Stephen W. Thiel

Formation of asymmetric polysulfone membranes by immersion precipitation. Part II. The effects of casting solution and gelation bath compositions on membrane structure and skin formation

Abstract The effects of casting solution and gelation bath compositions on the structure of membranes produced from the system polysulfone-N,N-dimethylacetamide-2-propanol have been studied. Membranes were prepared by immersion precipitation with no evaporation step. Membrane structures were determined by scanning electron microscopy. The observations are analyzed in the light of predictions made using the model developed in Part I of this study [J. Membrane Sci., 65 (1992) 213]. The concept of the critical flux ratio is introduced to facilitate the analysis of the phenomena leading to skin formation of asymmetric membranes. This concept provides the means for predicting, based on casting conditions, whether or not a membrane will have a dense skin.

Formation of asymmetric polysulfone membranes by immersion precipitation. Part I. Modelling mass transport during gelation

A model describing the formation of asymmetric membranes by immersion precipitation is presented. The model is used to predict the type of liquid-liquid phase separation (instantaneous or delayed) that occurs when polysulfone-N,N-dimethylacetamide-2-propanol casting solutions are immersed in a gelation bath. The model includes the thermodynamic interaction parameters determined in earlier work and the transport parameters determined by measuring solvent release and nonsolvent uptake of the cast film immersed into a gelation bath for different initial compositions of the cast film and gelation bath. The predictions of the model agree with the experimental observations.

TRANSPORT OF ETHANOL-WATER DIMERS IN PERVAPORATION THROUGH A SILICONE RUBBER MEMBRANE

Abstract Pervaporation experiments have been performed using a silicone rubber membrane with feed mixtures ranging from pure water to pure ethanol. The pervaporation process is modelled as coupled diffusion driven by activity gradients. Diffusion coefficients are assumed to be constant. Coupling effects are modelled by assuming that water clusters, practically immobilized within the silicone rubber membrane, break down in the presence of ethanol to form mobile ethanol-water dimers that contribute significantly to the overall water flux. Experimental data are consistent with the proposed model.

Investigation of the mechanism of protein adsorption on ordered mesoporous silica using flow microcalorimetry

The adsorption of bovine serum albumin (BSA) and lysozyme (LYS) on siliceous SBA-15 with 24 nm pores was studied using flow microcalorimetry; this is the first attempt to understand the thermodynamics of protein adsorption on SBA-15 using flow microcalorimetry. The adsorption mechanism is a strong function of protein structure. Exothermic events were observed when protein-surface interactions were attractive. Entropy-driven endothermic events were also observed in some cases, resulting from lateral protein-protein interactions and conformational changes in the adsorbed protein. The magnitudes of the enthalpies of adsorption for primary protein-surface interactions decrease with increased surface coverage, indicating the possibility of increased repulsion between adsorbed protein molecules. Secondary exothermic events were observed for BSA adsorption, presumably due to secondary adsorption made possible by conformational changes in the soft BSA protein. These secondary adsorption events were not observed for lysozyme, which is structurally robust. The results of this study emphasize the influence of solution conditions and protein structure on conformational changes of the adsorbed protein and the value of calorimetry in understanding protein-surface interactions.

Energetics of lysozyme adsorption on mesostructured cellular foam silica: Effect of salt concentration

The heat of lysozyme adsorption on mesostructured cellular foam (MCF) silica was measured using flow microcalorimetry (FMC) to investigate the influence of a neutral salt, sodium sulfate. At concentrations up to 0.5 M sodium sulfate, a complex initial exotherm was followed by an endotherm. Protein surface coverage, the magnitudes of the exothermic heat signals and the magnitudes of the net heat of adsorption increased with sodium sulfate concentration. These observations suggest that electrostatic interactions are the principal driving force at low ionic strengths; van der Waals interactions become dominant at higher salt concentrations. Each exotherm could be deconvoluted into two exotherms, indicating multiple modes of lysozyme attachment to the silica surface. The endothermic peak, associated with protein desorption, disappeared at the highest sodium sulfate concentration (1.0 M), indicating irreversible adsorption of the protein on the MCF silica surface. The data are consistent with an adsorption mechanism in which the initial attachment of lysozyme to the surface is followed by a reorientation and formation of a secondary or stronger attachment to the surface.

Energetics of protein adsorption on amine-functionalized mesostructured cellular foam silica.

