Dutt G B
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India.
J Chem Phys. 2008 Jul 7;129(1):014501. doi: 10.1063/1.2946705.
Rotational diffusion of two structurally similar ionic probes, rhodamine 110 and fluorescein, has been examined in nonionic reverse micellar system of Triton X-100/benzene-n-hexane/water as a function of mole ratio of the water to surfactant, W. This study has been undertaken to find out whether ionic and hydrophobic probes experience similar microenvironment in these reverse micelles. Experimental results indicate that, from W=0 to 3, the average reorientation time, which is a measure of the microviscosity experienced by the probe molecule, increases by 90% and 40% for rhodamine 110 and fluorescein, respectively, and from W=3 to 8, it decreases by 20% for both the probes. The increase in the average reorientation time with W has been rationalized on the basis of the flexible oxyethylene chains of the TX-100 surfactant being hydrogen bonded by the water molecules, which makes the core region less fluid. However, once the hydration of the oxyethylene chains is complete, further addition of water results in formation of water droplet; which renders the micelle-water interface in the core region less compact leading to a marginal decrease in the average reorientation time of the probe molecules. These explanations are consistent with the location of the probes and the structure of the Triton X-100/benzene-hexane/water reverse micelles. To compare how the microenvironment experienced by these ionic probes is different from the hydrophobic ones, results from our earlier work [J. Phys. Chem. B 108, 7944 (2004)] have been considered. Such a comparison revealed that both ionic and hydrophobic probes experience similar microenvironment in these reverse micelles until the hydration of the oxyethylene chains is complete. In case of hydrophobic probes, however, the onset of water droplet formation does not alter their microenvironment, which is due to their location in the reverse micellar cores.
在Triton X-100/苯 - 正己烷/水的非离子反胶束体系中，研究了两种结构相似的离子探针罗丹明110和荧光素的旋转扩散，该扩散是水与表面活性剂摩尔比W的函数。进行这项研究是为了弄清楚离子探针和疏水探针在这些反胶束中是否经历相似的微环境。实验结果表明，从W = 0到3，平均重取向时间（这是探针分子所经历的微粘度的一种度量），罗丹明110和荧光素分别增加了90%和40%，并且从W = 3到8，两种探针的平均重取向时间均下降了20%。平均重取向时间随W的增加可基于TX - 100表面活性剂的柔性氧乙烯链被水分子氢键合来解释，这使得核心区域流动性降低。然而，一旦氧乙烯链的水合作用完成，进一步加水会导致水滴形成；这使得核心区域的胶束 - 水界面不那么紧密，导致探针分子的平均重取向时间略有下降。这些解释与探针的位置以及Triton X - 100/苯 - 己烷/水反胶束的结构一致。为了比较这些离子探针所经历的微环境与疏水探针的微环境有何不同，考虑了我们早期工作[《物理化学杂志B》108, 第7944页(2004)]的结果。这样的比较表明，在氧乙烯链的水合作用完成之前，离子探针和疏水探针在这些反胶束中经历相似的微环境。然而，对于疏水探针，水滴形成的开始并没有改变它们的微环境，这是由于它们位于反胶束核心中。
