Ion transport through chemically induced pores in protein-free phospholipid membranes

We address the possibility of being able to induce the trafficking of salt ions and other solutes across cell membranes without the use of specific protein-based transporters or pumps. On the basis of realistic atomic-scale molecular dynamics simulations, we demonstrate that transmembrane ionic leakage can be initiated by chemical means, in this instance through addition of dimethyl sulfoxide (DMSO), a solvent widely used in cell biology. Our results provide compelling evidence that the small amphiphilic solute DMSO is able to induce transient defects (water pores) in membranes and to promote a subsequent diffusive pore-mediated transport of salt ions. The findings are consistent with available experimental data and offer a molecular-level explanation for the experimentally observed activities of DMSO solvent as an efficient penetration enhancer and a cryoprotectant, as well as an analgesic. Our findings suggest that transient pore formation by chemical means could emerge as an important general principle for therapeutics.     

Lateral diffusion in lipid membranes through collective flows

There is no comprehensive model for the dynamics of cellular membranes. Even mechanisms of basic dynamic processes, such as lateral diffusion of lipids, are poorly understood. Our atomic-scale molecular dynamics simulations support a novel, concerted mechanism for lipid diffusion. We find that a lipid and its nearest neighbors move in unison, forming loosely defined clusters. What is more, the motions of lipids are correlated over tens of nanometers: the lateral displacements of lipids in a given monolayer produce striking two-dimensional flow patterns. These flow patterns should have wide implications, affecting, for example, the formation of membrane domains, protein functionality, and action of lipases and drugs on membranes.     

Intrusion pressure to initiate flow through pores between spheres

In this work, clusters of three or four spheres were used to examine intrusion pressure. Polytetrafluoroethylene (PTFE) or polyamide 66 (PA66) spheres were arranged horizontally to create a single pore. Liquid drops of water or ethylene glycol were gently introduced from above. If the spheres were too large, drops flowed through as soon as they were deposited. If the spheres were too small, liquid was suspended in the neck of the pore and could not pass through; drops became unstable and fell to one side. Alternatively, if spheres of a certain size were chosen, then capillary forces initially prevented drops of lesser stature from breaking though. However, as these drops grew taller, they eventually reached a height where the gravitational force exceeded the capillary force and the liquid flowed through the pore. A simple model for intrusion pressure was derived. Estimates from the model agreed well with experimentally measured values.     

Interaction between amphiphilic peptides and phospholipid membranes

This brief review aims at providing some illustrative examples on the interaction between amphiphilic peptides and phospholipid membranes, an area of significant current interest. Focusing on antimicrobial peptides, factors affecting peptide–membrane interactions are addressed, including effects of peptide length, charge, hydrophobicity, secondary structure, and topology. Effects of membrane composition are also illustrated, including effects of membrane charge, nature of the polar headgroup, and presence of cholesterol and other sterols. Throughout, novel insights on the importance of peptide adsorption density on membrane stability are emphasized, as is the correlation between peptide adsorption, peptide-induced leakage in model liposome systems, peptide-induced lysis of bacteria, and bacteria killing.     

The transversal diffusion coefficient of phospholipid molecules through black lipid membranes

The possible formation of statistical pores through black lipid membranes may constitute a new mechanism for the explanation of transversal diffusion of lipid molecules through bilayers. In this work, we calculate the flip-flop diffusion coeficient, which is related to the mean number of statistical pores formed on the planar bilayer and the lateral diffusion coeficient. Its approximately calculated value is: D_ff = 10⁻³ m² s⁻¹.     

Reversible adsorption inside pores of ultrafiltration membranes

A range of experiments were performed on the dead-end ultrafiltration (UF) of poly(ethylene glycol) (PEG) of different molecular weights. Deviations from a linear dependence of the filtration rate with the applied membrane pressure difference were found. It is shown that these deviations are not caused by an osmotic pressure influence but determined by the reversible adsorption of PEG molecules inside the pores of the ultrafiltration membranes used. A theoretical model of the process is suggested, which describes the reversible adsorption inside the membrane pores and the corresponding reduction of the filtration velocity. Comparison of the theory predictions with experimental data on the ultrafiltration of PEG shows a good agreement between the theoretical predictions and experimental data. A theory is presented for calculation of the PEG rejection coefficient in the case of ultrafiltration.     

