Phase transitions in Langmuir monolayers of a rhodamine dye as studied by a second harmonic generation technique

Phase transitions in the Langmuir monolayer of a xanthene dye alkylated with two long hydrophobic chains have been studied by a second harmonic generation (SHG) technique at various temperatures. Owing to the sensitivity of the SHG technique to the molecular ordering at an interface, transitions between phases of different orientational organizations of dye chromophores have been observed in the shape of SHG signals. The method of the derivative of an SHG signal has been proposed to determine the points of phase transitions in the Langmuir monolayer. Obtained results are in good agreement with those derived from (π-A) isotherms. Several additional orientational phase transitions have been found upon compression of a liquid phase of the layer. Detailed comparison of the SHG signals with the (π-A) isotherms has allowed us to conclude that the phase transitions at a low surface pressure may not involve the rearrangement of dye chromophores. Formation of a denser phase likely occurs by means of the orientational rearrangement of flexible alkyl chain network of the layer. However, a transition to the solid phase is accompanied by profound changes in the chromophore arrangement. The amplitude of SHG signals generated by the solid phase increases with a decrease in the temperature, suggesting the formation of more ordered layers at lower temperatures.