Evolution of surface micro-structure and moisture sorption characteristics of spray-dried detergent powders

The role of water is critical to the solid state behaviour of complex formulated products such as spray-dried detergent powders. Understanding changes in water sorption behaviour due to interactions within the solid allows the influence of water sorption on the surface micro-structure to be better elucidated. This work investigates the effect of relative humidity (RH) on the micro-structural evolution and moisture sorption behaviour of model detergent powders containing the sodium salt of linear alkylbenzene sulphonate (NaLAS) and sodium sulphate. Scanning electron microscopy showed significant changes to surface morphology when the powders were exposed to 75% RH. However, the addition of sodium silicate to the formulation made the powders more susceptible to humidity-induced morphological changes. Surface crystal growth was seen at 54% RH and dramatic changes in structure were seen at 75% RH due to the emergence of the hydrated phase of sodium sulphate, i.e. mirabilite, which was confirmed by the X-ray diffraction. These differences are thought to be due to the increased hygroscopicity and water mobility conferred by the silicate and observed via sorption isotherm measurements and FTIR analysis, which suggest a change in structure due to a moisture-induced glass transition. Considerable differences in moisture absorption were observed between samples produced from the slurries differing in water content, highlighting the importance of micro-structure on moisture sorption behaviour. The experimental sorption isotherms were fitted to three mathematical models: Brunauer-Emmett-Teller (BET), Guggenheim-Anderson-de Boer (GAB) and Oswin. In nil-silicate powders, all three models presented a good fit to the experimental data with R2 ∼ 99%. However, in silicate containing powders, the GAB and Oswin fits were unsuccessful; the influence of the phase transition on the isotherm not being captured by the models.