Relation between hydraulic permeability and diffusion in homogeneous swollen membranes

The question of viscous flow versus molecular diffusion mechanisms for pressure-induced liquid transport through membranes is critically examined for the specific case of homogeneous swollen membranes. It is shown that previous attempts to compute diffusion coefficients from hydraulic permeabilities for such systems have used an equation which is grossly in error. It estimates diffusion coefficients which are orders of magnitude too high and often exceed self-diffusion coefficients. This has frequently led to the conclusion that viscous flow predominates. The origins of the errors in this equation are indicated, and a substitute equation is developed which gives diffusion coefficients well below that for self-diffusion when applied to literature data. As a result it is concluded that molecular diffusion is the dominant mechanism in homogeneous systems.     

Preparation of amylose gel and probe diffusion through the gel membranes

Hydro‐swollen amylose gels were prepared by chemical crosslinking with a polyethylene glycol diglycidyl ether. The degree of swelling of the amylose gel (q_w) increases with increasing the amount of crosslinker in the preparation, which is a result contrary to other cases of chemically crosslinked polymer gels. The diffusion coefficient of a fluorescein and a fluorescein‐dextran conjugate in the gel membranes was determined by the time lag method and increases as q_w of the gels increases.     

Halide diffusion in polyaniline membranes

The feasibility of application of polyaniline (PAni) as electrolyte in polymer–electrolyte–membrane fuel cells (PEMFC) was investigated. PAni was dissolved in N-methyl pyrrolidone (NMP), cast as emeraldine base membranes (EB) and then doped with halide acids. The proton conductivity was measured according to Hittorf. The chloride ion distribution within the membrane was evaluated using energy-dispersive-X-ray analysis (EDX) and the diffusion coefficient was calculated using EDX, impedance spectroscopy and photometric analysis. The specific resistance was determined using conventional four-point measurement. The proton conductivity of HCl_conc.-doped PAni is approximately half that of the reference material Nafion^®117. The diffusion coefficient for the chloride ion within the membrane was determined to be between 1.86 and 3.4 × 10⁻⁸ cm² s⁻¹. Although halide-doped membranes were found to be proton conducting, slow halide removal during fuel cell operation occurred causing a rapid decline in cell performance. It is, therefore, concluded that in order to prepare suitable PAni membranes for fuel cell applications, it will be necessary to dope PAni with acids in such a way that loss of the dopant can not occur. Doping with solid acids or large molecular acids (in order to reduce their diffusivity) may be a solution to the problem.     

Penetration of pores in membranes by plasma polymer forming species

Penetration of low‐temperature plasma polymer forming species through a microporous polycarbonate membrane was indicated by advancing contact angle measurement and scanning electron microscopoy of the membrane surface. No penetration of plasma polymer forming species was found in nylon and poly(vinylidene fluoride) membranes. This is attributed to the tortuous pore shapes of the nylon and poly‐(vinylidene fluoride) membranes compared to the straight cylindrical pore shape of the polycarbonate membrane.     

Transport of monomer surfactant molecules and hindered diffusion of micelles through porous membranes

Diffusion of Triton X-100 through Celgard 2500 membranes was examined. The pore permeability for monomers was 5.0 × 10⁻⁶ cm²/sec and it was measured for upstream concentrations below the CMC value of 2.29 × 10⁻⁴M at 30°C. This value is close to the monomer diffusion coefficient in bulk suggesting that the monomers do not experience significant hindrance due to the pore walls. The permeability of the surfactant drops abruptly within a narrow range of reservoir solution concentrations in the vicinity of the CMC. At concentrations 10 × CMC, the permeability coefficient becomes constant and equal to 3.9 × 10⁻⁷ cm²/sec which is the pore permeability for the Triton X-100 micelles. Compared to the diffusion coefficient of micelles in bulk water, the transport of micelles is hindered by the pore walls. In a 10-fold concentration range the micellar pore permeability is practically constant indicating no large change in micelle size. The chemical equilibrium model applied to surfactant diffusion in pores shows reasonable agreement over the entire range of the experimental data for reservoir concentrations from one-fifth times the CMC to 100 times the CMC.     

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.     

Contribution of convection,diffusion and migration to electrolyte transport through nanofiltration membranes

Transport mechanisms through nanofiltration membranes are investigated in terms of contribution of convection, diffusion and migration to electrolyte transport. A Donnan steric pore model, based on the application of the extended Nernst-Planck equation and the assumption of a Donnan equilibrium at both membrane-solution interfaces, is used. The study is focused on the transport of symmetrical electrolytes (with symmetric or asymmetric diffusion coefficients). The influence of effective membrane charge density, permeate volume flux, pore radius and effective membrane thickness to porosity ratio on the contribution of the different transport mechanisms is investigated. Convection appears to be the dominant mechanism involved in electrolyte transport at low membrane charge and/or high permeate volume flux and effective membrane thickness to porosity ratio. Transport is mainly governed by diffusion when the membrane is strongly charged, particularly at low permeate volume flux and effective membrane thickness to porosity ratio. Electromigration is likely to be the dominant mechanism involved in electrolyte transport only if the diffusion coefficient of coions is greater than that of counterions.     

