Sol/gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 1. Features of the i/n transition

The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10(-3) M/L. In this region of the phase diagrams, the system appears to be purely repulsive. The I/N transition shifts toward lower volume fractions for increasing particle anisotropy, and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic, and gel phases. The swelling law (separation distance vs swelling law) exhibits two regimes. For high volume fractions, the swelling law is one-dimensional as in layered systems and reveals the presence of isolated platelets. At lower volume fraction, distances scale as phi(-1/3), indicating isotropic volumic swelling. Finally, the experimental osmotic pressure curves can be satisfactorily reproduced by considering the interparticle distances between two charged planes whose effective charge is around 10% of the structural charge.     

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.     

Swelling of poly(ethylene oxide) gel in aqueous solutions of sodium dodecyl sulfate with added sodium chloride

The swelling behavior of poly(ethylene oxide) (PEO) gels in aqueous solutions of sodium dodecyl sulfate (SDS) with and without NaCl was investigated. In the absence of NaCl, PEO gels with different degrees of cross-linking began to swell from a concentration lower than the critical micelle concentration (cmc) of SDS, then showed sigmoidal enhancements of swelling in a higher SDS concentration region until the degrees of swelling reached maximum values. The SDS concentration at which the swelling began to appear was in reasonable agreement with the critical aggregation concentration (cac) value reported for the aqueous PEO system. For the cases where NaCl was present, the swelling behavior of PEO gel was different from that when NaCl was absent in the following way. The concentrations where the swelling begins to appear, and hence those where the degree of swelling rises steeply, decreased with an increase in NaCl concentration. The ultimate degrees of swelling at higher concentration regions also decreased with an increase in the NaCl concentration. The lowering of the SDS concentrations at which the PEO gel began to swell is in line with the decreases in the cmc of SDS solutions containing NaCl and also with the decreases in the cac of PEO solution. Electronic Publication     

Interactions between covalently crosslinked ethyl(hydroxyethyl)cellulose and SDS

The equilibrium swelling of chemically crosslinked gels based on ethyl(hydroxyethyl)cellulose (EHEC) in aqueous solutions of sodium dodecyl sulphate (SDS) was studied as a function of the SDS concentration at various temperatures and salt concentrations. Comparisons were made with gels based on poly-N-isopropylacrylamide (p-NIPA). Both polymers are known to form complexes with SDS above a critical association concentration (cac) of the surfactant, and both display a lower critical solution temperature (LCST) in water. For both types of gels, an increase in the equilibrium gel volume was seen with increasing SDS concentration above the cac, up to a maximum value when the SDS concentration in the external solution reached the critical micelle concentration (cmc). Above the cmc, the equilibrium gel volume decreased slowly with increasing SDS concentration. A volume collapse of the EHEC gels was observed in a temperature interval around the LCST of EHEC in solution. Above the cac, the collapse transition moved monotonically towards higher temperatures with added SDS. At lower SDS concentrations, however, the opposite trend was found. The swelling of the gel was less in the presence of salt and SDS, and a pronounced minimum in swelling appeared with added SDS when the salt concentration was sufficiently high (ca. 10 mmoles/l). Under these salt conditions, the LCST of the linear EHEC also passes through a deep minimum (below room temperature) on addition of SDS.     

The kinetics of poly(N-isopropylacrylamide) microgel latex formation

Conversion versus time curves were measured for poly(N-isopropylacrylamide) microgel latexes prepared by polymerization in water with sodium dodecyl sulfate, SDS. Polymerization rates increased with temperature with methylenebisacrylamide crosslinking monomer consumed faster thanN-isopropylacrylamide. The particle diameter decreased with increasing concentrations of SDS in the polymerization recipe and there was evidence that the rate of polymerization increased somewhat with SDS concentration. Particle formation occurred by homogeneous nucleation as micelles were absent.Comparison of particle size distributions from dynamic light scattering to those from a centrifugal sizer led to the conclusion that larger particles within a specific latex were less swollen with acetonitrile than were the smaller ones. This was interpreted as evidence for the polymer in larger particles having a higher crosslink density. Particle swelling was estimated from swelling ratios defined as the particle volume at 25 °C divided by the volume at 50 °C. In the absence of crosslinking poly(N-isopropylacrylamide) linear chains would disolve at 25 °C. The swelling results indicated that the average crosslink density in the particles decreased with conversion. This was explained by the observation that the methylenebisacrylamide was consumed more quickly and is typical of crosslinking in emulsion polymerization where polymer particles have high polymer concentrations at their birth.     

