Effect of proton concentration on membrane potential across a weak amphoteric polymer membrane

An amphoteric membrane consists of both positively and negatively fixed charge groups chemically bound to the polymer chains. If the external solution is changed from alkali to acid, it is possible to obtain an experimental result in which the membrane potential changes from positive to negative through the isoelectric point. It was characterized by examining the relationship between membrane potential and proton concentration (pH) obtained from both experimental and theoretical considerations. The Nernst-Planck flux equation and the Donnan equilibrium theory were also solved for a four-component system combined with the dissociation constant, in order to discuss the pH dependence of membrane potential in a weak amphoteric membrane by comparing the experimental results with the calculated results. It was proven that the calculated results substantially deviated from the theoretical results despite a similar tendency. Such a deviation was caused by the fact that the original theory disregarded the activity coefficient and the ionic mobility, which were dependent on the fixed charge concentration in a membrane. The original theoretical model was modified by adding the effect of a fixed charge group to the activity coefficient and ionic mobility. The calculated results using the modified model explained well the experimental results if the parameter called charge effectiveness, phi, was introduced into the equations. Introduction of phi into the prediction of membrane potential was already done by Kobatake et al. in a system of a strong polyelectrolyte monopolar membrane/salt aqueous solution. In this study, it was proved that phi can also be introduced into a weak amphoteric polymer membrane/salt aqueous solution system. Finally it was also concluded that the Donnan equilibrium and the Nernst-Planck flux equation were still applicable for examining the transport phenomena for the system of a weak amphoteric charged membrane and electrolyte solutions at various pH.     

Quantitative structure–permeability relationships at various pH values for neutral and amphoteric drugs and drug-like compounds$

Abstract

Human intestinal absorption is a key property for orally administered drugs and is dependent on pH. This study focuses on neutral and amphoteric compounds and their membrane permeabilities across the range of pH values found in the human intestine. The membrane permeability values for 15 neutral and 60 amphoteric compounds at pH 3, 5, 7.4 and 9 were measured using the parallel artificial membrane permeability assay (PAMPA). For each data series the quantitative structure–permeability relationships were developed and analysed. The results show that the membrane permeability of neutral compounds is attributed to a single structural characteristic, the hydrogen bond donor ability. Amphoteric compounds are more complex because of their chemical constitution, and therefore require three-parameter models to describe and predict membrane permeability. Analysis of the models for amphoteric compounds reveals that membrane permeability depends on multiple structural characteristics: the partition coefficient, hydrogen bond properties and the shape of the molecules. In addition to conventional validation strategies, two external compounds (isradipine and omeprazole) were tested and revealed very good agreement of pH profiles between experimental and predicted membrane permeability for all of the developed models. Selected QSAR models are available at the QsarDB repository (http://dx.doi.org/10.15152/QDB.184).     

Study on a novel antifouling polyamide–urea reverse osmosis composite membrane (ICIC–MPD): III. Analysis of membrane electrical properties

In this study, the electrical properties of the new polyamide–urea (ICIC–MPD) reverse osmosis composite membrane were analyzed via two self-made test cells. The electrical potential difference across membrane was measured via a perpendicular flow through mode potential difference measurement cell, and the electrical conductivity of membrane was tested by a tangential flow across mode conductivity measurement cell. Both streaming potential coefficient and gap between the upward and downward curves were determined by the plot of electrical potential difference versus up-loading and down-loading external pressure difference at both sides of membrane. It was found that pH of electrolyte solution has strong impact on streaming potential coefficient and electrical conductivity due to the dissociation of COOH group and protonation of NH₂ group of the active layer of ICIC–MPD membrane. It was also observed that both concentration of monomer 5-isocyanato-isophthaloyl chloride (ICIC) in the organic phase and contact time of organic phase with aqueous phase play an important role in salt rejection rate, gap between curves and electrical conductivity of the prepared ICIC–MPD membrane, and experimental results indicate that salt rejection rate of ICIC–MPD membrane is closely correlated to gap between curves at either polymerization condition. In addition, the effects of fouling behaviors on electrical potential difference and electrical conductivity of membrane were also discussed.     

两性喹诺酮抗菌药生理环境下独特的分子荷电性质

本文采用脂质体/水分配系统来考察pH、磷脂的种类和荷电性对药物与有序磷脂膜结合的影响.     

