Adsorption layer properties of alkyltrimethylammonium bromides at interfaces between water and different alkanes

•CnTAB molecules adsorb from the water phase at the water/oil interface.•The presence of hexane phase enhances the CnTAB adsorption.•Lengths of the oil and surfactant molecules define the adsorption layer structure.•Good theoretical fitting is obtained with the Frumkin Ionic Compressibility model.

We measured the interfacial tensions of aqueous solutions against different oil phases using drop profile analysis tensiometry (PAT-1, Sinterface Technologies, Germany) for decyl- and dodecyltrimethylammonium bromide (C10TAB and C12TAB) in phosphate buffer (10 mM, pH7). The following alkanes were used as oil phases: hexane, heptane, octane, nonane, decane, dodecane and tetradecane. The obtained equilibrium interfacial tension isotherms were fitted by the Frumkin Ionic Compressibility model (FIC). The surfactants adsorb at the water/oil interface in competition with the oil molecules. At high surfactant surface coverage this competitive adsorption is manifested in two ways. First, for short chain surfactants, the oil molecules are embedded into the adsorption layer. Second, for long chain surfactants, the short alkane chains of the oil molecules are squeezed out from the adsorption layer due to strong mutual interaction between surfactants’ chains.

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