Shifting of Fatty Acid Monolayer Phases Due to Ionization of the Headgroups

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 headgroup dissociation on the phase behavior of fatty acid monolayers is investigated in the absence of solutes that interact with the lipids. The comparison of the phase behavior of this system with that of a system, in which solutes are present that interact with the charged fatty acids, e.g., polymers or polyvalent metallic ions, should provide information on specific solute−lipid interactions separately from the effect of headgroup dissociation.

Phase behavior of Langmuir monolayers with ionic molecular heads: Molecular simulations

“…This is manifested in the shift of the phase coexistence lines to lower temperatures. These results are in qualitative agreement with experiments carried out in fatty acid monolayers with ionic head groups [45].…”

“…It has been found experimentally that the long-range repulsive interactions between molecular heads lead to the monolayer's rich structure and phase behavior. It is known that head-head interactions are modulated by partial dissociation when the subphase pH is varied [43][44][45]. For instance, in aqueous solutions of soaps with a pH between 7.0 and 10.0, the heads of long-chain fatty acids are partially ionized [12,43], while strong changes in the head-head interactions are expected when metal ions are present in the subphase [46][47][48].…”

“…Nonetheless, the untilted phases found at higher surface pressures values were attributed to the chains interactions [49]. Experimental studies of fatty-acid monolayers with ionized molecular heads encountered that the phase boundaries shifted to lower temperatures [45]. It was also found that the phosphate hydration crucially affected the interfacial behavior of phospholipids deposited on the air-water interface [52].…”

Fatty-Acid Monolayers at the Nematic/Water Interface: Phases and Liquid-Crystal Alignment

“…Numerous studies at the air/water interface have indicated the phase behavior of fatty-acid monolayers remains unchanged for pH values less than 9.0. [47][48][49] The pK a values for fatty acids in a monolayer state were measured to be markedly greater than that of a solution state though the reason for this remains unclear. 47 Attempts to carry out our studies at a lowered pH of 4.0 caused our monolayer to collapse as the temperature was lowered.…”

Effect of Degree, Type, and Position of Unsaturation on the pKa of Long-Chain Fatty Acids

“…Fatty acid monolayer phase transitions can also be studied using methods such as X-ray diffraction (31)(32)(33)(34)(35)(36), FT-Raman spectroscopy (35), and Brewster angle microscopy (BAM) (36,37). A correlation between fatty acid monolayer phase transition and solution pH has been shown using the BAM technique (37). Specifically, spread monolayers of fatty acids show maximum resistance to phase transition under compression when the pH of the solution equals the pK a , indicating that the L 2 phase, which is characteristic of fatty acid monolayers, is stabilized by the ion-dipole interactions between carboxylic acid and carboxylate headgroups (Fig.…”