The contributions of normal and anomalous osmosis to the osmotic effects arising across charged membranes with solutions of electrolytes

The osmotic effect arising across a porous membrane separating the solution of an electrolyte from water (or a more dilute solution) is ordinarily due to both normal osmosis, as it occurs also with non-electrolytes, and to "anomalous" osmosis. It is shown that the normal osmotic component cannot be measured quantitatively by the conventional comparison with a non-electrolytic reference solute. Anomalous osmosis does not occur with electroneutral membranes. Accordingly, with membranes which can be charged and discharged reversibly (without changes in geometrical structure), such as many proteinized membranes, the osmotic effects caused by an electrolyte can be measured both when only normal osmosis arises (with the membrane in the electroneutral state) and when normal as well as anomalous osmosis occurs (with the membrane in a charged state). The difference between these two effects is the true anomalous osmosis. Data are presented on the osmotic effects across an oxyhemoglobin membrane in the uncharged state at pH 6.75 and in two charged states, positive at pH 4.0 and negative at pH 10.0, with solutions of a variety of electrolytes using a concentration ratio of 2:1 over a wide range of concentrations. The rates of the movement of liquid across the membrane against an inconsequentially small hydrostatic head are recorded instead of, as conventional, the physiologically less significant pressure rises after a standard time.     

Separation of phenols and furfural by pervaporation and reverse osmosis membranes from biomass--superheated steam pyrolysis-derived aqueous solution

The separation of valuable chemicals from raw products, where a great number of chemicals coexist, is the key technology in biomass refinery. In this study, the applicability of membrane separation of valuable chemicals from our currently developed portable superheated steam (SHS) biomass pyrolysis process was demonstrated. Phenols (phenol, p-cresol, guaiacol, methyl guaiacol, and ethyl guaiacol), furfural, and acetone were successfully separated by pervaporation using the silicone rubber membrane from model solutions and an actual SHS derived aqueous solution. The solution was also concentrated effectively by reverse osmosis separation using a polyamide membrane. When a high concentration of SHS solution was fed to the pervaporation process, a phase-separated permeate was obtained, which indicated that the reverse osmosis concentration combined with pervaporation separation is useful for the superheated steam process.     

ON THE CAUSE OF THE INFLUENCE OF IONS ON THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES. I

1. In three previous publications it had been shown that electrolytes influence the rate of diffusion of pure water through a collodion membrane into a solution in three different ways, which can be understood on the assumption of an electrification of the water or the watery phase at the boundary of the membrane; namely, (a) While the watery phase in contact with collodion is generally positively electrified, it happens that, when the membrane has received a treatment with a protein, the presence of hydrogen ions and of simple cations with a valency of three or above (beyond a certain concentration) causes the watery phase of the double layer at the boundary of membrane and solution to be negatively charged. (b) When pure water is separated from a solution by a collodion membrane, the initial rate of diffusion of water into a solution is accelerated by the ion with the opposite sign of charge and retarded by the ion with the same sign of charge as that of the water, both effects increasing with the valency of the ion and a second constitutional quantity of the ion which is still to be defined. (c) The relative influence of the oppositely charged ions, mentioned in (b), is not the same for all concentrations of electrolytes. For lower concentrations the influence of that ion usually prevails which has the opposite sign of charge from that of the watery phase of the double layer; while in higher concentrations the influence of that ion begins to prevail which has the same sign of charge as that of the watery phase of the double layer. For a number of solutions the turning point lies at a molecular concentration of about M/256 or M/512. In concentrations of M/8 or above the influence of the electrical charges of ions mentioned in (b) or (c) seems to become less noticeable or to disappear entirely. 2. It is shown in this paper that in electrical endosmose through a collodion membrane the influence of electrolytes on the rate of transport of liquids is the same as in free osmosis. Since the influence of electrolytes on the rate of transport in electrical endosmose must be ascribed to their influence on the quantity of electrical charge on the unit area of the membrane, we must conclude that the same explanation holds for the influence of electrolytes on the rate of transport of water into a solution through a collodion membrane in the case of free osmosis. 3. We may, therefore, conclude, that when pure water is separated from a solution of an electrolyte by a collodion membrane, the rate of diffusion of water into the solution by free osmosis is accelerated by the ion with the opposite sign of charge as that of the watery phase of the double layer, because this ion increases the quantity of charge on the unit area on the solution side of the membrane; and that the rate of diffusion of water is retarded by the ion with the same sign of charge as that of the watery phase for the reason that this ion diminishes the charge on the solution side of the membrane. When, therefore, the ions of an electrolyte raise the charge on the unit area of the membrane on the solution side above that on the side of pure water, a flow of the oppositely charged liquid must occur through the interstices of the membrane from the side of the water to the side of the solution (positive osmosis). When, however, the ions of an electrolyte lower the charge on the unit area of the solution side of the membrane below that on the pure water side of the membrane, liquid will diffuse from the solution into the pure water (negative osmosis). 4. We must, furthermore, conclude that in lower concentrations of many electrolytes the density of electrification of the double layer increases with an increase in concentration, while in higher concentrations of the same electrolytes it decreases with an increase in concentration. The turning point lies for a number of electrolytes at a molecular concentration of about M/512 or M/256. This explains why in lower concentrations of electrolytes the rate of diffusion of water through a collodion membrane from pure water into solution rises at first rapidly with an increase in concentration while beyond a certain concentration (which in a number of electrolytes is M/512 or M/256) the rate of diffusion of water diminishes with a further increase in concentration.     

