The use of high sensitivity differential scanning calorimetry to characterise dilute aqueous dispersions of surfactants: 2. Further studies on polyoxyethylene alkyl ethers

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Polyoxyethylene alkyl ether surfactants have been investigated by use of differential scanning calorimetry (DSC) in the solid state, and high-sensitivity DSC (HSDSC) for dispersions in phosphate-buffered saline. The surfactants had either 16 or 18 carbons in their alkyl chain, and either 2, 3, 4 or 7 oxyethylene groups in their polar head groups. Two regions of transition were observed for each surfactant (pre-transition and main transition). The pre-transition was identical in both the 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 solid-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 hydrocarbon chain and the hydrophilic head group) and the nature of the transitions. For the main transition the structure of the dispersion was stabilised as the nature of the surfactant became more hydrophobic. There are clear analogies between the nature of these surfactants and the behaviour of lipid bilayers (e.g., both systems have two transition regions). The fact that the behaviour is similar may well allow the toxicity of these surfactants to be understood, if they form cooperative structures between the surfactant and the lipids.