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Comparative Studies of Film-Forming Properties of partially Fluorinated Tetracosanes

Broniatowski M., Miňones J., Sandez Macho I, Dynarowicz-Łątka P.

Polish Journal of Chemistry

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2005

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Vol. 79 / nr 6

1047-1061

EN

Contrary to simple n-alkanes of moderate molecular weight, which are not capable of Langmuir monolayer formation, partially fluorinated hydrocarbons of the general formula: F(CF2)m(CH2)nH (abbreviated as FmHn) were found to form monomolecular layers at the air/water interface. The aim of this study was to characterize several semifluorinated alkanes (SFA) containing 24 carbon atoms in total (tetracosanes) differing in the m/n ratio, as Langmuir monolayers at the free water surface. The following compounds have been investigated: F4H20, F6H18, F8H16, F10H14 and F12H12. Surface pressure (pi) and electric surface potential deltaV) isotherms were recorded in addition to quantitative Brewster angle microscopy results. The negative sign of deltaV evidenced for the orientation of all the semifluorinated tetracosanes investigated here, regardless the length of the hydrogenated segment, with their perfluorinated parts directed towards the air. As inferred from apparent dipole moment values and relative reflectivity results, tetracosanes with shorter perfluorinated fragment (F4H20 and F6H18) were found to take vertical orientation at the air/water interface, while those with longer perfluorinated moiety (F8H16, F10H14 and F12H12) remain tilted even in the vicinity of the film collapse. The observed differences may result from a different conformation of the perfluorinated chain, i.e. more fluorinated tetracosanes (F10H14 and F12H12) have their perfluorinated parts sufficiently long to form a helix, while less fluorinated derivatives (F4H20 and F6H18) may possibly exist in all-trans configuration. In particular, the perfluorinated fragment in F8H16 is considered to have an intermediate character between these two structures.

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Semifluorowane alkany (SFA) - właściwości w ciele stałym, roztworach i na granicach międzyfazowych

Broniatowski M., Dynarowicz-Łątka P.

Wiadomości Chemiczne

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2004

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[Z] 58, 7-8

535-569

EN

Semifluorinated n-alkanes (SFA), of the general formula F(CF2)m-(CH2)nH (abbreviated as FmHn) consist of a linear hydrocarbon segment linked to a fluorocarbon chain. These two units are highly incompatible, what arises from a very different physical and mechanical properties of hydrogenated and perfluorinated hydrocarbons. The presence of two opposing segments within one molecule makes semifluorinated alkanes a very interesting class of compounds, which show a particular behavior in bulk and at interfaces. Their highly asymmetric structure, arising from the incompatibility of the both constituent parts, results in surface activity of these molecules (so-called primitive surfactants) when dissolved in organic solvents, and allows for the Langmuir monolayer formation if spread at the air/water interface, despite of the absence of polar group. Since 1984, semifluorinated alkanes have been subject to many studies concerning their structure and physicochemical properties in bulk phase (solid and liquid) and at interfaces (oil/water; air/water). The present article reviews the results obtained so far.

3

Properties of Langmuir Monolayers from Perfluorohexyl-n-alkanes

Broniatowski M., Dynarowicz-Łątka P.

Polish Journal of Chemistry

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2004

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Vol.78 /nr 11-12

2149-2162

EN

A series of semifluorinated n-alkanes (SFAs), of the general formula: F(CF2)m(CH2)nH, (in short FmHn), where m = 6 and n = 16-20, have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were obtained under a variety of experimental conditions. The Langmuir monolayer experiments have been complemented with Brewster angle microscopy results, which enabled both a direct visualization of the monolayers structure and the estimation of their thicknesses at different stages of compression. Our results show, that these "non-classical" film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are capable of monolayer formation at the air/water interface. The negative sign of the measured surface potential, DeltaV, proves that SFA molecules are oriented at the air/water interface with their perfluorinated parts directed towards the air, independently on the length of hydrogenated moiety. The change of electric surface potential achieves the minimum value of ca. -0.75 V for all the investigated SFA. The minimum effective dipole moment is achieved for a molecule oriented at the angle of about 35 graduate to the surface normal. The relative intensity measurements allow one to conclude, that film molecules are oriented vertically in respect to the surface normal at the vicinity of collapse.

4

Perfluoroalkyl-n-eicosanes at the Air-Water Interface-A Monolayer Study

Broniatowski M., Dynarowicz-Łątka P.

Polish Journal of Chemistry

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2004

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Vol. 78 / nr 7

973-985

EN

Perfluoroalkyl-n-eicosanes of the general formula F(CF2)m(CH2)20H, where m = 4,6, 8,10,12 were spread at the air/water as Langmuir monolayers and studied at different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (-A isotherms) have been complemented with quantitative Brewster angle microscopy results, which enabled estimation of the film thickness at different stages of compression. Although the investigated molecules do not possess any polar group and are purely hydrophobic, they form stable monomolecular layers at the free water surface. The negative sign of the measured surface potential, Delta V, evidences for the orientation of the molecules with their perfluorinated parts exposed towards the air, independently of the length of the perfluorinated moiety. The relative intensity measurements allow one to conclude that the molecules with shorter perfluorinated part (F4H20, F6H20 and F8H20) are oriented almost vertically (in respect to the interface) in the vicinity of film collapse, while F10H20 and F12H20 are tilted to the water surface.