Estimation of polydispersity in polymers and colloids by field-flow fractionaion

Various methodologies of sedimentation, thermal, and steric field-flow fractionation for the estimation of the polydispersity in polymers and colloids are presented. These are based either on retention and/or on zone-spreading data. The reference materials used are nearly monodisperse and polydisperse submicron polystyrene and PVC latex beads, nearly monodisperse spherical particles of hematite, and polydisperse irregular particles of strengite (FePO₄·2H₂O) and PS polymers of various molecular weights. The results found are compared with those determined by other techniques or given by the manufacturers.     

Gas adsorption by a crystalline silicic acid

Crystalline silicic acids are prepared from alkali layer silicates by exchanging protons for the alkali ions. The acid H₂Si₂₀O₄₁ · xH₂O (parent material K₂Si₂₀O₄₁ · xH₂O) exhibits some outstanding gas adsorption properties which are related to the layer structure and the interlamellar microporosity. The external surface, about 20 m² g^–1, is estimated from nitrogen adsorption data after blocking the micropores. Slit-shaped ultramicropores (with diameters similiar to that of the nitrogen molecule) between the layers are widened to supermicropores near the crystal edges. During an adsorption run the nitrogen molecules penetrate more deeply into the ultramicropores. Nitrogen molecules strongly adsorbed in the ultramicropores are not desorbed at 77 K. Additional amounts of nitrogen are adsorbed by widening of the slit-shaped micropores at the crystal edges when pressure increases. This process proceeds slowly and is reversible.     

Effect of intercalation of metal ions on the colloidal and solid properties of vanadium pentaoxide hydrate,V2O5 nH2O

The colloidal stability of V₂O₅ nH₂O was studied on the basis of the measurements of critical flocculation concentration (CFC) by metal ions, amount of ions exchanged (or intercalated), and -potential. In total, the CFC values obeyed the Schulze Hardy law and strong Hofmeister's series was found in the systems including alkaline ions. The sequence of colloidal stability of V₂O₅ nH₂O in the electrolyte solutions was related to the intercalation of metal ions in the interlayer spaces of the solid. The largest CFC value for Li⁺ (87 mmol dm^–3) was explained by smaller affinity of Li⁺ to be intercalated in V₂O₅ nH₂O as well as smaller Hamaker constant of the intercalated solid compared to the other systems.Effect of intercalation of metal ions on the crystalline properties of the materials was measured by use of XRD and electron microscope. Under highly dehydrated condition the ions whose radii are smaller than 0.1 nm are captured in the structure of V₂O₅ nH₂O without changing interlayer distances, while those larger than 0.1 nm increase the interlayer distance. In a saturated H₂O vapor interlayer distances increased with increasing charge of intercalated ions. However, when intercalated with ions carrying the same valency the interlayer distances of the sample decreased with decrease in the hydration property of ions. Hydrolyzable Cr³⁺ gave exceptionally larger interlayer distances, both in a vacuum and in H₂O vapor.     

The influence of surfactants on latex flocculation with poly(diallyldimethyl ammonium chloride)

Aqueous latex was flocculated by mixtures of poly(diallyldimethyl ammonium chloride), PDADMAC, and anionic surfactants. Sodium dodecyl sulfate, (SDS), and Aerosol OT influenced flocculation whereas nonionic Tergitol NP-10 did not. The flocculation domains were correlated with properties of the polymer-surfactant complexFlocculation was never observed above the CMC of the corresponding surfactant solution without polymer or latex. At SDS concentrations greater than 10^–3.6 M the flocculation boundary corresponded to the first appearance of insoluble polymer-surfactant complex which was characterized by dynamic light scattering and microelectrophoresis. Under these conditions latex (diameter 570 nm) and dispersed polymer-surfactant complex particles (diameters between 30 and 2 000 nm) displayed simultaneous homo and heteroflocculation. The boundaries of the flocculation domains at low surfactant concentration were determined by the ratio of polymer to latex and by the net electrostatic charge of the soluble polymer-surfactant complex. On the other hand, the mechanisms controlling flocculation boundaries in the dispersed polymer-surfactant domain require further clarification.     