Electroosmosis in Membranes: Effects of Unstirred Layers and Transport Numbers: I. Theory

When a current is passed through a membrane system, differences in transport numbers between the membrane and the adjacent solutions will, in general, result in depletion and enhancement of concentrations at the membrane-solution interfaces. This will be balanced by diffusion back into the bulk solution, diffusion of solute back across the membrane itself, and osmosis resulting from these local concentration gradients. The two main results of such a phenomenon are (1) that there is a current-induced volume flow, which may be mistaken for electroosmosis, and (2) that there will generally develop transient changes in potential difference (PD) across membranes during and after the passage of current through them.     

Review on modelling and control of desalination system using reverse osmosis

Dissolved salts in seawater or brackish water are reduced to a potable level through separation techniques, like, distillation, multiple effect vapor compression, evaporation, or by membrane processes such as electro-dialysis reversal, nano-filtration, and reverse osmosis (RO). RO is the most widely used desalination process. Recent advances in RO technology has led to more efficient separation and now is the most cost effective process to operate. The performance of the reverse osmosis process is dependent on concentration of dissolved solids in the feed-water, feed-water pressure, and the membrane strength to withstand system pressure, membrane solute rejection, membrane fouling characteristics, and the required permeate solute concentration. RO is a promising tool that uses cellulose acetate (or) polyamide membrane and is widely chosen as the cost of production is reduced by the use of energy efficient and process control techniques. This paper presents a review on modelling, identification of parameters from single input-outputs and multi input/output lumped systems, dynamic modelling and control of desalination systems in the past twenty years by collecting more than 60 literatures.     

Concentration of ultrafiltered benzylpenicillin broths by reverse osmosis

Concentration of benzylpenicillin filtered broths purified by ultrafiltration and fermented broths clarified by ultrafiltration was carried out by reverse osmosis. This study was done using a reverse osmosis laboratory/pilot installation from Paterson Candy International Ltd. whose module has a tubular configuration and a membrane with a molecular weight cut-off of 100 Dalton (ZF99) made of a polyamide film with a rejection of 99% of NaCl. It was concluded that reverse osmosis is an adequate technique to concentrate benzylpenicillin ultrafiltered broths, leading to low losses of benzylpenicillin in the permeate and high benzylpenicillin recovery for high volumetric concentration factors.     

Centrifugal precipitation chromatography: principle, apparatus, and optimization of key parameters for protein fractionation by ammonium sulfate precipitation

A novel chromatographic system introduced here internally generates a concentration gradient of ammonium sulfate (AS) through a long separation channel under a centrifugal force field. Protein samples are exposed to a gradually increasing AS concentration and precipitated along the channel. Then, chromatographic elution is initiated by gradually decreasing the AS concentration in the gradient which causes the proteins to repeat dissolution and precipitation through the channel. Consequently, they are eluted out in the order of their solubility in the AS solution. The separation column consists of a pair of disks equipped with mutually mirror-imaged spiral grooves. A dialysis membrane is sandwiched between the disks to form two identical channels partitioned by the membrane. The disk assembly is mounted on the sealless continuous-flow centrifuge. When a concentrated AS solution is eluted through one channel and water through the other channel in an opposite direction, an exponential AS gradient is formed through the water channel. A series of basic experiments was performed to study the rates of AS transfer and osmosis through the membrane, and the operational parameters including elution time, revolution speed, inclination of gradient, and sample size were optimized using stable protein samples. Preliminary applications were successful in purification of monoclonal antibody from cell culture supernatant and an affinity separation of recombinant ketosteroid isomerase from a crude Escherichia coli lysate.     