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl₂, Na₂SO₄, and MgSO₄) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.     

Effect of electrolyte in silicone oil-in-water emulsions stabilised by fumed silica particles

Partially hydrophobised fumed silica particles are used to make silicone oil-in-water emulsions at natural pH of the aqueous phase. The stability and rheological properties of the emulsions and suspensions are studied at NaCl concentrations in the range 0-100 mM. It is found that all emulsions are very stable to coalescence irrespective of the NaCl concentration. However, a strong effect of electrolyte on the creaming and rheological properties is observed and linked to the particle interactions in aqueous suspensions. The creaming rate and extent are large at low electrolyte concentrations but both abruptly decrease at salt concentrations exceeding the critical flocculation concentration of the suspension (approximately 1 mM NaCl). The drastic improvement of the stability to creaming is attributed to the formation of a visco-elastic three-dimensional network of interconnected particles and emulsion droplets.     

Study of the colloidal stability of an amphoteric latex

We have studied the colloidal stability of an amphoteric latex (–COOH and NH ₂ surface groups) with a low-angle light scattering technique (nephelometry). Measurements were carried out at different pH values and electrolyte concentrations (NaCl or CaCl ₂). The results show a behaviour in agreement with DLVO theory when the pH of the medium is below the isoelectric point (i.e.p.) of the latex: at low ionic strengths the latex is stable, but it becomes completely unstable and coagulates when electrolyte concentration goes over a particular value (the critical coagulation concentration). However, when pH is raised above the isoelectric point, the latex is completely stable even at high electrolyte concentrations, showing a behaviour clearly opposite to the theory. This could be explained by means of an additional short-range repulsive "hydration force" due to the structure of water molecules that accompany hydrated cations around the hydrophilic latex surface. The morphology and electrical properties of the latex surface have been studied by different methods: dynamic light scattering, electrophoretic mobility, potentiometric and conductometric titrations.     

Microfiltration of amphoteric surfactant using ceramic membranes

The microfiltration of commercially available amphoteric surfactant using ceramic membranes has been investigated. Various combinations operating conditions such as pH, electrolyte and surfactant concentrations were employed. Zeta potential and adsorption isotherms were obtained for the components of membrane surfactant system as functions of pH using surfactant or indifferent electrolyte (KCl). The shift in the membrane isoelectric point induced by the surfactant is linked to the carboxylic groups present on the surfactant which are believed to play a dominant role in the net surface charge of the membrane. A minimum in the permeate flux was found at the pH corresponding to the isoelectric point of the zwitterionic surfactant. This behaviour is ascribed to the interactions occurring between the surfactant–surfactant molecules and the surfactant–membrane. The higher fluxes obtained at low pH as compared to high pH arise from different fouling mechanisms and ionic strengths. Lower fluxes were found when inorganic electrolytes were used in conjunction with surfactant. However, as the valency of the salt increases, flux behaviour of the zwitterionic surfactant (close to isoelectric point) does not vary whilst the cationic and anionic state of the surfactants are much more affected. Interactions between surfactant molecules as a result of the charge screening effects by the larger valence ions are encouraged. The permeate flux declines with an increasing surfactant concentration even though some concentrations fall under the critical micelle concentration (c.m.c.). This is attributed to concentration polarisation in which the accumulated surfactant concentration at the membrane surface could form a stable viscous phase which is resistant to permeate flow in the secondary layer next to the membrane surface. This paper demonstrates the role interactions such as surfactant–surfactant and surfactant–membrane play in influencing the filterability of surfactant solutions using ceramic membranes.     

Associative and segregative phase separations of gelatin/kappa-carrageenan aqueous mixtures

The effects of ionic strength, temperature, and pH on the phase separation behavior of type B pigskin gelatin/sodium-type kappa-carrageenan aqueous mixtures were investigated. Depending on the different combinations of temperature and sodium chloride (NaCl) concentration, the mixtures showed compatible, associative, and segregative phase separation behaviors. Additionally, a coexistence of associative and segregative (associative-co-segregative) phase separations was expected at low temperature and low NaCl concentration. These different phase separation events were observed using confocal scanning laser microscopy. Moreover, it was found that the segregative phase separation when alone is induced by the ordering of kappa-carrageenan chains, while that in the coexistence region is induced by the ordering of gelatin chains. pH had a significant effect on the associative phase separation, resulting in morphologies changing from compatible solution to liquid coacervate and further to solid precipitate with decreasing pH. These were attributed to the dramatic changes of the charge density of amphoteric gelatin during the pH decrease.     

