Foams are a colloidal phase of matter, containing a gas dispersed through a liquid. They find use in many applications and have excellent properties for alcohol-based hand rubs. However, producing foams in alcohol-water mixtures is challenging, and requires exotic siloxane polyethers surfactants. In addition, little is known about the structure-property relations which govern properties such as foamability in these systems. An empirical model was developed which connects chemical, interfacial and foaming behaviour for siloxane polyethers and facilitate the development of higher performance foaming agents.
A set of triblock siloxane polyether surfactants have been characterised. Foam fractionation and solvent extraction were shown to successfully remove impurities from the surfactant and improve performance. In addition, empirical rules were developed to identify a feasible region in which surfactants are predicted to disperse well in water-ethanol mixtures.
Several methods were explored to determine the critical micelle concentration for a series of siloxane polyethers. The critical micelle concentration was shown to decline rapidly as the size of the siloxane block and the relative proportion of water in the solvent are increased.
Foaming performance was explored using experimental design and regression modelling. Strong interaction effects were found between surfactant composition and ethanol content, with less siloxane-rich surfactants performing worse in ethanol-rich solutions. Significant differences in foamabilities are observed between different foaming methods.
Du Noüy ring tensiometry and the maximum bubble pressure method are used to measure dynamic surface tension at a range of surface ages. As predicted by the critical micelle concentration, water-rich solutions are shown to be more kinetically limited than ethanol-rich solutions. Surface pressures measured at the characteristic time for different foaming methods are shown to accurately predict foamability.
In conclusion, it is possible to draw direct links between surfactant composition, interfacial behaviour and foaming methods. Solvent composition and foaming method are shown to be important variables in designing a surfactant for foaming performance.
