Microcapsules. XIV. Effects of membrane materials and viscosity of aqueous phase on permeability of polyamide microcapsules toward electrolytes

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Interfacial Polymerization: From Chemistry to Functional Materials

Interfacial polymerization, where a chemical reaction is confined at the liquid-liquid or liquid-air interface, exhibits a strong advantage for the controllable fabrication of films, capsules, and fibers for use as separation membranes and electrode materials. Recent developments in technology and polymer chemistry have brought new vigor to interfacial polymerization. Here, we consider the history of interfacial polymerization in terms of the polymerization types: interfacial polycondensation, interfacial polyaddition, interfacial oxidative polymerization, interfacial polycoordination, interfacial supramolecular polymerization, and some others. The accordingly emerging functional materials are highlighted, as well as the challenges and opportunities brought by new technologies for interfacial polymerization. Interfacial polymerization will no doubt keep on developing and producing a series of fascinating functional materials.

Permeability to solutes of polyamide capsules with different chemical compositions

The permeability of nylon capsule membranes with different chemical compositions to solutes was studied at different temperatures. The rate of solute permeation through the membrane decreased with increasing hydrophobicity of the membrane in the temperature range of 293-323 K. Increase in the size of permeant molecules caused a decreased in their rate of permeation through all capsule membranes at all temperatures studied. Higher permeation rates were observed for all permeants at higher temperatures. The activation energy of permeation was evaluated for the permeants from the Arrhenius plots of their permeation data. From the values of activation energy evaluated the solute permeation through the nylon capsule membranes was demonstrated to proceed by a pore mechanism.

Microencapsulation within crosslinked polyethyleneimine membranes

A microencapsulation technique is proposed involving the formation of a polyethyleneimine (PEI) membrane crosslinked by an acid dichloride. The membranes were formed at pH 8 in a non-polar solvent, conditions which are better suited for the encapsulation of biocatalysts or fragile biochemicals than those using polyamide membranes. The mean diameter and size distribution of the PEI microcapsules were similar to that observed with nylon membranes. The resultant microcapsules were spherical, free-flowing with a strong membrane. The mass of membrane was seen to be independent of the reaction time (1-4 min), insensitive to the PEI concentration and proportional to the concentration of crosslinking agent.

Influence of total surface area of core material on yield of deposited coacervate

Influence of total surface area of core material on yield of deposited coacervate was evaluated. A simple coacervation method was employed solvent-evaporation method were used as a model core substance. A mixed polymer system--aqueous solution of gelatin and potassium salt of cellulose acetate phthalate--was used as a wall material. A sodium sulphate solution acted as a desolvating agent. Results obtained clearly show an opposite effect of the size of core material and derived total surface area on the yield of attached coacervate. While keeping the amount of core material constant, the amount of deposited coacervate increased both with the decreased size of core particles (increased total surface area), and increased concentration of desolvating agent. The proportion of wall material gradually increases with the increased concentration of sodium sulphate solution in the range from 1 to 9 per cent of the weight of microcapsules. Analysis of variance was used for the evaluation of obtained data.

Platelet adhesion on to polyamide microcapsules coated with lipid bilayer membrane

Polyamide microcapsules with diameters of 3-4 microns were coated with lipid bilayer membrane and their interaction with canine platelets was investigated. Platelet adhesion on to the microcapsules was significantly suppressed by the lipid-coating. Coating with dimyristoylphosphatidylcholine (liquid-crystalline state) reduced platelet adhesion on to the microcapsules to a greater extent than that with dipalmitoylphosphatidylcholine (gel state) at 37 degrees C. The surface properties of the microcapsule in adsorption of plasma proteins were also changed by lipid coating. The amount of gamma-globulin and fibrinogen adsorbed on to the microcapsule was slightly decreased by lipid coating, while the amount of adsorbed albumin was increased. Platelet adhesion on to the lipid-coated microcapsules was suppressed most strongly in the presence of gamma-globulin. Apparently platelet adhesion on to the polyamide microcapsules is controlled by the nature of lipid membrane and gamma-globulin adsorbed on to the microcapsules.

