![]() ![]() ![]() NA (3.270 g, ~3.3 mmol based on the above TAN value) was then added and boiling maintained. Thus, KOH (0.752 g, 13.0 mmol) was dissolved in ethanol (20 mL) and heated gently on a hot-plate until boiling. to prepare calcium salts of long-chain carboxylates. When the solution pH > p K a, the acidity constant of stearic acid, distinct stages in the formation of metal stearate layers were noted, and interpreted as : (i) deprotonation of adsorbed stearic acid (HSt), forming MSt 2 complexes at the interface (ii) nucleation of the MSt 2 complexes, allowing further acid adsorption and complexation (iii) partitioning of the three-dimensional MSt 2 structures into the oil phase.Ĭalcium naphthenate was synthesized based on a method used by Pereira et al. ![]() studied the evolution of the interfacial tension upon placing aqueous solutions of metal ions in contact with a decane solution of stearic acid (HSt). Early work identified that interfacial reactions involving metal ions and spread carboxylic acid monolayers at the air-water interface are relatively slow, with half-lives of several minutes. Indeed, the specificity of interfacial complexation between metal ions and oil-soluble components is important elsewhere in liquid–liquid extraction processes. Other studies into the LSE have suggested that interfacial properties should be sensitive to complexation across oil/water interfaces. This is also confirmed using a range of techniques and a synthetic tetra-acid, for which the strongest affinity was found for Ca 2+. Second, from the composition of the deposits, there should be a specific affinity between the organic acid and aqueous Ca 2+ ions, given the mixed cation composition of oilfield waters. First, if the tetra-acids are uncharged, it is likely that interactions must occur at an aqueous/oil interface. Regarding the formation of these species, two points are noteworthy. Following this discovery, other similar members were identified as being responsible for naphthenate deposits in diverse locations. In this case, the naphthenates are tetrameric species, the original C80-tetra-acid member being identified as C 80H 142O 8, molecular weight 1230 g/mol. Naphthenic acids also exhibit specific interactions with calcium in certain oilfields, where Ca 2+ coordination leads to precipitation and deposition of naphthenate-rich scale. The total acidity of the crude oil depends on its source, and oil-specific measures are often required to reduce potential damage when processing the highest acidity crudes (typically total acid number values in excess of approximately 0.8 mg KOH/g). The relevance of the present results to low salinity waterflooding as an enhanced crude oil recovery technique is also discussed.ĭuring crude oil refining, the acidity of NA (p K a ~ 5 ) is responsible for corrosion in distillation units under the high temperature process conditions. For example, “locking” acidic components at water/oil interfaces may be important for crude oil emulsion stability, or in bonding bulk oil to mineral surfaces through an aqueous phase, potentially relevant for carbonate reservoirs. Owing to the ubiquitous presence of Ca 2+ in oilfield waters, this finding has potential relevance to the surface chemistry underlying crude oil recovery. The results reflect the specific hydration and coordination chemistry of Ca 2+ seen in biology. Optimum concentrations of Ca 2+ and NA have been found to yield lower, time-dependent interfacial tensions, not evident for Mg 2+ and Sr 2+ or for several alkali metal ions studied. On the basis of dynamic interfacial tension measurements, Ca 2+ has been shown specifically to interact with naphthenic acid (NA) at the n-heptane/water interface, consistent with NA adsorption followed by interfacial complexation and formation of a more ordered interfacial film.
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