Kinetics of the processes of desorption from fatty acid monolayers

Abstract

The surface area, A, of contracting fatty acid monolayers was measured as a function of time, t, at constant surface pressure. In the initial temporal phase, ln A was linear with √t. In a subsequent steady-state phase, ln A was linear with t. The initial desorption coefficient for sodium palmitate, Ki, and the steady-state desorption coefficient, Ks, varied directly with surface pressure and subphase pH, and these desorption coefficients also varied with the composition of the subphase buffer. However, the Ks/Ki, ratio was independent of these variables. The diffusion coefficient, D25, for sodium palmitate calculated from desorption coefficient ratios was 4.8 ± 0.6 × 10–6 cm2/sec. This value was in reasonable agreement with D25 for sodium palmitate measured by time-lag diffusion, 3.7 ± 0.6 × 10–6 cm2/sec. D25 values obtained for a series of fatty acids suggested that higher members of the series diffused as small aggregates averaging two to four molecules in size. Kinetic and diffusion data both supported a model for the desorption process described by Ter Minassian-Saraga.

Keywords

• lauric acid
• myristic acid
• palmitic acid
• palmitoleic acid
• oleic acid
• surface pressure