Humidity-insensitive water evaporation from molecular complex fluids

TL;DR

This study demonstrates that water evaporation rates from concentrated molecular complex fluids are largely insensitive to ambient humidity due to concentration-dependent chemical activity and diffusion effects, especially when mutual diffusion coefficients are low.

Contribution

The paper provides a theoretical analysis showing that evaporation rates in complex fluids can be humidity-insensitive, highlighting the roles of chemical activity and diffusion coefficients.

Findings

Evaporation rate depends weakly on humidity at high solute concentrations.

A decrease in mutual diffusion coefficient enhances humidity insensitivity.

Concentration gradients shield the drying interface from humidity variations.

Abstract

We investigated theoretically water evaporation from concentrated supramolecular mixtures, such as solutions of polymers or amphiphilic molecules, using numerical resolutions of a one dimensional model based on mass transport equations. Solvent evaporation leads to the formation of a concentrated solute layer at the drying interface, which slows down evaporation in a long-time scale regime. In this regime, often referred to as the falling rate period, evaporation is dominated by diffusive mass transport within the solution, as already known. However, we demonstrate that, in this regime, the rate of evaporation does not also depend on the ambient humidity for many molecular complex fluids. Using analytical solutions in some limiting cases, we first demonstrate that a sharp decrease of the water chemical activity at high solute concentration, leads to evaporation rates which depend weakly on the humidity, as the solute concentration at the drying interface slightly depends on the humidity. However, we also show that a strong decrease of the mutual diffusion coefficient of the solution enhances considerably this effect, leading to nearly independent evaporation rates over a wide range of humidity. The decrease of the mutual diffusion coefficient indeed induces strong concentration gradients at the drying interface, which shield the concentration profiles from humidity variations, except in a very thin region close to the drying interface.