Colloidal Dimensions Research Articles

One possible explanation for the ?* relation in non-crystalline materials

The existence of natural occurring paracrystals, as an intermediate state of non-crystalline matter lying between crystals and gases, has been convincingly supported in recent years [1-3]. Paracrystalline materials can be detected by X-ray, neutron or electron diffraction and sometimes by electron microscopy [4]. One basic feature of real paracrystals is that they exhibit limited colloidal dimensions [2]. Paracrystalline materials cannot grow to macroscopic sizes mainly because of the gradual increase in the fluctuation of lattice points due to the presence of three dimensional statistically distributed motifs disturbing the lattice packing. When the lattice fluctuations reach values which are of the order of the average lattice spacing, d, the paracrystal has essentially reached its limiting coherent lattice size, /)=Nd. The study of non-crystalline materials showing a paracrystaUine behaviour has given rise to an empirical relation which states: the product of the square root of the number of N lattice planes within a paracrystal, N 1/2, times the value of the relative distance fluctuation (degree of distortion), g = Aa/cT, (A1 is the distance fluctuation between two neighbouring lattice planes)gN 1/2, is nearly a constant o~*. Fig. 1 illustrates the linear increase

Particulate carbon and other components of soot and carbon black

Soot is an unwanted by-product of combustion or pyrolysis of organic compounds. At least four morphologically distinct forms of particulate carbon can be recognized in various types of soot: 1. (1) aciniform carbon (AC), composed of spheroidal carbon particles fused together in aggregates of colloidal dimensions 2. (2) carbonaceous microgel (CM), in which spheroidal carbon particles are embedded in carbon or carbonaceous material 3. (3) carbon cenospheres 4. (4) coke and char fragments. We have examined 18 soots from various sources. Domestic chimney soots have very little AC but consist mainly of char fragments, CM, ash, and soluble organic fraction (SOF); whereas diesel soots consist mainly of AC and SOF. Carbon black is a manufactured product consisting almost entirely of AC. Carbon blacks can be distinguished from many soots by their morphology, and from all soots investigated in the present study by their purity. Thus, carbon blacks have less than 0.3% SOF removable by toluene, and less than 1% ash, whereas soots exceed at least one of these limits by several fold. Because of differences in origin, function, and composition, common nomenclature carbon black with soot is inappropriate.

A theoretical approach to the magnetic flocculation of weakly magnetic minerals

Energy of interaction of a system of mineral paramagnetic and diamagnetic particles of colloidal dimensions is calculated under the influence of external magnetic field according to DLVO theory. Conditions of rapid flocculation into a potential well are derived in terms of threshold magnetic field as a function of particle size and magnetic susceptibility. The effect of surface potential, Debye-Hückel reciprocal length parameter and Hamaker constant is briefly discussed. The values of the flocculating magnetic field correspond more closely to reality than other theoretical models.

Wechselwirkungskräfte in flüssigen Dünnschichten

AbstractThe forces operating in thin liquid films of colloidal dimensions are reviewed. These forces are: Van der Waals (dispersion‐), electrical and steric interactions. A discussion is given on their evaluation in situations of practical interest, including dynamic conditions. The review covers both symmetrical and asymmetrical films. A few examples are given to indicate possible applications.

The interaction of colloidal particles collected at fluid interfaces

Simple approximate expressions are derived for the meniscus forces acting between spherical and cylindrical bodies at a fluid interface using a superposition approximation due to Nicolson (Proc. Cambridge Philos. Soc.45, 288 (1949)). These expressions are correct to lowest order in the Bond number, B = (ϱB - ϱA)gR2/γAB, and are applicable to bodies that may be dissimilar in Bond number and wetting characteristics. Our results compare very favorably with the exact numerical calculations of Gifford and Scriven (Chem. Eng. Sci.26, 287 (1971)) for parallel cylinders (for B ≲ 10−1). The small Bond number expressions derived herein are directly applicable to the interaction between particles of colloidal dimensions collected at fluid interfaces. Some sample calculations are given to illustrate the importance of capillary forces in interfacial coagulation processes. The extension of the theory to higher Bond number is discussed briefly.

