Emulsion Filtration Through Surface Modified Ceramic Membranes

In this work we studied the influence of membrane hydrophobicity on the filtration of oil/water (O/W) emulsions with a dispersed phase content of 30% (V/V). The membrane filtration process was realized by using ceramic tubular hydrophilic or hydrophobic membranes with different mean pore size (0.2 pm, 1.2 pm, and 1.4 pm of mean pore radius). Hydrophobic character was obtained by modifying superficially the membrane surface with a very thin polymer layer. The results obtained showed that the emulsion viscosity and droplet size distribution depend on the shearing forces and transmembrane pressure. The operating conditions and the nature of the membrane surface/emulsion interaction are the main parameters which control the type and nature of emulsion changes, such as modification of the mean droplet size, concentration into oleic phase or breaking.     

Emulsion pathways to composite polymeric membranes for separation processes

A novel method for the preparation of selective composite membranes from emulsions is suggested. The dispersed phase is chosen to yield a polymer soluble in those components for which the membrane should be selective; the continuous phase, on the other hand, is selected to yield a polymer that is insoluble in any of the components of the mixture. Conventional emulsions (which have a maximum dispersed phase volume fraction of 0.74) or microemulsions can be employed to generate composites. However, concentrated emulsions which allow volume fractions as large as 0.99 are most suitable as precursors to selective and efficient membranes. These concentrated emulsions have the appearance of gels with a structure similar to that of foams. The relatively high permeabilities obtained with the resultant membranes are due to the small thickness of the films of the continuous phase. A concentrated emulsion of a hydrophobic (hydrophilic) monomer dispersed in a hydrophilic (hydrophobic) continuous phase is first prepared at room temperature, with suitable initiators in each phase for later polymerization, and with an appropriate dispersant in the continuous phase. To ensure the stability of the emulsion, the hydrophilic monomer is, in general, replaced by monomer plus water. On heating the gel at 50 °C, polymerization occurs in both phases and the emulsion transforms into a composite polymer membrane. As examples, composite membranes containing polystyrene as the dispersed phase and polyacrylamide as the continuous phase are used to separate toluene from cyclohexane, while other composite membranes containing acrylamide as the dispersed phase and a crosslinked polystyrene as the continuous phase are used to separate water from ethanol.Lecture presented at the Colburn Symposium, University of Delaware, October 19, 1988.     

Fouling and protein adsorption effect of low-temperature plasma treatment of membrane surfaces

Adsorption of proteins and the effect of the chemical nature of membrane surfaces on protein adsorption were investigated using¹⁴C-tagged albumin and several microporous membranes (polyvinilydene fluoride, PVDF; nylon; polypropylene, PP; and polycarbonate, PC). The membrane surfaces were modified by exposing them to low-temperature plasma of several different monomers (n-butane, oxygen, nitrogen alone or as mixtures) in a radiofrequency plasma reactor. Transients in the permeability of albumin solutions through the membranes and changes in flux of distilled water through the membranes before and after adsorption of albumin were used to investigate the role of protein adsorption on membrane fouling. The results show that the extent of adsorption of albumin on hydrophobic membranes was considerably more than that on hydrophilic membranes. The hydrophilic membranes were susceptible to electrostatic interactions and less prone to fouling. A pore-blocking model was successfully used to correlate the loss of water flux through pores of defined geometry     

Demulsification of water-in-oil emulsions by permeation through Shirasu-porous-glass (SPG) membranes

To investigate the effect of the droplet/pore size ratio on membrane demulsification, water-in-oil (W/O) emulsions with uniform-sized droplets was demulsified by permeation through Shirasu-porous-glass (SPG) membranes with a narrow pore size distribution at mean droplet/pore diameter ratios of 0.52–5.75. At transmembrane pressures above a critical pressure, the water droplets larger than the membrane pore size were demulsified, where the SPG membrane acted as a coalescer because the hydrophilic membrane surface had a high affinity for the water droplets. By contrast, at transmembrane pressures below the critical pressure, the larger water droplets were all retained by the membrane due to the sieving effect of the uniform-sized pores. When a W/O emulsion with a mean droplet diameter of 2.30 μm was allowed to permeate through a membrane with a mean pore diameter of 0.86 μm, the demulsification efficiency increased with increasing transmembrane pressure, to a maximum value of 91% at a transmembrane pressure of 392 kPa, and then decreased, while the transmembrane flux increased almost linearly with increasing transmembrane pressure. The demulsification efficiency was higher for higher water phase content and lower concentration of the surfactant, tetraglycerin condensed ricinoleic acid ester, in the emulsions due to the reduction of the emulsion stability.     

