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Interfacial tensions (IFT) between low salinity surfactant solutions of sodium dodecylbenzenesulfonate (SDBS) and sodium dioctylsulfosuccinate (Aerosol OT or AOT) and different oil phases were measured using the spinning drop method. The oil phases were either n-heptane, a 1:1 mixture of n-heptane and toluene or crude oils. The IFT was strongly affected by variation of the salinity and especially by the presence of calcium. IFT values between SDBS solutions and heptane, as well as all four crude oils, decreased when adding sodium and calcium. IFTs down to 32 μN/m for crude oil were measured in the presence of AOT solutions with low salinity electrolyte. In the IFT region below 1 mN/m the presence of small amounts of calcium could lower the IFT by more than 1 order of magnitude compared to solutions containing only sodium chloride with the same ion strength.

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