Relaxation of bimolecular layer films on water surfaces.

Ferric stearate, a three-tailed amphiphile, forms bimolecular layers on water surfaces. Molecules in the lower layer are in an "asymmetric" configuration, Fe-containing heads touching water and three hydrocarbon tails in air, while molecules in the upper layer are in a "symmetric" configuration, in pairs of "Y and inverted Y" disposition of tails about the Fe-bearing head. Pressure relaxation at constant area (pi- t curves) and area relaxation at constant pressure ( A- t curves) of this bimolecular layer can be modeled as a sum of three exponential decay terms with distinct time constants and weight factors. Relating the long-term decay with desorption of the total film thus indicates a remarkable long-term stability of the bimolecular layer film. An X-ray reflectivity study of the bimolecular films deposited horizontally on Si(001) at various conditions of relaxation shows no further growth along the vertical of any other layer. Under pressure relaxation molecules are transferred from the upper layer to the lower layer with a change from symmetric to asymmetric configuration, while under area relaxation the transfer is from the lower layer to the upper layer with a configurational change from symmetric to asymmetric.