Singh Prabhat K, Kumbhakar Manoj, Pal Haridas, Nath Sukhendu
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, India.
J Phys Chem B. 2008 Jul 3;112(26):7771-7. doi: 10.1021/jp800701v. Epub 2008 Jun 6.
Effect of electrostatic interaction on the location of a solubilized molecular probe with ionic character in a supramolecular assembly composed of a triblock copolymer, P123 ((ethylene oxide) 20-(propylene oxide) 70-(ethylene oxide) 20) and a cosurfactant cetyltrimethylammonium chloride (CTAC) in aqueous medium has been studied using steady-state and time-resolved fluorescence measurements. Coumarin-343 dye in its anionic form has been used as the molecular probe. In the absence of the surfactant, CTAC, the probe C343 prefers to reside at the surface region of the P123 micelle, showing a relatively less dynamic Stokes' shift, as a large part of the Stokes' shift is missed in the present measurements due to faster solvent relaxation at micellar surface region. As the concentration of CTAC is increased in the solution, the percentage of the total dynamic Stokes' shift observed from time-resolved measurements gradually increases until it reaches a saturation value. Observed results have been rationalized on the basis of the mixed micellar structure of the supramolecular assembly, where the hydrocarbon chain of the CTAC surfactant dissolves into the nonpolar poly(propylene oxide) (PPO) core of the P123 micelle and the positively charged headgroup of CTAC resides at the interfacial region between the central PPO core and the surrounding hydrated poly(ethylene oxide) (PEO) shell or the corona region. The electrostatic attraction between the anionic probe molecule and the positively charged surface of the PPO core developed by the presence of CTAC results in a gradual shift of the probe in the deeper region of the micellar corona region with an increase in the CTAC concentration, as clearly manifested from the solvation dynamics results.
在水介质中，使用稳态和时间分辨荧光测量方法，研究了静电相互作用对具有离子特性的增溶分子探针在由三嵌段共聚物P123（（环氧乙烷）20 -（环氧丙烷）70 -（环氧乙烷）20）和助表面活性剂十六烷基三甲基氯化铵（CTAC）组成的超分子聚集体中的位置的影响。阴离子形式的香豆素-343染料被用作分子探针。在没有表面活性剂CTAC的情况下，探针C343倾向于驻留在P123胶束的表面区域，显示出相对较小的动态斯托克斯位移，因为由于胶束表面区域更快的溶剂弛豫，本测量中错过了大部分斯托克斯位移。随着溶液中CTAC浓度的增加，从时间分辨测量中观察到的总动态斯托克斯位移的百分比逐渐增加，直到达到饱和值。基于超分子聚集体的混合胶束结构对观察结果进行了合理化解释，其中CTAC表面活性剂的烃链溶解到P123胶束的非极性聚环氧丙烷（PPO）核中，CTAC带正电的头基位于中央PPO核与周围水合聚环氧乙烷（PEO）壳或冠层区域之间的界面区域。CTAC的存在使阴离子探针分子与PPO核带正电的表面之间产生静电吸引，导致随着CTAC浓度的增加，探针在胶束冠层区域的更深区域逐渐移动，这从溶剂化动力学结果中可以清楚地看出。
