INTERFACIAL PROPERTIES OF SOME PROPYLENE GLYCOL MONOESTERS

The adsorption of some oil-soluble propylene glycol monoesters at the oil/water interface has been examined. The 1-monostearate ester (1-PGMS) has been studied in detail at the n-heptane/water interface. This compound is quite surface active, giving a surface pressure of 5 dynes/cm at a mole fraction concentration of ∼8 × 10-5. Even at such a low concentration there appears to be appreciable interaction among the 1-PGMS molecules in the interfacial phase, and the interface has the compressibility of a solid monolayer. The average area per 1-PGMS molecule is 58 Å2. At higher bulk concentrations the interfacial layer undergoes a condensation due to reorientation of the propylene glycol head groups in the interface. The interfacial layer also has properties of a solid monolayer, but the average area per 1-PGMS molecule is reduced to 20 Å2. It is postulated that, in the first solid monolayer, the head group of the surfactant molecule is oriented parallel to the interface, while in the second solid monolayer the head group is perpendicular to the interface. Similar adsorption behavior is seen for 1-PGMS at the benzene/water interface; calculated areas per molecule are meaningless in this case because of the mutual solubility of benzene and water. © 1968.