Study on the distribution of binary mixed counterions in surfactant adsorbed films by total reflection XAFS measurements

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF4). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB – dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) – 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.

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► In adsorbed surfactant films, Br− counterions are in two hydration states.
► BF4- ions are bound to surfactant ions preferentially to Br− ions.
► The specific ion effect is related to miscibility and distribution of counterions.