasting the behavior of conventional surfactants. The 2C12-2denGn dendrimers could be densely packed at the air/water interface owing to enhanced interaction between the dendrons and between the alkyl chains. Furthermore, these dendrimers formed spherical micelles at a concentration of 5.0 mmol dm-3 in solution (pH 9); the overall micelle size was not dependent on the generation number of the dendron, while as the generation number increased, the core radius of the micelle decreased, and the shell thickness of the micelle increased.

Physicochemical and solution properties of quaternary-ammonium-salt-type amphiphilic trimeric ionic liquids.

The quaternary-ammonium-salt-type amphiphilic compounds 3Cntris-s-Q X (star-type; carbon number between the central amino and ammonium groups s = 2, 3) and 3Cnlin-3-Q X (linear-type; carbon number between the hydrophilic groups s = 3), where n represents the alkyl chain length (n = 8, 10, 12, 14) and X represents a counterion [hexafluorophosphate, trifluoromethanesulfonate (OTf), bis(fluorosulfonyl)amide (FSA), and bis(trifluoromethanesulfonyl)amide (NTf2)], were synthesized. Except for 3C12tris-3-Q OTf, these trimeric compounds presented melting points lower than 100 °C and therefore are defined as ionic liquids. Among them, 3Cntris-3-Q NTf2 (n = 8, 12, 14) and 3Cnlin-3-Q NTf2 (n = 8, 10) presented melting points lower than 0 °C. The melting points of the amphiphilic trimeric ionic liquids (n = 8, 10), which were lower for the star- than for the linear-type compounds, were higher than those of the corresponding monomeric compounds but lower than those of the corresponding gemini samples. Moreover, the amphiphilic trimeric ionic liquids exhibited higher conductivities and lower viscosities than the corresponding gemini ionic liquids, while the star-type trimeric ionic liquids presented lower conductivities and higher viscosities than those of the linear-type compounds. The amphiphilic trimeric ionic liquids also readily adsorbed at the air/water interface and oriented themselves to form micelles in aqueous solution. This aggregation behavior was not observed in the monomeric and gemini ionic liquids.

Characterization and Solution Properties of Quaternary-Ammonium-Salt-Type Amphiphilic Gemini Ionic Liquids.

Quaternary-ammonium-salt-type amphiphilic gemini compounds (C m -2-C n X, where m and n represent the alkyl chain lengths; m = 4, 6, 8, 10; n = 2, 4, 6, 8, 10; m ≥ n; and X indicates the counterion BF4, PF6, OTf, FSA, or NTf2) were synthesized by the quaternization of N,N,N',N'-tetramethylethylenediamine and n-alkyl bromide and a subsequent ion-exchange reaction with five different counterions. For comparison, the corresponding monomeric compounds (C n X, n = 2, 4, 6, 8, and 10) were also synthesized. The melting points of the compounds were evaluated using differential scanning calorimetry, and those with melting points lower than 100 °C were treated as ionic liquids during the subsequent measurements. The amphiphilic gemini compounds exhibited the lowest melting points (44-49 °C) when bulky NTf2 - was the counterion and the degree of dissymmetry between the two alkyl chains was 0.4 < n/m < 0.75. However, their melting points were not similar to those of the monomeric compounds with NTf2 - and n = 4-10 (<29 °C). The gemini ionic liquids exhibited significantly lower conductivities and higher viscosities than those of the corresponding monomeric ionic liquids. This is because of the decrease in the mobility of the cation molecules caused by the gemini structure, in which the two monomeric compounds are connected by a spacer. The gemini ionic liquids also showed higher densities than those of the corresponding monomeric ionic liquids, owing to the dimer of the gemini structure. Further, the gemini ionic liquids were adsorbed readily at the air/water interface and oriented themselves but did not show the critical micelle concentration for the concentration range over which they could be dissolved in water. The amphiphilic monomeric and gemini ionic liquids also tended to form ion pairs in aqueous solutions, as the length of their alkyl chain was relatively short.

Superoxide Scavenging Activity of Gold, Silver, and Platinum Nanoparticles Capped with Sugar-based Nonionic Surfactants.