Analytical application of organic reagents in hydrophobic gel media-I General principle and the use of dithizone gel

Two types of hydrophobic gel particles containing dithizone were prepared, either by the swelling of low cross-linkage polystyrene beads with a dithizone solution in chlorobenzene, or by the gelatinization of a dithizone solution with dibenzalsorbitol. The extraction capacity of the gel particles for metal ions such as mercury, cadmium, zinc and lead from dilute aqueous solution, was investigated at various dithizone concentration and pH values. A column packed with such gel particles is found to be useful for the selective preconcentration or the selective trapping of specific metal ions at low concentration levels.     

Computer simulation of polymer networks: swelling by binary Lennard-Jones mixtures

The swelling of regular, tightly meshed model networks is investigated by a molecular-dynamics-Monte Carlo hybrid technique. The chemical equilibrium between two simulation boxes representing the gel phase and a solvent bath, respectively, is obtained by subjecting the Lennard-Jones particles of a binary mixture, serving as explicit solvent, to the particle transfer step of Gibbs ensemble-Monte Carlo. The swelling behavior, especially preferential absorption of a single component, whose dependence on temperature, pressure, and fluid composition is studied, also depends significantly on the size of the central simulation cell. These finite-size effects correlate well with those exhibited by the density of solvent-free (dry) networks. A theoretical expression, whose derivation is based on network elasticity (of dry networks) yields finite-size scaling behavior in good accord with simulation results for both dry networks and gels in contact with solvent baths. This expression can be used to extrapolate the swelling behavior of simulated finite systems to infinite system size.     

溶剂化团簇银的制备及表征

报导了在O/W,W/O,W/O/W以及LC这些有序分子集合体中制备Ag胶体的研究成果.利用W/O/W和LC可实现用高浓度AgNO3与N2H4·H2O反应制备出10nm以下的稳定Ag粒及Ag胶体.采用紫外可见、X射线衍射、电子显微技术对胶体及银粒进行了表征.发现团簇Ag外有层状结构.通过分析指出,层状结构体现了表面活性剂、助表面活性剂的有序双分子包复.     

Interaction of slightly cross-linked gels of poly(diallyldimethylammonium bromide) with sodium dodecyl sulfate. Diffusion of surfactant ions in gel

The dynamics of the changing microenvironment of the fluorescent probe pyrene in slightly cross-linked networks of poly(diallyldimethylmmonium bromide) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by fluorescence spectroscopy. Values of the spectral ratio I₃/I₁ for pyrene monomer included in SDS micelles in the swollen networks fall between the corresponding values for pyrene in water and for pyrene dissolved in SDS micelles in aqueous solution. In the narrow interval of the surfactant concentrations in the gel phase, the formation of pyrene excimers is observed. The values of the critical micelle concentration in the gel phase (ca. 5 × 10^?4 to 8 × 10^?4 mol/L) are tenfold lower than in aqueous solutions of the surfactant. The effective micellar diffusion coeffecient D in the gel phase increases with increasing swelling of the network. © 1993 John Wiley & Sons, Inc.     

Mesoscopic Simulation of Aggregates in Surfactant／Oil／Water Systems

The aggregates in sodium dedecylsulphate(SDS)/dimethylbenzene/water systems have been investigated using dissipative particles dynamic(DPD) simulation method.Through analyzing three-dimensional structures of aggregates,three simulated results are found.One is the phase separation,which is clearly observed by water density and the aggregates in the simulated cell;another is the water morphology in reverse micelle,which can be found through the isodensity slice of water including bound water,trapped water and bulky water;the third is about the water/oil interface,i.e.,ionic surfactant molecules,SDS,prefer to exist in the interface between water and oil phase at the low concentraion.     

