Mixed Langmuir monolayers of perfluorostearic acid and stearic acid studied by epifluorescence microscopy using fluorinated rhodamines and infrared reflection absorption spectroscopy (IRRAS)

摘要

The fluorophilicity of fluorinated rhodamine-based fluorescence dyes (F-rhodamines) functionalized with CH2-C3F7 (Rh-CH2-C3F7) or C2H4-C10F21 (Rh-C2H4-C10F21) at both amine groups was tested in mixed monolayers of stearic acid (SA) and perfluorostearic acid (FC18) on a Langmuir trough coupled with epifluorescence microscopy. The composition of the Langmuir monolayers was systematically changed from 100% FC18 to 100% SA in order to investigate the fluorophilicity and lipophilicity of the two F-rhodamines by observing their partitioning behavior within the different monolayers. The F-rhodamine bearing the longer perfluoroalkyl group and alkyl spacer (Rh-C2H4-C10F21) showed a substantially higher affinity to the FC18 rich phases in contrast to the F-rhodamine having the shorter perfluoroalkyl chain and alkyl spacer (Rh-CH2-C3F7) which was excluded from FC18 as well as from SA domains. The FC18, SA, and FC18/SA 50:50 (mol%) monolayers were further investigated by infrared reflection absorption spectroscopy (IRRAS) in order to investigate the phase behavior of FC18 and SA. It was observed that the orientations of FC18 and SA molecules in the pure and mixed monolayers do not change upon compression. Furthermore, a decrease in the order of the FC18 molecules in the mixed monolayer is observed upon the addition of SA. The opposite is not valid in a sense that no loss in order of the SA molecules was observed upon the addition of FC18. This shows that SA partitions more into FC18 phases than FC18 into SA domains. This is due to the weaker van der Waals forces present between the perfluoroalkyl chains in comparison with stronger ones existing between the alkyl chains.