Superspreading revealed

Materials World magazine
1 Feb 2010

A UK consortium is modelling how trisiloxane surfactants ‘superspread’ across hydrophobic surfaces at the nanoscale. The aim is to develop guidelines for synthesising safer surfactants and provide advice on how to control the stages of spreading to suit a particular application.

Aqueous trisiloxane solutions are tailored to enable superspreading of liquids over
substrates that would normally repel them. The surfactants are deposited at high concentrations above the critical wetting concentrations and spontaneously form vesicles that self-organise. This technology is exploited in sectors such as agriculture, where it enables chemicals to be absorbed by waxy leaves.

However, the toxicity of trisiloxanes makes them unsuitable for use in the pharmaceutical and personal care industries, explains Professor Victor Starov at Loughborough University, UK. ‘Currently [these industries] are unable to use superspreaders,’ he says, ‘but they are interested to understand how the liquids spread to try to develop something similar’.

A team at Loughborough and the University of Nottingham, UK, has characterised the nanoscale forces that occur during superspreading by depositing trisiloxane solutions onto hydrophobic Teflon surfaces. They measured the process using infrared and atomic force spectroscopy.

‘We established that the spreading behaviour proceeds in two stages – [a] fast stage, which is followed by the slower stage,’ explains Starov. ‘The duration of the first stage correlates with the duration of surfactant adsorption on the liquid-vapour interface.

‘The second stage is determined by a spontaneous transfer of surfactant molecules from the droplet surface onto a bare hydrophobic substrate in front of the moving three-phase contact line. The latter process results in partial hydrophilisation of the surface in front of the droplet [and] a reduction of total excess free energy of the whole droplet in spite of local increase in solid-vapour interfacial tension.’

By developing mathematical models of these steps, the researchers hope to inform future surfactant synthesis and optimise the superspreading process.

‘Varying the concentration and other parameters [will] control the duration of those two stages,’ says Starov. The chemists are working with a number of industry partners that remain undisclosed.