When an aqueous electrolyte is frozen, anions and cations are distributed between liquid and ice phases in different fashions. This partition imbalance is relaxed by the transfer of H+ and OH– to each phase, resulting in the acidification of the liquid phase when the cation is better distributed in the ice phase than the anion and in the basification in the opposite situation. In this work, a pH change in the liquid phase has been precisely evaluated by fluorescence ratiometry with pyranine as the pH probe. For frozen alkali chlorides (LiCl, NaCl, and KCl), the liquid phase is always basified by freezing due to the preferential partition of Cl– over the alkali metal cations. Changes in pH are quantitatively analyzed by a partition model, in which the distribution of an ion between the liquid and ice phases is determined by the partition coefficient. Since the concentration of a salt (i.e., ions) in the liquid phase in contact with ice becomes higher as freezing proceeds, the concentration of the ions in t...

Imbalance between Anion and Cation Distribution at Ice Interface with Liquid Phase in Frozen Electrolyte As Evaluated by Fluorometric Measurements of pH

The reaction of I− with methyl naphthalene-2-sulfonate (MeONs) is accelerated by the micellized sulfobetaine surfactants N-decyl, N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate. Concentrations of micellar-bound I− were determined by using ion-selective electrodes (ISE), and capillary electrophoresis. At low concentrations, I− incorporation fits Langmuir isotherms and is related to changes in micellar surface potentials. Rate effects of dilute KI are fitted quantitatively by a pseudophase model that describes I− binding in terms of a sorption isotherm, but at higher [KI], where the simple model predicts saturation, rates increase due to electrolyte invasion. This model considers transfer equilibria of both reactants between water and micelles and second-order rate constants in each pseudophase. Estimated second-order rate constants for reaction of MeONs with I− in the micellar pseudophase are 3.2- to 3.5-fold higher than the second-order rate constant, k2w, in water, depend...

Reactivity and models for anion distribution: specific iodide binding to sulfobetaine micelles.

Supra-long chain surfactants with double or triple quaternary ammonium headgroups

The hydration of ions in confined spaces, such as the interior of ion-exchange resins, micelles, and surface monolayers, is discussed on the basis of results obtained with X-ray absorption fine structure studies, electrophoresis, and ion-transfer voltammetry. The general trends are that anions are partly dehydrated therein, whereas cations are likely to keep their first hydration shells. For bromide ions, the hydration numbers under various circumstances have been determined. The extents of dehydration depend not only on the structure of the cationic sites electrostatically attracting bromide ions but also on whether the cationic sites are exposed to a solution or are effectively shielded from it. These findings will be useful for designing the systems for ionic recognition and separation.

Hydration of ions in confined spaces and ion recognition selectivity.

A coupled acoustic-gravity field is efficient for entrapping a particle at the position determined by its acoustic properties rather than its size. This field has been applied to the dynamic observation of ion-exchange reactions occurring in a single resin bead. The replacement of counterions in an ion-exchange resin induces changes in its acoustic properties, such as density and compressibility. Therefore, we can visually trace the advancement of an ion-exchange reaction as a time change in the levitation position of a resin bead entrapped in the field. Cation-exchange reactions occurring in resin beads with diameters of 40-120 microm are typically completed within 100-200 s. Ion-exchange equilibrium or kinetics is often evaluated with off-line chemical analyses, which require a batch amount of ion exchangers. Measurements with a single resin particle allow us to evaluate ion-exchange dynamics and kinetics of ions including those that are difficult to measure by usual off-line analyses. The diffusion properties of ions in resins have been successfully evaluated from the time change in the levitation positions of resin beads.

Coupled Acoustic-Gravity Field for Dynamic Evaluation of Ion Exchange with a Single Resin Bead

A novel zwitterionic surfactant, N-dodecyl-N,N,N‘,N‘-tetramethylethylenediammoniopropanesulfonate bromide (DEPB), has been synthesized, and Br- involved in the micellar system has been characterized by potentiometry, NMR, and X-ray absorption fine structure (XAFS). Although the dissociation degree of Br- from the micelle evaluated by potentiometry almost agrees with that determined by NMR, the former is significantly smaller than the latter over the entire range of concentrations of DEPB. This is explained by assuming that the bromide ions in the micellar system have several different peripheral structures. XAFS has given significant insight into the hydration structures of Br- involved in the system. Some of the bromide ions partitioned into the micelle are dehydrated and are directly bound by the ammonium groups in the DEP molecules. However, some of the bromide ions are still completely hydrated even when they are partitioned into the micelles. The average hydration number of the bromide ions directly ...

Characterization of Bromide Ions in Charge-Stacked Zwitterionic Micellar Systems

The solution properties and anion recognition selectivity of the N-dodecyl-N,N,N‘,N‘,-tetramethyldiethylenediammonio-propane sulfonate bromide (DEPB) and N-dodecyl-N,N,N‘,N‘,-tetramethyl-1,3-propyl...

Takeshi Aoki

Papers

Characterization of Bromide Ions in Charge-Stacked Zwitterionic Micellar Systems

Electrolyte-induced phase separation and charge reversal of cationic zwitterionic micelles.