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

omega-Fluoro-(omega-1)-hydroxy and omega-hydroxy-(omega-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 omega-hydroxylated methyl fluorohydroxyundecanoate, both regioisomeric methyl fluorohydroxytetradecanoates, and the omega-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 C-16-omega-hydroxylated fluorohydrine show stable monolayers and condensed phases at 20 degreesC. Increasing the subphase temperature to 30 degreesC, the methyl 19-fluoro-20-hydroxyeicosanoate shows a surface pressure-area isotherm comparable to that of the regioisomer at 20 degreesC. From these results we conclude an enlargement and enforcement of the hydrophobic part of the fluorohydrins due to a contribution from the (omega - 1)-F-atoms. The fluorescence microscopic investigations of both methyl fluorohydroxyoctadecanoates exhibit a liquid-expanded liquid-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. (C) 2002 Elsevier Science B.V. All rights reserved.