Search

We investigated miscibility of ionic liquids in the adsorbed film by surface tensiometry from the viewpoint of effect of the hydrogen bonding between cation and anion. The ion pair formation accompanied by two hydrogen-bond between cation and anion was indicated in 1-hexyl-3-methylimidazolium tetrafluoroborate (C6MIMBF4) system and also in hexylpyridinium tetrafluoroborate (C6PyBF4) system, the ion pairs of which structure is similar to that of C6MIMBF4 systems was suggested. The phase diagram of adsorption (PDA) of the ionic liquid–ionic liquid mixed system (C6PyBF4–C6MIMBF4) showed an ideal mixing. On the other hand, those of ionic liquid–alcohol mixed systems (C6MIMBF4–1-butanol, C6PyBF4–1-butanol) showed positive deviation in the adsorbed film. Judging from the values of excess Gibbs energy of adsorption of these systems, we concluded that the magnitude of the two hydrogen-bonds of C6PyBF4 and C6MIMBF4 is not significantly different, but the one between C6MIM+ and BF4− is slightly larger than that between C6Py+ and BF4−. The difference should be caused by the magnitude of difference of atomic charge distributions in the molecules owing to nitrogen atom in the ions, especially in the C(2)–H.

The temperature-dependent conformational states of a monolayer film of ethylene glycol monododecyl ether (C12E1) at the air/water interface have been investigated using ellipsometry, surface tension, external reflection–absorption FTIR spectroscopy and two-dimensional infrared (2DIR) correlation analysis. The ellipticity coefficients and the entropy associated with C12E1 adsorption changed almost discontinuously at certain temperatures, which manifested the interfacial phase transitions. The phase transition and coexistence of two phases were further clarified using 2DIR correlation analysis with temperature perturbation. The asynchronous correlation maps revealed that both bands of asymmetric and symmetric C–H stretching vibration in one-dimensional IR were split into two components, which confirmed the coexistence of two phases at the interface.

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 (DTABF4). 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 (HMIMBF4) 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.

The surface tension of aqueous solutions of tetraethyleneglycol octyl ether (C8E4) and octyl-β-d-maltopyranoside (OM) mixture was measured as a function of the total molality of surfactants and the composition of OM under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations, then phase diagrams of adsorption and micelle formation were constructed. From the analysis of the surface tension data, it was found that the C8E4 and OM molecules interact attractively in the adsorbed film and the excess Gibbs energy of adsorption can be compared with those observed in typical cationic–nonionic surfactant systems; nevertheless, they are mixed almost ideally in the mixed micelle. Judging from a negative excess surface area calculated by differentiating the excess Gibbs energy by the surface tension, we concluded that the attraction between C8E4 and OM molecules is a short-range one originated in the hydrogen bonding between them which favors the planar configuration.

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 s...

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.