Hi Ramendra

Good question.  Density ration in my opinion is almost irrelevant,  Membranes with different density rations can have the same average pore size with equal or different distributions and membranes with the same density ration can have different pore size distributions.  It all depends on the materials and how the membrane was prepared in the first place.  Look up some of Dr. Sourirajan's work and book on membranes for clues.

Dear Oleh

Thank you for the response.  

I am looking for explanation to the correlation Deff= Do*(rho_m/rho_a)*Chi. 

where, 

Deff= Effective diffusivity

Do= Diffusivity of water vapor in the dense membrane

rho_m= density of membrane

rho_a = density of air

Chi= partition coefficient (which is the ratio of water vapor concentration in the membrane to that in the air)

Such correlation has been used to model moisture transfer using hydrophilic membranes. I could not understand the physical significance of using the density ratio in this case.

Regards