04/02704 Revamping of diesel hydrodesulfurizers: options available and future research needs: Bej, S. K. Fuel Processing Technology, 2004, 85, (13), 1503–1517

Abstract

We have developed a model for microemulsions which relates surfactant structure to microemulsion droplet size, interracial tension, and phase behavior. The concept of curvature parameter, developed to describe microemulsion phase behavior (M. L. Robbins and J. Bock, J. Colloid Interface Sci.124, 462 (1988)), is expanded to cover interfacial tension, droplet size, and phase behavior correlations. The curvature parameter is made up of contributions from the thickness and volumes of surfactant polar heads and hydrocarbon chains at the water/oil interface. A set of equations is developed interrelating water and oil uptakes, droplet interfacial tensions, and droplet sizes via the curvature parameter and applied to equilibrated microemulsions prepared with mixtures of ethoxylated dinonyl phenol and the monoethanol amine salt of dodecyl o-xylene sulfonic acid in 11 decane/aqueous NaCl of varying salinity. The model predicts that droplet interfacial tensions should vary inversely with water and oil uptakes and that the square root of droplet tensions should vary linearly with the curvature parameters. Curvature parameters calculated from water and oil uptakes were found to plot linearly against salinity (see previous reference), suggesting that the square root of interracial tension should also plot linearly against salinity. Measured bulk interfacial tensions when plotted as √γ vs salinity gave linear plots as opposed to the conventional semilogarithmic plots which are highly curved. Measured bulk interfacial tensions varied inversely with water and oil uptakes and correlated very well with calculated droplet interfacial tensions using a sifigle empirical constant. Based on limited data, this constant appears to be universal for the systems tested.The current paper extends testing of theoretical correlations to include microemulsion droplet size. Droplet sizes in lower- (water-continuous) and upper- (oil-continuous) phase anionic microemulsions were measured as a function of aqueous salinity using dynamic (laser) light scattering on microemulsions prepared with the surfactant combination of monoethanol ammonium dodecyl ortho-xylene sulfonate/tertiary amyl alcohol (C12∗XS-MEA/TAA) in 11 oil/aqueous NaC1. The model predicts that micro-emulsion droplet size should be inversely proportional to the square root of the interfacial tension (√γ), which is in turn inversely related to water and oil uptakes (VwVs and VoVs. The value of the proportionality constant depends on the thickness and compressibility of the surfactant layer at the water/oil interface. Using the length of the C12∗XS-MEA molecule measured from molecular model projections, droplet sizes calculated from water and oil uptakes agree very well with those determined by dynamic light scattering experiments, lending further support to the model.