Retention of radionuclides by mobile humic compounds and soil particles

Particles of colloidal dimensions are important potential vehicles for the transport of radionuclide elements in soils and ground water. Distribution ratios of radionuclides are functions of the soils studied and particle size resolution of the techniques used. The distribution ratios for three broad classes of particles are analyzed: ionic particles containing radionuclides with radii less than 1 mm/ humic matter complexes with radii of 2-3 mm/ and larger soil particles bearing radionuclides, with radii of 10-60 mm.

Photophysical and Photochemical Processes in Micellar Systems

AbstractA fascinating feature inherent to aqueous surfactant solutions is the phenomenon of self‐organization: above a certain critical concentration (the critical micelle concentration, CMC) detergent molecules associate spontaneously to build up structural entities of colloidal dimensions called micelles. The architecture of these agglomerates is such that the interior contains the hydrophobic alkyl chain of the amphiphile while the hydrophilic head groups are located at the surface and are in contact with bulk water. In the case of ionic micelles the interface is charged giving rise to an electrical double layer and a potential difference of up to several hundred millivolts between the micellar pseudophase and water. Thus micellar systems are microheterogeneous in character: the electrostatic potential and polarity prevailing in the interior of the aggregate differ from those of the bulk aqueous phase. A particularly attractive aspect of photochemical studies in micellar systems is the possibility of organizing the reactants at a molecular level: by comparison of the data in micelles with similar data in homogeneous solution one can learn about the molecular details of a given reaction and establish which conditions favor one pathway or another. In simple surfactant systems differences in rate and efficiency of a reaction will often be controlled by local electrostatic potentials and the compartmentalization of the reagents within the surfactant aggregates. Through the latter effect the statistics of probe distribution over the micelles becomes important in controlling fast photochemical events. Functional micelles are distinguished by the fact that the surfactant molecule contains a group which itself participates in the photoprocess. These units are unique in that self‐assembly often introduces striking cooperative effects.

X-ray study of the structure of oriented polyethylene modified by graft polyvinylidene chloride

Polyethylene films were modified by liquid-phase graft copolymerization of vinylidene chloride. The structure of graft films elongated before and after grafting was examined by X-ray (large and small angle) directly during isometric annealing and after cooling in the isometric state. It was established that polyvinylidene chloride forms graft structures of colloidal dimensions which stabilize the combined oriented polyethylene macromolecules.

A mixed-valency lead oxyiodide

A new, mixed-valency lead oxyiodide of variable composition, Pb(II) 4− x Pb(IV) x O 4O 2 x , has been synthesized by different routes and identified by X-ray diffraction. The reaction of massicot (yellow, orthohombic PbO) with I 2 and water at room temperature produced a brown powder which consisted of crystals of almost colloidal dimensions with the above stoichiometry where 1< x<2. The crystals appear to have body-centered tetragonal symmetry with a 0 = 4.10 and c 0 = 13.6 A ̊ . The reaction of PbI 2 and Pb 3O 4 at 500°C produced better crystallized specimens most likely with similar stoichiometry, Pb 4O 4I 2 but always mixed with a considerable amount of at least one other phase, e.g. Pb(II) 5O 4I 2( = “4PbO·PbI 2”). The reaction of PbO 2 with PbI 2 in the temperature range of 430–550°C produced crystallized specimens with higher iodine content, e.g. Pb(II) 2.5Pb(IV) 1.5O 4I 3 (or 3 2 < × <2 in the above general formula), while an oxyiodide with the ideal composition, Pb(II) 2Pb(IV) 2O 4I 4 = 4PbOI (corresponding to BiOI) was never obtained. Some general reactions of the above compounds are described.

The significance of secondary iron and manganese oxides in geochemical exploration

The significance of secondary iron and manganese oxides in geochemical exploration is suggested by their common occurrence as coatings or concretions and as discrete particles of colloidal dimensions in soils and stream sediments and by their strong scavenging of important ore metals from the weathering zone. Chemical fractionation techniques are available to partition secondary oxides and their associated metals into different mineral phases and chemical forms by partial selective extraction. A knowledge of the distribution of metals in various chemical combinations is valuable to geochemical exploration in providing a proper selection of sample medium and extraction procedure and in helping evaluate the significance of metal anomalies. Studies of secondary Fe and Mn oxides and their relationships to the geochemistry of heavy metals should be further strengthened.