Interaction between an ion-penetrable particle and a solid particle. II. Criteria for heterocoagulation

The potential energy of the total interaction between two spherical colloidal particles of different nature is calculated, i. e., of an ion-penetrable particle and an ion-impenetrable solid particle having a constant surface potential or constant surface charge density. The criteria for heterocoagulation are derived. The obtained results suggest a possibility of selective coagulation in the mixed system.     

Precipitation of surfactant salts III. Metal perfluoroalkanecarboxylates

Precipitation of perfluorocarboxylic acids (n-C₃H₇OOH, n-C₇H₁₅COOH, and n-C₉F₁₉COOH) in aqueous solutions of different metal nitrates (K⁺, Ag⁺, Ca²⁺, Ba²⁺, Zn²⁺, Al³⁺, La³⁺) was studied by solubility and calorimetric measurements. The free energy of precipitation per single surfactant chain was found to depend on the chain length but not on the charge of the reacting cation. It was shown that the precipitation of perfluorocarboxylates of multivalent cations was governed by positive entropy change and a low enthalpic contribution, whereas the soaps of monovalent cations exhibited the opposite behavior. The entropy of precipitation was not affected by the chain length of the surfactants in the presence of monovalent cations, but it increased for cations of higher charge.Supported by the NSF Grant CHE-8619509.This paper is dedicated to Professor Armin Weiss on the occasion of his 60th birthday. One of the authors (E. M.) particularly appreciates many years of cooperation with Professor Weiss in editing Colloid and Polymer Science, as well as in many other scientific interactions.     

Preparation of muscovite with ultrahigh specific surface area by chemical cleavage

The specific surface area of a muscovite sample increases drastically after exposure to a LiNO₃ solution, e.g., from 3.4 m²/g, corresponding to platelets of ca. 200 silicate layers, to 295 m²/g (platelets of ca. 2–3 silicate layers) after treatment at 180°C under atmospheric pressure for 46 h. The efficiency of the cleavage process decreases with decreasing temperature (down to 50°C). The LiNO₃/H₂O weight ratio is also very important: at 130°C and a reaction time of 46 h, for instance, a value in the range of 1.7–1.8 leads to the highest specific surfaces. The cleaved products have the form of strong papers that disperse readily in water. During the cleaving procedure, not only the particle thickness, but also the diameter decreases. There is no evidence of damage or partial dissolution of the silicate structure after cleavage, by IR spectroscopy and yield. The use of LiCl also leads to an increase in specific surface area, but the effect is weaker than in the case of LiNO₃. Treatment with some other alkaline and alkaline earth nitrates and chlorides did not increase the specific surface area of muscovite significantly.     

Aggregation phenomena in the suspension polymerization of VCM

In the suspension polymerization of VCM, insoluble polymer particles are formed inside the monomer droplets. The growth and aggregation of these particles are responsible for important polymer properties, such as porosity. It is well established that the most characteristic polymer particles, the primary particles, are of a narrow distribution with a size (diameter) ranging from 0.10–0.20 m. This work studied the formation of primary particles based on the aggregation phenomena that take place inside a monomer droplet. This was done by formulating a population balance equation, which was based on the following considerations: a) polymerization occurs in both the monomer and the polymer phases; b) there is continuous formation of the basic particles in the monomer phase; c) the growth of the polymer particles occurs as a result of both polymerization in the polymer phase and aggregation of the particles; d) the colloidal properties of the particles that are responsible for the aggregation phenomena were considered to be the net result of attraction and repulsion energies.It was shown that for particles carrying a constant charge it was not possible to predict the formation of primary particles of size 0.10–0.20 m. The particle size distribution had a mode diameter equal to the diameter of the basic particles. Consequently, the particle charge was allowed to vary in a way proportional to the particle radius raised to a power coefficient. For values of the coefficient greater than zero, i. e., when the particle charge increased during polymerization, the aggregation of the basic particles was efficient enough to result in the formation of large primary particles.     

