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We performed a structural analysis of aggregates formed by two types of trimeric surfactants based on quaternary ammonium salts─linear-type 3C n lin- s -Q and star-type 3C n tris- 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 3C 12 lin-3-Q transitioned from gel solutions to worm-like micelles at high concentrations, and 3C 14 lin-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 3C n lin-3-Q increased with the increasing alkyl chain length. As the spacer chain length of 3C n lin- s -Q increased from 3 to 6, and as the spacer skeleton expanded from linear-type 3C n lin- s -Q to star-type 3C n tris- 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.

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 ( b C 7 - b C 9 EO 12 ). Similarly, homogeneous EO-type nonionic surfactants with linear double chains (C 8 -C 8 EO 12 ) and a linear single chain (C 18 EO 12 ) were used. b C 7 - b C 9 EO 12 and C 8 -C 8 EO 12 surfactants with branched hydrophobic chains or double chains increased the foam stability and suppressed the draining. Furthermore, they formed rod-like micelles, and C 18 EO 12 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 b C 7 - b C 9 EO 12 and C 8 -C 8 EO 12 , 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.

Textured hydrophobic surfaces that repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing. In many occasions, surface textures are oriented to direct rebounding droplets. Surface macrostructures (>100 μm) have often been explored to induce directional rebound. However, the influence of impact speed and detailed surface geometry on rebound is vaguely understood, particularly for small microstructures. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars. These insights improve our understanding of droplet impact on hydrophobic microstructures and may be useful for designing structured surfaces for controlling droplet mobility.

In this study, we synthesized quaternary ammonium salt-based gemini surfactants, 2C 12 (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 (2C 12 (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.

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C 4 FSA, C 8 FSA, and C 4 NTf 2 ) or gemini compounds (C 10 -2-C 4 FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C 12 EO 6 ), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C 12 TAB), and zwitterionic N -dodecyl- N , N -dimethyl-3-ammonio-1-propanesulfonate (C 12 Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA - ) or bis(trifluoromethanesulfonyl)imide (NTf 2 - ) 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 C 8 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, C 8 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 C 8 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 C 12 Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C 10 -2-C 4 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.

The impact of liquid drops on a rigid surface is central in cleaning, cooling, and coating processes in both nature and industrial applications. However, it is not clear how details of pores, roughness, and texture on the solid surface influence the initial stages of the impact dynamics. Here, we experimentally study drops impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We found that the difference between impact velocity and the capillary speed on a solid surface is a key factor of spreading asymmetry, where the capillary speed is determined by the friction at a moving three-phase contact line. The line-friction capillary number Ca f = μ f V 0 /σ (where μ f , V 0 , and σ are the line friction, impact velocity, and surface tension, respectively) is defined as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when Ca f ≪ 1, the droplet impact is asymmetric; the contact line speed in the direction against the inclination of the ridges is set by line friction, whereas in the direction with inclination, the contact line is pinned at acute corners of the ridges. When Ca f ≫ 1, the geometric details of nonsmooth surfaces play little role.

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 (C 12 -EtOH-βAla) and hydroxy group-free (C 12 -Me-βAla) 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.

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 C x EO y (where x and y represent alkyl and EO chain lengths, respectively) as the concentration of C x EO y increased. On the other hand, in the gemini amphiphilic ionic liquids, the surface tensions of C x EO y were lower than those of the monomeric ionic liquids. Consequently, both gemini amphiphilic ionic liquids and C x EO y 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 C x EO y , 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.

Gemini-type amphiphilic dendrimers featuring two dodecyl chains and two poly(amidoamine) dendrons (2C 12 -2denG n , 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 2C 12 -2denG n 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.

Gold nanoparticles were prepared in aqueous solutions containing four homogeneous polyoxyethylene (EO) alkyl ether type nonionic surfactants: octaoxyethylene dodecyl ether (C 12 EO 8 ), methoxyoctaoxyethylene dodecyl ether (C 12 EO 8 OMe), ethoxyoctaoxyethylene dodecyl ether (C 12 EO 8 OEt), and trioxypropylene-octaoxyethylene dodecyl ether (C 12 EO 8 PO 3 ). 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]:[Au 3+ ] = 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.

Homogeneous polyoxypropylene (PO)-polyoxyethylene (EO) alkyl ether type nonionic surfactants comprising alkyl, EO, and PO chains with identical chain length distributions (C n EO x PO y ; alkyl chain length n = 10, 12, 14, or 16; EO chain length x = 4, 6, or 8; and PO chain length y = 1, 2, or 3) were synthesized from homogeneous polyoxyethylene alkyl ether bromide and monosodium polyoxypropylene by Williamson ether synthesis. The adsorption and aggregation properties of these surfactants were characterized (cloud point, surface tension, dynamic light scattering, small-angle X-ray scattering, polarization microscopy, and cryogenic transmission electron microscopy) and compared to those of conventional homogeneous EO alkyl ether type nonionic surfactants (C n EO x ). The introduction of a PO chain to the EO terminal group of the C n EO x species lowered the cloud points, especially for x = 6. Contrary to our expectations, the C n EO x PO y surfactants adsorbed efficiently at the air/water interface, despite their complex structure (hydrophobic alkyl chain/hydrophilic EO chain/hydrophobic PO chain). They also displayed excellent micelle-forming ability in solution. Furthermore, the C n EO x species formed small micelles in solution at low concentrations and the structures transformed to hexagonal liquid crystals as the surfactant concentration increased. Conversely, C n EO x PO y maintained a micellar structure even at high concentrations. Notably, the introduction of a PO chain into the C n EO x surfactant controlled the formation of aggregates with a higher-order structure (hexagonal liquid crystals).