The Spreading of Oils on Water Part II. Non-Ionized Molecules Having Only One Polar Group

Abstract

Experiments are described on the spreading upon water of drops of oil containing small amounts of alcohols, esters, ketones, phenols, non-ionized acids, and non-ionized amines. It is shown that such polar molecules do not cause spreading unless the molal concentration exceeds a certain value, c0, which varies with the nature of the polar group, the chain length, the possible geometrical configurations of the molecule, and with the solubility in oil. The cause of their spreading is demonstrated to be a reversible adsorption at the oil-water interface associated with the osmotic pressure of the polar molecules dissolved in the oil. The average lifetime of adsorption of such molecules is shown to be only a small fraction of that of ionized molecules. A0, the interfacial area per adsorbed molecule at the film pressure of 11.3 dynes per cm, is given for a considerable number of saturated and unsaturated straight- and branched-chain acids and alcohols. The observed changes of A0 with chain length and polar group are explained on the basis of structural and kinetic considerations. Certain new spreading phenomena due to the formation of colloidal suspensions and crystalline precipitates of the polar compounds are described, and use is made of the latter to measure the solubility in oils of the various straight chain saturated acids and alcohols.