Phase Diagram of Monohexadecyl Phosphate Neutralized by L-Arginine: α-Gel Formation Ability

The aqueous phase diagram of monohexadecyl phosphate neutralized by L-arginine (C16MP-Arg) was studied using analytical techniques including differential scanning calorimetry (DSC) and small/wide-angle X-ray scattering (SWAXS) measurements. We found that the mixture could form an α-gel over wide ranges of concentrations and temperatures. The α-gel was thermodynamically stable at 25°C (below the phase transition temperature for hexagonal or lamellar liquid crystals), and did not transform to coagel for at least one year. This behavior is unique among anionic surfactant systems. Thus, the balance between hydrophilic and hydrophobic volumes is a key factor for determining whether a stable α-gel can be obtained. In this system, the relatively large volume of Arg relaxed the alkyl chains of C16MP and prevented their crystallization.