Semifluorinated thiols in Langmuir monolayers

A series of semifluorinated thiols of the general formula CF3(CF2)m-1(CH2)nSH (abbreviated to FmHnSH) have been synthesized and the Langmuir monolayers thoroughly characterized using surface pressure (π) and elec. surface potential (ΔV) measurements. These data have been complemented with Brewster angle microscopy (BAM) visualization of the monolayers structure and IR spectroscopy (PM-IRRAS) anal. of the alkyl chain conformation. The investigated thiols (namely F4H10SH, F8H6SH, F6H10SH, F10H6SH, F6H11SH, F8H10SH and F10H10SH) differ in the length of the hydrophobic chain as well as in the degree of fluorination. Although the -SH group cannot form strong hydrogen bonds with the water subphase, thereby causing only weak anchoring, it has been found that all the investigated thiols, except for F4H10SH, are capable of stable Langmuir monolayer formation. The investigated thiols can be divided into two categories - thiols with longer alkyl chain (F10H10SH, F10H6SH, F8H10SH) and compds. with shorter hydrophobic part (F6H10SH, F6H11SH, F8H6SH). The monolayers of the latter thiols have their equil. surface pressure (ESP) comparable with the collapse pressure (πC) of their monolayers; thus these films are stable within the whole range of compression. On the contrary, thiols with longer perfluorinated fragments have considerably lower ESP than the π C of their monolayers; therefore, both stable and metastable regions can be distinguished in their π-A isotherms. Interestingly, for F8H6SH, a smectic ordering of mols. can be obsd. in monolayers as visualized with BAM. By comparing film-forming properties of the studied semifluorinated thiols with previously studied semifluorinated alkanes it has been found that the presence of -SH group facilitates spreading at the free water surface. Semifluorinated thiols have been found to maintain their monomol. ordering in the presence of heavy metal cations in the aq. subphase, contrary to their hydrogenated analogs, which have been reported to crystallize under the same conditions.