Periodic formation/breakdown of lamellar aggregates with anionic cyanobiphenyl surfactants

International audience; This study reports unusual behaviour 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 micro-segregate 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 seconds. 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 as for the non-ethoxylated CB-B2ES analogue at high temperatures (≥ 65 ºC). However, for CB-B2ES polarity increased with decreasing temperature, in contrast to 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 the 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 cyanobiphenyl terminal tips, must play an important role promote this unusual behaviour.