AGGREGATION EFFECTS ON EVAPORATION AND THE AIR WATER INTERFACE OF DODECYLTRIMETHYLAMMONIUM HYDROXIDE SOLUTIONS/

The system dodecyltrimethylammonium hydroxide (DTAOH)-water was studie d by surface tension, ion-selective electrodes and evaporation in an e lectrobalance. Results confirmed earlier conclusions about a stepwise aggregation mechanism in DTAOH solutions. The aggregation process star ted 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 = (1,3 00 +/- 0.041) x 10(-3) mol dm(-3) there was a change in the surface an d 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 significa ntly 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 c hanges in structure were gradual. At the latter concentration there wa s a sudden change in the monolayer state at the air/water interface. w ith 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. Th is phenomenon is in agreement with literature information. The reducti on in the evaporation rate of water was mainly due to the reduction of the water activity, caused by colligative effects. The reduction of t he effective area available for evaporation had only a slight effect i n water evaporation.