The interactions between supported cationic surfactant bilayers were measured by colloidal probe atomic force spectroscopy and the eect of dierent halide salts was investigated. Di(alkyl iso-propyl ester) dimethyl ammonium methylsulfate (DIPED-MAMS) bilayers were fabricated by the vesicle fusion technique on muscovite mica.The interactions between the bilayers were measured in increasing concentrations of 1 NaCl, NaBr, NaI and CaCl 2 . In NaCl the bilayer interactions were repulsive at all concentrations investigated, and the Debye length and surface potential were observed to decrease with increasing concentration. The interactions were found to follow the Electrical Double Layer (EDL) component of DLVO theory well. However Van der Waals forces were not detected, instead a strong hydration repulsion was observed at short separations. CaCl 2 had a similar eect on the interactions as NaCl. NaBr and NaI were observed to be more ecient at decreasing the surface potential than the chloride salts, with the ecacy increasing with the ionic radius.

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