Takaya Sakai

Phase and rheological behaviour of viscoelastic wormlike micellar solutions formed in mixed nonionic surfactant systems

The phase and rheological behaviour of mixed nonionic surfactant system – polyoxyethylene cholesteryl ether (ChEO15)–alkanoyl-N-methylethanolamide (NMEA-n)–water was studied at 25 °C. With addition of NMEA to the ChEO15–water binary system, a micellar (Wm)–lamellar (Lα) phase transformation takes place at low ChEO15 concentration. Within the Wm phase region of the ChEO15–NMEA-12–water system, there exists a high-viscosity region consisting of a viscoelastic micellar solution of entangled wormlike micelles, as suggested by rheological measurements. These solutions obey the Maxwell model of a viscoelastic fluid. Substitution of NMEA-12 by its analogue with a longer hydrophobe (NMEA-16) in the mixed surfactant system increases the extent of micellar growth, however, a phase separation occurs before a highly viscoelastic micellar system is formed. This may be attributed to the increase in packing constraints in the lipophilic core of the micelle that favours unidimensional growth.

Thermodynamically Stable Vesicle Formation and Vesicle-to-Micelle Transition of Single-Tailed Anionic Surfactant in Water

The aggregation behavior of sodium 3,6,9,12,15-pentaoxa-heptacosanoate (AEC4-Na) in aqueous solution with increase of the concentration at 25 °C was investigated using differential scanning calorimetry, equilibrium surface tension, solubilization of an oil-soluble dye, steady-state fluorescence, dynamic light scattering, and freeze-fractured transmission electron microscopy. Vesicle formation of AEC4-Na preceded micelle formation below the critical micelle concentration (cmc). The vesicle-to-micelle transition was observed through a narrow concentration region above the cmc. The mean diameters of the vesicles and micelles were not affected by the concentration. All solutions over a wide range of concentrations were homogeneously transparent with a low Krafft point below 0 °C. These results indicate that the AEC4-Na vesicles have a thermodynamically stable structure. Vesicle formation may be caused by a pseudobinary mixed surfactant system composed of monomeric AEC4-Na and an acid soap that consists of a dimer complex formed between AEC4-Na and unneutralized AEC4-Na. The thermodynamic stability would then result from the inhibition of close intermolecular aggregation and flexibility of the molecular shape in the vesicles due to the oxyethylene units in AEC4-Na.

Precipitate deposition around CMC and vesicle-to-micelle transition of monopotassium monododecyl phosphate in water.

Monoalkyl phosphate (MAP) salts are a kind of bivalent anionic surfactants. The difference of properties between half-neutralized monosalt and completely neutralized disalt is very interesting. In this study, the aggregation behavior of monopotassium monododecyl phosphate (MAP-12K) in aqueous solution with an increase in concentration was investigated by surface tension (γ), elemental analysis, gas chromatography, differential scanning calorimetry, steady-state fluorescence, and negative strained transmission electron microscopy techniques. MAP-12K aqueous solution showed some characteristics: (I) Vesicle aggregates were formed at very dilute concentration (1.2 mM). (II) The precipitate of a highly hydrophobic dimer of MAP, which was quaternary neutralized by potassium, was generated only in a certain dilute concentration region (2.7-200 mM) around the critical micelle concentration (cmc = 20 mM). (III) Vesicles spontaneously translate into micelles at the cmc. (IV) In the higher concentration above 200 mM, the solution becomes homogeneous micellar solution. All of these uncommon characteristics are thought to be caused by the generation of the dimer, which is much more hydrophobic than dissolved MAP derivatives, in the complicated chemical equilibria based on the weakly acidic character of MAP. MAP-12K aqueous solution behaves as if it is a binary mixed surfactant solution of hydrophobic dialkyl surfactant and hydrophilic monoalkyl surfactant in spite of a single component solution.

Effect of Novel Alkanolamides on the Phase Behavior and Surface Properties of Aqueous Surfactant Solutions

Abstract The surface tension properties and phase behavior of a new series of alkanolamides, alkanoyl N‐methyl ethanolamides (NMEAs) and their mixtures with sodium dodecyl sulfate (SDS) were investigated. NMEAs alone do not form micelles in aqueous solutions but reduce considerably the surface tension until macroscopic phase separation occurs. According to Gibbs isotherms, the surface layer is less compact for the NMEA with the shortest alkanoyl chain. The critical micelle concentration (CMC) of SDS solutions is greatly reduced upon addition of a small amount of NMEA and the magnitude of this effect increases with the length of the alkanoyl group. The results indicate the presence of attractive interactions between SDS and NMEA molecules inside micelles. The mixing of SDS with NMEA‐16 causes a reduction in the melting temperature of the solid similar to freezing‐point depression in a binary system. On the other hand, the eutectic temperature is higher in SDS‐conventional dodecanoyl mono ethanolamide (DMA) systems in which the mixture is in a solid state at room temperature over a wide range of mixing fractions. Among NMEAs, surface tension decay is faster as the alkanoyl chain length decreases. Only for the dodecanoyl chain could a diffusion‐controlled adsorption be identified at low concentrations. When small amounts of NMEA are added to SDS aqueous solutions, the surface tension decay is retarded; however, and at long times a lower surface tension is reached. For NMEA/SDS and DMA/SDS systems, an adsorption barrier is likely present. The magnitude of this barrier seems to depend on the SDS/alkanolamide ratio.

Liquid Oil That Flows in Spaces of Aqueous Foam without Defoaming

A very interesting phenomenon has been observed in which foam formed from an aqueous fatty acid potassium salt solution spontaneously absorbs liquid oil immediately upon contact without defoaming. Although this phenomenon initially appeared to be based on capillary action, it was clarified that the liquid oil that flows in foam film did not wet the air/water interface. In this study, it is discussed why aqueous foam can spontaneously soak up liquid oil without defoaming using equilibrium surface tension, dynamic oil/water interfacial tension, and image analysis techniques. The penetration of oil was attributed both to the dynamic decrease in the surface tension at the oil/water interface and to Laplace pressure, depending on the curvature of the plateau border. Therefore, the foam does not absorb the oil, but the oil spontaneously penetrates the foam. This interesting behavior can be expected to be applied to aqueous detergents for liquid oil removal.

Effect of head groups on the phase transitions in Gibbs adsorption layers at the air–water interface

The adsorption kinetics and the surface phase behavior of four different amphiphiles, which are 2-hydroxyethyl laurate (2-HEL), dodecanoyl N-ethanolamide (NHEA-12), dodecanoyl N-methylethanolamide (NMEA-12) and tetradecanoyl N-methylethanolamide (NMEA-14), have been investigated at the air-water interface by film balance, surface tensiometer and Brewster angle microscopy (BAM). The former two amphiphiles show a first-order phase transition from a lower density liquid like phase to a higher density condensed phase in Gibbs adsorption layers. On the other hand, the latter two amphiphiles are unable to show such characteristics under any experimental conditions. The presence of a methyl group in the head group of NMEA-12 sterically hinders the molecules and resists the formation of any condensed phases. This steric hindrance is so high that even an increase in the chain length by two CH(2) groups in NMEA-14 does not allow the formation of condensed domains. Although, both 2-HEL and NHEA-12 are able to form the condensed phase, the domain morphology formed in these monolayers is different from each other. The domains of 2-HEL at lower temperatures are circular having a stripe texture, while those at higher temperatures show fingering patterns having uniform brightness. On the other hand, the domains of NHEA-12 are dendritic in shape. The presence of hydrogen bonding sites close to the interface should be responsible for the formation of such domains in NHEA-12.