THE USE OF HIGH-SENSITIVITY DIFFERENTIAL SCANNING CALORIMETRY TO CHARACTERIZE DILUTE AQUEOUS DISPERSIONS OF SURFACTANTS

Four synthetic surfactants, consisting of alkyl chains of either 16 or 18 carbon units, with a polar head of either three or seven ethylene oxide units (termed C16(EO)3, C18(EO)3, C16(EO)7 and C18(EO)7) have been studied using thermal analysis in both their hydrated and non-hydrated states. The non-hydrated samples were investigated using differential thermal analysis (DTA), and dilute aqueous dispersions were studied using high-sensitivity differential scanning calorimetry (HSDSC). The non-hydrated (EO)3 surfactants exhibited a single melting transition, whilst the equivalent experiment for the (EO)7 systems revealed a pre-melting transition, which could be a change in crystal structure. Multiple transitions were observed with all of the hydrated samples following HSDSC investigation. These transitions are probably related to different levels of hydration of a non-homogeneous dispersion of the surfactant in the water. The following general conclusions were reached: the surfactants exhibit a phase transition at a temperature which is predominantly determined by the alkyl chain length, but the increasing chain length of the head group (polyoxyethylene) causes a decrease in transition temperature. It has proved possible to estimate the probable contribution to the enthalpy of transition that is associated with a methylene group and an ethylene oxide unit. This allows predictions to be made regarding the behaviour of similar surfactants. It was concluded that HSDSC is potentially of great value for studies of dilute solutions/dispersions of surfactants.