Competition of Hydrophobic
Steroids with Sodium Dodecyl
Sulfate, Dodecyltrimethylammonium Bromide, or Dodecyl β‑d‑maltoside for the Dodecane/Water Interface
- Publication date
- Publisher
Abstract
The surface tension lowering abilities of insoluble steroids,
progesterone
and testosterone, were examined at the dodecane/water interface in
the presence and absence of surfactants, sodium dodecyl sulfate, dodecyltrimethylammonium
bromide, and dodecyl maltoside. In the absence of these surfactants,
the steroids significantly lowered the interfacial tension while exhibiting
no activity at the air/water and air/dodecane surfaces. Further, in
mixtures of surfactants and steroids, significant enhancement of interfacial
tension lowering was observed. At a sufficiently high concentration
of surfactant, no further lowering of tension was observed in the
presence of the steroids. The synergistic effects on interfacial tension
of steroids and surfactants were characterized by the free energy
of transfer to the interface of each solute based on a two-dimensional
solution equation of state. Assuming no significant interaction between
the steroids and the surfactants in the interface, predictions of
interfacial tensions were made based on the calculated free energies
of transfer and interfacial area occupied. Good agreement was found
between the predicted values and experimental values for interfacial
tension. The results of these studies show that progesterone and testosterone,
molecules not normally thought of as surface active, exhibit significant
interfacial activity and can successfully compete with surfactants
for the dodecane/water interface