Demulsification of water-in-oil emulsions via filtration through a hydrophilic polymer membrane

It is known that hydrophobic microfiltration membranes can be used for demulsification of oil-in-water (o/w) emulsion due to coalescence of oil droplets in membrane pores. This study demonstrates that a hydrophilic polymer membrane can be used for the demulsification of surfactant-stabilized water-in-oil (w/o) emulsions. The success of demulsification is dependent on the type of emulsions and membrane used. Membrane pore size and transmembrane pressure were found to affect demulsification efficiency (DM), while other factors, such as membrane thickness and initial water content have slight or almost no effect. A coalescence mechanism of the demulsification phenomenon is also discussed. The separation process is not based on sieving effects due to a difference in membrane pore size, but is determined by droplet interactions with membrane surface.     

微乳液聚合耦合共混法构建聚偏氟乙烯共混杂化膜

以苯乙烯和甲基丙烯酸甲酯混合物作为油相, 采用反相微乳液法制备了AgCl纳米粒子; 通过微乳液原位聚合油相单体得到包含AgCl纳米粒子的聚合乳液; 将聚合乳液与聚偏氟乙烯(PVDF)通过共混法构建了包含AgCl纳米粒子的PVDF共混杂化膜. 紫外-可见光谱、 透射电子显微镜(TEM)及扫描电子显微镜(SEM)等表征结果和超滤实验结果表明, 聚合乳液加入的同时引入了亲水性聚合物和表面亲水的AgCl纳米粒子, 不仅改善了PVDF共混杂化膜的孔隙率和平均孔径, 还显著增强了PVDF共混杂化膜的极性和亲水性, 最终提升了膜的水通量和抗污染性能; 过量聚合乳液加入后不能与PVDF材料均匀共混, 而且AgCl纳米粒子也会在膜中形成团聚物堵塞膜孔隙, 从而削弱了膜的水通量和抗污染性能.     

Demulsification of water-in-oil emulsion by using porous glass membrane

The hydrophilic porous glass membranes were used to demulsify water-in-oil emulsion, and demulsification efficiency can reach more than 96.2%. Effects of pore size of the membrane, transmembrane pressure and volumetric ratio of oil phase to internal aqueous phase in the emulsion on demulsification were investigated. It was found that pore size of membrane and transmembrane pressure can significantly affect demulsification efficiency. The smaller the pore size of the membrane, the better the demulsification efficiency. However, smaller pore size of the membrane has to be exerted a greater transmembrane pressure in order to make internal aqueous phase enter the membrane pore. Correspondingly, effect of transmembrane pressure on permeation flux of the droplets was also studied. In addition, recovered-oil phase by the demulsification were reused five times to extract cadmium from simulated aqueous waste. The results indicated that the extracting efficiency could arrive at 96.5%.     

Adsorption and structural change of beta-lactoglobulin at the diacylglycerol-water interface

Diacylglycerol (DAG)/water and triacylglycerol (TAG)/water emulsions were prepared using beta-lactoglobulin (beta-LG) as an emulsifier. The oil phase (20% in emulsion) was mixed with beta-LG solution (1% beta-LG in water, pH 7) to prepare the emulsions. A fine oil-in-water emulsion was produced from both DAG and TAG oils. The interfacial protein concentration of the TAG emulsion was higher than that of the DAG emulsion. The zeta potential of the DAG oil droplet was higher than that of the TAG oil droplet. The front-surface fluorescence spectroscopy results revealed that tryptophan residues in beta-LG moved to the more hydrophobic environment during the adsorption of protein on the oil droplet surfaces. Changes in secondary structure of beta-LG during the adsorption were determined by FT-IR spectroscopy. Decreases in the beta-sheet content concomitant with increases in the alpha-helix content were observed during the adsorption to the oil droplets, and the degree of structural change was greater for beta-LG in the TAG emulsion than in the DAG emulsion, indicating the increased unfolding of adsorbed beta-LG on the TAG oil droplet surface. Results of interfacial tension measurement supported this speculation, that is, the increased unfolding of the protein at the TAG-water interface. Trypsin- and proteinase K-catalyzed proteolysis was used to probe the topography of the adsorbed beta-LG on the oil droplet surface. SDS-PAGE analyses of liberated peptides after the proteolysis indicated the higher susceptibility of beta-LG adsorbed on the DAG oil droplet surface than on the TAG oil droplet surface. On the basis of all the results, we discussed the conformation of the adsorbed beta-LG on the two oil droplet surfaces.     

