The surface and solution properties of dihexadecyl dimethylammonium bromide

The surface adsorption behavior and solution aggregate microstructure of the dichain cationic surfactant dihexadecyl dimethylammonium bromide (DHDAB) have been studied using small angle neutron scattering (SANS), light scattering, neutron reflectivity (NR), and surface tension (ST). Using a combination of surface tension and neutron reflectivity, the DHDAB equilibrium surface excess at saturation adsorption has been measured as 2.60 +/- 0.05 x 10 (-10) mol.cm (-2). The values obtained by both methods are in good agreement and are consistent with the values reported for other dialkyl chain surfactants. The critical aggregation concentration (CAC) values obtained from both methods (NR and ST) are also in good agreement, with a mean value for the CAC of 4 +/- 2 x 10 (-5) M. The surface equilibrium is relatively slow, and this is attributed to monomer depletion in the near surface region, as a consequence of the long monomer residence times in the surfactant aggregates. The solution aggregate morphology has been determined using a combination of SANS, dynamic light scattering (DLS), cryogenic transmission electron microscopy (CryoTEM), and ultrasmall angle neutron scattering (USANS). Within the concentration range 1.5-80 mM, the aggregates are in the form of bilamellar vesicles with a lamellar " d-spacing" of the order of 900 A. The vesicles are relatively polydisperse with a particle size in the range 2000-4000 A. Above 80 mM, the bilamellar vesicles coexist with an additional L beta lamellar phase.     

Determination of cluster composition in heteroaggregation of binary particle systems by flow cytometry

Cluster composition in aggregation processes of multiple particle species can be dynamically determined by flow cytometry if particle populations are fluorescently labeled. By flow cytometric single particle analysis, aggregates can be characterized according to the exact amount of constituent particles, allowing the detailed and separate quantification of homo- and heteroaggregation. This contribution demonstrates the application of flow cytometry for the experimental detection of heteroaggregation in a binary particle mixture of oppositely charged polystyrene (PS) particles and Rhodamine-B labeled melamine-formaldehyde (MF-RhB) particles. Experiments with different particle concentration, temperature, mixing mode, ionic strength and particle mixing ratio are presented. Aggregation kinetics are enhanced with increasing particle concentration and temperature as well as by increased shear of mixing. These results represent well-known behavior published in previous investigations and validate the performance of flow cytometry for probing heteroaggregation processes. Physical insight with a novel level of detail is gained by the quantification of de- and restabilization phenomena. At low ionic strength, "raspberry"-type aggregates with PS cores are formed by primary heteroaggregation. At moderate particle number ratios, these aggregates are electrostatically destabilized and form more complex aggregates in a secondary heteroaggregation process. At high particle number ratios (> or =50:1), the raspberry-type aggregates are electrostatically restabilized and secondary heteroaggregation is prevented. The dynamic change of aggregate charge was verified by zeta-potential measurements. The elevation of salt concentration over several orders of magnitude retards aggregation dynamics, since attractive interparticle forces are diminished by an electrostatic double layer. This indicates that heteroaggregation induced by attractive interparticle forces is faster than aggregation due to random Brownian motion. Destabilization at high ionic strength is facilitated by charged ions and no longer by MF-RhB coverage. This results in a species independent one step aggregation process.     

Real space structure of opaque gel

The structure of the opaque poly(acrylamide) gels is studied by using a confocal laser scanning microscope. The polymer network of the gel consists of the fractal aggregate of the colloidal particles in the higher concentration region of the cross-linker. The diameter of the colloidal particle, which formed in the gel, increases from 180 to 420 nm with an increase of the concentration of cross-linker. On the other hand, the fractal dimensions of the aggregate remain constant, ranging from 1.5 to 1.7. The densities of the particle are calculated to be 0.7 and 1.2 x 103 kg/m3, which are >10 times larger than the average density of the polymer network of the gel. The results indicate that the monomer and the cross-linker are densely cross-linked into the particles.     

