On the adsorption and condensed film formation of dodecyl-, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromides at the mercury/electrolyte interface

The adsorption and condensed film formation of dodecyl (DTAB)-, tetradecyl (TTAB)-, hexadecyl (CTAB)-, and octadecyl (OTAB)-trimethylammonium bromides on the hanging mercury electrode is studied in KBr as supporting electrolyte, at various temperatures from 5 to 45 degrees C. A condensed film is formed at negative potentials and at room temperature only in the presence of CTAB. The decrease of the temperature favors the formation of the condensed film. A transition temperature is observed for the film formation. Capacity-time curves at the potentials where the film is formed show a nucleation and growth mechanism, with induction time depending not only on the final potential but also on the initial potential range, although it is in the desorption region. In this temperature range no film is observed for DTAB and TTAB. However, the film is observed for OTAB, but only at higher temperatures, and is more easily formed with increasing temperature. The film is formed in a certain potential region and the nucleation rate increases while moving toward more negative potentials. Hysteresis phenomena are observed during changes of scan direction. The capacity vs time curves for OTAB, where condensed film is formed, are treated using an Avrami plot formulation and have been explained as progressive one-dimensional nucleation with a decrease of the nucleation rate during the overall film formation. The results show a marked effect of the chain length of the alkyl chain on the film formation.