RESUMEN
Because their affinities for the oil and water phases vary considerably with the number of ethylene oxide units in their hydrophilic group, the ethoxylated nonionic species occurring in commercial products tend to behave in a non-collective way, with the low ethoxylation oligomers partitioning mostly in the oil phase. This results in a surfactant mixture at the interface which is more hydrophilic than the one which was introduced in the system in the first place. The pseudophase model is used to study the partitioning in Winsor III type systems, and to estimate the deviation of the interfacial mixture composition from the overall one. New results indicate that the selective partitioning into the oil phase increases when the oil phase becomes aromatic, when the total surfactant concentration decreases and when the water-to-oil ratio decreases.
RESUMEN
An "oil in water" formulation was optimized to determine chromium in heavy crude oil (HCO) and bitumen-in-water emulsion (Orimulsion-400(R)) samples by transversally heated electrothermal atomic absorption spectrometry (TH-ET AAS) using Zeeman effect background correction. The optimum proportion of the oil-water mixture ratio was 7:3 v/v (70 ml of oil as the internal phase) with a non-ionic surfactant concentration (Intan-100) in the emulsion of 0.2% w/w. Chromium was determined in different crude oil samples after dilution of the emulsions 1:9 v/v with a 0.2% w/w solution of surfactant in order to further reduce the viscosity from 100 to 1.6 cP and at the same time to bring the concentration of chromium within the working range of the ET AAS technique. The calibration graph was linear from 1.7 to 100 mug Cr l(-1). The sensitivity was of 0.0069 s l mug(-1), the characteristic mass (m(o)) was of 5.7 pg per 0.0044 s and the detection limit (3sigma) was of 0.52 mug l(-1). The relative standard deviation of the method, evaluated by replicate analyses of three crude oil samples varied in all cases between 1.5 and 2.6%. Recovery studies were performed on four Venezuelan crude oils, and the average chromium recovery values varied between 95.9-104.8, 90.6-107.6, 95.6-104.0 and 98.8-103.9% for the Cerro Negro, Crudo Hamaca and Boscán crude oils and for the Orimulsión(R)-400, respectively. The results obtained in this work for the Cerro Negro, Crudo Hamaca and Boscán crude oils and for the Orimulsión(R)-400 following the proposed procedure were of 0.448+/-0.008, 0.338+/-0.004 0.524+/-0.021 and 0.174+/-0.008 mg Cr l(-1), respectively, which were in good agreement with the values obtained by a tedious recommended standard procedure (respectively: 0.470+/-0.05, 0.335+/-0.080, 0.570+/-0.021 and 0.173+/-0.009 mg Cr l(-1)).
