Characteristics of Oil Droplets Stabilized by Mineral Particles: The Effect of Salinity

ABSTRACT The influence of salinity on the characteristics of oil droplets stabilized by mineral particles (oil-mineral aggregates – OMA) was studied in the laboratory using three different oils and a natural sediment. Size and concentration of oil droplets associated with negatively and positively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that the median droplet size increases rapidly from about 5 μm at zero salinity to double at salinity close to 1.2 ppt; decreases dramatically to about 5 μm at salinity 3.5 ppt and then increases slightly to 6 μm at the seawater salinity. The concentration of oil droplets also increases sharply when the salinity increases from zero to a critical aggregation salinity Scas, after which it stabilizes at its maximum value. The concentration of mineral-stabilized droplets is strongly affected by oil type at any salinity. When normalized to its maximum value, the concentration of droplets correlates well with normalized salinity S/Scas. A relationship is derived to predict the effect of salinity on the concentration of mineral-stabilized droplets. Size distributions of oil droplets follow similar trends, but their magnitudes depend on salinity and oil type. Self-similarity in droplet size distributions was shown when the data were plotted using normalized variables N/Nt and D/D50, where N is the number of droplets of diameter D, Nt is the total number of droplets and D50 the median size of the droplets. With these normalized variables, oil droplet size distributions measured in this study and those measured in field and laboratory under various conditions by different investigators fit the same curve regardless of the formation conditions of the droplets. A function is derived to calculate normalized cumulative size distributions of oil droplets.