Kinetic Switching of the Concentration/Separation Behavior of Microdroplets

This work demonstrates that the solute concentration inside 100 micrometer-sized aqueous microdroplets can be controlled by adjusting the time required for the aqueous nanometer-sized droplets (nanodroplet) or reverse micelles to pass over the surface of the microdroplet. The kinetics of molecular transport between the microdroplets and the nanodroplets was investigated by utilizing a microdroplet array, and on the basis of these results, a control over the concentration selectivity of the contents of the microdroplet was achieved. This method is operationally simple and can be potentially applied as a pretreatment method for microanalytical systems that require high-density microdroplet arrays. This method can also be utilized for parallel small sample analyses such as single cell analysis

Distribution and Adsorption of Ionic Species into a Liposome Membrane and Their Dependence upon the Species and Concentration of a Coexisting Counterion

The distribution of ions into a bilayer lipid membrane (BLM) and their adsorption on the BLM are investigated by extracting a hydrophobic cation, rhodamine 6G (R6G⁺), into a liposome through the dialysis membrane method. R6G⁺ distribution mainly depends upon the concentration of the coexisting anion and its species (Cl^–, Br^–, BF₄^–, ClO₄^–, and picrate). On the other hand, R6G⁺ adsorption on the BLM surface follows the Langmuir adsorption model and is independent of the coexisting anion in the aqueous phase. We propose an extraction model of ionic species into the BLM, to explain the dependence of extraction of ionic species upon the coexisting anion. In this model, an ion is distributed with a coexisting counterion into the BLM and then forms an ion pair in the BLM. Here, the ion adsorption equilibrium on the BLM surface is independent of the species and concentration of the coexisting counterion under the same ionic strength. On the basis of this model, we estimate the distribution constant of R6G⁺ and anion (<i>K</i>_D), the ion-pair formation constant in the BLM (<i>K</i>_ip), and the R6G⁺ adsorption constant on the BLM surface (<i>K</i>_ad). Even for an ultrathin membrane system, such as a BLM, R6G⁺ is distributed with a coexisting counterion and the distribution equilibrium of the ionic species at the water–BLM interface is analyzable similar to that at the water–organic solvent interface