Effect of water structure on the formation of coagels from ascorbyl-alkanoates

Ascorbyl-alkanoates (ascorbyl-O-CO-(CH₂)_n-2 CH₃, in brief ASCn) behave as anionic surfactants. They form coagels in aqueous dispersions at room temperature. On heating, these hydrated crystalline phases transform into either micellar solutions or gel phases, depending on the hydrophobic chain length. The different phase behavior reflects the interplay between interactions that involve the ionic headgroups and water and those between the hydrophobic chains. Solute-induced modification of water structure might then be expected to induce significant changes in the coagel-to-gel or coagel-to-micelle phase transitions. Such effects, induced by sucrose and by urea, are here explored over a wide region of the ASCn/water phase diagram (5% and 50% w/w). Differential scanning calorimetry and conductivity measurements are used to probe structure in 1% ASC12/water samples at different temperatures. The coagel phase transition temperature does not significantly increase with ASCn concentration, while the corresponding enthalpy change increases linearly with surfactant concentration. The addition of sucrose or urea to the ASCn aqueous dispersion induces changes in the phase transition temperatures. The phenomenon is related to the water structure changes due to these cosolutes.