Effect of the surface potential upon ion selectivity found in competitive inhibition of divalent cation uptake. A theoretical approach

The sequence of inhibition of cation uptake caused by a series of cationic inhibitors may shift dramatically on varying the experimental conditions. This sequence depends upon the concentration of the inhibitory cation, the concentration of the substrate cation, the charge density of the cell membrane, the ionic strength of the medium and the affinity of the substrate cation for the negative groups located on the cell membrane. A shift in sequence between inhibitory cations can only occur if one of the inhibitory cations shows a greater affinity for the translocation site than the other, while the affinity for the negative groups on the cell membrane is lower.     

Concentration and desalting by membrane processes of a natural pigment produced by the marine diatom Haslea ostrearia Simonsen

The marine microalga Haslea ostrearia, also called blue navicula, presents the unique peculiar property among the diatoms, to produce at its extremities a blue hydrosoluble pigment called marennine. It is presented the concentration and the desalting of the exocellular pigment by membrane processes (ultrafiltration, nanofiltration, reverse osmosis). Nanofiltration is particularly developed given the potential of this type of application both for the concentration of molecules and for desalting. It is shown the effect of velocity and pressure on performances of nanofiltration membranes. Permeation flux superior to 100 l h⁻¹ m⁻² (at 14.10⁵ Pa) are obtained with the Kiryat Weizmann membrane MP 20 (polyester coated with a polyacrylonitrile layer, cut-off 450 Da). For the desalting of the blue pigment solution, nanofiltration membranes present a few advantages: a low salt rejection (less than 10% at 14.10⁵ Pa) and a high pigment rejection (the nanofiltration membrane MP 20 retains more than 95% of the pigment). This membrane used in diafiltration mode allows an acceptable speed of desalting (700 g of salt eliminated per hour and per m² at 25.10⁵ Pa for a concentration of 18 g of salt per litre of solution).     

Sugar Syrup Concentration Using Reverse Osmosis Membranes

In sugar manufacturing industries, initially dilute syrup is obtained from the cane sugar or beetroot, which should be concentrated. In many factories, sugar syrup concentration is carried out using evaporation. This process has two main problems. Firstly, it consumes a huge amount of energy due to high latent heat of water and secondly, heating may decompose the sugar molecules resulting in low‐quality and dark‐colored sugar. Low energy consuming reverse osmosis may be employed for concentrating sugar syrup without decomposing the molecules, resulting in high‐quality sugar with low cost. In this study different commercial reverse osmosis membranes (DS, DSII, PVD, FT30, BW30) and one nanofiltration membrane (NF45) were used for sugar syrup concentration. The results show that nanofiltration NF45 membrane has no effect on sugar syrup concentration. The rejections of sugar using DSII and PVD reverse osmosis membranes vary between 23 % and 33 % for different operating conditions. DS membrane rejected around 10 % of the sugar molecules in best conditions. FT30 membrane initially showed better performance (55 %). However, the rejection was decreased during time (minimum 7 %). For BW30 membrane, the rejection of sugar was better (60 %) compared to the other membranes used in this work. For two‐stage processes (i.e. the permeate of the first stage used as a feed for the second stage) the highest rejection (88 %) was obtained.     

Membrane depolarization induced by transcellular osmosis in internodal cells ofNitella flexilis

Summary Membrane depolarization induced by transcellular osmosis was studied using internodal cells ofNitella flexilis. Transcellular osmosis was induced by using sorbitol or methanol as the osmotic agent. In the endosmotic cell half, the membrane often generated an action potential and depolarized further with a concomitant decrease in membrane resistance. This osmosis-induced depolarization was a graded response dependent on the external osmotic gradients. However, in the exosmotic cell half, both membrane potential and membrane resistance changed insignificantly. Membrane depolarization occurred also in cells made inexcitable by bathing in 0.1–1 mM KCl solution.Effects of temperature and internal osmotic pressure on osmosis-induced depolarization were investigated. The magnitude of depolarization at low temperature (2 or 4°C) was larger than that at room temperature (around 20°C). Membrane depolarization was accelerated by lowering the internal osmotic pressure and inhibited by raising it.Not only the plasmalemma but the tonoplast also responded significantly to endosmosis.     

