Temperature-dependent phase behaviour of dihydroxy octadecanoic acid methyl esters: Influence of stereochemistry and position of the second polar moiety

Racemic mixtures and enantiomers of amphiphiles based on methyl octadecanoa te with two vicinal hydroxy groups as a second polar moiety have been inves tigated at the air/water interface on ultrapure water. In contrast to methy l octadecanoate these bipolar substances show a distinct two-phase coexiste nce region. Earlier studies showed the dependence of the surface pressure-a rea (pi-A) isotherms and the structure and domain shapes of the condensed p hases on the position of the vicinal hydroxy groups within the hydrophobic alkyl chain of methyl octadecanoate. To obtain more information about the i nfluence of the insertion of a second polar moiety within an amphiphilic st ructure on film properties and film stability the temperature-dependent pha se behaviour is studied by measuring pi-A isotherms within a temperature ra nge of 275-313 K. Applying a suitable form of the Clausius-Clapeyron equati on to the pi-A isotherms, the enthalpy Delta H was calculated for the liqui d-expanded (LE) to liquid-condensed (LC) phase transition. From these calcu lations we conclude that the hydroxy groups at different positions along th e alkyl chain act as a disturbing moiety between the molecules of the conde nsed film and influence the hydrophobic interaction between the alkyl chain s for threo-dihydroxyoctadecanoate (DHO) with the hydroxy groups in the mid dle of the molecules. As a consequence the condensed monolayers are metasta ble and the more stable multilayered order is preferred. Films of the bolaa mphiphile methyl rac-17,18-DHO as well as films of methyl rac-threo-2,3-DHO show smaller effects on film properties. The observed effects are discusse d with respect to distearoylphosphatidylcholine (DSPC), which serves as a r eference. In contrast, racemic erythro-DHO form stable condensed monolayers . The hydroxy groups act not as a disturbing moiety but as a spanning moiet y between the particular amphiphiles by formation of hydrogen bonds. The en antio-enriched monolayers of all DHO investigated show a bent surface press ure-temperature course due to two different condensed phases which are in e quilibrium with the expanded phase. Therefore, the DHO show chiral discrimi nation with respect to thermodynamic investigations.