Coagels from Ascorbic Acid Derivatives

The 6-O-ascorbic acid alkanoates, ascorbyl-OCO(CH2)(n-2)CH3 (ASCn), behave as anionic surfactants in water. At low water content (10% w/v) aqueous dispersions also form coagels at room temperature. These hydrated crystalline phases transform to micellar solutions or gel phases, depending on the length of the hydrophobic chain in the surfactant. The formation and properties of these phases were studied through surface tension, differential scanning calorimetry (DSC), X-ray diffraction (XRD), conductivity, and environmental scanning electron microscopy (ESEM). On heating, the shorter ascorbyl alkanoates (n less than or equal to 10) exhibit a coagel to micelle phase transition. By contrast the longer homologues (n greater than or equal to 11) exhibit a coagel to gel transition. The temperature, DeltaH, and DeltaS of the transitions depend on the length of the aliphatic side chain. The different phase behavior is related to the interplay of the competing molecular interactions that involve the hydrophobic chains of the amphiphiles, their headgroups, and the solvent.