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 octadecanoate with two vicinal hydroxy groups as a second polar moiety have been investigated at the air/water interface on ultrapure water. In contrast to methyl octadecanoate these bipolar substances show a distinct two-phase coexistence region. Earlier studies showed the dependence of the surface pressure-area (pi-A) isotherms and the structure and domain shapes of the condensed phases on the position of the vicinal hydroxy groups within the hydrophobic alkyl chain of methyl octadecanoate. To obtain more information about the influence of the insertion of a second polar moiety within an amphiphilic structure on film properties and film stability the temperature-dependent phase behaviour is studied by measuring pi-A isotherms within a temperature range of 275-313 K. Applying a suitable form of the Clausius-Clapeyron equation to the pi-A isotherms, the enthalpy Delta H was calculated for the liquid-expanded (LE) to liquid-condensed (LC) phase transition. From these calculations we conclude that the hydroxy groups at different positions along the alkyl chain act as a disturbing moiety between the molecules of the condensed film and influence the hydrophobic interaction between the alkyl chains for threo-dihydroxyoctadecanoate (DHO) with the hydroxy groups in the middle of the molecules. As a consequence the condensed monolayers are metastable and the more stable multilayered order is preferred. Films of the bolaamphiphile 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 discussed with respect to distearoylphosphatidylcholine (DSPC), which serves as a reference. In contrast, racemic erythro-DHO form stable condensed monolayers. The hydroxy groups act not as a disturbing moiety but as a spanning moiety between the particular amphiphiles by formation of hydrogen bonds. The enantio-enriched monolayers of all DHO investigated show a bent surface pressure-temperature course due to two different condensed phases which are in equilibrium with the expanded phase. Therefore, the DHO show chiral discrimination with respect to thermodynamic investigations.