Properties of Langmuir Monolayers from Perfluorohexyl-n-alkanes

摘要

A series of semifluorinated n-alkanes (SFAs),of the general formula:F(CF_2)m(CH_2)_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,AV,proves that SFA molecules are oriented at the air/water interface with their per-fluorinated 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~o 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.