The Mechanism of Isotonic Water Transport

The mechanism by which active solute transport causes water transport in isotonic proportions across epithelial membranes has been investigated. The principle of the experiments was to measure the osmolarity of the transported fluid when the osmolarity of the bathing solution was varied over an eightfold range by varying the NaCl concentration or by adding impermeant non-electrolytes. An in vitro preparation of rabbit gall bladder was suspended in moist oxygen without an outer bathing solution, and the pure transported fluid was collected as it dripped off the serosal surface. Under all conditions the transported fluid was found to approximate an NaCl solution isotonic to whatever bathing solution used. This finding means that the mechanism of isotonic water transport in the gall bladder is neither the double membrane effect nor co-diffusion but rather local osmosis. In other words, active NaCl transport maintains a locally high concentration of solute in some restricted space in the vicinity of the cell membrane, and water follows NaCl in response to this local osmotic gradient. An equation has been derived enabling one to calculate whether the passive water permeability of an organ is high enough to account for complete osmotic equilibration of actively transported solute. By application of this equation, water transport associated with active NaCl transport in the gall bladder cannot go through the channels for water flow under passive conditions, since these channels are grossly too impermeable. Furthermore, solute-linked water transport fails to produce the streaming potentials expected for water flow through these passive channels. Hence solute-linked water transport does not occur in the passive channels but instead involves special structures in the cell membrane, which remain to be identified.     

ON THE CAUSE OF THE INFLUENCE OF IONS ON THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES. II

1. It had been shown in previous publications that when pure water is separated from a solution of an electrolyte by a collodion membrane the ion with the same sign of charge as the membrane increases and the ion with the opposite sign of charge as the membrane diminishes the rate of diffusion of water into the solution; but that the relative influence of the oppositely charged ions upon the rate of diffusion of water through the membrane is not the same for different concentrations. Beginning with the lowest concentrations of electrolytes the attractive influence of that ion which has the same sign of charge as the collodion membrane upon the oppositely charged water increases more rapidly with increasing concentration of the electrolyte than the repelling effect of the ion possessing the opposite sign of charge as the membrane. When the concentration exceeds a certain critical value the repelling influence of the latter ion upon the water increases more rapidly with a further increase in the concentration of the electrolyte than the attractive influence of the ion having the same sign of charge as the membrane. 2. It is shown in this paper that the influence of the concentration of electrolytes on the rate of transport of water through collodion membranes in electrical endosmose is similar to that in the case of free osmosis. 3. On the basis of the Helmholtz theory of electrical double layers this seems to indicate that the influence of an electrolyte on the rate of diffusion of water through a collodion membrane in the case of free osmosis is due to the fact that the ion possessing the same sign of charge as the membrane increases the density of charge of the latter while the ion with the opposite sign diminishes the density of charge of the membrane. The relative influence of the oppositely charged ions on the density of charge of the membrane is not the same in all concentrations. The influence of the ion with the same sign of charge increases in the lowest concentrations more rapidly with increasing concentration than the influence of the ion with the opposite sign of charge, while for somewhat higher concentrations the reverse is true.     

Osmosis and solute—Solvent drag

In 1903, George Hulett explained how solute alters water in an aqueous solution to lower the vapor pressure of its water. Hulett also explained how the same altered water causes osmosis and osmotic pressure when the solution is separated from liquid water by a membrane permeable to the water only. Hulett recognized that the solute molecules diffuse toward all boundaries of the solution containing the solute. Solute diffusion is stopped at all boundaries, at an open-unopposed surface of the solution, at a semipermeable membrane, at a container wall, or at the boundary of a solid or gaseous inclusion surrounded by solution but not dissolved in it. At each boundary of the solution, the solute molecules are reflected, they change momentum, and the change of momentum of all reflected molecules is a pressure, a solute pressure (i.e., a force on a unit area of reflecting boundary). When a boundary of the solution is open and unopposed, the solute pressure alters the internal tension in the force bonding the water in its liquid phase, namely, the hydrogen bond. All altered properties of the water in the solution are explained by the altered internal tension of the water in the solution. We acclaim Hulett's explanation of osmosis, osmotic pressure, and lowering of the vapor pressure of water in an aqueous solution. His explanation is self-evident. It is the necessary, sufficient, and inescapable explanation of all altered properties of the water in the solution relative to the same property of pure liquid water at the same externally applied pressure and the same temperature. We extend Hulett's explanation of osmosis to include the osmotic effects of solute diffusing through solvent and dragging on the solvent through which it diffuses. Therein lies the explanations of (1) the extravasation from and return of interstitial fluid to capillaries, (2) the return of luminal fluid in the proximal and distal convoluted tubules of a kidney nephron to their peritubular capillaries, (3) the return of interstitial fluid to the vasa recta, (4) return of aqueous humor to the episcleral veins, and (5) flow of phloem from source to sink in higher plants and many more examples of fluid transport and fluid exchange in animal and plant physiology. When a membrane is permeable to water only and when it separates differing aqueous solutions, the flow of water is from the solution with the lower osmotic pressure to the solution with the higher osmotic pressure.     

