Surface activity of a non-micelle forming compound containing a surface-active impurity

The purpose of this study is to characterize the surface activity of a water-soluble compound and its ability to form aggregates/micelles.

Aqueous solutions of the compound were prepared at various concentrations. Surface tension was determined using drop volume and Wilhelmy plate methods. Moreover, conductivity and osmolality measurements of aqueous solutions were also determined at various concentrations.

Even though the compound appeared to be surface active, no change in the slope was found of either molar conductivity (Scm(2)/mol) versus square root of concentration (R(2)=0.994) or osmolality (mOsm/kg) versus concentration (R(2)=0.999). Moreover, no clear critical micelle concentration was observed when surface tension was plotted versus log concentration. These results indicated no micelle formation in these solutions. In order to investigate this behavior further, the main impurity in the compound was also tested. Surface tension measurements of solutions containing different concentrations of the impurity indicated that the impurity was more surface active than the compound.

This study shows the importance of characterizing the behavior of surface-active compounds using multiple techniques. This work also emphasizes the importance of determining whether surface activity in aqueous solutions is due to the main compound, its impurities or both.