ANTIFOULING PROPERTIES OF POLYVINYL CHLORIDE) MEMBRANES MODIFIED BY AMPHIPHILIC COPOLYMERS P(MMA-A-MAA)

Three well-defined diblock copolymers of poly(methyl methacrylate-b-methacrylic acid)(P(MMA-b-MAA))were synthesized using atom transfer radical polymerization method and varying poly(methacrylic acid)(PMAA)chain lengths. These copolymers were blended with PVC to fabricate porous membranes via phase inversion process.Membrane morphologies were observed by scanning electron microscopy(SEM),and chemical composition changes of the membrane surfaces were measured by X-ray photoelectron spectroscopy(XPS).Static and dynamic protein adsorption experiments were used to evaluate antifouling properties of the blend membranes.It was found that,the blend membranes containing longer PMAA arm length showed lower static protein adsorption,higher water permeation flux and better protein solution flux recovery.     

Fouling reduction in poly(acrylonitrile-co-acrylamide) ultrafiltration membranes

Ultrafiltration membranes with similar pore sizes were prepared from acrylonitrile homopolymer and copolymers with increasing acrylamide content. The membranes containing acrylamide were more hydrophilic, had a smaller dispersion force component of the surface energy, and a smaller negative zeta potential than those prepared from the homopolymer. The effect of the differing surface chemistry of these membranes with similar pore sizes was examined by studying the ultrafiltration of bovine serum albumin (BSA) as a function of feed pH. The hydrophilic membranes showed higher permeate fluxes and flux recoveries than the hydrophobic membrane, in spite of their reduced repulsive electrostatic interaction. With increasing pH, protein transmission increased markedly for the acrylamide containing membranes whereas the transmission through the hydrophobic membrane remained low. These rejection data are explained by the combined effects of the increased hydrophilicity, decreased dispersive surface energy and reduced electrostatic repulsion of the acrylamide containing membranes.     

A versatile coating approach to fabricate superwetting membranes for separation of water-in-oil emulsions

Separation of oil/water mixtures, especially for the emulsified oil/water mixtures, is important because of the frequent occurrence of oil spill accidents. Utilizing superwetting porous membrane has become a promising approach to separate either surfactant-free or surfactant-stabilized emulsions. Herein we report a facile and versatile strategy for preparing hydrophobic/under-oil superhydrophobic membranes by coating the skeletons of the membranes with the poly[(3,3,3-trifluoropropyl)methylsiloxane] (PTFPMS) nanoparticles. The obtained membranes could be used to separate various waterin- oil emulsions with high flux and separation efficiency. In addition, owning to the outstanding resistance of PTFPMS to the most organic solvents or oils, the modified membranes exhibited the excellent reusability and the antifouling properties that were critical in the practical applications. Many commercially available membranes can be modified by such a simple method.     

Improving hydrophilicity and protein resistance of poly(vinylidene fluoride) membranes by blending with amphiphilic hyperbranched-star polymer

To endow hydrophobic poly(vinylidene fluoride) (PVDF) membranes with reliable hydrophilicity and protein resistance, an amphiphilic hyperbranched-star polymer (HPE-g-MPEG) with about 12 hydrophilic arms in each molecule was synthesized by grafting methoxy poly(ethylene glycol) (MPEG) to the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent and blended with PVDF to fabricate porous membranes via phase inversion process. The chemical composition changes of the membrane surface were confirmed by X-ray photoelectron spectroscopy (XPS), and the membrane morphologies were measured by scanning electron microscopy (SEM). Water contact angle, static protein adsorption, and filtration experiments were used to evaluate the hydrophilicity and anti-fouling properties of the membranes. It was found that MPEG segments of HPE-g-MPEG enriched at the membrane surface substantially, while the water contact angle decreased as low as 49 degrees for the membrane with a HPE-g-MPEG/PVDF ratio of 3/10. More importantly, the water contact angle of the blend membrane changed little after being leached continuously in water at 60 degrees C for 30 days, indicating a quite stable presence of HPE-g-MPEG in the blend membranes. Furthermore, the blend membranes showed lower static protein adsorption, higher water and protein solution fluxes, and better water flux recovery after cleaning than the pure PVDF membrane.     