Preparation of SnO2 supported membranes with ultrafine pores

The colloidal route of the sol-gel process was used to prepare supported SnO₂ membranes. The influence of the sol and monoelectrolyte concentrations on the formation of the gel layer by sol-casting on the top of macroporous α-Al₂O₃ support was described. The stability of the colloidal suspension as a function of the concentrations was analyzed from creep-recovery measurements. The calcined supported membranes were characterized by nitrogen adsorption-desorption isotherms and scanning electron microscopy. The set of results show that homogeneous membrane layers containing the smallest quantity of cracks are formed in a critical interval of sol (1.0<-[SnO₂]<-1.4 M) and electrolyte (2.0<-[Cl⁻]<-4.0 mM) concentrations. The samples prepared from concentrated suspensions present a lot of interconnected cracks which favors the peeling of the coated layer. The membranes have pores of average diameter of about 1 nm.     

Enhanced electrostatic modulation of ionic diffusion through carbon nanotube membranes by diazonium grafting chemistry

A membrane structure consisting of an aligned array of open ended carbon nanotubes (7 nm i.d.) spanning across an inert polymer matrix allows the diffusive transport of aqueous ionic species through CNT cores. The plasma oxidation process that opens CNTs tips inherently introduces carboxylic acid groups at the CNT tips, which allows for a limited amount of chemical functional at the CNT pore entrance. However for numerous applications, it is important to increase the density of carboxylic acid groups at the pore entrance for effective separation processes. Aqueous diazonium-based electrochemistry significantly increases the functional density of carboxylic acid groups. pH dependent dye adsorption–desorption and interfacial capacitance measurements indicate 5–6 times increase in functional density. To further control the spatial location of the functional chemistry, a fast flowing inert liquid column inside the CNT core is found to restrict the diazonium grafting to the CNT tips only. This is confirmed by the increased flux of positively charged with anionic functionality. The electrostatic enhancement of ion diffusion is readily screened in 0.1 M electrolyte solution consistent with the membrane pore geometry and increased functional density.     

An experimental and theoretical analysis of molecular separations by diffusion through ultrathin nanoporous membranes

Diffusion based separations are essential for laboratory and clinical dialysis processes. New molecularly thin nanoporous membranes may improve the rate and quality of separations achievable by these processes. In this work we have performed protein and small molecule separations with 15 nm thick porous nanocrystalline silicon (pnc-Si) membranes and compared the results to 1- and 3- dimensional models of diffusion through ultrathin membranes. The models predict the amount of resistance contributed by the membrane by using pore characteristics obtained by direct inspection of pnc-Si membranes in transmission electron micrographs. The theoretical results indicate that molecularly thin membranes are expected to enable higher resolution separations at times before equilibrium compared to thicker membranes with the same pore diameters and porosities. We also explored the impact of experimental parameters such as porosity, pore distribution, diffusion time, and chamber size on the sieving characteristics. Experimental results are found to be in good agreement with the theory, and ultrathin membranes are shown to impart little overall resistance to the diffusion of molecules smaller than the physical pore size cutoff. The largest molecules tested experience more hindrance than expected from simulations indicating that factors not incorporated in the models, such as molecule shape, electrostatic repulsion, and adsorption to pore walls, are likely important.     

Diffusiophoresis in a suspension of spherical particles with arbitrary double-layer thickness

The diffusiophoresis in a homogeneous suspension of identical dielectric spheres with an arbitrary thickness of the electric double layers in a solution of a symmetrically charged electrolyte with a constant imposed concentration gradient is analytically studied. The effects of particle interactions (or particle volume fraction) are taken into account by employing a unit cell model, and the overlap of the double layers of adjacent particles is allowed. The electrokinetic equations that govern the ionic concentration distributions, the electrostatic potential profile, and the fluid flow field in the electrolyte solution surrounding the charged sphere in a unit cell are linearized assuming that the system is only slightly distorted from equilibrium. Using a perturbation method, these linearized equations are solved with the surface charge density (or zeta potential) of the particle as the small perturbation parameter. Analytical expressions for the diffusiophoretic velocity of the dielectric sphere in closed form correct to the second order of its surface charge density or zeta potential are obtained from a balance between its electrostatic and hydrodynamic forces. Comparisons of the results of the cell model with different conditions at the outer boundary of the cell are made.