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.

No abstract

The adsorption dynamics of mixed adsorbed films of hexadecylpyridinium chloride (HPC) and sodium dodecyl sulfate (SDS) were investigated through external reflection absorption FTIR (ERA FTIR) and surface tension measurements, following two different experimental approaches. In the first approach (case 1), solutions of SDS and HPC, at the desired total concentration and composition, were first introduced into the measurement cell, and then, the measurement was started. In the second approach (case 2), the HPC solution, at a given concentration, was first introduced into the cell, and after adsorption equilibrium was reached, a certain volume of concentrated SDS solution (to attain the desired total concentration and composition) was added to the cell, and the measurement was continued. The experiments clearly showed that the adsorption rate, as well as the mechanism, differed significantly between the two approaches.There are many researches concerned with the adsorbed film of binary surfactant systems 1 from the thermodynamic 2,3 and spectroscopic 4,5 point of views, and the existence of the three distinguishable surface phases has been confirmed experimentally. The gaseous state is a 2D gas of adsorbed surfactant molecules. An increase in surface density often gives rise to a surface phase transition to the expanded (liquid-like) and the condensed (solid-like) state. These surface phase transitions and the properties of each surface state probably control the stability of colloidal dispersions such as foams and emulsions. Moreover, monolayer formation as well as multilayer formation have been reported in some systems. 6,7 Several anionic and cationic surfactant mixed systems form vesicles owing to ion pairing in aqueous solution.8 For the HPC SDS system, Kawai et al. reported multilayer formation at the air/water interface, 9 while our own studies using ERA FTIR measurements revealed that first the condensed monolayer with a hexagonally packed structure is formed, and then other layers accumulate beneath or over it to form the multilayer. 10 In the present study, we investigate the formation dynamics of both multilayers and condensed monolayers by measuring ERA FTIR and surface tension as a function of time, using two experimental approaches. In the first approach (case 1), mixed solutions of SDS and HPC were first introduced into the measurement cells, at the desired total concentration and composition, after which the measurement was started. In the second approach (case 2), the HPC solution at a given concentration was first introduced into the cell, and after the adsorption equilibrium was reached, the required volume of concentrated SDS solution was added to the cell to reach a certain total concentration and composition, and the measurement was continued.The surface tension was measured by the pendant bubble or drop method, at 298.15 K under atmospheric pressure. 11 The total molality and composition were fixed at the values where multilayer formation was reported, i.e.,X 2 ¼ 0:64 andm ¼ 1:9 and 1.4 mmol ...

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.

No abstract