Memory effects of monolayers and vesicles formed by the non-ionic surfactant, 2C 18E 12

Abstract

The behaviour of monolayers and bilayers formed by the dialkyl chain non-ionic surfactant, 1,2-di- O-octadecyl- rac-glycerol-3- ω-methoxydodecaethylene glycol (2C 18E 12) in water at 297 K has been investigated. Using a surface film balance (or Langmuir trough) the compression–expansion cycle of the 2C 18E 12 monolayer was found to be reversible when compressed to surface pressures ( π) less than 42 mN m −1. Compression of 2C 18E 12 monolayer to π greater than 42 mN m −1 above this resulted in a considerable hysteresis upon expansion with the π remaining high relative to that obtained upon compression, suggesting a time/pressure dependent re-arrangement of 2C 18E 12 molecules in the film. Morphology of the 2C 18E 12 monolayer, investigated using Brewster angle microscopy, was also found to depend upon monolayer history. Bright, randomly dispersed domains of 2C 18E 12 of approximately 5 μm in size were observed during compression of the monolayer to π less than 42 mN m −1. At π of 42 mN m −1 and above, the surfactant film appeared to be almost completely ‘solid-like.’ Regardless of the extent of compression of the monolayer film, expansion of the film caused formation of chains or ‘necklaces’ of individual surfactant domains, with the extent of chain formation dependent upon pressure of compression of the monolayer and the length of time held at that pressure. Irreversible effects on 2C 18E 12 vesicle size were also seen upon temperature cycling the vesicles through their liquid–crystalline phase transition temperature with vesicles shrinking in size and not returning to their original size upon standing at 298 K for periods of more than 24 h. No comparable hysteresis, time, pressure or temperature effects were observed with the monolayer or vesicles formed by the corresponding phospholipid, disteaorylphosphatidylcholine, under identical conditions. The effects observed with 2C 18E 12 are attributed to the ability of the polyoxyethylene head group to dehydrate and intrude into the hydrophobic chain region of the mono- and bilayers. These studies have important implications for the use of the vesicles formed by 2C 18E 12 as drug delivery vehicles.