纳米SiO2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用, 它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3), 并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述. 结果表明, 阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层, 从而增强了颗粒表面的疏水性是原位表面活性化的基础. 通过吸附CTAB和II-14-3, 颗粒的疏水性适当增强, 能吸附到正辛烷/水界面稳定O/W(1)型乳状液; 而通过吸附di-C12DMAB所形成的单分子层更加致密, 颗粒的疏水性进一步增强, 进而使乳状液从O/W(1)型转变为W/O型; 当表面活性剂浓度较高时, 由于链-链相互作用, 表面活性剂分子将在颗粒/水界面形成双层吸附, 使颗粒表面变得亲水而失去活性, 但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度. 因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时, 通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.     

Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

Effects of organic solvents on ultrafiltration polyamide membranes for the preparation of oil-in-water emulsions

Hydrophilic ultrafiltration membranes made of polyamide with molecular weight cutoff 10 and 50 kDa have been studied for the preparation of oil-in-water emulsions by a cross-flow membrane emulsification technique. Isooctane and phosphate buffer were used as disperse and continuous phase, respectively. The permeation of apolar isooctane through the polar hydrophilic membrane was achieved by pretreatment of membranes with a gradient of miscible solvents of decreasing polarity to remove water from the pores and replace it with isooctane. Four different procedures were investigated, based on the solvent mixture percentage and contact time with membranes. After pretreatment, the performance of the membranes in terms of pure isooctane permeate flux and emulsion preparation was evaluated. The influence of organic solvents on polyamide (PA) membranes has been studied by SEM analysis, which showed a clear change in the structure and morphology of the thin selective layers. The effects proved stronger for PA 10 kDa than for 50 kDa. In fact, similar pretreatment procedures caused larger pore size and pore size distribution for PA 10 kDa than for 50 kDa. The properties of emulsions in terms of droplet size distribution reflected the membrane pore sizes obtained after pretreatment. The correlation between pore size and droplet size, for the physicochemical and fluid dynamic conditions used, has been evaluated.     

Fabrication of blend hydrophilic polyamide imide (Torlon®)-sulfonated poly (ether ether ketone) hollow fiber membranes for oily wastewater treatment

Blend hydrophilic polyamide imide (PAI)-sulfonated poly (ether ether keton) (SPEEK) hollow fiber membranes were fabricated for oil-water emulsion separation. The structure and performance of the membranes were examined by FESEM analysis, N₂ permeation, overall porosity, collapsing pressure, water contact angle, pure water flux, molecular weight cutoff (MWCO), and oil rejection tests. By studying ternary phase diagrams of polymer/solvent-additive/water system, the higher phase-inversion rate was confirmed for the solutions prepared at higher PAI/SPEEK ratio. A more open structure with larger finger-likes was observed by increasing PAI/SPEEK ratio. Mean pore size of 81 nm, overall porosity of 79% and water contact angle of 58° were obtained for the improved membrane prepared by PAI/SPEEK ratio of 85/15. Increasing SPEEK ratio resulted in lower mechanical stability in terms of collapsing pressure. Pure water flux of about 2.5 times of the plain PAI membrane was found for the improved membrane. MWCO of 460 kDa was found for the improved blend membrane. From oil rejection test, all the membranes demonstrated an oil rejection of over 95%. The improved membrane showed a lower rate of permeate flux reduction compared to the plain membrane which was related to the smaller fouling possibility. Less fouling resistance of the improved membrane was related to the higher flux recovery ratio (about 92%). For all the membranes, the dominant fouling mechanism was found to be the cake filtration. The improved PAI-SPEEK hollow fiber membranes was found to be practical for ultrafiltration of oily wastewaters.     

Amphiphilic graft copolymers based on ultrahigh molecular weight poly(styrene-alt-maleic anhydride) with poly(ethylene glycol) side chains for surface modification of polyethersulfone membranes

Amphiphilic graft copolymers having ultrahigh molecular weight poly(styrene-alt-maleic anhydride) (SMA) backbones and methoxyl poly(ethylene glycol) (MPEG) grafts were synthesized via the esterification between anhydride groups with hydroxyl groups. The synthesized graft copolymers, SMA-g-MPEGs, were used as additives in the preparation of polyethersulfone (PES) membranes via phase inversion process. X-ray photoelectron spectroscopy (XPS) analysis showed the comb-like graft copolymers spontaneously segregated to membrane surface during membrane formation. Water contact angle measurements and water absorbance experiments indicated the PES/SMA-g-MPEG blend membranes were much more hydrophilic than pure PES membrane. The blend membranes had stronger protein adsorption resistance than pure PES membrane did. After washed using de-ionized water for 25 days, the blend membranes exhibited higher hydrophilicity and stronger protein adsorption resistance. This phenomenon was attributed to the further accumulation of SMA-g-MPEG additives on membrane surface in aqueous conditions. SMA-g-MPEGs can be well preserved in membrane near-surface and not lost during membrane washing due to their high molecular weight and comb-like architecture.     

