‘Clouding’ in Zwitterionic surfactant/water systems – the influence of additives on the upper consolute loop of the decyldimethylammonioethane sulfate/water system

The surfactant 2-decyldimethylammonioethane sulfate [C10H23 N+(CH3)2·CH2·CH2OSO3−] in water, exhibits an upper consolute loop with a critical temperature of approximately 78 °C and a very wide two phase area below this. We have investigated the influence of various additives (electrolytes, urea, amphiphiles) on this clouding behaviour. In addition we have also identified the liquid crystalline phases present in the system as well as making 1H nmr measurements of the micellar solutions to monitor alterations in the conformation of the head group. Unfortunately, most of the measurements were made on a surfactant sample containing 1% sodium bromide as an impurity. However, at most of the surfactant concentrations examined, this results in <10−2 mol dm−3 additional electrolyte – a minor component. The additives have very different effects on the clouding behaviour to those observed for polyoxyethylene surfactants. Electrolytes at concentrations of 0.1 mol dm−3 can give changes of up to 70 °C, whilst changes of more than 5 °C are rare for polyoxyethylene surfactants. In addition, the behaviour of cationic surfactants differs markedly from that of anionic surfactants, whilst even the simple exchange of 2H2O for normal water changes the cloud temperature by 8 °C (typically, approx. 2 °C is observed for polyoxyethylene surfactants). All the results indicate that the origin of the clouding behaviour arises from the head group conformation. An extended conformation leads to repulsive forces (arising from steric/hydration effects), whilst a ‘bent around’ configuration gives attractive forces (from interactions between zwitterions on adjacent micelles). The additives influence the cloud points by changing the balance of these head group conformations.