In the sorption Module, the fugacity (used to compute chemical potential of sorbates) is require for calculating gas (fluid) adsorption in porous media. I am confused how to determined the fugacity when a liquid adsorption involved. I think the activity should be used to calculate chemical potential if the sorbate is a liquid.For example, the  water at 1 atm, 298K is a liquid and has a very small fugacity. If fugacity is feeded to compuation, then very low sorption amount is got, which is not the case. Should I use the acvitiy other than fugacity in the compuation for liquid adsorption ? If not, how to determine the fugacity of liquid.