Investigation of fluid diffusion kinetics in nanochannels using micro-Raman spectrometry

Fluid diffusion kinetics in nanopores is crucial for energy conversion and utilization but influenced by complex pore structure and fluid-wall interactions. Traditional experiments are difficult to decouple diffusion in nanopores and micron-pores, molecular simulations are time-consuming when handling pores with diameters larger than 10 nm, and nanofluidic experiments <i>via</i> conventional optical methods face challenges in measuring fluid concentrations. Here, we report a novel Concentration Decay Method combining nanofluidics and microscopic Raman spectroscopy to investigate diffusion in nanochannels. A "channel-channel-cell" chip design enables real-time detection of fluid concentrations in microcells and measurement of diffusion coefficients in nanochannels, and a self-made temperature control module enables precise adjustment of fluid temperature. By this method, interdiffusion experiments of an <i>n</i>-octane-1-octene mixture and <i>n</i>-octane-cyclooctane mixture in nanochannels (depths = 21-173 nm) are conducted to explore oil diffusion in shale. We report that the oil diffusion in nanochannels still conforms to Fick's diffusion law, and the diffusion coefficients in channels with a minimum depth of 21 nm and at different temperatures (295-383 K) exhibit no obvious deviation from the bulk phase, suggesting that fluid-wall interactions have no significant effect on diffusion kinetics in our experiments. The consistency of the experimental results and classical predictions also validates the reliability of our Concentration Decay Method, which fills the gap in research on fluid diffusion in nanopores and has promising application prospects. Diffusion in nanochannels with more types of fluids, more complex channel structures and smaller depth of the channel can be furthered investigated by this method.