Estimation of surface area and counterion binding characteristics in fatty amine monolayers from desorption kinetics

The surface area per molecule of an un-ionized fatty amine is very similar to the surface area per molecule of an un-ionized fatty acid. Surface area increases with ionization in both fatty amine and fatty acid films. However, fatty amino cations have much smaller surface areas than the corresponding fatty acid anions. Thus counterion binding is stronger with fatty amine cations than with fatty acid anions. Surface area data show that counterion binding affinities for fatty amine cations decrease in the strong field sequence Cl- greater than Br- greater than I- greater than SCN-. Furthermore, surface areas in the presence of the most strongly bound counterions, Cl- and Br-, increase significantly with an increase in subphase ionic strength. These data are consistent with the formation of strong ion-pair bonds and their disruption with an increase in ionic strength. Fatty amine cations desorb as micelles with much lower relative diffusion coefficients than the corresponding fatty acid anions. Furthermore, relative diffusion coefficients for fatty amine cations are strongly dependent on the specific cation. These data show that fatty amine cations form larger micelles when they desorb in the presence of strongly bound counterions. Anions enhance the solubility of a fatty acid anion in the sequence Cl- less than I- less than SCN-, which is characteristic of chaotropic anions that disrupt water structure.