Plastoelasticity of high polymers based on the analysis of polyethylene stretched till natural draw ratio

A theory of the plastoelasticity of high polymers is developed, which takes into account the essential structural details explored with X-ray diffraction and electron micrographs. The structure consists of small microparacrystallites (mPC's), which are the nodules of a threedimensional network. This is deformed affine under stress. The theory takes into account the stretching phenomenon of affine transformation and retransformation down to colloidal dimensions and can solve therefore directly the differential equations of non-linear deformation behaviour. Within the microdomains of less than 0.1 mm diameter affine phenomena do not occur, because the microparacrystallites are disintegrated and plastically deformed. The theory is applied to the profile analysis of the shoulders in the neck of partically at room temperature drawn polyethylene. The retardation time of the microdomains reaches near the half width of the shoulder a minimum value of 2.5 min. Studying the stress-strain curve the rod firstly is deformed “homogeneously” till to an elongation of 8∼25% beyond the yield point. Till here those microdomains, which are plastically deformed, mostly are distributed randomly over the sample. In this way the deviation from the Hooke-law (linearity and immediate reversibility) can be explained, which becomes larger with increasing strain and increasing number of deformedmPC's. Above γ=25% cooperative deformations of adjacent microparacrystallites arise in planes orthogonal to the stretching direction as consequence of stress fields generated by the network knotted with plastically deformable knots. Now the macroscopically visible necking begins. At γ ∼ 2 both shoulders are separated and have gained their final profile, if the stretching velocity is constant and not too large and the temperature not too low. The Kelvin-elements, which represent the microdomains, are frozen in our example before they have reached the equilibrium elongation because outside of the shoulder the rod has become so cold that the plasticity of themPC's has decreased. This determines the so-called natural draw ratio. The deformation to natural draw ratio is absolutely reversible, if the time of passing of the microdomains the shoulder is small enough compared with the relaxation time of the Maxwell-elements. The effective Young-modulus in the phenomenological Hooke-part of the stress-strain curve has a maximal value of 5000 kp cm−2, whilst the entropy driven spring constant of the reversible deformable microdomains is much smaller with a value of 15 kp cm−2. The word “viscoelasticity” is not sufficiently descriptive.

Determination of structure parameters and size distribution of colloidal particles by means of small-angle X-ray scattering

A dilute silica suspension was used for examining the possibilities of using the small-angle X-ray scattering in the investigation of objects of colloid dimensions. The average radius of gyration, weight, volume and specific surface of the SiO2 particles were determined. The particle size distribution curve was determined by integral transformation of the scattering curve and by adjusting the parameters of empirical functions.

Microheterogeneity of anion exchange membranes

Anion exchange membranes (ferricyanide ion type) prepared by the “paste method” were treated with an aqueous solution of hydrogen peroxide, and the resultant microporous membranes had no ion exchange capacity but water permeability. By measuring the water permeability of the membranes and observing the electron microphotographs of the membrane surfaces, it was elucidated that the anion exchange membranes contained microheterogeneity of colloidal dimension.

Chlorophyll interactions and light conversion in photosynthesis.

Chlorophyll a is a molecule with an unusual combination of electron donor-acceptor properties. The Ring V keto C=O group can function as donor, the central Mg atom as acceptor. In the absence of extraneous nucleophiles, donor-acceptor interactions form chlorophyll dimers, (Chl.,), and oligomers, (Chl2),. With monofunctional electron donors, monomeric chlorophyll species Chl �9 L 1 and Chl �9 L 2 form. Bifunctional donors may cross-link chlorophylls through Mg atoms to form large polynuclear adduets of colloidal dimensions. Examination of the visible absorption spectra by computer deconvolution techniques indicates considerable similarity between bulk or antenna chlorophyll in the plant and (Chl2)n. Electron spin resonance studies suggest that ESR photo-signal I associated with the photosynthetic reaction center of photosynthetic organisms could arise in a special pair of chlorophyll molecules (Chl H20 Chl)- +. Introduclion There is universal agreement that chlorophyll is the primary photo-receptor in plants, but the details of the mechanisms whereby the energy of light quanta are made available for subsequent chemical purposes remain for the most part to be established. The term "chlorophyll" is generic for a small group of closely related substances invariably present in all organisms that carry out oxidation-reduction reactions with energy supplied by light. The principal chlorophylls are chlorophyll a (Chl a) and b (green plants, algae), bacteriochlorophyll (BChl, purple photosynthetic bacteria), and chlorobium chlorophyll (green photosynthetic bacteria). Chlorophyll q and c~ It, 2] (diatoms, marine brown algae) and chlorophyll d (marine red algae) are minor or auxiliary chlorophylls. Chlorophyll a is without exception present in all organisms that carry out photosynthesis with the evolution of molecular oxygen. As the most widely distributed and most important of the chlorophylls, the discussion here will for the most part concern chlorophyll a. The chlorophylls are cyclic tetrapyrroles (Fig. t) and thus belong to the family of porphyrin compounds, which have a very important role in biology as active participants in respiratory pigments, electron transport agents, and oxidative enzymes. The chlorophylls, however, differ in some important respects from the true porphyrins. Although the methyl, ethyl, vinyl and propionic acid side-chains in the chlorophylls are characteristic of porphyrins, the alicyclic Ring V, which contains a keto C=O function at position C-9, is unique to the chlorophylls. The

