Surfactants are used in a wide range of home and personal care products, foods, beverages and cosmetics; our explainer article gives more information on their structure and properties. Naturally occurring surfactants are becoming increasingly important in the home and personal care industries. These 'biosurfactants' can be used to manufacture biodegradable/bio-sustainable products, and are in line with Unilever's “Sustainable Living Programme". As an intermediate stage before they are used more widely, these biosurfactants are likely to be initially used in parallel with conventional synthetic surfactants, and therefore it is important to understand how the two interact.
Through a collaborative DTI grant with the universities of Oxford, Ulster and Newcastle, Unilever has used small angle neutron scattering (SANS) and neutron reflectivity (NR) at ISIS to understand the behaviour of mixtures natural and synthetic surfactants. As part of that study, in 2017 they used SANS on the LOQ and SANS2D instruments to probe the structure and self-assembly characteristics of mixtures of nonionic and anionic surfactants with the rhamnolipid biosurfactant. The results enabled the scientists to create a detailed phase diagrams. Complementary NR measurements on the adsorption of these multi-component mixtures provided a detailed assessment of their surface properties. These results enabled them to predict how different combinations would perform in future formulations, and so shorten the route to new product formulations [1].
More recently through a collaborative Innovate UK grant with a number of partners, Unilever scientists, in collaboration with the University of Oxford and ISIS, studied another but rather different type of biosurfactant, the saponin escin. The initial focus was on the adsorption behaviour at the air/water interface using NR on the SURF and INTER beamlines. NR is a particularly powerful tool for investigating surfactant mixing at interfaces, as adsorbed amounts and the surface composition can be measured directly over a wide range of surfactant concentrations. The saponin, Escin, has a molecular structure that is quite different to most synthetic surfactants and many other biosurfactants. The unusual molecular structure of the saponins gives rise to a high surface activity and results in some other unusual surface properties. These surface properties have led to its traditional use as an emulsifier and foam stabiliser in foods and beverages.
NR measurements of saponin / synthetic surfactant mixtures adsorbed at the air / water interface were made using deuterium labelled and hydrogenated synthetic surfactants (synthesised at the ISIS Deuteration Facility) to establish the surface composition and structure. The results showed that the nature of the interaction and the mixing depended critically upon the structure of synthetic surfactant and its specific interaction with the saponin molecule. The results were quantified and explained in terms of the latest developments in the thermodynamic models of non-ideal mixing [2].
The results of this study are invaluable to the wider understanding of the interaction of saponins with a wide range of conventional surfactants, and to the possibility of their wider use in a range of different product formulations. The study has also provided a basis for broader investigations of saponin / surfactant mixing at interfaces and in self-assembled aggregates.
Further information
The full papers can be found online at:
[1] http://dx.doi.org/10.1016/j.jcis.2016.10.071
[2] https://doi.org/10.1016/j.jcis.2020.04.061
