Journal of Lipid Research, Vol 17, 197-202, Copyright © 1976 by Lipid Research, Inc.
Estimation of surface area and counterion binding characteristics in fatty amine monolayers from desorption kinetics
GS Patil, RH Matthews and DG Cornwell
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.
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