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 (DTABF(4)). 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 (HMIMBF(4)) 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.

Surface adsorption and aggregate formation of cationic gemini surfactant and long-chain alcohol mixtures.

We measured the surface tension of aqueous solutions of octanol-butandiyl-1,4-bis(decyldimethylammonium bromide) using the drop-volume technique at 298.15 K under atmospheric pressure as a function of the total molality and bulk composition. The results of the surface tension measurements, which were analyzed by originally developed thermodynamic equations, suggested that octanol molecules filled the spaces among the hydrophobic chains of gemini surfactants and formed a densely packed monolayer with them in the adsorbed film. The turbidity of aqueous solutions was also measured to construct the concentration-composition diagram with the surface tension data. A transmission electron microscope was used to determine the aggregate morphology in the aqueous solutions. Disc-like micelle and microemulsion regions were found on the diagram prior to the spherical micelle formation; nevertheless, the butandiyl-1,4-bis(decyldimethylammonium bromide) itself formed only spherical (or small ellipsoid) micelles in the concentration range measured. We also studied the relationship between synergism and molecular packing in the aggregates.