Coupling between transport phenomena and conformational transitions: Isothermal water transport and “asymmetric osmotic pressure” across a “membrane phase” promoted by a helix-coil transition

A membrane system is described consisting of an aqueous concentrated solution of poly-L-lysine bounded by two rigid conventional membranes permeable to water and to small ions but impervious to the polymer. When a large and steady pH difference is maintained between two external isoosmotic solutions α and β, a vertical volume flow may be observed from the acid solution to the basic solution across the membrane. The flow may give rise to a pressure difference between the basic and the acid solution corresponding to a non-equilibrium steady state of the system. These effects, first reported by Liquori et al. [1] are extensively analyzed from a thermodynamic point of view.It is shown that the pH gradient across the membrane maintains a conformational gradient of poly-L-lysine which is known to undergo a pH regulated helix-coil transition. The latter gradient in turn determines a gradient of chemical potential of water within the membrane phase acting as the main driving force of the volume flow.Using the flux equations of irreversible thermodynamics in connection with a model of helix-coil transition proposed by the author [3,4] compact equations are derived for the isobaric volume flow across the membrane phase and the steady state osmotic pressure in agreement with the experimental results.Possible implications of this study in connection with active transport phenomena across biological membranes are discussed.     

The effect of membrane potential on the development of chemical osmotic pressure in compacted clay

When clay soils are subjected to salt concentration gradients, various interrelated processes come into play. It is known that chemical osmosis induces a water flow and that a membrane potential difference develops that counteracts diffusive flow of solutes and osmotic flow of water. In this paper, we present the results of experiments on the influence of membrane potential on chemical osmotic flow and diffusion of solutes and we show how we are able to derive the membrane potential value from theory. Moreover, the simultaneous development of water pressure, salt concentration and membrane potential difference are simulated using a model for combined chemico-electroosmosis in clays. A new method for short-circuiting the clay sample is employed to assess the influence of electrical effects on flow of water and transport of solutes.     

Transport processes in responding lipid membranes: a possible mechanism for the pH gradient in the stratum corneum

The "acidic mantle" of the skin surface has been related to several essential functions of the skin, although the origin of the acidity is still obscure. In this paper, we investigate how different transport processes can influence the local proton concentration inside a membrane consisting of oriented lipid bilayers. This system is chosen as a simple model of the extracellular lipids in the upper layer of the skin, the stratum corneum. We present a theoretical model for diffusional transport over the membrane in the presence of an osmotic gradient and a gradient in CO(2), taking into account the influence of these gradients on the lipid structure and the local electrostatics. We are also discussing the complications in applying the concept of pH to the stratum corneum. From this, we make the following conclusions: (i) The definition of pH in the stratum corneum is ambiguous, and thus, all statements regarding pH should always be related to a clear definition. (ii) A natural definition of pH in the stratum corneum can be proposed which takes into account local heterogeneity, local charges, and the fact that the stratum corneum is not in thermodynamical equilibrium. (iii) Diffusive transport across an oriented bilayer stack in the presence of an osmotic gradient and/or a gradient in CO(2) can give rise to a substantial gradient in pH. (iv) The results from the simplified model can be correlated to experimental observations of pH in the stratum corneum.     

Unsteady state flux response: a method to determine the nature of the solute and gel layer in membrane filtration

The unsteady-state permeate flux response to a step change in transmembrane pressure is shown to result in unique flux–pressure profiles for the three types of solutes common in membrane ultrafiltration (UF): (a) solutes which exert an osmotic pressure but do not form a ‘gel’; (b) solutes which do not exert an osmotic pressure but form a ‘gel’ and (c) solutes which exert an osmotic pressure and also form a ‘gel’. It is also shown that for stirred cell UF, changes in the bulk feed solution properties (concentration, volume) are negligible on the time scale needed to attain a stable permeate flux. Unsteady-state permeate flux measurements could therefore be made at short filtration times so that the results would not be masked by changes in bulk properties.     

