A new approach to lamellar phases (Lα) in water – non-ionic surfactant systems

We present new results on two unusual features concerning lamellar phases (Lα) in binary aqueous solutions of non-ionic surfactants. The first striking feature is the formation of highly dilute Lα phases down to 1 wt % of surfactant, which has been observed for a small number of non-ionic surfactants which are all of the alkyl polyglycol ether (C i E j ) type. So far, in binary H2O – C i E j systems either the absence or the presence of a dilute Lα phase has been reported. In the latter case, the dilute and the concentrated Lα phase are always connected continuously. However, for one particular silane surfactant, namely (CH3)3Si(CH2)6(OCH2CH2)5OCH3, two disconnected Lα phases were observed. Systematic investigations of the phase behaviour of the binary H2O – C10E4 system as well as of the pseudobinary H2O – C10E4/C10E5 systems enabled us to conclude that: (a) the disconnected Lα phase is a general feature of non-ionic surfactants and (b) there are no structural differences between the connected and the disconnected Lα phases. We propose a mechanism for the disconnection of the Lα phase.