Role of Water in the Lyotropic Liquid Crystalline Mesophase of Lithium Salts and Non-ionic Surfactants.

The lyotropic liquid crystalline (LLC) mesophase forms upon evaporation of water from aqueous solutions of LiX salts (X is Cl, Br, NO, or SCN) and a surfactant [CH(OCHCH)OH, abbreviated as CE]. The LiX/CE/HO aqueous solutions have been monitored (during evaporation of their excess water to obtain stable LLC mesophases) by gravimetric, spectroscopic, and conductivity measurements to elucidate the role of water in these mesophases. The water/salt molar ratio in stable mesophases changes from 1.5 to 8.0, depending on the counteranion of the salt and the ambient humidity of the laboratory. The LiX/CE/HO LLC mesophases lose water at lower humidity levels and absorb water at higher humidity levels. The LiCl-containing mesophase holds as few as four structural water molecules per LiCl, whereas the LiNO mesophase holds 1.5 waters per salt (least among those assessed). This ratio strongly depends on the atmospheric humidity level; the water/LiX mole ratio increases by 0.08 ± 0.01 HO in the LLC mesophases per percent humidity unit. Surprisingly, the LLC mesophases are stable (no salt leaching) in broad humidity (10-85%) and salt/surfactant mole ratio (2-10 LiX/CE) ranges. Attenuated total reflectance Fourier transform infrared spectroscopic data show that the water molecules in the mesophase interact with salt species more strongly in the LiCl mesophase and more weakly in the case of the nitrate ion, which is evident by the shift of the O-H stretching band of water. The O-H stretching peak position in the mesophases decreases in the order ν > ν > ν > ν and accords well with the HO/LiX mole ratio. The conductivity of the LLC mesophase also responds to the amount of water as well as the nature of the counteranion (X). The conductivity decreases in the order σ > σ > σ > σ at low salt mole ratios and in the order σ > σ > σ > σ at higher ratios due to structural changes in the mesophase.