van der Loop Tibert H, Ottosson Niklas, Lotze Stephan, Kentzinger Emmanuel, Vad Thomas, Sager Wiebke F C, Bakker Huib J, Woutersen Sander
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG Amsterdam, The Netherlands.
J Chem Phys. 2014 Nov 14;141(18):18C535. doi: 10.1063/1.4898380.
We study the reorientation dynamics of liquid water confined in nanometer-sized reverse micelles of spherical and cylindrical shape. The size and shape of the micelles are characterized in detail using small-angle x-ray scattering, and the reorientation dynamics of the water within the micelles is investigated using GHz dielectric relaxation spectroscopy and polarization-resolved infrared pump-probe spectroscopy on the OD-stretch mode of dilute HDO:H2O mixtures. We find that the GHz dielectric response of both the spherical and cylindrical reverse micelles can be well described as a sum of contributions from the surfactant, the water at the inner surface of the reversed micelles, and the water in the core of the micelles. The Debye relaxation time of the core water increases from the bulk value τ(H2O) of 8.2 ± 0.1 ps for the largest reverse micelles with a radius of 3.2 nm to 16.0 ± 0.4 ps for the smallest micelles with a radius of 0.7 nm. For the nano-spheres the dielectric response of the water is approximately ∼6 times smaller than expected from the water volume fraction and the bulk dielectric relaxation of water. We find that the dielectric response of nano-spheres is more attenuated than that of nano-tubes of identical composition (water-surfactant ratio), whereas the reorientation dynamics of the water hydroxyl groups is identical for the two geometries. We attribute the attenuation of the dielectric response compared to bulk water to a local anti-parallel ordering of the molecular dipole moments. The difference in attenuation between nano-spheres and nano-cylinders indicates that the anti-parallel ordering of the water dipoles is more pronounced upon spherical than upon cylindrical nanoconfinement.
我们研究了限制在球形和圆柱形纳米级反胶束中的液态水的重新取向动力学。使用小角X射线散射详细表征了胶束的尺寸和形状，并使用GHz介电弛豫光谱和偏振分辨红外泵浦 - 探测光谱对稀HDO:H₂O混合物的OD伸缩模式研究了胶束内水的重新取向动力学。我们发现，球形和圆柱形反胶束的GHz介电响应都可以很好地描述为表面活性剂、反胶束内表面的水以及胶束核心中的水的贡献之和。对于半径为3.2 nm的最大反胶束，核心水的德拜弛豫时间从本体值τ(H₂O) = 8.2 ± 0.1 ps增加到半径为0.7 nm的最小胶束的16.0 ± 0.4 ps。对于纳米球，水的介电响应比根据水的体积分数和水的本体介电弛豫预期的小约6倍。我们发现，相同组成（水 - 表面活性剂比）的纳米球的介电响应比纳米管的更衰减，而两种几何形状的水羟基的重新取向动力学是相同的。我们将与本体水相比介电响应的衰减归因于分子偶极矩局部反平行排列。纳米球和纳米圆柱之间衰减的差异表明，水偶极的反平行排列在球形纳米限制下比在圆柱形纳米限制下更明显。