Dendritic polyamine architectures with lipophilic shells as nanocompartments for polar guest molecules: A comparative study of their transport behavior

Dendritic core-shell architectures were synthesized by simple melt reactions of polyethylenimine (PEI) with different fatty acids. These systems were investigated towards their ability to encapsulate various guest molecules. Parameters, such as the length of the attached alkyl chain, size of the polymer core, concentration of the core-shell architecture in solution, pH, and nature of the guest molecule were investigated and compared. Guest molecules that bear anionic groups, such as carboxylate, phosphate, sulfonate, or acidic OH groups, as present in phenol units, are readily encapsulated and transferred from the aqueous phase to the organic phase because of multiple anionic–cationic interactions. Hyperbranched polymer architectures exhibit enhanced encapsulation properties when compared with their linear counterparts. In case of the hyperbranched system PEI₂₅C16 amide higher transport capacities were observed at lower concentrations of the polymer, for example, 26 guest molecules at 10⁻⁵ mol L⁻¹ versus 143 at 10⁻⁶ mol L⁻¹, suggesting formation of larger aggregates. The aggregation behavior of these polar nanocompartments were investigated at different concentrations by AFM showing particle aggregates of typically 250 nm at a concentration of 10⁻⁵ M. The individual particle sizes in these aggregates are similar to the particle size at 10⁻⁶ M concentration, typically 4.0–5.5 nm (AFM height). This indicates that aggregate formation takes place at concentrations higher than 10⁻⁸ M and transport might be mediated by small aggregates rather than unimolecular particles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2287–2303, 2007     

Electrochemical, microscopic, and spectroscopic characterization of prevesicle nanostructures and vesicles on mixed cationic surfactant systems

Several experimental techniques (conductivity, zeta potential, transmission electronic microscopy, and steady-state fluorescence spectroscopy) have been used to study the formation of mixed colloidal aggregates consisting of a cationic double-chain surfactant, di-dodecyldimethylammonium bromide (di-C12DMAB), and a single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C10TAB, and/or tetradecyltrimethylammonium bromide, C14TAB). Special interest has been devoted to the prevesicle domain, within which the formation of aggregated nanostructures was first reported in our laboratory. For that purpose, studies have been carried out on the very dilute region by means of conductivity experiments, confirming the existence of two critical aggregation concentrations in that concentration domain: the so-called mixed critical aggregate concentration, CAC, and the mixed critical vesicle concentration, CVC. By carrying out TEM experiments on negatively stained samples, we were surprised to find a number of aggregates without a clear aggregation pattern and with a variety of sizes and shapes at concentrations below CAC, where only monomers were expected. However, the nanoaggregates found at concentrations between CAC and CVC, also by TEM microscopy, show a clear and ordered "fingerprint"-like aggregation pattern similar to the liquid-crystalline phases reported for DNA-liposome complexes and/or DNA packed with viral capsids. Finally, at total surfactant concentrations above CVC, the aggregates were confirmed, by means of cryo-TEM micrographs and zeta potential measurements, to be essentially unilamellar spherical vesicles with a medium polydispersity and a net-averaged surface density charge of around 12 x 10(-3) C m(-2). The fluorescence emission of two probes, TNS (anionic) and PRODAN (nonionic), allows for the analysis of the micropolarity and microviscosity of the different microenvironments present in aqueous surfactant solutions where the above-mentioned vesicle and prevesicle aggregates are present.     