Steric repulsion of polyoxyethylene groups for emulsion stability

Rapid coalescence was studied on liquid paraffin emulsion stabilized with a series of poly(oxyethylene) dodecyl ethers [C₁₂H₂₅ (EO),n=1, 2, 3, 4, 5, 6, 7, 8] and of poly(oxyethylene) nonylphenyl ethers [C₉H₁₉(EO) _n ,n=2, 4, 5, 6, 12]. The turbidity of emulsion was measured as a function of the solution pHs at constant ionic strength of 0.1 mol dm^–3.As a result, it was found that the emulsions (which were formed with C₁₂H₂₅(EO) _n surfactants having less than four oxyethylene groups, or with C₉H₁₉ (EO) _n surfactants having less than six oxyethylene groups) brought about rapid coalescence in the bulk pH between 2.03.5, which corresponded to the zero point of charges for the emulsions of the present systems. According to the Tadros treatment for emulsion flocculation, the total flocculation potennual was estimated as a function of the distance relative to the number of oxyethelene groups in the surfactants. The critical coalescence energy was obtained as –343 ×10^–19 J for the C₁₂H₂₅(EO) _n surfactants and –2.14×10^–19) J for the C₉H₁₉ (EO) _n surfactants. Furthermore, the formation of a hole for coalescence was estimated under the simple assumption that the coalescence was caused only by the energy dissipation.     

Amorphous solid-like distribution of polydisperse particles of colloidal clay and microcrystalline cellulose in deionized suspension

A metallurgical microscope is used to directly observe the amorphous solid-like structures of deionized suspensions of highly polydispersed colloids in sedimentation equilibrium. The colloids used are colloidal clay of bentonite and microcrystalline cellulose (MCC). The two-dimensional distance distribution functions of the amorphous solid-like structures at relatively diluted and concentrated suspensions resemble those of the liquid-like and gas-like structures of monodispersed spheres, respectively. The center-to-center interparticle distances (D) in the amorphous solid-like structures are explained by the effective hard-sphere model; a colloidal particle is coated with electrical double layers. The maximum length of the width of the double layers (Debye length,D ₁) observed is ca. 1n at very dilute suspensions.D andD ₁ continue to decrease as the initial concentration of the particles increases, and from these data rigidities are estimated to be 0.12 and 0.09 Pa for bentonite and MCC suspensions, respectively. The log [viscosity] of bentonite suspensions begins to increase linearly as log [shear rate] decreases with a slope close to –1, which supports the solid-like nature of the suspensions. These experimental results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the particles are both essential for the appearance of the amorphous solid-like structures.     

Stability of chromium dioxide particles in aqueous media

An investigation was carried out to characterize the dissolution behavior of stabilized and unstabilized CrO₂ particles in water, used in magnetic recording. Special attention was paid to the measurement of Cr(VI) concentration in the contact solution and to the elucidation of the effects of the stabilizing treatment by x-ray photoelectron spectroscopy (XPS). The experimental results indicate that both stabilized and unstabilized CrO₂ particles predominantly release hexavalent chromium when in contact with water. The solubility of freshly stabilized CrO₂ is considerably smaller than that of unstabilized CrO₂, but the difference in solubility diminishes with aged particles. The XPS study indicated that stabilization of CrO₂ particles with a bisulfite solution results in the formation of Cr₂O₃ or a Cr₂O₃-like product on the particle. However, it appears that Cr(III) oxide coating does not uniformly form on all particles.     

