The evaluation of the physical characteristics of the Siallon silica cell was aimed at answering two questions: How durable is the cell? and How is the hydrocarbon held within the interior? In most cases at least two different analytical or instrumental techniques were employed for each step in order to confirm the answers by separate methods. Samples of motor oil that had been encapsulated with the Siallon process were used as the basis sample for most of the evaluations of morphology.
The first item considered was the exact crystalline structure of the silica cell. The generally accepted structures of silica are crystalline forms of quartz, cristobalite, and tridymite, of which only the quartz is thermodynamically stable . Amorphous silica can be formed by the very slow cooling of molten silica, resulting in a random array of three-dimensional units or chains of Si02- When the Siallon silica cells are examined with a microscope under plane-polarized light, the cells appear to be highly colored. Under plane-polarized light, crystalline materials transmit the light without distortion and appear as bright or white areas. It was obvious that the Siallon cells were amorphous silica. This was reaffirmed by the use of selected area electron diffraction, where all of the samples show an amorphous haze, with the exception of the trace of a salt formed as a by-product of the reaction that was confirmed to be sylvite, a common form of NaCl.
The next step was evaluation of the surface characteristics of single silica cells. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed. The SEM micrographs show the surface to be fairly regular in appearance, nonporous, and with a solid monolithic character. The TEM micrographs show that the silica cell is essentially a solid particle rather than a hollow sphere. This left the question of how the hydrocarbon was held within the silica cell. It was obvious from the SEM work that there was encapsulation of the hydrocarbon rather than adsorption, as the surface was nonporous, and also obvious that it was not a zeolite type of open-lattice structure that would lead to absorption. The next step then became the cutting of thin sections of single silica cells using a diamond knife in an ultramicrotome. The sections were first evaluated with SEM at 15,000x, 30,000x, and 60,000x magnification and shown to be a random mix of solid matter and void space. They were then analyzed by EDXA, which showed that the solid matter on the interior of the cell was essentially pure Si02. To further evaluate the configuration of the interior, the SEM micrographs of the thin sections were computer-enhanced; the results show a three-dimensional, mazelike, random honeycomb structure very much like a closed-cell urethane foam.
The results of the chemical and morphological investigations can be summarized as follows:
The Siallon reaction product is essentially pure silica—this provides a material with negligible water solubility, extreme hardness and durability, and resistance to acids and alkali. The silica is a stable amorphous form and as such is not prone to fracture or cleavage along crystal planes.
The surface is solid and nonporous and will thus prevent or retard leaching out of the encapsulated hydrocarbon.
The interior of the micrometer-sized silica cell is a mazelike configuration of almost infinite tortuosity within which the hydrocarbon is firmly embedded.
Was this article helpful?