Calorimetric effects in rapidly frozen micellar solutions below the macroscopic melting point

Small samples of aqueous solutions of three cetyltrimethylammonium (CTA) salts and of sodium dodecyl sulfate (SDS) were rapidly frozen by plunging into liquid Nitrogen. The frozen samples were heated and recooled in a differential scanning calorimeter between −50 and −5°C, i.e., below the macroscopic melting point. Upon first heating an exothermic process was observed in CTA salts (but not in SDS) at between −20 and −30°C. The process was assigned to crystallization of micellar microdomains that had not taken place upon rapid freezing. The mobility for crystallization in the frozen environment was possible because of local melting point depression. In the presence of sufficient sodium chloride the local melting was proven by according endo- and exothermic DSC peaks upon heating and recooling, respectively.