(627e) Effect of Structural Variations of Sugar-Based Surfactants (alkyl polyglucosides) on Interfacial and Surface Tension

Sugar-based surfactants produced from renewable raw materials such as starch and fats and their derivatives have attracted considerable interest due to their sustainable nature and biocompatibility. Amongst these “green surfactants” we have studied the particular family of Alkyl glucosides (AG) and alkyl polyglucosides (APG). APG surfactants are non-ionic surfactants characterised by the length of the alkyl chain tail (hydrophobic region) and the average number of glycose units linked to itforming part of the head group (hydrophilic region). While the general structure of an APG consists of only one sugar-based head group and a linear alkyl chain tail group, the overarching interest is to explore other possible morphologies, the incorporation of side-chains, other chemical groups (e.g. aromatic rings) to enhance the “tunability” of the overall surfactant [1].

In order to assess the effect of the different morphologies on the interfacial and surface tension, large scale atomistic canonical Molecular Dynamics (MD) simulations were performed for systems consisting of amphiphiles at the flat interfaces of water + n-decane and water + air. APG surfactants were modelled using the polymer compatible force field (pcff+). The values of interfacial tension were calculated using the Irving-Kirkwood method [2], through the integration of the difference between the normal and tangential pressure profiles against the coordinate perpendicular to the interface.

The surfactant with a single-glucoside head and a linear tail with 12 carbon atoms (APG₁₂) is used as a reference and benchmark to assess the difference with the other structural variations. This APG can reduce the interfacial tension for the water/oil system to close-to-zero values, while for the water/air system the tension is reduced to around 30 mN/m. Simulations have been run for over 30 different combinatorial variants of APG₁₂ changing either the structure of the tail group, the head or both. Two of the more promising alternatives are shown in Figure 1, corresponding to a single-glucoside head and a branched tail with 12 carbon atoms (isomer), where one branch has 8 carbon atoms and the other one has 4 carbons. This structure can reach a lower critical micelle concentration, decreasing the interfacial tension faster than the regular APG₁₂ configuration. Another example of an improved morphology consists of using a double-glucoside head and a linear tail with 12 carbon atoms. The surfactant with two glucoside rings shows a good performance at low surface coverages, decreasing the tension even faster than the branched structure. However, with a wider hydrophilic head, the system reaches the point of surface saturation before the other two structures.

One of our key findings is that branching of the tails provides a more stable interface with a lower critical micelle concentration. Amongst the options for branching tails, the more symmetrical the branches, the lower the surface coverage needed to reach the saturation point. Simulations provide key evidence on how the structure of the surfactant plays an important role in the stability of the interface. In particular, for APG’s, given the bulkier nature of the headgroup, the surfactant benefits from tail groups that fill the space more evenly. This space-filling is typically carried out by adding co-surfactants to the formulations but can be easily engineered within the main surfactant.

This work encourages the validation of these findings through experiments and showcases the potential that molecular modelling has for the optimization of molecular fluids towards the ultimate deployment of bio-based surfactants in industrial, pharma and commercial applications.

[1] T. O. Gaudin et al., “Impact of the chemical structure on amphiphilic properties of sugar-based surfactants: A literature overview,” Advances in Colloid and Interface Science, 270, 87–100 (2019).

[2] J. H. Irving and J. G. Kirkwood. Journal of Chemical Physics, 18:817–829, 1950.