507—Electron transport through human epithelial amniotic membrane in vitro: Comparative study with artificial membranes (bilayer and millipore filters)

The transfer of solutes and water across the amniotic membrane, in vivo, is controlled by hydraulic, osmotic and electrochemical factors. In this study the electrochemical component is specified in vitro, and the mechanism of ion transport across the amnion is characterized by comparison with artificial membranes (bilayers and Teflon millipore filters). The electrochemical study shows that the amniotic conductance depends on time and salt concentration; the cationic transference number is comprised between 0.75 and 0.85; the electro-osmotic potentials are negative; the I(V) curves are linear; the activation conductance energy ranges between 4 and 8 kcal degree^?1 mole^?1 and the cation selectivity sequence is: RbCsK>Na>Li. The pharmacological agents (ouabain, amiloride, dinitrophenol) decrease gNa. These results suggest: (1) that the amnion has fixed neutral sites in external epithelial cell membranes and on intercellular channels with, under certain conditions, negative sites in greater number than positive sites; (2) that the amnion is comparable to negative artificial membranes; (3) that the paracellular pathway is more important than the transcellular pathway; (4) that there are specific sites on the external membranes and in intercellular channels.     

Isothermal and non-isothermal water transport in porous membranes. II. The steady state

Thermoosmotic behaviour was studied in simple systems constituted of grossly porous hydrophobic membranes permeated by distilled water. Attention was focused on steady-state conditions, characterized by absence of net transmembrane volume flow, obtained equilibrating thermoosmotic pressure with an external counterpressure. Comparison of hydraulic and thermoosmotic fluxes with steady-state pressure gives insight in the peculiar thermofluidodynamics of volume flow in non-isothermal membrane channels. This investigation was extended to volume transport caused by combination of the two thermodynamic forces constituted of the temperature and pressure gradients, synergic or antagonistic across the membrane. The experimental findings can be fruitfully compared with theoretical predictions of the system's behaviour derived from different approaches. Results obtained with six different membrane types, under a wide range of experimental conditions, lend support to the “thermal radiation pressure theory” which attributes the various effects of matter transport produced by a temperature gradient to the transfer of momentum from drifting thermal excitations to atoms and molecules in the material crossed by heat flow.     

The dependence of electro-osmotic flow on current density and time

The dependence of the water transport number on current density is examined for three membranes whose characteristics cover a wide spectrum: poly(vinylbenzenesulfonate), porous Vycor glass and cellulose. Experiments and theory show that non-linear volume—time plots in electro-osmotic experiments arise from displacements of the membrane in the electric field, and that reliable water transport numbers can be obtained at a given current density. When the current density is varied, experiments show that the observed water transport number can: (a) increase at low current densities because of osmotic flow superimposed on water transport by the electric field; (b) decrease at higher current densities because of accumulation of salt in the membrane; (c) decrease more at current densities near and above the limiting value because of an increased contribution of hydrogen and hydroxide ions to transport. These phenomena arise from a combination of diffusion films at both membrane—solution interfaces and from the dependence of counteflon and water transport numbers on external salt concentration.     

Effect of colloids on salt transport in crossflow nanofiltration

The role of colloid deposition on the performance of a salt-rejecting NF membrane was evaluated by modeling salt transport using a two-layer transport model, which quantified the relative contributions of advection and diffusion in the cake and the membrane layers, and the effects of flux on the membrane sieving coefficient. The model was able to accurately describe how the measured permeate concentration, rejection, osmotic pressure, and flux decline varied with time. The two-layer model confirmed that the Peclet number in the cake layer was about an order of magnitude higher than that in the membrane layer, leading to significant concentration polarization at the membrane surface, as shown by others. However, the cake layer also increased overall resistance, which resulted in flux decline during constant pressure operation. Flux decline caused an increase in the actual sieving coefficient, leading to higher solute flux, lower observed rejection, and thus lower the bulk concentration. These coupled phenomena tended to mitigate the increase in concentration polarization caused by the cake. Therefore, as predicted by the model and verified by experiment, the osmotic pressure does not increase monotonically as the cake grows, and in fact can decrease when the cake layer is thick and the flux decline is significant. In our experimental system, the pressure drop across the cake layer, which was proportional to the cake thickness, was significant under the conditions studied. The effects of cake-enhanced osmotic pressure analyzed here are lower than those observed in previous studies, possibly because the transport model employed explicitly accounts for the effect of flux decline due to cake growth on the membrane sieving coefficient, and possibly because we used a somewhat different methodology to estimate cake porosity.     

