Liquid crystals: Cool transitions

Compressed carbon dioxide is used to control the self-assembly of surfactants at ambient temperatures without cosurfactants.

The ability of surfactants to self-assemble into different structures is used in fields such as detergency, agrochemicals, drug delivery and chemical engineering. The transition from one type of structure —or phase— to another has potential uses in many applications. However, such transitions usually occur due to changes in temperature or the addition of cosurfactants—both of which could be detrimental to the application.

Compressed CO₂ has been shown to have an effect on this phase behaviour, and Buxing Han and colleagues¹ in China have now shown that it can be used to reversibly switch the lamellar liquid-crystal phase of the sodium bis(2-ethylhexyl) sulphosuccinate (AOT)/water system into solutions of micelles, at ambient temperature and without the need for cosurfactants. In the absence of CO₂this phase transition occurs above 140°C in the composition range studied.

Fig. 1: Transmission electron microscope images of a, gold plates prepared in AOT/H₂O lamellar liquid crystal (1:1 by weight) without CO₂, and b, gold particles prepared in the AOT/H₂O/CO₂ system at 4.98 MPa.

The lamellar phase comprises planar layers of surfactant molecules with their heads and tails respectively aligned, and the micelles are spherical aggregates. The researchers suggest that when compressed CO₂ is applied to the lamellar phase of the AOT/water system, the CO₂ molecules enter the area between the tails of the AOT molecules, making it difficult for them to align in parallel. The surfactant layers then bend to form spherical aggregates with the CO₂ molecules sitting in their cores. Decreasing the pressure of CO₂ then reverses this process and the AOT molecules once again form lamellar layers. The CO₂ can be easily removed from the process and recycled.

Han and colleagues also showed that the CO₂/AOT/water system could be used as a template for the synthesis of gold or silica nanostructures, namely, flat gold plates or silica film formed in the absence of CO₂, but spherical gold nanoparticles or hollow silica spheres were produced when CO₂ was added. An advantage of using CO₂ for these syntheses is that no reducing agent or catalyst was needed.

The ready availability, low-cost, non-toxicity, non-flammable nature and recyclability of CO₂ makes it an attractive means of controlling the phase behaviour of surfactant assemblies for many applications, and the researchers suggest that it could result in unique properties. They plan future work on studying the effect of CO₂ on other surfactant aggregates.