Bipolar/monopolar conformational transitions of selected hydroxyoctadecanoic acids and esters: A fluorescence microscopy study

The bipolar/monopolar conformational transitions of the hydroxyoctadecanoic acids (HOAs) [6-hydroxy-9-hydroxy- and 12-hydroxyoctadecanoic acid (6HOA, 9HOA, and 12HOA)] plus those of the methyl and ethyl esters of 12HOA have been studied using classical surface pressure (II)/area per molecule (A) isotherms and fluorescence microscopy techniques. Most studies were carried out using identical compressional rates (5 Az mol-l min-l) and identical amounts of a fluid-phase soluble fluorescent probe 1-palmitoyl-2-[6-(7-nitro-2-1-3-benzoxadiazol-4-yl)amino] caproyl phosphatidylcholine (NBD-PC); however, the effect of both compressional rate and probe concentration was investigated. For 6HOA, condensed-phase domains are roughly diamond-shaped with both nucleation and apparent crystal growth being observed. Larger, essentially hexagonal, crystals were observed for 9HOA. For 12HOA the extent of nucleation was much greater, producing elongated needlelike crystals quite different from either 6- or 9HOA. This difference is ascribed to a greater stability of the condensed state, possibly due to better packing and enhanced hydrogen bonding. The highly brittle nature of the needlelike crystals support this interpretation. On the basis of the pi/A isotherms the 12HOA methyl ester condensed state shows little indication of diminished stability vis-a-vis 12HOA, but the ethyl ester does. However, both esters show substantially larger crystals, indicating that growth is favored over nucleation, and it is proposed that the reduced hydrogen bonding of the ester polar group is a primary cause.