Department of Chemistry, Stanford University, Stanford, California 94305, United States.
J Phys Chem B. 2011 Oct 13;115(40):11658-70. doi: 10.1021/jp206903k. Epub 2011 Sep 22.
Water dynamics inside of reverse micelles made from the surfactant Aerosol-OT (AOT) were investigated by observing spectral diffusion, orientational relaxation, and population relaxation using two-dimensional infrared (2D IR) vibrational echo spectroscopy and pump-probe experiments. The water pool sizes of the reverse micelles studied ranged in size from 5.8 to 1.7 nm in diameter. It is found that spectral diffusion, characterized by the frequency-frequency correlation function (FFCF), significantly changes as the water pool size decreases. For the larger reverse micelles (diameter 4.6 nm and larger), the 2D IR signal is composed of two spectral components: a signal from bulk-like core water, and a signal from water at the headgroup interface. Each of these signals (core water and interfacial water) is associated with a distinct FFCF. The FFCF of the interfacial water layer can be obtained using a modified center line slope (CLS) method that has been recently developed. The interfacial FFCFs for large reverse micelles have a single exponential decay (∼1.6 ps) to an offset plus a fast homogeneous component and are nearly identical for all large sizes. The observed ∼1.6 ps interfacial decay component is approximately the same as that found for bulk water and may reflect hydrogen bond rearrangement of bulk-like water molecules hydrogen bonded to the interfacial water molecules. The long time offset arises from dynamics that are too slow to be measured on the accessible experimental time scale. The influence of the chemical nature of the interface on spectral diffusion was explored by comparing data for water inside reverse micelles (5.8 nm water pool diameter) made from the surfactants AOT and Igepal CO-520. AOT has charged, sulfonate head groups, while Igepal CO-520 has neutral, hydroxyl head groups. It is found that spectral diffusion on the observable time scales is not overly sensitive to the chemical makeup of the interface. An intermediate-sized AOT reverse micelle (water pool diameter of 3.3 nm) is analyzed as a large reverse micelle because it has distinct core and interface regions, but its core region is more constrained than bulk water. The interfacial FFCF for this intermediate-sized reverse micelle is somewhat slower than those found for the larger reverse micelles. The water nanopools in the smaller reverse micelles cannot be separated into core and interface regions. In the small reverse micelles, the FFCFs are biexponential decays to an offset plus a fast homogeneous component. Each small reverse micelle exhibits an ∼1 ps decay time, which may arise from local hydrogen bond fluctuations and a slower, ∼6-10 ps decay, which is possibly due to slow hydrogen bond rearrangement of noninterfacial water molecules or topography fluctuations at the interface.
通过观察二维红外（2D IR）振动回声光谱和泵浦探测实验中的光谱扩散、取向弛豫和粒子数弛豫，研究了由表面活性剂 Aeroso l-OT（AOT）形成的反胶束内的水动力学。所研究的反胶束的水腔尺寸范围为 5.8 至 1.7nm 直径。结果发现，水腔尺寸减小会显著改变以频率-频率相关函数（FFCF）为特征的光谱扩散。对于较大的反胶束（直径 4.6nm 及以上），2D IR 信号由两个光谱分量组成：来自类似体相的核心水的信号和来自头部基团界面的水的信号。这些信号中的每一个（核心水和界面水）都与一个独特的 FFCF 相关联。可以使用最近开发的改进中心线斜率（CLS）方法来获得界面水层的 FFCF。大反胶束的界面 FFCF 具有单指数衰减（1.6ps）到一个偏移量加上一个快速的均匀分量，并且对于所有大尺寸几乎相同。观察到的1.6ps 界面衰减分量与体相水的相同，可能反映了与界面水氢键合的类似体相水分子的氢键重排。长时偏移量来自在可访问的实验时间尺度上测量太慢的动力学。通过比较由表面活性剂 AOT 和 Igepal CO-520 形成的反胶束（5.8nm 水腔直径）内水的光谱扩散数据，研究了界面化学性质对光谱扩散的影响。AOT 具有带电的磺酸盐头基，而 Igepal CO-520 具有中性的羟基头基。结果发现，在可观察的时间尺度上，光谱扩散对界面的化学组成并不敏感。一个中等大小的 AOT 反胶束（水腔直径为 3.3nm）被分析为一个大反胶束，因为它具有明显的核心和界面区域，但它的核心区域比体相水更受限制。对于这种中等大小的反胶束，界面 FFCF 比那些大反胶束慢一些。较小的反胶束中的纳米水腔不能分为核心和界面区域。在小反胶束中，FFCF 是双指数衰减到一个偏移量加上一个快速的均匀分量。每个小反胶束都表现出约 1ps 的衰减时间，这可能是由于局部氢键波动和较慢的~6-10ps 衰减，这可能是由于非界面水分子的缓慢氢键重排或界面处的形貌波动引起的。