Improving hydrophilicity and protein antifouling of electrospun poly(vinylidenefluoride-hexafluoropropylene) nanofiber membranes

<正>Poly(vinylidenefluoride-hexafluoropropylene)(PVDF-HFP) nanofiber membranes with improved hydrophilicity and protein fouling resistance via surface graft copolymerization of hydrophilic monomers were prepared.The surface modification involves atmospheric pressure glow discharge plasma(APGDP) pretreatment followed by graft copolymerization of poly(ethylene glycol) methyl ether methacrylate(PEGMA).The success of the graft modification with PEGMA on the PVDF-HFP fibrous membrane is ascertained by X-ray photoelectron spectroscopy(XPS) and attenuated total reflectance Fourier transform infrared measurements(ATR-FTIR).The hydrophilic property of the nanofiber membranes is assessed by water contact angle measurements.The results show that the PEGMA grafted PVDF-HFP nanofiber membrane has a water contact angle of 0°compared with the pristine value of 132°.The protein adsorption was effectively reduced after PEGMA grafting on the PVDF-HFP nanofiber membrane surface.The PEGMA polymer grafting density on the PVDF-HFP membrane surface is measured by the gravimetric method,and the filtration performance is characterized by the measurement of water flux.The results indicate that the water flux of the grafted PVDF-HFP fibrous membrane increases significantly with the increase of the PEGMA grafting density.     

Protein folding at emulsion oil/water interfaces

It has long been known that proteins change their conformation upon adsorption to emulsion oil/water interfaces. However, it is only recently that details of the specifics of these structural changes have emerged. The development of synchrotron radiation circular dichroism (SRCD), combined with advances in FTIR spectroscopy, has allowed the secondary and tertiary structure of proteins adsorbed at emulsion oil/water interfaces to be studied. SRCD in particular has provided quantitative information and has enabled new insights into the mechanisms and forces driving protein structure re-arrangement to be achieved.The extent of conformational re-arrangement of proteins at emulsion interfaces is influenced by several factors including; the inherit flexibility of the protein, the distribution of hydrophobic/hydrophilic domains within the protein sequence and the hydrophobicity of the oil phase. In general, proteins lose much of their tertiary structure upon adsorption to the oil/water interface and have considerable amounts of non-native secondary structure. Two key conformations have been identified in the structure of proteins at interfaces, intermolecular β-sheet and α-helix. The preferred conformation appears to be the α-helix which is the most compact amphipathic conformation at the oil/water interface. The polarity of the oil phase can have a considerable influence on the degree of protein conformational re-arrangement because it acts as a solvent for hydrophobic amino acids. The new conformation of proteins at interfaces also means that proteins undergo less heat induced re-arrangement at interfaces than in solution. Different conformations of proteins at interfaces impact on emulsification capability, emulsion stability and protein/emulsion digestion. Hence advances in the understanding of protein conformation at interfaces can help to identify suitable proteins and conditions for the preparation of emulsion based food products.     

Simple, reversible emulsion system switched by pH on the basis of chitosan without any hydrophobic modification

Chitosan without hydrophobic modification is not a good emulsifier itself. However, it has a pH-tunable sol-gel transition due to free amino groups along its backbone. In the present work, a simple reversible Pickering emulsion system based on the pH-tunable sol-gel transition of chitosan was developed. At pH > 6.0, as adjusted by NaOH, chitosan was insoluble in water. Chitosan nanoparticles or micrometer-sized floccular precipitates were formed in situ. These chitosan aggregates could adsorb at the interface of oil and water to stabilize the o/w emulsions, so-called Pickering emulsions. At pH < 6.0, as adjusted by HCl, chitosan was soluble in water. Demulsification happened. Four organic solvents (liquid paraffin, n-hexane, toluene, and dichloromethane) were chosen as the oil phase. Reversible emulsions were formed for all four oils. Chitosan-based Pickering emulsions could undergo five cycles of emulsification-demulsification with only a slight increase in the emulsion droplet size. They also had good long-term stability for more than 2 months. Herein, we give an example of chitosan without any hydrophobic modification to act as an effective emulsifier for various oil-water systems. From the results, we have determined that natural polymers with a stimulus-responsive sol-gel transition should be a good particulate emulsifier. The method for in situ formation of pH-responsive Pickering emulsions based on chitosan will open up a new route to the preparation of a wide range of reversible emulsions.