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A method to determine the critical concentration Rayleigh number in isothermal membrane transport processes was worked out. This method is based on equations which include the diffusive permeability coefficient, Tks; solution parameters (density, D; kinematic viscosity, <; diffusion coefficient, Dks) and solute permeability of the complex: were determined in a single-membrane cell boundary layer/membrane/boundary layer ( ). Coefficients Tks and system which consisted of a flat polymeric dialysis membrane and glucose in 0.2 mol.l!1 aqueous solutions. Water was placed on one side of the membrane. The opposite side of the membrane was exposed to the solution. Two configurations (A and B) of a single-membrane-cell system in a gravitational field were studied. In configuration A water was placed in a compartment above the membrane and the solution below. In configuration B the arrangement of water and solution was reversed. The calculation results show that the critical value of the Rayleigh number concentration ( ) in configuration A is = 277.1 and in configuration B, = 225.2.

The method of evaluation presented for the Rayleigh concentration number ( ) for isothermal membrane transport is based on the formula This method requires determination of the diffusive permeability coefficient for the membrane (Ts), concentration polarization coefficient ( ), diffusion coefficient for solute in solution (Dss), density (D) and kinematic viscosity (<) for solutions. The data of the Rayleigh concentration number make it possible to observe the kinetics of the solute transport through the membrane and the concentration boundary layers. It is known that this number defines transition conditions from pure to convective diffusion. The results presented in this paper can be important for recognition of the influence of the mechanism of hypo- and hypergravity on passive transmembrane transport in biological systems and provide new information on the subject, which is called the membrane version of Rayleigh-Taylor instability.