Pyridine sorption from aqueous solution by montmorillonite and kaolinite

Pyridine sorption from aqueous solution onto highly-purified sodium kaolinite and sodium montmorillonite of colloidal dimensions was studied by sterile, radiochemical procedures. The process is attributed to cationic exchange with the extent of pyridium ion sorption depending upon the pH of the solution. Sorption occurs from acidic solutions with maxima occurring for sodium kaolinite at approximately pH 5.5 and for sodium montmorillonite at approximately pH 4.0. Pyridine molecules do not sorb at pH > 7. Sorption is described by the empirical Freundlich relationship. Isotherms were obtained at 1° and 24°C with sorption greater at 1°C. Equilibria were achieved within 24 h with significant sorption occurring in less than 17 min. Equilibrium intervals did not vary over clay: pyridine ratios of 12.15 to 6.25 × 10 4

Sorption and desorption of pyridine-clay in aqueous solution

Sterile, radiochemical procedures were applied in a study of the cationic exchange processes occurring during pyridine sorption onto highly-purified sodium kaolinite and sodium montmorillonite particulates of colloidal dimensions. Sodium desorption is directly related to increased protonation. Hydrogen ion exchange dominates at pH < 3. Maximum pyridium sorption occurs at pH 4.0–5.5. At pH > 7.0 neither pyridium nor hydrogen exchange occurs. Previously sorbed pyridium desorbs from the clays as a function of time and solution pH with maximum desorption at pH 1 and 11 and minimum near pH = p K a = 5.25. Pyridine desorption is much slower than sorption at comparable pH and clay:organic ratio. The extent of desorption is also directly related to the number of stages and/or the volume of solution. Particulate charge, zeta potential, changes are greatest between pH 1–4 with coagulation and charge reversal at pH ≅ 2 if the pyridine concentration is < cation exchange capacity, CEC. Pyridine concentration in excess of CEC induces coagulation through the “cage effect”.

Viscoelastic and Ultimate Tensile Properties of Styrene-Butadiene-Styrene Block Copolymers

Abstract A study was made of the stress—strain and ultimate properties in simple tension of an elastomeric styrene—butadiene—styrene block copolymer (Kraton 101) and also of a similar material (Thermolastic 226) that contains about 35% plasticizer as well as inorganic pigments. Stress—strain data were obtained at crosshead speeds from 0.02 to 20 in./min at temperatures from − 40 to 60° C. The relaxation rate, derived from the data at constant extension rates, was about 8% per decade of time for both materials at temperatures from − 40 to about 40° C and at extensions from about 20% up to 400%. Above − 30° C, the shift factor log aT was found to vary linearly with temperature. These findings indicate that the time and temperature dependence of the mechanical properties results primarily from the plastic (or viscoelastic) characteristics of the styrene domains. The tensile strength for Kraton 101 below 40° C is somewhat greater than 4000 psi, sensibly independent of extension rate and temperature. For the highly plasticized Thermolastic 226, the tensile strength at an extension rate of 1.0 min−1 increases from 2200 psi at 0° C to 3600 psi at − 40° C. Above 40° C for Kraton 101 and above 0° C for Thermolastic 226, the tensile strengths are dependent on extension rate and temperature owing to the increased ductility of the styrene domains. The high strength of these materials results from the uniformly dispersed styrene domains of colloidal dimensions. To obtain a crack of sufficient size to satisfy an energetic criterion for self-sustained high-speed propagation, domains must be disrupted. The plastic characteristics of the domains have a controlling effect on crack growth and thus on the ultimate properties of the materials. The strength and extensibility of other elastomers are considered in relation to those of the block copolymers.

