The Spreading of Oils on Water Part I. Ionized Molecules Having Only One Polar Group

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Abstract
The spreading phenomena on water of drops of mineral oils containing a wide variety of organic acids and amines having but one polar group have been studied under conditions where dissociation at the oil‐water interface could most readily occur. Particular emphasis is placed on the homologous series as an aid in separating the variables involved. Various phenomena of edge diffusion are described and a qualitative theory is presented. Edge diffusion is shown not to be a serious source of error in measuring the cross‐sectional areas of the adsorbed molecules if there are over 13 carbon atoms per straight chain. The different types of film spreading and break‐up effects are described and the various physical and chemical factors involved are discussed. The changes in the spreading effects due to varying the pH and to the presence in the water of Ca++, Cu++, Pb++, Fe+++, Al+++, La+++, and Th++++ on the interfacial film are described. It is shown that only the long chain monobasic saturated acids form rigid films when affected by metallic ions, while other acids are either condensed somewhat or are unaffected. Short chain metallic salts of these fatty acids are often oil soluble and have brief lifetimes of adsorption. An experimental proof is given that dissociated long chain acids and amines have infinite lifetimes of adsorption. Evidence is given for concluding that the lifetime is short for molecules having less than 14 carbon atoms. The effect of Ca++ on the spreading of oleic‐stearic acid mixtures is given in some detail and also a theory of the results. Reasons are given for concluding that the adsorbed acid molecules form two‐dimensional film complexes with hydrocarbon molecules derived from the oil.

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