Surface activity of a non-micelle forming compound containing a surface-active impurity.
OBJECTIVE
The purpose of this study is to characterize the surface activity of a water-soluble compound and its ability to form aggregates/micelles.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Al-Maaieh
Ahmad
A
Pharmaceutical Sciences Research and Development, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285, USA. al-maaieh_ahmad@lilly.com
Aburub
Aktham
A
D004340:Drug Contamination; D004553:Electric Conductivity; D008823:Micelles; D009994:Osmolar Concentration; D004364:Pharmaceutical Preparations / Q000737:chemistry; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*
D008823:Micelles; D004364:Pharmaceutical Preparations; D013501:Surface-Active Agents