Characterizing Water Behavior in α-Gel (α-Type Hydrated Crystal) Formed from Monohexadecyl Phosphate and L-Arginine

Monohexadecyl phosphate, neutralized by L-arginine (C16MP-Arg), forms an α-gel (α-type hydrated crystal) with water. In this study, we characterized the behavior of water in the C16MP-Arg α-gel system by means of small/wide angle X-ray scattering (SWAXS), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and ¹H nuclear magnetic resonance (NMR) spectroscopy. An increased water concentration resulted in an increase in the d-spacing of the lamellar bilayers as well as an increased wavenumber for the O-H stretching vibration peak. In addition, the melting enthalpy increased with increasing water concentration, while freezing was not observed below a water concentration of 20 wt%. These results suggest that the overall properties of water changed as a function of its concentration in the sample. ¹H-NMR spin-spin relaxation time (T₂) measurements further suggest that the protons of water and C16MP-Arg can be classified into three components (low-, middle-, and high-T₂ components) as a function of the temperature and concentration. The low-T₂ component mainly arose from the protons of C16MP-Arg alkyl chains, and its mobility increased with increasing temperature. The high-T₂ component arose from the protons of water. The water behaved as "bound water" for the C16MP-Arg headgroups at -30°C and a water concentration of 20 wt%, and the mobility increased with increasing temperature and water concentration. These changes suggest that an increased water concentration results in an increased amount of water being incorporated between the C16MP-Arg lamellar bilayers as well as in spaces surrounded by α-gel domains.