Aggregation effects on evaporation and the air/water interface of dodecyltrimethylammonium hydroxide solutions

The system dodecyltrimethylammonium hydroxide (DTAOH)-water was studied by surface tension, ion-selective electrodes and evaporation in an electrobalance. Results confirmed earlier conclusions about a stepwise aggregation mechanism in DTAOH solutions. The aggregation process started at a total concentration C-T = (2.51 +/- 0.10) x 10(-4) mol dm(-3)) which probably corresponds to the formation of dimers. At C-T = (1,300 +/- 0.041) x 10(-3) mol dm(-3) there was a change in the surface and evaporation behavior, corresponding to the formation of small, fully ionized aggregates which grew with increasing concentration. At C-T = (1.108 +/- 0.010) x 10(-2) mol dm(-3) the formation of true micelles with hydroxide counterions in the Stern layer did not change significantly the evaporation and adsorption behavior. This means that between this concentration and C-T = (3.02 +/- 0.28)x 10(-2) mol dm(-3), the changes in structure were gradual. At the latter concentration there was a sudden change in the monolayer state at the air/water interface. with a strong surfactant desorption, and a major change in evaporation behavior. The changes are compatible with the formation of few, large aggregates reducing the total concentration of kinetically independent solute units, which in turn increased the activity of the solvent. This phenomenon is in agreement with literature information. The reduction in the evaporation rate of water was mainly due to the reduction of the water activity, caused by colligative effects. The reduction of the effective area available for evaporation had only a slight effect in water evaporation.