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Ion-pair amphiphiles (IPAs) are neotericpseudo-double-tailed compoundswith potential as a novel substitute of phospholipid. IPA, synthesizedby stoichiometric/equimolar mixing of aqueous solution of hexadecyltrimethylammoniumbromide (HTMAB) and sodium dodecyl sulfate (SDS), was used as a potentialsubstituent of naturally occurring phospholipid, soylecithin (SLC).Vesicles were prepared using SLC and IPA in different ratios alongwith cholesterol. The impact of IPA on SLC was examined by way ofsurface pressure (π)–area (A) measurements.Associated thermodynamic parameters were evaluated; interfacial miscibilitybetween the components was found to depend on SLC/IPA ratio. Solutionbehavior of the bilayers, in the form of vesicles, was investigatedby monitoring the hydrodynamic diameter, zeta potential, and polydispersityindex over a period of 100 days. Size and morphology of the vesicleswere also investigated by electron microscopic studies. Systems comprising20 and 40 mol % IPA exhibited anomalous behavior. Thermal behaviorof the vesicles, as scrutinized by differential scanning calorimetry,was correlated with the hydrocarbon chain as well as the headgrouppacking. Entrapment efficiency (EE) of the vesicles toward the cationicdye methylene blue (MB) was also evaluated. Vesicles were smart enoughto entrap the dye, and the efficiency was found to vary with IPA concentration.EE was found to be well above 80% for some stable dispersions. Suchformulations thus could be considered to have potential as novel drugdelivery systems. [ABSTRACT FROM AUTHOR]