Deposition of Oil Drops on a Glass Substrate in Relation to the Process of Washing

The attachment of emulsion drops to glass substrates is investigated in relation to the redeposition of oil drops in the process of washing. It turns out that the drops of a surfactant-stabilized oil-in-water emulsion cannot be attached to an immersed glass plate simply by the buoyancy force. However, the same drops can be deposited on the plate when the latter is pulled out of the emulsion, i.e., when the drops are pressed against the substrate by a receding meniscus. We measured the amount of the oily deposit as a function of the pH, ionic strength, and composition of an amphoteric-anionic surfactant mixture. The enhanced oil deposition at low pH correlates with the domain in which the emulsion drops and the solid substrate bear opposite electric charges. This was established by zeta-potential measurements with oil drops and glass particles. The anionic surfactant brings negative surface charge to the oil droplets and suppresses the oil deposition on the negatively charged glass. With the increase of the fraction of the amphoteric surfactant in the mixture, the zeta-potential is converted from negative to positive, and the oil deposition grows almost linearly with the potential. In general, the deposition of oil drops by a receding meniscus is governed by an interplay of electrostatic and hydrodynamic factors. Copyright 2000 Academic Press.     

Tuning the Electrochemical Properties of Novel Asymmetric Integral Sulfonated Polysulfone Cation Exchange Membranes

In this study, novel asymmetric integral cation exchange membranes were prepared by the wet phase inversion of sulfonated polysulfone (SPSf) solutions. SPSf with different degrees of sulfonation (DS) was synthesized by variation in the amount of chlorosulfonic acid utilized as a sulfonating agent. The characterization of SPSf samples was performed using FTIR and ¹H-NMR techniques. SPSf with a DS of 0.31 (0.67 meq/g corresponding ion exchange capacity) was chosen to prepare the membranes, as polymers with a higher DS resulted in poor mechanical properties and excessive swelling in water. By a systematic study, the opportunity to tune the properties of SPSf membranes by acting on the composition of the polymeric solution was demonstrated. The effect of two different phase inversion parameters, solvent type and co-solvent ratio, were investigated by morphological and electrochemical characterization. The best properties (permselectivity of 0.86 and electrical resistance of 6.3 Ω∙cm²) were obtained for the membrane prepared with 2-propanol (IPA):1-Methyl-2-pyrrolidinone (NMP) in a 20:80 ratio. This membrane was further characterized in different solution concentrations to estimate its performance in a Reverse Electrodialysis (RED) operation. Although the estimated generated power was less than that of the commercial CMX (Neosepta) membrane, used as a benchmark, the tailor-made membrane can be considered as a cost-effective alternative, as one of the main limitations to the commercialization of RED is the high membrane price.     

Influence of the structural properties of poly(vinylidene fluoride) membranes on the heterogeneous nucleation rate of protein crystals

In this study, the influence of the morphological parameters of microporous poly(vinylidene fluoride) (PVDF) membranes on the heterogeneous nucleation rate of hen egg white lysozyme (HEWL) crystals has been investigated. Experiments have been carried out on membranes prepared by non-solvent-induced phase inversion method, using PVDF-co-hexafluoropropylene (Kynarflex 2800) and PVDF homopolymer (Kinar 460), and adding LiCl and poly(vinylpyrrolidone) (PVP) in order to modulate the pore structure. From a theoretical point of view, the free Gibbs energy balance for the formation of a critical nucleus has been adapted to nonporous surfaces, thus obtaining a mathematical correlation between the energy nucleation barrier, the membrane porosity, and the contact angle between protein solution and polymeric substrate. The energetic barrier to heterogeneous nucleation was found to increase at higher contact angles-according to the prediction of classical theory-and to decrease at higher porosity. For instance, the predicted deltaG(het)/deltaG(hom) ratio for PVDF-Kynarflex (PVP 2.5%) membrane with porosity of 0.11 was 0.30, 35% lower with respect to the value calculated by the Volmer equation for a dense polymeric matrix having the same contact angle (87.4 +/- 5.8 degrees). In addition, the effect of the membrane pore size, porosity, and thickness on the removal rate of solvent have been discussed. For example, the transmembrane flux through PVDF-Kynar (LiCl 5.0%) membrane was 12% inferior than the one measured under the same experimental conditions through PVDF-Kynarflex (LiCl 7.5%) membrane, the latter having similar pore size and thickness but higher porosity (0.44 vs 0.32). The possibility to achieve rapidly a high level of supersaturation is expected to increase the nucleation rate. In general, measurements performed during crystallization tests carried out at pH 4.5 in NaAc 0.05 M buffer with different precipitant (NaCl) concentrations agree with the predicted trends.