Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8561, Japan.
Langmuir. 2013 Jun 25;29(25):7618-28. doi: 10.1021/la400376g. Epub 2013 Jun 10.
High-pressure small-angle neutron scattering (HP-SANS) studies were conducted to investigate nanostructures and interfacial properties of water-in-supercritical CO2 (W/CO2) microemulsions with double-fluorocarbon-tail anionic surfactants, having different fluorocarbon chain lengths and linking groups (glutarate or succinate). At constant pressure and temperature, the microemulsion aqueous cores were found to swell with an increase in water-to-surfactant ratio, W0, until their solubilizing capacities were reached. Surfactants with fluorocarbon chain lengths of n = 4, 6, and 8 formed spherical reversed micelles in supercritical CO2 even at W0 over the solubilizing powers as determined by phase behavior studies, suggesting formation of Winsor-IV W/CO2 microemulsions and then Winsor-II W/CO2 microemulsions. On the other hand, a short C2 chain fluorocarbon surfactant analogue displayed a transition from Winsor-IV microemulsions to lamellar liquid crystals at W0 = 25. Critical packing parameters and aggregation numbers were calculated by using area per headgroup, shell thickness, the core/shell radii determined from SANS data analysis: these parameters were used to help understand differences in aggregation behavior and solubilizing power in CO2. Increasing the microemulsion water loading led the critical packing parameter to decrease to ~1.3 and the aggregation number to increase to >90. Although these parameters were comparable between glutarate and succinate surfactants with the same fluorocarbon chain, decreasing the fluorocarbon chain length n reduced the critical packing parameter. At the same time, reducing chain length to 2 reduced negative interfacial curvature, favoring planar structures, as demonstrated by generation of lamellar liquid crystal phases.
采用高压小角中子散射（HP-SANS）研究了具有不同氟碳链长和连接基团（戊二酸酯或琥珀酸酯）的双氟碳尾阴离子表面活性剂的超临界 CO2（W/CO2）微乳液的纳米结构和界面性质。在恒压和恒温和温度下，随着水与表面活性剂的比值（W0）的增加，发现微乳液水核会膨胀，直到达到其增溶能力。即使在超过由相行为研究确定的增溶能力的 W0 下，具有 n = 4、6 和 8 的氟碳链长的表面活性剂在超临界 CO2 中也形成了球形反胶束，表明形成了 Winsor-IV W/CO2 微乳液，然后是 Winsor-II W/CO2 微乳液。另一方面，短 C2 链氟碳表面活性剂类似物在 W0 = 25 时从 Winsor-IV 微乳液转变为层状液晶。通过使用每个头部基团的面积、壳层厚度、从 SANS 数据分析确定的核/壳半径来计算临界堆积参数和聚集数：这些参数用于帮助理解在 CO2 中聚集行为和增溶能力的差异。增加微乳液的水负载会导致临界堆积参数降低至约 1.3，聚集数增加至>90。尽管这些参数在具有相同氟碳链的戊二酸酯和琥珀酸酯表面活性剂之间是可比的，但降低氟碳链长 n 会降低临界堆积参数。同时，将链长降低到 2 会降低负的界面曲率，有利于平面结构，如层状液晶相的生成所示。
