Osmosis in small pores: a molecular dynamics study of the mechanism of solvent transport

Abstract

Osmosis through semi–permeable pores is a complex process by which solvent is driven by its free energy gradient towards a solute–rich reservoir. We have studied osmotic flow across a semi–permeable cylindrical pore using hard–sphere molecular dynamics which simulates osmosis in the absence of attractive forces between solute and solvent. In addition, we recorded the rates of pressure–driven solvent flow and the diffusive flow of labelled solvent under concentration gradients. It is apparent that there are differences, which are radius dependent, between viscous and diffusive solvent permeabilities in small pores. The osmotic flow rate is decreased by allowing solute entry into part of the pore, an effect which is not due to solute obstruction. The flow rate is dependent on the structure of the pore, which for asymmetric pores leads, surprisingly, to flow asymmetry or osmotic rectification. In the absence of any possible viscous rectification at these very low flow rates the effect correlates with changes between diffusive and pressure flows created by the presence of solute, an effect which has been predicted from thermodynamic arguments. The geometry of a semi–permeable pore in relation to the solute size is therefore required to predict the osmotic flow rate, a departure from the classical picture. Finally, by extracting transport parameters from simulations with pure solvent, we examine the departure of observed flow rate from that predicted by continuum mechanics, obtaining drag coefficients which we compare with those derived from hydrodynamics alone.

Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

“…For discussion of some of the difficulties see e.g. [ 53 , 57 , 58 ]. Bulat and Klarica [ 59 ], alongside their consideration of brain fluid movements, have proposed that in peripheral capillaries water moves down the resultant of the hydrostatic and total osmotic gradients and that the resulting change in total osmotic pressure rather than the colloid osmotic pressure within plasma limits the water fluxes and thus filtration.…”

A universal route to pattern formation in multicellular systems

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

“…Recent MD studies of osmosis in the aquaporins GlpF (Zhu et al 2002) and AQP1 (Zhu et al 2004a,b) have yielded a conductance similar to that found in experiments. Assumption (ii) has been challenged for narrow, as opposed to very narrow, pores (Hill 1994(Hill , 1995 and recently an MD simulation with hard spheres, simulating osmotic and hydraulic flow in straight pores, has shown that both viscous and diffusive modes of water transfer can exist with different rate coefficients in the same pore (Kim et al 2005). In these simulations, the osmotic flow is determined from the volume flow at zero pressure difference.…”