Use of anionic surfactants for selective polishing of silicon dioxide over silicon nitride films using colloidal silica-based slurries

This paper introduces and investigates a new two-step method by further addition of acid in the basic step. By this method high Rw (molar ratio of water/TEOS) systems (Rw reaches 250) can be gelled without volume loss. Below the coagulation point, a larger amount of HCl addition in the basic step (the system remains basic) and higher ultimate base concentration lead to faster gelation, which might be attributed to the change of electrolyte concentration, ion strength and pH value of the system, while a larger amount of water causes longer gelation times due to dilution effects.

Effect of HCl in a two-step sol-gel process using TEOS

Alkali silicate gels, and in particular, lithium silicate gels, have been studied over the last ten years because they represent a new approach to preparing fast ion conductors.1Initially, the idea was that gel-prepared alkali silicates and conventional melted alkali silicates were chemically and structurally identical.2, 3 This turns out not to be the case.4The fact that gel-prepared glasses and conventional glasses are not identical in all ways has set off an explosion of experimental studies on gels.5, 6 At the same time, this fact has created an expectation that physical properties, especially transport properties, will not be the same in gel-prepared and conventional silicates of similar composition. Based on this expectation, the interest in transport properties has increased and many fast ion conductors from gels have been synthesized and characterized.7

Applications of AC Complex Impedance Spectroscopy to Fast Ion Conducting Lithium Silicate Gels

The origin of life on Earth required myriads of chemical and physical processes. These include the formation of the planet and its geological structures, the formation of the first primitive chemicals, reaction, and assembly of these primitive chemicals to form more complex or functional products and assemblies, and finally the formation of the first cells (or protocells) on early Earth, which eventually evolved into modern cells. Each of these processes presumably occurred within specific prebiotic reaction environments, which could have been diverse in physical and chemical properties. While there are resources that describe prebiotically plausible environments or nutrient availability, here, we attempt to aggregate the literature for the various physicochemical properties of different prebiotic reaction microenvironments on early Earth. We introduce a handful of properties that can be quantified through physical or chemical techniques. The values for these physicochemical properties, if they are known, are then presented for each reaction environment, giving the reader a sense of the environmental variability of such properties. Such a resource may be useful for prebiotic chemists to understand the range of conditions in each reaction environment, or to select the medium most applicable for their targeted reaction of interest for exploratory studies.

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A Physicochemical Consideration of Prebiotic Microenvironments for Self-Assembly and Prebiotic Chemistry

Na2O-SiO2 sols were prepared using silicon alkoxide and colloidal silica precursors with sodium nitrate as the salt. The experimental procedure carried out was changed depending on the precursors used. Several tests with dialysis membranes were performed in order to study the possibility of incorporating Na+-ions into the sols. These ions were introduced into silica sols contained in membranes from an exterior solution. In addition, washing tests were carried out with Na2O-SiO2 sols contained inside the membranes and with several exterior washing solutions. Resistivity of the washing solutions was measured against time. The washing solutions were changed periodically. After drying and heat-treatment, Na2O content in the gels was determined using flame photometry. The crystalline phases formed were detected by X-ray diffraction. The results show a small incorporation of alkaline ions in Na2O-SiO2 gels, even in those samples prepared through the dialysis membranes tests.

The problem of incorporating high levels of sodium nitrate into silica gels without segregation

Silica suspensions (CabOSil) can be made to gel without loss of water by adjusting the ionic strength of the suspension with inert electrolyte ions. In this paper we examine this critical gelation concentration (CGC) for the nitrate salts of lithium, sodium and potassium. The pH of the suspensions is held constant at its point of maximum stability (pH = 5.85) and the ionic strength was varied (0.01–0.263 M) to determine the CGC. All of the systems gelled through water evaporation without loosing more than 10% of their volume. While this small volume change results in less sensitivity in determining the CGC, it can be said that lithium, sodium and potassium ions show a lyotropic trend. When lithium, which is the largest hydrated ion tested, is the counter-ion, the sol loses more water before gelling than in the presence of sodium which in turn has to lose more water than when potassium is the counter-ion. A similar lyotropic trend was found in the cleaning of these sols before gelation and in the ability of the gels to retain their salts upon drying and calcination to 200°C without evidence of salt segregation.

M Tomić

Papers

The critical gelling point in silica gels containing lithium, sodium and potassium