目的:极性非水介质的氢键键合能力显著影响表面活性剂在这些介质中的自组装行为。
背景:甘油,非水氢键溶剂,由于其理想的物理性质,广泛用于工业配方。表面活性剂在此类应用中无处不在;然而,表面活性剂在甘油中的自组装还没有很好的理解。
方法:使用一系列成像技术研究了凝胶相的微观结构:偏振光显微镜(PLM),共聚焦激光扫描显微镜(CLSM),和环境扫描电子显微镜(ESEM)。使用粘度测定法和振荡流变学测量研究了凝胶的流变性质。使用小角度中子散射(SANS)进行了进一步的纳米结构表征。
结果:我们观察到在低至~2wt%的临界凝胶化浓度(CGC)下,在SDS和甘油混合物中意外地形成微原纤维凝胶;在水性系统中未观察到这种SDS凝胶化。凝胶的微观结构由长度为几毫米且平均直径为D〜0.5μm的微纤维组成。凝胶相中的纤维在粘度测量中表现出剪切诱导的排列,振荡测试表明凝胶是粘弹性的,具有弹性主导的行为。拟合SANS曲线显示室温下凝胶微纤维中的层状纳米结构,在临界凝胶化温度以上转化为圆柱形胶束溶液,TCG~45℃
结论:这些前所未有的观察结果突出了在水性和非水性H键溶剂中明显不同的自组装行为,这一点目前还没有得到很好的理解。破译这种自组装行为是在基本层面上进一步理解自组装的关键。
OBJECTIVE: Hydrogen-bonding capacities of polar nonaqueous media significantly affect self-assembly behaviours of surfactants in these media.
BACKGROUND: Glycerol, a nonaqueous hydrogen-bonding solvent, is widely used in industrial formulations due to its desirable physical properties. Surfactants are ubiquitous in such applications; however, surfactant self-assembly in glycerol is not well understood.
METHODS: The microscopic structure of the gel phase was studied using a series of imaging techniques: polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and environmental scanning electron microscopy (ESEM). The rheological properties of the gel were studied using viscometry and oscillation rheology measurements. Further nano-structural characterisation was carried out using small-angle neutron scattering (SANS).
RESULTS: We have observed the unexpected formation of a microfibrillar gel in SDS and glycerol mixtures at a critical gelation concentration (CGC) as low as ~2 wt%; such SDS gelation has not been observed in aqueous systems. The microscopic structure of the gel consisted of microfibres some mm in length and with an average diameter of D ~ 0.5 μm. The fibres in the gel phase exhibited shear-induced alignment in the viscometry measurements, and oscillation tests showed that the gel was viscoelastic, with an elastic-dominated behaviour. Fitting to SANS profiles showed lamellar nano-structures in the gel microfibres at room temperature, transforming into cylindrical-micellar solutions above a critical gelation temperature, TCG ~ 45 °C.
CONCLUSIONS: These unprecedented observations highlight the markedly different self-assembly behaviours in aqueous and nonaqueous H-bonding solvents, which is not currently well understood. Deciphering such self-assembly behaviour is key to furthering our understanding of self-assembly on a fundamental level.