James W. Peterson and John C. Berg’
Department of Chemical Engineering, BF- 10, University of Washington, Seattle, Washington 98 195

An investigation is made of fractionation which occurs in oil mixtures as they spread over water under the control of surface forces. Its Occurrence is suggested in the literature, but the evidence is scant and fragmentary. Carefully controlled experiments using mixtures of poly（dimethylsi1oxane） and tetradecane were undertaken to verify or disprove the existence of the phenomenon. The results provide positive evidence of fractionation and indicate that a preferential spreading mechanism is involved. Additional fractionation measurements are made of a mixture of toluene, octane, and decane. This system also exhibits fractionation trends, indicating preferential spreading. A mathematical model is developed which describes the process as a multistage, batch-charged separation and yields good fits to the fractionation data. Oil film thickness profiles needed in the model were obtained by a new method using polychromatic interference fringes.