A Physical Interpretation of the Phenomenological Coefficients of Membrane Permeability

A "translation" of the phenomenological permeability coefficients into friction and distribution coefficients amenable to physical interpretation is presented. Expressions are obtained for the solute permeability coefficient ω and the reflection coefficient σ for both non-electrolytic and electrolytic permeants. An analysis of the coefficients is given for loose membranes as well as for dense natural membranes where transport may go through capillaries or by solution in the lipoid parts of the membrane. Water diffusion and filtration and the relation between these and capillary pore radius of the membrane are discussed. For the permeation of ions through the charged membranes equations are developed for the case of zero electrical current in the membrane. The correlation of σ with ω and L_p for electrolytes resembles that for non-electrolytes. In this case ω and σ depend markedly on ion concentration and on the charge density of the membrane. The reflection coefficient may assume negative values indicating anomalous osmosis. An analysis of the phenomena of anomalous osmosis was carried out for the model of Teorell and Meyer and Sievers and the results agree with the experimental data of Loeb and of Grim and Sollner. A set of equations and reference curves are presented for the evaluation of ω and σ in the transport of polyvalent ions through charged membranes.     

Studies on the electrical properties of a single plant cell; internodal cell of Nitella flexilis

Impedance changes of single plant cells of Nitella flexilis were studied under different environmental conditions. With the analysis presented changes in resistance of the protoplasmic membrane and of cell sap can be studied independently and simultaneously. Under "transcellular osmosis," the resistance of the protoplasmic membrane and of the cell sap increase at the part of the cell where water enters, while they decrease where water goes out. Ethanol of low concentration (below 8 per cent) first decreases and later increases the resistance of the protoplasmic membrane. Concentrated ethanol (over 10 per cent), however, brings about a large decrease in resistance of the protoplasmic membrane. Its time course is not simple, but undulatory changes occur. When ethanol is applied to one part of the cell, the resistance of the protoplasmic membrane shows a different type of change, which may be attributed to the local osmotic effect of ethanol; injury generally occurs with comparatively low concentration. Methanol, ethanol, and propanol have almost the same effect upon the cell, while butanol is toxic at the same concentration. When the cell dies, the resistance of the protoplasmic membrane decreases greatly, while the resistance of the cell sap increases to a level (several hundred kilo ohms or more), expected when external solution and cell sap are freely mixed with each other.     

Characteristics of Tryptophan Accumulation by Isolated Rat Forebrain Synaptosomes

Uptake of 10 microM L-tryptophan into isolated rat brain synaptosomes was studied to assess its effect on the rate of serotonin synthesis from tryptophan. The initial rate of uptake was rapid, being two orders of magnitude above the rate of tryptophan hydroxylation. Uptake was highly concentrative, the concentration ratio across the plasma membrane at equilibrium being approximately 9. This concentration ratio was decreased to about 1 in the presence of high concentrations of amino acids transported by the L-type neutral amino acid uptake system. A mixture of the large neutral amino acids at physiological concentrations decreased the internal tryptophan concentration to 58% of that in their absence. Large tryptophan concentration ratios were observed in experiments in which Na+ in the medium was replaced with choline+. The concentrative uptake of tryptophan was energy-dependent, being decreased by inclusion of cyanide and omission of glucose. The concentration gradient was abolished by veratridine or rotenone. Time courses of the changes in ATP content and tryptophan concentration ratio on addition of these and other agents established that tryptophan uptake is probably not driven by ATP hydrolysis or efflux of other amino acids, but by the plasma membrane potential.     

