Marjorie J. Vold

Computer simulation of floc formation in a colloidal suspension

Aggregates which form by successive random addition of individual spherical particles without internal rearrangement as the floc grows are shown to have a roughly isometric “core” in which the density diminishes slowly from 54 volume per cent at the center to 54 m−0.29 volume per cent at the edge of the core, where m is the number of primary particles encompassed. The particles not included in the core form projecting “tentacles” which give the total aggregate a rough surface and a mean extent about five particle diameters larger than the core diameter. The fraction of total particles within the core increases with increasing floe size and is nearly unity for visible flocs. The model is capable of accounting for the hitherto anomalously large sediment volume of silica sols in organic solvents.

A numerical approach to the problem of sediment volume

Abstract The observed sediment volume of 3.5 c.c./gram for moist spheres in hydrocarbon solvents is shown to correspond exactly to a model in which each sphere coheres rigidly to the first sphere it contacts, but is inconsistent with strong forces operative over an appreciable range of distance. The mean number of neighbors for each sphere in this type of packing is 2.0. Sediment structure is more sensitive to the breadth of the size distribution than is sediment volume.

The application of the mass law to the aggregation of colloidal electrolytes

Abstract It has been shown that experimental data for colloidal electrolytes which are in accord with the predictions of the mass law for a single micellar species are capable also of an entirely similar description in terms of the mass law for a distribution of micellar species. The results also suggest that in this distribution large micelles are favored, and that as the concentration of colloidal electrolyte increases, the proportion of bound counter ion tends rapidly toward a constant value which is not very different for different colloidal electrolytes.

Polymorphic transformations of calcium stearate and calcium stearate monohydrate

Abstract Anhydrous calcium stearate undergoes transformations at 65°, 86°, 123°, 150°, and 195°C. It melts with decomposition at 350°C. The substance can be obtained at room temperature either as a stable crystal, an unstable crystal (by quenching from between 123° and 150°), or a vitreous form (by quenching from above 150°). Differential heating and cooling curves and X-ray diffraction patterns of quenched samples at room temperature are presented. The set of transformations is partially analogous to that for sodium stearate, the resemblance being greater at low temperatures and less marked at high temperatures where, with the hydrocarbon chains irregularly packed and relatively mobile, the difference in binding power between sodium and calcium ions becomes important. Calcium stearate forms a monohydrate whose decomposition temperature at atmospheric pressure is about 110°C. Calcium stearates of technical quality resemble the pure material but tend to assume the vitreous form more readily and to crystallize completely only with difficulty.

Crystal forms of anhydrous calcium stearate derivable from calcium stearate monohydrate

Abstract Anhydrous calcium stearate can be obtained at room temperature in any one of three forms, or mixtures thereof, by dehydration of calcium stearate monohydrate. Calcium stearate VIA is believed to be the stable modification. Calcium stearate VIH is an anhydrous crystalline powder having the same X-ray diffraction pattern as calcium stearate monohydrate. Calcium stearate VIN is relatively amorphous with respect to interchain spacings although it exhibits a crystalline long spacing. Samples of calcium stearate hydrate which yield calcium stearate VIH upon dehydration lack a second order thermal transition at 48°C. characteristic of calcium stearate hydrate samples which yield calcium stearate VIA upon dehydration. Evidence is presented supporting the interpretation that transformation of the hydrate type structure to the anhydrous type is inhibited by impurities in the calcium stearate, particularly adsorbed calcium salts and foreign fatty acid anions.

The flocculation of aqueous sols of a graphitized carbon black by a potential-determining electrolyte

Summary The rates of flocculation of carbon black sols stabilized by sodium β-naphtalene sulfonate were measured by means of the accompanying decrease in light absorbance. Values were observed of the zeta-potential over the same concentration range. The surfactant is presumed to stabilize the sol by formation of a diffuse electric double layer around the particles. However, an excess of surfactant beyond that leading to the maximum stability results in increased rates of flocculation. The behavior can be described qualitatively by DLVO theory but the theory in its present form does not give satisfactory quantitative agreement with experiment. The disagreement may be ascribed to failure of the sols to match the required premises of monodispersity and spherical shape of particles or constant surface potential during the act of aggregation, etc. Alternatively, the use of rates of decrease of light absorbance rather than the more accurate but tedious particle counting methods to measure rates of flocculation may make the disagreement illusory. Thirdly, it is possible that the, procedures used for calculating the expected behavior are inadequate despite qualitative satisfaction.

A comparative study of the X-ray diffraction patterns and thermal transitions of metal soaps

SummaryX-ray diffraction patterns at room temperature for soaps of barium, calcium, and lithium were found to be characteristic for different soaps of the same cation. This is true not only for palmitates and stearates but also (with minor variations) for mixed soaps containing up to at least 50% oleate, for pure calcium oleate, and even for lithium 12-hydroxystearate.Soaps of the same cation in mixtures approximating the composition of natural tallow fatty acids appear to form mixed crystals. The succession of phases formed on heating shows strongly the influence of the oleate in the mixture.Calcium resinates appear to crystallize imperfectly but definitely with a “head to head” arrangment of the abietate radicals in contrast to the acids which give evidence of a head to tail arrangement.