Surface density measurement of the bromide ion by the total-reflection X-ray absorption fine structure technique at the air/aqueous dodecyltrimethylammonium bromide solution interface

The X-ray absorption fine structure technique was applied to the aqueous dodecyltrimethylammonium bromide solution surface under the total reflection condition at 275 K. The Br K-edge absorption jump (J) value was evaluated and compared to the surface density (Gamma(1)(H)) of surfactants calculated from the surface otension versus concentration curve at 275.15 K. An excellent correlation between the J and Gamma(1)(H) values was found, and their dependences on concentration were not of a simple Langmuir type but of the Frumkin type. This should be indicative that a phase transition between the gaseous and expanded states takes place in the adsorbed film, claimed previously by us. Furthermore, the J value shows a stepwise increase at around the critical micelle concentration (cmc). This may be attributed to a change of the structure of the adsorbed film, such as staggered arrangement of surfactant ions close to the cmc to minimize the electric repulsion between their headgroups, or that of the electrical double layer. The results obtained in this study are compared to those evaluated from the radiotracer method applied to the aqueous tritiated sodium dodecyl sulfate solution surface.