Measuring mutual diffusion coefficients in aqueous binary mixtures with unidimensional drying cells. Issue 2 (15th December 2022)

• Record Type: Journal Article
• Authors: Roger, Kevin; Atasi, Omer; Lalanne, Benjamin
• Is Part Of: Physical chemistry chemical physics. Volume 25:Issue 2(2023)
• Page Start: 994
• Page End: 998
• Publication Date: 2022-12-15
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/d2cp03118c
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Unidirectional drying of a mixture leads to the build-up of a concentration gradient in the vicinity of the air/liquid interface, which can be transformed analytically into an expression of the concentration-dependent mutual diffusion coefficient. Abstract : Chemical diffusion is a mass transport process caused by thermally generated motions of species. In a binary mixture, the diffusion of one species in one direction involves the diffusion of another species in the opposite direction, which corresponds to a single mutual diffusion coefficient. Here, we report a simple and general method to measure such coefficients in binary liquid mixtures, using the PNIPAM/water system as a study case. Experimentally, we show how a simple unidirectional drying cell coupled with a spatially-resolved characterization method such as Raman microscopy can yield concentration gradients developing in between two boundaries of known and constant chemical potential. Acquiring such gradients over time leads to a time-set that is shown to collapse to a single master curve using a change of variable. Such a scaling law offers a self-checking frame for solving analytically the diffusion–advection equation. As a result, we show that a simple analytical formula relates the measured concentration gradient with the concentration-dependent mutual diffusion coefficient. In the PNIPAM/water system, the mutual diffusion coefficient sharply decreases at low water content. Our work thus highlights the importance of considering the concentration-dependence of the mutual diffusion coefficient in complex aqueous solutions and provides a method to measure it.