Comparison of the DSC curves obtained for aqueous solutions of nonionic and ionic surfactants

An interpretation of the differential scanning calorimetry (DSC) curves that were obtained for aqueous solutions of three nonionic surfactants, n-C8DPO, -C9DPO, and -C10DPO, was recently reported (Kresheck, C;. C. Langmuir 2000, 16, 3067) and reanalyzed. These investigations have now been extended to include three cationic, DTAB,TTAB, and DPB, and one anionic, SDecS, surfactants. The curves obtained for the, ionic surfactants exhibited the same pattern as for the nonionic surfactants. Below the critical micelle concentration cmc, the heat capacity of the solutions gradually decreased and did not exhibit any systematic variation within the limits of experimental error (+/- 0.2 mcal/deg). Solutions which were 2-3 times more concentrated than the cme exhibited curvature that resembled the temperature dependence of the cmc. However, studies with solutions of intermediate concentrations produced curves with either distinct maxima, minima, or both. These slope changes all occurred at temperatures where the cme became equal to that of the concentration of surfactant in the cell. All of the heating curves were reversible and independent of scan rate. PL small change in the shape of the DSC curves at surfactant concentrations well above the cmc may result from micellar growth. A simple two-state model that was used to describe the data for nonionic surfactants could also be used to describe similar data for ionic surfactants.