Journal of Colloid Science最新文献

A new technique for estimating diffusion rates in solid materials from transient sorption data was developed and applied to the diffusion of gases in polymer films. Specifically, the analysis developed here applies to solutes which are sorbed according to a particular isotherm: one described by a linear component (Henry's law) and a commonly observed nonlinear component (Langmuir equation). The linear component corresponds physically to gas dissolved in the amorphous regions of the polymer; the nonlinear, to gas trapped in polymer microvoids, analogous to the adherence of gas molecules to sites on the surface of porous adsorbents.

By using this equilibrium isotherm together with the assumptions that the gas trapped in microvoids is immobilized, and that the driving force for diffusion is the concentration gradient of dissolved molecules, a mathematical description of transient sorption was developed; this diffusion model consisted of a nonlinear partial differential equation. With the use of a finite-difference technique, solutions to the equation were obtained; these were next applied to data for the solution of CO₂ in a one mil “Mylar” polyester film at 40°C. The diffusivity was estimated to be 1.74 × 10⁻⁹ cm.²/sec. and the precision of the method was estimated at ±12%.

In summary, the method developed has been successfully applied, and the proposed diffusion mechanism was thereby validated. It is believed that this method has wide potential applicability not only to the study of diffusion in polymers but also to the measurement of effective diffusivities of gases in porous catalysts.

A new class of ion exchange membranes has been developed for possible use in conversion of brackish water to fresh water. These membranes are based primarily on inorganic ion exchangers and have small quantities of extremely stable high temperature resistant organic polymers as binders.

The best cation exchange membranes to date utilize zirconium phosphate as the exchange material and Teflon or Kynar as the binder. The optimum anion exchange membranes have hydrous thorium oxide as the exchange material and the same organic binders.

The membranes are still in an early state of development but can be expected to show certain advantages over organic membranes. These include stability to high temperatures and to wetting-drying cycles. Resistance values and transference numbers of the membranes have not yet reached those of the organic ion exchange membranes but are in the process of improvement.

The effect of various salts on the miscibility gap of the glycerol triacetate (GTA)water system was examined at 25°C. Whereas addition of NaCl and KC1 reduces the mutual solubility of GTA and water, the addition of certain other salts, e.g., perchlorates, thiocyanates, iodides, bromides, salicylates, and zinc chloride, increases the mutual solubility of GTA and water to such an extent that complete miscibility may result.

For the anions listed the “salting-in” effects are, in general, insensitive to the cation used or its charge. Plots of equivalents of salt required to produce miscibility vs. wt. percent GTA showed curves with maxima usually at about 40% GTA. Miscibility at the maximum required ca. 1 mole of perchlorate or salicylate ions per kilogram of solvent; for thiocyanates, iodides, bromides, and zinc chloride, the corresponding concentrations were usually larger.

For several salts measurements were made in the temperature range 2° to 75°C. The mutual solubility of GTA and water in the absence of salts increases slightly with increasing temperature. Less NaC₇H₅O₃ was required for miscibility at a higher temperature. However, for Mg(ClO₄)₂, KCNS, NaI, and CdBr₂, more salt was required for miscibility at a higher temperature.

In order to develop better semipermeable membranes for reverse osmosis, the mechanism of water transport must be determined. An evaluation of existing theories led to the hypothesis that degree of crosslinking and polar group content of a polymer were important factors in determining its semipermeability.

This hypothesis was tested by preparing a series of crosslinked reinforced mem branes by polymerization in a thin film between two plates. These membranes were copolymers of ethylene glycol monomethacrylate (EGM), ethyl methacrylate (EM), and ethylene glycol dimethacrylate (EGD). High-pressure reverse osmosis experiments with a sodium chloride solution and water sorption tests were then conducted and the results were correlated with the degree of crosslinking and hydroxyl content of the membranes.

For copolymers of EGM and EGD, the degree of salt rejection rose as the membrane was more tightly crosslinked, then dropped off sharply. The rise was attributed to a decrease in translational motion of the polymer chain segments relative to one another, with associated hindrance to ion diffusion. The fall was due to monomersolvent incompatibility during polymerization with attendant precipitation of the polymer forming under these conditions, resulting in macrovoids in the membrane structure. As a result, water flux increased as the membrane was more highly crosslinked. Crosslinking had a great effect on water sorption.

Copolymers containing EM showed virtually none of the semipermeability of EGM polymers. Water flux dropped off very rapidly, and salt rejection was not measurable. Perhaps most striking was the fact that water sorption tests showed a direct proportionality between hydroxyl content of the polymer and water sorption capacity.

In aqueous systems, fatty acid salts below their critical micelle concentration (CMC) diffuse through collodion membranes of graded porosity at a rate which varies indirectly as the chain length of the fatty acids and directly as the porosity of the membranes. For a given salt the rate is somewhat less than for a nonelectrolyte of the same molecular weight.

When the concentration of the fatty acid salt is above the CMC, the rate is dependent on the size of the micelles relative to the pore size. Phospholipids, which exist in solution as very large micelles, are virtually impermeable even in very porous membranes. Cholesterol or benzene solubilized in lipid micelles can be transported through porous membranes, but the pores must be large enough to accommodate the micelles.

A mechanism is proposed for the ionic fluxes in excitable membranes based on recent studies of monolayer permeation and of interfacial ionic transport. The excitable membrane is assumed to be a homogeneous lipid bilayer with anionic groups on the inner and outer surfaces and nonselectively adsorbed cations at these sites. The bound ions are in high concentration at the membrane surface and they cause the enhanced ionic fluxes when released during activity. The free ions, which are in equilibrium with the adsorbed ions, give rise to the resting ionic fluxes. Starting with the steady state ionic concentration gradients, the current due to “depolarizing electrodes” causes the concentration of potassium ions near the outer surface to increase, and it is assumed that these ions exchange with and release the bound sodium ions. Because of the increased sodium concentration at the membrane surface, there is an increased sodium ion flux across the membrane. This flux causes a release and a reverse flux of potassium ions by a similar process. The fluxes have been calculated in terms of several membrane parameters, and they can account qualitatively for the ion fluxes and permeability ratios at rest and during activity, the actions of some ions and pharmacologic agents, the effects of ion depletion, etc. Therefore, the mechanism provides a basis for explaining the ionic events at excitable membranes in terms of physical processes at the membrane level.

The time course of swelling of a membrane during water imbibition is explained using time-independent properties of the membrane and an equation developed from physicochemical considerations. The corneal stroma is an example of a membrane for which sufficient experimental data are available to compare theory and experiment. When account is taken of the uncertainties in both theory and experimental data, there is good agreement between the predicted and observed time course of membrane swelling.