Toward predictive permeabilities: Experimental measurements and multiscale simulation of methanol transport in Nafion

A polymer membrane's permeability to solutes determines its suitability for various applications: a permeability value is essential for predicting performance in diverse contexts. Using aqueous methanol permeation through Nafion as an example, we describe a methodology for determining membrane permeability that accounts for boundary layer effects and the possibility of swelling. For the materials and apparatus used herein, analysis of a permeance measurement and computational fluid dynamics simulations show that the mass transfer boundary layer is on the order of ones to tens of microns. The data are used to develop and validate a multiscale model describing solute permeation through a hydrated membrane as a series of physical mechanistic steps: reversible adsorption from solution at the membrane interface, diffusion driven by a concentration gradient within the membrane, and reversible desorption into solution at the opposite membrane interface. The validated model is used to predict methanol transport across a solar-driven CO₂ reduction device and to assess the impact of polymer changes on the measured value. The approach of combining experimental data, computational fluid dynamics, and the mechanistic multiscale model is expected to provide more accurate analysis of membrane permeation data in cases with polymer swelling or unusual device geometries, among others.     

Artificial liposome functionalized with cytochrome c3 as a model for H2-metabolizing bacteria

The artificial liposome functionalized with cyt c₃ from Desulfovibrio vulgaris Miyazaki transports electrons across the membrane from the external H₂ to K₃Fe(CN)₆ in its interior in the presence of colloidal Pt. This electron transport simultaneously produces the considerable H⁺ gradient across the membrane large enough for the ATP synthesis.     

Molecular ferries: membrane carriers that promote phospholipid flip-flop and chloride transport

The facilitated transport of ionic or polar solutes through biological membranes is an essential process for cellular life, and a major technical goal of the pharmaceutical industry. Synthetic receptors with affinities for anions are shown to act as molecular ferries and facilitate the movement of chloride ions and salts across vesicle and cell membranes. A process that competes with chloride transport is phospholipid translocation or flip-flop. This has led to the development of synthetic scramblases that can alter the transmembrane distribution of phospholipids and induce biological responses such as membrane enzyme activation. The facilitated translocation of phospholipids with multiply-charged head groups, like phosphatidylserine, is a difficult supramolecular challenge that requires a complementary, multitopic receptor with appropriate amphiphilicity.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic-Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion-exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent-induced phase separation (NIPS) and spin-coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m⁻² under a 50-fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Monte Carlo simulation of osmotic equilibria

We present a Metropolis Monte Carlo simulation algorithm for the Tpπ-ensemble, where T is the temperature, p is the overall external pressure, and π is the osmotic pressure across the membrane. The algorithm, which can be applied to small molecules or sorption of small molecules in polymer networks, is tested for the case of Lennard-Jones interactions.     

Efficient Solar-osmotic Power Generation from Bioinspired Anti-fouling 2D WS2 Composite Membranes

Nanofluidic reverse electrodialysis provides an attractive way to harvest osmotic energy. However, most attention was paid to monotonous membrane structure optimization to promote selective ion transport, while the role of external fields and relevant mechanisms are rarely explored. Here, we demonstrate a Kevlar-toughened tungsten disulfide (WS₂) composite membrane with bioinspired serosa-mimetic structures as an efficient osmotic energy generator coupling light. As a result, the output power could be up to 16.43 W m⁻² under irradiation, outperforming traditional two-dimensional (2D) membranes. Both the experiment and simulation uncover that the generated photothermal and photoelectronic effects could synergistically promote the confined ion transport process. In addition, this membrane also possesses great anti-fouling properties, endowing its practical application. This work paves new avenues for sustainable power generation by coupling solar energy.     