The energetics of lysozyme adsorption on aminopropyl-grafted MCF silica (MCF-NH2) are compared to the trends observed during lysozyme adsorption on native MCF silica using flow microcalorimetry (FMC). Surface modification on MCF silica affects adsorption energetics significantly. All thermograms consist of two initial exothermic peaks and one later endothermic peak, but the heat signal trends of MCF-NH2 are opposite from those observed for adsorption onto native MCF silica in salt solutions of sodium acetate and sodium sulfate. At low ionic strength (0.01 M), LYS adsorption onto MCF-NH2 was accompanied by a large exotherm followed by a desorption endotherm. With increasing ionic strength (0.1 and 3.01 M), the magnitude of the thermal signal decreased and the total process became less exothermic. Also a higher protein loading of 14 μmol g(-1) was obtained at low ionic strength in batch adsorption isotherm measurements. Taken together, the FMC thermograms and batch adsorption isotherms reveal that MCF-NH2 has the nature of an ion exchange adsorbent, even though lysozyme and the aminopropyl ligands have like net charges at the adsorption pH. Reduced electrostatic interaction, reduced Debye length, and increased adsorption-site competition attenuate exothermicity at higher ionic strengths. Thermograms from flow microcalorimetry (FMC) give rich insight into the mechanisms of protein adsorption. A two-step adsorption mechanism is proposed in which negatively charged surface amino acid side chains on the lysozyme surface make an initial attachment to surface aminopropyl ligands by electrostatic interaction (low ionic strength) or van der Waals interaction (high ionic strength). Secondary attachments take place between protruding amino acid side chains and silanol groups on the silica surface. The reduced secondary adsorption heat is attributed to the inhibitory effect of the enhanced steric barrier of aminopropyl group on MCF silica.

Interphase transfer kinetics of uranium using the drop method, Lewis cell, and Kenics mixer

The rate constants for the interphase transfer of uranium between 3.5 M HNO/sub 3/ and tributyl phosphate (TBP) - normal hydrocarbon diluent solutions have been measured using the single drop method, Lewis cell method, and a Kenics mixer - centrifugal separator. Rate constants obtained by all methods were the same within experimental error. The variables studied that affect the rate constants include the TBP concentration, the acidity and total neutral nitrate concentrations of the aqueous phase, and temperature. Results of these tests indicate that the rate controlling mechanism is chemical reaction at the interface.

Comparative thermodynamic and spectroscopic properties of water interaction with human stratum corneum

Background/purpose: The water content of skin has a significant impact on skin properties; sufficient hydration is necessary to keep the skin supple, flexible, and smooth. To understand more completely the water retention properties of the human skin barrier, physical macroscopic properties must be related to the structural organization of the stratum corneum (SC). Water, lipids, and natural moisturizing factor (NMF) influence the molecular structures that affect the properties of SC, including water sorption and binding enthalpy. In the research reported here, isothermal microcalorimetry was used to study the interaction of water vapor with isolated human SC in intact, delipidized, and water‐washed delipidized forms to identify the influences of the principal components of SC on water sorption. The calorimetric data are interpreted in conjunction with spectroscopic results to identify the conformational changes in keratins induced by lipid and NMF removal and to assess the influence of these changes on water binding in SC.

Multicomponent effects in the pressure-driven membrane separation of dilute solutions of nonelectrolytes

Abstract The rejection of mannitol, 1,6-hexanediol, and 1-pentanol from dilute binary and ternary aqueous solutions was measured using asymmetric cellulose acetate membranes. For the binary systems, mannitol rejection was greater than 1,6-hexanediol rejection, which was in turn greater than 1-pentanol rejection. The presence of either 1,6-hexanediol or l-pentanol caused an increase in mannitol rejection; the presence of mannitol caused a decrease in 1-pentanol rejection, l-Pentanol rejection is independent of the presence of 1,6-hexanediol. 1,6-Hexanediol rejection is independent of the presence of either mannitol or l-pentanol. The observed multicomponent effects are not caused by frictional coupling of solute fluxes; a physical model for the membrane that is consistent with the observed multicomponent effects is proposed.

Anaerobic toxicity of cationic silver nanoparticles

The microbial toxicity of silver nanoparticles (AgNPs) stabilized with different capping agents was compared to that of Ag(+) under anaerobic conditions. Three AgNPs were investigated: (1) negatively charged citrate-coated AgNPs (citrate-AgNPs), (2) minimally charged polyvinylpyrrolidone coated AgNPs (PVP-AgNPs) and (3) positively charged branched polyethyleneimine coated AgNPs (BPEI-AgNPs). The AgNPs investigated in this experiment were similar in size (10-15nm), spherical in shape, but varied in surface charge which ranged from highly negative to highly positive. While, at AgNPs concentrations lower than 5mgL(-1), the anaerobic decomposition process was not influenced by the presence of the nanoparticles, there was an observed impact on the diversity of the microbial community. At elevated concentrations (100mgL(-1) as silver), only the cationic BPEI-AgNPs demonstrated toxicity similar in magnitude to that of Ag(+). Both citrate and PVP-AgNPs did not exhibit toxicity at the 100mgL(-1) as measured by biogas evolution. These findings further indicate the varying modes of action for nanoparticle toxicity and represent one of the few studies that evaluate end-of-life management concerns with regards to the increasing use of nanomaterials in our everyday life. These findings also highlight some of the concerns with a one size fits all approach to the evaluation of environmental health and safety concerns associated with the use of nanoparticles.