Fluidity modulation of phospholipid bilayers by electrolyte ions: insights from fluorescence microscopy and microslit electrokinetic experiments

Fluidity and charging of supported bilayer lipid membranes (sBLMs) prepared from 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) were studied by fluorescence recovery after photobleaching (FRAP) and microslit electrokinetic measurements at varying pH and ionic composition of the electrolyte. Measurements in neutral electrolytes (KCl, NaCl) revealed a strong correlation between the membrane fluidity and the membrane charging due to unsymmetrical water ion adsorption (OH(-) ? H(3)O(+)). The membrane fluidity significantly decreased below the isoelectric point of 3.9, suggesting a phase transition in the bilayer. The interactions of both chaotropic anions and strongly kosmotropic cations with the zwitterionic lipids were found to be related with nearly unhindered lipid mobility in the acidic pH range. While for the chaotropic anions the observed effect correlates with the increased negative net charge at low pH, no correlation was found between the changes in the membrane fluidity and charge in the presence of kosmotropic cations. We discuss the observed phenomena with respect to the interaction of the electrolyte ions with the lipid headgroup and the influence of this process on the headgroup orientation and hydration as well as on the lipid packaging.     

Behaviour of a cobalt spinel ultrafiltration membrane during salt filtration with different ionic strengths

The characterisation of the surface charge of a cobalt spinel ultrafiltration membrane was investigated in the presence of electrolyte solutions by means of streaming potential measurements. The selectivity of a membrane towards different salts depends on the electrostatic interactions between the species in solution and the charge of the membrane surface. Firstly, the values of the streaming potential coefficient were measured during the filtration of NaCl, CaCl₂, Na₂SO₄ and CaSO₄ as a function of pH and ionic strength. The iso-electric point was determined for each electrolyte. Secondly, the rejection rates of the electrolytes were measured by ionic chromatography. The rejection was explained by a correlation with the surface charge of the membrane.     

Adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide from simple (1-1) electrolyte solutions

The adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide (delta-MnO2) has been studied using two methods, viz., isotherms at constant pH in the presence of buffer solution and pH variation in the absence of buffer solution from a fixed metal ion concentration. While the adsorption isotherm experiments were carried out in 0.5 M NaCl only, pH variation or batch titration experiments were carried out in 0.5 M NaCl, 0.01 M NaCl, and 0.01 M KNO3 solutions. The complex nature of adsorption isotherms at constant pH values indicates that adsorption of all the cations is non-Langmuirian (Freundlich) and takes place on the highly heterogeneous oxide surface with different binding energies. The proton stoichiometry derived from isotherms at two close pH values varies between 0.3 and 0.8. The variation of fractional adsorption with pH indicates that the background electrolyte solution influences the adsorption of cations through either metal-like or ligand-like complexes with Cl-, the former showing a low adsorption tendency. The proton stoichiometry values derived from the Kurbatov-type plot varies not only with the electrolyte solution but also with the adsorbate/adsorbent ratio. The variation of fractional adsorption with pH can be modeled either with the formation of the SOM+ type or with a combination of SOM+ and SOMOH type complexes, depending upon the cation and electrolyte medium. The equilibrium constants obtained from Kurbatov-type plots are found to be most suitable in these model calculations. Adsorption calculated on the basis of ternary surface metal-chlorocomplex formation exhibits very low values.     

Investigation of amphoteric polybenzimidazole (PBI) nanofiltration hollow fiber membrane for both cation and anions removal

The removal of both anions (phosphate, arsenate, arsenite and borate ions) and cations (copper ions) has been investigated by employing a lab-developed amphoteric polybenzimidazole (PBI) nanofiltration (NF) hollow fiber membrane. The amphoteric characteristics are due to the imidazole group within PBI molecules that makes the PBI NF membrane have an isoelectric point near pH 7.0 and show different charge signs based on the media pH. Investigations on the rejection capability of typical anions, e.g. phosphate, arsenate, arsenite, borate anions and typical heavy metal cations, e.g. copper ions, reveal that the PBI NF membrane exhibits impressive rejection performance for various ions removal. However, their rejections are strongly dependent on the chemical nature of electrolytes, solution pHs and the feed concentrations. The experimental results are analyzed by using the Spiegler–Kedem model with the transport parameters of the reflection coefficient (σ) and the solute permeability (P). The PBI NF membrane may have potential to be used in industrial removal of various environmentally unfriendly ion species.     

