Iwao Watanabe

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.

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.

Preferential adsorption of cationic surfactant mixture studied by bromide ion selective total-reflection XAFS measurement

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions XjH of individual ions j ( j = HTA+, Cl(-), DTA+, and Br (-)) were evaluated by combining the surface excess concentration of Br(-) estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA+ and Br(-) are preferentially adsorbed to DTA+ and Cl(-) at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient fi+/-(H,p) of component i and excess Gibbs energy of adsorption ĝprH,E. Then, the magnitude of ĝprH,E was compared with that of ĝprH,E attributable to intrinsic interaction between ions.

An Abnormally Large EXAFS Debye‐Waller Factor for a Mo–O Bond in Hexamolybdate

Temperature dependence of EXAFS was analyzed for Mo6O192 and W6O192− compounds. The Fourier transform peak for bridging Mo–O bond is quite smaller than that for other Mo–O bonds in Mo6O192−. Such a phenomenon was not observed in W6O192−. That reason of small peak is abnormally large Debye‐Waller factor for this bond, which comes from the large static fluctuation of the bridging Mo–O bonds.