Trapping of chemical carcinogens with magnetic polyethyleneimine microcapsules: II. Effect of membrane and reactant structures

Effects of the membrane structure and reactant type on the trapping of carcinogens and other reactive species by semipermeable magnetic polyethyleneimine (PEI) microcapsules are investigated. A series of these microcapsules with poly(hexamethyleneterephthalamide) membranes was prepared by interfacial polymerization with an 8-fold variation of hexamethylenediamine concentration in the aqueous emulsion phase. Although little change was found in the encapsulation of PEI (within the microcapsule core) and magnetite, the microcapsule membrane showed a 6-fold alteration in regard to mass and associated PEI. All the microcapsule types tested were capable of trapping N-methyl-N-nitrosourea and fluorescein isothiocyanate as covalent-binding probes, and eosin and tetrasodium copper phthalocyanine tetrasulfonic acid (CPTS) as ionic-binding probes. Very rapid penetration and reaction of eosin and CPTS with the membranes was demonstrated, with apparent saturation of membrane binding affecting the overall trapping. Differences in the site and quantity of binding were ascribed to the following factors: the core:membrane distribution of incorporated PEI; the probe molecular weight; the reaction or adsorption of the probes with the microcapsule membrane; the probe stability in aqueous solution; and the amount of probe used. These probes represent the chemical-physical features of many known carcinogens or metabolites; together with previous data, these results indicate the potential usefulness of this type of microcapsule for trapping carcinogens (and their metabolites) as covalently or ionically bound products.

Permeability of ethylcellulose microcapsules towards phenobarbital

Inward permeation from the surrounding medium of phenobarbital through the wall of water-loaded ethylcellulose microcapsules was investigated as a function of capsule size under the conditions of constant total capsule volume and constant total capsule surface area. The experimental data obtained were analysed in terms of capsule wall density and drug partition coefficient. The drug permeability coefficients calculated according to an equation derived from Fick's first law of diffusion were found to increase with decreasing capsule size in both constant total capsule volume and constant total capsule surface area experiments. The wall density and the drug partition coefficient also exhibited the same trend. Based on these findings, it was concluded that the drug permeation through ethylcellulose microcapsule membrane occurs predominantly by a solution-diffusion mechanism.

A controlled-release anti-inflammatory drug. Studies on microcapsules

A procedure to obtain a controlled-release microencapsulated anti-inflammatory drug based on a solvent evaporation method is described. The present method makes use of ethylcellulose as the polymer and methylene chloride as solvent. The evaporation of solvent is controlled by means of an air stream. Variations in the preparative procedure and their effects on capsule dimensions and permeabilities were studied. The release behavior of the drug is determined, and two different diffusion constants are also determined: 7.0 X 10(-10) cm2/s and 1.2 X 10(-10) cm2/s, corresponding to low and high release time. Based on these results it is proposed that these microcapsules have a nonhomogeneous polymeric wall, and are more porous in the outer surface. This model might be applicable to the microcapsules obtained by means of the solvent evaporation method.

Effect of capsule size on permeability of gelatin-acacia microcapsules toward sodium chloride

The effect of capsule size on the permeability of gelatin-acacia microcapsules toward sodium chloride was investigated. Gelatin-acacia microcapsules containing olive oil were prepared by phase separation. The encapsulated olive oil was extracted with acetone and the acetone-loaded microcapsules dispersed in acetone were fractionated by a series of mesh screens. The core material of acetone than was replaced by water. The permeability of each capsule fraction toward sodium chloride was estimated from the change in electrical conductance with time of the mixture of microcapsule suspension and sodium chloride solution. The permeability decreased with decreasing capsule size. Structured water in and around the capsule wall may be the cause of the observed size effect.

Preparation and evaluation of microencapsulated ion-exchange resin beads

Ion-exchange resin beads in the benzoate form were coated by several microencapsulation techniques to alter and improve characteristics, especially the control of drug release, of this type of drug delivery system. The most successful techniques included polymer-polymer interaction, temperature change, and nonsolvent addition. The microencapsulated beads then were studied with respect to the release rate of the organic anion to determine the effects of microencapsulation. The release rate of the organic anion could be controlled over a wide range, depending on the encapsulating material characteristics. Factors affecting the extent and rate of release as result of microencapsulation are discussed.