Measurement of Multicomponent Diffusion in Liquids Using Raman Microspectroscopy and Microfluidics

Diffusion is the rate-limiting mass transport step in many (bio)chemical processes. Diffusion data is therefore necessary to design unit operations, e.g., extraction. However, diffusion data on multicomponent mixtures in liquids are scarce, as diffusion measurements are intrinsically time-consuming and laborious. Also, several measurements are required for one diffusion coefficient matrix in a multicomponent mixture.

Microfluidics promises to reduce experiment time and experimental effort, while Raman spectroscopy reduces the number of necessary experiments for multicomponent mixtures. To reduce the measurement effort for diffusion coefficients in multicomponent mixtures, microfluidics and Raman microspectroscopy were combined in this work for the first time for the measurement of diffusion coefficient matrices in multicomponent mixtures: Two liquids of different concentrations co-flow in parallel in a microfluidic H-cell, while changes of concentration due to diffusion are quantified locally using Raman microspectroscopy. From these changes of concentration, the diffusion coefficient matrix is determined in a least squares procedure using a convection-diffusion-model. Thereby, diffusion coefficient matrices can be determined from a single experiment, while the known initial concentrations together with quantitative Raman spectroscopy spare additional calibration measurements. The experiment takes less than 1h and the sample consumption is below 6mL.

The developed method to determine diffusion coefficients was validated for binary and ternary systems. Additionally, new diffusion data was measured for a binary, a ternary and a quaternary system. Two and three experiments are sufficient to determine diffusion coefficient matrices with accuracy and precision for ternary and quaternary systems respectively. Systems used for validation agree with literature data. Thus, diffusion coefficient matrices of multicomponent mixture can be provided with small effort and in short time to enable the design of unit operations.