Formulating liquid detergents with naturally derived surfactants—Phase behaviour, crystallisation and rheo-stability of primary alkyl sulphates based on coconut oil

Abstract

There has been a drive to replace synthetic petrochemical-based linear alkylbenzene sulphonates (LAS) in liquid detergents with renewable equivalents such as the coconut oil-based primary alkyl sulphate (cocoPAS). However, liquids containing cocoPAS “solidify” on storage at low ambient temperatures because of the natural tendency of PAS to crystallise in such conditions. This paper explores possible ways of formulating PAS-based liquid detergents with improved low temperature stability. To understand the mechanisms of the “solidification” we studied the phase behaviour of a simplified commercial PAS/water system along with full formulations containing cocoPAS, non-ionic surfactant, soap and electrolytes. We employed an array of techniques including optical microscopy, SAXS, WAXS, electron and light microscopy DSC, rheometry, ultracentrifgation and conductimetry to describe the phase behaviour of both the binary and mixed systems. The simplified commercial PAS system behaves in a manner very similar to SDS, forming hexagonal, intermediate and lamellar phases. For full formulations containing sodium dodecyl sulphate (SDS), there was a slow precipitation of SDS crystals within the lamellar regions of the “liquid”. In cocoPAS formulations there was no crystallisation, but a partial transition of lamellar (L α) phase to gel (L β) phase, as confirmed by a characteristic X-ray reflection at 4.2 Å. We found that the gel formation could be effectively prevented by increasing the fatty acid to soap ratio of the formulations by lowering the pH close to the p K a of fatty acids in bilayers. We speculate here that this approach most likely operates via preventing the PAS head group from close packing in the lamellar bilayers by a simple ‘dilution’ effect, reducing the electrostatic repulsion between the ionic groups and increasing the number of components in the bilayers.