Takanori Takiue

Multilayer formation of the fluoroalkanol-ω-hydrogenated fluorocarbon mixture at the hexane/water interface studied by interfacial tensiometry and X-ray reflection

Novel multilayer formation of fluorocarbon compounds at the hexane/water interface was investigated from the viewpoint of intermolecular interaction and miscibility of molecules in the adsorbed film. The two kinds of mixed systems were employed: 1H,1H,2H,2H-perfluorododecanol (FC12OH)-1H-perfluorodecane (HFC10) (System A) and 1-icosanol (C20OH)-HFC10 (System B). The interfacial tension γ between the hexane solution and water was measured as a function of total concentration m and the composition of HFC10 in the mixture X(2) at 298.15 K under atmospheric pressure. X-ray reflectivity (XR) measurement was performed at BL37XU in SPring-8 as a function of scattering vector Q(z). In both systems, the γ vs m curves except for the pure HFC10 system have a break at low concentrations, which corresponds to the gaseous-condensed monolayer transition for System A and the expanded-condensed monolayer for System B. The remarkable difference between the two systems was that the curves in a limited bulk composition range (0.45 ≤ X(2) ≤ 0.9) of System A show another break at high concentrations close to the solubility limit. The total interfacial density above this break point was around 7-11 μmol m(-2), suggesting the spontaneous molecular piling to form a multilayer. The phase diagrams of adsorption in the condensed monolayer indicated that the film composition of HFC10 is negative in System B but definitely positive above X(2) ≥ 0.45 in System A. This clearly shows that HFC10 molecules are miscible with FC12OH but immiscible with C20OH in the condensed monolayer. Thus, it is likely that the mixing of HFC10 with FC12OH in the condensed monolayer induces multilayer formation. The X-ray reflectivity normalized by Fresnel reflectivity R/R(F) vs Q(z) plot in the condensed monolayer of System A was fitted by a one-slab model with uniform electron density and thickness. The electron density profile was almost the same as that of the pure FC12OH system. The plot in the multilayer, on the other hand, was fitted well by the two-slab model with different electron densities and thicknesses. The electron density profile showed that the multilayer consists of two layers, one of which has slightly higher electron density than the bulk hexane phase and piles on the lower layer with almost the same electron density as the condensed FC12OH monolayer.

Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface

To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)-dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF(4) was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF(4)(-) ions, but the composition of Br(-) ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr-DTABF(4) mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)-1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixtures, where a stronger hydrogen-bonding exists between BF(4)(-) and HMIM(+) ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions.

Miscibility and Distribution of Counterions of Imidazolium Ionic Liquid Mixtures at the Air/Water Surface

The miscibility and distribution of Br(-) and BF(4)(-) ions of imidazolium ionic liquid mixtures, 1-hexyl-3-methylimidazolium bromide (HMIMBr) + 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)), at the air/water surface were investigated by surface tensiometry and the total-reflection XAFS (TRXAFS) method. Tensiometry showed that the surface density of BF(4)(-) was much larger than that of Br(-), the adsorbed films of the HMIMBr-HMIMBF(4) mixture were greatly enriched in BF(4)(-) at all surface tensions, and the excess Gibbs energy of adsorption was positive. However, TRXAFS revealed that the Br ions were all in the free-Br state solvated by six water molecules in the mixed adsorbed film. Entropy-originated nonideal mixing, where a kind of segregation of the counterion distribution takes place in the interfacial region, was suggested in the mixture.

Calorimetric Study of Micelle Formation in Polyethylene Glycol Monooctyl Ether Solution

The micellar solutions of polyethylene glycol monooctyl ethers C8Ej (j = 3,4,5) were investigated by employing the high-precision isothermal titration microcalorimeter at seven temperatures from 10° to 40°C. From the thermodynamic analysis of the experimental results, the differential enthalpies of solution of monomer and micelle of C8Ej were obtained separately; they are negative and increase with increasing temperature and decrease with the number j. It was found that C8Ej molecules interact with water molecules more strongly in the monomeric than in micellar state. The enthalpy of micelle formation of C8Ej was positive and decreased with rising temperature, while they increased slightly with increasing j. By comparing the results of ethylene glycol oligomers (C0Ej) with those of C8Ej, it seems reasonable to suppose that the driving force of micelle formation of C8Ej is mainly the increment of entropy caused by dehydration of hydrophilic parts at higher temperatures, while the increment caused by dehydration of hydrophobic parts becomes dominant at lower temperatures in the micellization process.

