Effect of line tension on domain formation of fluorinated compounds at alkane/water interface

Condensed domain formation of fluorinated compounds (alcohol and ester) in adsorbed films at alkane/water interfaces was investigated by interfacial tensiometry, X-ray reflectometry, and BAM observation. The results obtained are explained by the effects of contact energy and dipole-dipole interaction, which are competitive contributions to the line tension acting at the domain boundary. Estimation of the two contributions indicated that the former is more dominant than the latter for the domain formation. In the adsorbed films of fluorinated alcohol, F₈H₂OH, at alkane/water interfaces, the domain formation was suppressed with increasing alkane chain length due to an increase in contact energy. The mixing of hybrid alcohol with fluorocarbon and hydrocarbon chains of the same carbon number, F₆H₆OH, in adsorbed film reduces contact energy. This stabilizes the dispersion of many domains with small size. On the other hand, when F₈H₂OH molecules are mixed with a cationic surfactant, C₁₂TAB, in the adsorbed film, an attractive dipole-ion interaction between the hydrophilic groups relaxes the repulsive dipole-dipole interaction between F₈H₂OH molecules, resulting in the formation of domains with smooth boundary and large size. Furthermore, fluorinated ester, FC₁₂Me molecules form flower shaped domains with intricate boundary mainly due to a larger dipole moment of FC₁₂Me than F₈H₂OH.