Abstract
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 yet 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 temperatures of arachidic acid monolayers is investigated in detail in the range from 2 to 45 °C 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 headgroups. The impressive correlation between the L2/Ov nearest neighbor to next nearest neighbor (NN/NNN) transition pressure and the position correlation length as functions of the pH value suggests that the L2/Ov NN/NNN transition in fatty acid monolayers is strongly influenced or even originates in headgroup bonding interactions. The influence of h...
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