Hindered transmembrane protein transport in hollow-fibre devices

Protein transport in the extracapillary and intracapillary spaces (ECS and ICS) as well as across the membranes of hollow-fibre devices was investigated. Regenerated-cellulose ultrafiltration membrane cartridges were loaded with solutions of myoglobin and operated in the closed-shell mode for a period of several days. The transmembrane leakage and spatial distribution of myoglobin were dependent on the protein loading, the ICS flow rate and the presence of bovine serum albumin in the ECS. A mathematical model was developed to characterise the time-dependent protein redistribution, taking into account the effects of osmotic pressure, fibre expansion under wet conditions and the fibre-free manifold regions of the ECS. According to the model, the transmembrane transport of protein can be described using one semi-empirical parameter, the membrane constant, defined as the ratio of the square of pore tortuosity to surface porosity, from which the osmotic reflection coefficient, the partition coefficient, and the diffusive and convective hindrance factors can be calculated. The experimental data were used to test the model and to find the values of the membrane constant for the system under consideration.     

Quantifying the effect of membrane potential in chemical osmosis across bentonite membranes by virtual short-circuiting

Clay liners are charged membranes and show semipermeable behavior regarding the flow of fluids, electrical charge, chemicals and heat. At zero gradients of temperature and hydrostatic pressure, a salt concentration gradient across a compacted clay sample induces not only an osmotic flux of water and diffusion of salt across the membrane but also an electrical potential gradient, defined as membrane potential. Laboratory experiments were performed on commercially available bentonite samples in a rigid-wall permeameter connected to two electrically insulated fluid reservoirs filled with NaCl solutions of different concentrations and equipped with Ag/AgCl electrodes to measure the electrical potential gradient. The effect of membrane potential could be cancelled out by short-circuiting the clay with the so-called virtual shortcut. The potential gradient across the sample is brought to zero with a negative feedback circuit. It was observed that the water flux and the diffusion of Cl- were hindered by the occurrence of a membrane potential, indicating that an electroosmotic counterflow is induced. Flow parameters were calculated with modified coupled flow equations of irreversible thermodynamics. They were in excellent agreement with values reported in the literature. Comparing the method of short-circuiting with a study elsewhere, where the electrodes were physically short-circuited, it was shown that the virtual shortcut is more appropriate because physically short-circuiting induces additional effects that are attributed to the fluxes.     

Transport of ions through the oil phase of W(1)/O/W(2) double emulsions

Using a capillary video microscopy technique, the ion transport at liquid-liquid interfaces and through a surfactant-containing emulsion liquid membrane was visually studied by preparing a double emulsion globule within the confined space of a thin-walled, transparent, cylindrical microtube. NaCl and AgNO(3) were selected as the model reactants and were prepared to form a NaCl/AgNO(3) pair across the oil film. By observing and measuring the formed AgCl deposition, it was found that both Cl(-) and Ag(+) could transport through a thick oil film and Ag(+) was transported faster than Cl(-). Interestingly, the ion transport was significantly retarded when the oil film became extremely thin (<1 microm). The results suggested that the transport of ions mainly depends on the "reverse micelle transport" mechanism, in which reverse micelles with entrapped ions and water molecules can be formed in a thick oil film and their construction will get impeded if the oil film becomes extremely thin, leading to different ion transport rates in these two cases. The direction of ion transport depends on the direction of the osmotic pressure gradient across the oil film and the ion transport is independent of the oil film thickness in the investigated thick range. Ions with smaller Pauling radii are more easily entrapped into the formed reverse micelles and therefore will be transported faster through the oil film than bigger ions. Oil-soluble surfactants facilitate ion transport; however, too much surfactant in the oil film will slow down the ion migration. In addition, this study showed no support for the "molecular diffusion" mechanism of ion transport through oils.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic‐Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion‐exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent‐induced phase separation (NIPS) and spin‐coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m^?2 under a 50‐fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.