Metal nanoparticles have the ability to remove superoxide via changes in the surface electronic states at the large surface area. Gold, silver, and platinum nanoparticles were prepared in the presence of three sugar-based nonionic surfactants using NaBH4 as a reducing agent. The surfactants (glycosyloxyethyl methacrylate: xGEMA) contain sugar oligomers of various lengths (x), are biodegradable, and act as protecting groups for the nanoparticles. Three types of xGEMA were used: dodecyl and hexadecyl chains containing amphiphilic oligomers (C12-3.0GEMA and C16-3.2GEMA) and multi-dodecyl chain with multiple sugar side chains (1.8C12-4.7GEMA). We found that the type of nonionic surfactant affected the size of the nanoparticles. The average size of the gold, silver, and platinum nanoparticles ranged from 1.9 to 6.6 nm depending on the surfactant. The trend in the size of gold nanoparticles in relation to the chosen surfactants was different from that for the silver and platinum nanoparticles. Moreover, the gold nanoparticles did not show effective antioxidant activity for superoxide, whereas the silver and platinum nanoparticles removed superoxide to a certain extent. The general order for superoxide scavenging activity increased in the following order: gold < platinum < silver. In particular, the largest size of silver nanoparticles capped with C16-3.2GEMA had a similar ability for the removal of superoxide as superoxide dismutase (ca. 3999 unit/mg) on the basis of the mass concentration.

Structure and Catalytic Activities of Gold Nanoparticles Protected by Homogeneous Polyoxyethylene Alkyl Ether Type Nonionic Surfactants.

Gold nanoparticles were prepared in aqueous solutions containing four homogeneous polyoxyethylene (EO) alkyl ether type nonionic surfactants: octaoxyethylene dodecyl ether (C12EO8), methoxyoctaoxyethylene dodecyl ether (C12EO8OMe), ethoxyoctaoxyethylene dodecyl ether (C12EO8OEt), and trioxypropylene-octaoxyethylene dodecyl ether (C12EO8PO3). The sizes of obtained gold nanoparticles were almost independent of the terminal group in the EO surfactants; and the average sizes of nanoparticles prepared by surfactants with hydroxy, methoxy, ethoxy, and trioxypropylene terminal groups at [surfactant]:[Au3+] = 1:1 were 5.1 ± 1.2, 8.1 ± 1.4, 6.4 ± 2.1, and 8.6 ± 2.9 nm, respectively. The gold nanoparticles easily aggregated together according to the increasing hydrophobicity of hydroxy < methoxy ethoxy < trioxypropylene terminal groups. Highly stable dispersed nanoparticles were observed with hydroxy group in the EO terminal group. On the other hand, introducing hydrophobic moiety to the hydroxy group resulted in aggregated nanoparticles because of the interaction between the hydrophobic groups of a protective agent for the gold nanoparticles. For the reduction reaction of p-nitrophenol and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging reaction, catalytic activities of the prepared gold nanoparticles decreased by the introduction of methoxy, ethoxy, or trioxypropylene to the hydroxy group of the EO type surfactant. Thus, a significant correlation was observed between the structure of gold nanoparticles and their catalytic activities.

Rheo-SANS study on relationship between micellar structures and rheological behavior of cationic gemini surfactants in solution.

HYPOTHESIS: Gemini surfactant 12-2-12 (dimethylene-1,2-bis(dodecyl dimethylammonium bromide)) solutions are known to show shear thickening and thinning under salt-free conditions. Because the rheological behavior of the wormlike micelles formed by 12-2-12 in solution is related to their structure, we expected that changes to the precursor structure would affect their rheological behaviors. It is also important to understand the effect of the introduction of a spacer group in the 12-2-12 molecules on the rheological behavior and structure of the wormlike micelles under shear flow. EXPERIMENTS: Simultaneous small-angle neutron scattering and rheological measurements (Rheo-SANS) of the 12-2-12 solutions were performed. We exhaustively studied the structural changes in the wormlike micelles upon increasing shear rate. FINDINGS: We found that the wormlike micelles were oriented in the direction of the flow due to elongation and that changes to the precursor of the wormlike micelles did not affect the shear thickening. As a precursor structural change of shear thinning, the wormlike micelles elongated while maintaining their orientation. We found that an increase in the molecular curvature of the 12-2-12 due to the introduction of a spacer-group contributed to the unusual rheological behaviors of the wormlike micelles in a solution under shear flow.