Abstract
The solubilization of tributyrin by rodlike diheptanoylphosphatidylcholine (diheptanoyl-PC) micelles has been studied by small-angle neutron-scattering techniques. A rod to sphere transition is observed from the measured angular distribution of neutron-scattering intensities. The transition is from the rodlike micelle to a globular mixed micelle which has tributyrin molecules at the core surrounded by an outer shell of diheptanoyl-PC molecules. When the molar ratio of tributyrin to diheptanoyl-PC is increased beyond 0.18, all rodlike diheptanoyl-PC micelles are transformed into globular mixed micelles. The size of these globular mixed micelles increases with increasing tributyrin concentration in the simple micelle region up to the solubility limit. The small-angle neutron scattering data were analyzed to obtain the structural parameters of the mixed micelles. It is found that the growth of the mixed micelles does not follow a constant available surface area model or a constant palisade layer thickness model. If the model is modified to include tributyrin molecules at the surface of the mixed micelle, the growth behavior can be explained. For the tributyrin/diheptanoyl-PC molar ratio less than 0.18, rodlike micelles are found to coexist with globular mixed micelles. By fitting the neutron scattering data in the middle Q range, it is possible to determine the size and number density of the mixed micelles in the coexistence region. The results of analyzing the small-angle neutron scattering data in the coexistence region show that some tributyrin molecules are solubilized by the rodlike dipheptanoyl-PC micelles and only part of the added tributyrin forms globular micelles. These globular mixed micelles, in coexistence with rodlike micelles, do not grow with increasing tributyrin concentration, in contrast to the results found in the simple mixed micelle region. © 1990 American Chemical Society.