Ajith V J, Patil Shivprasad
Department of Physics, Indian Institute of Science Education and Research Pune, Pune 411008, Maharashtra, India.
Langmuir. 2022 Jan 25;38(3):1034-1044. doi: 10.1021/acs.langmuir.1c02550. Epub 2022 Jan 10.
Diffusion of tracer dye molecules in water confined to the nanoscale is an important subject with a direct bearing on many technological applications. It is not yet clear, however, if the dynamics of water in hydrophilic as well as hydrophobic nanochannels remains bulk-like. Here, we present diffusion measurement of a fluorescent dye molecule in water confined to the nanoscale between two hydrophilic surfaces whose separation can be controlled with a precision of less than a nm. We observe that the fluorescence intensities correlate over fast (∼30 μs) and slow (∼1000 μs) time components. The slow time scale is due to adsorption of fluorophores to the confining walls, and it disappears in the presence of 1 M salt. The fast component is attributed to diffusion of dye molecules in the gap. It is found to be bulk-like for sub-10 nm separations and indicates that the viscosity of water under confinement remains unaltered up to a confinement gap as small as ∼5 nm. Our findings contradict some of the recent measurements of diffusion under nanoconfinement; however, they are consistent with many estimates of self-diffusion using molecular dynamics simulations and measurements using neutron scattering experiments.
示踪染料分子在局限于纳米尺度的水中的扩散是一个重要课题，与许多技术应用直接相关。然而，目前尚不清楚亲水性和疏水性纳米通道中的水动力学是否仍类似本体水。在此，我们展示了一种荧光染料分子在局限于纳米尺度的水中的扩散测量，该水处于两个亲水性表面之间，其间距可控制在小于1纳米的精度。我们观察到荧光强度在快速（约30微秒）和慢速（约1000微秒）时间分量上相关。慢速时间尺度归因于荧光团吸附到限制壁上，并且在1M盐存在时消失。快速分量归因于染料分子在间隙中的扩散。发现在间距小于10纳米时它类似本体扩散，这表明在局限条件下水的粘度在小至约5纳米的局限间隙内保持不变。我们的发现与最近一些关于纳米局限条件下扩散的测量结果相矛盾；然而，它们与许多使用分子动力学模拟的自扩散估计以及使用中子散射实验的测量结果一致。