Effects of surfactants on the phase transition of poly(N-isopropylacrylamide) in water

The effects of both anionic (sodium dodecyl sulfate, SDS) and cationic (dodecylpyridine bromide, DPB) surfactants on the phase transition of narrowly distributed poly(N-isopropylacrylamide) (PNIPAM) microgel particles were investigated by laser light scattering. The addition of SDS swells the particles and increases the phase transition temperature, while DPB has a much smaller effect. This difference cannot be due to an association between the surfactant hydrophobic tail and PNIPAM because DPB and SDS have an identical hydrophobic tail. The amide groups in PNIPAM are slightly protonized in deionized water (pH ∼ 5.5). Our results contradict a previous prediction that oppositely charged surfactants will collapse a polyelectrolyte gel. After adding SDS, a two-step phase transition of the PNIPAM gel is observed. This suggests that SDS forms micelles inside the microgel with the help of the immobilized counter ions on the gel network. The SDS micelles are broken into individual SDS molecules in the first step of phase transition, while in the second step individual SDS molecules are gradually expelled. Surfactant effects on the microgel particles are compared with those of individual PNIPAM chains. © 1996 John Wiley & Sons, Inc.     

Property and Application of BACy‐Based Functional Hydrogels

Conventional polyacrylamide hydrogels prepared from the free radical polymerization between acrylamide and N,N′‐methylenebisacrylamide (NMBA) have been frequently used in the biochemical technique like the sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) to resolve protein mixtures. In this study, we have prepared an alternative polyacrylamide hydrogel from the cross‐linking of acrylamide and N,N′‐bisacrylylcystamine (BACy). In addition, we have compared the BACy‐based hydrogel with the NMBA‐based polyacrylamide hydrogel for their physical properties such as swelling ratio, shear modulus, crosslink density and morphology. Moreover, we further determined whether BACy‐based polyacrylamide hydrogel could be applied to SDS‐PAGE and proteomics research. The results showed that this type of hydrogel is capable of separating proteins and facilitates further in‐gel protein digestion and the following protein identifications by mass spectrometry. In summary, our study provides a basis for the putative application of BACy‐based hydrogels.     

多孔壳聚糖接枝聚丙烯酸/钠基蒙脱土复合高吸水凝胶的制备及其对Pb2+的吸附

以十二烷基苯磺酸钠(SDS)胶束为孔模板,过硫酸铵(APS)引发,N,N'-亚甲基双丙烯酰胺(MBA)交联,壳聚糖(CS)与丙烯酸(AA)、钠基蒙脱土(Na-MMT)在水溶液中接枝共聚,成功制备了多孔壳聚糖接枝聚丙烯酸/钠基蒙脱土(CS-g-PAA/Na-MMT)复合高吸水凝胶,提出了SDS胶束致孔机理。扫描电子显微镜(SEM)分析表明,添加SDS的样品,凝胶表面出现多孔结构。通过考察SDS浓度对复合高吸水凝胶平衡吸水倍率和吸水速率的影响,发现当SDS浓度为1.5 mmol/L时,复合凝胶在蒸馏水和生理盐水中的平衡吸水倍率相对于空白样分别提高53.9%和35.3%,初始溶胀速率常数K_is也由空白样的1.2652 g/(g·s) 提高到5.1680 g/(g·s)。多孔结构也使复合凝胶对Pb²⁺的吸附速率加快,在10 min内即可达到饱和吸附量的95%,30 min完全达到吸附平衡。     

On the Modeling of Polyelectrolyte Gels

Summary: Ionic polymer gels are very attractive actuation materials with a great similarity to biological contractile tissues. They consist of a polymer network with bound charged groups and a liquid phase with mobile ions. Absorption and delivery of solvent lead to a considerably large change of volume. This swelling mechanism results from the equilibrium of different forces such as osmotic pressure forces, electrostatic forces and viscoelastic restoring forces and can be triggered by chemical (change of salt concentration or pH in the solution), thermal or electrical stimulation. In the present work, chemically and electrically stimulated electrolyte polymer gels in a solution bath are investigated. To describe the different phenomena occurring in these gels adequately, the modeling can be conducted on different scales. If only the global macroscopic behavior is of interest, the statistical theory which is capable to describe the global swelling ratio, is sufficient. By refining the scale, the mesoscopic coupled multi-field theory can be applied. Here, the chemical field is described by a convection-diffusion equation for the different mobile species. The electric field is directly obtained by solving the Poisson equation in the gel and solution domain. The mechanical field is formulated by the momentum equation. By further refining the scale, the whole structure can be investigated on the microscale by the discrete element (DE) method. In this model, the material is represented by distributed particles comprising a certain amount of mass; the particles interact with each other mechanically by a truss or beam network of massless elements. The mechanical behavior, i.e. the dynamics of the system, is followed by solving the Newton's equations of motion while the chemical field, i.e. the ion movement inside the gel and from the gel to the solution, is described by diffusion equations for the different mobile particles. All three formulations can give chemical, (electrical) and mechanical unknowns and all rely on the assumption that the concentration differences between the different regions of the gel and between gel and solution form the osmotic pressure difference, which is a main cause for the mechanical deformation of the polyelectrolyte gel film.     

