Laboratory of Food Proteins and Colloids, School of Food Science and Engineering, Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, South China University of Technology, Guangzhou 510640, China.
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China.
Soft Matter. 2024 Feb 7;20(6):1173-1185. doi: 10.1039/d3sm00973d.
Glycyrrhizic acid (GA), a naturally derived food-grade saponin molecule, is a promising alternative to synthetic surfactants for stabilizing multiphase systems including emulsions and foams, due to its biological activity and surface-active properties. Understanding the interfacial behavior of GA, particularly in relation to its complex self-assembly behaviors in water induced by multiple environmental stimuli, is crucial to its application in multiphase systems. In this study, we comprehensively investigate the interfacial structure and rheological properties of GA systems, as a function of pH and temperature, through Langmuir-Blodgett films combined with atomic force microscopy, interfacial particle tracking, adsorption kinetics, stress-relaxation behavior and interfacial dilatational rheology. The variation of solution pH provokes pronounced changes in the interfacial properties of GA. At pH 2 and 4, GA fibril aggregates/fibrils adsorb rapidly, followed by rearrangement into large lamellar and rod-like structures, forming a loose and heterogeneous fibrous network at the interface, which exhibit a stretchable gel-like behavior. In contrast, GA at pH 6 and 8, featuring micelles or monomers in solutions, adsorb slowly to the interface and re-assemble partially into small micelle-like or irregular structures, which lead to a dense and homogeneous interfacial layer with stiffer glassy-like responses. With successively elevated temperature, the GA structures (pH 4) at the interface break into smaller fragments and further adsorption is promoted. Upon cooling, the interfacial tension of GA further decreases and a highly elastic interfacial layer may be formed. The diverse GA assemblies in bulk solution impart them with rich and intriguing interfacial behaviors, which may provide valuable mechanistic insights for the development of novel edible soft matter stabilized by GA.
甘草酸（GA）是一种天然衍生的食品级皂苷分子，由于其生物活性和表面活性特性，是稳定多相体系（包括乳液和泡沫）的合成表面活性剂的有前途的替代品。了解 GA 的界面行为，特别是在多种环境刺激诱导下其复杂的自组装行为，对于其在多相体系中的应用至关重要。在这项研究中，我们通过 Langmuir-Blodgett 薄膜结合原子力显微镜、界面粒子跟踪、吸附动力学、应力松弛行为和界面扩张流变学，全面研究了 GA 体系的界面结构和流变性质，作为 pH 和温度的函数。溶液 pH 的变化会引起 GA 界面性质的明显变化。在 pH 2 和 4 时，GA 纤维状聚集物/纤维快速吸附，然后重新排列成大的层状和棒状结构，在界面形成一个松散且不均匀的纤维状网络，表现出可拉伸的凝胶状行为。相比之下，GA 在 pH 6 和 8 时，在溶液中呈现胶束或单体，缓慢吸附到界面并部分重新组装成小胶束状或不规则结构，导致界面层致密且均匀，具有刚性玻璃状响应。随着温度的升高，界面上的 GA 结构（pH 4）会破裂成更小的碎片，并进一步促进吸附。冷却时，GA 的界面张力进一步降低，可能会形成高度弹性的界面层。GA 在本体溶液中的多种组装形式赋予其丰富而有趣的界面行为，这可能为开发由 GA 稳定的新型可食用软物质提供有价值的机制见解。
