THE USE OF HIGH-SENSITIVITY DIFFERENTIAL SCANNING CALORIMETRY TO CHARACTERIZE DILUTE AQUEOUS DISPERSIONS OF SURFACTANTS .2. FURTHER-STUDIESON POLYOXYETHYLENE ALKYL ETHERS

Polyoxyethylene alkyl ether surfactants have been investigated by use of differential scanning calorimetry (DSC) in the solid state, and hig h-sensitivity DSC (HSDSC) for dispersions in phosphate-buffered saline . The surfactants had either 16 or 18 carbons in their alkyl chain, an d either 2, 3, 4 or 7 oxyethylene groups in their polar head groups. T wo regions of transition were observed for each surfactant (pre-transi tion and main transition). The pre-transition was identical in both th e solid and dispersed state and was seen to be due to many cooperating molecules. It was argued that the pre-transition was related to a sol id-state structure. The main transition was significantly different in the aqueous environment than in the dry state. Relationships existed between the structure of the surfactants (both the length of the hydro carbon chain and the hydrophilic head group) and the nature of the tra nsitions. For the main transition the structure of the dispersion was stabilised as the nature of the surfactant became more hydrophobic. Th ere are dear analogies between the nature of these surfactants and the behaviour of lipid bilayers (e.g., both systems have two transition r egions). The fact that the behaviour is similar may well allow the tox icity of these surfactants to be understood, if they form cooperative structures between the surfactant and the lipids.