Water Sorption Isotherms

Quick Start

Water sorption isotherms can be fitted to the GAB (Guggenheim- Anderson-de Boer) model. If you have a mix of materials (here we just use two) a weight-fraction average of the two seems to be an adequate approximation for the mixture.

Water Sorption Isotherms


A water sorption isotherm shows the amount (or fraction, I have left the units vague) of water absorbed versus the water activity, aw or, if you prefer, %RH/100. To get these isotherms you need to be able to measure the weight increase as the %RH of the air is changed, allowing enough time for equilibrium to be reached before stepping to the next level. There is often hysteresis (not shown) between the values measured going from 0 to 100% and 100% going down to 0, with the descending isotherm showing more sorption that the ascending one.

The GAB model has three parameters: M which is what would be expected from a monolayer coverage, typically in the 0.05 to 0.2 range, then C which is in the 1-50 range, and K which is an adjustment parameter, typically in the 0.5 to 1 range. When K=1 the equation is formally equivalent to the well-known BET isotherm. The GAB model calculates the Sorption, S for a given water activity, aw. Because experimental data and GAB both have problems above 0.95 activity, the graph stops at that point.


Play with the two sets of three values to describe your different components (you can check values with the mouse readout) then alter the Wt%1 slider to see how the isotherm changes.