Observing individual thermal motions of ions, and molecules in water with light

TL;DR

This paper demonstrates observing individual ions and molecules in water through light extinction, analyzing their thermal motions to extract diffusion constants and particle sizes, aligning with macroscopic measurements.

Contribution

It introduces a method to observe and analyze thermal motions of individual particles in water using light extinction, providing new insights at the single-particle level.

Findings

Measured spectra match theoretical predictions based on random motions.

Extracted diffusion constants are consistent with previous macroscopic data.

Estimated particle sizes align with known values.

Abstract

We observe thermal motions of ions, and molecules in water through light extinction, at the individual particle level. The motions appear as time dependent intensity variations, characterized through their averaged spectra. Theoretical spectrum derived from random motions of one particle describes these measured spectra. This theory is used to extract diffusion constants of liquid mixtures and solutions, that correspond to binary diffusion, and thermal diffusion, which are consistent with previous macroscopic measurements. We also estimate the sizes of the particles.