丝素膜上药物渗透量对溶液PH值的响应

蚕丝丝素蛋白膜是一种具有弱碱性和弱酸性的两性荷电膜。因此在丝素膜－水溶液体系中，水溶液ｐＨ值变化会影响丝素膜上溶质的渗透速度和渗透量。丝素膜的等电点ｐＨ≈４．５。丝素膜上药物渗透实验的结果表明：在ｐＨ＝３．０～９．０的范围内，当溶液的ｐＨ＜４．５时，丝素膜带正电，正离子苄三甲氯化铵的渗透系数下降；当溶液的ｐＨ＞４．５时，丝素膜带负电，负离子酚磺酸钠的渗透系数明显下降。中性分子间苯二酚在丝素膜上的透过不受外部溶液ｐＨ值变化的影响。当溶液的ｐＨ＞８或ｐＨ＞４．５时，离子化的药物５－氟尿嘧啶或维生素Ｃ的渗透系数明显变小，这是因为两者分别在ｐＨ＝８（ｐＫａ＝８．０）和ｐＨ＝４．５（ｐＫａ＝４．２５）以上变成带电离子。这表明离子化药物在蚕丝丝素膜上的渗透速度和渗透量对溶液ｐＨ值变化有较好的响应特性。     

High-Performance Aqueous Size-Exclusion Chromatography Using Diol-Bonded Porous Glass Packing Materials. Retention Behavior of Some Proteins

Abstract

Retention volume of proteins increased or decreased with increasing phosphate buffer or neutral electrolyte concentrations in the mobile phase. This variation suppressed or accelerated by changing pH values in the mobile phase. The behavior of proteins can be interpreted by knowing isoelectric points (pI) of proteins and pKa value of the residual silanol groups on the surface of diol-bonded porous glasses. Positively charged surface of proteins below pH 8.0 (cytochrome c, lysozyme) retarded the elution by the ion-adsorption effects and negatively charged proteins around pH 7.0 (egg albumin, bovin serum albumin) eluted earlier than expected by the ion-exclusion effects. These effects suppressed by increasing phosphate buffer and neutral electrolyte concentrations in the mobile phase. Size-exclusion separation was attained in the mobile phase over 0.1 M phosphates and 0.1 M NaCl concentrations at pH 7.0. Mcllvaine buffer and Gomori buffer showed opposite action to proteins for retention comparing with Soerensen phosphate buffer. Potassium thiocyanate showed the different action for retention of proteins comparing with other neutral electrolytes and acted like sodium dodecyl sulphonate.     

Studies on electrochemical characterization and performance prediction of cellulose acetate and Zeocarb-225 composite membranes in aqueous NaCl solutions

We have mixed cellulose acetate and Zeocarb-225 in different ratios, leading to the preparations of Membrane-1 and Membrane-2. Membrane potential, water content, and conductance measurements have been carried out to estimate and analyze the data in terms of equilibria and important electrochemical parameters. The Donnan equilibrium has been incorporated to estimate the activity coefficient of counterions, y(p)M, and solute, y(+/-)M in the membrane phase along with the parameter, so called varphi expressing non-ideality. Dependence of the extent of hydrophilicity of both membranes on mean electrolyte concentrations has been examined. Selectivity in membranes is discussed in terms of dissociation equilibria, K(d)s and K(d)f. It has been found that membrane surface charge density sigma(s) increases with increasing of external NaCl concentration. Dependence of water transport number and cationic transport number on electrolyte concentration shows a similar trend of variation. At higher mean concentration of electrolyte, water transport number in Membrane-2 has a negative value. Membrane-2 has a higher value of water transport number than Membrane-1. The entropy production due to solute and water transport has been quantified for both the membranes in the light of nonequilibrium thermodynamics. The various type of interactions such as solute-membrane, solute-water, and water-membrane are analyzed in terms of friction coefficients (f(ij)) of Spiegler's frictional pore model. In our case, an f(wm) < f(sm) < f(sw)-like trend is observed in both membranes. These frictional coefficients show close dependence on external electrolyte concentrations. Pore potential values of Membrane-1 and Membrane-2 have been worked out using the Poisson-Boltzmann equation. In both systems pore potential values increase with increasing mean electrolyte concentrations. The transport through Membrane-1 and Membrane-2 tends to follow diffusion-control criteria, i.e., (D(+/-) . C. d/D(+/-)M C(M) . delta) > 2. A slightly higher value of solute rejection is found in Membrane-2.     

