Dynamic properties of fatty acid monomolecular films

Abstract

Saturated fatty acids ranging from fifteen to twenty carbons in alkyl chain length were spread as insoluble monomolecular films and compressed continually at constant velocities. As these surface films assume close-packing they exhibit a spontaneous loss in surface pressure when held at constant area per molecule or when compressed beyond an apparent collapse area, the greater the surface pressure compressed to the greater the rate of instability. Increasing chain length increases film stability but different behavior for odd and even carbon compounds has been noted for collapse pressures and for the rate of surface pressure loss. Comparison of saturated fatty acids with their methyl esters and their corresponding [alpha]-hydroxy acids indicates a change in stability in the order: [alpha]-hydroxy acids > fatty acids > methyl esters. From the evidence obtained it is concluded that three-dimensional lens formation occurs when the film enters the region of instability before an apparent collapse pressure is reached. The rate of molecular expulsion from the surface and the rate of lens formation determine the apparent collapse pressure and the rate of surface pressure loss at any area per molecule. This helps to explain the well-known effect of compression rate on the apparent collapse pressure of surface films.