高机械性能的聚(丙烯酸-co-甲基丙烯酸十八酯)疏水缔合凝胶的溶胀行为研究

以丙烯酸(AA)、甲基丙烯酸十八酯(OMA)、十二烷基硫酸钠(SDS)为原料,采用胶束共聚的方法合成了疏水缔合(HA)凝胶.在HA凝胶内部,表面活性剂SDS与疏水单体OMA组成的增溶胶束起到物理交联作用,将亲水的聚合物链交联起来.通过单向拉伸试验证实了该凝胶具有较高的机械性能.此外,也测试了HA凝胶在不同pH值溶液中的溶胀行为.结果显示,HA凝胶具有特殊的溶胀行为,其溶胀过程可以分为凝胶溶蚀、溶胀平衡和凝胶瓦解3个阶段.在强酸性条件下,凝胶的溶胀被抑制,没有出现凝胶瓦解阶段.在强碱性条件下,凝胶的溶胀被促进,溶胀平衡阶段被越过.盐的存在也会抑制HA凝胶的溶胀,但在SDS溶液中,溶液中的SDS会促使凝胶中的疏水改性聚合物溶解到溶液中去,组成新的缔合结构,而使溶液增稠。     

Specific interactions of poly(4-vinyl phenol) gel with cationic and anionic surfactants

Binding behaviors of ionic surfactants (decyl- and dodecyltrimethylammonium bromide (C(10)TAB, C(12)TAB), sodium decane sulfonate (SDeSo), and sodium dodecyl sulfate (SDS)) to poly(4-vinyl phenol) (P4VPh) gel were investigated to elucidate a specific swelling behavior that has been found for P4VPh gel in aqueous solutions of tetraalkylammonium salts. With increasing cationic surfactant concentration, P4VPh gel significantly deswelled and then remarkably reswelled at a concentration somewhat below the respective cmc values. On the other hand, in the case of the anionic surfactants, the gel only showed a marked swelling at a concentration just below the respective cmc values. A similar charge-specific behavior of the surfactants was also found for the P4VPh dispersion system studied with a UV-vis spectroscopy; namely, in the cationic surfactant-P4VPh systems, the turbidity of the dispersion first increased with increasing the surfactant concentration and then decreased. This result suggests that aggregation of P4VPh particles first occurred and finally the particles were solubilized. A red shift followed by a blue shift observed for a pi-pi absorption of phenol at around 278 nm was also consistent with the aggregation-solubilization behavior. In the anionic surfactant-P4VPh system, however, only solubilization of the polymer particle was observed, and the UV peak only showed a blue shift. All these results in the gel and the dispersion systems strongly suggest that the cation-pi interaction is involved in the binding of the cationic surfactants to P4VPh.     

Effect of colloidal stabilizer on the shape of polystyrene/poly(methyl methacrylate) composite particles prepared in aqueous medium by the solvent evaporation method

Effects of the kind and concentration of stabilizers on the nonspherical shape of polystyrene (PS)/poly(methyl methacrylate) (PMMA) composite particles prepared by release of toluene from PS/PMMA/toluene droplets dispersed in stabilizer aqueous solution were examined. In the case of poly(vinyl alcohol), the surfaces of the obtained particles always had a single dimple. In the case of sodium dodecyl sulfate (SDS), the shapes of the composite particles changed from the dimple, via acorn, to spherical with increasing SDS concentration. It was clarified that the dimple and acorn shapes of the PS/PMMA composite particles were caused by contraction of the PS phase after hardening of the PMMA phase in excentered core-shell and hemisphere morphologies, respectively, which were formed by phase separation during toluene evaporation.     

