Periodic Formation/Breakdown of Lamellar Aggregates with Anionic Cyanobiphenyl Surfactants.

This study reports unusual behavior of aqueous-phase lamellar aggregates with a new class of hybrid surfactant, CB-B2ES, having mesogenic units {6-[4-(4-cyanophenyl)phenyloxy]hexyl} and temperature-sensitive oxyethylated (butoxyethoxyethyl) tails. These tails are poorly miscible and likely to microsegregate if the surfactant molecules assemble. Lamellar aggregates appear at CB-B2ES concentrations higher than 5 wt % and were found to undergo repeat formation/breakdown periodically at 30 °C, with an average domain lifetime of ∼10 s. To investigate effects of the temperature-sensitive oxyethylene units on the hydrophilic/lipophilic balance (HLB) of the CB-B2ES bilayers, a fluorescence probe 1-pyrene-carboxaldehide was solubilized in the mixtures to sense the micro-environmental polarities. Fluorimetric measurements suggested that the polarity of CB-B2ES bilayers is very similar to that of the non-ethoxylated CB-B2ES analogue at high temperatures (≥65 °C). However, for CB-B2ES, polarity increased with a decreasing temperature, in contrast with the small decrease in polarity observed for analogous non-ethoxylated bilayers. This is consistent with increased hydration of the oxyethylene units in CB-B2ES bilayers at low temperatures. The periodic formation/breakdown and cooling-induced hydrophilicity of the CB-B2ES lamellar aggregates did not appear in the non-hybrid and/or non-ethoxylated surfactant systems. Therefore, the combination of two unsymmetrical tails, one containing oxyethylene units and the other containing cyanobiphenyl terminal tips, must play an important role promoting this unusual behavior.