New regioisomeric fluorohydroxy fatty acid methyl esters and their phase behaviour at the air–water interface

ω-Fluoro-(ω−1)-hydroxy and ω-hydroxy-(ω−1)-fluoro fatty acid methyl esters with different length of the alkyl chain were synthesised by hydrofluorination of the corresponding epoxides. The phase behaviour of these fluorohydrins at the air–water interface has been investigated by film balance and fluorescence microscopy. The shorter chain ω-hydroxylated methyl fluorohydroxyundecanoate, both regioisomeric methyl fluorohydroxytetradecanoates, and the ω-fluorinated methyl fluorohydroxyhexadecanoate form unstable films without condensed phases. The methyl 11-fluoro-10-hydroxyundecanoate with the shortest hydrophobic part of the chain shows no surface activity even at small molecular areas. Long-chain fluorohydroxylated fatty acid methyl esters with 18 and 20 C-atoms and the C16-ω-hydroxylated fluorohydrine show stable monolayers and condensed phases at 20 °C. Increasing the subphase temperature to 30 °C, the methyl 19-fluoro-20-hydroxyeicosanoate shows a surface pressure–area isotherm comparable to that of the regioisomer at 20 °C. From these results we conclude an enlargement and enforcement of the hydrophobic part of the fluorohydrins due to a contribution from the (ω−1)-F-atoms. The fluorescence microscopic investigations of both methyl fluorohydroxyoctadecanoates exhibit a liquid-expanded/iquid-condensed phase transition. Compared to methyl 17,18-dihydroxyoctadecanoate the methyl 17-fluoro-18-hydroxyoctadecanoate forms similarly shaped liquid-condensed domains. Removing the fluorine atom at position 17 the surface pressure–area isotherm does not show a two-phase coexistence region. In conclusion, a single fluorine atom within an amphiphilic structure influences significantly the phase behaviour at the air–water interface.