贵阳污灌区菜地土壤团聚体中有机碳和重金属的含量特征及相关性分析

以贵阳某污灌区菜地土壤为研究对象,分别采用微波消解-电感耦合等离子体质谱(ICP-MS)法和水合热重铬酸钾氧化分光光度法分析不同粒径土壤团聚体中重金属和有机碳的含量特征,并对有机碳和重金属的相关性进行分析。结果表明,以2mm粒径团聚体的含量为最高,约占75%。Cu、Zn、Cd和Pb在0.25～0.5mm粒径团聚体中含量最高,Cr在5～8mm粒径团聚体中含量最高,As在不同粒径团聚体中的含量变化不大,重金属含量随土层深度增大而减小。土壤重金属富集因子表现为CuPbCdZnCrAs,Cu、Zn、Cd和Pb在0.25～0.5mm粒径团聚体中分布因子最高,而在5～8mm粒径团聚体中重金属的质量负载因子最大。土壤有机碳含量随团聚体粒径的增大表现为先增大后减小,不同粒径团聚体中Cu、Cd、Pb和As含量与有机碳含量均呈显著正相关(p0.05)。污灌区菜地土壤Cr、As在5～8mm粒径团聚体中富集现象显著,Cu、Zn、Cd、Pb在0.25～0.5mm粒径团聚体中均表现出显著的富集特征。6种重金属在5～8mm粒径团聚体中的质量负载因子均为最高,表明6种重金属在5～8mm粒径团聚体中的贡献最大。     

Surface activity and self-aggregation ability of three cationic quaternized aminocalix[4]arenes

The self-aggregation ability of three amphiphilic cationic calix[4]arenes possessing four quaternary amino groups (aminoCAs) was investigated using a variety of methods. All of the studied compounds possess high aggregation ability. Their critical aggregation concentration (CAC) values in water are in the 0.0009–0.04 % (w/v) concentration range. Several size populations of aggregates were detected by DLS for all three CAs, and restructuring of aggregates was observed to be dependent on concentration. Particles formed above CAC were attributed to formation of vesicular structures (vesicles). The coexistence of other type of aggregates (presumably micelles) with vesicles was observed in the aqueous solution of CAs 2 and 3 from concentrations of 0.5 and 0.8 % (w/v), respectively. The filtration procedure was found to be a significant factor since the obtained data from filtered and unfiltered samples was different. The particle sizes obtained by TEM measurements were somewhat correlated with the DLS data for unfiltered CAs solutions. An analysis of the aggregate composition was undertaken by a size-exclusion method using semi-permeable cellophane membranes with different MWCO. A negative deviation from linearity of permeability flux profile starting from 0.8 % (w/v) concentration of donor phase indicated that the fraction of large aggregates at this point is significant enough that the molecules could not easily permeate through the membranes.     

Solvent effect on the aggregate of fluorinated gemini surfactant at silica surface

The aggregate states of partially fluorinated gemini surfactant [(CF3)2CF(CF2)2(CH2)10N(CH3)2]2(CH2)6Br2 (C(F)(5)C10-C6-C10C(F)(5)) on silica surface were investigated with atomic force microscopy (AFM) and water contact angle (CA) measurement by analyzing the effects of bulk concentration and adsorption time on stack state. On surfactant-adsorbed silica surfaces, there was a flat surface layer interspersed with some scattering surfactant aggregates. In the case of short adsorption times, the aggregates would be hemisphere. In the case of long adsorption times, the aggregates would be present in the form of bilayers. With the increase of bulk concentration, the adsorbed amount was enlarged and the surface layer became more compact. The formation of patchy bilayer aggregates indicated the saturation of the surface layer. Furthermore, organic solvent effects on the aggregate state of the surfactant on a silica surface were studied with four organic solvents, including n-hexane, dehydrated ethanol, 1,1,2-trichloro-1,2,2-trifluoroethane, and toluene. With the treatment of different organic solvents, the hemisphere aggregates on the surface layer can rearrange into spherical bilayer, rodlike monolayer, and branched rodlike monolayer aggregates, respectively. The polarity of solvents and affinity of organic solvents for surfactant molecules may have a great impact on the stack state of the fluorinated gemini surfactant molecules.     

