Kinetics of the processes of desorption from fatty acid monolayers

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 radical t. In a subsequent steady-state phase, ln A was linear with t. The initial desorption coefficient for sodium palmitate, K(i), and the steady-state desorption coefficient, K(s), varied directly with surface pressure and subphase pH, and these desorption coefficients also varied with the composition of the subphase buffer. However, the K(s)/K(i) ratio was independent of these variables. The diffusion coefficient, D(25), for sodium palmitate calculated from desorption coefficient ratios was 4.8 +/- 0.6 x 10(-6) cm(2)/sec. This value was in reasonable agreement with D(25) for sodium palmitate measured by time-lag diffusion, 3.7 +/- 0.6 x 10(-6) cm(2)/sec. D(25) 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.