Crystal structure induced by mixture of Water/Organic/antagonistic Salts

Self-organization is an important topic of material physics. It is caused by a property (amphipathicity) to have both liking water part (hydrophilic) and part liking oil (hydrophobic) in surfactant molecules. On the other hand, “Salts” that is sodium chloride ionizes to cation and anion but they attract water molecule selective. Therefore, we think that a Mixture of Water/Organic Solvent/salt don´t form ordered structure like the microphase separated structure. However, we discovered that antagonistic salts, which are composed of hydrophilic and hydrophobic ions, induce multi-lamellar vesicles in a mixture of water/organic solvent although no surfactants are contained. In this case, hydrophobic ions preferentially attract organic solvents around themselves. In the present study, we investigated the structural properties of the mixture of water / 2,6-dimethylpyridine / NaBPh4 by polarization microscope at room temperature. Then, crystal structures with a scale of 1 μm to 100μm are observed. The number of the crystals increases with increasing the concentration of NaBPh4. On the other hand, the overall shape of the crystal does not change with the salt concentration. We assumed that the crystal structure is composed of water, 2,6-dimethylpyridine, and NaBPh4.