Shifting of fatty acid monolayer phases due to ionization of the headgroups

Monolayers of simple fatty acids exhibit a rich polymorphism with at least seven solid phases. The reason for such a variety of phases is not known ye t and therefore is the object of the current investigation. In the present work the pH of the subphase is varied between 2 and 10.5 and the influence of the degree of dissociation on phase transition pressures and temperature s of arachidic acid monolayers is investigated in detail in the range from 2 to 45 degreesC by means of Brewster-angle microscopy and surface pressure -area isotherms. The shift of phase boundaries is ascribed to the change in the steric, electrostatic, and bonding interactions between the lipid head groups. The impressive correlation between the L-2/v nearest neighbor to ne xt nearest neighbor (NN/NNN) transition pressure and the position correlati on length as functions of the pH value suggests that the L-2/Ov NN/NNN tran sition in fatty acid monolayers is strongly influenced or even originates i n headgroup bonding interactions. The influence of headgroup dissociation o n the phase behavior of fatty acid monolayers is investigated in the absenc e of solutes that interact with the lipids. The comparison of the phase 'be havior of this system with that of a system, in which solutes are present t hat interact with the charged fatty acids, e.g., polymers or polyvalent met allic ions, should provide information on specific solute-lipid interaction s separately from the effect of headgroup dissociation.