Aggregation behavior of polyoxyethylene (20) sorbitan monolaurate (tween 20) in imidazolium based ionic liquids

Surface tension measurements were carried out for the solutions of polyoxyethylene (20) sorbitan monolaurate (Tween 20) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF 4) and hexafluorophosphate (bmimPF 6) at various temperatures. Two transition points were found in the surface tension-concentration curves at each temperature. The freeze-fracture transmission electron microscopy revealed that two kinds of particles with different sizes are formed at the concentrations of each transition point. Thus, the surfactant concentrations of the two transition points are regarded as critical aggregation concentrations, CAC 1 and CAC 2. From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of the aggregate formation, Delta G agg (0), Delta H agg (0), and Delta S agg (0). The thermodynamic parameters related to CAC 1 are almost independent of temperature. On the other hand, as for the aggregate formation at CAC 2, a positiveDelta S agg (0) contributes to a negative Delta G agg (0) at low temperature, while a negative Delta H agg (0) contributes to a negative Delta G agg (0) at high temperature. The behavior of the thermodynamic parameters as a function of temperature, combined with the variation of (1)H NMR chemical shifts of the bmim (+) protons as a function of the surfactant concentration, demonstrated that the aggregates formed at CAC 1 are nanodroplets of Tween 20 segregated from the solution phase, while those formed at CAC 2 are similar to the usual surfactant micelles formed in aqueous solution.     

Flocculation mechanism induced by cationic polymers investigated by light scattering

Three cationic polymers with molecular weights and charge densities of 3.0 x 10(5) g/mol and 10%, 1.1 x 10(5) g/mol and 40%, and 1.2 x 10(5) g/mol and 100% were chosen as flocculants to aggregate silica particles (90 nm), under various conditions, including change in polymer dosage, particle concentration, background electrolyte concentration, and shear rate. The size and structure of flocs produced were determined using the static light scattering technique. On the basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has been found that the polymer charge density plays an important role in determining the flocculation mechanism. Polymers with a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of charge neutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring about bridging mechanism at the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readily be affected by the particle concentration, and an increase in particle concentration results in the formation of larger but looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. It has also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.     

Effect of Electrode Pattern on the Column Structure and Yield Stress of Electrorheological Fluids

For electrorheological (ER) suspensions, the aggregate structures of particles were observed in electric fields by the use of transparent cells with different electrode patterns. Although the suspension is dispersed to noninteracting particles without electric fields, many aggregates are formed on the electrode surface in electric fields. Since the dipole–dipole interactions cause chain structures of particles and equilibrium conformations of chains are always aligned with electric field, the aggregates indicate the presence of columns spanning the electrode gap. The particle concentration in columns which are developed between parallel-plate electrodes is about 22 vol %. In striped electrodes, the particles construct striped aggregates along the electrodes and no particles remain in the insulating region. The particle concentration in striped aggregates is about 35 vol %. The nonuniformity of electric field is responsible for the high particle concentration. The increase in particle concentration of column lead to the high yield stress of electrified suspension. Therefore, the ER performance of suspension as an overall response can be improved by the electrode design.     

萘和丹酰基修饰的扇形树枝状分子聚集体的能量转移行为

分别对1-3代聚(酰胺-胺)(PAMAM)结构的dendron分子的外端基和focal point进行了修饰,得到了外端基为萘(给体)色团、焦点(focal point)为丹酰(受体)色团的树枝状化合物Dan-ABπ-Nap(n=2,4,8).利用荧光光谱测定了不同浓度下所得一系列树枝状分子在水中的荧光强度,并计算了它...     

Amplified light scattering and emission of silver and silver core-silica shell particles

Side versus forward light scattergrams, and fluorescence (488 nm excitation) intensity versus particle count histograms were gathered for bare, R6G-coated, and silica-R6G-coated silver particles of 150-200 nm diameter, one-by-one by flow cytometry. Fluorescence emission intensity of the composite particles monotonically increased and then reached a plateau with greater R6G concentrations, as measured by flow cytometry. Fluorescence amplification factors of up to 3.5x10(3) were estimated by reference to measurements on core-shell particles with silica instead of silver cores. Huge surface enhanced Raman scattering (SERS) intensities, at least 10(14)-fold greater than normal Raman scattering intensities, were observed with 633 nm excitation for molecules such as rhodamine 6G (R6G) on the same single particles of silver. Although routine transmission (TEM) and scanning (SEM) electron microscopies showed gross structures of the bare and coated particles, high-resolution field emission scanning electron microscopy (FE-SEM), revealed Brownian roughness describing quantum size and larger structures on the surface of primary colloidal silver particles. These silver particles were further characterized by extinction spectra and zeta potentials. Structural and light scattering observations that are reported herein were used to tentatively propose a new hierarchical model for the mechanism of SERS.     

Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X2 相似文献   

Fate of excited probes in micellar systems

This article presents studies on the photophysical and photochemical behavior of probes within micellar systems: organized emulsifier/polymer aggregates; the intra- and interpolymer association of amphiphilic polymers; monomer-swollen micelles (microdroplets); and the interfacial layer. Pyrene (Py) as a probe is particularly attractive because of its ability to measure the polarity of its microenvironment. Dipyme yields information on the microviscosity of micellar systems. Probes such as laurdan and prodan can be used to explore the surface characteristics of micelles or microdroplets. The dansyl group has a special photophysical property that gives information about the local polarity and mobility (viscosity) of the microenvironment. The organized association of amphiphilic polymer and emulsifier introduces a heterogeneity in the local concentration of the reactants. This heterogeneity also results from the attractive interaction between hydrophilic monomer and emulsifier in the case when the monomer carries a positive charge and the counterpart a negative one, and vice versa. Some emulsifiers can bind to the amphiphilic copolymers by simple partitioning between the aqueous phase and the polymer--non-cooperative association. The interaction between micelles (microdroplets) and charged polymers leads to the formation of mixed micelles. Binding emulsifiers to these polymers was detected at emulsifier concentrations much below the critical micellar concentration (CMC). Emulsifiers often interact cooperatively with polymers at the critical aggregation concentration (CAC) below the CMC, forming micelle-like aggregates within the polymer. The CAC can be taken as a measure of interaction between the emulsifier and polymer. A decrease in the monomer fluorescence intensity of probe-labeled polymer results from increased excimer formation, or higher aggregates within the unimolecular polymeric micelles. An increase in the monomer fluorescence intensity of probe-labeled polymer within the micellar system can be ascribed to shielding of the probe chromophores by emulsifier micelles. The quenching of probe emission by (un)charged hydrophilic monomer depends on partitioning of the monomer between the aqueous phase and the micelles. Penetration of reactants into the interfacial layer determines the quenching of the hydrophobic probe by hydrophilic quencher, or vice versa. Quenching depends on the thickness, density and charge of the interfacial layer. Compartmentalization prevents the carbonyl compound and unsaturated monomer from coming into sufficiently close contact to allow singlet or triplet-monomer interaction. All negatively charged carbonyl probe molecules are quenched with significantly lower rates than the parent neutral hydrophobic benzophenone molecules, which were located further inside the aggregates. This results from the different conformation and allocation of reactants within the micellar system. In the reverse micelles, quenching depends on the amount of water in the interfacial layer and the total area of the water/oil interface.     

Effect of sphingomyelinase-mediated generation of ceramide on aggregation of low-density lipoprotein

This study addresses the response-to-retention hypothesis, which states that the subendothelial retention of atherogenic lipoproteins is the necessary and sufficient condition for the initiation of atherosclerosis. Here we focus on the relationship between the generation of ceramide in the low-density lipoprotein (LDL) phospholipid monolayer and the resulting aggregation of LDL particles. This study provides the first measurement of neutral, Mg (2+)-dependent Sphingomyelinase (Smase)-mediated ceramide formation from LDL-sphingomyelin and does so for a range of enzyme concentrations (0-0.22 units Smase/mL). The kinetics of ceramide generation was measured using a fluorescence assay for the above enzyme concentrations with a fixed substrate concentration (0.33 mg LDL/mL). The kinetics of LDL aggregate formation was measured by dynamic light scattering (DLS, method of cumulants) for identical enzyme concentrations. Ceramide concentration profiles were fit with a modification of the Michaelis-Menten model ( k a = 1.11 x 10 (-1) microM (-1) min (-1), k -a = 6.54 x 10 (2) microM (-1) min (-1), k 1 = 3.33 x 10 (1) microM (-1) min (-1), k -1 = 1.41 x 10 (-2) min (-1), k cat = 8.05 x 10 (1) min (-1), K M = 2.418 microM, k deact = 4.66 x 10 (-2) microM (-1) min (-1)) that accounts for the effects of enzyme attachment to the LDL monolayer and for deactivation of Smase due to product inhibition. LDL aggregation is described by a mass action model as explained in previous studies. A key result of this work is the finding that LDL aggregate size depends directly on ceramide concentration and is independent of enzyme concentration. This study demonstrates how principles of colloid science are relevant to important biomedical problems.     

