Investigating the molecular orientation and configuration in thin films of soft solid formulations

Soft solid formulations, such as solid fabric enhancer (SFE), are widely used in consumer goods industry, of which the interfacial characteristics underpin the performance and sustainability of the formulated products. Our previous studies show that the orientation of molecules at the solid/ambient interface controls the mechanical properties, which in turn influence the final product properties and performance at customer, understanding interfacial conformation of the polymer and surfactants is crucial. SFE formulations, composed of quaternary ammonium ester (EQ) and fatty acid (FA) were investigated by force spectroscopy based on atomic force microscopy (AFM) in the preliminary work. Two types of materials (A/B) were studied and showed different behaviour with formulations of type A materials showing AFM retract curves with multiple pull-offs events. X-ray photoelectron spectroscopy (XPS) was unable to differentiate between the formulation components. However, all samples studied present a characteristics break-through event in the approach curves, implying that the AFM tip passes through different molecular layers as it penetrates the coating. We propose to use neutron reflectometry in combination with differential isotopic labelling of FA and EQ components to resolve the compositional distribution as a function of distance and investigate how the interfacial structure changes as a function of humidity, mimicking conditions that are relevant to the actual product.