Viscoelastic and ultimate tensile properties of styrene‐butadiene‐styrene block copolymers

AbstractA study was made of the stress–strain and ultimate properties in simple tension of an elastomeric styrene‐butadiene‐styrene block copolymer (Kraton 101) and also of a similar material (Thermolastic 226) that contains about 35% plasticizer as well as inorganic pigments. Stress–strain data were obtained at crosshead speeds from 0.02 to 20 in./min at temperatures from ‐40 to 60°C. The relaxation rate, derived from the data at constant extension rates, was about 8% per decade of time for both materials at temperatures from −40 to about 40°C and at extensions from about 20% up to 400%. Above −30°C, the shift factor log aT was found to vary linearly with temperature. These findings indicate that the time and temperature dependence of the mechanical properties results primarily from the plastic (or viscoelastic) characteristics of the styrene domains. The tensile strength for Kraton 101 below 40°C is somewhat greater than 4000 psi, sensibly independent of extension rate and temperature. For the highly plasticized Thermolastic 226, the tensile strength at an extension rate of 1.0 min−1 increases from 2200 psi at 0°C to 3600 psi at −40°C. Above 40°C for Kraton 101 and above 0°C for Thermolastic 226, the tensile strengths are quite dependent on extension rate and temperature owing to the increased ductility of the styrene domains. The high strength of these materials results from the uniformly dispersed styrene domains of colloidal dimensions. To obtain a crack of sufficient size to satisfy an energetic criterion for self‐sustained high‐speed propagation, domains must be disrupted. The plastic characteristics of the domains have a controlling effect on crack growth and thus on the ultimate properties of the materials. The strength and extensibility of other elastomers are considered in relation to those of the block copolymers.

Rheological characteristics of polymeric microcrystal-gels

Novel polymeric microcrystal-gels containing discrete, colloidal particles constitute a unique class of materials. They differ from gels where the network of chain molecules are held together either by primary bonds or by thermally reversible linkages due to secondary attractive forces. The microcrystals can be obtained from fiber-forming polymers by a controlled chemical degradation of the amorphous regions, followed by mechanical shear in order to free the microcrystals from their matrix. These particles of colloidal dimensions gel in various media above a certain critical concentration. Polymers from which such microcrystal-gels have been prepared include cellulose (natural and regenerated), amylose, magnesium silicate, nylon, and collagen. The rheological properties of the microcrystal-gels are highly dependent on the particle size distribution, particularly on the fraction under 1 micron in maximum dimension. They are characterized by unusual, extremely high yield values resulting from a three-dimensional network structure. This structure breaks down instantaneously as the elastic limits of the network are surpassed, and will rebuild at velocities different for each microcrystal-gel. The flow curves indicate rarely encountered pseudoplastic-rheopeatic behavior. The apparent viscosities decrease sharply with increasing rates of shear.

Relation of Heat-induced Changes in Protein-salt Constituents to Astringency in Milk Systems

Selected milk systems were heated at varying levels and subsequently examined for astringent flavor response, as well as by high speed centrifugation, Sephadex gel filtration, polyacrylamide gel electrophoresis, electron microscopy, and chemical analyses, to determine changes in the size, shape, and composition of protein-salt aggregate particles present.Astringency was produced by heating various skimmilk, whey, and ultrafiltrate systems. Astringency development in whey and ultrafiltrate systems was accompanied by aggregation and formation of large protein-salt and salt particles, respectively. Heating skimmilk to produce astringency resulted in the association of calcium phosphate and whey proteins with the caseinate system without noticeably altering the size, shape, or electron density of the caseinate-phosphate micelles. Although particle size is not the sole factor related to astringency, it appears that milk components associated with astringency development must attain critical colloidal dimensions, to impart the astringent response. The effective size and chemical nature of heat-altered whey protein-salt constituents and their association with the caseinate-phosphate micelles are indicated as the primary origin of astringent flavor in milk.