Anomalous osmosis through a model membrane constructed with potassium polystyrenesulfonate solution

It is reported that anomalous osmosis can be observed through liquid membranes which are made by placing a potassium polystyrenesulfonate solution between two inert cellophane membranes. No anomalous osmosis is observed with pure water instead of polyelectrolyte solution.     

INFLUENCE OF THE CONCENTRATION OF ELECTROLYTES ON THE ELECTRIFICATION AND THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES

1. When a watery solution is separated from pure water by a collodion membrane, the initial rate of diffusion of water into the solution is influenced in an entirely different way by solutions of electrolytes and of non-electrolytes. Solutions of non-electrolytes, e.g. sugars, influence the initial rate of diffusion of water through the membrane approximately in direct proportion to their concentration, and this. influence begins to show itself under the conditions of our experiments when the concentration of the sugar solution is above M/64 or M/32. We call this effect of the concentration of the solute on the initial rate of diffusion of water into the solution the gas pressure effect. 2. Solutions of electrolytes show the gas pressure effect upon the initial rate of diffusion also, but it commences at a somewhat higher concentration than M/64; namely, at M/16 or more (according to the nature of the electrolyte). 3. Solutions of electrolytes of a lower concentration than M/16 or M/8 have a specific influence on the initial rate of diffusion of water through a collodion membrane from pure solvent into solution which is not found in the case of the solutions of non-electrolytes and which is due to the fact that the particles of water diffuse in this case through the membrane in an electrified condition, the sign of the charge depending upon the nature of the electrolyte in solution, according to two rules given in a preceding paper. 4. In these lower concentrations the curves representing the influence of the concentration of the electrolyte on the initial rate of diffusion of water into the solution rise at first steeply with an increase in the concentration, until a maximum is reached at a concentration of M/256 or above. A further increase in concentration causes a drop-in the curve and this drop increases with a further increase of concentration until that concentration of the solute is reached in which the gas pressure effect begins to prevail; i.e., above M/16. Within a range of concentrations between M/256 and M/16 or more (according to the nature of the electrolyte) we notice the reverse of what we should expect on the basis of van''t Hoff''s law; namely, that the attraction of a solution of an electrolyte for water diminishes with an increase in concentration. 5. We wish to make no definite assumption concerning the origin of the electrification of water and concerning the mechanism whereby ions influence the rate of diffusion of water particles through collodion membranes from pure solvent to solution. It will facilitate, however, the presentation of our results if it be permitted to present them in terms of attraction and repulsion of the charged particles of water by the ions. With this reservation we may say that in the lowest concentrations attraction of the electrified water particles by the ions with the opposite charge prevails over the repulsion of the electrified water particles by the ions with the same sign of charge as that of the water; while beyond a certain critical concentration the repelling action of the ion with the same sign of charge as that of the water particles upon the latter increases more rapidly with increasing concentration of the solute than the attractive action of the ion with the opposite charge. 6. It is shown that negative osmosis, i.e. the diminution of the volume of the solution of acids and of alkalies when separated by collodion membranes from pure water, occurs in the same range of concentrations in which the drop in the curves of neutral salts occurs, and that it is due to the same cause; namely, the repulsion of the electrified particles of water by the ion with the same sign of charge as that of the water. This conclusion is supported by the fact that negative osmosis becomes pronounced when the ion with the same sign of charge as that of the electrified particles of water carries more than one charge.     

Zwitterionic polymers functionalised nanoporous graphene for water desalination: a molecular dynamics study

In this paper, one nanoporous graphene grafting several zwitterionic polymer chains was designed as the osmosis membrane for seawater desalination. Using molecular dynamics simulation, the efficiency and mechanism of salt rejection were discussed. The simulated results showed that the zwitterionic polymer chains on nanoporous graphene can form the charge channel to block Na⁺ and Cl^? ions pass through, and the slat rejection efficiency of functionalised graphene can reach to about 90%. In the simulation, the steric hindrance and electrostatic interaction are the main factors for the salt rejection. With time evolution, the charge channel formed by the soft polymer chains can decrease the effective pore area of membrane, leading to the increase of steric hindrance; the positive and negative centres of polymer chains can adsorb Na⁺ and Cl^? ions under electrostatic interaction in the solution, contributing into the increase of charge density above the membrane. These conclusions are consistent with experimental report. Our designed osmosis membrane about the graphene is helpful for improving the potential application of defect graphene in water desalination and reducing the trouble of obtaining appropriate graphene sheet with small aperture.