Messaoud Taoufik, Duplâtre Gilles, Waton Gilles, Michels Bernard
Institut de Recherches Subatomiques, UMR 7500 CNRS/IN2P3-Université Louis Pasteur, B.P. 28, 67037, Strasbourg Cedex 2, France.
Phys Chem Chem Phys. 2005 Nov 21;7(22):3839-44. doi: 10.1039/b510439d. Epub 2005 Sep 15.
The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores.
基于正电子湮没寿命谱（PALS）测定胶束平均核半径R(core)和聚集数N(ag)的新方法，被应用于研究三嵌段普朗尼克P84共聚物水溶液在超过凝胶化点（334 K）时随温度（T）的变化。PALS谱中出现两个长寿命成分，分别归因于本体水中的三重态正电子素（o-Ps(aq)）和胶束有机核中的三重态正电子素（o-Ps(org)）。在各种拟合参数中，只有后者的寿命tau4以及胶束参数R(core)和N(ag)，在升至334 K时先增加，然后下降，揭示了凝胶化的发生。与相变情况不同的是，在334 K时没有任何参数显示出突然变化，而溶液的宏观粘度却急剧增加。这归因于正电子素对溶液微观粘度敏感这一事实。在转变点，形成P84胶束有机核的聚氧丙烯聚集体的性质没有受到很大影响。此外，实验得到的N(ag)值与根据R(core)值计算出的球形胶束的N(ag)值一致，这表明P84核在凝胶化后的形状没有显著变化。凝胶化的开始是由于溶液中氢键相互作用减少，随之聚氧丙烯（PPO）基团之间的相互作用相对增加，最初这些基团形成P84胶束的冠层，以胶束间模式进行。PPO基团在其初始位置之外受到的这种增强的吸引，将导致形成胶束的表面活性剂分子数量减少，即334 K以上时R(core)和N(ag)均下降。从这些数据中，可以获得关于胶束核内局部粘度和表面张力的更多信息。
