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.

软固体配方（如固体织物柔顺剂（solid fabric enhancer, SFE））在消费品行业中应用广泛，其界面特性是决定配方产品性能与可持续性的核心支撑。我们此前的研究表明，固-环境界面处的分子取向决定了材料的力学性能，进而影响最终产品的属性与用户端使用表现，因此解析聚合物与表面活性剂的界面构象至关重要。本团队前期工作采用基于原子力显微镜（atomic force microscopy, AFM）的力光谱技术，对由季铵酯（quaternary ammonium ester, EQ）与脂肪酸（fatty acid, FA）复配而成的SFE配方展开了研究。本次研究涉及两类材料（A、B），二者表现出截然不同的行为：A型材料的AFM回缩曲线呈现出多个脱附峰事件。X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）无法区分该配方中的不同组分，但所有被测样品的AFM趋近曲线均呈现出典型的穿透事件，这表明AFM探针在穿透涂层的过程中，依次穿过了不同的分子层。本研究拟采用中子反射术，并结合FA与EQ组分的差分同位素标记技术，以解析组分分布随距离的变化规律，并探究界面结构随湿度的演化情况，从而模拟与实际产品使用场景相符的环境条件。