Abstract
This project investigates the effect of surfactant mixtures on Marangoni transport of colloidal particles adsorbed at the oil/water interface using a narrow-gap, radial stagnation point flow cell. Marangoni transport is created by the transient interfacial tension gradient established when surfactants adsorb from a solution in a laminar flow to an initially clean oil/water interface. The magnitude of Marangoni velocity for surfactant mixtures was compared with the Marangoni velocity for single component surfactants which comprise the surfactant mixtures. The surfactant pair is cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium dodecyl sulfate (SDS), and the oil is olive oil. To achieve a certain peak Marangoni velocity, the lowest concentration required was for CTAB as a single component surfactant among all the tested surfactant solutions including surfactant mixtures at molar ratios SDS/CTAB = 10/90 and 90/10, pure SDS, and pure CTAB. Since in a system with Marangoni synergism a particular peak Marangoni velocity can be achieved by a mixture having lower total surfactant concentration than the concentration of either pure surfactant needed to produce that same peak Marangoni velocity, none of these surfactant mixtures yields a positive Marangoni synergism in my measured peak velocity range. Instead, for SDS/CTAB = 90/10 composition, SDS/CTAB mixtures yield negative synergism, where the larger concentration of mixture than either pure surfactant is required to produce a same peak Marangoni velocity within my measured velocity range. After measuring the corresponding interfacial tension for each solution, we found that interfacial tension and Marangoni transport synergism do not occur at the same compositions. Further anionic fatty acids in olive oil form complexes with CTAB at the interface, which alter the Marangoni transport.
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