Small-angle neutron scattering of colloidal silica dispersions in a non-polar solvent

Small-angle neutron scattering studies were performed on dilute dispersions of colloidal silica spheres in mixtures of h₁₂- and d₁₂-cyclohexane. The particles consisted of a SiO₂-core and a layer of stearyl alcohol molecules terminally attached with a chemical bond (Si-O) to the particle surface. The contrast variation method was applied to reveal the internal structure of the particles. The matchpoints determined with this method were in accordance with those calculated from the mass density of the particles and the atomic composition, as determined with elemental analysis.For a detailed interpretation of the scattering curves, we assumed that the particles were spherosymmetrical and consisted of two concentric layers. With the relation we derived between the radius of gyration and the reciprocal contrast for such a model, it was possible to determine all the parameters characterizing the particle in terms of this model. The model calculations performed using these parameters fitted very well to the experimental intensities for high contrasts. For lower contrasts, the fit was somewhat less good. This is probably due to random fluctuations in the scattering length density within the particle core.The different radii as found by neutron scattering, agreed very well with those determined using other techniques, such as light scattering (static and dynamic) and electron microscopy.     

碳包覆LiFe0.5Co0.5PO4固溶体正极材料的制备及其电化学性能

分别以四水磷酸铁（FePO₄·4H₂O）和二水草酸亚铁（FeC₂O₄·2H₂O）为铁源,采用简单便捷的流变相法制备了碳包覆LiFe_0.5Co_0.5PO₄固溶体材料（LiFe_0.5Co_0.5PO₄/C,简称为LFCP/C）。采用X射线衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、恒流充放电等测试手段对复合材料的物相、形貌结构和电化学性能进行了表征和测试。结果表明,2种铁源得到的材料均为橄榄石晶型结构且结晶度良好,二者在颗粒尺寸分布、碳包覆效果和电化学性能方面具有显著的差别。用作锂离子电池正极材料时,以FeC₂O₄·2H₂O为原料得到的LFCP/C具有更优异的电性能：在2.5~5.0 V电压范围内,0.1C倍率下（1C=150 mA·g^-1）,放电比容量为137.5 mAh·g^-1,在10C仍具有57.6 mAh·g^-1的放电比容量;0.5C循环100次后容量仍保持78.1%。该样品更佳的电化学性能主要得益于其更小的平均颗粒尺寸,更高的比表面积和理想的碳包覆效果。     

Polymer adsorption on fine particles; the effects of particle size and its stability

The effects of particle size on polyacrylamide (PAAm,M _w =59×10⁴, 500×10⁴) adsorption were investigated using a series of well-characterized hematite (-Fe₂O₃) dispersions. The -Fe₂O₃ particles with highly monodisperse and nearly spherical shape ranged in radius from 23 nm to 300 nm. the maximum amount of PAAm adsorption (M _m ) in each system, showed a steady increase with decreasing particle radius and was influenced strongly by particle concentrations in the medium. Furthermore, it was realized that the diameter of -Fe₂O₃ particles after treatment with PAAm under different particle concentrations decreased with increasing particle concentration. The relation between particle concentration in the medium and particle size after treatment was also influenced by the medium pH, i.e., at the medium pH close to the isoelectric point of -Fe₂O₃ particles (pH^o=9.2), the particle size after treatment increased with increasing particle concentration. All these results suggest that in the system of ultra-fine particles, the mixing process between particle-particle and polymerparticle will play an important role on the conformation of adsorbed polymer layer.     

A small-angle neutron scattering study on polydisperse silica spheres coated with polyisobutene

Neutron-scattering studies at small angles are performed on dilute dispersions of small, polydisperse silica spheres coated with polyisobutene in mixtures of h₁₂ and d₁₂-cyclohexane. The contrast variation method is applied to reveal the internal structure of the compound particles. For a detailed interpretation of the scattering curves, it is assumed that the particles consist of spherical silica cores with concentric PIB-layers into which solvent molecules can penetrate. Also the polydispersity of the particle cores is taken into account. Model calculations fit the experimental curves fairly well except for the curves near the matching point, and at higher wavevectors, where experimental errors are relatively large due to the smallness of the scattering of the particles compared to that of the background.     

