Surface crystallization and phase transitions of the adsorbed film of F(CF 2 ) 12 (CH 2 ) 16 H at the surface of liquid tetradecane

Abstract

The interaction between a long chain alkane, tetradecane (abbreviated H14), molecule and a semi-fluorinated alkane, 1-perfluorododecyl-hexadecane F(CF2)12(CH2)16H (abbreviated F12H16), molecule at the air/ H14 solution interface was studied by measuring the surface tension of the H14 solutions of F12H16 as a function of temperature and bulk concentration under atmospheric pressure. Pure liquid H14 freezes without forming a condensed film at its surface. Nevertheless, a very small amount of F12H16 initiates the surface freezing of H14. In contrast to the F12H16-hexadecane (abbreviated H16) system, the condensed monolayer of H14 has a finite solubility of F12H16 in the F12H16-H14 system. By further increasing the bulk concentration of F12H16, the F12 chains of the F12H16 molecules form the other closely packed condensed state. Hence, as in the case of the H16 system, the H14 system also exhibits a surface hetero-azeotrope behavior in the lower temperature region. Below the surface hetero-azeotropic point, the condensed H14 monolayer containing a small amount of F12H16 is completely replaced by the condensed monolayer of F12H16. At 2 °C, for example, a surface of H14 solution of F12H16 covered with a gaseous film of F12H16 is replaced by a condensed H14 monolayer containing an almost gaseous state of F12H16, and is then completely replaced by the condensed monolayer of F12H16 with increasing bulk concentration. Above the temperature of the triple point for the F12H16 monolayer, the F12H16-H14 system exhibits a gaseous, expanded, and condensed state.