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NMR studies of carboxylic acids : an investigation of head group behaviour in lyotropic and nematic phases Delikatny, Edward James


The orientational order of the methylene segments adjacent to the head group in the lamellar liquid crystalline phase of potassium palmitate/D₂0 was investigated using multinuclear magnetic resonance. Previous studies had shown that the orientational order near the soap--water interface decreased with decreasing temperature, contrary to intuition. To investigate this phenomenon, three isotopically substituted species of palmitic acid were synthesized: palmitic acid-d₃₁, 1-¹³C-2,2-H₂- palmitic acid-d₂₉, and 2,2,3,3-H₄- palmitic acid-d₂₇. These compounds were treated in two complementary fashions: the acids were dissolved in the liquid crystal 4 - (octyloxy) - benzoic acid (p - OOBA) and the corresponding potassium salts were dispersed in D₂0 at a constant water concentration. Dipolar and quadrupolar couplings were obtained from the ¹H, ¹³C, and ²H nmr spectra of these molecules, in nematic and lamellar liquid crystalline phases, as a function of temperature. In a parallel study, nmr spectra of acetic, propionic, and butyric-2,2 - d₂ acids dissolved in p - OOBA were recorded. In order to observe ¹H - ¹H dipolar couplings, a two dimensional spin echo (π/2 - t₁/2 - π - t₁/2 - echo) was necessary to remove heteronuclear dipolar couplings to the chain deuterons. A refocussing pulse applied simultaneously to the spins allowed observation of the heteronuclear ¹H - ¹³C dipolar couplings in the carbon 13 labelled compound. The complete orientational order matrix of the alpha methylene segment of potassium paImitate/D₂0 and of palmitic acid/p - OOBA was determined from the dipolar and quadrupolar couplings. As the temperature is decreased from 110°C to a temperature just above the gel-liquid crystalline phase transition (45°C), the orientation of the methylene segment of potassium palmitate is rotated by 3° towards a configuration in which the first C - C bond is parallel to the bilayer normal. This is in direct agreement with a previous model, the Abdollal model of lipid - water interaction, in which the decrease in orientational order was postulated to be a strictly geometric effect arising from electrostatic interactions of the lipid with the water. A mean field equilibrium statistical mechanical model, based on the Samulski Inertial Frame Model, was developed to simulate the experimental dipolar and quadrupolar nmr couplings. In potassium palmitate, electrostatic interactions, approximately constant at higher temperatures, increase dramatically as the phase transition is approached. In contrast mean field steric repulsive forces remain constant over the entire temperature range studied. This evidence also supports the Abdollal model of lipid - water interaction. The electrostatic interaction was shown to be of greater importance in the orientational ordering of the solutes in the liquid crystal than in potassium palmitate and this was attributed to intermolecular H - bonding between solute and p - OOBA. The ordering of the head group of carboxylic acids dissolved in p - OOBA was demonstrated to be remarkably similar regardless of the chain length of the solute.

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