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A solute flow model of the equations for a double-membrane system, in which a series of two (Ml and Mr) vertically mounted flat, microporous and symmetric polymer membranes separate three compartments (l, m, r) containing the non-homogeneous, non-ionic solutions was developed. Solution concentrations fulfilled the condition Ckl > Ckm > Ckr . The intermembrane compartment (m) consists of an infinitesimally layer of solution. The volume of the compartments (l, m, r) fulfill the condition Vl = Vr ≈ 170Vm. The experimental tests were performed for binary (aqueous solutions of glucose or ethanol) or ternary (glucose solutions in 0.75 mol·l–1 solution of ethanol or ethanol solutions in 0.1 mol·l–1 aqueous solution of glucose) solutions. The linear dependencies of the solute flux on the concentration difference in binary solutions and non-linear — in ternary solutions — were obtained. It is shown that the double-membrane system has amplifying properties of solute flows. The results obtained during experiments were interpreted in the categories of convective instability, which increased the value of diffusive permeability coefficient of the system: concentration boundary layer/membrane/ concentration boundary layer.

The following conclusions arise from the performed investigations: 1. Concentration boundary layers are the main factor reducing the passive osmotic and diffusive transport in a double-membrane osmotic-diffusive cell. By generating natural or forced convective flows in the near-membrane areas, the gravitational field limits the process of concentration boundary layer creation, increasing the osmotic and diffusive membrane transport. 2. The measure of this reduction are values of coefficients ψ11, ψ21, ψ12 and ψ22 which depend l r on the values of coefficients ϑ1 , ϑ1 , ϑl2 , ϑr and l r l r l r l r ω11 , ω11 , ω21 , ω21 , ω12 , ω12 , ω22 , ω22 . 3. The intermembrane compartment in twomembrane osmotic-diffusive cell is the place of accumulation or depletion of solute. In connection with this, the solute concentration in a stationary state can be greater or lower than at the initial moment. Both convective mixing and forced mixing of solutions do not influence on the value of this concentration. 4. A double-membrane osmotic-diffusive cell with the membranes positioned in the vertical planes has a property of diffusive flux amplification. The measure of this property is the diffusive flux amplification coefficient.