Osmosis in leaky pores: the role of pressure

Osmosis in leaky pores is often considered to involve gradients of pressure that originate from concentration gradients at the pore wall. These postulated surface pressures are incompatible with the Boltzmann equation and result from an incorrect application of the Gibbs-Duhem equation to binary solutions near surfaces. Such pressures would give rise to tensions in the surface phase that are not observed experimentally. Application of the argument to the swelling of porous systems and gels shows that this phenomenon cannot involve internal pressures within the fluid phase.