Swelling equilibria and volume phase transition of polyelectrolyte gel with strongly dissociated groups

The peculiarities of the swelling equilibrium and volume phase transition of polyelectrolyte copolymer gels consisting of acrylamide and 2-acrylamido-2-methyl-1-propane sulphonic acid (AAm/AMPS) have been studied as a function of copolymer composition and gel structure. Equilibrium swelling for AAm/AMPS gels in buffer solutions, pure water and aqueous salt (NaCl) solutions were investigated. The swelling curves were calculated using the Hasa–Ilavský–Dušek theory and obtained experimentally. The agreement is reasonably good, and the HID theory can be used to estimate the swelling and structural parameter of AAm/AMPS gels. The volume phase transition has been generated by changing the solvent composition by progressive substitution of water by acetone. The critical interaction parameter corresponding to collapse depends strongly on the composition of the gel, since the extent of collapse is determined by composition and cross-link density of the gel.     

疏水改性智能水凝胶P(NIPA-co-DiAB)的合成及其温敏行为

以N-异丙基丙烯酰胺(NIPA)和N,N-双烯丙基苄胺(DiAB)为共聚单体、N,N-亚甲基双丙烯酰胺(BIS)为交联剂、十二烷基硫酸钠(SDS)为表面活性剂、过硫酸铵(APS)-四甲基乙二胺(TMEDA)为氧化还原引发体系,采用自由胶束交联共聚法合成了疏水基团为芳香基的疏水改性温敏性智能水凝胶P(NIPA-co-DiAB)。研究了DiAB摩尔分数(x(DiAB))对水凝胶溶胀性能的影响。 在初始溶胀阶段,随着x(DiAB)由0增大至3%,P(NIPA-co-DiAB)水凝胶的溶胀行为由Fickian扩散转变为non-Fickian扩散。x(DiAB)分别为0、1%、2%和3%时,P(NIPA-co-DiAB)水凝胶的平衡溶胀率SR0在蒸馏水中分别为63.6、93.5、141.6和167.4,在0.01 mol/L SDS溶液中分别为63.1、71.0、59.0和77.5,在CTAB溶液中分别为37.6、42.2、44.1和60.0,在Triton X-100溶液中分别为30.9、49.4、68.5和88.3。 结果表明,P(NIPA-co-DiAB)水凝胶的(SR0)大于PNIPA水凝胶,且在蒸馏中比在0.01 mol/L表面活性剂溶液中要大。 加入0.01 mol/L Triton X-100、CTAB或SDS后,PNIPA水凝胶的体积相变温度或较低临界溶解温度(LCST)由32.5 ℃分别增加至35.4、45.6和80 ℃。P(NIPA-co-DiAB)水凝胶的LCST由32.0～32.5 ℃分别增加至34.7～35.6 ℃、45.8~46.2 ℃和80 ℃。 加入表面活性剂能增加P(NIPA-co-DiAB)水凝胶的体积相变温度,高的体积相变温度与DiAB含量无关。     

Double phase inversion of emulsions containing layered double hydroxide particles induced by adsorption of sodium dodecyl sulfate

A liquid paraffin-water emulsion was investigated using layered double hydroxide (LDH) particles and sodium dodecyl sulfate (SDS) as emulsifiers. Both emulsifiers are well-known to stabilize oil-in-water (o/w) emulsions. Surprisingly, a double phase inversion of the emulsion containing LDH particles is induced by the adsorption of SDS. At a constant LDH concentration, the emulsion is o/w type when SDS concentrations are low. At intermediate SDS concentrations, the first emulsion inversion from o/w to w/o occurs, which is attributed to the enhanced hydrophobicity of LDH particles caused by the desorption of the second layer of surfactant, leaving a densely packed SDS monolayer on the LDH exterior surfaces. The second inversion from water-in-oil (w/o) to o/w occurs at higher SDS concentrations, which may be due to the competitive adsorption at the oil/water interfaces between the LDH particles modified by the SDS bilayers and the free SDS molecules in the bulk solution, but the free SDS molecules dominate and determine the emulsion type. Laser-induced fluorescent confocal micrographs clearly confirm the adsorption of LDH particles on the surfaces of the initial o/w and intermediate w/o emulsion droplets, whereas no LDH particles were adsorbed on the final o/w emulsion droplet surfaces. Also, transmission electron microscopy (TEM) observations indicate that the shape of the final o/w emulsions is similar to that of the monomeric SDS-stabilized emulsion but different from that of the initial o/w emulsions. The adsorption behavior of SDS on LDH particles in water was investigated to offer an explanation for the emulsion double phase inversion. The zeta potential results show that the particles will flocculate first and then redisperse following surfactant addition. Also, X-ray diffraction (XRD) measurements indicate that SDS adsorption on the LDH interior surfaces will be complete at intermediate concentrations.