Liquid crystalline phases of linear alkylbenzene sulphonate in spray-dried detergent powders studied by small-angle X-ray scattering, TEM, and ATR-IR spectroscopy

Abstract

Spray-dried detergent granules have a complex and multi-scale structure, in which submicron sized crystals of inorganic salts along with multi-lamellar stacks of surfactant molecules are dispersed within a matrix phase. In this study, two model formulations based on sodium linear alkylbenzene sulphonate (NaLAS) and sodium sulphate were used to investigate the molecular structure of stacked lamellae. The parameters of lamellar d-spacing, bilayer thickness (dHH) and water layer thickness (dW) were determined using small-angle X-ray scattering. Four sets of lamellae were detected in nil-silicate detergent powders conditioned at 33 % RH (relative humidity). The exposure of these samples to a higher RH level (75 %) resulted in a reduced number of coexisting lamellar phases by two. This was accompanied by an increase in lamellar d-spacings and a considerable reduction in bilayer thicknesses (dHH). The reduction in number of sets of lamellae was explained by an increase in the fluidity of the lamellar phases and disappearance of water-poor solid polymorphs. This was consistent with the results of FTIR for nil-silicate samples, indicating an increase in the conformational disorder of the alkyl chains at a higher RH value. Interestingly, in the presence of sodium silicate the lamellar phases display a greater degree of swelling at a relatively low RH value (33 % RH) as compared with those in the absence of sodium silicate. Nonetheless, the thickness of water layer was found to slightly decrease at a higher RH (75 %), which was attributed to the moisture-induced glass transition and kosmotropic effect of sodium silicates.