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Published online: 10 December 2008 | doi:10.1038/nchina.2008.291

Phase transitions: Activated by carbon dioxide

Tim Reid

Introduction

© (2008) Wiley Interscience

Surfactants — molecules each comprising a hydrophilic head group and a hydrophobic tail group — have the ability to self-assemble into various structural forms, such as micelles, vesicles and liquid crystals, which are widely used as 'soft templates' for the synthesis of different materials. Buxing Han and co-workers at the Chinese Academy of Sciences in Beijing¹ have discovered a surfactant that can switch from the ordered liquid-crystal phase to the micelle phase, simply by using CO₂.

When surfactant molecules called AOT are placed in water, they form a lamellar liquid crystal — a bilayer in which the hydrophilic heads are in contact with water on each side of the bilayer (pictured). Previous studies have shown that AOT can change into micelles, but only at temperatures above 140 °C.

Han and co-workers achieved the same phase transition at room temperature, by adding compressed CO₂ gas. It seems that the CO₂ molecules move between layered molecules of AOT to break up the liquid crystal structure and form micelles. The process is easily reversed by reducing the CO₂ pressure.

The researchers used the AOT–water system to shape gold into both plates and nanoparticles, and to produce thin films and tiny hollow spheres made from silica. It represents a cheap, environmentally friendly method of material synthesis for several diverse fields.

The authors of this work are from:
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China.

Reference

1. Zhang, J., Han, B., Li, W., Zhao, Y. & Hou, M. Reversible switching of lamellar liquid crystals into micellar solutions using CO₂. Angew. Chem. Int. Ed. doi:10.1002/anie.200803753 (2008). | Article |