Osmosis in semi-permeable pores: an examination of the basic flow equations based on an experimental and molecular dynamics study

Abstract

Classically ‘semi-permeable’ pores are generally considered to mediate osmotic flow at a rate dependent upon the hydraulic conductance of the pore and the difference in water potential. The shape or size of the solute molecules is not considered to exert a first-order effect on the flow rate nor is the hydraulic conductance thought to be solute dependent. By the experimental measurement of osmosis in the biological pore AQP (aquaporin) and hard-sphere molecular dynamics simulation of a model pore, we show here that the solute radius can have a profound effect on the osmotic flow rate, causing it to decline steeply with decreasing solute radius.Using a simple non-equilibrium thermodynamic theory, we propose that an additional ‘osmotic flow coefficient’ is required to describe flows in semi-permeable structures such as AQPs, and that the fall in flow rate with radius represents a conversion from hydraulic to diffusive water flow due to increasing penetration of the pore by the solute. The interaction between the pore geometry and the solute size cannot, therefore, be overlooked, although for every solute the system obeys the criterion for semi-permeability required by basic thermodynamics. The osmotic pore theory therefore reveals a novel and potentially rich structure that remains to be explored in full.