体积排阻色谱和激光光散射对大豆蛋白热诱导聚集体的研究

采用体积排阻色谱(SEC-HPLC)和激光光散射(LLS)研究了不同浓度和离子强度下大豆蛋白热诱导聚集体的分子量分布和粒径分布。在离子强度为0时,SEC-HPLC的结果表明,热处理后的蛋白溶液主要由3部分组成,即聚集体、中间体和未聚集部分。聚集体部分随着浓度增加而逐渐增加;LLS的结果表明:体系有不均一的粒径分布,且浓度增加时体系的平均粒径增加。上述样品在较高离子强度下加热时,SEC-HPLC和LLS的结果都证明溶液中的中间体部分逐渐消失。因此,控制浓度和离子强度可以制备不同性质的大豆蛋白聚集体。     

Optimized solvent-exchange synthesis method for C60 colloidal dispersions

An existing solvent exchange method used to produce aqueous suspensions of fullerene C(60) aggregates (nC(60)) using the solvents toluene, tetrahydrofuran, acetone, and water, has been optimized for producing 75 nm diameter particles. Numerous synthesis parameters were evaluated for their effects on colloid yield and particle size distribution. Varying the relative volumes used of the intermediate solvents relative to the initial toluene volume allowed the controlled tuning of the resulting particle size up to a diameter of 210 nm. The resulting suspensions produced 10-20 ppm concentrations and reduced the residual organic solvents to less than the detection limit of 1 ppm.     

Preparation and Use of Polyurethane Enzyke Particles

The preparation and use of immobilized enzyme particles by disrersior and radiation polymerization technique in polyurethane prepolymer using polylene-2,4-diisocyanate, 2-hydroryethyl methacrylate, pentamethylene glycol, and cellulase are presented. The enzyme was added to form the particles (10–100C μm in diameter) under stirring of the mixture containing the prepolymer and enzyme solution, in which the enzyme was covalently bound to the surface of the particle. The size of the particles varied with the concentration of the enzyme and monomer. The enzymatic activity of the particles varied with the particle diameter, composition of monomer, and dispersion temperature.     

Redispergation of TiO2 particles in aqueous solutions

The colloidal stability of TiO2 dispersions in aqueous solutions was studied. Aqueous solutions of ATLAS G-3300 (1.57 x 10(-3) mol/l), TRITON X-100 (5 x 10(-5) mol/l), and PMAA (4 x 10(-6) and 5.81 x 10(-3) mol/l) have been used as medium for redispergation of TiO2 particles. Stability of dispersions was investigated at different pH values by two different methods. By using analytical centrifuge the sedimentation velocity of TiO2 particles was directly measured and by means of light scattering the particle size of dispersed particles has been monitored. Combination of these two methods allowed determination of the aggregation degree of TiO2 particles as well as structure of the aggregates formed in aqueous phase. It has been found that redispergation process does not provide complete separation of virgin TiO2 particles. Even in the case of stable dispersions some aggregates were found, which consisted of 2-4 virgin TiO2 particles. With increasing colloidal stability of dispersions aggregates appear to be spherically shaped. In the system where TRITON X-100 was used, formation of secondary aggregates by fusion of primary ones was observed.