Mechanism of monovalent and divalent ion mobility in Nafion membrane: An atomistic simulation study.

Polymer cation exchange membranes (CEMs) are widely used in water treatment processes. The fundamental factors that control the separation and selectivity of cations with different valences in CEMs are not fully understood. In this study, we use atomistic molecular dynamics simulations to investigate the underlying molecular mechanisms that control the mobility of cations with different valences in Nafion membranes. Our results indicate structural differences in binding of monovalent and divalent cations, which gives rise to differences in ion diffusion in Nafion. Monovalent cations are shown to be "territorially" bound, i.e., bound while partially hydrated, to the fixed charge groups whereas divalent cations are shown to be "site" bound, i.e., bound while fully dehydrated, to the charge groups on the polymer. This difference in binding structure gives rise to differences in transport characteristics of cations in Nafion.

Effect of Dielectric Saturation on Ion Activity Coefficients in Ion Exchange Membranes.

Polymeric ion exchange membranes are used in water purification processes to separate ions from water. The distribution and transport of ionic species through these membranes depend on a variety of factors, including membrane charge density, morphology, chemical structure, and the specific ionic species present in the fluid. The electrical potential distribution between membranes and solutions is typically described using models based on Donnan theory. An extension of the original theory is proposed to account for the nonideal behavior of ions both in the fluid and in the membrane as well to provide a more robust description of interactions of solutes with fixed charge groups on the polymer backbone. In this study, the variation in dielectric permittivity in the membrane medium with electric field strength is taken into account in a model based on Gouy-Chapman double-layer theory to provide a more accurate description of ion activity coefficients in an ion exchange membrane. A semianalytical model is presented that accounts for the variation in dielectric permittivity of water in a charged polymer membrane. A comparison of this model with Manning's counterion condensation model clearly demonstrates that by incorporating changes in water dielectric permittivity with electric field strength, much better agreement with experiments can be obtained over a range of salt concentrations for different ions.

Experimental measurement of settling velocity of spherical particles in unconfined and confined surfactant-based shear thinning viscoelastic fluids.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data. The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.(1) Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

A family of alkyl sulfate gemini surfactants. 1. Characterization of surface properties.

The fundamental aqueous and surface properties of a family of sulfate gemini surfactants have been characterized. The critical micelle concentrations (cmc) were determined by both electrical conductivity and surface tension methods. The cmc values were found to be two orders of magnitude lower than those measured for single tail surfactants. The cmc values depend primarily on the surfactant tail length, and relatively little on the spacer length and solution temperature. The surface tension measurements suggest that current family of gemini surfactants have higher tendency to spontaneously adsorb at the air-water interface and thus are much more efficient in reducing surface tension than conventional single-chain surfactants. Thermodynamic calculations of Gibbs free energies for micellization and adsorption indicate surface adsorption is promoted more than micellization for these sulfate gemini surfactants. This type of molecules may therefore be very efficient and cost-effective in applications that require ultra-low interfacial tensions and high interfacial activities.

A family of alkyl sulfate gemini surfactants. 2. Water-oil interfacial tension reduction.

A family of sulfate gemini surfactants that has great potential for various industrial applications was previously prepared and characterized in our lab. The unique and versatile structure of these surfactants has endowed them with properties that are attractive for enhanced oil recovery (EOR). A detailed experimental study was carried out and is presented here on the water-oil interfacial properties of these novel molecules. This series of seven sulfate gemini surfactants of different hydrophobic tail and spacer group lengths shows systematic trends in interfacial tension (IFT) reduction with changing solution conditions. These molecules exhibit extraordinary aqueous stability even in high salinity and hard brines. Ultra-low interfacial tension values were measured with low surfactant concentrations. The results from this study showed the potential of utilizing these surfactants at low concentrations and in harsh reservoir environments.