Amphoteric character of polyelectrolyte complex particles as revealed by isotachophoresis and viscometry

Isotachophoresis and viscometric measurements were performed on aqueous dispersions of non-stoichiometric polyelectrolyte complexes in order to elucidate the surface charge situation of the complex particles in dependence on component charge density, ratio of cationic to anionic groups in the complex, and pH and ionic strengths of the ambient medium. Components for complex formation were acryl-based anionic and cationic polyelectrolytes of the pendent type. From our results, an amphoteric character of the polyelectrolyte complex particles can be concluded, with an isoelectric point characterized by zero mobility and a minimum in reduced viscosity _spec/c of the particle dispersion, and with the sign of net surface charge depending on ambient pH and component charge density. The influence of ionic strength on the _spec/c vs pH plots can be interpreted by assuming a competition between salting-out and electrostatic shielding effects. No correlation could be established between the overall molar ratio of cationic to anionic groups and the isoelectric point of the complex particles, which obviously indicates a different composition of surface and bulk of the polyelectrolyte complex particles.     

Capillary rise of liquids in rectangular tubings

The capillary rise of liquids was investigated in square capillary tubings of different dimensions (e. g. 300 m · 300 m to 1000 m · 1000 m) in the temperature range 25° to 35°C.The data were fitted to an equation:=1/2 · ·g · (S · (C ·H/2 +C ·S)) where is the surface tension of the liquid,S is the side length of the square tubing,H is the capillary rise,C (= 1.089) is a capillary constant.     

Preparation of Forsterite (Mg2SiO4) Powders Via an Aqueous Route Using Magnesium Salts and Silicon Tetrachloride (SiCl4)

Forsterite (Mg₂SiO₄) powders were prepared by mixing SiCl₄ with aqueous solutions of either Mg(CH₃COO)₂·4H₂O or Mg(NO₃)₂·6H₂O and heating the powdered gel. The powders were characterised using thermal analysis (DTA and TGA), X-ray diffraction (XRD), nitrogen adsorption surface area analysis (BET) and transmission electron microscopy (TEM). On heating, MgO and enstatite (MgSiO₃) were observed in addition to forsterite. On heating to 1200°C, forsterite was the dominant phase in the powders produced from Mg(NO₃)₂·6H₂O, and MgO was the dominant phase in the powders produced from Mg(CH₃COO)₂·4H₂O. The primary particle sizes of these powders were between 100 and 500 nm, which remained the same on heat treatment. However, higher temperatures gave rise to an increase in the size and densities of the agglomerates of primary particles.     

The effect of particle concentration on zeta potential in extremely dilute solutions

The electroporetic mobility of hexadecane particles in water and in very dilute CTAB solutions was measured. The technique of microelectrophoresis was applied. Zeta potential was calculated according to the Hückel formula, applying Henry's correction factors. Electrokinetic charge density was calculated according to the formula used by Delgado et al., and previously discovered empiricaly by Loeb et al. and derived mathematicaly by Ohshima et al. It was found that the particle concentration in emulsion limits their charge in solutions of very low ion concentration (10^–7:10^–6 M), because the greater the particle concentration, the smaller the extent of the ions adsorbed at the surface of one particle. The zeta potential was found to be independent of particle concentration if the ratio of the number of bulk ions to particle number is not lower than 1.6×10⁶. This ratio depends on particle size, and the value 1.6×10⁶ relates to particles of diameter 1.6 m.     

CPS K 221 Model of proton longitudinal magnetic relaxation in hydrated crosslinked poly(methacrylic acid)

Proton longitudinal magnetic relaxation time (T₁) measurements have been made at 30 MHz over a wide range of temperature for crosslinked poly(methacrylic acid), PMA, hydrated with H₂O as well as with D₂O. From the point of view of nuclear magnetic relaxation, PMA hydrogel is a multiregion system in which three proton regions (a, b, c) can be distinguished. Region a is regarded as to be formed by the nonexchangeable polymer protons, region b by the protons of -COOH · H₂O combinations, and region c by the protons of remaining water molecules. Cross relaxation between polymer and water protons and a log normal distribution of correlation times have been assumed to take place. Temperature dependences of the T₁ time for the particular regions have been determined, from which the distribution width parameter, the second moment and the intramolecular proton-proton distance for sorbed water have been calculated.