Chemical Engineering Science | Year: 2010
Unified description of mass, charge and heat transport in a solution in the presence of temperature gradient is suggested. The description is based on the idea that the rate of mass transport is proportional to the gradient of physicochemical potential, which is a more general function than electrochemical potential and includes the product of molar entropy and temperature. The proportionality coefficient is the product of Einstein's mobility and concentration. The approach explains the dependence of the Soret coefficient on concentration, molar entropy, and temperature. It also explains the reciprocal process (Dufour effect) and temperature-induced charge separation (Seebeck effect). In addition to usual reciprocal relation L21=L12 it gives the relation of different phenomenological coefficients of the Onsager description. In its turn, description of heat transport driven in opposite directions by concentration, pressure, and voltage gives classic equilibrium relations for mass transport. © 2010 Elsevier Ltd. All rights reserved.
Chemical Engineering Science | Year: 2010
General yet simple description of chemical transport processes in non-isolated system is suggested. It is based on extended Teorell equation and just two fundamental parameters: physicochemical potential and Einstein's mobility. Using mobility it is possible to compare the rates of all major linear transport phenomena, including pressure-driven migration and also nonideal and multicomponent diffusion. Relationship with the Stefan-Maxwell approach and Onsager's linear thermodynamics is demonstrated and physical interpretation of both diagonal and off-diagonal phenomenological coefficients is suggested. Imposing boundary conditions for transport equations allows description of transport in homogeneous membranes caused by several concurrent driving factors, such as concentration, pressure, and voltage. Differences of barodiffusion, membrane filtration, and reverse osmosis are considered. For a porous membrane an expression for the pressure-driven volumetric flux through the pores as a function of mobility and pore size is derived. It is explained why hydraulic flow prevails in submicron pores and why diffusion is the dominant mechanism in reverse osmosis if the pressure difference is not too high. The theory naturally leads to the solution-diffusion model equations but does not need usual assumptions of constant pressure across the membrane and the pressure jump only at one surface. Internal pressure and mechanical stress gradients within a membrane exist and can be useful in a description of rheology of aging polymer membranes. A new equation for concurrent diffusion and hydraulic transport is derived and two possible molecular mechanisms leading to the Kedem-Katchalsky equations for reverse osmosis membranes are suggested. Finally, electrokinetic processes are described and their similarity to concentration- and pressure-driven transport is discussed. © 2009 Elsevier Ltd. All rights reserved.
General Physiology and Biophysics | Year: 2013
Aquaporin attracted attention not only of physiologists and biophysicists, but also of chemical engineers. Here we critically analyze a paper describing aquaporin-based artificial membranes, suggested for forward osmosis-based water purification (Wang et al. 2012, Small 8, pp. 1185-1190). Related papers published later by the same group are also discussed. We indicate recently developed general approach to describe membrane transport, membrane permeability and selectivity, which is applicable for forward osmosis. In addition, we also mention our papers describing simple nitrocellulose-based membranes, which have selective aqueous channels without proteins, but successfully imitate many properties of biomembranes.
Kocherginsky N.M.,Biomime |
Lvovich V.F.,Cleveland Clinic
Langmuir | Year: 2010
Earlier we have shown that many important properties of ionic aqueous channels in biological membranes can be imitated using simple biomimetic membranes. These membranes are composed of mixed cellulose ester-based filters, impregnated with isopropyl myristate or other esters of fatty acids, and can be used for high-throughput drug screening. If the membrane separates two aqueous solutions, combination of relatively hydrophilic polymer support with immobilized carboxylic groups results in the formation of thin aqueous layers covering inner surface of the pores, while the pore volume is filled by lipid-like substances. Because of these aqueous layers biomimetic membranes even without proteins have a cation/anion ion selectivity and specific (per unit of thickness) electrical properties, which are similar to typical properties of biological membranes. Here we describe frequency-dependent impedance of the isopropyl myristate-impregnated biomimetic membranes in the 4-electrode arrangement and present the results as Bode and Nyquist diagrams. When the membranes are placed in deionized water, it is possible to observe three different dispersion processes in the frequency range 0.1 Hz to 30 kHz. Only one dispersion is observed in 5 mM KH2PO4 solution. It is suggested that these three dispersion features are determined by (a) conductivity in aqueous structures/channels, formed near the internal walls of the filter pores at high frequencies, (b) dielectric properties of the whole membrane at medium frequencies, determined by polymer support, aqueous layers and impregnating oil, and, finally, (c) by the processes in hydrated liquid crystal structures formed in pores by impregnating oil in contact with water at low frequencies. © 2010 American Chemical Society.
Kocherginsky N.,Biomime |
Gruebele M.,Urbana University
Journal of Chemical Physics | Year: 2013
Starting with the continuity and Smoluchowski equations, we write the mass flux for a system out of equilibrium in terms of the physicochemical potential μg. μg is a coarse-grained analog of the chemical potential in the presence of forces that drive the system out of equilibrium. The expression for flux in terms of μg allows for a macroscopic derivation of the Onsager reciprocal relations for the case of transport by diffusion and drift in single or multi-component systems, without recourse to microscopic fluctuations or equations of motion. Transport coefficients for any time reversal-invariant properties now are expressed in terms of only partial molar derivatives and mobilities (diffusion coefficients). The thermodynamic derivation cannot treat time reversal. © 2013 American Institute of Physics.