X‐Ray Studies of Surfactant Ordering and Interfacial Phases at the Water‐Oil Interface

X‐ray reflectivity studies of surfactants at the water‐oil interface yield structural information with sub‐nanometer resolution. In this presentation, we reviewed recent X‐ray reflectivity measurements of the interface between water and a hexane solution of the hydrocarbon alkanol CH3(CH2)19OH and fluorocarbon alkanol CF3(CF2)7(CH2)2OH. The mixed system exhibits three monolayer phases, two of which are similar to single surfactant phases. A transition from a liquid monolayer to a solid monolayer occurs with increasing temperature. This unusual phase transition and the qualitative features of the phase diagram are predicted by an appropriate superposition of the behavior of the two single surfactant systems.

Spontaneous vesicle formation of mixtures of double-chain cationic surfactants with a different counterion.

Aggregate formation of a didodecyldimethylammonium bromide (DDAB) and didodecyldimethylammonium chloride (DDAC) mixture in aqueous solution was investigated. The concentration vs composition diagram of aggregate formation was constructed by analyzing the surface tension, turbidity, and electrical conductivity data. The cryogenic transmission electron microscopy was applied to several representative points in the diagram and provided information of the morphology of aggregates. The sequence of monomer (m) - m + small aggregate (A) - m + A + vesicle (V) - m + V was concluded with increasing total concentration of surfactants at all mixing ratios. The compositions of counterions in A and V were estimated on the basis of thermodynamic consideration and examined from the viewpoint of asymmetry of constituents and uneven distribution between outer and inner monolayers of a vesicle bilayer. Vesicle surfaces were suggested to abound in chloride ions compared to bulk solution, which is opposite to spherical micelle surfaces.

Surface Adsorption and Aggregate Formation of Cationic Gemini Surfactant and Long-Chain Alcohol Mixtures

We measured the surface tension of aqueous solutions of octanol-butandiyl-1,4-bis(decyldimethylammonium bromide) using the drop-volume technique at 298.15 K under atmospheric pressure as a function of the total molality and bulk composition. The results of the surface tension measurements, which were analyzed by originally developed thermodynamic equations, suggested that octanol molecules filled the spaces among the hydrophobic chains of gemini surfactants and formed a densely packed monolayer with them in the adsorbed film. The turbidity of aqueous solutions was also measured to construct the concentration-composition diagram with the surface tension data. A transmission electron microscope was used to determine the aggregate morphology in the aqueous solutions. Disc-like micelle and microemulsion regions were found on the diagram prior to the spherical micelle formation; nevertheless, the butandiyl-1,4-bis(decyldimethylammonium bromide) itself formed only spherical (or small ellipsoid) micelles in the concentration range measured. We also studied the relationship between synergism and molecular packing in the aggregates.

Probing the self-aggregation behavior and counter ion distribution of a copper surfactant complex

In the present work, a new copper surfactant complex [Cu(BrCl2)(C12H25N(CH3)3)] was synthesized and characterized. Its organization at the air–water interface and in bulk aqueous medium was investigated. The aggregation behavior and adsorption pattern of the metallosurfactant revealed important information about the molecular organization and counter ion distribution, which until now was unknown. The critical micellization concentration and optical thickness of the copper surfactant were determined using surface tension, cyclic voltammetry and ellipsometry techniques. The X-ray absorption fine structure technique was applied to the copper surfactant solution under both total reflection (TRXAFS) and transmission conditions to understand the ion organization at the interface and in the bulk, respectively. The Br and Cu K-edge absorption revealed the presence of Br− ions and the absence of Cu2+ ions at the interface and in the inner core of micelles. Furthermore, Br− ions were found to exist in two solvation states, designated as “free-Br” and “bound-Br”. Even the surface excess concentration of the ions was precisely determined from the adsorbed film of copper surfactant using external reflection absorption (ERA)-FTIR spectroscopy.

Effect of alkane chain length and counterion on the freezing transition of cationic surfactant adsorbed film at alkane mixture - water interfaces

Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane-water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film. Furthermore, we demonstrated that the freezing transition temperature of cationic surfactant adsorbed film was independent of the kind of counterion.

Study on the distribution of binary mixed counterions in surfactant adsorbed films by total reflection XAFS measurements

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF(4)). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB - dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) - 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.