Ion Exchange and Protonation Equilibria of an Amphoteric Ion-exchange Resin in the Presence of Simple Salt

The influence of simple salts on the ion exchange and protonation equilibria of an amphoteric ion-exchange resin, which has strong base and weak acid moieties in a single functional group fixed onto the styrene-DVB matrix, has been investigated. Concentrations of ionic species in the amphoteric ion-exchange resin in equilibrium with various sodium salt solutions were estimated by (23)Na NMR spectroscopy. For the NaClO(4) system, the ratio of sodium ion concentration in the resin phase to that in the equilibrium solution was greater than 1 and increased with a decrease in the salt concentration. In contrast to an ordinary cation-exchange resin, the ion exchange behavior of Mg(2+) and Ca(2+) on the amphoteric ion-exchange resin showed a marked dependence on the kinds of salts: the distribution coefficients for the NaCl system were independent of the salt concentration, while the log D vs. log[Na(+)] plots for the NaClO(4) system showed linear relationships with slopes being neither -2 nor 0. Apparent protonation constants of the carboxylate in the functional group of the resin in equilibrium with NaClO(4) solutions were greater than those with NaCl solutions. The ion exchange and protonation properties of the amphoteric ion-exchange resin were elucidated on the basis of the information about the salt concentrations in the resin phase estimated by the NMR method.     

Influence of poly(ethylene glycol) on the partition of a charged spin probe

The influence of poly(ethylene glycol) on the partition of a charged long chain spin probe between membranes and an external phase is studied. The partition coefficient is derived from the ESR spectra. Membranes of different properties are used (egg lecithin liposomes, erythrocytes) to differentiate between the influence of the external phase and the specific properties of the membrane.The partition coefficient is decreased in an exponential manner on increasing the PEG concentration, which indicates a lowering of the thermodynamic stability of the membranes. The determination of the change in the difference of the chemical potential is dependent on the PEG concentration.The membrane destabilization induced by PEG is caused in an indirect manner by a change of the chemical potential difference as result of the changed water structure and the osmotic pressure, surface tension and hydration of the membrane. This destabilization could be connected with the high fusogenic activity of PEG.     

羧甲基纤维素钠/蒙脱土悬浮体系的流变性能及其影响因素

考察了蒙脱土(MT)质量分数(w)、羧甲基纤维素钠(CMC)/MT质量比(R)、电解质(NaCl、MgCl2和AlCl3)、pH值和温度对MT或CMC/MT悬浮体系流变性能和触变性的影响。 结果表明,随w增加,纯MT悬浮体系的流体类型由近牛顿流体向塑型流体变化,触变类型则由无触变、正触变向复合触变性转变;而CMC/MT悬浮体系仍保持MT体系的正触变塑型流体特征,但屈服值τ0和稠度指数K增加,滞后环面积S和电动电势|ζ|先增加后减小,表现出同步变化。 电解质和实验温度均不改变CMC/MT悬浮体系的正触变性塑型流体特征,但电解质使τ0和K增加,而温度增加使τ0和K减小。 随介质pH值的增加,CMC/MT体系由负触变性变为正触变性,|ζ|增大,S先增加后减小,且在pH=8.46时达到最大。     

丝素膜修饰电极pH电荷选择效应的研究

研究了丝素膜的带电特性,其等电点在pH4.5附近,利用丝系膜在等电点前后的两性荷电特性和电荷之间的静电作用,研制了丝素膜修饰石墨电极,多巴胺在膜中的表观扩散系数为2.65×10^-7cm^2/s,其电极反应异相电子转移速率常数为8.9×10^-6cm/s,电极用于神经递质类化合物体系的测定中,验证了此修饰电极的pH电荷选择效应。