Li Zifu, Richtering Walter, Ngai To
Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 2G6, AB, Canada.
Soft Matter. 2014 Sep 7;10(33):6182-91. doi: 10.1039/c4sm00888j.
Understanding the interfacial properties of soft poly(N-isopropylacrylamide) (PNIPAM) microgels covering an oil-water interface is essential for engineering stimuli-responsive emulsions stabilized by soft microgel particles. This study presents a systematic study on the interfacial properties of the PNIPAM-microgel-laden heptane-water interface as a function of temperature. We measured the interfacial tensions as well as dilatational rheology properties of the microgel-laden heptane-water interface using a pendant drop tensiometer. From fresh droplet experiments, the anomalous interfacial tension minima of the microgels covered oil-water interface were observed around the volume phase transition temperature (VPTT) of the PNIPAM microgels. Such interfacial tension minima are observable regardless of the microgel aggregates. Both dynamic and static parameters contributed to the observed interfacial tension minima around VPTT. The PNIPAM microgel deformability dynamically dominated the microgel spreading at the heptane-water interface in the early states, while PNIPAM microgel packing and interactions dominated the final static equilibrium states. Combining the interfacial tension and the dilatational rheology properties, we propose that the microgels would approach three distinctive states at temperatures below, around, and above VPTT at the heptane-water interface. Single droplet experiments further demonstrate that there exists an irreversible transition among these three states. The results of this study deepen our understanding of soft, porous, and deformable microgels' behaviors at the oil-water interface and have important implications for engineering microgels as stimuli-responsive emulsion stabilizers.
了解覆盖油水界面的柔软聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶的界面性质，对于设计由柔软微凝胶颗粒稳定的刺激响应乳液至关重要。本研究对负载PNIPAM微凝胶的庚烷-水界面的界面性质随温度的变化进行了系统研究。我们使用悬滴张力仪测量了负载微凝胶的庚烷-水界面的界面张力以及拉伸流变学性质。通过新鲜液滴实验，在PNIPAM微凝胶的体积相变温度（VPTT）附近观察到微凝胶覆盖的油水界面出现异常的界面张力最小值。无论微凝胶聚集体如何，这种界面张力最小值都是可观察到的。动态和静态参数都导致了在VPTT附近观察到的界面张力最小值。在早期状态下，PNIPAM微凝胶的可变形性动态主导了微凝胶在庚烷-水界面的铺展，而PNIPAM微凝胶的堆积和相互作用主导了最终的静态平衡状态。结合界面张力和拉伸流变学性质，我们提出微凝胶在庚烷-水界面温度低于、接近和高于VPTT时会接近三种不同的状态。单液滴实验进一步证明这三种状态之间存在不可逆转变。本研究结果加深了我们对柔软、多孔且可变形的微凝胶在油水界面行为的理解，对将微凝胶设计为刺激响应乳液稳定剂具有重要意义。
